Probability of stress-corrosion fracture under random loading.

NASA Technical Reports Server (NTRS)

Yang, J.-N.

1972-01-01

A method is developed for predicting the probability of stress-corrosion fracture of structures under random loadings. The formulation is based on the cumulative damage hypothesis and the experimentally determined stress-corrosion characteristics. Under both stationary and nonstationary random loadings, the mean value and the variance of the cumulative damage are obtained. The probability of stress-corrosion fracture is then evaluated using the principle of maximum entropy. It is shown that, under stationary random loadings, the standard deviation of the cumulative damage increases in proportion to the square root of time, while the coefficient of variation (dispersion) decreases in inversed proportion to the square root of time. Numerical examples are worked out to illustrate the general results.

Experimental and computational correlation of fracture parameters KIc, JIc, and GIc for unimodular and bimodular graphite components

NASA Astrophysics Data System (ADS)

Bhushan, Awani; Panda, S. K.

2018-05-01

The influence of bimodularity (different stress ∼ strain behaviour in tension and compression) on fracture behaviour of graphite specimens has been studied with fracture toughness (KIc), critical J-integral (JIc) and critical strain energy release rate (GIc) as the characterizing parameter. Bimodularity index (ratio of tensile Young's modulus to compression Young's modulus) of graphite specimens has been obtained from the normalized test data of tensile and compression experimentation. Single edge notch bend (SENB) testing of pre-cracked specimens from the same lot have been carried out as per ASTM standard D7779-11 to determine the peak load and critical fracture parameters KIc, GIc and JIc using digital image correlation technology of crack opening displacements. Weibull weakest link theory has been used to evaluate the mean peak load, Weibull modulus and goodness of fit employing two parameter least square method (LIN2), biased (MLE2-B) and unbiased (MLE2-U) maximum likelihood estimator. The stress dependent elasticity problem of three-dimensional crack progression behaviour for the bimodular graphite components has been solved as an iterative finite element procedure. The crack characterizing parameters critical stress intensity factor and critical strain energy release rate have been estimated with the help of Weibull distribution plot between peak loads versus cumulative probability of failure. Experimental and Computational fracture parameters have been compared qualitatively to describe the significance of bimodularity. The bimodular influence on fracture behaviour of SENB graphite has been reflected on the experimental evaluation of GIc values only, which has been found to be different from the calculated JIc values. Numerical evaluation of bimodular 3D J-integral value is found to be close to the GIc value whereas the unimodular 3D J-value is nearer to the JIc value. The significant difference between the unimodular JIc and bimodular GIc indicates that GIc should be considered as the standard fracture parameter for bimodular brittle specimens.

The Influence of Specimen Type on Tensile Fracture Toughness of Rock Materials

NASA Astrophysics Data System (ADS)

Aliha, Mohammad Reza Mohammad; Mahdavi, Eqlima; Ayatollahi, Majid Reza

2017-03-01

Up to now, several methods have been proposed to determine the mode I fracture toughness of rocks. In this research, different cylindrical and disc shape samples, namely: chevron bend (CB), short rod (SR), cracked chevron notched Brazilian disc (CCNBD), and semi-circular bend (SCB) specimens were considered for investigating mode I fracture behavior of a marble rock. It is shown experimentally that the fracture toughness values of the tested rock material obtained from different test specimens are not consistent. Indeed, depending on the geometry and loading type of the specimen, noticeable discrepancies can be observed for the fracture toughness of a same rock material. The difference between the experimental mode I fracture resistance results is related to the magnitude and sign of T-stress that is dependent on the geometry and loading configuration of the specimen. For the chevron-notched samples, the critical value of T-stress corresponding to the critical crack length was determined using the finite element method. The CCNBD and SR specimens had the most negative and positive T-stress values, respectively. The dependency of mode I fracture resistance to the T-stress was shown using the extended maximum tangential strain (EMTSN) criterion and the obtained experimental rock fracture toughness data were predicted successfully with this criterion.

Evaluation of retention and fracture resistance of different fiber reinforced posts: An in vitro study.

PubMed

Pruthi, Varun; Talwar, Sangeeta; Nawal, Ruchika Roongta; Pruthi, Preeti Jain; Choudhary, Sarika; Yadav, Seema

2018-01-01

The aim of this study was to evaluate retention & fracture resistance of different fibre posts. 90 extracted human permanent maxillary central incisors were used in this study. For retention evaluation, after obturation, post space preparation was done in all root canals and posts were cemented under three groups. Later, the posts were grasped & pulled out from the roots with the help of a three-jaw chuck at a cross-head speed of 5mm/min. Force required to dislodge each post was recorded in Newtons. To evaluate the fracture behavior of posts, artificial root canals were drilled into aluminium blocks and posts were cemented. Load required to fracture each post was recorded in Newtons. The results of the present study show the mean retention values for Fibrekleer Parallel post were significantly greater than those for Synca Double tapered post & Bioloren Tapered post. The mean retention values of the Double tapered post & the tapered post were not statistically different. The Synca Double tapered post had the highest mean load to fracture, and this value was significantly higher than those of FibreKleer Parallel & Bioloren Tapered post. The mean fracture resistance values of Parallel & tapered post were not statistically different. This study showed parallel posts to have better retention than tapered and double tapered posts. Regarding the fracture resistance, double tapered posts were found to be better than parallel and tapered posts.

Comparison of fracture resistance and fracture characterization of bilayered zirconia/fluorapatite and monolithic lithium disilicate all ceramic crowns.

PubMed

Altamimi, Abdulaziz M; Tripodakis, Aris Petros; Eliades, George; Hirayama, Hiroshi

2014-01-01

To compare the fracture resistance between bilayered zirconia/ fluorapatite and monolithic lithium disilicate heat-pressed crowns and characterize the mode of fracture failure. Thirty crown samples were sequentially fitted on a mandibular right first molar metal replica of an ivory prepared molar tooth. The crown specimens were divided in three groups (A, B, and C; n = 10 for each group). Group A consisted of bilayered zirconia/fluorhapatite pressed-over crowns with standard design crown copings (0.7 mm uniform thickness), Group B of bilayered zirconia/fluorhapatite with anatomical design crown copings, and Group C of lithium disilicate monolithic crowns. The samples were then dynamically loaded under water for 100,000 cycles with a profile of 250 N maximum load at 1,000 N/s rate and 2.0 Hz frequency. Loading was performed with a steel ball (5 mm in diameter) coming into contact with the test crown, loading to maximum, holding for 0.2 s, unloading and lifting off 0.5 mm. The samples were then fractured under static loading, in order to determine the ultimate crown strength. Analysis of the recorded fracture load values was carried out with one-way analysis of variance (ANOVA) followed by Tukey tests. Fractured specimens were examined by stereomicroscopy and scanning electron microscopy. The fracture loads measured were (N, means and standard deviations): Group A: 561.87 (72.63), Group B: 1,014.16 (70.18) and Group C: 1,360.63 (77.95). All mean differences were statistically significant (P < 0.001). Catastrophic fractures occurred in Group C, whereas mainly veneer fractures were observed in Groups A and B. In the present study, the heat-pressed monolithic lithium-disilicate crowns showed more fracture resistance than zirconia/fluorapatite pressed-over crowns. Within the bilayered groups, the anatomical zirconia coping design presented increased ceramic fracture resistance.

Fracture load of metal-ceramic, monolithic, and bi-layered zirconia-based posterior fixed dental prostheses after thermo-mechanical cycling.

PubMed

López-Suárez, Carlos; Castillo-Oyagüe, Raquel; Rodríguez-Alonso, Verónica; Lynch, Christopher D; Suárez-García, María-Jesús

2018-06-01

This study aims to evaluate the fracture load of differently fabricated 3-unit posterior fixed dental prostheses (FDPs) with an intermediate pontic. Fifty sets of two stainless-steel abutments were randomly assigned to five groups (n = 10 each) depending on the material and technique used for manufacturing the FDPs: (1) Metal-ceramic (MC, control); (2) Lava Zirconia (LZ, bi-layered); (3) Lava Plus (LM, monolithic); (4) VITA In-Ceram YZ (YZ, bi-layered); and (5) IPS e-max ZirCAD (ZZ, bi-layered). After being luted to the dies, all FDPs were submitted to thermo-mechanical cycling (120,000 masticatory cycles, 50 N; plus 774 thermal cycles of 5 °C/55 °C, dwell time: 30 s). Samples were then subjected to a three-point bending test until fracture in a universal testing machine (cross-head speed: 1 mm/min). Fracture load of the veneering ceramic (VF) and total fracture load (TF) were recorded. Microstructure and failure patterns were assessed. Data was analysed using one-way ANOVA and Tukey HSD post-hoc tests (α = 0.05). MC restorations recorded higher VF and TF values than did zirconia FDPs (p = 0.0001), which showed no between-group differences. Within the bi-layered groups, TF was significantly higher than VF. LM pieces registered lower average grain size than did LZ specimens (p = 0.001). Overall, the connector was the weakest part. All of the groups tested could withstand clinical chewing forces in terms of average fracture load. Zirconia-based samples performed similarly to each other, but showed lower mean fracture load values than did metal-ceramic ones. Monolithic zirconia may be recommendable for solving the chipping problem. Copyright © 2018 Elsevier Ltd. All rights reserved.

Fracture properties of concrete specimens made from alkali activated binders

NASA Astrophysics Data System (ADS)

Šimonová, Hana; Kucharczyková, Barbara; Topolář, Libor; Bílek, Vlastimil, Jr.; Keršner, Zbyněk

2017-09-01

The aim of this paper is to quantify crack initiation and other fracture properties - effective fracture toughness and specific fracture energy - of two types of concrete with an alkali activated binder. The beam specimens with a stress concentrator were tested in a three-point bending test after 28, 90, and 365 days of maturing. Records of fracture tests in the form of load versus deflection (P-d) diagrams were evaluated using effective crack model and work-of-fracture method and load versus mouth crack opening displacement (P-CMOD) diagrams were evaluated using the Double-K fracture model. The initiation of cracks during the fracture tests for all ages was also monitored by the acoustic emission method. The higher value of monitored mechanical fracture parameters of concrete with alkali activated blast furnace slag were achieved with substitution blast furnace slag by low calcium fly ash in comparison with substitution by cement kiln dust.

Fracture toughness determination using spiral-grooved cylindrical specimen and pure torsional loading

DOEpatents

Wang, Jy-An; Liu, Kenneth C.

2003-07-08

A method for determining fracture toughness K.sub.IC of materials ranging from metallic alloys, brittle ceramics and their composites, and weldments. A cylindrical specimen having a helical V-groove with a 45.degree. pitch is subjected to pure torsion. This loading configuration creates a uniform tensile-stress crack-opening mode, and a transverse plane-strain state along the helical groove. The full length of the spiral groove is equivalent to the thickness of a conventional compact-type specimen. K.sub.IC values are determined from the fracture torque and crack length measured from the test specimen using a 3-D finite element program (TOR3D-KIC) developed for the purpose. In addition, a mixed mode (combined tensile and shear stress mode) fracture toughness value can be determined by varying the pitch of the helical groove. Since the key information needed for determining the K.sub.IC value is condensed in the vicinity of the crack tip, the specimen can be significantly miniaturized without the loss of generality.

Dynamic fatigue performance of implant-abutment assemblies with different tightening torque values.

PubMed

Xia, Dandan; Lin, Hong; Yuan, Shenpo; Bai, Wei; Zheng, Gang

2014-01-01

Implant-abutment assemblies are usually subject to long-term cyclic loading. To evaluate the dynamic fatigue performance of implant-abutment assemblies with different tightening torque values, thirty implant-abutment assemblies (Zimmer Dental, Carlsbad, CA, USA) were randomly assigned to three tightening groups (24 Ncm; 30 Ncm; 36 Ncm), each consisted of 10 implants. Five specimens from each group were unscrewed, and their reverse torque values recorded. The remaining specimens were subjected to a load between 30 N~300 N at a loading frequency of 15 Hz for 5 × 10(6) cycles. After fatigue tests, residual reverse torque values were recorded if available. In the 24 Ncm tightening group, all the implants fractured at the first outer thread of the implant after fatigue loading, with fatigue crack propagation at the fractured surface showed by SEM observation. For the 30 Ncm and 36 Ncm tightening groups, a statistical significant difference (p<0.05) between the unloaded and loaded groups was revealed. Compared with the unloaded specimens, the specimens went through fatigue loading had decreased reverse torque values. It was demonstrated that insufficient torque will lead to poor fatigue performance of dental implant-abutment assemblies and abutment screws should be tightened to the torque recommended by the manufacturer. It was also concluded that fatigue loading would lead to preload loss.

Fracture strength of lithium disilicate crowns compared to polymer-infiltrated ceramic-network and zirconia reinforced lithium silicate crowns.

PubMed

Sieper, Kim; Wille, Sebastian; Kern, Matthias

2017-10-01

The aim of this study was to evaluate the fracture strength of crowns made from current CAD/CAM materials. In addition the influence of crown thickness and chewing simulation on the fracture strength was evaluated. Crowns were fabricated from lithium disilicate, zirconia reinforced lithium silicate (ZLS-ceramic) and a polymer-infiltrated ceramic-network (PICN) with an occlusal thickness of 1.0mm or 1.5mm, respectively (n=16). Crowns were cemented on composite dies. Subgroups of eight specimens were loaded with 5kg in a chewing simulator for 1,200,000 cycles with thermal cycling. Finally, all specimens were loaded until fracture in a universal testing machine. Three-way ANOVA was used to detect statistical interaction. Differences regarding the materials were tested with two-way ANOVA, following one-way ANOVA and a post-hoc Tukey's-Test. All crowns survived the chewing simulation. The material had a significant influence on the fracture resistance (p≤0.05). Lithium disilicate achieved the highest values of fracture strength in almost all groups followed by ZLS-ceramic. PICN achieved the lowest values of fracture strength. Chewing simulation increased the fracture strength of thick lithium disilicate crown significantly. Greater occlusal thickness of all crown materials resulted in higher crown fracture strength before chewing simulation. After chewing simulation occlusal thickness of lithium disilicate and PICN crowns had no significant influence on the fracture strength. All crowns revealed fracture strength above the clinically expected loading forces. Therefore the durability of the tested CAD/CAM materials seems promising also in an occlusal thickness of 1.0mm. Copyright © 2017 Elsevier Ltd. All rights reserved.

Fracture resistance of zirconia-based implant abutments after artificial long-term aging.

PubMed

Alsahhaf, Abdulaziz; Spies, Benedikt Christopher; Vach, Kirstin; Kohal, Ralf-Joachim

2017-02-01

To investigate the survival rate, fracture strength, bending moments, loading to fracture and fracture modes of different designs of zirconia abutments after dynamic loading with thermocycling, and compare these values to titanium abutments. A total of 80 abutment samples were divided into 5 test groups of 16 samples in each group. The study included the following groups, "Group 1" CAD/CAM produced all-zirconia abutments, "Group 2" titanium abutments, "Group 3" zirconia-abutments adhesively luted to a titanium base, "Group 4" prefabricated all-zirconia abutments and "Group 5" zirconia-abutments glass soldered to a titanium base. Half the number of samples in each group was exposed to 1.2 million loading cycles (5-years simulation) in the chewing simulator. The samples that survived the artificial aging were later tested for fracture strength in a universal testing machine. The remaining 8 samples of the group were directly tested for fracture strength. All samples exposed to the 5-years artificial aging survived except of six samples in one group (Group 1). The surviving samples were later fracture tested in the universal testing machine. The bending moments (Ncm) values were as follow: Exposed groups: "Group 1" 94.5Ncm; "Group 2" 599.2Ncm; "Group 3" 477.5Ncm; "Group 4" 314.4Ncm; "Group 5" 509.4Ncm. Non-exposed groups: "Group 1" 269.3Ncm; "Group 2" 474.2Ncm; "Group 3" 377.6Ncm; "Group 4" 265.4Ncm; "Group 5" 372.4Ncm. Except in Group 1, the values were higher in the exposed groups, although, statistically there was no difference (p>0.05). The one-piece ZrO2-abutment group (Group 1 and Group 4) exhibited lower values, while the two-piece ZrO2-abutment groups (Group 3 and Group 5) showed similar values and fracture modes like the titanium abutment group. The titanium abutment group showed the highest values of bending moments among all groups. The implant-abutment connection area appeared to influence the bending moment value and the fracture mode of the tested abutment groups, and it was found to be the weakest part of an internal connection one-piece zirconia abutment. The titanium base in the two-piece zirconia abutment worked as a substitute for the weakest part of the abutment. Therefore, the titanium base can reinforce the fracture strength of a zirconia abutment. Copyright © 2016 Elsevier Ltd. All rights reserved.

Fatigue loading and R-curve behavior of a dental glass-ceramic with multiple flaw distributions.

PubMed

Joshi, Gaurav V; Duan, Yuanyuan; Della Bona, Alvaro; Hill, Thomas J; St John, Kenneth; Griggs, Jason A

2013-11-01

To determine the effects of surface finish and mechanical loading on the rising toughness curve (R-curve) behavior of a fluorapatite glass-ceramic (IPS e.max ZirPress) and to determine a statistical model for fitting fatigue lifetime data with multiple flaw distributions. Rectangular beam specimens were fabricated by pressing. Two groups of specimens (n=30) with polished (15 μm) or air abraded surface were tested under rapid monotonic loading in oil. Additional polished specimens were subjected to cyclic loading at 2 Hz (n=44) and 10 Hz (n=36). All fatigue tests were performed using a fully articulated four-point flexure fixture in 37°C water. Fractography was used to determine the critical flaw size and estimate fracture toughness. To prove the presence of R-curve behavior, non-linear regression was used. Forward stepwise regression was performed to determine the effects on fracture toughness of different variables, such as initial flaw type, critical flaw size, critical flaw eccentricity, cycling frequency, peak load, and number of cycles. Fatigue lifetime data were fit to an exclusive flaw model. There was an increase in fracture toughness values with increasing critical flaw size for both loading methods (rapid monotonic loading and fatigue). The values for the fracture toughness ranged from 0.75 to 1.1 MPam(1/2) reaching a plateau at different critical flaw sizes based on loading method. Cyclic loading had a significant effect on the R-curve behavior. The fatigue lifetime distribution was dependent on the flaw distribution, and it fit well to an exclusive flaw model. Copyright © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Fatigue loading and R-curve behavior of a dental glass-ceramic with multiple flaw distributions

PubMed Central

Joshi, Gaurav V.; Duan, Yuanyuan; Bona, Alvaro Della; Hill, Thomas J.; John, Kenneth St.; Griggs, Jason A.

2013-01-01

Objectives To determine the effects of surface finish and mechanical loading on the rising toughness curve (R-curve) behavior of a fluorapatite glass-ceramic (IPS e.max ZirPress) and to determine a statistical model for fitting fatigue lifetime data with multiple flaw distributions. Materials and Methods Rectangular beam specimens were fabricated by pressing. Two groups of specimens (n=30) with polished (15 μm) or air abraded surface were tested under rapid monotonic loading in oil. Additional polished specimens were subjected to cyclic loading at 2 Hz (n=44) and 10 Hz (n=36). All fatigue tests were performed using a fully articulated four-point flexure fixture in 37°C water. Fractography was used to determine the critical flaw size and estimate fracture toughness. To prove the presence of R-curve behavior, non-linear regression was used. Forward stepwise regression was performed to determine the effects on fracture toughness of different variables, such as initial flaw type, critical flaw size, critical flaw eccentricity, cycling frequency, peak load, and number of cycles. Fatigue lifetime data were fit to an exclusive flaw model. Results There was an increase in fracture toughness values with increasing critical flaw size for both loading methods (rapid monotonic loading and fatigue). The values for the fracture toughness ranged from 0.75 to 1.1 MPa·m1/2 reaching a plateau at different critical flaw sizes based on loading method. Significance Cyclic loading had a significant effect on the R-curve behavior. The fatigue lifetime distribution was dependent on the flaw distribution, and it fit well to an exclusive flaw model. PMID:24034441

Effect of cements on fracture resistance of monolithic zirconia crowns

PubMed Central

Nakamura, Keisuke; Mouhat, Mathieu; Nergård, John Magnus; Lægreid, Solveig Jenssen; Kanno, Taro; Milleding, Percy; Örtengren, Ulf

2016-01-01

Abstract Objectives The present study investigated the effect of cements on fracture resistance of monolithic zirconia crowns in relation to their compressive strength. Materials and methods Four different cements were tested: zinc phosphate cement (ZPC), glass-ionomer cement (GIC), self-adhesive resin-based cement (SRC) and resin-based cement (RC). RC was used in both dual cure mode (RC-D) and chemical cure mode (RC-C). First, the compressive strength of each cement was tested according to a standard (ISO 9917-1:2004). Second, load-to-failure test was performed to analyze the crown fracture resistance. CAD/CAM-produced monolithic zirconia crowns with a minimal thickness of 0.5 mm were prepared and cemented to dies with each cement. The crown–die samples were loaded until fracture. Results The compressive strength of SRC, RC-D and RC-C was significantly higher than those of ZPC and GIC (p < 0.05). However, there was no significant difference in the fracture load of the crown between the groups. Conclusion The values achieved in the load-to-failure test suggest that monolithic zirconia crowns with a minimal thickness of 0.5 mm may have good resistance against fracture regardless of types of cements. PMID:27335900

Modeling of crack growth under mixed-mode loading by a molecular dynamics method and a linear fracture mechanics approach

NASA Astrophysics Data System (ADS)

Stepanova, L. V.

2017-12-01

Atomistic simulations of the central crack growth process in an infinite plane medium under mixed-mode loading using Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), a classical molecular dynamics code, are performed. The inter-atomic potential used in this investigation is the Embedded Atom Method (EAM) potential. Plane specimens with an initial central crack are subjected to mixed-mode loadings. The simulation cell contains 400,000 atoms. The crack propagation direction angles under different values of the mixity parameter in a wide range of values from pure tensile loading to pure shear loading in a wide range of temperatures (from 0.1 K to 800 K) are obtained and analyzed. It is shown that the crack propagation direction angles obtained by molecular dynamics coincide with the crack propagation direction angles given by the multi-parameter fracture criteria based on the strain energy density and the multi-parameter description of the crack-tip fields. The multi-parameter fracture criteria are based on the multi-parameter stress field description taking into account the higher order terms of the Williams series expansion of the crack tip fields.

Fracture resistance and reliability of new zirconia posts.

PubMed

Oblak, Cedomir; Jevnikar, Peter; Kosmac, Tomaz; Funduk, Nenad; Marion, Ljubo

2004-04-01

The radicular portion of zirconia endodontic posts often need to be reshaped to achieve a definitive form and may be airborne-particle abraded to improve adhesion during luting. Therefore, the surface of the tetragonal zirconia ceramics may be transformed and damaged, influencing the mechanical properties of the material. This study compared the fracture resistance of prefabricated zirconia posts with a new retentive post-head after different surface treatments. Experimental zirconia posts of 2 different diameters, 1.3 mm and 1.5 mm, were produced from commercially available zirconia powder. A cylindro-conical outline form was used for the root portion of the system and a post-head with 3 retentive rings was designed. Sixty posts of each diameter were divided into 3 groups (n=20). Group 1 was ground with a coarse grit diamond bur; Group 2 was airborne-particle abraded with 110-microm fused alumina particles, and Group 3 was left as-received (controls). Posts were luted into the root-shaped artificial canals with the Clearfil adhesive system and Panavia 21 adhesive resin luting agent. The posts were loaded in a universal testing machine at an inclination of 45 degrees with the constant cross-head speed of 1 mm/min. The fracture load (N) necessary to cause post fracture was recorded, and the statistical significance of differences among groups was analyzed with 1-way ANOVA followed by the Fischer LSD test (alpha=.05). The variability was analyzed using Weibull statistics. Load to fracture values of all zirconia posts depended primarily on post diameter. Mean fracture loads (SD) in Newtons were 518.4 (+/-101.3), 993.6 (+/-224.1), and 622.7 (+/-110.3) for Groups 1 through 3, respectively, for thicker posts, and 385.9 (+/-110.3), 627.0 (+/-115.1), and 451.2 (+/-81.4) for Groups 1 through 3, respectively, for thinner posts. Airborne-particle-abraded posts exhibited significantly higher resistance to fracture (P<.05) than those in the other 2 groups for diameters 1.3 mm and 1.5 mm. Grinding reduced Weibull modulus compared with controls, and the values were 4.1 and 6.5 for thicker and thinner posts, respectively. Within the limitations of this study, the results suggest that grinding leads to a significant drop in load to fracture of zirconia posts, whereas airborne-particle abrasion increased the fracture load.

The Extravehicular Suit Impact Load Attenuation Study for Use in Astronaut Bone Fracture Prediction

NASA Technical Reports Server (NTRS)

Lewandowski, Beth E.; Gilkey, Kelly M.; Sulkowski, Christina M.; Samorezov, Sergey; Myers, Jerry G.

2011-01-01

The NASA Integrated Medical Model (IMM) assesses the risk, including likelihood and impact of occurrence, of all credible in-flight medical conditions. Fracture of the proximal femur is a traumatic injury that would likely result in loss of mission if it were to happen during spaceflight. The low gravity exposure causes decreases in bone mineral density which heightens the concern. Researchers at the NASA Glenn Research Center have quantified bone fracture probability during spaceflight with a probabilistic model. It was assumed that a pressurized extravehicular activity (EVA) suit would attenuate load during a fall, but no supporting data was available. The suit impact load attenuation study was performed to collect analogous data. METHODS: A pressurized EVA suit analog test bed was used to study how the offset, defined as the gap between the suit and the astronaut s body, impact load magnitude and suit operating pressure affects the attenuation of impact load. The attenuation data was incorporated into the probabilistic model of bone fracture as a function of these factors, replacing a load attenuation value based on commercial hip protectors. RESULTS: Load attenuation was more dependent on offset than on pressurization or load magnitude, especially at small offsets. Load attenuation factors for offsets between 0.1 - 1.5 cm were 0.69 +/- 0.15, 0.49 +/- 0.22 and 0.35 +/- 0.18 for mean impact forces of 4827, 6400 and 8467 N, respectively. Load attenuation factors for offsets of 2.8 - 5.3 cm were 0.93 +/- 0.2, 0.94 +/- 0.1 and 0.84 +/- 0.5, for the same mean impact forces. Reductions were observed in the 95th percentile confidence interval of the bone fracture probability predictions. CONCLUSIONS: The reduction in uncertainty and improved confidence in bone fracture predictions increased the fidelity and credibility of the fracture risk model and its benefit to mission design and operational decisions.

In vitro study of fracture load and fracture pattern of ceramic crowns: a finite element and fractography analysis.

PubMed

Campos, Roberto Elias; Soares, Carlos José; Quagliatto, Paulo S; Soares, Paulo Vinícius; de Oliveira, Osmir Batista; Santos-Filho, Paulo Cesar Freitas; Salazar-Marocho, Susana M

2011-08-01

This in vitro study investigated the null hypothesis that metal-free crowns induce fracture loads and mechanical behavior similar to metal ceramic systems and to study the fracture pattern of ceramic crowns under compressive loads using finite element and fractography analyses. Six groups (n = 8) with crowns from different systems were compared: conventional metal ceramic (Noritake) (CMC); modified metal ceramic (Noritake) (MMC); lithium disilicate-reinforced ceramic (IPS Empress II) (EMP); leucite-reinforced ceramic (Cergogold) (CERG); leucite fluoride-apatite reinforced ceramic (IPS d.Sign) (SIGN); and polymer crowns (Targis) (TARG). Standardized crown preparations were performed on bovine roots containing NiCr metal dowels and resin cores. Crowns were fabricated using the ceramics listed, cemented with dual-cure resin cement, and submitted to compressive loads in a mechanical testing machine at a 0.5-mm/min crosshead speed. Data were submitted to one-way ANOVA and Tukey tests, and fractured specimens were visually inspected under a stereomicroscope (20×) to determine the type of fracture. Maximum principal stress (MPS) distributions were calculated using finite element analysis, and fracture origin and the correlation with the fracture type were determined using fractography. Mean values of fracture resistance (N) for all groups were: CMC: 1383 ± 298 (a); MMC: 1691 ± 236 (a); EMP: 657 ± 153 (b); CERG: 546 ± 149 (bc); SIGN: 443 ± 126 (c); TARG: 749 ± 113 (b). Statistical results showed significant differences among groups (p < 0.05) represented by different lowercase letters. Metal ceramic crowns presented fracture loads significantly higher than the others. Ceramic specimens presented high incidence of fractures involving either the core or the tooth, and all fractures of polymer crown specimens involved the tooth in a catastrophic way. Based on stress and fractographic analyses it was determined that fracture occurred from the occlusal to the cervical direction. Within the limitations of this study, the results indicated that the use of ceramic and polymer crowns without a core reinforcement should be carefully evaluated before clinical use due to the high incidence of failure with tooth involvement. This mainly occurred for the polymer crown group, although the fracture load was higher than normal occlusal forces. High tensile stress concentrations were found around and between the occlusal loading points. Fractographic analysis indicated fracture originating from the load point and propagating from the occlusal surface toward the cervical area, which is the opposite direction of that observed in clinical situations. © 2011 by The American College of Prosthodontists.

Zirconia-implant-supported all-ceramic crowns withstand long-term load: a pilot investigation.

PubMed

Kohal, Ralf-J; Klaus, Gerold; Strub, Jörg R

2006-10-01

The purpose of this pilot investigation was to test whether zirconia implants restored with different all-ceramic crowns would fulfill the biomechanical requirements for clinical use. Therefore, all-ceramic Empress-1 and Procera crowns were cemented on zirconia implants and exposed to the artificial mouth. Afterwards, the fracture strength of the all-ceramic implant-crown systems was evaluated. Conventional titanium implants restored with porcelain-fused-to-metal (PFM) crowns served as controls. Sixteen titanium implants with 16 PFM crowns and 32 zirconia implants with 16 Empress-1 crowns and 16 Procera crowns each--i.e., three implant-crown groups--were used in this investigation. The titanium implants were fabricated using the ReImplant system and the zirconia implants using the Celay system. The upper left central incisor served as a model for the fabrication of the implants and the crowns. Eight samples of each group were submitted to a long-term load test in the artificial mouth (1.2 million chewing cycles). Subsequently, a fracture strength test was performed with seven of the eight crowns. The remaining eight samples of each group were not submitted to the long-term load in the artificial mouth but were fracture-tested immediately. One loaded and one unloaded sample of each group were evaluated regarding the marginal fit of the crowns. All test samples survived the exposure to the artificial mouth. Three Empress-1 crowns showed cracks in the area of the loading steatite ball. The values for the fracture load in the titanium implant-PFM crown group without artificial loading ranged between 420 and 610 N (mean: 531.4 N), between 460 and 570 N (mean: 512.9 N) in the Empress-1 crown group, and in the Procera crown group the values were between 475 and 700 N (mean: 575.7 N) when not loaded artificially. The results when the specimens were loaded artificially with 1.2 million cycles were as follows: the titanium implant-PFM crowns fractured between 440 and 950 N (mean: 668.6 N), the Empress-1 crowns between 290 and 550 N (mean: 410.7 N), and the Procera crowns between 450 and 725 N (mean: 555.5 N). No statistically significant differences could be found among the groups without artificial load. The fracture values for the PFM and the Procera crowns after artificial loading were statistically significantly higher than that for the loaded Empress-1 crowns. There was no significant difference between the PFM crown group and the Procera group. Within the limits of this pilot investigation, it seems that zirconia implants restored with the Procera crowns possibly fulfill the biomechanical requirements for anterior teeth. However, further investigations with larger sample sizes have to confirm these preliminary results. As three Empress-1 crowns showed crack development in the loading area of the steatite balls in the artificial mouth, their clinical use on zirconia implants has to be questioned.

Effect of overglazed and polished surface finishes on the compressive fracture strength of machinable ceramic materials.

PubMed

Asai, Tetsuya; Kazama, Ryunosuke; Fukushima, Masayoshi; Okiji, Takashi

2010-11-01

Controversy prevails over the effect of overglazing on the fracture strength of ceramic materials. Therefore, the effects of different surface finishes on the compressive fracture strength of machinable ceramic materials were investigated in this study. Plates prepared from four commercial brands of ceramic materials were either surface-polished or overglazed (n=10 per ceramic material for each surface finish), and bonded to flat surfaces of human dentin using a resin cement. Loads at failure were determined and statistically analyzed using two-way ANOVA and Bonferroni test. Although no statistical differences in load value were detected between polished and overglazed groups (p>0.05), the fracture load of Vita Mark II was significantly lower than those of ProCAD and IPS Empress CAD, whereas that of IPS e.max CAD was significantly higher than the latter two ceramic materials (p<0.05). It was concluded that overglazed and polished surfaces produced similar compressive fracture strengths irrespective of the machinable ceramic material tested, and that fracture strength was material-dependent.

Effect of Measured Welding Residual Stresses on Crack Growth

NASA Technical Reports Server (NTRS)

Hampton, Roy W.; Nelson, Drew; Doty, Laura W. (Technical Monitor)

1998-01-01

Welding residual stresses in thin plate A516-70 steel and 2219-T87 aluminum butt weldments were measured by the strain-gage hole drilling and X-ray diffraction methods. The residual stress data were used to construct 3D strain fields which were modeled as thermally induced strains. These 3D strain fields were then analyzed with the WARP31) FEM fracture analysis code in order to predict their effect on fatigue and on fracture. For analyses of fatigue crack advance and subsequent verification testing, fatigue crack growth increments were simulated by successive saw-cuts and incremental loading to generate, as a function of crack length, effects on crack growth of the interaction between residual stresses and load induced stresses. The specimen experimental response was characterized and compared to the WARM linear elastic and elastic-plastic fracture mechanics analysis predictions. To perform the fracture analysis, the plate material's crack tearing resistance was determined by tests of thin plate M(T) specimens. Fracture analyses of these specimen were performed using WARP31D to determine the critical Crack Tip Opening Angle [CTOA] of each material. These critical CTOA values were used to predict crack tearing and fracture in the weldments. To verify the fracture predictions, weldment M(T) specimen were tested in monotonic loading to fracture while characterizing the fracture process.

Law of damage accumulation and fracture criteria in highly filled polymer materials

NASA Astrophysics Data System (ADS)

Bykov, D. L.; Kazakov, A. V.; Konovalov, D. N.; Mel'nikov, V. P.; Milyokhin, Yu. M.; Peleshko, V. A.; Sadovnichii, D. N.

2014-09-01

We present the results of a large series of experiments aimed at the study of laws of damage accumulation and fracture in highly filled polymer materials under loading conditions of various types: monotone, repeated, low- and high-cycle, with varying type of stress state, dynamic (in general, more than 50 programs implemented on specimens from one lot of material). The data obtained in these test allow one to make conclusions about the constitutive role of the attained maximum of strain intensity when estimating the accumulated damage in the process of uniaxial tension by various programs (in particular, an additional cyclic deformation below the preliminary attained strain maximum does not affect the limit values of strain and stress in the subsequent active extension), about the strong influence of the stress state on the deformation and fracture, about the specific features of the nonlinear behavior of the material under the shock loading conditions and its influence on the repeated deformation. All tests are described (with an accuracy acceptable in practical calculations, both with respect to stresses and strains in the process of loading and at the moment of fracture) in the framework of the same model of nonlinear viscoelasticity with the same set of constants. The constants of the proposed model are calculated according to a relatively simple algorithm by using the results of standard uniaxial tension tests with constant values of the strain rate and hydrostatic pressure (each test for 2-3 levels of these parameters chosen from the ranges proposed in applications, each loading lasts until the fracture occurs, and one of the tests contains an intermediate interval of total loading and repeated loading) and one axial shock compression test if there are dynamic problems in the applications. The model is based on the use of the criterion fracture parameter which, in the class of proportional loading processes, is the sum of partial increments of the strain intensity on active segments of the process (where the strain intensity is at its historical maximum) with the form of the stress state and the intensity of strain rates taken into account.

Biomechanical comparison of orthogonal versus parallel double plating systems in intraarticular distal humerus fractures.

PubMed

Atalar, Ata C; Tunalı, Onur; Erşen, Ali; Kapıcıoğlu, Mehmet; Sağlam, Yavuz; Demirhan, Mehmet S

2017-01-01

In intraarticular distal humerus fractures, internal fixation with double plates is the gold standard treatment. However the optimal plate configuration is not clear in the literature. The aim of this study was to compare the biomechanical stability of the parallel and the orthogonal anatomical locking plating systems in intraarticular distal humerus fractures in artificial humerus models. Intraarticular distal humerus fracture (AO13-C2) with 5 mm metaphyseal defect was created in sixteen artificial humeral models. Models were fixed with either orthogonal or parallel plating systems with locking screws (Acumed elbow plating systems). Both systems were tested for their stiffness with loads in axial compression, varus, valgus, anterior and posterior bending. Then plastic deformation after cyclic loading in posterior bending and load to failure in posterior bending were tested. The failure mechanisms of all the samples were observed. Stiffness values in every direction were not significantly different among the orthogonal and the parallel plating groups. There was no statistical difference between the two groups in plastic deformation values (0.31 mm-0.29 mm) and load to failure tests in posterior bending (372.4 N-379.7 N). In the orthogonal plating system most of the failures occurred due to the proximal shaft fracture, whereas in the parallel plating system failure occurred due to the shift of the most distal screw in proximal fragment. Our study showed that both plating systems had similar biomechanical stabilities when anatomic plates with distal locking screws were used in intraarticular distal humerus fractures in artificial humerus models. Copyright © 2016 Turkish Association of Orthopaedics and Traumatology. Production and hosting by Elsevier B.V. All rights reserved.

The Relationship Between Constraint and Ductile Fracture Initiation as Defined by Micromechanical Analyses

NASA Technical Reports Server (NTRS)

Panontin, Tina L.; Sheppard, Sheri D.

1994-01-01

The use of small laboratory specimens to predict the integrity of large, complex structures relies on the validity of single parameter fracture mechanics. Unfortunately, the constraint loss associated with large scale yielding, whether in a laboratory specimen because of its small size or in a structure because it contains shallow flaws loaded in tension, can cause the breakdown of classical fracture mechanics and the loss of transferability of critical, global fracture parameters. Although the issue of constraint loss can be eliminated by testing actual structural configurations, such an approach can be prohibitively costly. Hence, a methodology that can correct global fracture parameters for constraint effects is desirable. This research uses micromechanical analyses to define the relationship between global, ductile fracture initiation parameters and constraint in two specimen geometries (SECT and SECB with varying a/w ratios) and one structural geometry (circumferentially cracked pipe). Two local fracture criteria corresponding to ductile fracture micromechanisms are evaluated: a constraint-modified, critical strain criterion for void coalescence proposed by Hancock and Cowling and a critical void ratio criterion for void growth based on the Rice and Tracey model. Crack initiation is assumed to occur when the critical value in each case is reached over some critical length. The primary material of interest is A516-70, a high-hardening pressure vessel steel sensitive to constraint; however, a low-hardening structural steel that is less sensitive to constraint is also being studied. Critical values of local fracture parameters are obtained by numerical analysis and experimental testing of circumferentially notched tensile specimens of varying constraint (e.g., notch radius). These parameters are then used in conjunction with large strain, large deformation, two- and three-dimensional finite element analyses of the geometries listed above to predict crack initiation loads and to calculate the associated (critical) global fracture parameters. The loads are verified experimentally, and microscopy is used to measure pre-crack length, crack tip opening displacement (CTOD), and the amount of stable crack growth. Results for A516-70 steel indicate that the constraint-modified, critical strain criterion with a critical length approximately equal to the grain size (0.0025 inch) provides accurate predictions of crack initiation. The critical void growth criterion is shown to considerably underpredict crack initiation loads with the same critical length. The relationship between the critical value of the J-integral for ductile crack initiation and crack depth for SECT and SECB specimens has been determined using the constraint-modified, critical strain criterion, demonstrating that this micromechanical model can be used to correct in-plane constraint effects due to crack depth and bending vs. tension loading. Finally, the relationship developed for the SECT specimens is used to predict the behavior of circumferentially cracked pipe specimens.

Fracture resistance and failure mode of posterior fixed dental prostheses fabricated with two zirconia CAD/CAM systems

PubMed Central

López-Suárez, Carlos; Gonzalo, Esther; Peláez, Jesús; Rodríguez, Verónica

2015-01-01

Background In recent years there has been an improvement of zirconia ceramic materials to replace posterior missing teeth. To date little in vitro studies has been carried out on the fracture resistance of zirconia veneered posterior fixed dental prostheses. This study investigated the fracture resistance and the failure mode of 3-unit zirconia-based posterior fixed dental prostheses fabricated with two CAD/CAM systems. Material and Methods Twenty posterior fixed dental prostheses were studied. Samples were randomly divided into two groups (n=10 each) according to the zirconia ceramic analyzed: Lava and Procera. Specimens were loaded until fracture under static load. Data were analyzed using Wilcoxon´s rank sum test and Wilcoxon´s signed-rank test (P<0.05). Results Partial fracture of the veneering porcelain occurred in 100% of the samples. Within each group, significant differences were shown between the veneering and the framework fracture resistance (P=0.002). The failure occurred in the connector cervical area in 80% of the cases. Conclusions All fracture load values of the zirconia frameworks could be considered clinically acceptable. The connector area is the weak point of the restorations. Key words:Fixed dental prostheses, zirconium-dioxide, zirconia, fracture resistance, failure mode. PMID:26155341

Sensitivity Analysis of the Bone Fracture Risk Model

NASA Technical Reports Server (NTRS)

Lewandowski, Beth; Myers, Jerry; Sibonga, Jean Diane

2017-01-01

Introduction: The probability of bone fracture during and after spaceflight is quantified to aid in mission planning, to determine required astronaut fitness standards and training requirements and to inform countermeasure research and design. Probability is quantified with a probabilistic modeling approach where distributions of model parameter values, instead of single deterministic values, capture the parameter variability within the astronaut population and fracture predictions are probability distributions with a mean value and an associated uncertainty. Because of this uncertainty, the model in its current state cannot discern an effect of countermeasures on fracture probability, for example between use and non-use of bisphosphonates or between spaceflight exercise performed with the Advanced Resistive Exercise Device (ARED) or on devices prior to installation of ARED on the International Space Station. This is thought to be due to the inability to measure key contributors to bone strength, for example, geometry and volumetric distributions of bone mass, with areal bone mineral density (BMD) measurement techniques. To further the applicability of model, we performed a parameter sensitivity study aimed at identifying those parameter uncertainties that most effect the model forecasts in order to determine what areas of the model needed enhancements for reducing uncertainty. Methods: The bone fracture risk model (BFxRM), originally published in (Nelson et al) is a probabilistic model that can assess the risk of astronaut bone fracture. This is accomplished by utilizing biomechanical models to assess the applied loads; utilizing models of spaceflight BMD loss in at-risk skeletal locations; quantifying bone strength through a relationship between areal BMD and bone failure load; and relating fracture risk index (FRI), the ratio of applied load to bone strength, to fracture probability. There are many factors associated with these calculations including environmental factors, factors associated with the fall event, mass and anthropometric values of the astronaut, BMD characteristics, characteristics of the relationship between BMD and bone strength and bone fracture characteristics. The uncertainty in these factors is captured through the use of parameter distributions and the fracture predictions are probability distributions with a mean value and an associated uncertainty. To determine parameter sensitivity, a correlation coefficient is found between the sample set of each model parameter and the calculated fracture probabilities. Each parameters contribution to the variance is found by squaring the correlation coefficients, dividing by the sum of the squared correlation coefficients, and multiplying by 100. Results: Sensitivity analyses of BFxRM simulations of preflight, 0 days post-flight and 365 days post-flight falls onto the hip revealed a subset of the twelve factors within the model which cause the most variation in the fracture predictions. These factors include the spring constant used in the hip biomechanical model, the midpoint FRI parameter within the equation used to convert FRI to fracture probability and preflight BMD values. Future work: Plans are underway to update the BFxRM by incorporating bone strength information from finite element models (FEM) into the bone strength portion of the BFxRM. Also, FEM bone strength information along with fracture outcome data will be incorporated into the FRI to fracture probability.

Design, fabrication, and characterization of laminated hydroxyapatite-polysulfone composites

NASA Astrophysics Data System (ADS)

Wilson, Clifford Adams, II

There exists a need to develop devices that can be used to replace hard tissues, such as bone, in load-bearing areas of the body. An ideal hard tissue replacement device is one that stimulates growth of natural tissues, and is slowly resorbed by the body. The implant is also required to have elastic modulus, strength, and toughness values similar to the tissues being replaced. Hydroxyapatite (HA) is the primary mineral phase of bone and has the potential for use in biomedical applications because it stimulates cell growth and is resorbable. Unfortunately, HA is a relatively low strength, low toughness material, which limits its application to only low load-bearing regions of the body. In order to apply HA to greater load-bearing areas of the body, strength and toughness must be improved through the formation of a composite structure. The goal of this study to show that a composite structure formed from HA and a biocompatible polymer can be fabricated with strength and toughness values that are within the range necessary for load-bearing biomedical applications. Therefore, Polysulfone-HA composites were developed and tested. Polysulfone (PSu) is a hard, glassy polymer that has been shown to be biocompatible. Composites were fabricated through a combination of tape casting, solvent casting, and lamination. Monolithic HA and laminate specimens were tested in biaxial flexure. A unique laminate theory solution was developed to characterize stress distributions for laminates. Failure loads, failure stress, work of fracture, and apparent toughness were compared for the laminates against monolithic HA specimens. Initial testing results showed that laminates had a failure stress of 60 +/- 10, which is a 170% improvement over the 22 +/- 2 MPa failure stress for monolithic HA. The work of fracture was improved by 5500% from 11 +/- 2 for the monolithic HA to 612 +/- 240 for the laminates. Work of fracture values gave the laminates an apparent fracture toughness of 7.2 MPa•m1/2 compared to 0.6 MPa•m1/2 for the monolithic HA. Laminates with different geometries were built and tested in an attempt to optimize the strength and toughness of the composites. Laminate behavior was characterized as a function of initial flaw size, HA layer thickness, PSu layer thickness, and stressing rate. The failure stress of the laminates was maximized at a value of 108 +/- 14 MPa, which is a 400% improvement over monolithic HA, and close to the 12-160 MPa range reported for bone. The work of fracture of laminates was maximized at 724 +/- 206 J/m2, which is a 6400% improvement over monolithic HA, and yields an apparent fracture toughness value of 7.5 MPa•m1/2. This apparent toughness value is within the 2-12 MPa•m1/2 range for bone, and an 1100% improvement over the fracture toughness of monolithic HA.

Simulation of fatigue fracture of TiNi shape memory alloy samples at cyclic loading in pseudoelastic state

NASA Astrophysics Data System (ADS)

Belyaev, Fedor S.; Volkov, Aleksandr E.; Evard, Margarita E.; Khvorov, Aleksandr A.

2018-05-01

Microstructural simulation of mechanical behavior of shape memory alloy samples at cyclic loading in the pseudoelastic state has been carried out. Evolution of the oriented and scattered deformation defects leading to damage accumulation and resulting in the fatigue fracture has been taken into account. Simulations were performed for the regime of loading imitating that for endovascular stents: preliminary straining, unloading, deformation up to some mean level of the strain and subsequent mechanical cycling at specified strain amplitude. Dependence of the fatigue life on the loading parameters (pre-strain, mean and amplitude values of strain) has been obtained. The results show a good agreement with available experimental data.

Finite element modeling of the influence of hand position and bone properties on the Colles' fracture load during a fall.

PubMed

Buchanan, Drew; Ural, Ani

2010-08-01

Distal forearm fracture is one of the most frequently observed osteoporotic fractures, which may occur as a result of low energy falls such as falls from a standing height and may be linked to the osteoporotic nature of the bone, especially in the elderly. In order to prevent the occurrence of radius fractures and their adverse outcomes, understanding the effect of both extrinsic and intrinsic contributors to fracture risk is essential. In this study, a nonlinear fracture mechanics-based finite element model is applied to human radius to assess the influence of extrinsic factors (load orientation and load distribution between scaphoid and lunate) and intrinsic bone properties (age-related changes in fracture properties and bone geometry) on the Colles' fracture load. Seven three-dimensional finite element models of radius were created, and the fracture loads were determined by using cohesive finite element modeling, which explicitly represented the crack and the fracture process zone behavior. The simulation results showed that the load direction with respect to the longitudinal and dorsal axes of the radius influenced the fracture load. The fracture load increased with larger angles between the resultant load and the dorsal axis, and with smaller angles between the resultant load and longitudinal axis. The fracture load also varied as a function of the load ratio between the lunate and scaphoid, however, not as drastically as with the load orientation. The fracture load decreased as the load ratio (lunate/scaphoid) increased. Multiple regression analysis showed that the bone geometry and the load orientation are the most important variables that contribute to the prediction of the fracture load. The findings in this study establish a robust computational fracture risk assessment method that combines the effects of intrinsic properties of bone with extrinsic factors associated with a fall, and may be elemental in the identification of high fracture risk individuals as well as in the development of fracture prevention methods including protective falling techniques. The additional information that this study brings to fracture identification and prevention highlights the promise of fracture mechanics-based finite element modeling in fracture risk assessment.

Acoustic and Electrical Signal Emission recordings when marble specimens are subjected to compressional mechanical stress

NASA Astrophysics Data System (ADS)

Triantis, Dimos; Stavrakas, Ilias; Hloupis, George; Ninos, Konstantinos; Vallianatos, Filippos

2013-04-01

The detection of Acoustic Emissions (AE) and Electrical Signals (ES) has been proved as a valuable experimental method to characterize the mechanical status of marble specimens when subjected to mechanical stress. In this work, marble specimens with dimensions 10cm x 4cm x 4cm where subjected to sequential loading cycles. The maximum stress of each loading was near the vicinity of fracture and was maintained for a relatively long time (th=200s). Concurrently to the mechanical tests, AE and ES were recorded. Specifically, two AE sensors and five ES sensors were installed on the surface of the specimens and the detected emissions were stored on a PC. The recordings show that AE and ES provide information regarding the damage spreading and location in the bulk of the specimen. Specifically, when the mechanical stress was maintained constant at the high stress value during each loading cycle the cumulative number of the AE hits become gradually less reaching a minimum after the first three loading cycles, indicating the existence of the Kaiser effect. During the eighth loading cycle the AE hits show a significant increase that became maximum at the ninth cycle before where failure occured. A similar behavior was observed for the cumulative energy. A b-value analysis was conducted following both Aki's and Gutenberg-Richter relations on the amplitudes of the AE hits. The b-values were found to increase during the three first loading cycles while consequently they were practically constant until reaching the two final loading cycles where they became gradually lower. The ES significantly increases during the stress increase of each cycle and gradually restores at a background level when the applied stress is maintained constant near the vicinity of fracture. It was observed that the background restoration level becomes gradually higher during the first four loading cycles. Consequently, during the next three loading cycles the background level is maintained practically constant. During the two final loading cycles the background restoration level significantly increases indicating the upcoming fracture. Acknowledgments. This work was supported by the THALES Program of the Ministry of Education of Greece and the European Union in the framework of the project entitled "Integrated understanding of Seismicity, using innovative Methodologies of Fracture mechanics along with Earthquake and non extensive statistical physics - Application to the geodynamic system of the Hellenic Arc. SEISMO FEAR HELLARC".

Prediction of fracture healing under axial loading, shear loading and bending is possible using distortional and dilatational strains as determining mechanical stimuli.

PubMed

Steiner, Malte; Claes, Lutz; Ignatius, Anita; Niemeyer, Frank; Simon, Ulrich; Wehner, Tim

2013-09-06

Numerical models of secondary fracture healing are based on mechanoregulatory algorithms that use distortional strain alone or in combination with either dilatational strain or fluid velocity as determining stimuli for tissue differentiation and development. Comparison of these algorithms has previously suggested that healing processes under torsional rotational loading can only be properly simulated by considering fluid velocity and deviatoric strain as the regulatory stimuli. We hypothesize that sufficient calibration on uncertain input parameters will enhance our existing model, which uses distortional and dilatational strains as determining stimuli, to properly simulate fracture healing under various loading conditions including also torsional rotation. Therefore, we minimized the difference between numerically simulated and experimentally measured courses of interfragmentary movements of two axial compressive cases and two shear load cases (torsional and translational) by varying several input parameter values within their predefined bounds. The calibrated model was then qualitatively evaluated on the ability to predict physiological changes of spatial and temporal tissue distributions, based on respective in vivo data. Finally, we corroborated the model on five additional axial compressive and one asymmetrical bending load case. We conclude that our model, using distortional and dilatational strains as determining stimuli, is able to simulate fracture-healing processes not only under axial compression and torsional rotation but also under translational shear and asymmetrical bending loading conditions.

Higher dietary glycemic index and glycemic load values increase the risk of osteoporotic fracture in the PREvención con DIeta MEDiterránea (PREDIMED)-Reus trial.

PubMed

García-Gavilán, Jesús Francisco; Bulló, Mònica; Camacho-Barcia, Lucia; Rosique-Esteban, Nuria; Hernández-Alonso, Pablo; Basora, Josep; Martínez-González, Miguel Angel; Estruch, Ramón; Fitó, Montserrat; Salas-Salvadó, Jordi

2018-06-01

High glucose and insulin concentrations seem to have a negative impact on bone health. However, the relation between the dietary glycemic index (DGI) and the dietary glycemic load (DGL), which has proved to be effective at modulating blood glucose concentrations after carbohydrate consumption, has yet to be explored in relation to bone health. The aim of the study was to examine the associations between the DGI or DGL and the risk of osteoporotic-related fractures in an elderly Mediterranean population. The study was conducted in 870 subjects aged 55-80 y at high cardiovascular disease risk participating in the PREvención con DIeta MEDiterránea (PREDIMED)-Reus study. The DGI and DGL were estimated from validated food-frequency questionnaires with the use of the international glycemic index and glycemic load values, with glucose as reference. Data on osteoporotic fractures were acquired from a systematic review of medical records. We used Cox proportional hazard models to assess the risk of osteoporotic fracture according to tertiles of average DGI and DGL. A total of 114 new cases of osteoporotic-related fractures were documented after a mean follow-up of 8.9 y. Participants in the highest tertile of DGI and DGL had a significantly higher risk of osteoporotic fractures than those in the lowest tertile after adjusting for potential confounders (HR: 1.80; 95% CI: 1.03, 3.15 and HR: 3.20; 95% CI: 1.25, 8.18, respectively). A high DGI and DGL are associated with a higher risk of osteoporosis-related fractures in an elderly Mediterranean population at high cardiovascular disease risk. This trial was registered at isrctn.com as ISRCTN35739639.

Strain hardening and fracture behavior during tension of directionally solidified high-nitrogen austenitic steel

NASA Astrophysics Data System (ADS)

Maier, Galina; Astafurova, Elena; Melnikov, Eugene; Moskvina, Valentina; Galchenko, Nina

2017-12-01

The effect of grain orientation relative to tensile load on the strain hardening behavior and fracture mechanism of directionally solidified high-nitrogen steel Fe-20Cr-22Mn-1.5V-0.2C-0.6N (in wt %) was studied. The tensile samples oriented along the longitudinal direction of columnar grains demonstrated the improved mechanical properties compared to specimens with the transversal directions of columnar grains: the values of tensile strength and strain-to-fracture were as high as 1080 MPa and 22%, respectively, for tension along the columnar grains and 870 MPa and 11%, respectively, for the tension transversal to the columnar grains. The change in the grain orientation relative to the tensile load varies a fracture mode of the steel. The fraction of the transgranular fracture was higher in the samples with longitudinal directions of the columnar grains compared to the transversal ones.

The relationship between fibre post geometry and flexural properties: an assessment through a modified three-point bending test.

PubMed

Soares, L P; de Vasconcellos, A B; da Silva, A H Monteiro da Fonseca Thomé; Sampaio, E M; Vianna, G A de Deus Carneiro

2010-12-01

The aim of this study was to investigate the flexural properties of five types of fiber-reinforced dowels using a modified three-point bending test. Fiber-reinforced resin dowels were tested by a modified three-point bending test associated with models for cylindrical and conical simple-supported beams. The fracture load ranged from 86 to 246 N and the flexural strength from 423 to 1192 MPa. FRC Postec had significantly higher flexural strength and fracture loads values. Thus, the present study demonstrated higher flexural strength values for the FRC Postec fibre posts, suggesting that this system would present a better response to the forces of mastication.

Nanomechanical characterization of alumina coatings grown on FeCrAl alloy by thermal oxidation.

PubMed

Frutos, E; González-Carrasco, J L; Polcar, T

2016-04-01

This work studies the feasibility of using repetitive-nano-impact tests with a cube-corner tip and low loads for obtaining quantitative fracture toughness values in thin and brittle coatings. For this purpose, it will be assumed that the impacts are able to produce a cracking, similar to the pattern developed for the classical fracture toughness tests in bulk materials, and therefore, from the crack developed in the repetitive impacts it will be possible to evaluate the suitability of the classical indentation models (Anstins and Laugier) for measuring fracture toughness. However, the length of this crack has to be lower than 10% of the total coating thickness to avoid substrate contributions. For this reason, and in order to ensure a small plastic region localized at the origin of the crack tip, low load values (or small distance between the indenter tip and the surface) have to be used. In order to demonstrate the validity of this technique, repetitive-nano-impact will be done in a fine and dense oxide layer (α-Al2O3), which has been developed on the top of oxide dispersion strengthened (ODS) FeCrAl alloys (PM 2000) by thermal oxidation at elevated temperatures. Moreover, it will be shown how it is possible to know with each new impact the crack geometry evolution from Palmqvist crack to half-penny crack, being able to study the proper evolution of the different values of fracture toughness in terms of both indentation models and as a function of the strain rate, ε̇, decreasing. Thereby, fracture toughness values for α-Al2O3 layer decrease from ~4.40MPam , for high ϵ̇ value (10(3)s(-1)), to ~3.21MPam, for quasi-static ϵ̇ value (10(-3)s(-1)). On the other hand, ϵ̇ a new process to obtain fracture toughness values will be analysed, when the classical indentation models are not met. These values are typically found in the literature for bulk α-Al2O3, demonstrating the use of repetitive-nano-impact tests which not only provide qualitative information about fracture resistance of the materials but it also can be used to obtain quantitative information as fracture toughness values in the case of brittle materials. Copyright © 2016 Elsevier Ltd. All rights reserved.

Fracture Anisotropy and Toughness in the Mancos Shale: Implications for crack-growth geometry

NASA Astrophysics Data System (ADS)

Chandler, M. R.; Meredith, P. G.; Brantut, N.; Crawford, B. R.

2013-12-01

The hydraulic fracturing of gas-shales has drawn attention to the fundamental fracture properties of shales. Fracture propagation is dependent on a combination of the in-situ stress field, the fracturing fluid and pressure, and the mechanical properties of the shale. However, shales are strongly anisotropic, and there is a general paucity of available experimental data on the anisotropic mechanical properties of shales in the scientific literature. The mode-I stress intensity factor, KI, quantifies the concentration of stress at crack tips. The Fracture Toughness of a linear elastic material is then defined as the critical value of this stress intensity factor; KIc, beyond which rapid catastrophic crack growth occurs. However, shales display significant non-linearity, which produces hysteresis during experimental cyclic loading. This allows for the calculation of a ductility coefficient using the residual displacement after successive loading cycles. From this coefficient, a ductility corrected Fracture Toughness value, KIcc can be determined. In the Mancos Shale this ductility correction can be as large as 60%, giving a Divider orientation KIcc value of 0.8 MPa.m0.5. Tensile strength and mode-I Fracture Toughness have been experimentally determined for the Mancos Shale using the Brazil Disk and Short-Rod methodologies respectively. The three principal fracture orientations; Arrester, Divider and Short-Transverse were all analysed. A significant anisotropy is observed in the tensile strength, with the Arrester value being 1.5 times higher than the Short-Transverse value. Even larger anisotropy is observed in the Fracture Toughness, with KIcc in the Divider and Arrester orientations being around 1.8 times that in the Short-Transverse orientation. For both tensile strength and fracture toughness, the Short-Transverse orientation, where the fracture propagates in the bedding plane in a direction parallel to the bedding, is found to have significantly lower values than the other two orientations. This anisotropy and variability in fracture properties is seen to cause deviation of the fracture direction during experiments on Arrester and Short-Transverse oriented samples, and can be expected to influence the geometry of propagating fractures. A comparison between the anisotropic tensile strength of the material and the crack-tip stress field in a transversely isotropic material has been used to develop a crack-tip deflection criterion in terms of the elasticity theory of cracks. This criterion suggests that a small perturbation in the incident angle of a mode-I crack propagating perpendicular to the bedding is likely to lead to a substantial deflection towards bedding-parallel (Short-Transverse) propagation. Further experimental work is currently underway on anisotropic Fracture Toughness measurements at elevated pressures and temperatures, simulating conditions in Shale Gas reservoirs at depths up to around 4km.

Effect of water temperature on cyclic fatigue properties of glass-fiber-reinforced hybrid composite resin and its fracture pattern after flexural testing.

PubMed

Kuroda, Soichi; Shinya, Akikazu; Vallittu, Pekka K; Nakasone, Yuji; Shinya, Akiyoshi

2013-02-01

To evaluate in vitro the influence of dynamic loading applied to a glass-fiber-reinforced hybrid composite resin on its flexural strength in a moist, simulated oral environment. Three-point flexural strength specimens were subjected to cyclic loading in water at 37°C and 55°C to investigate the influence of immersion temperature on impact fatigue properties. Specimens were subjected to cyclic impact loading at 1 Hz for up to 5 × 105 cycles to obtain the number of cycles to failure, the number of unbroken specimens after 5 × 105 cycles, and the residual flexural strength of unbroken specimens. Maximum loads of 100, 200, and 300 N were chosen for both the non-reinforced and the glass-fiber reinforced hybrid composite resins. The mean residual flexural strength for 100 N impact loading at temperatures of 37°C and 55°C was 634 and 636 MPa, respectively. All specimens fractured at fewer than 5 × 105 cycles for loads of 200 and 300 N. Reduced numbers of cycles to fracture and lower fatigue values were observed as both the maximum load and immersion temperature increased.

Development plates for stable internal fixation: Study of mechanical resistance in simulated fractures of the mandibular condyle.

PubMed

Celegatti Filho, Tóride Sebastião; Rodrigues, Danillo Costa; Lauria, Andrezza; Moreira, Roger William Fernandes; Consani, Simonides

2015-01-01

To develop Y-shaped plates with different thicknesses to be used in simulated fractures of the mandibular condyle. Ten plates were developed in Y shape, containing eight holes, and 30 synthetic polyurethane mandible replicas were developed for the study. The load test was performed on an Instron Model 4411 universal testing machine, applying load in the mediolateral and anterior-posterior positions on the head of the condyle. Two-way ANOVA with Tukey testing with a 5% significance level was used. It was observed that when the load was applied in the medial-lateral plate of greater thickness (1.5 mm), it gave the highest strength, while in the anteroposterior direction, the plate with the highest resistance was of the lesser thickness (0.6 mm). A plate with a thickness of 1.5 mm was the one with the highest average value for all displacements. In the anteroposterior direction, the highest values of resistance were seen in the displacement of 15 mm. After comparing the values of the biomechanical testing found in the scientific literature, it is suggested that the use of Y plates are suitable for use in subcondylar fractures within the limitations of the study. Copyright © 2014 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.

Experimental and numerical investigation into the influence of loading conditions in biomechanical testing of locking plate fracture fixation devices

PubMed Central

MacLeod, A.; Simpson, A. H. R. W.

2018-01-01

Objectives Secondary fracture healing is strongly influenced by the stiffness of the bone-fixator system. Biomechanical tests are extensively used to investigate stiffness and strength of fixation devices. The stiffness values reported in the literature for locked plating, however, vary by three orders of magnitude. The aim of this study was to examine the influence that the method of restraint and load application has on the stiffness produced, the strain distribution within the bone, and the stresses in the implant for locking plate constructs. Methods Synthetic composite bones were used to evaluate experimentally the influence of four different methods of loading and restraining specimens, all used in recent previous studies. Two plate types and three screw arrangements were also evaluated for each loading scenario. Computational models were also developed and validated using the experimental tests. Results The method of loading was found to affect the gap stiffness strongly (by up to six times) but also the magnitude of the plate stress and the location and magnitude of strains at the bone-screw interface. Conclusions This study demonstrates that the method of loading is responsible for much of the difference in reported stiffness values in the literature. It also shows that previous contradictory findings, such as the influence of working length and very large differences in failure loads, can be readily explained by the choice of loading condition. Cite this article: A. MacLeod, A. H. R. W. Simpson, P. Pankaj. Experimental and numerical investigation into the influence of loading conditions in biomechanical testing of locking plate fracture fixation devices. Bone Joint Res 2018;7:111–120. DOI: 10.1302/2046-3758.71.BJR-2017-0074.R2. PMID:29363522

Finite element method for analysis of stresses arising in the skull after external loading in cranio-orbital fractures.

PubMed

Wanyura, Hubert; Kowalczyk, Piotr; Bossak, Maciej; Samolczyk-Wanyura, Danuta; Stopa, Zygmunt

2012-01-01

The craniofacial skeleton remains not fully recognised as far as its mechanical resistance properties are concerned. Heretofore, the only available information on the mechanism of cranial bone fractures came from clinical observations, since the clinical evaluation in a living individual is practically impossible. It seems crucial to implement computer methods of virtual research into clinical practice. Such methods, which have long been used in the technical sciences, may either confirm or disprove previous observations. The aim of the study was to identify the areas of stress concentrations caused by external loads, which can lead to cranio-orbital fractures (COF), by the finite element method (FEM). For numerical analysis, a three-dimensional commercially available geometrical model of the skull was used which was imported into software of FEM. Computations were performed with ANSYS 12.1 Static Structural module. The loads were applied laterally to the frontal squama, the zygomatic process and partly to the upper orbital rim to locate dangerous concentration of stresses potentially resulting in COF. Changes in the area of force application revealed differences in values, quality and the extent of the stress distribution. Depending on the area of force application the following parameters would change: the value and area of stresses characteristic of COF. The distribution of stresses obtained in this study allowed definition of both the locations most vulnerable to fracture and sites from which fractures may originate or propagate.

Residual Strength Predictions with Crack Buckling

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Gullerud, A. S.; Dodds, R. H., Jr.; Hampton, R. W.

1999-01-01

Fracture tests were conducted on middle crack tension, M(T), and compact tension, C(T), specimens of varying widths, constructed from 0.063 inch thick sheets of 2024-T3 aluminum alloy. Guide plates were used to restrict out-of-plane displacements in about half of the tests. Analyses using the three-dimensional, elastic-plastic finite element code WARP3D simulated the tests with and without guide plates using a critical CTOA fracture criterion. The experimental results indicate that crack buckling reduced the failure loads by up to 40%. Using a critical CTOA value of 5.5 deg., the WARP3D analyses predicted the failure loads for the tests with guide plates within +/- 10% of the experimentally measured values. For the M(T) tests without guide plates, the WARP3D analyses predicted the failure loads for the 12 and 24 inch tests within 10%, while over predicting the failure loads for the 40 inch wide tests by about 20%.

A comparative biomechanical analysis of implants for the stabilization of proximal humerus fractures.

PubMed

Füchtmeier, B; May, R; Fierlbeck, J; Hammer, J; Nerlich, M

2006-01-01

A new humerus nail (Sirus) has become available for the treatment of fractures of the proximal humerus. The aim of the study was the biomechanical comparison of this implant with established systems. 12 matched pairs of human humeri were employed for testing. Three randomized groups were formed (n = 4 pairs). A bending moment of 7.5 Nm and a torsional moment of 8.3 Nm were applied in a test of five loading cycles to all intact bones. Loading was consistently initiated at the distal epiphysis. The consequent deformation at the distal epiphysis was continuously recorded. Then an osteotomy with a defect of 5 mm was created to simulate an unstable subcapital humerus fracture. For paired comparison, one of each pair was stabilized with the Sirus proximal humerus nail. The other side was fixed with a reference implant. The following groups were created.: Group I: Sirus versus Proximal humerus nail (PHN) with spiral blade. Group II: Sirus versus PHILOS plate. Group III: Sirus versus 4.5 mm AO T-plate. The proximal humerus nail (Sirus) demonstrated significantly higher stiffness values than the reference implants for both bending and torsional load. The following values were recorded at a bending moment of 7.5 Nm: Sirus 14.2 mm, PHN 20.7 mm, PHILOS plate 28.1 mm, 4.5 AO T-plate 29.3 mm p < 0.0012). The values at 8.3 Nm torsional moment were: Sirus 8.5 degrees , PHN 12.3 degrees , PHILOS plate 16.4 degrees , 4.5 AO T-Platte 15.6 degrees (p < 0.002). The intramedullary load carriers were biomechanically superior when compared to the plating systems in the fracture model presented here. Supplementary, the Sirus Nail showed higher stiffness values than the PHN. However, the latter are gaining in importance due to the possibility of minimal invasive implantation. Whether this will be associated with functional advantages requires further clinical investigation.

3D Experimental Measurement of Lattice Strain and Fracture Behavior of Sand Particles Using Synchrotron X-Ray Diffraction and Tomography

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cil, Mehmet B.; Alshibli, Khalid A.; Kenesei, Peter

3D synchrotron X-ray diffraction (3DXRD) and synchrotron micro-computed tomography (SMT) techniques were used to measure and monitor the lattice strain evolution and fracture behavior of natural Ottawa sand particles subjected to 1D compression loading. The particle-averaged lattice strain within sand particles was measured using 3DXRD and then was used to calculate the corresponding lattice stress tensor. In addition, the evolution and mode of fracture of sand particles was investigated using high-resolution 3D SMT images. The results of diffraction data analyses revealed that the major principal component of the lattice strain or stress tensor increased in most of the particles asmore » the global applied compressive load increased until the onset of fracture. Particle fracture and subsequent rearrangements caused significant variation and fluctuations in measured lattice strain/stress values from one particle to another and from one load step to the next one. SMT image analysis at the particle-scale showed that cracks in fractured sand particles generally initiate and propagate along the plane that connects the two contact points. Fractured particles initially split into two or three major fragments followed by disintegration into multiple smaller fragments in some cases. In conclusion, microscale analysis of fractured particles showed that particle position, morphology, the number and location of contact points play a major role in the occurrence of particle fracture in confined comminution of the sand assembly.« less

3D Experimental Measurement of Lattice Strain and Fracture Behavior of Sand Particles Using Synchrotron X-Ray Diffraction and Tomography

DOE PAGES

Cil, Mehmet B.; Alshibli, Khalid A.; Kenesei, Peter

2017-05-27

3D synchrotron X-ray diffraction (3DXRD) and synchrotron micro-computed tomography (SMT) techniques were used to measure and monitor the lattice strain evolution and fracture behavior of natural Ottawa sand particles subjected to 1D compression loading. The particle-averaged lattice strain within sand particles was measured using 3DXRD and then was used to calculate the corresponding lattice stress tensor. In addition, the evolution and mode of fracture of sand particles was investigated using high-resolution 3D SMT images. The results of diffraction data analyses revealed that the major principal component of the lattice strain or stress tensor increased in most of the particles asmore » the global applied compressive load increased until the onset of fracture. Particle fracture and subsequent rearrangements caused significant variation and fluctuations in measured lattice strain/stress values from one particle to another and from one load step to the next one. SMT image analysis at the particle-scale showed that cracks in fractured sand particles generally initiate and propagate along the plane that connects the two contact points. Fractured particles initially split into two or three major fragments followed by disintegration into multiple smaller fragments in some cases. In conclusion, microscale analysis of fractured particles showed that particle position, morphology, the number and location of contact points play a major role in the occurrence of particle fracture in confined comminution of the sand assembly.« less

Three-dimensional finite-element analysis of chevron-notched fracture specimens

NASA Technical Reports Server (NTRS)

Raju, I. S.; Newman, J. C., Jr.

1984-01-01

Stress-intensity factors and load-line displacements were calculated for chevron-notched bar and rod fracture specimens using a three-dimensional finite-element analysis. Both specimens were subjected to simulated wedge loading (either uniform applied displacement or uniform applied load). The chevron-notch sides and crack front were assumed to be straight. Crack-length-to-specimen width ratios (a/w) ranged from 0.4 to 0.7. The width-to-thickness ratio (w/B) was 1.45 or 2. The bar specimens had a height-to-width ratio of 0.435 or 0.5. Finite-element models were composed of singularity elements around the crack front and 8-noded isoparametric elements elsewhere. The models had about 11,000 degrees of freedom. Stress-intensity factors were calculated by using a nodal-force method for distribution along the crack front and by using a compliance method for average values. The stress intensity factors and load-line displacements are presented and compared with experimental solutions from the literature. The stress intensity factors and load-line displacements were about 2.5 and 5 percent lower than the reported experimental values, respectively.

Mechanical properties and three-body wear of dental restoratives and their comparative flowable materials.

PubMed

Schultz, Sabine; Rosentritt, Martin; Behr, Michael; Handel, Gerhard

2010-01-01

To compare wear performance and resistance to crack propagation (K1C) of commercial restorative materials and their flowable variations. A potential correlation between three-body wear and fracture toughness, modulus of elasticity, fracture work, Vickers hardness, and filler content was investigated. Seven restoratives (five composites, one ormocer, and one compomer) and their corresponding flowable materials were used to determine and compare the three-body wear with a bolus of millet-seed shells and rice food (Willytec). The wear characteristics were measured by profilometry after 50,000, 100,000, 150,000, and 200,000 loading cycles. The fracture toughness value, K1C (MPam1/2), for each single-edged notched specimen was measured in a three-point bending test (universal testing machine 1446, Zwick). Fracture work and modulus of elasticity were calculated from the load curves. Vickers hardness was measured (HV hardness tester, Zwick) according to DIN 50133. The veneering composite Sinfony (3M ESPE) was used as a reference material. Heavily filled composites experienced less wear than their flowable variations. The nanofiller composites revealed better wear results than hybrid composites, compomers, and ormocers. After 200,000 load cycles, the lowest wear rates were detected for Grandio (14 microm; Voco), and the highest mean values were found for Dyract AP (104 microm; Dentsply DeTrey). The values for fracture toughness (K1C) ranged from 0.82 to 3.64 MPam1/2. Highest K1C data was exhibited by the nanocomposite Nanopaq (Schutz Dental). All tested restorative materials exhibited higher fracture toughness than their low-viscosity variations. The wear resistance of the newer generation composites with incorporated nanofiller or microfiller particles increased to a high extent. Flowables show less resistance against wear and crack propagation because of their lower filler content. The reduced mechanical properties limit their use as a restorative to small noncontact, low-stress clinical situations.

Fracture load of ceramic restorations after fatigue loading.

PubMed

Baladhandayutham, Balasudha; Lawson, Nathaniel C; Burgess, John O

2015-08-01

A clinician must decide what ceramic coping and veneer material to prescribe based on the amount of tooth reduction possible and the desired esthetic outcome of the restoration. The purpose of this in vitro study was to compare the fracture strength of monolithic and bilayered lithium disilicate (IPS e.max) and zirconia (LAVA) crowns at clinically relevant thicknesses after load cycling. Crowns (n=8) were fabricated from 6 groups: 1.2-mm monolithic lithium disilicate, 1.5-mm monolithic lithium disilicate, 1.5-mm bilayered lithium disilicate with hand-layered veneer, 0.6 mm monolithic zirconia, 1.2-mm bilayered zirconia with hand-layered veneer, and 1.2-mm bilayered zirconia with milled veneer (dimension represents thickness at the occlusal pit). Crowns were cemented to identical milled resin dies with resin-modified glass ionomer cement. Cemented crowns were stored at 37°C for 24 hours and load cycled for 200,000 cycles at 25 N at a rate of 40 cycles/minute. The ultimate fracture load for each specimen was measured in a universal testing machine. Data were analyzed with a 1-way ANOVA and Tukey honest significant difference post hoc analysis (α=.05). Mean ±SD fracture load values were 1465 ±330 N for monolithic lithium disilicate (1.2-mm thickness) and 2027 ±365 N (1.5-mm thickness) and 1732 ±315 N for bilayered hand-veneered lithium disilicate (1.5-mm thickness). Fracture loads were 1669 ±311 N for monolithic zirconia crowns (0.6mm thickness), 2625 ±300 N for zirconia milled-veneered (1.2-mm thickness), and 2655 ±590N for zirconia hand-veneered crowns (1.2mm thickness). One-way ANOVA showed a statistically significant difference among the groups (P<.01). Veneered zirconia crowns showed the highest fracture strength, 1.2-mm hand veneered zirconia was similar to that of 1.5-mm monolithic zirconia, and all other groups were not statistically different. Crowns of 1.2-mm bilayered zirconia had higher fracture loads than 0.6-mm zirconia or 1.2-mm lithium disilicate monolithic crowns. Copyright © 2015 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

Fracture load of complete-arch implant-supported prostheses reinforced with nylon-silica mesh: An in vitro study.

PubMed

Gonçalves, Fernanda de Cássia Papaiz; Amaral, Marina; Borges, Alexandre Luiz Souto; Gonçalves, Luiz Fernando Martins; Paes-Junior, Tarcisio José de Arruda

2018-04-01

Complete-arch implant-supported prostheses without a framework have a high risk of failure: a straightforward and inexpensive reinforcement material, such as nylon mesh, could improve their longevity. The purpose of this in vitro study was to evaluate a nylon-silica mesh compound on the fracture strength of acrylic resin and the fracture load of complete-arch implant-supported prostheses. Twenty-four complete mandibular arch implant-supported prostheses were divided into 2 groups according to cantilever length (molar and premolar) and subdivided into another 2 subgroups according to the presence or absence of reinforcing mesh. The specimens were submitted to a maximum load-to-fracture test in a universal testing machine, with a 100-N load cell, a 2 mm/min crosshead speed, and a spherical metal tip diameter of 4 mm at different points (molar and premolar). These were submitted to 1-way analysis of variance for repeated measurement and the post hoc Tukey multiple comparison test (α=.05). The mean maximum load ±standard deviation for the molar group was 393.4 ±95.0 N with reinforcement and 305.4 ±76.3 N without reinforcement (P=.02); and for the premolar group was 1083.3 ±283.7 N with reinforcement and 605.3 ±90.5 N without reinforcement (P=.001). Reinforcement with nylon mesh increased the mean maximum load of implant-supported complete-arch prostheses at both cantilever lengths. The cantilever to the premolar (5 mm) presented the highest maximum load values to fracture. Copyright © 2017 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

[Relative fracture toughness of differents dental ceramics].

PubMed

Pagani, Clovis; Miranda, Carolina Baptista; Bottino, Marco Cícero

2003-03-01

Although ceramics present high compressive strength, they are brittle materials due to their low tensile strength so they have lower capacity to absorb shocks. This study evaluated the fracture toughness of different ceramic systems, which refers to the ability of a friable material to absorb defformation energy. Three ceramic systems were investigated. Ten cylindrical samples (5,0mm x 3,0mm), were obtained from each ceramic material as follows: G1- 10 samples of Vitadur Alpha (Vita-Zahnfabrik); G2- 10 samples of IPS Empress2 (Ivoclar-Vivadent); G3- 10 samples of In-Ceram Alumina (Vita-Zahnfabrik). Fracture toughness values were collected upon indentation tests that were performed under a heavy load. A microhardness tester (Digital Microhardness Tester FM) utilized a 500gf load cell during 10seconds to perform four impressions on each sample. Statistically significant results were observed (ANOVA and Kruskal-Wallis tests). In-Ceram Alumina presented the highest median toughness values (2,96N/m3/2), followed by Vitadur Alpha (2,08N/m3/2) and IPS Empress2 (1,05N/m3/2). It may be concluded that different ceramic systems present distinct fracture toughness values, thus In-Ceram is capable of absorbing superior stress when compared to Vitadur Alpha and IPS Empress2.

Effect of metallurgical structure and properties on adhesion and friction behavior of cobalt alloys

NASA Technical Reports Server (NTRS)

Keller, D. V., Jr.; Shatynski, S.; Vedamanikam, P. M.

1972-01-01

The metallurgical structure and some of the mechanical properties of two cobalt alloys, cobalt-50% iron and cobalt-25% molybdenum-10% chromium, were determined under various heat treated conditions. The mechanical properties of the bcc disordered Co-50Fe alloy, which was found to be very brittle, indicated an exceedingly low fracture strength, low hardness, and very weak grain boundary strength. Ordering by suitable heat treatment only produced a more brittle material with a lower fracture strength and a slightly higher hardness value. Work hardening was found to produce a finer grain structure and a greater grain boundary strength. Tensile properties were examined. It was found that the Co-25Mo-10Cr alloy was difficult to place in the alpha Co solid solution condition, which limited the ability to use precipitation as a hardening reaction. Over two hundred adhesion cycles from zero contact load, to maximum load, to fracture were conducted between couples for each of the above alloys in an ultrahigh vacuum system which would permit the sample surfaces to be cleaned of all contaminant layers. In the Co-50Fe case, the calculated fracture stress from the adhesion tests showed values in the range of 80 to 150 k.s.i., which is about ten times greater than the values from tension tests.

Fracture Behavior of Zr-BASED Bulk Metallic Glass Under Impact Loading

NASA Astrophysics Data System (ADS)

Shin, Hyung-Seop; Kim, Ki-Hyun; Oh, Sang-Yeob

The fracture behavior of a Zr-based bulk amorphous metal under impact loading using subsize V-shaped Charpy specimens was investigated. Influences of loading rate on the fracture behavior of amorphous Zr-Al-Ni-Cu alloy were examined. As a result, the maximum load and absorbed fracture energy under impact loading were lower than those under quasi-static loading. A large part of the absorbed fracture energy in the Zr-based BMG was consumed in the process for crack initiation and not for crack propagation. In addition, fractographic characteristics of BMGs, especially the initiation and development of shear bands at the notch tip were investigated. Fractured surfaces under impact loading are smoother than those under quasi-static loading. The absorbed fracture energy appeared differently depending on the appearance of the shear bands developed. It can be found that the fracture energy and fracture toughness of Zr-based BMG are closely related with the extent of shear bands developed during fracture.

Analysis and comparison of the biomechanical properties of univalved and bivalved cast models.

PubMed

Crickard, Colin V; Riccio, Anthony I; Carney, Joseph R; Anderson, Terrence D

2011-01-01

Fiberglass casts are frequently valved to relieve the pressure associated with upper extremity swelling after a surgical procedure or when applied after reduction of a displaced fracture in a child. Although different opinions exist regarding the valving of casts, no research to date has explored the biomechanical effects of this commonly used technique. As cast integrity is essential for the maintenance of fracture reduction, it is important to understand whether casts are structurally compromised after valving. Understanding the effects of valving on cast integrity may help guide clinicians in the technique of valving while minimizing the potential for a loss of fracture reduction. Thirty standardized cylindrical fiberglass cast models were created. Ten models were left intact, 10 were univalved, and 10 were bivalved. All the models were mechanically tested by a 3-point bending apparatus secured to a biaxial materials testing system. Load to failure and bending stiffness were recorded for each sample. Differences in load of failure and bending stiffness were compared among the groups. Unvalved cast models had the highest failure load and bending stiffness, whereas bivalved casts showed the lowest value for both failure load and bending stiffness. Univalved casts had a failure load measured to be between those of unvalved and bivalved cast models. Analysis of variance showed significance when failure load and bending stiffness data among all the groups were compared. A post hoc Bonferroni statistical analysis showed significance in bending stiffness between intact and bivalved models (P < 0.01), intact and univalved models (P < 0.01), but no significant difference in bending stiffness between univalved and bivalved models (P > 0.01). Differences in measured failure load values were found to be statistically significant among all cast models (P < 0.01). Valving significantly decreases the bending stiffness and load to failure of fiberglass casts. Univalved casts have a higher load to failure than bivalved casts. Valving adversely alters the structural integrity of fiberglass casts. This may impair a cast's ability to effectively immobilize an extremity or maintain a fracture reduction.

Post-fatigue fracture resistance of metal core crowns: press-on metal ceramic versus a conventional veneering system

PubMed Central

Agustín-Panadero, Rubén; Campos-Estellés, Carlos; Labaig-Rueda, Carlos

2015-01-01

Background The aim of this in vitro study was to compare the mechanical failure behavior and to analyze fracture characteristics of metal ceramic crowns with two veneering systems – press-on metal (PoM) ceramic versus a conventional veneering system – subjected to static compressive loading. Material and Methods Forty-six crowns were constructed and divided into two groups according to porcelain veneer manufacture. Group A: 23 metal copings with porcelain IPS-InLine veneering (conventional metal ceramic). Group B: 23 metal copings with IPS-InLine PoM veneering porcelain. After 120,000 fatigue cycles, the crowns were axially loaded to the moment of fracture with a universal testing machine. The fractured specimens were examined under optical stereomicroscopy and scanning electron microscope. Results Fracture resistance values showed statistically significant differences (Student’s t-test) regarding the type of ceramic veneering technique (p=0.001): Group A (conventional metal ceramics) obtained a mean fracture resistance of 1933.17 N, and Group B 1325.74N (Press-on metal ceramics). The most common type of fracture was adhesive failure (with metal exposure) (p=0.000). Veneer porcelain fractured on the occlusal surface following a radial pattern. Conclusions Metal ceramic crowns made of IPS InLine or IPS InLine PoM ceramics with different laboratory techniques all achieved above-average values for clinical survival in the oral environment according to ISO 6872. Crowns made with IPS InLine by conventional technique resisted fracture an average of 45% more than IPS InLine PoM fabricated with the press-on technique. Key words:Mechanical failure, conventional feldspathic, pressable ceramic, chewing simulator, thermocycling, compressive testing, fracture types, scanning electron microscope. PMID:26155346

In vitro analysis of the fracture resistance of CAD-CAM monolithic zirconia molar crowns with different occlusal thickness.

PubMed

Sorrentino, Roberto; Triulzio, Clementina; Tricarico, Maria Gabriella; Bonadeo, Giovanni; Gherlone, Enrico Felice; Ferrari, Marco

2016-08-01

To compare the fracture resistance and mode of failure of CAD-CAM monolithic zirconia crowns with different occlusal thickness. Forty CAD-CAM monolithic zirconia crowns with different occlusal thickness were randomly distributed into 4 experimental groups: 2.0mm (group 1), 1.5mm (group 2), 1.0mm (group 3) and 0.5mm (group 4). The restorations were cemented onto human molars with a self-adhesive resin cement. The specimens were loaded until fracture; the fracture resistance and mode of failure were recorded. The data were statistically analyzed with the one-way ANOVA followed by the Fisher׳s Exact test with Bonferroni׳s correction (p=0.05). The fracture resistance values of all the specimens exceeded the maximum physiological occlusal loads in molar regions. All the crowns showed cohesive microcracks of the zirconia core; only 1 crown with a thickness of 0.5mm was interested by a complete fracture. The occlusal thickness of CAD-CAM monolithic zirconia crowns did not influence either the fracture resistance and the mode of failure of the restorations; the occlusal thickness of CAD-CAM monolithic zirconia crowns can be reduced up to a lower bound of 0.5mm keeping a sufficient strength to withstand occlusal loads; CAD-CAM monolithic zirconia crowns showed sufficient fracture resistance to be used in molar regions, even in a thin configuration (0.5mm). Copyright © 2016 Elsevier Ltd. All rights reserved.

Effect of core ceramic grinding on fracture behaviour of bilayered lithium disilicate glass-ceramic under two loading schemes.

PubMed

Wang, Xiao-Dong; Jian, Yu-Tao; Guess, Petra C; Swain, Michael V; Zhang, Xin-Ping; Zhao, Ke

2014-11-01

The purpose of this in vitro study was to evaluate the effect of core ceramic grinding on the fracture behaviour of bilayered lithium disilicate glass-ceramic (LDG) under two loading schemes. Interfacial surfaces of sandblasted LDG disks (A) were ground with 220 (B), 500 (C) and 1200 (D) grit silicon carbide (SiC) sandpapers, respectively. Surface roughness and topographic analysis were performed using a profilometer and a scanning electron microscopy (SEM), and then underwent retesting after veneer firing. Biaxial fracture strength (σf) and Weibull modulus (m) were calculated either with core in tension (subgroup t) or in compression (subgroup c). Failure modes were observed by SEM, and loading induced stress distribution was simulated and analyzed by finite element analysis. Statistical data analysis was performed using Kruskal-Wallis, one-way ANOVA, and paired test at a significance level of 0.05. As the grits size of SiC increased, LDG surface roughness decreased from group A to D (p<0.001), which remained unchanged after veneer firing. No difference in σf (p=0.41 for subgroups At-Dt; p=0.11 for subgroups Ac-Dc), m values as well as failure modes was found among four subgroups for both loading schemes. Specimens in subgroup t showed higher σf (p<0.001) and m values than subgroup c. Stress distribution between loading schemes did not differ from each other. Cracks, as the dominant failure mode initiated from bottom tensile surface. No sign of interfacial cracking or delamination was observed for all groups. Technician grinding changed surface topography of LDG ceramic material, but was not detrimental to the bilayered system strength after veneer application. LDG bilayered system was more sensitive to fracture when loaded with veneer porcelain in tension. Within the limitations of the simulated grinding applied, it is concluded that veneer porcelain can be applied directly after technician grinding of LDG ceramic as it has no detrimental effect on the strength of bilayered structures. The connector areas of LDG fixed dental prosthesis are more sensitive to fracture compared with single crowns, and should be fabricated with more caution. Copyright © 2014 Elsevier Ltd. All rights reserved.

Fracture toughness of SiC/Al metal matrix composite

NASA Technical Reports Server (NTRS)

Flom, Yury; Parker, B. H.; Chu, H. P.

1989-01-01

An experimental study was conducted to evaluate fracture toughness of SiC/Al metal matrix composite (MMC). The material was a 12.7 mm thick extrusion of 6061-T6 aluminum alloy with 40 v/o SiC particulates. Specimen configuration and test procedure conformed to ASTM E399 Standard for compact specimens. It was found that special procedures were necessary to obtain fatigue cracks of controlled lengths in the preparation of precracked specimens for the MMC material. Fatigue loading with both minimum and maximum loads in compression was used to start the precrack. The initial precracking would stop by self-arrest. Afterwards, the precrack could be safely extended to the desired length by additional cyclic tensile loading. Test results met practically all the E399 criteria for the calculation of plane strain fracture toughness of the material. A valid K sub IC value of the SiC/Al composite was established as K sub IC = 8.9 MPa square root of m. The threshold stress intensity under which crack would cease to grow in the material was estimated as delta K sub th = 2MPa square root of m for R = 0.09 using the fatigue precracking data. Fractographic examinations show that failure occurred by the micromechanism involved with plastic deformation although the specimens broke by brittle fracture. The effect of precracking by cyclic loading in compression on fracture toughness is included in the discussion.

Effect of notch position on fracture energy for foamed concrete

NASA Astrophysics Data System (ADS)

Naqiuddin Zamri, Mohd; Rahman, Norashidah Abd; Jaini, Zainorizuan Mohd; Shamila Bahador, Nurul

2017-11-01

Foamed concrete is one of the lightweight concrete used to replace normal concrete. Foamed concrete has potential as a building construction material in Malaysia due to low density range. However, the behavior of fracture energy on foamed concrete still under investigation. Therefore, a study to determine the fracture energy of foamed concrete was conducted. In this study, foamed concrete fracture energy was obtained using the three-point bending test methods develop by RILEM and Hillerborg. A total of 12 beams with different types of notch and positions of notch were tested on the load-deflection condition. In addition, a total of 9 cube samples were cast to support the result of fracture energy by using model from Bazant and Becq-Giraudon and Comite Euro-International du Beton (CEB). Results showed the far the position of the notch from midpoint, the higher the value of fracture energy. In this study, the value of fracture energy ranges between 15 N/m and 40 N/m.

Peri-anterior cruciate ligament reconstruction femur fracture: a biomechanical analysis of the femoral tunnel as a stress riser.

PubMed

Han, Yung; Sardar, Zeeshan; McGrail, Scott; Steffen, Thomas; Martineau, Paul A

2011-12-01

Sixteen case reports of distal femur fractures as post-operative complications after anterior cruciate ligament (ACL) reconstruction have been described in the literature. The femoral tunnel has been suggested as a potential stress riser for fracture formation. Additionally, double bundle ACL reconstructions may compound this risk. This is the first biomechanical study to examine the significance of a stress riser effect of the femoral tunnel(s) after ACL reconstruction. The hypotheses tested in this study are that the femoral tunnel acts as a stress riser for fracture and that this effect increases with the size of the tunnel (8 mm vs. 10 mm) and with the number of tunnels (1 vs. 2). Femoral tunnels simulating single bundle (SB) hamstring graft (8 mm), bone-patellar tendon-bone graft (10 mm), and double bundle (DB) ACL reconstruction (7, 6 mm) were drilled in fourth-generation saw bones. These three experimental groups and a control group consisting of native saw bones without tunnels were loaded to failure. All fractures occurred through the tunnels in the DB group, whereas fractures did not consistently occur through the tunnels in the SB groups. The mean fracture load was 6,145N ± 471N in the native group, 5,691N ± 198N in the 8 mm SB group, 5,702N ± 282N in the 10 mm SB group, and 4,744N ± 418N in the DB group. The mean fracture load for the DB group was significantly lower when compared to the native, 8 mm SB, and 10 mm SB groups independently (P value = 0.0016, 0.0060, and 0.0038, respectively). The mean fracture loads for neither SB groups were not significantly different from the native group. An anatomically placed femoral tunnel in single bundle ACL reconstruction in our experimental model was not a significant stress riser to fracture, whereas the two femoral tunnels in double bundle ACL reconstruction significantly decreased load to failure. The results support the sparsity of reported peri-ACL reconstruction femur fractures in single femoral tunnel techniques. However, the increased fracture risk in double bundle ACL reconstruction may be a cause for concern and impact patient selection.

Stability Study of Anthropomorphic Robot Antares under External Load Action

NASA Astrophysics Data System (ADS)

Kodyakov, A. S.; Pavlyuk, N. A.; Budkov, V. Yu; Prakapovich, R. A.

2017-01-01

The paper presents the study of the behavior of the major structural elements of the lower limbs of anthropomorphic robot Antares under the influence of different types of loads (torsion, fracture). We have determined the required values for actuators torques for motion of the robot in space. The maximum values of torques are 5 Nm and 5.2 Nm respectively, and are able to withstand the upper and lower leg structures.

Experimental Determination of the Fracture Toughness and Brittleness of the Mancos Shale, Utah.

NASA Astrophysics Data System (ADS)

Chandler, Mike; Meredith, Phil; Crawford, Brian

2013-04-01

The hydraulic fracturing of Gas-Shales has become a topic of interest since the US Shale Gas Revolution, and is increasingly being investigated across Europe. A significant issue during hydraulic fracturing is the risk of fractures propagating further than desired into aquifers or faults. This occured at Preese Hall, UK in April and May 2011 when hydraulic fractures propagated into an adjacent fault causing 2.3ML and 1.7ML earthquakes [1]. A rigorous understanding of how hydraulic fractures propagate under in-situ conditions is therefore important for treatment design, both to maximise gas accessed, and to minimise risks due to fracture overextension. Fractures will always propagate along the path of least resistance, but the direction and extent of this path is a complex relationship between the in-situ stress-field, the anisotropic mechanical properties of the rock, and the pore and fracturing pressures [2]. It is possible to estimate the anisotropic in-situ stress field using an isolated-section hydraulic fracture test, and the pore-pressure using well logs. However, the anisotropic mechanical properties of gas-shales remain poorly constrained, with a wide range of reported values. In particular, there is an extreme paucity of published data on the Fracture Toughness of soft sediments such as shales. Mode-I Fracture Toughness is a measure of a material's resistance to dynamic tensile fracture propagation. Defects such as pre-existing microcracks and pores in a material can induce high local stress concentrations, causing fracture propagation and material failure under substantially lower stress than its bulk strength. The mode-I stress intensity factor, KI, quantifies the concentration of stress at the crack tip. For linear elastic materials the Fracture Toughness is defined by the critical value of this stress intensity factor; KIc, beyond which rapid catastrophic crack growth occurs. However, rocks such as shales are relatively ductile and display significant non-linearity. This produces hysteresis during cyclic loading, allowing for the calculation of a brittleness coefficient using the residual displacement after successive loading cycles. This can then be used to define a brittleness corrected Fracture Toughness, KIcc. We report anisotropic KIcc values and a variety of supporting measurements made on the Mancos Shale in the three principle Mode-I crack orientations (Arrester, Divider and Short-Transverse) using a modified Short-Rod sample geometry. The Mancos is an Upper Cretaceous shale from western Colorado and eastern Utah with a relatively high siliclastic content for a gas target formation. The Short-Rod methodology involves the propagation of a crack through a triangular ligament in a chevron-notched cylindrical sample [3]. A very substantial anisotropy is observed in the loading curves and KIcc values for the three crack orientations, with the Divider orientation having KIcc values 25% higher than the other orientations. The measured brittleness for these Mancos shales is in the range 1.5-2.1; higher than for any other rocks we have found in the literature. This implies that the material is extremely non-linear. Increases in KIcc with increasing confining pressure are also investigated, as Shale Gas reservoirs occur at depths where confining pressure may be as high as 35MPa and temperature as high as 100oC. References [1] C.A. Green, P. Styles & B.J. Baptie, "Preese Hall Shale Gas Fracturing", Review & Recommendations for Induced Seismic Mitigation, 2012. [2] N.R. Warpinski & M.B. Smith, "Rock Mechanics and Fracture Geometry", Recent advances in Hydraulic Fracturing, SPE Monograms, Vol. 12, pp. 57-80, 1990. [3] F. Ouchterlony, "International Society for Rock Mechanics Commision on Testing Methods: Suggested Methods for Determining the Fracture Toughness of Rock", International Journal of Rock Mechanics and Mining Science & Geomechanics Abstracts, Vol. 25, 1988.

[Odontoid bending stiffness after anterior fixation with a single lag screw: biomechanical study].

PubMed

Buchvald, P; Čapek, L; Barsa, P

2015-01-01

PURPOSE OF THE STUDY The aim of the experiment was to compare the bending stiffness of an intact odontoid process with bending stiffness after its simulated type II fracture was fixed with a single lag screw. The experiment was done with a desire to answer the question of whether a single osteosynthetic screw is sufficient for good fixation of a type II odontoid fracture. MATERIAL AND METHODS The C2 vertebrae of six cadavers were used. With simultaneous measurement of odontoid bending stiffness, the occurrence of a fracture (type IIA, Grauer's modification of the Anderson- D'Alonzo classification) was simulated using action exerted by a tearing machine in the direction perpendicular to the odontoid axis. Each odontoid fracture was subsequently treated by direct osteosynthesis with a single lag screw inserted in the axial direction by a standard surgical procedure in order to provide conditions similar to those achieved by routine surgical management. The treated odontoid process was subsequently subjected to the same tearing machine loading as applied to it at the start of the experiment. The bending stiffness measured was then compared with that found before the fracture occurred. The results were statistically evaluated by the t-test for paired samples at the level of significance α = 0.05. RESULTS The average value of bending stiffness for odontoid processes of intact vertebrae at the moment of fracture occurrence was 318.3 N/mm. After single axial lag screw fixation of the fracture, the average bending stiffness for the odontoid processes treated was 331.3 N/mm. DISCUSSION Higher values of bending stiffness after screw fixation were found in all specimens and, in comparison with the values recorded before simulated fractures, the increase was statistically significant. CONCLUSIONS The results of our measurements suggest that the single lag screw fixation of a type IIA odontoid fracture will provide better stability for the fracture fragment-C2 body complex on antero-posterior perpendicular loading than can be found in intact C2 vertebrae. Key words: odontoid fracture, odontoid fixation, bending stiffness, lag screw.

Marginal adaptation, fracture load and macroscopic failure mode of adhesively luted PMMA-based CAD/CAM inlays.

PubMed

Ender, Andreas; Bienz, Stefan; Mörmann, Werner; Mehl, Albert; Attin, Thomas; Stawarczyk, Bogna

2016-02-01

To evaluate marginal adaptation, fracture load and failure types of CAD/CAM polymeric inlays. Standardized prepared human molars (48) were divided into four groups (n=12): (A) PCG (positive control group); adhesively luted glass-ceramic inlays, (B) TRX; CAD/CAM polymeric inlays luted using a self-adhesive resin cement, (C) TAC; CAD/CAM polymeric inlays luted using a conventional resin cement, and (D) NCG (negative control group); direct-filled resin-based composite restorations. All specimens were subjected to a chewing simulator. Before and after chewing fatigue, marginal adaptation was assessed at two interfaces: (1) between dental hard tissues and luting cement and (2) between luting cement and restoration. Thereafter, the specimens were loaded and the fracture loads, as well as the failure types, were determined. The data were analysed using three- and one-way ANOVA with post hoc Scheffé test, two sample Student's t-test (p<0.05). Before and after chewing fatigue, marginal adaptation for interface 1 showed significantly better results for TRX and PCG than for TAC (p=0.001-0.02) and NCG (p=0.001-0.047). For interface 2, marginal adaptation for TAC was significantly inferior to TRX (p<0.001) and PCG (p<0.001). Chewing fatigue had a negative impact on the marginal adaptation of TAC and NCG. No significant differences in fracture load were found between all tested groups. Self-adhesive luted polymeric CAD/CAM inlays showed similar marginal adaptation and fracture load values compared to adhesively luted glass-ceramic inlays. Copyright © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Rock Fracture Toughness Under Mode II Loading: A Theoretical Model Based on Local Strain Energy Density

NASA Astrophysics Data System (ADS)

Rashidi Moghaddam, M.; Ayatollahi, M. R.; Berto, F.

2018-01-01

The values of mode II fracture toughness reported in the literature for several rocks are studied theoretically by using a modified criterion based on strain energy density averaged over a control volume around the crack tip. The modified criterion takes into account the effect of T-stress in addition to the singular terms of stresses/strains. The experimental results are related to mode II fracture tests performed on the semicircular bend and Brazilian disk specimens. There are good agreements between theoretical predictions using the generalized averaged strain energy density criterion and the experimental results. The theoretical results reveal that the value of mode II fracture toughness is affected by the size of control volume around the crack tip and also the magnitude and sign of T-stress.

Accurate Critical Stress Intensity Factor Griffith Crack Theory Measurements by Numerical Techniques

PubMed Central

Petersen, Richard C.

2014-01-01

Critical stress intensity factor (KIc) has been an approximation for fracture toughness using only load-cell measurements. However, artificial man-made cracks several orders of magnitude longer and wider than natural flaws have required a correction factor term (Y) that can be up to about 3 times the recorded experimental value [1-3]. In fact, over 30 years ago a National Academy of Sciences advisory board stated that empirical KIc testing was of serious concern and further requested that an accurate bulk fracture toughness method be found [4]. Now that fracture toughness can be calculated accurately by numerical integration from the load/deflection curve as resilience, work of fracture (WOF) and strain energy release (SIc) [5, 6], KIc appears to be unnecessary. However, the large body of previous KIc experimental test results found in the literature offer the opportunity for continued meta analysis with other more practical and accurate fracture toughness results using energy methods and numerical integration. Therefore, KIc is derived from the classical Griffith Crack Theory [6] to include SIc as a more accurate term for strain energy release rate (𝒢Ic), along with crack surface energy (γ), crack length (a), modulus (E), applied stress (σ), Y, crack-tip plastic zone defect region (rp) and yield strength (σys) that can all be determined from load and deflection data. Polymer matrix discontinuous quartz fiber-reinforced composites to accentuate toughness differences were prepared for flexural mechanical testing comprising of 3 mm fibers at different volume percentages from 0-54.0 vol% and at 28.2 vol% with different fiber lengths from 0.0-6.0 mm. Results provided a new correction factor and regression analyses between several numerical integration fracture toughness test methods to support KIc results. Further, bulk KIc accurate experimental values are compared with empirical test results found in literature. Also, several fracture toughness mechanisms are discussed especially for fiber-reinforced composites. PMID:25620817

The Fatigue Behavior of Built-Up Welded Beams of Commercially Pure Titanium

NASA Astrophysics Data System (ADS)

Patnaik, Anil; Poondla, Narendra; Bathini, Udaykar; Srivatsan, T. S.

2011-10-01

In this article, the results of a recent study aimed at evaluating, understanding, and rationalizing the extrinsic influence of fatigue loading on the response characteristics of built-up welded beams made from commercially pure titanium (Grade 2) are presented and discussed. The beams were made from welding plates and sheets of titanium using the pulsed gas metal arc welding technique to form a structural beam having an I-shaped cross section. The welds made for the test beams of the chosen metal were fillet welds using a matching titanium filler metal wire. The maximum and minimum load values at which the built-up beams were cyclically deformed were chosen to be within the range of 22-45% of the maximum predicted flexural static load. The beams were deformed in fatigue at a stress ratio of 0.1 and constant frequency of 5 Hz. The influence of the ratio of maximum load with respect to the ultimate failure load on fatigue performance, quantified in terms of fatigue life, was examined. The percentage of maximum load to ultimate load that resulted in run-out of one million cycles was established. The overall fracture behavior of the failed beam sample was characterized by scanning electron microscopy observations to establish the conjoint influence of load severity, intrinsic microstructural effects, and intrinsic fracture surface features in governing failure by fracture.

Subcritical crack growth of selected aerospace pressure vessel materials

NASA Technical Reports Server (NTRS)

Hall, L. R.; Bixler, W. D.

1972-01-01

This experimental program was undertaken to determine the effects of combined cyclic/sustained loads, stress level, and crack shape on the fatigue crack growth rate behavior of cracks subjected to plane strain conditions. Material/environment combinations tested included: 2219-T87 aluminum plate in gaseous helium, room air, and 3.5% NaCl solution at room temperature, liquid nitrogen, and liquid hydrogen; 5Al-2.5 Sn (ELI) titanium plate in liquid nitrogen and liquid hydrogen and 6AL-4V (ELI) STA titanium plate in gaseous helium and methanol at room temperature. Most testing was accomplished using surface flawed specimens instrumented with a clip gage to continuously monitor crack opening displacements at the specimen surface. Tapered double cantilever beam specimens were also tested. Static fracture and ten hour sustained load tests were conducted to determine fracture toughness and apparent threshold stress intensity values. Cyclic tests were performed using sinusoidal loading profiles at 333 MHz (20 cpm) and trapezoidal loading profiles at both 8.3 MHz (0.5 cpm) and 3.3 MHz (0.2 cpm). Data were evaluated using modified linear elastic fracture mechanics parameters.

Fracture analysis of stiffened panels under biaxial loading with widespread cracking

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Dawicke, D. S.

1995-01-01

An elastic-plastic finite-element analysis with a critical crack-tip-opening angle (CTOA) fracture criterion was used to model stable crack growth and fracture of 2024-T3 aluminum alloy (bare and clad) panels for several thicknesses. The panels had either single or multiple-site damage (MSD) cracks subjected to uniaxial or biaxial loading. Analyses were also conducted on cracked stiffened panels with single or MSD cracks. The critical CTOA value for each thickness was determined by matching the failure load on a middle-crack tension specimen. Comparisons were made between the critical angles determined from the finite-element analyses and those measured with photographic methods. Predicted load-against-crack extension and failure loads for panels under biaxial loading, panels with MSD cracks, and panels with various number of stiffeners were compared with test data, whenever possible. The predicted results agreed well with the test data even for large-scale plastic deformations. The analyses were also able to predict stable tearing behavior of a large lead crack in the presence of MSD cracks. The analyses were then used to study the influence of stiffeners on residual strength in the presence of widespread fatigue cracking. Small MSD cracks were found to greatly reduce the residual strength for large lead cracks even for stiffened panels.

Fracture analysis of stiffened panels under biaxial loading with widespread cracking

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1995-01-01

An elastic-plastic finite-element analysis with a critical crack-tip opening angle (CTOA) fracture criterion was used to model stable crack growth and fracture of 2024-T3 aluminum alloy (bare and clad) panels for several thicknesses. The panels had either single or multiple-site damage (MSD) cracks subjected to uniaxial or biaxial loading. Analyses were also conducted on cracked stiffened panels with single or MSD cracks. The critical CTOA value for each thickness was determined by matching the failure load on a middle-crack tension specimen. Comparisons were made between the critical angles determined from the finite-element analyses and those measured with photographic methods. Predicted load-against-crack extension and failure loads for panels under biaxial loading, panels with MSD cracks, and panels with various numbers of stiffeners were compared with test data whenever possible. The predicted results agreed well with the test data even for large-scale plastic deformations. The analyses were also able to predict stable tearing behavior of a large lead crack in the presence of MSD cracks. The analyses were then used to study the influence of stiffeners on residual strength in the presence of widespread fatigue cracking. Small MSD cracks were found to greatly reduce the residual strength for large lead cracks even for stiffened panels.

Indentation fracture assessment of residual stress in Si{sub 3}N{sub 4}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, K.H.; Liu, K.C.; Sentella, M.

1996-12-31

The measurement of residual stress in Si{sub 3}N{sub 4} ceramics was examined using the indentation technique while a bar specimen with a square cross-section was loaded in tension, and an indentation was created by means of a Vicker`s indenter. The stress applied to the specimen ranged from 0 to 98.8 MPa. The crack length and the shape of the crack were measured by both optical and scanning electron microscopes. Results of the tests indicate that the indentation fracture method can be used to accurately determine the residual stress existing in the material as well as to predict the K{sub c}more » value of the material. The indentation load must be higher than a critical value in order to develop a well-defined penny-shaped crack. For the Si{sub 3}N{sub 4} this critical load is approximately 3 kg. A geometric constant is an important factor for the calculation of the residual stress.« less

Evaluation of the Fracture Toughness of a SMSS Subjected to Common Heat Treatment Cycles in an Aggressive Environment

NASA Astrophysics Data System (ADS)

Pieta, G.; Leite, R.; Kwietniewski, C.; Clarke, T.; Strohaecker, T.

2010-12-01

Supermartensitic stainless steels (SMSS) are an alternative to corrosion-prone carbon steels and expensive duplex stainless steels in offshore tubing applications for the oil and gas industry. Due to their differentiated alloying, SMSS exhibit superior toughness, corrosion resistance, and weldability properties when compared with another viable option, conventional martensitic stainless steels. However, when cathodically protected in a seawater environment they can be susceptible to embrittlement due to hydrogen charging. In the present study, SMSS samples were removed from deep water pipelines and their fracture toughness in the as-received condition and with different heat treatments was evaluated. Tests were carried out in air and in harsh environmental and loading conditions, which were ensured by subjecting specimens to cathodic overprotection, simulating effects seen in structures with complex geometries, and to incremental step loads in a synthetic seawater environment, thus favoring hydrogen diffusion to the precrack tip. The fracture surfaces of the specimens were analyzed in order to identify hydrogen-induced embrittlement and fracture toughness values of specimens tested in air were compared to values obtained in environment-assisted experiments. The influence of microstructure was evaluated by control of the retained austenite and δ-ferrite contents of the specimens. The results show a significant drop in the fracture toughness of steel in the studied environment, with a fracture mode which is clearly more brittle and dependent on microstructural characteristics of the samples.

Effect of surface preparation on the failure load of a highly filled composite bonded to the polymer-monomer matrix of a fiber-reinforced composite.

PubMed

Shimizu, Hiroshi; Tsue, Fumitake; Chen, Zhao-Xun; Takahashi, Yutaka

2009-04-01

The purpose of the present study was to evaluate the effect of surface preparation on the maximum fracture load value of a highly filled composite bonded to the polymer-monomer matrix of a fiber-reinforced composite. A polymer-monomer matrix was made by mixing urethane dimethacrylate and triethyleneglycol dimethacrylate at a ratio of 1:1 with camphorquinone and 2-dimethylaminoethyl methacrylate as a light initiator. The matrix was then polymerized in a disk-shaped silicone mold with a light-polymerizing unit. The flat surfaces of the polymer-monomer matrix disk were prepared in one of the following ways: (1) without preparation; (2) application of silane coupling agent; or (3) application of matrix liquid and prepolymerization. A highly filled composite material was applied and polymerized with a light-polymerizing unit. Additional test specimens made entirely of the polymer-monomer matrix were fabricated as references; the disk and cylinder were fabricated in one piece using a mold specially made for the present study (group 4). Half the specimens were thermocycled up to 10,000 times in water with a 1-minute dwell time at each temperature (5 degrees C and 55 degrees C). The maximum fracture load values were determined using a universal testing machine (n = 10). The maximum fracture loads for group 3 were significantly enhanced both before and after thermocycling, whereas the maximum fracture loads of group 2 were significantly enhanced before thermocycling (p < 0.05); however, the failure loads decreased for all groups after thermocycling (p < 0.05). All the specimens in groups 1 and 2 debonded during thermocycling. The failure load of group 3 was significantly lower than that of group 4 both before and after thermocycling (p < 0.05). Within the limitations of the current in vitro study, the application and prepolymerization of a mixed dimethacrylate resin liquid prior to the application of a highly filled composite was an effective surface preparation for the polymer-monomer matrix of a fiber-reinforced composite; however, the bond durability may be insufficient.

Stability of prototype two-piece zirconia and titanium implants after artificial aging: an in vitro pilot study.

PubMed

Kohal, Ralf-Joachim; Finke, Hans Christian; Klaus, Gerold

2009-12-01

Zirconia oral implants are a new topic in implant dentistry. So far, no data are available on the biomechanical behavior of two-piece zirconia implants. Therefore, the purpose of this pilot investigation was to test in vitro the fracture strength of two-piece cylindrical zirconia implants after aging in a chewing simulator. This laboratory in vitro investigation comprised three different treatment groups. Each group consisted of 16 specimens. In group 1, two-piece zirconia implants were restored with zirconia crowns (zirconia copings veneered with Triceram; Esprident, Ispringen, Germany), and in group 2 zirconia implants received Empress 2 single crowns (Ivoclar Vivadent AG, Schaan, Liechtenstein). The implants, including the abutments, in the two zirconia groups were identical. In group 3, similar titanium implants were reconstructed with porcelain-fused-to-metal crowns. Eight samples of each group were submitted to artificial aging with a long-term load test in the artificial mouth (chewing simulator). Subsequently, all not artificially aged samples and all artificially aged samples that survived the long-term loading of each group were submitted to a fracture strength test in a universal testing machine. For the pairwise comparisons in the different test groups with or without artificial loading and between the different groups at a given artificial loading condition, the Wilcoxon rank-sum test for independent samples was used. The significance level was set at 5%. One sample of group 1 (veneer fracture), none of group 2, and six samples of group 3 (implant abutment screw fractures) failed while exposed to the artificial mouth. The values for the fracture strength after artificial loading with 1.2 million cycles for group 1 were between 45 and 377 N (mean: 275.7 N), in group 2 between 240 and 314 N (mean: 280.7 N), and in the titanium group between 45 and 582 N (mean: 165.7 N). The fracture strength results without artificial load for group 1 amounted to between 270 and 393 N (mean: 325.1 N), for group 2 between 235 and 321 N (mean: 281.8 N), and between 474 and 765 N (mean: 595.2 N) for the titanium group. The failure mode during the fracture testing in the zirconia implant groups was a fracture of the implant head and a bending/fracture of the abutment screw in the titanium group. Within the limits of this pilot investigation, the biomechanical stability of all tested prototype implant groups seems to be - compared with the possibly exerted occlusal forces - borderline for clinical use. A high number of failures occurred already during the artificial loading in the titanium group at the abutment screw level. The zirconia implant groups showed irreparable implant head fractures at relatively low fracture loads. Therefore, the clinical use of the presented prototype implants has to be questioned.

Effect of fiber post length and abutment height on fracture resistance of endodontically treated premolars prepared for zirconia crowns.

PubMed

Lin, Jie; Matinlinna, Jukka Pekka; Shinya, Akikazu; Botelho, Michael George; Zheng, Zhiqiang

2018-04-01

The purpose of this study was to compare the fracture resistance, mode of fracture, and stress distribution of endodontically treated teeth prepared with three different fiber post lengths and two different abutment heights, using both experimental and finite element (FE) approaches. Forty-eight human maxillary premolars with two roots were selected and endodontically treated. The teeth were randomly distributed into six equally sized groups (n = 8) with different combinations of post lengths (7.5, 11, and 15 mm) and abutment heights (3 and 5 mm). All the teeth restored with glass fiber post (Rely X Fiber Post, 3M ESPE, USA) and a full zirconia crown. All the specimens were thermocycled and then loaded to failure at an oblique angle of 135°. Statistical analysis was performed for the effects of post length and abutment height on failure loads using ANOVA and Tukey's honestly significant difference test. In addition, corresponding FE models of a premolar restored with a glass fiber post were developed to examine mechanical responses. The factor of post length (P < 0.01) had a significant effect on failure load. The abutment height (P > 0.05) did not have a significant effect on failure load. The highest mean fracture resistance was recorded for the 15 mm post length and 5 mm abutment height test group, which was significantly more resistant to fracture than the 7.5 mm post and 5 mm abutment height group (P < 0.05). The FE analysis showed the peak compression and tension stress values of 7.5 mm post length were higher than that of 11 and 15 mm post length. The stress value of remaining tooth decreased as the post length was increased. Within the limitations of this experimental and FE analysis study, increasing the post length inside the root of endodontically treated premolar teeth restored with glass-fiber posts increase the fracture resistance to non-axial forces. Failure mode is more favorable with reduced abutment heights.

Virtual stress testing of fracture stability in soldiers with severely comminuted tibial fractures.

PubMed

Petfield, Joseph L; Hayeck, Garry T; Kopperdahl, David L; Nesti, Leon J; Keaveny, Tony M; Hsu, Joseph R

2017-04-01

Virtual stress testing (VST) provides a non-invasive estimate of the strength of a healing bone through a biomechanical analysis of a patient's computed tomography (CT) scan. We asked whether VST could improve management of patients who had a tibia fracture treated with external fixation. In a retrospective case-control study of 65 soldier-patients who had tibia fractures treated with an external fixator, we performed VST utilizing CT scans acquired prior to fixator removal. The strength of the healing bone and the amount of tissue damage after application of an overload were computed for various virtual loading cases. Logistic regression identified computed outcomes with the strongest association to clinical events related to nonunion within 2 months after fixator removal. Clinical events (n = 9) were associated with a low tibial strength for compression loading (p < 0.05, AUC = 0.74) or a low proportion of failed cortical bone tissue for torsional loading (p < 0.005, AUC = 0.84). Using post-hoc thresholds of a compressive strength of four times body-weight and a proportional of failed cortical bone tissue of 5%, the test identified all nine patients who failed clinically (100% sensitivity; 40.9% positive predictive value) and over three fourths of those (43 of 56) who progressed to successful healing (76.8% specificity; 100% negative predictive value). In this study, VST identified all patients who progressed to full, uneventful union after fixator removal; thus, we conclude that this new test has the potential to provide a quantitative, objective means of identifying tibia-fracture patients who can safely resume weight bearing. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:805-811, 2017. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.

The effect of osteoporotic vertebral fracture on predicted spinal loads in vivo.

PubMed

Briggs, Andrew M; Wrigley, Tim V; van Dieën, Jaap H; Phillips, Bev; Lo, Sing Kai; Greig, Alison M; Bennell, Kim L

2006-12-01

The aetiology of osteoporotic vertebral fractures is multi-factorial, and cannot be explained solely by low bone mass. After sustaining an initial vertebral fracture, the risk of subsequent fracture increases greatly. Examination of physiologic loads imposed on vertebral bodies may help to explain a mechanism underlying this fracture cascade. This study tested the hypothesis that model-derived segmental vertebral loading is greater in individuals who have sustained an osteoporotic vertebral fracture compared to those with osteoporosis and no history of fracture. Flexion moments, and compression and shear loads were calculated from T2 to L5 in 12 participants with fractures (66.4 +/- 6.4 years, 162.2 +/- 5.1 cm, 69.1 +/- 11.2 kg) and 19 without fractures (62.9 +/- 7.9 years, 158.3 +/- 4.4 cm, 59.3 +/- 8.9 kg) while standing. Static analysis was used to solve gravitational loads while muscle-derived forces were calculated using a detailed trunk muscle model driven by optimization with a cost function set to minimise muscle fatigue. Least squares regression was used to derive polynomial functions to describe normalised load profiles. Regression co-efficients were compared between groups to examine differences in loading profiles. Loading at the fractured level, and at one level above and below, were also compared between groups. The fracture group had significantly greater normalised compression (p = 0.0008) and shear force (p < 0.0001) profiles and a trend for a greater flexion moment profile. At the level of fracture, a significantly greater flexion moment (p = 0.001) and shear force (p < 0.001) was observed in the fracture group. A greater flexion moment (p = 0.003) and compression force (p = 0.007) one level below the fracture, and a greater flexion moment (p = 0.002) and shear force (p = 0.002) one level above the fracture was observed in the fracture group. The differences observed in multi-level spinal loading between the groups may explain a mechanism for increased risk of subsequent vertebral fractures. Interventions aimed at restoring vertebral morphology or reduce thoracic curvature may assist in normalising spine load profiles.

Method of Evaluating Hydrogen Embrittlement Susceptibility of Tempered Martensitic Steel Showing Intergranular Fracture

NASA Astrophysics Data System (ADS)

Matsumoto, Yu; Takai, Kenichi

2018-02-01

A stress application method in delayed fracture susceptibility tests was investigated using 1450 MPa class tempered martensitic steel. Its fracture mode under hydrogen charging was mainly intergranular because of its relatively small Si content of 0.21 mass pct. The conditions for consistency in fracture strength between tensile tests and constant load tests (CLTs) were clarified: first, to conduct hydrogen precharging before stress application; and second, to choose a sufficiently low crosshead speed in tensile tests. When hydrogen precharging was not conducted before CLTs, the fracture strength was higher than the values in CLTs with hydrogen charging and in tensile tests. If the crosshead speed was too high, the fracture strength obtained was higher than the values in CLTs. The dependence of the fracture strength on crosshead speed was seen for both notched and smooth bar specimens. These results suggested that plastic deformation, i.e., dislocation motion, was related to intergranular fracture with a tear pattern as well as to quasi-cleavage fracture. In addition, cathodic electrolysis in an alkaline solution containing NaOH should be used as the hydrogen charging method to avoid the effects of corrosion.

Crack classification and evolution in anisotropic shale during cyclic loading tests by acoustic emission

NASA Astrophysics Data System (ADS)

Wang, Miaomiao; Tan, Chengxuan; Meng, Jing; Yang, Baicun; Li, Yuan

2017-08-01

Characterization and evolution of the cracking mode in shale formation is significant, as fracture networks are an important element in shale gas exploitation. In this study we determine the crack modes and evolution in anisotropic shale under cyclic loading using the acoustic emission (AE) parameter-analysis method based on the average frequency and RA (rise-time/amplitude) value. Shale specimens with bedding-plane orientations parallel and perpendicular to the axial loading direction were subjected to loading cycles with increasing peak values until failure occurred. When the loading was parallel to the bedding plane, most of the cracks at failure were shear cracks, while tensile cracks were dominant in the specimens that were loaded normal to the bedding direction. The evolution of the crack mode in the shale specimens observed in the loading-unloading sequence except for the first cycle can be divided into three stages: (I) no or several cracks (AE events) form as a result of the Kaiser effect, (II) tensile and shear cracks increase steadily at nearly equal proportions, (III) tensile cracks and shear cracks increase abruptly, with more cracks forming in one mode than in the other. As the dominant crack motion is influenced by the bedding, the failure mechanism is discussed based on the evolution of the different crack modes. Our conclusions can increase our understanding of the formation mechanism of fracture networks in the field.

Effect of framework design on crown failure.

PubMed

Bonfante, Estevam A; da Silva, Nelson R F A; Coelho, Paulo G; Bayardo-González, Daniel E; Thompson, Van P; Bonfante, Gerson

2009-04-01

This study evaluated the effect of core-design modification on the characteristic strength and failure modes of glass-infiltrated alumina (In-Ceram) (ICA) compared with porcelain fused to metal (PFM). Premolar crowns of a standard design (PFMs and ICAs) or with a modified framework design (PFMm and ICAm) were fabricated, cemented on dies, and loaded until failure. The crowns were loaded at 0.5 mm min(-1) using a 6.25 mm tungsten-carbide ball at the central fossa. Fracture load values were recorded and fracture analysis of representative samples were evaluated using scanning electron microscopy. Probability Weibull curves with two-sided 90% confidence limits were calculated for each group and a contour plot of the characteristic strength was obtained. Design modification showed an increase in the characteristic strength of the PFMm and ICAm groups, with PFM groups showing higher characteristic strength than ICA groups. The PFMm group showed the highest characteristic strength among all groups. Fracture modes of PFMs and of PFMm frequently reached the core interface at the lingual cusp, whereas ICA exhibited bulk fracture through the alumina core. Core-design modification significantly improved the characteristic strength for PFM and for ICA. The PFM groups demonstrated higher characteristic strength than both ICA groups combined.

Structural Design Parameters for Germanium

NASA Technical Reports Server (NTRS)

Salem, Jon; Rogers, Richard; Baker, Eric

2017-01-01

The fracture toughness and slow crack growth parameters of germanium supplied as single crystal beams and coarse grain disks were measured. Although germanium is anisotropic (A* 1.7), it is not as anisotropic as SiC, NiAl, or Cu. Thus the fracture toughness was similar on the 100, 110, and 111 planes, however, measurements associated with randomly oriented grinding cracks were 6 to 30 higher. Crack extension in ring loaded disks occurred on the 111 planes due to both the lower fracture energy and the higher stresses on stiff 111 planes. Germanium exhibits a Weibull scale effect, but does not exhibit significant slow crack growth in distilled water. (n 100), implying that design for quasi static loading can be performed with scaled strength statistics. Practical values for engineering design are a fracture toughness of 0.69 0.02 MPam (megapascals per square root meter) and a Weibull modulus of m 6 2. For well ground and reasonable handled coupons, average fracture strength should be greater than 40 megapascals. Aggregate, polycrystalline elastic constants are Epoly 131 gigapascals, vpoly 0.22.

A comparison of wire- and Kevlar-reinforced provisional restorations.

PubMed

Powell, D B; Nicholls, J I; Yuodelis, R A; Strygler, H

1994-01-01

Stainless steel wire 0.036 inch in diameter was compared with Kevlar 49 polyaramid fiber as a means of reinforcing a four-unit posterior provisional fixed restoration with 2 pontics. Three reinforcement patterns for wire and two for Kevlar 49 were evaluated and compared with the control, which was an unreinforced provisional restoration. A central tensile load was placed on the cemented provisional restoration and the variables were measured: (1) the initial stiffness; (2) the load at initial fracture; and (3) the unit toughness, or the energy stored in the beam at a point where the load had undergone a 1.0-mm deflection. Statistical analysis showed (1) the bent wire configuration had a significantly higher initial stiffness (P < or = .05), (2) there was no difference between designs for load at initial fracture, and (3) the bent wire had a significantly higher unit toughness value (P < or = .05).

Examination of ceramic restoration adhesive coverage in cusp-replacement premolar using acoustic emission under fatigue testing.

PubMed

Chang, Yen-Hsiang; Yu, Jin-Jie; Lin, Chun-Li

2014-12-13

This study investigates CAD/CAM ceramic cusp-replacing restoration resistance with and without buccal cusp replacement under static and dynamic cyclic loads, monitored using the acoustic emission (AE) technique. The cavity was designed in a typical MODP (mesial-occlusal-distal-palatal) restoration failure shape when the palatal cusp has been lost. Two ceramic restorations [without coverage (WOC) and with (WC) buccal cuspal coverage with 2.0 mm reduction in cuspal height] were prepared to perform the fracture and fatigue tests with normal (200 N) and high (600 N) occlusal forces. The load versus AE signals in the fracture and fatigue tests were recorded to evaluate the restored tooth failure resistance. The results showed that non-significant differences in load value in the fracture test and the accumulated number of AE signals under normal occlusal force (200 N) in the fatigue test were found between with and without buccal cuspal coverage restorations. The first AE activity occurring for the WOC restoration was lower than that for the WC restoration in the fracture test. The number of AE signals increased with the cyclic load number. The accumulated number of AE signals for the WOC restoration was 187, higher than that (85) for the WC restoration under 600 N in the fatigue test. The AE technique and fatigue tests employed in this study were used as an assessment tool to evaluate the resistances in large CAD/CAM ceramic restorations. Non-significant differences in the tested fracture loads and accumulated number of AE signals under normal occlusal force (200 N) between different restorations indicated that aggressive treatment (with coverage preparation) in palatal cusp-replacing ceramic premolars require more attention for preserving and protecting the remaining tooth.

New data on the kinetics and governing factors of the spall fracture of metals

NASA Astrophysics Data System (ADS)

Kanel, G. I.; Razorenov, S. V.; Garkushin, G. V.; Savinykh, A. S.

2018-01-01

This paper presents two examples of significant departures from usual trends of varying the resistance to spall fracture (spall strength) with changing loading history, load duration and peak shock stress. In experiments with vanadium single crystals we observed an important decrease of spall strength when increasing the shock stress. This was interpreted in terms of disruption of the matter homogeneity as a result of its twinning at shock compression. In experiments with 12Kh18N10T austenitic stainless steel we observed a sharp increase of recorded spall strength value when short load pulses of a triangular profile were replaced by shock pulses of long duration having a trapezoidal shape. This anomaly is associated with formation of the deformation-induced martensitic phase.

Fracture loads of all-ceramic crowns under wet and dry fatigue conditions.

PubMed

Borges, Gilberto A; Caldas, Danilo; Taskonak, Burak; Yan, Jiahau; Sobrinho, Lourenco Correr; de Oliveira, Wildomar José

2009-12-01

The aim of this study was to test the hypothesis that fracture loads of fatigued dental ceramic crowns are affected by testing environment and luting cement. One hundred and eighty crowns were prepared from bovine teeth using a lathe. Ceramic crowns were prepared from three types of ceramic systems: an alumina-infiltrated ceramic, a lithia-disilicate-based glass ceramic, and a leucite-reinforced ceramic. For each ceramic system, 30 crowns were cemented with a composite resin cement, and the remaining 30 with a resin-modified glass ionomer cement. For each ceramic system and cement, ten specimens were loaded to fracture without fatiguing. A second group (n = 10) was subjected to cyclic fatigue and fracture tested in a dry environment, and a third group (n = 10) was fatigued and fractured in distilled water. The results were statistically analyzed using one-way ANOVA and Tukey HSD test. The fracture loads of ceramic crowns decreased significantly after cyclic fatigue loading (p

Acoustic Emission Monitoring of Multicell Reinforced Concrete Box Girders Subjected to Torsion

PubMed Central

Bagherifaez, Marya; Behnia, Arash; Majeed, Abeer Aqeel; Hwa Kian, Chai

2014-01-01

Reinforced concrete (RC) box girders are a common structural member for road bridges in modern construction. The hollow cross-section of a box girder is ideal in carrying eccentric loads or torques introduced by skew supports. This study employed acoustic emission (AE) monitoring on multicell RC box girder specimens subjected to laboratory-based torsion loading. Three multicell box girder specimens with different cross-sections were tested. The aim is to acquire AE analysis data indicative for characterizing torsion fracture in the box girders. It was demonstrated through appropriate parametric analysis that the AE technique could be utilized to effectively classify fracture developed in the specimens for describing their mechanical behavior under torsion. AE events localization was presented to illustrate the trend of crack and damage propagation in different stages of fracture. It could be observed that spiral-like patterns of crack were captured through AE damage localization system and damage was quantified successfully in different stages of fracture by using smoothed b-value analysis. PMID:25180203

Characterization of crack growth under combined loading

NASA Technical Reports Server (NTRS)

Feldman, A.; Smith, F. W.; Holston, A., Jr.

1977-01-01

Room-temperature static and cyclic tests were made on 21 aluminum plates in the shape of a 91.4x91.4-cm Maltese cross with 45 deg flaws to develop crack growth and fracture toughness data under mixed-mode conditions. During cyclic testing, it was impossible to maintain a high proportion of shear-mode deformation on the crack tips. Cracks either branched or turned. Under static loading, cracks remained straight if shear stress intensity exceeded normal stress intensity. Mixed-mode crack growth rate data compared reasonably well with published single-mode data, and measured crack displacements agreed with the straight and branched crack analyses. Values of critical strain energy release rate at fracture for pure shear were approximately 50% higher than for pure normal opening, and there was a large reduction in normal stress intensity at fracture in the presence of high shear stress intensity. Net section stresses were well into the inelastic range when fracture occurred under high shear on the cracks.

Comparison of effects of different screw materials in the triangle fixation of femoral neck fractures.

PubMed

Gok, Kadir; Inal, Sermet; Gok, Arif; Gulbandilar, Eyyup

2017-05-01

In this study, biomechanical behaviors of three different screw materials (stainless steel, titanium and cobalt-chromium) have analyzed to fix with triangle fixation under axial loading in femoral neck fracture and which material is best has been investigated. Point cloud obtained after scanning the human femoral model with the three dimensional (3D) scanner and this point cloud has been converted to 3D femoral model by Geomagic Studio software. Femoral neck fracture was modeled by SolidWorks software for only triangle configuration and computer-aided numerical analyses of three different materials have been carried out by AnsysWorkbench finite element analysis (FEA) software. The loading, boundary conditions and material properties have prepared for FEA and Von-Misses stress values on upper and lower proximity of the femur and screws have been calculated. At the end of numerical analyses, the best advantageous screw material has calculated as titanium because it creates minimum stress at the upper and lower proximity of the fracture line.

Dynamic Fracture Properties of Rocks Subjected to Static Pre-load Using Notched Semi-circular Bend Method

NASA Astrophysics Data System (ADS)

Chen, Rong; Li, Kang; Xia, Kaiwen; Lin, Yuliang; Yao, Wei; Lu, Fangyun

2016-10-01

A dynamic load superposed on a static pre-load is a key problem in deep underground rock engineering projects. Based on a modified split Hopkinson pressure bar test system, the notched semi-circular bend (NSCB) method is selected to investigate the fracture initiation toughness of rocks subjected to pre-load. In this study, a two-dimensional ANSYS finite element simulation model is developed to calculate the dimensionless stress intensity factor. Three groups of NSCB specimen are tested under a pre-load of 0, 37 and 74 % of the maximum static load and with the loading rate ranging from 0 to 60 GPa m1/2 s-1. The results show that under a given pre-load, the fracture initiation toughness of rock increases with the loading rate, resembling the typical rate dependence of materials. Furthermore, the dynamic rock fracture toughness decreases with the static pre-load at a given loading rate. The total fracture toughness, defined as the sum of the dynamic fracture toughness and initial stress intensity factor calculated from the pre-load, increases with the pre-load at a given loading rate. An empirical equation is used to represent the effect of loading rate and pre-load force, and the results show that this equation can depict the trend of the experimental data.

Sliding contact fracture of dental ceramics: Principles and validation

PubMed Central

Ren, Linlin; Zhang, Yu

2014-01-01

Ceramic prostheses are subject to sliding contact under normal and tangential loads. Accurate prediction of the onset of fracture at two contacting surfaces holds the key to greater long-term performance of these prostheses. In this study, building on stress analysis of Hertzian contact and considering fracture criteria for linear elastic materials, a constitutive fracture mechanics relation was developed to incorporate the critical fracture load with the contact geometry, coefficient of friction and material fracture toughness. Critical loads necessary to cause fracture under a sliding indenter were calculated from the constitutive equation, and compared with the loads predicted from elastic stress analysis in conjunction with measured critical load for frictionless normal contact—a semi-empirical approach. The major predictions of the models were calibrated with experimentally determined critical loads of current and future dental ceramics after contact with a rigid spherical slider. Experimental results conform with the trends predicted by the models. PMID:24632538

In vitro evaluation of the fracture resistance and microleakage of porcelain laminate veneers bonded to teeth with composite fillings after cyclic loading

PubMed Central

Sadighpour, Leyla; Fallahi Sichani, Babak; Kharazi Fard, Mohamd Javad

2014-01-01

PURPOSE There is insufficient data regarding the durability of porcelain laminate veneers bonded to existing composite fillings. The aim of the present study was to evaluate the fracture resistance and microleakage of porcelain laminate veneers bonded to teeth with existing composite fillings. MATERIALS AND METHODS Thirty maxillary central incisors were divided into three groups (for each group, n=10): intact teeth (NP), teeth with class III composite fillings (C3) and teeth with class IV cavities (C4). Porcelain laminate veneers were made using IPS-Empress ceramic and bonded with Panavia F2 resin cement. The microleakage of all of the specimens was tested before and after cyclic loading (1 × 106 cycles, 1.2 Hz). The fracture resistance values (N) were measured using a universal testing machine, and the mode of failure was also examined. The statistical analyses were performed using one-way ANOVA and Tukey post hoc tests (α=.05). RESULTS There was a significant difference in the mean microleakage of group C4 compared with group NT (P=.013). There was no significant difference in the fracture loads among the groups. CONCLUSION The microleakage and failure loads of porcelain laminate veneers bonded to intact teeth and teeth with standard class III composite fillings were not significantly different. PMID:25177471

Single versus double row suture anchor fixation for greater tuberosity fractures - a biomechanical study.

PubMed

Seppel, Gernot; Saier, Tim; Martetschläger, Frank; Plath, Johannes E; Guevara-Alvarez, Alberto; Henschel, Julia; Winkler, Martin; Augat, Peter; Imhoff, Andreas B; Buchmann, Stefan

2017-12-01

Fractures of the humeral greater tuberosity (GT) are a frequent injury progressively treated with arthroscopic suture anchor repair. Yet, no biomechanical study has been performed comparing fixation strength of arthroscopic single- (SR) vs. double row (DR) fixation. Standardized fractures of the greater tuberosity were created in 12 fresh frozen proximal humeri. After random assignation to the SR or DR group the fixed humeri were tested applying cyclic loading to the supraspinatus and infraspinatus tendon. Load to failure and fragment displacement were assessed by means of an electrodynamic material testing machine using an optical tracking system. Load to failure values were higher in the DR group (649 N; ±176) than in the SR group (490 N; ±145) however without statistical significance (p = .12). In greater tuberosity displacement of 3-5 mm surgical treatment is recommended. The fixing constructs in this study did not reach displacement landmarks of 3 or 5 mm before construct failure as shown in previous studies. Thus the applied traction force (N) at 1 mm displacement was analyzed. In the SR group the load at 1 mm displacement was 277 N; ±46 compared to 260 N; ±62 in the DR group (p = .65). The results suggest that both techniques are viable options for refixation of greater tuberosity fractures. Laboratory study.

Residual Strength Analyses of Monolithic Structures

NASA Technical Reports Server (NTRS)

Forth, Scott (Technical Monitor); Ambur, Damodar R. (Technical Monitor); Seshadri, B. R.; Tiwari, S. N.

2003-01-01

Finite-element fracture simulation methodology predicts the residual strength of damaged aircraft structures. The methodology uses the critical crack-tip-opening-angle (CTOA) fracture criterion to characterize the fracture behavior of the material. The CTOA fracture criterion assumes that stable crack growth occurs when the crack-tip angle reaches a constant critical value. The use of the CTOA criterion requires an elastic- plastic, finite-element analysis. The critical CTOA value is determined by simulating fracture behavior in laboratory specimens, such as a compact specimen, to obtain the angle that best fits the observed test behavior. The critical CTOA value appears to be independent of loading, crack length, and in-plane dimensions. However, it is a function of material thickness and local crack-front constraint. Modeling the local constraint requires either a three-dimensional analysis or a two-dimensional analysis with an approximation to account for the constraint effects. In recent times as the aircraft industry is leaning towards monolithic structures with the intention of reducing part count and manufacturing cost, there has been a consistent effort at NASA Langley to extend critical CTOA based numerical methodology in the analysis of integrally-stiffened panels.In this regard, a series of fracture tests were conducted on both flat and curved aluminum alloy integrally-stiffened panels. These flat panels were subjected to uniaxial tension and during the test, applied load-crack extension, out-of-plane displacements and local deformations around the crack tip region were measured. Compact and middle-crack tension specimens were tested to determine the critical angle (wc) using three-dimensional code (ZIP3D) and the plane-strain core height (hJ using two-dimensional code (STAGS). These values were then used in the STAGS analysis to predict the fracture behavior of the integrally-stiffened panels. The analyses modeled stable tearing, buckling, and crack branching at the integral stiffener using different values of critical CTOA for different material thicknesses and orientation. Comparisons were made between measured and predicted load-crack extension, out-of-plane displacements and local deformations around the crack tip region. Simultaneously, three-dimensional capabilities to model crack branching and to monitor stable crack growth of multiple cracks in a large thick integrally-stiffened flat panels were implemented in three-dimensional finite element code (ZIP3D) and tested by analyzing the integrally-stiffened panels tested at Alcoa. The residual strength of the panels predicted from STAGS and ZP3D code compared very well with experimental data. In recent times, STAGS software has been updated with new features and now one can have combinations of solid and shell elements in the residual strength analysis of integrally-stiffened panels.

A study of environmental characterization of conventional and advanced aluminum alloys for selection and design. Phase 2: The breaking load test method

NASA Technical Reports Server (NTRS)

Sprowls, D. O.; Bucci, R. J.; Ponchel, B. M.; Brazill, R. L.; Bretz, P. E.

1984-01-01

A technique is demonstrated for accelerated stress corrosion testing of high strength aluminum alloys. The method offers better precision and shorter exposure times than traditional pass fail procedures. The approach uses data from tension tests performed on replicate groups of smooth specimens after various lengths of exposure to static stress. The breaking strength measures degradation in the test specimen load carrying ability due to the environmental attack. Analysis of breaking load data by extreme value statistics enables the calculation of survival probabilities and a statistically defined threshold stress applicable to the specific test conditions. A fracture mechanics model is given which quantifies depth of attack in the stress corroded specimen by an effective flaw size calculated from the breaking stress and the material strength and fracture toughness properties. Comparisons are made with experimental results from three tempers of 7075 alloy plate tested by the breaking load method and by traditional tests of statistically loaded smooth tension bars and conventional precracked specimens.

Microstructural effects on fracture toughness of polycrystalline ceramics in combined mode I and mode II loading

NASA Technical Reports Server (NTRS)

Singh, D.; Shetty, D. K.

1988-01-01

Fracture toughness of polycrystalline alumina and ceria partially-stabilized tetragonal zirconia (CeO2-TZP) ceramics were assessed in combined mode I and mode II loading using precracked disk specimens in diametral compression. Stress states ranging from pure mode I, combined mode I and mode II, and pure mode II were obtained by aligning the center crack at specific angles relative to the loading diameter. The resulting mixed-mode fracture toughness envelope showed significant deviation to higher fracture toughness in mode II relative to the predictions of the linear elastic fracture mechanics theory. Critical comparison with corresponding results on soda-lime glass and fracture surface observations showed that crack surface resistance arising from grain interlocking and abrasion was the main source of the increased fracture toughness in mode II loading of the polycrystalline ceramics. The normalized fracture toughness for pure mode II loading, (KII/KIc), increased with increasing grain size for the CeO2-TZP ceramics. Quantitative fractography confirmed an increased percentage of transgranular fracture of the grains in mode II loading.

A novel ex vivo model of compressive immature rib fractures at pathophysiological rates of loading.

PubMed

Beadle, Nicola; Burnett, Timothy L; Hoyland, Judith A; Sherratt, Michael J; Freemont, Anthony J

2015-11-01

Compressive rib fractures are considered to be indicative of non-accidental injury (NAI) in infants, which is a significant and growing issue worldwide. The diagnosis of NAI is often disputed in a legal setting, and as a consequence there is a need to model such injuries ex vivo in order to characterise the forces required to produce non-accidental rib fractures. However, current models are limited by type of sample, loading method and rate of loading. Here, we aimed to: i) develop a loading system for inducing compressive fractures in whole immature ribs that is more representative of the physiological conditions and mechanism of injury employed in NAI and ii) assess the influence of loading rate and rib geometry on the mechanical performance of the tissue. Porcine ribs (5-6 weeks of age) from 12 animals (n=8 ribs/animal) were subjected to axial compressive load directed through the anterior-posterior rib axis at loading rates of 1, 30, 60 or 90 mm/s. Key mechanical parameters (including peak load, load and percentage deformation to failure and effective stiffness) were quantified from the load-displacement curves. Measurements of the rib length, thickness at midpoint, distance between anterior and posterior extremities, rib curvature and fracture location were determined from radiographs. This loading method typically produced incomplete fractures around the midpoint of the ribs, with 87% failing in this manner; higher loads and less deformation were required for ribs to completely fracture through both cortices. Loading rate, within the range of 1-90 mm/s, did not significantly affect any key mechanical parameters of the ribs. Load-displacement curves displaying characteristic and quantifiable features were produced for 90% of the ribs tested, and multiple regression analyses indicate that, in addition to the geometrical variables, there are other factors such as the micro- and nano-structure that influence the measured mechanical data. A reproducible method of inducing fractures in a consistent location in immature porcine ribs has been successfully developed. Fracture appearance may be indicative of the amount of load and deformation that produced the fracture, which is an important finding for NAI, where knowledge of the aetiology of fractures is vital. Characteristic rib behaviour independent of loading rate and, to an extent, rib geometry has been demonstrated, allowing further investigation into how the complex micro- and nano-structure of immature ribs influences the mechanical performance under compressive load. This research will ultimately enable improved characterisation of the loading pattern involved in non-accidental rib fractures. Copyright © 2015 Elsevier Ltd. All rights reserved.

Electrothermal fracturing of tensile specimens

NASA Technical Reports Server (NTRS)

Blinn, H. O.; Hanks, J. G.; Perkins, H. P.

1970-01-01

Pulling device consisting of structural tube, connecting rod, spring-loaded nuts, loading rod, heating element, and three bulkheads fractures tensile specimens. Alternate heating and cooling increases tensile loading by increments until fracturing occurs. Load cell or strain gage, applied to pulling rod, determines forces applied.

Fracture of Structural Materials under Dynamic Loading

DTIC Science & Technology

1981-03-25

in character- izing the dynamic fracture resistance of materials, and in designing equipment and procedures for measuring dynamic fracture toughness...useful in assessing the safety of structures under dynamic loads, in characterizing the dyraamic fracture resistance of materials, and in designing ...I INTRODUCTION Structures used by the United States Air Force must be designed to resist catastrophic fracture when subjected ti dynamic loads. For

The Metacarpal Locked Intramedullary Nail: Comparative Biomechanical Evaluation of New Implant Design for Metacarpal Fractures.

PubMed

Boonyasirikool, Chinnakart; Tanakeatsakul, Sakkarin; Niempoog, Sunyarn

2015-04-01

The optimal fixation of metacarpal fracture should provide sufficient stability to permit early functionfor all types of fracture. However; it must preserve surrounding soft tissue during application and not require secondary removal due to its prominence. The prototype of metacarpal locked intramedullary nail (MCLN) was designed by our institute aiming to achieve those allfeatures. To biomechanically test our newly designed, locked metacarpal nail and compare with common current available fixation methods. Thirty chicken humeri were devided into 3 groups (n = 1 per group) according tofixation techniques: MCLN, 1.5 mm miniplate (Synthes), and Kirschner wire. After complete fixation, all specimens were osteotomized at mid-shaft creating transverse fractures. Five specimens from each group were tested by load of failure under axial compression, and another five under bending force. In axial compression model, the loads tofailure in MCLN group was greatest (460 ± 17 N), which was significant higher than the Kirschner wire group. The MCLN group also showed the highest load to failure in bending test (341 ± 10 N). This value reaches statistical significance when compared with plate and Kirschner wire groups. The MCLN construct provided higher stability than miniplate and Kirschner wire fixation both in axial and bending mode. Together with the minimally invasive and soft tissue-friendly design concept, this study suggests that MCLN is promising fixation option for metacarpal fracture.

Prediction of risk of fracture in the tibia due to altered bone mineral density distribution resulting from disuse: a finite element study.

PubMed

Gislason, Magnus K; Coupaud, Sylvie; Sasagawa, Keisuke; Tanabe, Yuji; Purcell, Mariel; Allan, David B; Tanner, K Elizabeth

2014-02-01

The disuse-related bone loss that results from immobilisation following injury shares characteristics with osteoporosis in post-menopausal women and the aged, with decreases in bone mineral density leading to weakening of the bone and increased risk of fracture. The aim of this study was to use the finite element method to: (i) calculate the mechanical response of the tibia under mechanical load and (ii) estimate of the risk of fracture; comparing between two groups, an able-bodied group and spinal cord injury patients group suffering from varying degrees of bone loss. The tibiae of eight male subjects with chronic spinal cord injury and those of four able-bodied age-matched controls were scanned using multi-slice peripheral quantitative computed tomography. Images were used to develop full three-dimensional models of the tibiae in Mimics (Materialise) and exported into Abaqus (Simulia) for calculation of stress distribution and fracture risk in response to specified loading conditions - compression, bending and torsion. The percentage of elements that exceeded a calculated value of the ultimate stress provided an estimate of the risk of fracture for each subject, which differed between spinal cord injury subjects and their controls. The differences in bone mineral density distribution along the tibia in different subjects resulted in different regions of the bone being at high risk of fracture under set loading conditions, illustrating the benefit of creating individual material distribution models. A predictive tool can be developed based on these models, to enable clinicians to estimate the amount of loading that can be safely allowed onto the skeletal frame of individual patients who suffer from extensive musculoskeletal degeneration (including spinal cord injury, multiple sclerosis and the ageing population). The ultimate aim is to reduce fracture occurrence in these vulnerable groups.

Stress and stability of plate-screw fixation and screw fixation in the treatment of Schatzker type IV medial tibial plateau fracture: a comparative finite element study.

PubMed

Huang, Xiaowei; Zhi, Zhongzheng; Yu, Baoqing; Chen, Fancheng

2015-11-25

The purpose of this study is to compare the stress and stability of plate-screw fixation and screw fixation in the treatment of Schatzker type IV medial tibial plateau fracture. A three-dimensional (3D) finite element model of the medial tibial plateau fracture (Schatzker type IV fracture) was created. An axial force of 2500 N with a distribution of 60% to the medial compartment was applied to simulate the axial compressive load on an adult knee during single-limb stance. The equivalent von Mises stress, displacement of the model relative to the distal tibia, and displacement of the implants were used as the output measures. The mean stress value of the plate-screw fixation system was 18.78 MPa, which was significantly (P < 0.001) smaller than that of the screw fixation system. The maximal value of displacement (sum) in the plate-screw fixation system was 2.46 mm, which was lower than that in the screw fixation system (3.91 mm). The peak stress value of the triangular fragment in the plate-screw fixation system model was 42.04 MPa, which was higher than that in the screw fixation model (24.18 MPa). But the mean stress of the triangular fractured fragment in the screw fixation model was significantly higher in terms of equivalent von Mises stress (EVMS), x-axis, and z-axis (P < 0.001). This study demonstrated that the load transmission mechanism between plate-screw fixation system and screw fixation system was different and the stability provided by the plate-screw fixation system was superior to the screw fixation system.

Fracture analysis near the mid-ocean plate boundary, Reykjavik-Hvalfjördur area, Iceland

NASA Astrophysics Data System (ADS)

Jefferis, Robert G.; Voight, Barry

1981-07-01

The geometry and thermal history of fractures have been determined at 59 stations from Reykjavik to Hvalfjördur in southwestern Iceland. The data provide information on crustal stress regimes in the vicinity of mid-ocean ridges. Two major, generalized fracture orientations are present (1) a northeast system, trend 010°-030°, except on Akranes where the orientation is 040°-060° (2) a broad east—west system containing one or more sets with strike between 070°-130°. Thermal history of the host rock and fractures was determined from secondary minerals in vugs and fractures. The thermal history indicates that the northeast fracture set opened while the area was within the relatively hot axial zone of active volcanism and rifting. Some of the east—west trending fractures also opened at this time but many formed later, after the area had begun to cool and drift from the active zone. The northeast fracture set is essentially parallel to the trend of dikes and normal faults in southwestern Iceland. They have been interpreted as extension fractures (resulting in about 0.4% maximum extension) forming generally from the same stress field associated with normal faulting and dike injection in the active zone. Fracturing in an east-west direction (estimated 0.1% maximum extension), mainly near the edge and outside the active zone, indicates a reorientation of this stress field. The dominant mechanism related to the origin of the east—west fractures may be thermoelastic stresses arising from axial and basal accretion and cooling of lithospheric plates. Both fracture systems are inferred to have formed, in the Griffiths idealization, under nearly biaxial effective compressive loading on the order of 200 bar. The discrepancy between this value and the kilobar-order strengths of short-time laboratory tests reflects such factors as high temperature stress corrosion and fatigue. Fracture propagation is assumed to have been stable, but governed primarily by lateral load-diminishing mechanisms rather than by progressive loading. These relaxation mechanisms may have been episodic (northeast-system fissure swarm activity) or steady-state (thermoelastic contraction) in time.

Mechanical characterization of composite repairs for fiberglass wind turbine blades

NASA Astrophysics Data System (ADS)

Chawla, Tanveer Singh

While in service, wind turbine blades experience various modes of loading. An example is impact loading in the form of hail or bird strikes, which might lead to localized damage or formation of cracks a few plies deep on the blade surface. One of the methods to conduct repairs on wind turbine blades that are damaged while in service is hand lay-up of the repair part after grinding out the damaged portion and some of its surrounding area. The resin used for such repairs usually differs from the parent plate resin in composition and properties such as gel time, viscosity, etc. As a result the properties of the repaired parts are not the same as that of the undamaged blades. Subsequent repetitive loading can be detrimental to weak repairs to such an extent so as to cause delamination at the parent-repair bondline causing the repairs to eventually fall off the blade. Thus the strength and toughness of the repair are of critical importance. Initial part of this work consists of an effort to increase repair strength by identifying an optimum hand layup repair resin for fiberglass wind turbine blades currently being manufactured by a global company. As delamination of the repair from the parent blade is a major concern and unidirectional glass fibers along with a polymer resin are used to manufacture blades under consideration, testing method detailed in ASTM D 5528 (Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites) was followed to determine propagation fracture toughness values of the prospective vinyl ester repair resin candidates. These values were compared to those for a base polyester repair resin used by the company. Experimental procedure and results obtained from the above mentioned testing using double cantilever beam (DCB) specimens are detailed. Three new repair resins were shortlisted through mode I testing. It was also found that variation in the depth of the ground top ply of the parent part affects the propagation fracture toughness values of the repair. Repairs conducted on surfaces with partially ground top plies possess higher fracture toughness values than those conducted on surfaces with complete top plies ground off. The three top repair resin candidates were then evaluated against the base repair resin under fatigue loading. The specimen configuration and testing method were chosen so as to be able to test hand layup repairs under tension -- tension cyclic loading. It was observed that all three new repair resins perform better than the base repair resin. The selection of the optimum repair resin was based on results from mode I and fatigue testing. Global manufacturing regulations and standards were also of prime concern. The final new repair resin is being used by the company in all of its plants over the globe. The balance of this work involves study of the effect of mixed mode I -- mode II loading on the strength of repairs conducted on fiber reinforced composite parts using hand lay-up technique. The specimens for this part were similar to those manufactured for mode I testing but with different dimensions and layup. They were made and tested in accordance with ASTM D 6671 (Standard Test Method for Mixed Mode I -- Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites). Comparison was made between the fracture toughness of the above chosen optimum repair resin and the base repair resin. At least two levels of mode mixture GII/G (Mode II fracture toughness / Mode I and II fracture toughness) were examined. Also, two levels of grinding were considered (complete ply vs. partial ply ground off) in order to establish the influence of varying top-ply grinding depths on the strength of hand layup repairs conducted on fiberglass composite structures. The results of this work have the potential to improve the repair process for current fiberglass wind turbine blades.

Stability of mechanical joints in launching vehicles: Local and global stationary values of energy density

NASA Astrophysics Data System (ADS)

Chue, Ching-Hwei

A method was developed for predicting the behavior of mechanical joints in launch vehicles with particular emphasis placed on how the combined effects of loading, geometry, and materials could be optimized in terms of structure instability and/or integrity. What was considered to be essential is the fluctuation of the volume energy density with time in the structure. The peaks and valleys of the volume energy density function will be associated with failure by fracture and/or yielding while the distance between their local and global stationary values govern the structure instability. The Solid Rocket Booster (SRB) of the space shuttle was analyzed under axisymmetric and non-axisymmetric loadings. A semi-analytical finite element program was developed for solving the case of non-axisymmetric loading. Following a dynamic stress analysis, contours of the volume energy density in the structure were obtained as a function of time. The magnitudes and locations of these stationary values were then calculated locally and globally and related to possible failure by fracture. In the case of axisymmetric flight, the local and global instability behavior do not change appreciably. Fluctuations in the energy density and the dynamic stability length parameter become appreciable when the non-axisymmetric loads are considered. The magnitude of the energy in the shell structure is sensitive to alterations in the gas pressure induced by the solid propellant.

Simplified data reduction methods for the ECT test for mode 3 interlaminar fracture toughness

NASA Technical Reports Server (NTRS)

Li, Jian; Obrien, T. Kevin

1995-01-01

Simplified expressions for the parameter controlling the load point compliance and strain energy release rate were obtained for the Edge Crack Torsion (ECT) specimen for mode 3 interlaminar fracture toughness. Data reduction methods for mode 3 toughness based on the present analysis are proposed. The effect of the transverse shear modulus, G(sub 23), on mode 3 interlaminar fracture toughness characterization was evaluated. Parameters influenced by the transverse shear modulus were identified. Analytical results indicate that a higher value of G(sub 23) results in a low load point compliance and lower mode 3 toughness estimation. The effect of G(sub 23) on the mode 3 toughness using the ECT specimen is negligible when an appropriate initial delamination length is chosen. A conservative estimation of mode 3 toughness can be obtained by assuming G(sub 23) = G(sub 12) for any initial delamination length.

Biomechanical analysis of fixation of middle third fractures of the clavicle.

PubMed

Drosdowech, Darren S; Manwell, Stuart E E; Ferreira, Louis M; Goel, Danny P; Faber, Kenneth J; Johnson, James A

2011-01-01

This biomechanical study compares four different techniques of fixation of middle third clavicular fractures. Twenty fresh-frozen clavicles were randomized into four groups. Each group used a different fixation device (3.5 Synthes reconstruction plate, 3.5 Synthes limited contact dynamic compression plate, 3.5 Synthes locking compression plate, and 4.5 DePuy Rockwood clavicular pin). All constructs were mechanically tested in bending and torque modes both with and without a simulated inferior cortical defect. Bending load to failure was also conducted. The four groups were compared using an analysis of variance test. The plate constructs were stiffer than the pin during both pure bending and torque loads with or without an inferior cortical defect. Bending load to failure with an inferior cortical defect revealed that the reconstruction plate was weaker compared with the other three groups. The limited contact and locking plates were stiffer than the reconstruction plate but demonstrated statistical significance only with the cortical defect. As hypothesized, the 3.5 limited contact dynamic compression plate and 3.5 locking compression plate demonstrated the greatest resistance to bending and torque loads, especially in the presence of simulated comminution of a middle third clavicular fracture. The reconstruction plate demonstrated lower stiffness and strength values compared with the other plates, especially with a cortical defect, whereas the pin showed poor resistance to bending and torque loads in all modes of testing. This information may help surgeons to choose the most appropriate method of fixation when treating fractures of the middle third of the clavicle.

On the feasibility of the Chevron Notch Beam method to measure fracture toughness of fine-grained zirconia ceramics.

PubMed

Kailer, Andreas; Stephan, Marc

2016-10-01

The fracture toughness determination of fine-grained zirconia ceramics using the chevron notched beam method (CNB) was investigated to assess the feasibility of this method for quality assurance and material characterization. CNB tests were performed using four different yttria-stabilized zirconia ceramics under various testing modes and conditions, including displacement-controlled and load-rate-controlled four point bending to assess the influence of slow crack growth and identify most suitable test parameters. For comparison, tests using single-edge V-notch beams (SEVNB) were conducted. It was observed that the CNB method yields well-reproducible results. However, slow crack growth effects significantly affect the measured KIC values, especially when slow loading rates are used. To minimize the effect of slow crack growth, the application of high loading rates is recommended. Despite a certain effort needed for setting up a sample preparation routine, the CNB method is considered to be very useful for measuring and controlling the fracture toughness of zirconia ceramics. Copyright © 2016 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Effect of Solute Segregation on Fracture Toughness in a Ni-Cr Steel

NASA Astrophysics Data System (ADS)

Kameda, Jun

1981-12-01

A study has been made of the influence of intergranular solute segregation on fracture toughness K1c in a series of Ni-Cr steels individually doped with Sb, Sn, and P. By means of toughness measurements in steels having two different intergranular Sb distributions, of measurements of acoustic emissions and of scanning electron micrographs of a load-interrupted and post-test-fatigued specimen, the values of K1c, computed from the “pop-in” load of the load vs clip gauge displacement curves, are found to represent the formation of many patches of contiguous intergranular microcracks ahead of the precrack. The present experiments demonstrate that in the early stage of solute segregation, K1c decreases more substantially than does the strength of grain boundaries σ* (measured in the notched bar tests), although the embrittlement effects of metalloid elements are the same order for both K1c and σ*. A proposed model for the stress-gradient-control of brittle fracture supports the finding that the measurements of K1c give a distorted view of the progress of intergranular embrittlement.

Mechanisms of fracture of ring samples made of FCC metals on loading with magnetic-pulse method

NASA Astrophysics Data System (ADS)

Morozov, Viktor; Kats, Victor; Savenkov, Georgiy; Lukin, Anton

2018-05-01

Results of study of deformation and fracture of ring-shaped samples made of thin strips of cuprum, aluminum and steel in wide range of loading velocity are presented. Three developed by us schemes of magnetic-pulse method are used for the samples loading. The method of samples fracture with the high electrical resistance (e.g. steel) is proposed. Crack velocity at the sample fracture is estimated. Fracture surfaces are inspected. Mechanisms of dynamic fracture of the sample arere discussed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kelekis, Alexios, E-mail: akelekis@med.uoa.gr; Filippiadis, Dimitrios K., E-mail: dfilippiadis@yahoo.gr; Vergadis, Chrysovalantis, E-mail: valvergadis@yahoo.gr

PurposeThrough a prospective comparison of patients with vertebral fractures and normal population, we illustrate effect of percutaneous vertebroplasty (PV) upon projection of load distribution changes.MethodsVertebroplasty group (36 symptomatic patients with osteoporotic vertebral fractures) was evaluated on an electronic baropodometer registering projection of weight bearing areas on feet. Load distribution between right and left foot (including rear-front of the same foot) during standing and walking was recorded and compared before (group V1) and the day after (group V2) PV. Control group (30 healthy asymptomatic volunteers-no surgery record) were evaluated on the same baropodometer.ResultsMean value of load distribution difference between rear-front ofmore » the same foot was 9.45 ± 6.79 % (54.72–45.28 %) upon standing and 14.76 ± 7.09 % (57.38–42.62 %) upon walking in the control group. Respective load distribution values before PV were 16.52 ± 11.23 and 30.91 ± 19.26 % and after PV were 10.08 ± 6.26 and 14.25 ± 7.68 % upon standing and walking respectively. Mean value of load distribution variation between the two feet was 6.36 and 14.6 % before and 4.62 and 10.4 % after PV upon standing and walking respectively. Comparison of load distribution variation (group V1–V2, group V1-control group) is statistically significant. Comparison of load distribution variation (group V2-control group) is not statistically significant. Comparison of load distribution variation among the two feet is statistically significant during walking but not statistically significant during standing.ConclusionsThere is a statistically significant difference when comparing load distribution variation prior vertebroplasty and that of normal population. After vertebroplasty, this difference normalizes in a statistically significant way. PV is efficient on equilibrium-load distribution improvement as well.« less

PATIENT-SPECIFIC FINITE ELEMENT ANALYSIS OF CHRONIC CONTACT STRESS EXPOSURE AFTER INTRA-ARTICULAR FRACTURE OF THE TIBIAL PLAFOND

PubMed Central

Li, Wendy; Anderson, Donald D.; Goldsworthy, Jane K.; Marsh, J. Lawrence; Brown, Thomas D.

2008-01-01

SUMMARY The role of altered contact mechanics in the pathogenesis of post-traumatic osteoarthritis (PTOA) following intra-articular fracture remains poorly understood. One proposed etiology is that residual incongruities lead to altered joint contact stresses that, over time, predispose to PTOA. Prevailing joint contact stresses following surgical fracture reduction were quantified in this study using patient-specific contact finite element (FE) analysis. FE models were created for 11 ankle pairs from tibial plafond fracture patients. Both (reduced) fractured ankles and their intact contralaterals were modeled. A sequence of 13 loading instances was used to simulate the stance phase of gait. Contact stresses were summed across loadings in the simulation, weighted by resident time in the gait cycle. This chronic exposure measure, a metric of degeneration propensity, was then compared between intact and fractured ankle pairs. Intact ankles had lower peak contact stress exposures that were more uniform, and centrally located. The series-average peak contact stress elevation for fractured ankles was 38% (p=0.0015; peak elevation was 82%). Fractured ankles had less area with low contact stress exposure than intacts, and a greater area with high exposure. Chronic contact stress overexposures (stresses exceeding a damage threshold) ranged from near zero to a high of 18 times the matched intact value. The patient-specific FE models utilized in this study represent substantial progress towards elucidating the relationship between altered contact stresses and the outcome of patients treated for intra-articular fractures. PMID:18404662

A maximum entropy fracture model for low and high strain-rate fracture in TinSilverCopper alloys

NASA Astrophysics Data System (ADS)

Chan, Dennis K.

SnAgCu solder alloys exhibit significant rate-dependent constitutive behavior. Solder joints made of these alloys exhibit failure modes that are also rate-dependent. Solder joints are an integral part of microelectronic packages and are subjected to a wide variety of loading conditions which range from thermo-mechanical fatigue to impact loading. Consequently, there is a need for non-empirical rate-dependent failure theory that is able to accurately predict fracture in these solder joints. In the present thesis, various failure models are first reviewed. But, these models are typically empirical or are not valid for solder joints due to limiting assumptions such as elastic behavior. Here, the development and validation of a maximum entropy fracture model (MEFM) valid for low strain-rate fracture in SnAgCu solders is presented. To this end, work on characterizing SnAgCu solder behavior at low strain-rates using a specially designed tester to estimate parameters for constitutive models is presented. Next, the maximum entropy fracture model is reviewed. This failure model uses a single damage accumulation parameter and relates the risk of fracture to accumulated inelastic dissipation. A methodology is presented to extract this model parameter through a custom-built microscale mechanical tester for Sn3.8Ag0.7Cu solder. This single parameter is used to numerically simulate fracture in two solder joints with entirely different geometries. The simulations are compared to experimentally observed fracture in these same packages. Following the simulations of fracture at low strain rate, the constitutive behavior of solder alloys across nine decades of strain rates through MTS compression tests and split-Hopkinson bar are presented. Preliminary work on using orthogonal machining as novel technique of material characterization at high strain rates is also presented. The resultant data from the MTS compression and split-Hopkinson bar tester is used to demonstrate the localization of stress to the interface of solder joints at high strain rates. The MEFM is further extended to predict failure in brittle materials. Such an extension allows for fracture prediction within intermetallic compounds (IMCs) in solder joints. It has been experimentally observed that the failure mode shifts from bulk solder to the IMC layer with increasing loading rates. The extension of the MEFM would allow for prediction of the fracture mode within the solder joint under different loading conditions. A fracture model capable of predicting failure modes at higher strain rates is necessary, as mobile electronics are becoming ubiquitous. Mobile devices are prone to being dropped which can induce loading rates within solder joints that are much larger than experienced under thermo-mechanical fatigue. A range of possible damage accumulation parameters for Cu6Sn 5 is determined for the MEFM. A value within the aforementioned range is used to demonstrate the increasing likelihood of IMC fracture in solder joints with larger loading rates. The thesis is concluded with remarks about ongoing work that include determining a more accurate damage accumulation parameter for Cu6Sn 5 IMC, and on using machining as a technique for extracting failure parameters for the MEFM.

Couple stresses and the fracture of rock.

PubMed

Atkinson, Colin; Coman, Ciprian D; Aldazabal, Javier

2015-03-28

An assessment is made here of the role played by the micropolar continuum theory on the cracked Brazilian disc test used for determining rock fracture toughness. By analytically solving the corresponding mixed boundary-value problems and employing singular-perturbation arguments, we provide closed-form expressions for the energy release rate and the corresponding stress-intensity factors for both mode I and mode II loading. These theoretical results are augmented by a set of fracture toughness experiments on both sandstone and marble rocks. It is further shown that the morphology of the fracturing process in our centrally pre-cracked circular samples correlates very well with discrete element simulations. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

Combination fracturing/gravel-packing completion technique on the Amberjack, Mississippi Canyon 109 field

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hannah, R.R.; Park, E.I.; Porter, D.A.

1994-11-01

This paper describes a one-step fracturing/gravel-pack (frac-and-pack) completion procedure conducted on the BP Exploration Amberjack platform beginning in early 1992. This platform is 35 miles southwest of Venice, LA. The first four completions on this platform had an average positive skin values of 21. The goal of the frac-and-pack procedure was to reduce these skins to nearly zero. In total, 24 frac-and-pack operations were performed. Details of the fracture design, prefracture testing, fracture design and execution, and production response and a continuing optimization program are discussed. The fractures were performed with the screens in place with the gravel pack aftermore » the fracturing operation. The treatments were designed for the tip screenout technique to create wide fractures and to provide proppant loadings exceeding 8 lbm/ft. This paper presents the trend of the declining skin values, along with a discussion of time-dependent skins. The changes in fluids, breakers, and proppants are also presented. The average skin on 14 frac-and-pack completions was 5.3. The average skin on the final eight completions was 0.2.« less

Comparative evaluation of fracture resistance under static and fatigue loading of endodontically treated teeth restored with carbon fiber posts, glass fiber posts, and an experimental dentin post system: an in vitro study.

PubMed

Ambica, Khetarpal; Mahendran, Kavitha; Talwar, Sangeeta; Verma, Mahesh; Padmini, Govindaswamy; Periasamy, Ravishankar

2013-01-01

This investigation sought to compare the fracture resistance under static and fatigue loading of endodontically treated teeth restored with fiber-reinforced composite posts and experimental dentin posts milled from human root dentin by using computer-aided design/computer-aided manufacturing. Seventy maxillary central incisors were obturated and divided into 4 groups: control group without any post (n = 10), carbon fiber post group (n = 20), glass fiber post group (n = 20), and dentin post group (n = 20). Control group teeth were prepared to a height of 5 mm. In all other teeth, post space was prepared; a post was cemented, and a core build-up was provided. Half the samples from each group were statistically loaded until failure, and the remaining half were subjected to cyclic loading, followed by monostatic load until fracture. One-way analysis of variance and Bonferroni multiple comparisons revealed a significant difference among test groups. The control group demonstrated highest fracture resistance (935.03 ± 33.53 N), followed by the dentin post group (793.12 ± 33.69 N), glass fiber post group (603.44 ± 46.67 N), and carbon fiber post group (497.19 ± 19.27 N) under static loading. These values reduced to 786.69 ± 29.64 N, 646.34 ± 26.56 N, 470 ± 36.34 N, and 379.71 ± 13.95 N, respectively, after cyclic loading. Results suggest that human dentin can serve as post material under static and fatigue loading. Although at an early stage in research, the use of dentin posts in root-filled teeth looks promising. Copyright © 2013 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

Mechanical resistance of zirconium implant abutments: A review of the literature

PubMed Central

Vaquero-Aguilar, Cristina; Torres-Lagares, Daniel; Jiménez-Melendo, Manuel; Gutiérrez-Pérez, José L.

2012-01-01

The increase of aesthetic demands, together with the successful outcome of current implants, has renewed interest in the search for new materials with enough mechanical properties and better aesthetic qualities than the materials customarily used in implanto-prosthetic rehabilitation. Among these materials, zirconium has been used in different types of implants, including prosthetic abutments. The aim of the present review is to analyse current scientific evidence supporting the use of this material for the above mentioned purposes. We carried out the review of the literature published in the last ten years (2000 through 2010) of in vitro trials of dynamic and static loading of zirconium abutments found in the databases of Medline and Cochrane using the key words zirconium abutment, fracture resistance, fracture strength, cyclic loading. Although we have found a wide variability of values among the different studies, abutments show favourable clinical behaviour for the rehabilitation of single implants in the anterior area. Such variability may be explained by the difficulty to simulate daily mastication under in vitro conditions. The clinical evidence, as found in our study, does not recommend the use of implanto-prosthetic zirconium abutments in the molar area. Key words: Zirconium abutment, zirconium implant abutment, zirconia abutment, fracture resistance, fracture strength, cyclic loading. PMID:22143702

Specimen-specific modeling of hip fracture pattern and repair.

PubMed

Ali, Azhar A; Cristofolini, Luca; Schileo, Enrico; Hu, Haixiang; Taddei, Fulvia; Kim, Raymond H; Rullkoetter, Paul J; Laz, Peter J

2014-01-22

Hip fracture remains a major health problem for the elderly. Clinical studies have assessed fracture risk based on bone quality in the aging population and cadaveric testing has quantified bone strength and fracture loads. Prior modeling has primarily focused on quantifying the strain distribution in bone as an indicator of fracture risk. Recent advances in the extended finite element method (XFEM) enable prediction of the initiation and propagation of cracks without requiring a priori knowledge of the crack path. Accordingly, the objectives of this study were to predict femoral fracture in specimen-specific models using the XFEM approach, to perform one-to-one comparisons of predicted and in vitro fracture patterns, and to develop a framework to assess the mechanics and load transfer in the fractured femur when it is repaired with an osteosynthesis implant. Five specimen-specific femur models were developed from in vitro experiments under a simulated stance loading condition. Predicted fracture patterns closely matched the in vitro patterns; however, predictions of fracture load differed by approximately 50% due to sensitivity to local material properties. Specimen-specific intertrochanteric fractures were induced by subjecting the femur models to a sideways fall and repaired with a contemporary implant. Under a post-surgical stance loading, model-predicted load sharing between the implant and bone across the fracture surface varied from 59%:41% to 89%:11%, underscoring the importance of considering anatomic and fracture variability in the evaluation of implants. XFEM modeling shows potential as a macro-level analysis enabling fracture investigations of clinical cohorts, including at-risk groups, and the design of robust implants. © 2013 Published by Elsevier Ltd.

DXA and pQCT predict pertrochanteric and not femoral neck fracture load in a human side-impact fracture model.

PubMed

Gebauer, Matthias; Stark, Olaf; Vettorazzi, Eik; Grifka, Joachim; Püschel, Klaus; Amling, Michael; Beckmann, Johannes

2014-01-01

The validity of dual energy X-ray absorptiometry (DXA) and peripheral quantitative computed tomography (pQCT) measurements as predictors of pertrochanteric and femoral neck fracture loads was compared in an experimental simulation of a fall on the greater trochanter. 65 proximal femora were harvested from patients at autopsy. All specimens were scanned with use of DXA for areal bone mineral density and pQCT for volumetric densities at selected sites of the proximal femur. A three-point bending test simulating a side-impact was performed to determine fracture load and resulted in 16 femoral neck and 49 pertrochanteric fractures. Regression analysis revealed that DXA BMD trochanter was the best variable at predicting fracture load of pertrochanteric fractures with an adjusted R(2) of 0.824 (p < 0.0001). There was no correlation between densitometric parameters and the fracture load of femoral neck fractures. A significant correlation further was found between body weight, height, femoral head diameter, and neck length on the one side and fracture load on the other side, irrespective of the fracture type. Clinically, the DXA BMD trochanter should be favored and integrated routinely as well as biometric and geometric parameters, particularly in elderly people with known osteoporosis at risk for falls. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.

A novel pillar indentation splitting test for measuring fracture toughness of thin ceramic coatings

DOE PAGES

Sebastiani, Marco; Johanns, K. E.; Herbert, Erik G.; ...

2014-05-16

Fracture toughness is an important material property that plays a role in determining the in-service mechanical performance and adhesion of thin ceramic films. Unfortunately, measuring thin film fracture toughness is affected by influences from the substrate and the large residual stresses that can exist in the films. In this paper, we explore a promising new technique that potentially overcomes these problems based on nanoindentation testing of micro-pillars produced by focused ion beam milling of the films. By making the pillar diameter approximately equal to its length, the residual stress in the pillar’s upper portion is almost fully relaxed, and whenmore » indented with a sharp Berkovich indenter, the pillars fracture by splitting at reproducible loads that are readily quantified by a sudden displacement excursion in the load displacement behavior. Cohesive finite element simulations are used to analyze and develop, for a given material, a simple relation between the critical load at failure, pillar radius, and fracture toughness. The main novel aspect of this work is that neither crack geometries nor crack sizes need to be measured post test. Furthermore, the residual stress can be measured at the same time with toughness, by comparing the indentation results from the stress-free pillars and the as-deposited film. The method is tested on three different hard coatings formed by physical vapor deposition: titanium nitride, chromium nitride, and a CrAlN/Si 3N 4 nanocomposite. Results compare well to independently measured values of fracture toughness for the three brittle films. The technique offers several benefits over existing methods.« less

Impact of double-tiered subchondral support procedure with a polyaxial locking plate on the stability of distal radius fractures using fresh cadaveric forearms: Biomechanical and radiographic analyses.

PubMed

Tsutsui, Sadaaki; Kawasaki, Keikichi; Yamakoshi, Ken-Ichi; Uchiyama, Eiichi; Aoki, Mitsuhiro; Inagaki, Katsunori

2016-09-01

The present study compared the changes in biomechanical and radiographic properties under cyclic axial loadings between the 'double-tiered subchondral support' (DSS) group (wherein two rows of screws were used) and the 'non-DSS' (NDSS) group (wherein only one row of distal screws was used) using cadaveric forearm models of radius fractures fixed with a polyaxial locking plate. Fifteen fresh cadaveric forearms were surgically operated to generate an Arbeitsgemeinschaft für Osteosynthesefragen (AO) type 23-C2 fracture model with the fixation of polyaxial volar locking plates. The model specimens were randomized into two groups: DSS (n = 7) and NDSS (n = 8). Both the groups received 4 locking screws in the most distal row, as is usually applied, whereas the DSS group received 2 additional screws in the second row inserted at an inclination of about 15° to support the dorsal aspect of the dorsal subchondral bone. Cyclic axial compression test was performed (3000 cycles; 0-250 N; 60 mm/min) to measure absolute rigidity and displacement, after 1, 1000, 2000 and 3000 cycles, and values were normalized relative to cycle 1. These absolute and normalized values were compared between those two groups. Radiographic images were taken before and after the cyclic loading to measure changes in volar tilt (ΔVT) and radial inclination (ΔRI). The DSS group maintained significantly higher rigidity and lower displacement values than the NDSS group during the entire loading period. Radiographic analysis indicated that the ΔVT values of the DSS group were lower than those of the NDSS group. In contrast, the fixation design did not influence the impact of loading on the ΔRI values. Biomechanical and radiographic analyses demonstrated that two rows of distal locking screws in the DSS procedure conferred higher stability than one row of distal locking screws. Copyright © 2016 The Japanese Orthopaedic Association. Published by Elsevier B.V. All rights reserved.

Influence of the supporting die structures on the fracture strength of all-ceramic materials.

PubMed

Yucel, Munir Tolga; Yondem, Isa; Aykent, Filiz; Eraslan, Oğuz

2012-08-01

This study investigated the influence of the elastic modulus of supporting dies on the fracture strengths of all-ceramic materials used in dental crowns. Four different types of supporting die materials (dentin, epoxy resin, brass, and stainless steel) (24 per group) were prepared using a milling machine to simulate a mandibular molar all-ceramic core preparation. A total number of 96 zirconia cores were fabricated using a CAD/CAM system. The specimens were divided into two groups. In the first group, cores were cemented to substructures using a dual-cure resin cement. In the second group, cores were not cemented to the supporting dies. The specimens were loaded using a universal testing machine at a crosshead speed of 0.5 mm/min until fracture occurred. Data were statistically analyzed using two-way analysis of variance and Tukey HSD tests (α = 0.05). The geometric models of cores and supporting die materials were developed using finite element method to obtain the stress distribution of the forces. Cemented groups showed statistically higher fracture strength values than non-cemented groups. While ceramic cores on stainless steel dies showed the highest fracture strength values, ceramic cores on dentin dies showed the lowest fracture strength values among the groups. The elastic modulus of the supporting die structure is a significant factor in determining the fracture resistance of all-ceramic crowns. Using supporting die structures that have a low elastic modulus may be suitable for fracture strength tests, in order to accurately reflect clinical conditions.

Effect of finite element model loading condition on fracture risk assessment in men and women: the AGES-Reykjavik study.

PubMed

Keyak, J H; Sigurdsson, S; Karlsdottir, G S; Oskarsdottir, D; Sigmarsdottir, A; Kornak, J; Harris, T B; Sigurdsson, G; Jonsson, B Y; Siggeirsdottir, K; Eiriksdottir, G; Gudnason, V; Lang, T F

2013-11-01

Proximal femoral (hip) strength computed by subject-specific CT scan-based finite element (FE) models has been explored as an improved measure for identifying subjects at risk of hip fracture. However, to our knowledge, no published study has reported the effect of loading condition on the association between incident hip fracture and hip strength. In the present study, we performed a nested age- and sex-matched case-control study in the Age Gene/Environment Susceptibility (AGES) Reykjavik cohort. Baseline (pre-fracture) quantitative CT (QCT) scans of 5500 older male and female subjects were obtained. During 4-7years follow-up, 51 men and 77 women sustained hip fractures. Ninety-seven men and 152 women were randomly selected as controls from a pool of age- and sex-matched subjects. From the QCT data, FE models employing nonlinear material properties computed FE-strength of the left hip of each subject in loading from a fall onto the posterolateral (FPL), posterior (FP) and lateral (FL) aspects of the greater trochanter (patent pending). For comparison, FE strength in stance loading (FStance) and total femur areal bone mineral density (aBMD) were also computed. For all loading conditions, the reductions in strength associated with fracture in men were more than twice those in women (p≤0.01). For fall loading specifically, posterolateral loading in men and posterior loading in women were most strongly associated with incident hip fracture. After adjusting for aBMD, the association between FP and fracture in women fell short of statistical significance (p=0.08), indicating that FE strength provides little advantage over aBMD for identifying female hip fracture subjects. However, in men, after controlling for aBMD, FPL was 424N (11%) less in subjects with fractures than in controls (p=0.003). Thus, in men, FE models of posterolateral loading include information about incident hip fracture beyond that in aBMD. © 2013.

Toughening and healing of continuous fibre reinforced composites with bis-maleimide based pre-pregs

NASA Astrophysics Data System (ADS)

Kostopoulos, V.; Kotrotsos, A.; Tsantzalis, S.; Tsokanas, P.; Christopoulos, A. C.; Loutas, T.

2016-08-01

Unidirectional (UD) pre-pregs containing self-healing materials based on Diels-Alder reaction bis-maleimide (BMI) polymers were successfully incorporated on the mid-plane of UD carbon fibre reinforced polymers. The fracture toughness of these composites and the introduced healing capability were measured under mode I loading. The interlaminar fracture toughness was enhanced considerably, since the maximum load (P max) of the modified composite increased approximately 1.5 times and the mode I fracture energy (G IC) displayed a significant increase of almost 3.5 times when compared to the reference composites. Furthermore the modified composites displayed a healing efficiency (HE) value of about 30% for P max and 20% for G IC after the first healing, appearing to be an almost stable behaviour after the third healing cycle. The HE displayed a decrease of 20% and 15% for P max and G IC values, respectively, after the fifth healing cycle. During the tests, the monitored acoustic emission (AE) activity of the samples showed that there is no significant difference due to the presence of BMI polymer in terms of AE hits. Moreover, optical microscopy not only showed that the epoxy matrix at the interface is partly infiltrated by the BMI polymer, but it also revealed the presence of pulled out fibres at the fractured surface, indicating ductile behaviour.

Biomechanical outcome of proximal femoral nail antirotation is superior to proximal femoral locking compression plate for reverse oblique intertrochanteric fractures: a biomechanical study of intertrochanteric fractures.

PubMed

Ma, Jian-Xiong; Wang, Jie; Xu, Wei-Guo; Yu, Jing-Tao; Yang, Yang; Ma, Xin-Long

2015-01-01

Reverse obliquity intertrochanteric fractures are a challenge for orthopedic surgeons. The optimal internal fixation for repairing this type of unstable intertrochanteric fractures remains controversial. This study aimed to compare the biomechanical properties in axial load and cyclical axial load of proximal femoral nail antirotation (PFNA) and proximal femoral locking compression plate (PFLCP) for fixation of reverse obliquity intertrochanteric fractures. Sixteen embalmed cadaver femurs were sawed to simulate reverse obliquity intertrochanteric fracture and instrumented with PFNA or PFLCP. Axial loads and axial cyclic loads were applied to the femoral head by an Instron tester. If the implant-femur constructs did not fail, axial failure load was added to the remaining implant-femur constructs. Mean axial stiffness for PFNA was 21.10% greater than that of PFLCP. Cyclic axial loading caused significantly less (p=0.022) mean irreversible deformation in PFNA (3.43 mm) than in PFLCP (4.34 mm). Significantly less (p=0.002) mean total deformation was detected in PFNA (6.16 mm) than in PFLCP (8.67 mm). For fixing reverse obliquity intertrochanteric fractures, PFNA is superior to PFLCP under axial load.

Load-bearing properties of minimal-invasive monolithic lithium disilicate and zirconia occlusal onlays: finite element and theoretical analyses

PubMed Central

Ma, Li; Guess, Petra C.; Zhang, Yu

2013-01-01

Objectives The aim of this study was to test the hypothesis that monolithic lithium disilicate glass-ceramic occlusal onlay can exhibit a load-bearing capacity that approaches monolithic zirconia, due to a smaller elastic modulus mismatch between the lithium disilicate and its supporting tooth structure relative to zirconia. Methods Ceramic occlusal onlays of various thicknesses cemented to either enamel or dentin were considered. Occlusal load was applied through an enamel-like deformable indenter or a control rigid indenter. Flexural tensile stress at the ceramic intaglio (cementation) surface—a cause for bulk fracture of occlusal onlays—was rigorously analyzed using finite element analysis and classical plate-on-foundation theory. Results When bonded to enamel (supported by dentin), the load-bearing capacity of lithium disilicate can approach 75% of that of zirconia, despite the flexural strength of lithium disilicate (400 MPa) being merely 40% of zirconia (1000 MPa). When bonded to dentin (with the enamel completely removed), the load-bearing capacity of lithium disilicate is about 57% of zirconia, still significantly higher than the anticipated value based on its strength. Both ceramics show slightly higher load-bearing capacity when loaded with a deformable indenter (enamel, glass-ceramic, or porcelain) rather than a rigid indenter. Significance When supported by enamel, the load-bearing property of minimally invasive lithium disilicate occlusal onlays (0.6 to 1.4 mm thick) can exceed 70% of that of zircona. Additionally, a relatively weak dependence of fracture load on restoration thickness indicates that a 1.2 mm thin lithium disilicate onlay can be as fracture resistant as its 1.6 mm counterpart. PMID:23683531

Fracture mode during cyclic loading of implant-supported single-tooth restorations.

PubMed

Hosseini, Mandana; Kleven, Erik; Gotfredsen, Klaus

2012-08-01

Fracture of veneering ceramics in zirconia-based restorations has frequently been reported. Investigation of the fracture mode of implant-supported ceramic restorations by using clinically relevant laboratory protocols is needed. This study compared the mode of fracture and number of cyclic loads until veneering fracture when ceramic and metal ceramic restorations with different veneering ceramics were supported by implants. Thirty-two implant-supported single-tooth restorations were fabricated. The test group was composed of 16 ceramic restorations of zirconia abutment-retained crowns with zirconia copings veneered with glass-ceramics (n=8) and feldspathic ceramics (n=8). The control group was composed of 16 metal ceramic restorations of titanium abutment-retained crowns with gold alloy copings veneered with glass (n=8) and feldspathic ceramics (n=8). The palatal surfaces of the crowns were exposed to cyclic loading of 800 N with a frequency of 2 Hz, which continued to 4.2 million cycles or until fracture of the copings, abutments, or implants. The number of cycles and the fracture modes were recorded. The fracture modes were analyzed by descriptive analysis and the Mann-Whitney test (α=.05). The differences in loading cycles until veneering fracture were estimated with the Cox proportional hazards analysis. Veneering fracture was the most frequently observed fracture mode. The severity of fractures was significantly more in ceramic restorations than in metal ceramic restorations. Significantly more loading cycles until veneering fracture were estimated with metal ceramic restorations veneered with glass-ceramics than with other restorations. The metal ceramic restorations demonstrated fewer and less severe fractures and resisted more cyclic loads than the ceramic restorations, particularly when the metal ceramic crowns were veneered with glass-ceramics. Copyright © 2012 The Editorial Council of the Journal of Prosthetic Dentistry. Published by Mosby, Inc. All rights reserved.

Acoustic emission testing of composite vessels under sustained loading

NASA Technical Reports Server (NTRS)

Lark, R. F.; Moorhead, P. E.

1978-01-01

Acoustic emissions (AE) generated from Kevlar 49/epoxy composite pressure vessels subjected to sustained load-to-failure tests were studied. Data from two different transducer locations on the vessels were compared. It was found that AE from vessel wall-mounted transducers showed a wide variance from those for identical vessels subjected to the same pressure loading. Emissions from boss-mounted transducers did, however, yield values that were relatively consistent. It appears that the signals from the boss-mounted transducers represent an integrated average of the emissions generated by fibers fracturing during the vessel tests. The AE from boss-mounted transducers were also independent of time for vessel failure. This suggests that a similar number of fiber fractures must occur prior to initiation of vessel failure. These studies indicate a potential for developing an AE test procedure for predicting the residual service life or integrity of composite vessels.

Mathematical modelling of bone adaptation of the metacarpal subchondral bone in racehorses.

PubMed

Hitchens, Peta L; Pivonka, Peter; Malekipour, Fatemeh; Whitton, R Chris

2018-06-01

In Thoroughbred racehorses, fractures of the distal limb are commonly catastrophic. Most of these fractures occur due to the accumulation of fatigue damage from repetitive loading, as evidenced by microdamage at the predilection sites for fracture. Adaptation of the bone in response to training loads is important for fatigue resistance. In order to better understand the mechanism of subchondral bone adaptation to its loading environment, we utilised a square root function defining the relationship between bone volume fraction [Formula: see text] and specific surface [Formula: see text] of the subchondral bone of the lateral condyles of the third metacarpal bone (MCIII) of the racehorse, and using this equation, developed a mathematical model of subchondral bone that adapts to loading conditions observed in vivo. The model is expressed as an ordinary differential equation incorporating a formation rate that is dependent on strain energy density. The loading conditions applied to a selected subchondral region, i.e. volume of interest, were estimated based on joint contact forces sustained by racehorses in training. For each of the initial conditions of [Formula: see text] we found no difference between subsequent homoeostatic [Formula: see text] at any given loading condition, but the time to reach equilibrium differed by initial [Formula: see text] and loading condition. We found that the observed values for [Formula: see text] from the mathematical model output were a good approximation to the existing data for racehorses in training or at rest. This model provides the basis for understanding the effect of changes to training strategies that may reduce the risk of racehorse injury.

Fracture modes of high modulus graphite/epoxy angleplied laminates subjected to off-axis tensile loads

NASA Technical Reports Server (NTRS)

Sinclair, J. H.

1980-01-01

Angelplied laminates of high modulus graphite fiber/epoxy were studied in several ply configurations at various tensile loading angles to the zero ply direction in order to determine the effects of ply orientations on tensile properties, fracture modes, and fracture surface characteristics of the various plies. It was found that fracture modes in the plies of angleplied laminates can be characterized by scanning electron microscope observation. The characteristics for a given fracture mode are similar to those for the same fracture mode in unidirectional specimens. However, no simple load angle range can be associated with a given fracture mode.

Fracture behavior of human molars.

PubMed

Keown, Amanda J; Lee, James J-W; Bush, Mark B

2012-12-01

Despite the durability of human teeth, which are able to withstand repeated loading while maintaining form and function, they are still susceptible to fracture. We focus here on longitudinal fracture in molar teeth-channel-like cracks that run along the enamel sidewall of the tooth between the gum line (cemento-enamel junction-CEJ) and the occlusal surface. Such fractures can often be painful and necessitate costly restorative work. The following study describes fracture experiments made on molar teeth of humans in which the molars are placed under axial compressive load using a hard indenting plate in order to induce longitudinal cracks in the enamel. Observed damage modes include fractures originating in the occlusal region ('radial-median cracks') and fractures emanating from the margin of the enamel in the region of the CEJ ('margin cracks'), as well as 'spalling' of enamel (the linking of longitudinal cracks). The loading conditions that govern fracture behavior in enamel are reported and observations made of the evolution of fracture as the load is increased. Relatively low loads were required to induce observable crack initiation-approximately 100 N for radial-median cracks and 200 N for margin cracks-both of which are less than the reported maximum biting force on a single molar tooth of several hundred Newtons. Unstable crack growth was observed to take place soon after and occurred at loads lower than those calculated by the current fracture models. Multiple cracks were observed on a single cusp, their interactions influencing crack growth behavior. The majority of the teeth tested in this study were noted to exhibit margin cracks prior to compression testing, which were apparently formed during the functional lifetime of the tooth. Such teeth were still able to withstand additional loading prior to catastrophic fracture, highlighting the remarkable damage containment capabilities of the natural tooth structure.

THE EFFECT OF STRAIN RATE ON FRACTURE TOUGHNESS OF HUMAN CORTICAL BONE: A FINITE ELEMENT STUDY

PubMed Central

Ural, Ani; Zioupos, Peter; Buchanan, Drew; Vashishth, Deepak

2011-01-01

Evaluating the mechanical response of bone under high loading rates is crucial to understanding fractures in traumatic accidents or falls. In the current study, a computational approach based on cohesive finite element modeling was employed to evaluate the effect of strain rate on fracture toughness of human cortical bone. Two-dimensional compact tension specimen models were simulated to evaluate the change in initiation and propagation fracture toughness with increasing strain rate (range: 0.08 to 18 s−1). In addition, the effect of porosity in combination with strain rate was assessed using three-dimensional models of microcomputed tomography-based compact tension specimens. The simulation results showed that bone’s resistance against the propagation of fracture decreased sharply with increase in strain rates up to 1 s−1 and attained an almost constant value for strain rates larger than 1 s−1. On the other hand, initiation fracture toughness exhibited a more gradual decrease throughout the strain rates. There was a significant positive correlation between the experimentally measured number of microcracks and the fracture toughness found in the simulations. Furthermore, the simulation results showed that the amount of porosity did not affect the way initiation fracture toughness decreased with increasing strain rates, whereas it exacerbated the same strain rate effect when propagation fracture toughness was considered. These results suggest that strain rates associated with falls lead to a dramatic reduction in bone’s resistance against crack propagation. The compromised fracture resistance of bone at loads exceeding normal activities indicates a sharp reduction and/or absence of toughening mechanisms in bone during high strain conditions associated with traumatic fracture. PMID:21783112

Numerical simulation of the fracture process in ceramic FPD frameworks caused by oblique loading.

PubMed

Kou, Wen; Qiao, Jiyan; Chen, Li; Ding, Yansheng; Sjögren, Göran

2015-10-01

Using a newly developed three-dimensional (3D) numerical modeling code, an analysis was performed of the fracture behavior in a three-unit ceramic-based fixed partial denture (FPD) framework subjected to oblique loading. All the materials in the study were treated heterogeneously; Weibull׳s distribution law was applied to the description of the heterogeneity. The Mohr-Coulomb failure criterion with tensile strength cut-off was utilized in judging whether the material was in an elastic or failed state. The simulated loading area was placed either on the buccal or the lingual cusp of a premolar-shaped pontic with the loading direction at 30°, 45°, 60°, 75° or 90° angles to the occlusal surface. The stress distribution, fracture initiation and propagation in the framework during the loading and fracture process were analyzed. This numerical simulation allowed the cause of the framework fracture to be identified as tensile stress failure. The decisive fracture was initiated in the gingival embrasure of the pontic, regardless of whether the buccal or lingual cusp of the pontic was loaded. The stress distribution and fracture propagation process of the framework could be followed step by step from beginning to end. The bearing capacity and the rigidity of the framework vary with the loading position and direction. The framework loaded with 90° towards the occlusal surface has the highest bearing capacity and the greatest rigidity. The framework loaded with 30° towards the occlusal surface has the least rigidity indicating that oblique loading has a major impact on the fracture of ceramic frameworks. Copyright © 2015 Elsevier Ltd. All rights reserved.

Application of the boundary elements method for modeling of the fracture of cylindrical bodies by hydraulic fracturing

NASA Astrophysics Data System (ADS)

Legan, M. A.; Blinov, V. A.; Larichkin, A. Yu; Novoselov, A. N.

2017-10-01

Experimental study of hydraulic fracturing of thick-walled cylinders with a central circular hole was carried out using the machine that creates a high oil pressure. Experiments on the compression fracture of the solid cylinders by diameter and rectangular parallelepipeds perpendicular to the ends were carried out with a multipurpose test machine Zwick / Roell Z100. Samples were made of GF-177 material based on cement. Ultimate stresses in the material under study were determined for three types of stress state: under compression, with a pure shear on the surface of the hole under frecking conditions and under a compound stress state under conditions of diametral compression of a solid cylinder. The value of the critical stress intensity factor of GF-177 material was obtained. The modeling of the fracturing process taking into account the inhomogeneity of the stress state near the hole was carried out using the boundary elements method (in the variant of the fictitious load method) and the gradient fracture criterion. Calculation results of the ultimate pressure were compared with values obtained analytically on the basis of the Lame solution and with experimental data.

Effects of Simulated Functional Loading Conditions on Dentin, Composite, and Laminate Structures

PubMed Central

Walker, Mary P.; Teitelbaum, Heather K.; Eick, J. David; Williams, Karen B.

2008-01-01

Use of composite restorations continues to increase, tempered by more potential problems when placed in posterior dentition. Thus, it is essential to understand how these materials function under stress-bearing clinical conditions. Since mastication is difficult to replicate in the laboratory, cyclic loading is frequently used within in vitro evaluations but often employs traditional fatigue testing, which typically does not simulate occlusal loading because higher stresses and loading frequencies are used, so failure mechanisms may be different. The present investigation utilized relevant parameters (specimen size; loading frequency) to assess the effects of cyclic loading on flexural mechanical properties and fracture morphology of (coronal) dentin, composite, and dentin-adhesive-composite “laminate” structures. Incremental monitoring of flexural modulus on individual beams over 60,000 loading cycles revealed a gradual increase across materials; post-hoc comparisons indicated statistical significance only for 1 versus 60k cycles. Paired specimens were tested (one exposed to 60k loading cycles, one to static loading only), and comparisons of flexural modulus and strength showed statistically significantly higher values for cyclically-loaded specimens across materials, with no observable differences in fracture morphology. Localized reorganization of dentin collagen and polymer chains could have increased flexural modulus and strength during cyclic loading, which may have implications toward the life and failure mechanisms of clinical restorations and underlying tooth structure. PMID:18823019

[The long-term fracture resistance of orthodontic nickel-titanium wires].

PubMed

Drescher, D; Bourauel, C; Sonneborn, W; Schmuth, G P

1994-01-01

This study reports on the long-term fracture resistance of orthodontic nickel titanium wires, a material property that has not been investigated thoroughly, yet. A computer-controlled apparatus was designed to perform long-term bending tests. The investigated material comprised 9 nickel titanium wires (dimensions 0.016", round and 0.016" x 0.022", rectangular) as well as a stainless steel and a beta-titanium wire that were included as reference. Compared with the steel wire, the nickel titanium wires exhibited 2- to 5-fold higher yield forces in bending. At a specified deflection angle, the generated bending forces of the nickel titanium wires reached one half to one fourth of the values of steel. The fracture resistance under longterm loading was determined using the Wöhler-method. After 10(5) loadings, 0.016" nickel titanium wires were subject to break failure, if forces exceed values greater than 1.2 to 3.1 N. Steel and TMA wires could be loaded with forces of up to 4.4 and 3.7 N, respectively. The 0.016" x 0.022"-rectangular wires allowed forces of approximately twice this magnitude. Elastic fatigue of the superelastic specimens "Memorywire", "Rematitan Lite", and "Sentalloy medium" showed up as hardening of the wire by up to 70%. Material degradation lead to a severe deformation of the hysteresis loop and to plastic deformation. Work-hardened martensitic NiTi wires did not show these effects to this extent.

Joining characteristics of titanium-based orthodontic wires connected by laser and electrical welding methods.

PubMed

Matsunaga, Junko; Watanabe, Ikuya; Nakao, Noriko; Watanabe, Etsuko; Elshahawy, Waleed; Yoshida, Noriaki

2015-01-01

This study investigated the possibility of electrical and laser welding to connect titanium-based alloy (beta-titanium and nickel-titanium) wires and stainless-steel or cobalt-chromium alloy wires for fabrication of combination arch-wires. Four kinds of straight orthodontic rectangular wires (0.017 × 0.025 inch) were used: stainless-steel (S-S), cobalt-chromium (Co-Cr), beta-titanium alloy (β-Ti), and nickel-titanium (Ni-Ti). Homogeneous and heterogeneous end-to-end joints (15 mm long each) were made by electrical welding and laser welding. Non-welded wires (30 mm long) were also used as a control. Maximum loads at fracture (N) and elongation (%) were measured by conducting tensile test. The data (n = 10) were statistically analyzed using analysis of variance/Tukey test (P < 0.05).The S-S/S-S and Co-Cr/Co-Cr specimens showed significantly higher values of the maximum load (ML) at fracture and elongation (EL) than those of the Ni-Ti/Ni-Ti and β-Ti/β-Ti specimens for electrical welding and those of the S-S/S-S and Co-Cr/Co-Cr specimens welded by laser. On the other hand, the laser-welded Ni-Ti/Ni-Ti and β-Ti/β-Ti specimens exhibited higher values of the ML and EL compared to those of the corresponding specimens welded by electrical method. In the heterogeneously welded combinations, the electrically welded Ni-Ti/S-S, β-Ti/S-S and β-Ti/Co-Cr specimens showed significantly (P < 0.05) higher ML and EL than those of the corresponding specimens welded by laser. Electrical welding exhibited the higher values of maximum load at fracture and elongation for heterogeneously welded combinations than laser-welding.

Monolithic and bi-layer CAD/CAM lithium-disilicate versus metal-ceramic fixed dental prostheses: comparison of fracture loads and failure modes after fatigue.

PubMed

Schultheis, Stefan; Strub, Joerg R; Gerds, Thomas A; Guess, Petra C

2013-06-01

The authors analyzed the effect of fatigue on the survival rate and fracture load of monolithic and bi-layer CAD/CAM lithium-disilicate posterior three-unit fixed dental prostheses (FDPs) in comparison to the metal-ceramic gold standard. The authors divided 96 human premolars and molars into three equal groups. Lithium-disilicate ceramic (IPS-e.max-CAD) was milled with the CEREC-3-system in full-anatomic FDP dimensions (monolithic: M-LiCAD) or as framework (Bi-layer: BL-LiCAD) with subsequent hand-layer veneering. Metal-ceramic FDPs (MC) served as control. Single-load-to-failure tests were performed before and after mouth-motion fatigue. No fracture failures occurred during fatigue. Median fracture loads in [N], before and after fatigue were, respectively, as follows: M-LiCAD, 1,298/1,900; BL-LiCAD, 817/699; MC, 1,966/1,818. M-LiCAD and MC FPDs revealed comparable fracture loads and were both significantly higher than BL-LiCAD. M-LiCAD and BL-LiCAD both failed from core/veneer bulk fracture within the connector area. MC failures were limited to ceramic veneer fractures exposing the metal core. Fatigue had no significant effect on any group. Posterior monolithic CAD/CAM fabricated lithium-disilicate FPDs were shown to be fracture resistant with failure load results comparable to the metal-ceramic gold standard. Clinical investigations are needed to confirm these promising laboratory results. Monolithic CAD/CAM fabricated lithium-disilicate FDPs appeared to be a reliable treatment alternative for the posterior load-bearing area, whereas FDPs in bi-layer configuration were susceptible to low load fracture failure.

Coseismic Damage Generation in Fault Zones by Successive High Strain Rate Loading Experiments

NASA Astrophysics Data System (ADS)

Aben, F. M.; Doan, M. L.; Renard, F.; Toussaint, R.; Reuschlé, T.; Gratier, J. P.

2014-12-01

Damage zones of active faults control both resistance to rupture and transport properties of the fault. Hence, knowing the rock damage's origin is important to constrain its properties. Here we study experimentally the damage generated by a succession of dynamic loadings, a process mimicking the stress history of a rock sample located next to an active fault. A propagating rupture generates high frequency stress perturbations next to its tip. This dynamic loading creates pervasive damage (pulverization), as multiple fractures initiate and grow simultaneously. Previous single loading experiments have shown a strain rate threshold for pulverization. Here, we focus on conditions below this threshold and the dynamic peak stress to constrain: 1) if there is dynamic fracturing at these conditions and 2) if successive loadings (cumulative seismic events) result in pervasive fracturing, effectively reducing the pulverization threshold to milder conditions. Monzonite samples were dynamically loaded (strain rate > 50 s-1) several times below the dynamic peak strength, using a Split Hopkinson Pressure Bar apparatus. Several quasi-static experiments were conducted as well (strain rate < 10-5-s). Samples loaded up to stresses above the quasi-static uniaxial compressive strength (qsUCS) systematically fragmented or pulverized after four successive loadings. We measured several damage proxies (P-wave velocity, porosity), that show a systematic increase in damage with each load. In addition, micro-computed tomography acquisition on several damage samples revealed the growth of a pervasive fracture network between ensuing loadings. Samples loaded dynamically below the qsUCS failed along one fracture after a variable amount of loadings and damage proxies do not show any a systematic trend. Our conclusions is that milder dynamic loading conditions, below the dynamic peak strength, result in pervasive dynamic fracturing. Also, successive loadings effectively lower the pulverization threshold of the rock. However, the peak loading stress must exceed the qsUCS of the rock, otherwise quasi-static fracturing occurs. Pulverized rocks found in the field are therefore witnesses of previous large earthquakes.

Modeling propellant-based stimulation of a borehole with peridynamics

DOE PAGES

Panchadhara, Rohan; Gordon, Peter A.; Parks, Michael L.

2017-02-27

A non-local formulation of classical continuum mechanics theory known as peridynamics is used to study fracture initiation and growth from a wellbore penetrating the subsurface within the context of propellant-based stimulation. The principal objectives of this work are to analyze the influence of loading conditions on the resulting fracture pattern, to investigate the effect of in-situ stress anisotropy on fracture propagation, and to assess the suitability of peridynamics for modeling complex fracture formation. In peridynamics, the momentum equation from the classical theory of solid mechanics is replaced by a non-local analogue, which results in an integrodifferential conservation equation. A continuummore » material is discretized with a set of material points that interact with all other points within a specified distance. Interactions between points are governed by bonds that can deform and break depending on loading conditions. The accumulated breakage of bonds gives rise to a picture of complex growth of fractures that is seen as a key advantage in the peridynamic representation of discontinuities. It is shown that the loading rate significantly influences the number and ex- tent of fractures initiated from a borehole. Results show that low loading rates produce fewer but longer fractures, whereas high loading rates produce numerous shorter fractures around the borehole. The numerical method is able to predict fracture growth patterns over a wide range of loading and stress conditions. Our results also show that fracture growth is attenuated with increasing in-situ confining stress, and, in the case of confining stress anisotropy, fracture extensions are largest in the direction perpendicular to the minimum compressive stress. Since the results are in broad qualitative agreement with experimental and numerical studies found in the literature, suggesting that peridynamics can be a powerful tool in the study of complex fracture network formation.« less

Modeling propellant-based stimulation of a borehole with peridynamics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Panchadhara, Rohan; Gordon, Peter A.; Parks, Michael L.

A non-local formulation of classical continuum mechanics theory known as peridynamics is used to study fracture initiation and growth from a wellbore penetrating the subsurface within the context of propellant-based stimulation. The principal objectives of this work are to analyze the influence of loading conditions on the resulting fracture pattern, to investigate the effect of in-situ stress anisotropy on fracture propagation, and to assess the suitability of peridynamics for modeling complex fracture formation. In peridynamics, the momentum equation from the classical theory of solid mechanics is replaced by a non-local analogue, which results in an integrodifferential conservation equation. A continuummore » material is discretized with a set of material points that interact with all other points within a specified distance. Interactions between points are governed by bonds that can deform and break depending on loading conditions. The accumulated breakage of bonds gives rise to a picture of complex growth of fractures that is seen as a key advantage in the peridynamic representation of discontinuities. It is shown that the loading rate significantly influences the number and ex- tent of fractures initiated from a borehole. Results show that low loading rates produce fewer but longer fractures, whereas high loading rates produce numerous shorter fractures around the borehole. The numerical method is able to predict fracture growth patterns over a wide range of loading and stress conditions. Our results also show that fracture growth is attenuated with increasing in-situ confining stress, and, in the case of confining stress anisotropy, fracture extensions are largest in the direction perpendicular to the minimum compressive stress. Since the results are in broad qualitative agreement with experimental and numerical studies found in the literature, suggesting that peridynamics can be a powerful tool in the study of complex fracture network formation.« less

Effect of ceramic infrastructure on the failure behavior and stress distribution of fixed partial dentures.

PubMed

Borba, Márcia; Duan, Yuanyuan; Griggs, Jason A; Cesar, Paulo F; Della Bona, Álvaro

2015-04-01

The effect of the ceramic infrastructure (IS) on the failure behavior and stress distribution of fixed partial dentures (FPDs) was evaluated. Twenty FPDs with a connector cross-section of 16 mm(2) were produced for each IS and veneered with porcelain: (YZ) Vita In-Ceram YZ/Vita VM9 porcelain; (IZ) Vita In-Ceram Zirconia/Vita VM7 porcelain; (AL) Vita In-Ceram AL/Vita VM7 porcelain. Two experimental conditions were evaluated (n = 10). For control specimens, load was applied in the center of the pontic at 0.5 mm/min until failure, using a universal testing machine, in 37°C deionized water. For mechanical cycling (MC) specimens, FPDs were subjected to MC (2 Hz, 140 N, 10(6) cycles) and subsequently tested as described for the control group. For YZ, an extra group of 10 FPDs were built with a connector cross-section of 9 mm(2) and tested until failure. Fractography and FEA were performed. Data were analyzed by ANOVA and Tukey's test (α = 0.05). YZ16 showed the greatest fracture load mean value, followed by YZ16-MC. Specimens from groups YZ9, IZ16, IZ16-MC, AL16 and AL16-MC showed no significant difference for the fracture load. The failure behavior and stress distribution of FPDs was influenced by the type of IS. AL and IZ FPDs showed similar fracture load values but different failure modes and stress distribution. YZ showed the best mechanical behavior and may be considered the material of choice to produce posterior FPDs as it was possible to obtain a good mechanical performance even with a smaller connector dimension (9 mm(2)). Copyright © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Recovery of physical activity levels in adolescents after lower limb fractures: a longitudinal, accelerometry-based activity monitor study.

PubMed

Ceroni, Dimitri; Martin, Xavier; Lamah, Léopold; Delhumeau, Cécile; Farpour-Lambert, Nathalie; De Coulon, Geraldo; Ferrière, Victor Dubois

2012-07-25

In adolescents, loss of bone mineral mass usually occurs during phases of reduced physical activity (PA), such as when an injured extremity spends several weeks in a cast. We recorded the PA of adolescents with lower limb fractures during the cast immobilization, at 6 and at 18 months after the fracture, and we compared these values with those of healthy controls. Fifty adolescents with a first episode of limb fracture and a control group of 50 healthy cases were recruited for the study through an advertisement placed at the University Children's Hospital of Geneva, Switzerland. PA was assessed during cast immobilization and at 6- and 18-month follow-up by accelerometer measurement (Actigraph(®) 7164, MTI, Fort Walton Beach, FL, USA). Patients and their healthy peers were matched for gender and age. Time spent in PA at each level of intensity was determined for each participant and expressed in minutes and as a percentage of total valid time. From the 50 initial teenagers with fractures, 44 sustained functional evaluations at 6 months follow-up, whereas only 38 patients were studied at 18 months. The total PA count (total number of counts/min) was lower in patients with lower limb fractures (-62.4%) compared with healthy controls (p<0.0001) during cast immobilization. Similarly, time spent in moderate-to-vigorous PA was lower by 76.6% (p<0.0001), and vigorous PA was reduced by 84.4% (p<0.0001) in patients with cast immobilization for lower limb injuries compared to healthy controls values. At 6 and 18 months after the fracture, the mean PA level of injured adolescents was comparable to those of healthy teenagers (-2.3%, and -1.8%, respectively).Importantly, we observed that time spent in vigorous PA, which reflects high-intensity forces beneficial to skeletal health, returned to similar values between both groups from the six month follow-up in adolescents who sustained a fracture. However, a definitive reduction in time spent in moderate PA was observed among patients with a lower limb fracture at 18 months, when comparing with healthy controls values (p = 0.0174). As cast immobilization and reduced PA are known to induce bone mineral loss, this study provides important information to quantify the decrease of skeletal loading in adolescents with limb fractures. The results of this study demonstrate that the amount of skeletal loading returns to normal values in adolescents with lower limb fractures after bone healing and is probably linked to an overall better pattern of functional recovery among this age group. When comparing both populations of adolescents, a definitive decrease in time spent in moderate-to-vigorous PA was observed among patients with a lower limb fracture at 18 months and may suggest a modification of lifestyle. The high rate of missing data (26.5%) due to above all non compliance with monitor wearing among teenagers complicates the data analysis, and requires a more cautious interpretation of the results. Future studies using accelerometer to monitor PA in adolescents should therefore include strategies for improving the rate of adherence and minimizing the ratio of missing data.

Fractographic study of epoxy fractured under mode I loading and mixed mode I/III loading

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ren, Fei; Wang, Jy-An John; Bertelsen, Williams D.

2011-01-01

Fiber reinforced polymeric composite materials are widely used in structural components such as wind turbine blades, which are typically subject to complicated loading conditions. Thus, material response under mixed mode loading is of great significance to the reliability of these structures. Epoxy is a thermosetting polymer that is currently used in manufacturing wind turbine blades. The fracture behavior of epoxy is relevant to the mechanical integrity of the wind turbine composite materials. In this study, a novel fracture testing methodology, the spiral notch torsion test (SNTT), was applied to study the fracture behavior of an epoxy material. SNTT samples weremore » tested using either monotonic loading or cyclic loading, while both mode I and mixed mode I/III loading conditions were used. Fractographic examination indicated the epoxy samples included in this study were prone to mode I failure even when the samples were subject to mixed mode loading. Different fatigue precracks were observed on mode I and mixed mode samples, i.e. precracks appeared as a uniform band under mode I loading, and a semi-ellipse under mixed mode loading. Fracture toughness was also estimated using quantitative fractography.« less

The influence of impact direction and axial loading on the bone fracture pattern.

PubMed

Cohen, Haim; Kugel, Chen; May, Hila; Medlej, Bahaa; Stein, Dan; Slon, Viviane; Brosh, Tamar; Hershkovitz, Israel

2017-08-01

The effect of the direction of the impact and the presence of axial loading on fracture patterns have not yet been established in experimental 3-point bending studies. To reveal the association between the direction of the force and the fracture pattern, with and without axial loading. A Dynatup Model POE 2000 (Instron Co.) low energy pendulum impact machine was utilized to apply impact loading on fresh pig femoral bones (n=50). The bone clamp shaft was adjusted to position the bone for three-point bending with and without additional bone compression. Four different directions of the force were applied: anterior, posterior, lateral, and medial. The impacted aspect can be distinguished from the non-impacted aspects based on the fracture pattern alone (the most fractured one); the impact point can be identified on bare bones (the area from which all oblique lines radiate and/or the presence of a chip fragment). None of our experiments (with and without compression) yielded a "true" butterfly fracture, but instead, oblique radiating lines emerged from the point of impact (also known as "false" butterfly). Impacts on the lateral and anterior aspects of the bones produce more and longer fracture lines than impacts on the contralateral side; bones subjected to an impact with axial loading are significantly more comminuted and fragmented. Under axial loading, the number of fracture lines is independent of the impact direction. Our study presents an experimental model for fracture analysis and shows that the impact direction and the presence of axial loading during impact significantly affect the fracture pattern obtained. Copyright © 2017 Elsevier B.V. All rights reserved.

Effects of age and loading rate on equine cortical bone failure.

PubMed

Kulin, Robb M; Jiang, Fengchun; Vecchio, Kenneth S

2011-01-01

Although clinical bone fractures occur predominantly under impact loading (as occurs during sporting accidents, falls, high-speed impacts or other catastrophic events), experimentally validated studies on the dynamic fracture behavior of bone, at the loading rates associated with such events, remain limited. In this study, a series of tests were performed on femoral specimens obtained post-mortem from equine donors ranging in age from 6 months to 28 years. Fracture toughness and compressive tests were performed under both quasi-static and dynamic loading conditions in order to determine the effects of loading rate and age on the mechanical behavior of the cortical bone. Fracture toughness experiments were performed using a four-point bending geometry on single and double-notch specimens in order to measure fracture toughness, as well as observe differences in crack initiation between dynamic and quasi-static experiments. Compressive properties were measured on bone loaded parallel and transverse to the osteonal growth direction. Fracture propagation was then analyzed using scanning electron and scanning confocal microscopy to observe the effects of microstructural toughening mechanisms at different strain rates. Specimens from each horse were also analyzed for dry, wet and mineral densities, as well as weight percent mineral, in order to investigate possible influences of composition on mechanical behavior. Results indicate that bone has a higher compressive strength, but lower fracture toughness when tested dynamically as compared to quasi-static experiments. Fracture toughness also tends to decrease with age when measured quasi-statically, but shows little change with age under dynamic loading conditions, where brittle "cleavage-like" fracture behavior dominates. Copyright © 2010 Elsevier Ltd. All rights reserved.

Fracture resistance of pulpless teeth restored with post-cores and crowns.

PubMed

Hayashi, Mikako; Takahashi, Yutaka; Imazato, Satoshi; Ebisu, Shigeyuki

2006-05-01

The present study was designed to test the null hypothesis that there is no difference in the fracture resistance of pulpless teeth restored with different types of post-core systems and full coverage crowns. Extracted human upper premolars were restored with a fiber post, prefabricated metallic post or cast metallic post-core. Teeth with full crown preparations without post-core restorations served as a control. All teeth were restored with full coverage crowns. A 90-degree vertical or 45-degree oblique load was applied to the restored teeth with a crosshead speed of 0.5 mm/min, and the fracture loads and mode of fracture were recorded. Under the condition of vertical loading, the fracture load of teeth restored with the cast metallic post-cores was greatest among the groups (two-factor factorial ANOVA and Scheffe's F test, P<0.05). All fractures in teeth restored with all types of post-core systems propagated in the middle portions of roots, including the apices of the posts. Under the condition of oblique loading, the fracture load of teeth restored with pre-fabricated metallic posts was significantly smaller than that in other groups. Two-thirds of fractures in the fiber post group propagated within the cervical area, while most fractures in other groups extended beyond the middle of the roots. From the results of the present investigations, it was concluded that under the conditions of vertical and oblique loadings, the combination of a fiber post and composite resin core with a full cast crown is most protective of the remaining tooth structure.

Mode I Fracture Toughness of Rock - Intrinsic Property or Pressure-Dependent?

NASA Astrophysics Data System (ADS)

Stoeckhert, F.; Brenne, S.; Molenda, M.; Alber, M.

2016-12-01

The mode I fracture toughness of rock is usually regarded as an intrinsic material parameter independent of pressure. However, most fracture toughness laboratory tests are conducted only at ambient pressure. To investigate fracture toughness of rock under elevated pressures, sleeve fracturing laboratory experiments were conducted with various rock types and a new numerical method was developed for the evaluation of these experiments. The sleeve fracturing experiments involve rock cores with central axial boreholes that are placed in a Hoek triaxial pressure cell to apply an isostatic confining pressure. A polymere tube is pressurized inside these hollow rock cylinders until they fail by tensile fracturing. Numerical simulations incorporating fracture mechanical models are used to obtain a relation between tensile fracture propagation and injection pressure. These simulations indicate that the magnitude of the injection pressure at specimen failure is only depending on the fracture toughness of the tested material, the specimen dimensions and the magnitude of external loading. The latter two are known parameters in the experiments. Thus, the fracture toughness can be calculated from the injection pressure recorded at specimen breakdown. All specimens had a borehole diameter to outer diameter ratio of about 1:10 with outer diameters of 40 and 62 mm. The length of the specimens was about two times the diameter. Maximum external loading was 7.5 MPa corresponding to maximum injection pressures at specimen breakdown of about 100 MPa. The sample set tested in this work includes Permian and Carboniferous sandstones, Jurassic limestones, Triassic marble, Permian volcanic rocks and Devonian slate from Central Europe. The fracture toughness values determined from the sleeve fracturing experiments without confinement using the new numerical method were found to be in good agreement with those from Chevron bend testing according to the ISRM suggested methods. At elevated confining pressures, the results indicate a significant positive correlation between fracture toughness and confining pressure for most tested rock types.

The viscoelastic behavior of notched glassy polymers

NASA Technical Reports Server (NTRS)

Crook, R. A.; Letton, Alan

1993-01-01

In the bulk, glassy polymers exhibit a nonlinear viscoelastic response during deformation. Stress or strain induced damage (i.e. crazing, microshear banding) results in the production of nonrecoverable work and observed nonlinearity. Stress or strain dependent shift factors have been used to mathematically model the mechanical behavior of these polymers. Glassy polymers that have been notched, may exhibit very different load displacement response compared to the same material under bulk deformation. If a sharp notch is introduced into the body then loaded, the load displacement trace may appear to be single-valued in the absence of viscoelasticity and crack growth. This suggests the volume of damaged material is small compared to the overall dimensions of the specimen. The ability to produce a single-valued load-load-line displacement trace through the use of the Correspondence Principle may prove to be useful for fracture of viscoelastic materials.

Micromechanisms of Crack Growth in Ceramics and Glasses in Corrosive Environments.

DTIC Science & Technology

1980-05-01

Resistance Mecanique du Verre et les Moyens de l’Amelioree, Union Scientifique Continentale du Verre , Charleroix, Belgium, (1962). 8. B. A. Proctor, I...exhibit similar types of delayed failure curves. Failure occurs most rapidly at high loads. Below a critical value of the load known as the stress...fracture for the three types of materials differ greatly. Polymers and metals have plastic zones at their crack tips, so that stress corrosion

Modeling of the fracture behavior of spot welds using advanced micro-mechanical damage models

NASA Astrophysics Data System (ADS)

Sommer, Silke

2010-06-01

This paper presents the modeling of deformation and fracture behavior of resistance spot welded joints in DP600 steel sheets. Spot welding is still the most commonly used joining technique in automotive engineering. In overloading situations like crash joints are often the weakest link in a structure. For those reasons, crash simulations need reliable and applicable tools to predict the load bearing capacity of spot welded components. Two series of component tests with different spot weld diameters have shown that the diameter of the weld nugget is the main influencing factor affecting fracture mode (interfacial or pull-out fracture), load bearing capacity and energy absorption. In order to find a correlation between nugget diameter, load bearing capacity and fracture mode, the spot welds are simulated with detailed finite element models containing base metal, heat affected zone and weld metal in lap-shear loading conditions. The change in fracture mode from interfacial to pull-out or peel-out fracture with growing nugget diameter under lap-shear loading was successfully modeled using the Gologanu-Leblond model in combination with the fracture criteria of Thomason and Embury. A small nugget diameter is identified to be the main cause for interfacial fracture. In good agreement with experimental observations, the calculated pull-out fracture initiates in the base metal at the boundary to the heat affected zone.

The influence of nail blocking conditions in cattle femoral fractures.

PubMed

Paolucci, Leopoldo A; Las Casas, Estevam B; Faleiros, Rafael R; Paz, Cahuê F R; Rocha Junior, Sergio S

2018-05-07

To investigate the effect of different fixation strategies of the intramedullary interlocking nail (IIN) on the mechanical behavior of a polymeric implant applied for femoral fracture fixation in calves, and to evaluate the performance of a glass fiber-reinforced polymer applied in a bovine femoral fracture reduction system, five Holstein male animals with a mean weight (±SD) of 62.8 ± 20.4 kg and aged 74 ± 15 were used to generate biomechanical parameters for this study. Twelve models of the fractured bovine femur, simulating a simple oblique fracture, were developed for use during the simulations. The models were divided into three groups, with each group of four models being associated with a different fixation strategy. Models were used to simulate the loading conditions corresponding to a calf in the transition (decubitus position to static position) condition. The maximum stresses found in each set (bone/implant) were compared with the reference stresses of each nail material. Maximum implant stresses were found in the screws and at the interface between the screw and the nail. The performance of implants was influenced by the material and fixation strategy, which can be confirmed by the stress values found in the set. The analysis indicated that the composite nail is able to withstand the loading demands in all fixation strategies. The finite element analysis (FEA) demonstrated that all polymeric materials analyzed provided sufficient resistance to withstand the loading forces imposed to the femur when an adequate blocking strategy was applied. Copyright © 2018 Elsevier Ltd. All rights reserved.

Statistical analysis of biomechanical properties of the adult skull and age-related structural changes by sex in a Japanese forensic sample.

PubMed

Torimitsu, Suguru; Nishida, Yoshifumi; Takano, Tachio; Koizumi, Yoshinori; Makino, Yohsuke; Yajima, Daisuke; Hayakawa, Mutsumi; Inokuchi, Go; Motomura, Ayumi; Chiba, Fumiko; Otsuka, Katsura; Kobayashi, Kazuhiro; Odo, Yuriko; Iwase, Hirotaro

2014-01-01

The purpose of this research was to investigate the biomechanical properties of the adult human skull and the structural changes that occur with age in both sexes. The heads of 94 Japanese cadavers (54 male cadavers, 40 female cadavers) autopsied in our department were used in this research. A total of 376 cranial samples, four from each skull, were collected. Sample fracture load was measured by a bending test. A statistically significant negative correlation between the sample fracture load and cadaver age was found. This indicates that the stiffness of cranial bones in Japanese individuals decreases with age, and the risk of skull fracture thus probably increases with age. Prior to the bending test, the sample mass, the sample thickness, the ratio of the sample thickness to cadaver stature (ST/CS), and the sample density were measured and calculated. Significant negative correlations between cadaver age and sample thickness, ST/CS, and the sample density were observed only among the female samples. Computerized tomographic (CT) images of 358 cranial samples were available. The computed tomography value (CT value) of cancellous bone which refers to a quantitative scale for describing radiodensity, cancellous bone thickness and cortical bone thickness were measured and calculated. Significant negative correlation between cadaver age and the CT value or cortical bone thickness was observed only among the female samples. These findings suggest that the skull is substantially affected by decreased bone metabolism resulting from osteoporosis. Therefore, osteoporosis prevention and treatment may increase cranial stiffness and reinforce the skull structure, leading to a decrease in the risk of skull fractures. Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.

Acoustic emission characterization of steel fibre reinforced concrete during bending

NASA Astrophysics Data System (ADS)

Aggelis, D. G.; Soulioti, D. V.; Sapouridis, N.; Barkoula, N. M.; Paipetis, A. S.; Matikas, T. E.

2010-04-01

The acoustic emission (AE) behaviour of steel fibre reinforced concrete is studied in this paper. The experiments were conducted in four-point bending with concurrent monitoring of AE signals. The sensors used, were of broadband response in order to capture a wide range of fracturing phenomena. The results indicate that AE parameters undergo significant changes much earlier than the final fracture of the specimens, even if the AE hit rate seems approximately constant. Specifically, the Ib-value which takes into account the amplitude distribution of the recent AE hits decreases when the load reaches about 60-70 % of its maximum value. Additionally, the average frequency of the signals decreases abruptly when a fracture incident occurs, indicating that matrix cracking events produce higher frequencies than fibre pull-out events. It is concluded that proper study of AE parameters enables the characterization of structural health of large structures in cases where remote monitoring is applied.

The fracture load and failure types of veneered anterior zirconia crowns: an analysis of normal and Weibull distribution of complete and censored data.

PubMed

Stawarczyk, Bogna; Ozcan, Mutlu; Hämmerle, Christoph H F; Roos, Malgorzata

2012-05-01

The aim of this study was to compare the fracture load of veneered anterior zirconia crowns using normal and Weibull distribution of complete and censored data. Standardized zirconia frameworks for maxillary canines were milled using a CAD/CAM system and randomly divided into 3 groups (N=90, n=30 per group). They were veneered with three veneering ceramics, namely GC Initial ZR, Vita VM9, IPS e.max Ceram using layering technique. The crowns were cemented with glass ionomer cement on metal abutments. The specimens were then loaded to fracture (1 mm/min) in a Universal Testing Machine. The data were analyzed using classical method (normal data distribution (μ, σ); Levene test and one-way ANOVA) and according to the Weibull statistics (s, m). In addition, fracture load results were analyzed depending on complete and censored failure types (only chipping vs. total fracture together with chipping). When computed with complete data, significantly higher mean fracture loads (N) were observed for GC Initial ZR (μ=978, σ=157; s=1043, m=7.2) and VITA VM9 (μ=1074, σ=179; s=1139; m=7.8) than that of IPS e.max Ceram (μ=798, σ=174; s=859, m=5.8) (p<0.05) by classical and Weibull statistics, respectively. When the data were censored for only total fracture, IPS e.max Ceram presented the lowest fracture load for chipping with both classical distribution (μ=790, σ=160) and Weibull statistics (s=836, m=6.5). When total fracture with chipping (classical distribution) was considered as failure, IPS e.max Ceram did not show significant fracture load for total fracture (μ=1054, σ=110) compared to other groups (GC Initial ZR: μ=1039, σ=152, VITA VM9: μ=1170, σ=166). According to Weibull distributed data, VITA VM9 showed significantly higher fracture load (s=1228, m=9.4) than those of other groups. Both classical distribution and Weibull statistics for complete data yielded similar outcomes. Censored data analysis of all ceramic systems based on failure types is essential and brings additional information regarding the susceptibility to chipping or total fracture. Copyright © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Addition of a suture anchor for coracoclavicular fixation to a superior locking plate improves stability of type IIB distal clavicle fractures.

PubMed

Madsen, Wes; Yaseen, Zaneb; LaFrance, Russell; Chen, Tony; Awad, Hani; Maloney, Michael; Voloshin, Ilya

2013-06-01

The purpose of this study was to determine the effect of coracoclavicular (CC) fixation on biomechanical stability in type IIB distal clavicle fractures fixed with plate and screws. Twelve fresh-frozen matched cadaveric specimens were used to create type IIB distal clavicle fractures. Dual-energy x-ray absorptiometry (DEXA) scans ensured similar bone quality. Group 1 (6 specimens) was stabilized with a superior precontoured distal clavicle locking plate and supplemental suture anchor CC fixation. Group 2 (6 specimens) followed the same construct without CC fixation. Each specimen was cyclically loaded in the coronal plane at 40 to 80 N for 17,500 cycles. Load-to-failure testing was performed on the specimens that did not fail cyclic loading. Outcome measures included mode of failure and the number of cycles or load required to create 10 mm of displacement in the construct. All specimens (12 of 12) completed cyclic testing without failure and underwent load-to-failure testing. Group 1 specimens failed at a mean of 808.5 N (range, 635.4 to 952.3 N), whereas group 2 specimens failed at a mean of 401.3 N (range, 283.6 to 656.0 N) (P = .005). Group 1 specimens failed by anchor pullout without coracoid fracture (4 of 6) and distal clavicle fracture fragment fragmentation (1 of 6); one specimen did not fail at the maximal load the materials testing machine was capable of exerting (1,000 N). Group 2 specimens failed by distal clavicle fracture fragment fragmentation (3 of 6) and acromioclavicular (AC) joint displacement (1 of 6); 2 specimens did not fail at the maximal load of the materials testing machine. During cyclic loading, type IIB distal clavicle fractures with and without CC fixation remain stable. CC fixation adds stability to type IIB distal clavicle fractures fixed with plate and screws when loaded to failure. CC fixation for distal clavicle fractures is a useful adjunct to plate-and-screw fixation to augment stability of the fracture. Copyright © 2013 Arthroscopy Association of North America. Published by Elsevier Inc. All rights reserved.

Influence of preparation design and ceramic thicknesses on fracture resistance and failure modes of premolar partial coverage restorations

PubMed Central

Guess, Petra C.; Schultheis, Stefan; Wolkewitz, Martin; Zhang; Strub, Joerg R.

2015-01-01

Statement of problem Preparation designs and ceramic thicknesses are key factors for the long-term success of minimally invasive premolar partial coverage restorations. However, only limited information is presently available on this topic. Purpose The aim of this in vitro study was to evaluate the fracture resistance and failure modes of ceramic premolar partial coverage restorations with different preparation designs and ceramic thicknesses. Material and methods Caries-free human premolars (n= 144) were divided into 9 groups. Palatal onlay preparation comprised reduction of the palatal cusp by 2 mm (Palatal-Onlay-Standard), 1 mm (Palatal-Onlay-Thin), or 0.5 mm (Palatal-Onlay-Ultra-Thin). Complete-coverage onlay preparation additionally included the buccal cusp (Occlusal-Onlay-Standard; Occlusal-Onlay-Thin; Occlusal-Onlay-Ultra-Thin). Labial surface preparations with chamfer reductions of 0.8 mm (Complete-Veneer-Standard), 0.6 mm (Complete-Veneer-Thin) and 0.4 mm (Complete-Veneer-Ultra-Thin) were implemented for complete veneer restorations. Restorations were fabricated from a pressable lithium-disilicate ceramic (IPS-e.max-Press) and cemented adhesively (Syntac-Classic/Variolink-II). All specimens were subjected to cyclic mechanical loading (F= 49 N, 1.2 million cycles) and simultaneous thermocycling (5°C to 55°C) in a mouth-motion simulator. After fatigue, restorations were exposed to single-load-to-failure. Two-way ANOVA was used to identify statistical differences. Pair-wise differences were calculated and P-values were adjusted by the Tukey–Kramer method (α= .05). Results All specimens survived fatigue. Mean (SD) load to failure values (N) were as follows: 837 (320/Palatal-Onlay-Standard), 1055 (369/Palatal-Onlay-Thin), 1192 (342/Palatal-Onlay-Ultra-Thin), 963 (405/Occlusal-Onlay-Standard), 1108 (340/Occlusal-Onlay-Thin), 997 (331/Occlusal-Onlay-Ultra-Thin), 1361 (333/Complete-Veneer-Standard), 1087 (251/Complete-Veneer-Thin), 883 (311/Complete-Veneer-Ultra-Thin). Palatal-onlay restorations revealed a significantly higher fracture resistance with ultra-thin thicknesses than with standard thicknesses (P=.015). Onlay restorations were not affected by thickness variations. Fracture loads of standard complete veneers were significantly higher than thin (P=.03) and ultra-thin (P<.001) restorations. Conclusions In this in vitro study, the reduction of preparation depth to 1.00 and 0.5 mm did not impair fracture resistance of pressable lithium-disilicate ceramic onlay restorations but resulted in lower failure loads in complete veneer restorations on premolars. PMID:24079561

FEMCAM Analysis of SULTAN Test Results for ITER Nb3SN Cable-conduit Conductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yuhu Zhai, Pierluigi Bruzzone, Ciro Calzolaio

2013-03-19

Performance degradation due to filament fracture of Nb3 Sn cable-in-conduit conductors (CICCs) is a critical issue in large-scale magnet designs such as ITER which is currently being constructed in the South of France. The critical current observed in most SULTAN TF CICC samples is significantly lower than expected and the voltage-current characteristic is seen to have a much broader transition from a single strand to the CICC. Moreover, most conductors exhibit the irreversible degradation due to filament fracture and strain relaxation under electromagnetic cyclic loading. With recent success in monitoring thermal strain distribution and its evolution under the electromagnetic cyclicmore » loading from in situ measurement of critical temperature, we apply FEMCAM which includes strand filament breakage and local current sharing effects to SULTAN tested CICCs to study Nb3 Sn strain sensitivity and irreversible performance degradation. FEMCAM combines the thermal bending effect during cool down and the EM bending effect due to locally accumulating Lorentz force during magnet operation. It also includes strand filament fracture and related local current sharing for the calculation of cable n value. In this paper, we model continuous performance degradation under EM cyclic loading based on strain relaxation and the transition broadening upon cyclic loading to the extreme cases seen in SULTAN test data to better quantify conductor performance degradation.« less

Bending moments of zirconia and titanium implant abutments supporting all-ceramic crowns after aging.

PubMed

Mühlemann, Sven; Truninger, Thomas C; Stawarczyk, Bogna; Hämmerle, Christoph H F; Sailer, Irena

2014-01-01

To test the fracture load and fracture patterns of zirconia abutments restored with all-ceramic crowns after fatigue loading, exhibiting internal and external implant-abutment connections as compared to restored and internally fixed titanium abutments. A master abutment was used for the customization of 5 groups of zirconia abutments to a similar shape (test). The groups differed according to their implant-abutment connections: one-piece internal connection (BL; Straumann Bonelevel), two-piece internal connection (RS; Nobel Biocare ReplaceSelect), external connection (B; Branemark MkIII), two-piece internal connection (SP, Straumann StandardPlus) and one-piece internal connection (A; Astra Tech AB OsseoSpeed). Titanium abutments with internal implant-abutment connection (T; Straumann Bonelevel) served as control group. In each group, 12 abutments were fabricated, mounted to the respective implants and restored with glass-ceramic crowns. All samples were embedded in acrylic holders (ISO-Norm 14801). After aging by means of thermocycling in a chewing simulator, static load was applied until failure (ISO-Norm 14801). Fracture load was analyzed by calculating the bending moments. Values of all groups were compared with one-way ANOVA followed by Scheffé post hoc test (P-value<0.05). Failure mode was analyzed descriptively. The mean bending moments were 464.9 ± 106.6 N cm (BL), 581.8 ± 172.8 N cm (RS), 556.7 ± 128.4 N cm (B), 605.4 ± 54.7 N cm (SP), 216.4 ± 90.0 N cm (A) and 1042.0 ± 86.8 N cm (T). No difference of mean bending moments was found between groups BL, RS, B and SP. Test group A exhibited significantly lower mean bending moment than the other test groups. Control group T had significantly higher bending moments than all test groups. Failure due to fracture of the abutment and/or crown occurred in the test groups. In groups BL and A, fractures were located in the internal part of the connection, whereas in groups RS and SP, a partial deformation of the implant components occurred and cracks and fractures of the zirconia abutment were detected. The differently connected zirconia abutments exhibited similar bending moments with the exception of one group. Hence, the type of connection only had a minor effect on the stability of restored zirconia abutments. In general, restored titanium abutments exhibited the highest bending moments. © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

Carbon-enhanced electrical conductivity during fracture of rocks

NASA Astrophysics Data System (ADS)

Roberts, J. J.; Duba, A. G.; Mathez, E. A.; Shankland, T. J.; Kinzler, R.

1999-01-01

Changes in electrical resistance during rock fracture in the presence of a carbonaceous atmosphere have been investigated using Nugget sandstone and Westerly granite. The experiments were performed in an internally heated, gas-pressure vessel with a load train that produced strain rates between 10-6 and 10-5 s-1. Samples were deformed at temperatures of 354° to 502°C and pressures of 100 to 170 MPa in atmospheres of Ar or mixtures of 95% CO2 with 5% CO or 5% CH4, compositions that are well within the field of graphite stability at the run conditions. In experiments using Nugget sandstone, resistance reached a minimum value when the maximum temperature was achieved and good electrode contact was made. The resistance then increased as the experiment continued, probably due to dry out of the sample, a change in the oxidation state of the Fe-oxide associated with the cement, or destruction of current-bearing pathways. At approximately 200-MPa end load, the rock sample failed. Plots of load and resistance versus time show several interesting features. In one experiment, for example, as the end load reached about 175 MPa, resistance stopped increasing and remained fairly constant for a period of approximately 0.5 hour. During loading, the end load displayed small decreases that were simultaneous with small decreases in resistance; when the end load (and the displacement) indicated rock failure, resistance decreased dramatically, from ˜150 MΩ to 100 MΩ. In a single experiment, the Westerly granite also showed a decrease in resistance during dilatancy. The nature and distribution of carbon in the run products were studied by electron microprobe and time-of-flight secondary-ion mass spectroscopy (TOP-SIMS). Carbon observed by mapping with the former is clearly observed on micro-cracks that, based on the microtexture, are interpreted to have formed during the deformation. The TOF-SIMS data confirm the electron-probe observations that carbon is present on fracture surfaces. These observations and experimental results lead to the hypothesis that as microfractures open in the time leading up to failure along a fracture, carbon is deposited as a continuous film on the new, reactive mineral surfaces, and this produces a decrease in resistance. Subsequent changes in resistance occur as connectivity of the initial fracture network is altered by continued deformation. Such a process may explain some electromagnetic effects associated with earthquakes.

Fatigue Life Prediction of Fiber-Reinforced Ceramic-Matrix Composites with Different Fiber Preforms at Room and Elevated Temperatures

PubMed Central

Li, Longbiao

2016-01-01

In this paper, the fatigue life of fiber-reinforced ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply, 2D (two dimensional), 2.5D and 3D CMCs at room and elevated temperatures in air and oxidative environments, has been predicted using the micromechanics approach. An effective coefficient of the fiber volume fraction along the loading direction (ECFL) was introduced to describe the fiber architecture of preforms. The statistical matrix multicracking model and fracture mechanics interface debonding criterion were used to determine the matrix crack spacing and interface debonded length. Under cyclic fatigue loading, the fiber broken fraction was determined by combining the interface wear model and fiber statistical failure model at room temperature, and interface/fiber oxidation model, interface wear model and fiber statistical failure model at elevated temperatures, based on the assumption that the fiber strength is subjected to two-parameter Weibull distribution and the load carried by broken and intact fibers satisfies the Global Load Sharing (GLS) criterion. When the broken fiber fraction approaches the critical value, the composites fatigue fracture. PMID:28773332

Mechanical design optimization of bioabsorbable fixation devices for bone fractures.

PubMed

Lovald, Scott T; Khraishi, Tariq; Wagner, Jon; Baack, Bret

2009-03-01

Bioabsorbable bone plates can eliminate the necessity for a permanent implant when used to fixate fractures of the human mandible. They are currently not in widespread use because of the low strength of the materials and the requisite large volume of the resulting bone plate. The aim of the current study was to discover a minimally invasive bioabsorbable bone plate design that can provide the same mechanical stability as a standard titanium bone plate. A finite element model of a mandible with a fracture in the body region is subjected to bite loads that are common to patients postsurgery. The model is used first to determine benchmark stress and strain values for a titanium plate. These values are then set as the limits within which the bioabsorbable bone plate must comply. The model is then modified to consider a bone plate made of the polymer poly-L/DL-lactide 70/30. An optimization routine is run to determine the smallest volume of bioabsorbable bone plate that can perform and a titanium bone plate when fixating fractures of this considered type. Two design parameters are varied for the bone plate design during the optimization analysis. The analysis determined that a strut style poly-L-lactide-co-DL-lactide plate of 690 mm2 can provide as much mechanical stability as a similar titanium design structure of 172 mm2. The model has determined a bioabsorbable bone plate design that is as strong as a titanium plate when fixating fractures of the load-bearing mandible. This is an intriguing outcome, considering that the polymer material has only 6% of the stiffness of titanium.

Shock wave treatment shows dose-dependent enhancement of bone mass and bone strength after fracture of the femur.

PubMed

Wang, Ching-Jen; Yang, Kuender D; Wang, Feng-Sheng; Hsu, Chia-Chen; Chen, Hsiang-Ho

2004-01-01

Shock wave treatment is believed to improve bone healing after fracture. The purpose of this study was to evaluate the effect of shock wave treatment on bone mass and bone strength after fracture of the femur in a rabbit model. A standardized closed fracture of the right femur was created with a three-point bending method in 24 New Zealand white rabbits. Animals were randomly divided into three groups: (1) control (no shock wave treatment), (2) low-energy (shock wave treatment at 0.18 mJ/mm2 energy flux density with 2000 impulses), and (3) high-energy (shock wave treatment at 0.47 mJ/mm2 energy flux density with 4000 impulses). Bone mass (bone mineral density (BMD), callus formation, ash and calcium contents) and bone strength (peak load, peak stress and modulus of elasticity) were assessed at 12 and 24 weeks after shock wave treatment. While the BMD values of the high-energy group were significantly higher than the control group (P = 0.021), the BMD values between the low-energy and control groups were not statistically significant (P = 0.358). The high-energy group showed significantly more callus formation (P < 0.001), higher ash content (P < 0.001) and calcium content (P = 0.003) than the control and low-energy groups. With regard to bone strength, the high-energy group showed significantly higher peak load (P = 0.012), peak stress (P = 0.015) and modulus of elasticity (P = 0.011) than the low-energy and control groups. Overall, the effect of shock wave treatment on bone mass and bone strength appears to be dose dependent in acute fracture healing in rabbits.

Experimental analysis of quasi-static and dynamic fracture initiation toughness of gy4 armor steel material

NASA Astrophysics Data System (ADS)

Ren, Peng; Guo, Zitao

Quasi-static and dynamic fracture initiation toughness of gy4 armour steel material are investigated using three point bend specimen. The modified split Hopkinson pressure bar (SHPB) apparatus with digital image correlation (DIC) system is applied to dynamic loading experiments. Full-field deformation measurements are obtained by using DIC to elucidate on the strain fields associated with the mechanical response. A series of experiments are conducted at different strain rate ranging from 10-3 s-1 to 103 s-1, and the loading rate on the fracture initiation toughness is investigated. Specially, the scanning electron microscope imaging technique is used to investigate the fracture failure micromechanism of fracture surfaces. The gy4 armour steel material fracture toughness is found to be sensitive to strain rate and higher for dynamic loading as compared to quasi-static loading. This work is supported by National Nature Science Foundation under Grant 51509115.

Fracture strength of three all-ceramic systems: Top-Ceram compared with IPS-Empress and In-Ceram.

PubMed

Quran, Firas Al; Haj-Ali, Reem

2012-03-01

The purpose of this study was to investigate the fracture loads and mode of failure of all-ceramic crowns fabricated using Top-Ceram and compare it with all-ceramic crowns fabricated from well-established systems: IPS-Empress II, In-Ceram. Thirty all-ceramic crowns were fabricated; 10 IPS-Empress II, 10 In-Ceram alumina and 10 Top-Ceram. Instron testing machine was used to measure the loads required to introduce fracture of each crown. Mean fracture load for In-Ceram alumina [941.8 (± 221.66) N] was significantly (p > 0.05) higher than those of Top-Ceram and IPS-Empress II. There was no statistically significant difference between Top-Ceram and IPS-Empress II mean fracture loads; 696.20 (+222.20) and 534 (+110.84) N respectively. Core fracture pattern was highest seen in Top- Ceram specimens.

Assessment of a novel biomechanical fracture model for distal radius fractures

PubMed Central

2012-01-01

Background Distal radius fractures (DRF) are one of the most common fractures and often need surgical treatment, which has been validated through biomechanical tests. Currently a number of different fracture models are used, none of which resemble the in vivo fracture location. The aim of the study was to develop a new standardized fracture model for DRF (AO-23.A3) and compare its biomechanical behavior to the current gold standard. Methods Variable angle locking volar plates (ADAPTIVE, Medartis) were mounted on 10 pairs of fresh-frozen radii. The osteotomy location was alternated within each pair (New: 10 mm wedge 8 mm / 12 mm proximal to the dorsal / volar apex of the articular surface; Gold standard: 10 mm wedge 20 mm proximal to the articular surface). Each specimen was tested in cyclic axial compression (increasing load by 100 N per cycle) until failure or −3 mm displacement. Parameters assessed were stiffness, displacement and dissipated work calculated for each cycle and ultimate load. Significance was tested using a linear mixed model and Wald test as well as t-tests. Results 7 female and 3 male pairs of radii aged 74 ± 9 years were tested. In most cases (7/10), the two groups showed similar mechanical behavior at low loads with increasing differences at increasing loads. Overall the novel fracture model showed a significant different biomechanical behavior than the gold standard model (p < 0,001). The average final loads resisted were significantly lower in the novel model (860 N ± 232 N vs. 1250 N ± 341 N; p = 0.001). Conclusion The novel biomechanical fracture model for DRF more closely mimics the in vivo fracture site and shows a significantly different biomechanical behavior with increasing loads when compared to the current gold standard. PMID:23244634

Fracture-resistant monolithic dental crowns.

PubMed

Zhang, Yu; Mai, Zhisong; Barani, Amir; Bush, Mark; Lawn, Brian

2016-03-01

To quantify the splitting resistance of monolithic zirconia, lithium disilicate and nanoparticle-composite dental crowns. Fracture experiments were conducted on anatomically-correct monolithic crown structures cemented to standard dental composite dies, by axial loading of a hard sphere placed between the cusps. The structures were observed in situ during fracture testing, and critical loads to split the structures were measured. Extended finite element modeling (XFEM), with provision for step-by-step extension of embedded cracks, was employed to simulate full failure evolution. Experimental measurements and XFEM predictions were self-consistent within data scatter. In conjunction with a fracture mechanics equation for critical splitting load, the data were used to predict load-sustaining capacity for crowns on actual dentin substrates and for loading with a sphere of different size. Stages of crack propagation within the crown and support substrate were quantified. Zirconia crowns showed the highest fracture loads, lithium disilicate intermediate, and dental nanocomposite lowest. Dental nanocomposite crowns have comparable fracture resistance to natural enamel. The results confirm that monolithic crowns are able to sustain high bite forces. The analysis indicates what material and geometrical properties are important in optimizing crown performance and longevity. Copyright © 2015 Academy of Dental Materials. All rights reserved.

FRACTURE-RESISTANT MONOLITHIC DENTAL CROWNS

PubMed Central

Zhang, Yu; Mai, Zhisong; Barani, Amir; Bush, Mark; Lawn, Brian

2016-01-01

Objective To quantify the splitting resistance of monolithic zirconia, lithium disilicate and nanoparticle-composite dental crowns. Methods Fracture experiments were conducted on anatomically-correct monolithic crown structures cemented to standard dental composite dies, by axial loading of a hard sphere placed between the cusps. The structures were observed in situ during fracture testing, and critical loads to split the structures were measured. Extended finite element modeling (XFEM), with provision for step-by-step extension of embedded cracks, was employed to simulate full failure evolution. Results Experimental measurements and XFEM predictions were self consistent within data scatter. In conjunction with a fracture mechanics equation for critical splitting load, the data were used to predict load-sustaining capacity for crowns on actual dentin substrates and for loading with a sphere of different size. Stages of crack propagation within the crown and support substrate were quantified. Zirconia crowns showed the highest fracture loads, lithium disilicate intermediate, and dental nanocomposite lowest. Dental nanocomposite crowns have comparable fracture resistance to natural enamel. Significance The results confirm that monolithic crowns are able to sustain high bite forces. The analysis indicates what material and geometrical properties are important in optimizing crown performance and longevity. PMID:26792623

Experimental and Numerical Study on the Cracked Chevron Notched Semi-Circular Bend Method for Characterizing the Mode I Fracture Toughness of Rocks

NASA Astrophysics Data System (ADS)

Wei, Ming-Dong; Dai, Feng; Xu, Nu-Wen; Liu, Jian-Feng; Xu, Yuan

2016-05-01

The cracked chevron notched semi-circular bending (CCNSCB) method for measuring the mode I fracture toughness of rocks combines the merits (e.g., avoidance of tedious pre-cracking of notch tips, ease of sample preparation and loading accommodation) of both methods suggested by the International Society for Rock Mechanics, which are the cracked chevron notched Brazilian disc (CCNBD) method and the notched semi-circular bend (NSCB) method. However, the limited availability of the critical dimensionless stress intensity factor (SIF) values severely hinders the widespread usage of the CCNSCB method. In this study, the critical SIFs are determined for a wide range of CCNSCB specimen geometries via three-dimensional finite element analysis. A relatively large support span in the three point bending configuration was considered because the fracture of the CCNSCB specimen in that situation is finely restricted in the notch ligament, which has been commonly assumed for mode I fracture toughness measurements using chevron notched rock specimens. Both CCNSCB and NSCB tests were conducted to measure the fracture toughness of two different rock types; for each rock type, the two methods produce similar toughness values. Given the reported experimental results, the CCNSCB method can be reliable for characterizing the mode I fracture toughness of rocks.

Disadvantages of interfragmentary shear on fracture healing--mechanical insights through numerical simulation.

PubMed

Steiner, Malte; Claes, Lutz; Ignatius, Anita; Simon, Ulrich; Wehner, Tim

2014-07-01

The outcome of secondary fracture healing processes is strongly influenced by interfragmentary motion. Shear movement is assumed to be more disadvantageous than axial movement, however, experimental results are contradictory. Numerical fracture healing models allow simulation of the fracture healing process with variation of single input parameters and under comparable, normalized mechanical conditions. Thus, a comparison of the influence of different loading directions on the healing process is possible. In this study we simulated fracture healing under several axial compressive, and translational and torsional shear movement scenarios, and compared their respective healing times. Therefore, we used a calibrated numerical model for fracture healing in sheep. Numerous variations of movement amplitudes and musculoskeletal loads were simulated for the three loading directions. Our results show that isolated axial compression was more beneficial for the fracture healing success than both isolated shearing conditions for load and displacement magnitudes which were identical as well as physiological different, and even for strain-based normalized comparable conditions. Additionally, torsional shear movements had less impeding effects than translational shear movements. Therefore, our findings suggest that osteosynthesis implants can be optimized, in particular, to limit translational interfragmentary shear under musculoskeletal loading. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.

Effect of initial delamination on Mode 1 and Mode 2 interlaminar fracture toughness and fatigue fracture threshold

NASA Technical Reports Server (NTRS)

Murri, Gretchen Bostaph; Martin, Roderick H.

1991-01-01

Static and fatigue double-cantilever beam (DCB) and end-notch flexure (ENF) tests were conducted to determine the effect of the simulated initial delamination in interlaminar fracture toughness, G(sub c), and fatigue fracture threshold, G(sub th). Unidirectional, 24-ply specimens of S2/SP250 glass/epoxy were tested using Kapton inserts of four different thickness - 13, 25, 75, and 130 microns, at the midplane at one end, or with tension or shear precracks, to simulate an initial delamination. To determine G(sub c), the fatigue fracture threshold below which no delamination growth would occur in less than 1 x 10(exp 6) cycles, fatigue tests were conducted by cyclically loading specimens until delamination growth was detected. Consistent values of model 1 fracture toughness, G(sub Ic), were measured from DCB specimens with inserts of thickness 75 microns or thinner, or with shear precracks. The fatigue DCB tests gave similar values of G(sub Ith) for the 13, 25, and 75 microns specimens. Results for the shear precracked specimens were significantly lower that for specimens without precracks. Results for both the static and fatigue ENF tests showed that measured G(IIc) and G(IIth) values decreased with decreasing insert thickness, so that no limiting thickness could be determined. Results for specimens with inserts of 75 microns or thicker were significantly higher than the results for precracked specimens or specimens with 13 or 25 microns inserts.

The effects of the sagittal plane malpositioning of the patella and concomitant quadriceps hypotrophy on the patellofemoral joint: a finite element analysis.

PubMed

Aksahin, Ertugrul; Kocadal, Onur; Aktekin, Cem N; Kaya, Defne; Pepe, Murad; Yılmaz, Serdar; Yuksel, H Yalcin; Bicimoglu, Ali

2016-03-01

Anterior knee pain is a common symptom after intramedullary nailing in tibia shaft fracture. Moreover, patellofemoral malalignment is also known to be a major reason for anterior knee pain. Patellofemoral malalignment predisposes to increased loading in patellar cartilage. In the previous study, we have demonstrated the quadriceps atrophy and patellofemoral malalignment after intramedullary nailing due to tibia shaft fracture. In this study, our aim was to clarify the effects of quadriceps atrophy and patellofemoral malalignment with the pathologic loading on the joint cartilage. Mesh models of patellofemoral joint were constructed with CT images and integrated with soft tissue components such as menisci and ligaments. Physiological and sagittal tilt models during extension and flexion at 15°, 30° and 60° were created generating eight models. All the models were applied with 137 N force to present the effects of normal loading and 115.7 N force for the simulation of quadriceps atrophy. Different degrees of loading were applied to evaluate the joint contact area and pressure value with the finite element analysis. There was increased patellofemoral contact area in patellar tilt models with respect to normal models. The similar loading patterns were diagnosed in all models at 0° and 15° knee flexion when 137 N force was applied. Higher loading values were obtained at 30° and 60° knee flexions in sagittal tilt models. Furthermore, in the sagittal tilt models, in which the quadriceps atrophy was simulated, the loadings at 30° and 60° knee flexion were higher than in the physiological ones. Sagittal malalignment of the patellofemoral joint is a new concept that results in different loading patterns in the patellofemoral joint biomechanics. This malalignment in sagittal plane leads to increased loading values on the patellofemoral joint at 30° and 60° of the knee flexions. This new concept should be kept in mind during the course of diagnosis and treatment in patients with anterior knee pain. Definition of the exact biomechanical effects of the sagittal tilting will lead to the development of new treatment modalities.

Flexural properties of fiber reinforced root canal posts.

PubMed

Lassila, Lippo V J; Tanner, Johanna; Le Bell, Anna-Maria; Narva, Katja; Vallittu, Pekka K

2004-01-01

Fiber-reinforced composite (FRC) root canal posts have been introduced to be used instead of metal alloys and ceramics. The aim of this study was to investigate the flexural properties of different types of FRC posts and compare those values with a novel FRC material for dental applications. Seventeen different FRC posts of various brands (Snowpost, Carbopost, Parapost, C-post, Glassix, Carbonite) and diameters, (1.0-2.1 mm) and a continuous unidirectional E-glass FRC polymerized by light activation to a cylindrical form (everStick, diameter 1.5 mm) as a control material were tested. The posts (n=5) were stored at room's humidity or thermocycled (12.000 x, 5 degrees C/55 degrees C) and stored in water for 2 weeks before testing. A three-point bending test (span=10 mm) was used to measure the flexural strength and modulus of FRC post specimens. Analysis of ANOVA revealed that thermocycling, brand of material and diameter of specimen had a significant effect (p<0.001) on the fracture load and flexural strength. The highest flexural strength was obtained with the control material (everStick, 1144.9+/-99.9 MPa). There was a linear relationship between fracture load and diameter of posts for both glass fiber and carbon fiber posts. Thermocycling decreased the flexural modulus of the tested specimens by approximately 10%. Strength and fracture load decreased approximately 18% as a result of thermocycling. Considerable variation can be found in the calculated strength values of the studied post brands. Commercial prefabricated FRC posts showed lower flexural properties than an individually polymerised FRC material.

Evaluation of an injectable hydrogel and polymethyl methacrylate in restoring mechanics to compressively fractured spine motion segments.

PubMed

Balkovec, Christian; Vernengo, Andrea J; Stevenson, Peter; McGill, Stuart M

2016-11-01

Compressive fracture can produce profound changes to the mechanical profile of a spine segment. Minimally invasive repair has the potential to restore both function and structural integrity to an injured spine. Use of both hydrogels to address changes to the disc, combined with polymethyl methacrylate (PMMA) to address changes to the vertebral body, has the potential to facilitate repair. The purpose of this investigation was to determine if the combined use of hydrogel injection and PMMA could restore the mechanical profile of an axially injured spinal motion segment. This is a basic science study evaluating a combination of hydrogel injection and vertebroplasty on restoring mechanics to compressively injured porcine spine motion segments. Fourteen porcine spine motion segments were subject to axial compression until fracture using a dynamic servohydraulic testing apparatus. Rotational and compressive stiffness was measured for each specimen under the following conditions: initial undamaged, fractured, fatigue loading under compression, hydrogel injection, PMMA injection, and fatigue loading under compression. Group 1 received hydrogel injection followed by PMMA injection, whereas Group 2 received PMMA injection followed by hydrogel injection. This study was funded under a Natural Sciences and Engineering Research Council of Canada discovery grant. PMMA injection was found to alter the compressive stiffness properties of axially injured spine motion segments, restoring values from Groups 1 and 2 to 89.3%±29.3% and 81%±27.9% of initial values respectively. Hydrogel injection was found to alter the rotational stiffness properties, restoring specimens in Groups 1 and 2 to 151.5%±81% and 177.2%±54.9% of initial values respectively. Prolonged restoration of function was not possible, however, after further fatigue loading. Using this repair technique, replication of the mechanism of injury appears to cause a rapid deterioration in function of the motion segments. Containment of the hydrogel appears to be an issue with large breaches in the end plate, as it is posited to migrate into the cancellous bone of the vertebral body. Future work should attempt to evaluate methods in fully sealing the disc space. Copyright © 2016 Elsevier Inc. All rights reserved.

Biomechanical analysis of tension band fixation for olecranon fracture treatment.

PubMed

Kozin, S H; Berglund, L J; Cooney, W P; Morrey, B F; An, K N

1996-01-01

This study assessed the strength of various tension band fixation methods with wire and cable applied to simulated olecranon fractures to compare stability and potential failure or complications between the two. Transverse olecranon fractures were simulated by osteotomy. The fracture was anatomically reduced, and various tension band fixation techniques were applied with monofilament wire or multifilament cable. With a material testing machine load displacement curves were obtained and statistical relevance determined by analysis of variance. Two loading modes were tested: loading on the posterior surface of olecranon to simulate triceps pull and loading on the anterior olecranon tip to recreate a potential compressive loading on the fragment during the resistive flexion. All fixation methods were more resistant to posterior loading than to an anterior load. Individual comparative analysis for various loading conditions concluded that tension band fixation is more resilient to tensile forces exerted by the triceps than compressive forces on the anterior olecranon tip. Neither wire passage anterior to the K-wires nor the multifilament cable provided statistically significant increased stability.

In-vitro development of a temporal abutment screw to protect osseointegration in immediate loaded implants.

PubMed

García-Roncero, Herminio; Caballé-Serrano, Jordi; Cano-Batalla, Jordi; Cabratosa-Termes, Josep; Figueras-Álvarez, Oscar

2015-04-01

In this study, a temporal abutment fixation screw, designed to fracture in a controlled way upon application of an occlusal force sufficient to produce critical micromotion was developed. The purpose of the screw was to protect the osseointegration of immediate loaded single implants. Seven different screw prototypes were examined by fixing titanium abutments to 112 Mozo-Grau external hexagon implants (MG Osseous®; Mozo-Grau, S.A., Valladolid, Spain). Fracture strength was tested at 30° in two subgroups per screw: one under dynamic loading and the other without prior dynamic loading. Dynamic loading was performed in a single-axis chewing simulator using 150,000 load cycles at 50 N. After normal distribution of obtained data was verified by Kolmogorov-Smirnov test, fracture resistance between samples submitted and not submitted to dynamic loading was compared by the use of Student's t-test. Comparison of fracture resistance among different screw designs was performed by the use of one-way analysis of variance. Confidence interval was set at 95%. Fractures occurred in all screws, allowing easy retrieval. Screw Prototypes 2, 5 and 6 failed during dynamic loading and exhibited statistically significant differences from the other prototypes. Prototypes 2, 5 and 6 may offer a useful protective mechanism during occlusal overload in immediate loaded implants.

Assessment of function-graded materials as fracture fixation bone-plates under combined loading conditions using finite element modelling.

PubMed

Fouad, H

2011-05-01

In previous work by Fouad (Medical Engineering and Physics 2010 [23]), 3D finite element (FE) models for fractured bones with function-graded (FG) bone-plates and traditional bone-plates made of stainless steel (SS) and titanium (Ti) alloy were examined under compressive loading conditions using the ABAQUS Code. In this study, the effects of the presence of the torsional load in addition to the compressive load on the predicted stresses of the fracture fixation bone-plate system are examined at different healing stages. The effects on the stress on the fracture site when using contacted and non-contacted bone-plate systems are also studied. The FE modelling results indicate that the torsional load has significant effects on the resultant stress on the fracture fixation bone-plate system, which should be taken into consideration during the design and the analysis. The results also show that the stress shielding at the fracture site decreases significantly when using FG bone-plates compared to Ti alloy or SS bone-plates. The presence of a gap between the bone and the plate results in a remarkable reduction in bone stress shielding at the fracture site. Therefore, the significant effects of using an FG bone-plate with a gap and the presence of torsional load on the resultant stress on the fracture fixation bone-plate system should be taken into consideration. Copyright © 2010 IPEM. Published by Elsevier Ltd. All rights reserved.

The tolerance of the femoral shaft in combined axial compression and bending loading.

PubMed

Ivarsson, B Johan; Genovese, Daniel; Crandall, Jeff R; Bolton, James R; Untaroiu, Costin D; Bose, Dipan

2009-11-01

The likelihood of a front seat occupant sustaining a femoral shaft fracture in a frontal crash has traditionally been assessed by an injury criterion relying solely on the axial force in the femur. However, recently published analyses of real world data indicate that femoral shaft fracture occurs at axial loads levels below those found experimentally. One hypothesis attempting to explain this discrepancy suggests that femoral shaft fracture tends to occur as a result of combined axial compression and applied bending. The current study aims to evaluate this hypothesis by investigating how these two loading components interact. Femoral shafts harvested from human cadavers were loaded to failure in axial compression, sagittal plane bending, and combined axial compression and sagittal plane bending. All specimens subjected to bending and combined loading fractured midshaft, whereas the specimens loaded in axial compression demonstrated a variety of failure locations including midshaft and distal end. The interaction between the recorded levels of applied moment and axial compression force at fracture were evaluated using two different analysis methods: fitting of an analytical model to the experimental data and multiple regression analysis. The two analysis methods yielded very similar relationships between applied moment and axial compression force at midshaft fracture. The results indicate that posteroanterior bending reduces the tolerance of the femoral shaft to axial compression and that that this type of combined loading therefore may contribute to the high prevalence of femoral shaft fracture in frontal crashes.

Comparison of the fracture resistances of glass fiber mesh- and metal mesh-reinforced maxillary complete denture under dynamic fatigue loading.

PubMed

Im, So-Min; Huh, Yoon-Hyuk; Cho, Lee-Ra; Park, Chan-Jin

2017-02-01

The aim of this study was to investigate the effect of reinforcing materials on the fracture resistances of glass fiber mesh- and Cr-Co metal mesh-reinforced maxillary complete dentures under fatigue loading. Glass fiber mesh- and Cr-Co mesh-reinforced maxillary complete dentures were fabricated using silicone molds and acrylic resin. A control group was prepared with no reinforcement (n = 15 per group). After fatigue loading was applied using a chewing simulator, fracture resistance was measured by a universal testing machine. The fracture patterns were analyzed and the fractured surfaces were observed by scanning electron microscopy. After cyclic loading, none of the dentures showed cracks or fractures. During fracture resistance testing, all unreinforced dentures experienced complete fracture. The mesh-reinforced dentures primarily showed posterior framework fracture. Deformation of the all-metal framework caused the metal mesh-reinforced denture to exhibit the highest fracture resistance, followed by the glass fiber mesh-reinforced denture ( P <.05) and the control group ( P <.05). The glass fiber mesh-reinforced denture primarily maintained its original shape with unbroken fibers. River line pattern of the control group, dimples and interdendritic fractures of the metal mesh group, and radial fracture lines of the glass fiber group were observed on the fractured surfaces. The glass fiber mesh-reinforced denture exhibits a fracture resistance higher than that of the unreinforced denture, but lower than that of the metal mesh-reinforced denture because of the deformation of the metal mesh. The glass fiber mesh-reinforced denture maintains its shape even after fracture, indicating the possibility of easier repair.

Application of Acoustic Emission on the Characterization of Fracture in Textile Reinforced Cement Laminates

PubMed Central

Blom, J.; Wastiels, J.; Aggelis, D. G.

2014-01-01

This work studies the acoustic emission (AE) behavior of textile reinforced cementitious (TRC) composites under flexural loading. The main objective is to link specific AE parameters to the fracture mechanisms that are successively dominating the failure of this laminated material. At relatively low load, fracture is initiated by matrix cracking while, at the moment of peak load and thereafter, the fiber pull-out stage is reached. Stress modeling of the material under bending reveals that initiation of shear phenomena can also be activated depending on the shape (curvature) of the plate specimens. Preliminary results show that AE waveform parameters like frequency and energy are changing during loading, following the shift of fracturing mechanisms. Additionally, the AE behavior of specimens with different curvature is very indicative of the stress mode confirming the results of modeling. Moreover, AE source location shows the extent of the fracture process zone and its development in relation to the load. It is seen that AE monitoring yields valuable real time information on the fracture of the material and at the same time supplies valuable feedback to the stress modeling. PMID:24605050

Application of acoustic emission on the characterization of fracture in textile reinforced cement laminates.

PubMed

Blom, J; Wastiels, J; Aggelis, D G

2014-01-01

This work studies the acoustic emission (AE) behavior of textile reinforced cementitious (TRC) composites under flexural loading. The main objective is to link specific AE parameters to the fracture mechanisms that are successively dominating the failure of this laminated material. At relatively low load, fracture is initiated by matrix cracking while, at the moment of peak load and thereafter, the fiber pull-out stage is reached. Stress modeling of the material under bending reveals that initiation of shear phenomena can also be activated depending on the shape (curvature) of the plate specimens. Preliminary results show that AE waveform parameters like frequency and energy are changing during loading, following the shift of fracturing mechanisms. Additionally, the AE behavior of specimens with different curvature is very indicative of the stress mode confirming the results of modeling. Moreover, AE source location shows the extent of the fracture process zone and its development in relation to the load. It is seen that AE monitoring yields valuable real time information on the fracture of the material and at the same time supplies valuable feedback to the stress modeling.

Fatigue limits of monolithic Y-TZP three-unit-fixed dental prostheses: effect of grinding at the gingival zone of the connector

PubMed Central

Amaral, Marina; Rocha, Regina FV; Melo, Renata Marques; Pereira, Gabriel KR; Zhang, Yu; Valandro, Luiz Felipe; Bottino, Marco Antonio

2017-01-01

Objectives To determine the fatigue limits of three-unit monolithic zirconia FDPs before and after grinding of the gingival areas of connectors with diamond burs. Material and Methods FDPs were milled from pre-sintered blocks of zirconia simulating the absence of the first mandibular molar. Half of the specimens were subjected to grinding, simulating clinical adjustment, and all of them were subjected to glazing procedure. Additional specimens were manufactured for roughness analysis. FDPs were adhesively cemented onto glass-fiber reinforced epoxy resin abutments. Fatigue limits and standard deviations were obtained using a staircase fatigue method (n = 20, 100,000 loading cycles/5 Hz). The initial test load was 70% of the mean load-to-fracture (n = 3) and load increments were 5% of the initial test load for both the control and ground specimens. Data were compared by Student’s T-test (α ≤ 0.05). Results Both the control and ground groups exhibited similar values of load-to-fracture and fatigue limits. Neither the surface treatments nor ageing affected the surface roughness of the specimens. Conclusions The damage induced by grinding with fine-grit diamond bur in the gingival area of the connectors did not decrease the fatigue limit of the three-unit monolithic zirconia FDP. PMID:28494273

In vitro resistance to fracture of roots obturated with Resilon or gutta-percha.

PubMed

Monteiro, Jeanne; de Ataide, Ida de Noronha; Chalakkal, Paul; Chandra, Pavan Kumar

2011-06-01

There have been varied results from studies comparing postendodontic fracture resistance between teeth obturated with Resilon or gutta-percha. This study was performed to evaluate the fracture resistance of roots obturated by using Resilon (RealSeal system) or gutta-percha (with AH Plus sealer). Eighty extracted human mandibular single-rooted premolars stored in 10% formalin were used in the study. They were prepared by using a crown-down technique, debrided with NaOCl, ethylenediaminetetraacetic acid, and sterile water and divided into 4 groups. Obturation was performed by using the lateral condensation method. The negative control group consisted of unfilled specimens, and the positive control group consisted of those obturated with flowable, dual-cure composite resin. All root specimens were stored for 2 weeks in 100% humidity to allow complete setting of the sealer. Each specimen was mounted in acrylic in a polyvinyl ring and tested for fracture resistance with the Universal testing machine. The loading fixture of the machine was mounted with its spherical tip aligned with the center of the canal opening of each root. A vertical loading force was applied until it fractured the root. The force values were subjected to statistical analysis including analysis of variance and Fisher least significant difference testing. Teeth obturated with Resilon were more resistant to fracture than those obturated with gutta-percha. The difference was found to be highly significant (P=.00001). Resilon increased the resistance to fracture of single-rooted teeth in vitro. Copyright © 2011 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

A comprehensive update on current fixation options for two-part proximal humerus fractures: a biomechanical investigation.

PubMed

Yoon, Richard S; Dziadosz, Daniel; Porter, David A; Frank, Matthew A; Smith, Wade R; Liporace, Frank A

2014-03-01

Recent advancements in implant technology offer updated options for surgical management that have been rapidly adopted into clinical practice. The objective of this study is to biomechanically test and compare the current fixation options available for surgical fixation of two-part proximal humerus fractures and establish load to failure and stiffness values. Sixteen match-paired (32 total) fresh-frozen, cadaveric specimens were randomized to receive 1 of 4 fixation constructs following creation of an AO/OTA Type 11A3 (two-part) proximal humerus fractures. Fixation constructs tested consisted of 3.5 mm fixed angle plate (3.5-FAP), 4.5 mm fixed angle plate (4.5-FAP), humeral intramedullary nail (IMN), and a humeral intramedullary nail with a fixed angle blade (IMN-FAB). Specimen bone density was measured to ensure no adequate, non-osteoporotic bone. Constructs were tested for stiffness and ultimate load to failure and compared via one-way ANOVA analysis with subsequent post hoc Tukey HSD multiple group comparison statistical analysis. The IMN-FAB construct was significantly stiffer than the 3.5-FAP construct (123.8 vs. 23.9, p<0.0001), the 4.5-FAP construct (123.8 vs. 33.3, p<0.0001) and the IMN construct (123.8 vs. 60.1, p=0.005). The IMN-FAB construct reported a significantly higher load to failure than the 3.5-FAB construct (4667.3 N vs. 1756.9 N, p<0.0001), and the 4.5-FAP construct (4667.3 N vs. 2829.4 N, p=0.019, Table 2). The IMN construct had a significantly higher load to failure than the 3.5-FAP construct (3946.8 vs. 1756.9, p=0.001, Table 2). Biomechanical testing of modern fixation options for two-part proximal humerus fracture exhibited that the stiffest and highest load to failure construct was the IMN-FAB followed by the IMN, 3.5-FAP and then the 4.5-FAP constructs. However, prospective clinical trials with longer-term follow-up are required for definitive assessment of the ideal fixation construct for surgical management of two-part proximal humerus fractures. Copyright © 2013 Elsevier Ltd. All rights reserved.

Initiation and propagation of mixed mode fractures in granite and sandstone

NASA Astrophysics Data System (ADS)

Rück, Marc; Rahner, Roman; Sone, Hiroki; Dresen, Georg

2017-10-01

We investigate mixed mode fracture initiation and propagation in experimentally deformed granite and sandstone. We performed a series of asymmetric loading tests to induce fractures in cylindrical specimens at confining pressures up to 20 MPa. Loading was controlled using acoustic emission (AE) feedback control, which allows studying quasi-static fracture propagation for several hours. Location of acoustic emissions reveals distinct differences in spatial-temporal fracture evolution between granite and sandstone samples. Before reaching peak stress in experiments performed on granite, axial fractures initiate first at the edge of the indenter and then propagate through the entire sample. Secondary inclined fractures develop during softening of the sample. In sandstone, inclined shear fractures nucleate at peak stress and propagate through the specimen. AE source type analysis shows complex fracturing in both materials with pore collapse contributing significantly to fracture growth in sandstone samples. We compare the experimental results with numerical models to analyze stress distribution and energy release rate per unit crack surface area in the samples at different stages during fracture growth. We thereby show that for both rock types the energy release rate increases approximately linearly during fracture propagation. The study illuminates how different material properties modify fracture initiation direction under similar loading conditions.

Biomechanical Analysis of the Efficacy of Locking Plates during Cyclic Loading in Metacarpal Fractures

PubMed Central

Meffert, Rainer H.; Raschke, Michael J.; Blunk, Torsten; Ochman, Sabine

2014-01-01

Purpose. To analyse the biomechanical characteristics of locking plates under cyclic loading compared to a nonlocking plate in a diaphyseal metacarpal fracture. Methods. Oblique diaphyseal shaft fractures in porcine metacarpal bones were created in a biomechanical fracture model. An anatomical reduction and stabilization with a nonlocking and a comparable locking plate in mono- or bicortical screw fixation followed. Under cyclic loading, the displacement, and in subsequent load-to-failure tests, the maximum load and stiffness were measured. Results. For the monocortical screw fixation of the locking plate, a similar displacement, maximum load, and stiffness could be demonstrated compared to the bicortical screw fixation of the nonlocking plate. Conclusions. Locking plates in monocortical configuration may function as a useful alternative to the currently common treatment with bicortical fixations. Thereby, irritation of the flexor tendons would be avoided without compromising the stability, thus enabling the necessary early functional rehabilitation. PMID:24757429

Immediate loading of unsplinted implants in the anterior mandible for overdentures: 3-year results.

PubMed

Roe, Phillip; Kan, Joseph Y K; Rungcharassaeng, Kitichai; Lozada, Jaime L

2011-01-01

This 3-year study evaluated the implant survival rate, peri-implant tissue response, prosthetic maintenance, and prosthetic complications in a series of patients who received two immediately loaded unsplinted threaded implants to retain a mandibular overdenture. Eight completely edentulous patients were evaluated clinically and radiographically immediately after implant placement, at 3 months, and at 1, 2, and 3 years after implant placement. Data were analyzed using repeated-measures one-way analysis of variance and the Wilcoxon signed rank test at a significance level of α = .05. At 3 years, all implants remained osseointegrated (16/16), with an overall mean marginal bone change of -0.58 ± 0.39 mm and a mean Periotest value of -7.19 ± 0.54. The modified Plaque Index scores showed marked improvement in oral hygiene during the first year, but some relapse was observed thereafter. Prosthetic maintenance and complications included replacement of the attachment inserts, abutment loosening, dislodgement of the attachment housing, overdenture reline, denture tooth fracture, and overdenture base fracture. This 3-year study suggests that, despite less than ideal oral hygiene and a high incidence of complete/partial fracture of overdentures, favorable implant survival rate and peri-implant tissue responses can be achieved in mandibular overdentures retained with two immediately loaded unsplinted threaded implants.

Fracture characterization of human cortical bone under mode II loading using the end-notched flexure test.

PubMed

Silva, F G A; de Moura, M F S F; Dourado, N; Xavier, J; Pereira, F A M; Morais, J J L; Dias, M I R; Lourenço, P J; Judas, F M

2017-08-01

Fracture characterization of human cortical bone under mode II loading was analyzed using a miniaturized version of the end-notched flexure test. A data reduction scheme based on crack equivalent concept was employed to overcome uncertainties on crack length monitoring during the test. The crack tip shear displacement was experimentally measured using digital image correlation technique to determine the cohesive law that mimics bone fracture behavior under mode II loading. The developed procedure was validated by finite element analysis using cohesive zone modeling considering a trapezoidal with bilinear softening relationship. Experimental load-displacement curves, resistance curves and crack tip shear displacement versus applied displacement were used to validate the numerical procedure. The excellent agreement observed between the numerical and experimental results reveals the appropriateness of the proposed test and procedure to characterize human cortical bone fracture under mode II loading. The proposed methodology can be viewed as a novel valuable tool to be used in parametric and methodical clinical studies regarding features (e.g., age, diseases, drugs) influencing bone shear fracture under mode II loading.

Comminuted supracondylar femoral fractures: a biomechanical analysis comparing the stability of medial versus lateral plating in axial loading.

PubMed

Briffa, Nikolai; Karthickeyan, Raju; Jacob, Joshua; Khaleel, Arshad

2016-11-01

The aim of this study was to compare the biomechanical properties of medial and lateral plating of a medially comminuted supracondylar femoral fracture. A supracondylar femoral fracture model comparing two fixation methods was tested cyclically in axial loading. One-centimetre supracondylar gap osteotomies were created in six synthetic femurs approximately 6 cm proximal to the knee joint. There were two constructs investigated: group 1 and group 2 were stabilized with an 8-hole LC-DCP, medially and laterally, respectively. Both construct groups were axially loaded. Global displacement (total length), wedge displacement, bending moment and strain were measured. Medial plating showed a significantly decreased displacement, bending moment and strain at the fracture site in axial loading. Medial plating of a comminuted supracondylar femur fracture is more stable than lateral plating.

Effect of Stress Triaxiality on the Flow and Fracture of Mg Alloy AZ31

NASA Astrophysics Data System (ADS)

Kondori, Babak; Benzerga, A. Amine

2014-07-01

The microscopic damage mechanisms operating in a hot-rolled magnesium alloy AZ31B are investigated under both uniaxial and controlled triaxial loadings. Their connection to macroscopic fracture strains and fracture mode (normal vs shear) is elucidated using postmortem fractography, interrupted tests, and microscopic analysis. The fracture locus (strain-to-failure vs stress triaxiality) exhibits a maximum at moderate triaxiality, and the strain-to-failure is found to be greater in notched specimens than in initially smooth ones. A transition from twinning-induced fracture under uniaxial loading to microvoid coalescence fracture under triaxial loading is evidenced. It is argued that this transition accounts in part for the observed greater ductility in notched bars. The evolution of plastic anisotropy with stress triaxiality is also investigated. It is inferred that anisotropic plasticity at a macroscopic scale suffices to account for the observed transition in the fracture mode from flat (triaxial loading) to shear-like (uniaxial loading). Damage is found to initiate at second-phase particles and deformation twins. Fracture surfaces of broken specimens exhibit granular morphology, coarse splits, twin-sized crack traces, as well as shallow and deep dimples, in proportions that depend on the overall stress triaxiality and fracture mode. An important finding is that AZ31B has a greater tolerance to ductile damage accumulation than has been believed thus far, based on the fracture behavior in uniaxial specimens. Another finding, common to both tension and compression, is the increase in volumetric strain, the microscopic origins of which remain to be elucidated.

Effect of cyclic fatigue on the fracture toughness of Polyoxymethylene

NASA Astrophysics Data System (ADS)

Ramoa, B.; Berer, M.; Schwaiger, M.; Pinter, G.

2017-05-01

Polymers are used in a wide range of applications and their properties are dependent upon the morphological development during processing and the specimen configuration which in turn define the mechanical properties. In this context fatigue and monotonic testing are part of the standard procedure to assess relevant mechanical and material parameters to ensure a better part design. The present work addresses the performance issues of a real component made of Polyoxymethylene (POM) which is subjected to cyclic loads from intermediate levels to high peak values inside a damping mechanism. For this linear elastic fracture mechanics concepts were used to characterize the behavior of a POM homopolymer resin used in this application. Injection molded compact tension specimens, with sharp and blunt notches, were tested under a combination of cyclic and monotonic loads and the fracture surfaces were examined. The critical stress intensity factor obtained by monotonic tests was evaluated as a function of the cycle number, where an increase after the first 1000 cycles followed by a continuous decrease with higher numbers of cycles was observed. A variation of approximately 50% and 70% were obtained along the duration of the tests for the sharp and blunt notch specimens, respectively. In light of the obtained results, a discussion is presented considering the dynamic specimen compliance and the structural features observed on the fracture surfaces in combination with the fracture mechanical response.

The stability of a hip fracture determines the fatigue of an intramedullary nail.

PubMed

Eberle, S; Bauer, C; Gerber, C; von Oldenburg, G; Augat, P

2010-01-01

The purpose of this study was to address the question of how the stability of a proximal hip fracture determines the fatigue and failure mechanism of an intramedullary implant. To answer this question, mechanical experiments and finite element simulations with two different loading scenarios were conducted. The two load scenarios differed in the mechanical support of the fracture by an artificial bone sleeve, representing the femoral head and neck. The experiments confirmed that an intramedullary nail fails at a lower load in an unstable fracture situation in the proximal femur than in a stable fracture. The nails with an unstable support failed at a load 28 per cent lower than the nails with a stable support by the femoral neck. Hence, the mechanical support of a fracture is crucial to the fatigue failure of an implant. The simulation showed why the fatigue fracture of the nail starts at the aperture of the lag screw. It is the location of the highest von Mises stress, which is the failure criterion for ductile materials.

Damage accumulation of bovine bone under variable amplitude loads.

PubMed

Campbell, Abbey M; Cler, Michelle L; Skurla, Carolyn P; Kuehl, Joseph J

2016-12-01

Stress fractures, a painful injury, are caused by excessive fatigue in bone. This study on damage accumulation in bone sought to determine if the Palmgren-Miner rule (PMR), a well-known linear damage accumulation hypothesis, is predictive of fatigue failure in bone. An electromagnetic shaker apparatus was constructed to conduct cyclic and variable amplitude tests on bovine bone specimens. Three distinct damage regimes were observed following fracture. Fractures due to a low cyclic amplitude loading appeared ductile ( 4000 μ ϵ ), brittle due to high cyclic amplitude loading (> 9000 μ ϵ ), and a combination of ductile and brittle from mid-range cyclic amplitude loading (6500 -6750 μ ϵ ). Brittle and ductile fracture mechanisms were isolated and mixed, in a controlled way, into variable amplitude loading tests. PMR predictions of cycles to failure consistently over-predicted fatigue life when mixing isolated fracture mechanisms. However, PMR was not proven ineffective when used with a single damage mechanism.

Influence of preparation depths on the fracture load of customized zirconia abutments with titanium insert.

PubMed

Joo, Han-Sung; Yang, Hong-So; Park, Sang-Won; Kim, Hyun-Seung; Yun, Kwi-Dug; Ji, Min-Kyung; Lim, Hyun-Pil

2015-06-01

This study evaluated the fracture load of customized zirconia abutments with titanium insert according to preparation depths, with or without 5-year artificial aging. Thirty-six identical lithium disilicate crowns (IPS e.max press) were fabricated to replace a maxillary right central incisor and cemented to the customized zirconia abutment with titanium insert on a 4.5×10 mm titanium fixture. Abutments were fabricated with 3 preparation depths (0.5 mm, 0.7 mm, and 0.9 mm). Half of the samples were then processed using thermocycling (temperature: 5-55℃, dwelling time: 120s) and chewing simulation (1,200,000 cycles, 49 N load). All specimens were classified into 6 groups depending on the preparation depth and artificial aging (non-artificial aging groups: N5, N7, N9; artificial aging groups: A5, A7, A9). Static load was applied at 135 degrees to the implant axis in a universal testing machine. Statistical analyses of the results were performed using 1-way ANOVA, 2-way ANOVA, independent t-test and multiple linear regression. The fracture loads were 539.28 ± 63.11 N (N5), 406.56 ± 28.94 N (N7), 366.66 ± 30.19 N (N9), 392.61 ± 50.57 N (A5), 317.94 ± 30.05 N (A7), and 292.74 ± 37.15 N (A9). The fracture load of group N5 was significantly higher than those of group N7 and N9 (P<.017). Consequently, the fracture load of group A5 was also significantly higher than those of group A7 and A9 (P<.05). After artificial aging, the fracture load was significantly decreased in all groups with various preparation depths (P<.05). The fracture load of a single anterior implant restored with lithium disilicate crown on zirconia abutment with titanium insert differed depending on the preparation depths. After 5-year artificial aging, the fracture loads of all preparation groups decreased significantly.

Influence of preparation depths on the fracture load of customized zirconia abutments with titanium insert

PubMed Central

Joo, Han-Sung; Yang, Hong-So; Park, Sang-Won; Kim, Hyun-Seung; Yun, Kwi-Dug; Ji, Min-Kyung

2015-01-01

PURPOSE This study evaluated the fracture load of customized zirconia abutments with titanium insert according to preparation depths, with or without 5-year artificial aging. MATERIALS AND METHODS Thirty-six identical lithium disilicate crowns (IPS e.max press) were fabricated to replace a maxillary right central incisor and cemented to the customized zirconia abutment with titanium insert on a 4.5×10 mm titanium fixture. Abutments were fabricated with 3 preparation depths (0.5 mm, 0.7 mm, and 0.9 mm). Half of the samples were then processed using thermocycling (temperature: 5-55℃, dwelling time: 120s) and chewing simulation (1,200,000 cycles, 49 N load). All specimens were classified into 6 groups depending on the preparation depth and artificial aging (non-artificial aging groups: N5, N7, N9; artificial aging groups: A5, A7, A9). Static load was applied at 135 degrees to the implant axis in a universal testing machine. Statistical analyses of the results were performed using 1-way ANOVA, 2-way ANOVA, independent t-test and multiple linear regression. RESULTS The fracture loads were 539.28 ± 63.11 N (N5), 406.56 ± 28.94 N (N7), 366.66 ± 30.19 N (N9), 392.61 ± 50.57 N (A5), 317.94 ± 30.05 N (A7), and 292.74 ± 37.15 N (A9). The fracture load of group N5 was significantly higher than those of group N7 and N9 (P<.017). Consequently, the fracture load of group A5 was also significantly higher than those of group A7 and A9 (P<.05). After artificial aging, the fracture load was significantly decreased in all groups with various preparation depths (P<.05). CONCLUSION The fracture load of a single anterior implant restored with lithium disilicate crown on zirconia abutment with titanium insert differed depending on the preparation depths. After 5-year artificial aging, the fracture loads of all preparation groups decreased significantly. PMID:26140169

Mechanical characterization of stomach tissue under uniaxial tensile action.

PubMed

Jia, Z G; Li, W; Zhou, Z R

2015-02-26

In this article, the tensile properties of gastric wall were investigated by using biomechanical test and theoretical analysis. The samples of porcine stomach strips from smaller and greater curvature of the stomach were cut in longitudinal and circumferential direction, respectively. The loading-unloading, stress relaxation, strain creep, tensile fracture tests were performed at mucosa-submucosa, serosa-muscle and intact layer, respectively. Results showed that the biomechanical properties of the porcine stomach depended on the layers, orientations and locations of the gastric wall and presented typical viscoelastic, nonlinear and anisotropic mechanical properties. During loading-unloading test, the stress of serosa-muscle layer in the longitudinal direction was 15-20% more than that in the circumferential direction at 12% stretch ratio, while it could reach about 40% for the intact layer and 50% for the mucosa-submucosa layer. The results of stress relaxation and strain creep showed that the variation degree was obviously faster in the circumferential direction than that in the longitudinal direction, and the ultimate residual values were also different for the different layers, orientations and locations. In the process of fracture test, the serosa-muscle layer fractured firstly followed by the mucosa-submucosa layer when the intact layer was tested, the longitudinal strips firstly began to fracture and the required stress value was about twice as much as that in the circumferential strips. The anisotropy and heterogeneity of mechanical characterization of the porcine stomach were related to its complicated geometry, structure and functions. The results would help us to understand the biomechanics of soft organ tissue. Copyright © 2015 Elsevier Ltd. All rights reserved.

Biomechanical analysis on fracture risk associated with bone deformity

NASA Astrophysics Data System (ADS)

Kamal, Nur Amalina Nadiah Mustafa; Som, Mohd Hanafi Mat; Basaruddin, Khairul Salleh; Daud, Ruslizam

2017-09-01

Osteogenesis Imperfecta (OI) is a disease related to bone deformity and is also known as `brittle bone' disease. Currently, medical personnel predict the bone fracture solely based on their experience. In this study, the prediction for risk of fracture was carried out by using finite element analysis on the simulated OI bone of femur. The main objective of this research was to analyze the fracture risk of OI-affected bone with respect to various loadings. A total of 12 models of OI bone were developed by applying four load cases and the angle of deformation for each of the models was calculated. The models were differentiated into four groups, namely standard, light, mild and severe. The results show that only a small amount of load is required to increase the fracture risk of the bone when the model is tested with hopping conditions. The analysis also shows that the torsional load gives a small effect to the increase of the fracture risk of the bone.

A Biomechanical Comparison of Distal Fixation for Bridge Plating in a Distal Radius Fracture Model.

PubMed

Alluri, Ram K; Bougioukli, Sofia; Stevanovic, Milan; Ghiassi, Alidad

2017-09-01

To compare the biomechanical properties of second versus third metacarpal distal fixation when using a radiocarpal spanning distraction plate in an unstable distal radius fracture model. Biomechanical evaluation of the radiocarpal spanning distraction plate comparing second versus third metacarpal distal fixation was performed using a standardized model of an unstable wrist fracture in 10 matched-pair cadaveric specimens. Each fixation construct underwent a controlled cyclic loading protocol in flexion and extension. The resultant displacement and stiffness were calculated at the fracture site. After cyclic loading, each specimen was loaded to failure. The stiffness, maximum displacement, and load to failure were compared between the 2 groups. Cyclic loading in flexion demonstrated that distal fixation to the third metacarpal resulted in greater stiffness compared with the second metacarpal. There was no significant difference between the 2 groups with regards to maximum displacement at the fracture site in flexion. Cyclic loading in extension demonstrated no significant difference in stiffness or maximum displacement between the 2 groups. The average load to failure was similar for both groups. Fixation to the third metacarpal resulted in greater stiffness in flexion. All other biomechanical parameters were similar when comparing distal fixation to the second or third metacarpal in distal radius fractures stabilized with a spanning internal distraction plate. The treating surgeon should choose distal metacarpal fixation primarily based on fracture pattern, alignment, and soft tissue integrity. If a stiffer construct is desired, placement of the radiocarpal spanning plate at the third metacarpal is preferred. Copyright © 2017 American Society for Surgery of the Hand. Published by Elsevier Inc. All rights reserved.

Comparison of the load at fracture of Turkom-Cera to Procera AllCeram and In-Ceram all-ceramic restorations.

PubMed

AL-Makramani, Bandar M A; Razak, Abdul A A; Abu-Hassan, Mohamed I

2009-08-01

This study investigated the occlusal fracture resistance of Turkom-Cerafused alumina compared to Procera AllCeram and In-Ceram all-ceramic restorations. Sixmaster dies were duplicated from the prepared maxillary first premolar tooth using nonprecious metal alloy (Wiron 99). Ten copings of 0.6 mm thickness were fabricated from each type of ceramic, for a total of thirty copings. Two master dies were used for each group, and each of them was used to lute five copings. All groups were cemented with resin luting cement Panavia F according to manufacturer's instructions and received a static load of 5 kg during cementation. After 24 hours of distilled water storage at 37 degrees C, the copings were vertically compressed using a universal testing machine at a crosshead speed of 1 mm/min. The results of the present study showed the following mean loads at fracture: Turkom-Cera (2184 +/- 164 N), In-Ceram (2042 +/- 200 N), and Procera AllCeram (1954 +/- 211 N). ANOVA and Scheffe's post hoc test showed that the mean load at fracture of Turkom-Cera was significantly different from Procera AllCeram (p < 0.05). Scheffe's post hoc test showed no significant difference between the mean load at fracture of Turkom-Cera and In-Ceram or between the mean load at fracture of In-Ceram and Procera AllCeram. Because Turkom-Cera demonstrated equal to or higher loads at fracture than currently accepted all-ceramic materials, it would seem to be acceptable for fabrication of anterior and posterior ceramic crowns.

Spartan Release Engagement Mechanism (REM) stress and fracture analysis

NASA Technical Reports Server (NTRS)

Marlowe, D. S.; West, E. J.

1984-01-01

The revised stress and fracture analysis of the Spartan REM hardware for current load conditions and mass properties is presented. The stress analysis was performed using a NASTRAN math model of the Spartan REM adapter, base, and payload. Appendix A contains the material properties, loads, and stress analysis of the hardware. The computer output and model description are in Appendix B. Factors of safety used in the stress analysis were 1.4 on tested items and 2.0 on all other items. Fracture analysis of the items considered fracture critical was accomplished using the MSFC Crack Growth Analysis code. Loads and stresses were obtaind from the stress analysis. The fracture analysis notes are located in Appendix A and the computer output in Appendix B. All items analyzed met design and fracture criteria.

Comparison of the fracture resistances of glass fiber mesh- and metal mesh-reinforced maxillary complete denture under dynamic fatigue loading

PubMed Central

2017-01-01

PURPOSE The aim of this study was to investigate the effect of reinforcing materials on the fracture resistances of glass fiber mesh- and Cr–Co metal mesh-reinforced maxillary complete dentures under fatigue loading. MATERIALS AND METHODS Glass fiber mesh- and Cr–Co mesh-reinforced maxillary complete dentures were fabricated using silicone molds and acrylic resin. A control group was prepared with no reinforcement (n = 15 per group). After fatigue loading was applied using a chewing simulator, fracture resistance was measured by a universal testing machine. The fracture patterns were analyzed and the fractured surfaces were observed by scanning electron microscopy. RESULTS After cyclic loading, none of the dentures showed cracks or fractures. During fracture resistance testing, all unreinforced dentures experienced complete fracture. The mesh-reinforced dentures primarily showed posterior framework fracture. Deformation of the all-metal framework caused the metal mesh-reinforced denture to exhibit the highest fracture resistance, followed by the glass fiber mesh-reinforced denture (P<.05) and the control group (P<.05). The glass fiber mesh-reinforced denture primarily maintained its original shape with unbroken fibers. River line pattern of the control group, dimples and interdendritic fractures of the metal mesh group, and radial fracture lines of the glass fiber group were observed on the fractured surfaces. CONCLUSION The glass fiber mesh-reinforced denture exhibits a fracture resistance higher than that of the unreinforced denture, but lower than that of the metal mesh-reinforced denture because of the deformation of the metal mesh. The glass fiber mesh-reinforced denture maintains its shape even after fracture, indicating the possibility of easier repair. PMID:28243388

In-vitro development of a temporal abutment screw to protect osseointegration in immediate loaded implants

PubMed Central

2015-01-01

PURPOSE In this study, a temporal abutment fixation screw, designed to fracture in a controlled way upon application of an occlusal force sufficient to produce critical micromotion was developed. The purpose of the screw was to protect the osseointegration of immediate loaded single implants. MATERIALS AND METHODS Seven different screw prototypes were examined by fixing titanium abutments to 112 Mozo-Grau external hexagon implants (MG Osseous®; Mozo-Grau, S.A., Valladolid, Spain). Fracture strength was tested at 30° in two subgroups per screw: one under dynamic loading and the other without prior dynamic loading. Dynamic loading was performed in a single-axis chewing simulator using 150,000 load cycles at 50 N. After normal distribution of obtained data was verified by Kolmogorov-Smirnov test, fracture resistance between samples submitted and not submitted to dynamic loading was compared by the use of Student's t-test. Comparison of fracture resistance among different screw designs was performed by the use of one-way analysis of variance. Confidence interval was set at 95%. RESULTS Fractures occurred in all screws, allowing easy retrieval. Screw Prototypes 2, 5 and 6 failed during dynamic loading and exhibited statistically significant differences from the other prototypes. CONCLUSION Prototypes 2, 5 and 6 may offer a useful protective mechanism during occlusal overload in immediate loaded implants. PMID:25932315

Experimental Investigation on Deformation Failure Characteristics of Crystalline Marble Under Triaxial Cyclic Loading

NASA Astrophysics Data System (ADS)

Yang, Sheng-Qi; Tian, Wen-Ling; Ranjith, P. G.

2017-11-01

The deformation failure characteristics of marble subjected to triaxial cyclic loading are significant when evaluating the stability and safety of deep excavation damage zones. To date, however, there have been notably few triaxial experimental studies on marble under triaxial cyclic loading. Therefore, in this research, a series of triaxial cyclic tests was conducted to analyze the mechanical damage characteristics of a marble. The post-peak deformation of the marble changed gradually from strain softening to strain hardening as the confining pressure increased from 0 to 10 MPa. Under uniaxial compression, marble specimens showed brittle failure characteristics with a number axial splitting tensile cracks; in the range of σ 3 = 2.5-7.5 MPa, the marble specimens assumed single shear fracture characteristics with larger fracture angles of about 65°. However, at σ 3 = 10 MPa, the marble specimens showed no obvious shear fracture surfaces. The triaxial cyclic experimental results indicate that in the range of the tested confining pressures, the triaxial strengths of the marble specimens under cyclic loading were approximately equal to those under monotonic loading. With the increase in cycle number, the elastic strains of the marble specimens all increased at first and later decreased, achieving maximum values, but the plastic strains of the marble specimens increased nonlinearly. To evaluate quantitatively the damage extent of the marble under triaxial cyclic loading, a damage variable is defined according to the irreversible deformation for each cycle. The evolutions of the elastic modulus for the marble were characterized by four stages: material strengthening, material degradation, material failure and structure slippage. Based on the experimental results of the marble specimens under complex cyclic loading, the cohesion of the marble decreased linearly, but the internal friction angles did not depend on the damage extent. To describe the peak strength characteristics of the marble specimens under complex cyclic loadings with various deformation positions, a revised strength criterion for damaged rocks is offered.

Scratch Testing of Hot-Pressed Monolithic Chromium Diboride (CrB2) and CrB2 + MoSi2 Composite

NASA Astrophysics Data System (ADS)

Bhatt, B.; Murthy, T. S. R. Ch.; Singh, K.; Sashanka, A.; Vishwanadh, B.; Sonber, J. K.; Sairam, K.; Nageswara Rao, G. V. S.; Srinivasa Rao, T.; Kain, Vivekanand

2017-10-01

The tribological performance of hot-pressed monolithic CrB2 and a newly developed CrB2 + 20 vol.% MoSi2 composite was investigated by using scratch test. The test was carried out under progressive loading ranging from 0.9 to 30 N over a scratch distance of 3 mm. In situ values of coefficient of friction (COF), depth of penetration and acoustic emission were recorded. The wear volume and fracture toughness were also calculated. COF of both materials is increased with increasing the scratch length and progressive load. COF of the composite was observed to be slightly higher compared to the monolithic CrB2. The wear volume of the composite is 60% higher compared to monolithic CrB2. Fracture toughness values of 2.48 and 2.81 MPa m1/2 were calculated for monolithic CrB2 and CrB2 + 20 vol.% MoSi2 composite, respectively. Microstructural characterization indicates that the abrasive wear is the dominant wear mechanism in both the materials.

Fracture resistance and primary failure mode of endodontically treated teeth restored with a carbon fiber-reinforced resin post system in vitro.

PubMed

Raygot, C G; Chai, J; Jameson, D L

2001-01-01

This study was undertaken to characterize the fracture resistance and mode of fracture of endodontically treated incisors restored with cast post-and-core, prefabricated stainless steel post, or carbon fiber-reinforced composite post systems. Ten endodontically treated teeth restored with each technique were subjected to a compressive load delivered at a 130-degree angle to the long axis until the first sign of failure was noted. The fracture load and the mode of fracture were recorded. The failure loads registered in the three groups were not significantly different. Between 70%, and 80% of teeth from any of the three groups displayed fractures that were located above the simulated bone level. The use of carbon fiber-reinforced composite posts did not change the fracture resistance or the failure mode of endodontically treated central incisors compared to the use of metallic posts.

Double torsion fracture mechanics testing of shales under chemically reactive conditions

NASA Astrophysics Data System (ADS)

Chen, X.; Callahan, O. A.; Holder, J. T.; Olson, J. E.; Eichhubl, P.

2015-12-01

Fracture properties of shales is vital for applications such as shale and tight gas development, and seal performance of carbon storage reservoirs. We analyze the fracture behavior from samples of Marcellus, Woodford, and Mancos shales using double-torsion (DT) load relaxation fracture tests. The DT test allows the determination of mode-I fracture toughness (KIC), subcritical crack growth index (SCI), and the stress-intensity factor vs crack velocity (K-V) curves. Samples are tested at ambient air and aqueous conditions with variable ionic concentrations of NaCl and CaCl2, and temperatures up to 70 to determine the effects of chemical/environmental conditions on fracture. Under ambient air condition, KIC determined from DT tests is 1.51±0.32, 0.85±0.25, 1.08±0.17 MPam1/2 for Marcellus, Woodford, and Mancos shales, respectively. Tests under water showed considerable change of KIC compared to ambient condition, with 10.6% increase for Marcellus, 36.5% decrease for Woodford, and 6.7% decrease for Mancos shales. SCI under ambient air condition is between 56 and 80 for the shales tested. The presence of water results in a significant reduction of the SCI from 70% to 85% compared to air condition. Tests under chemically reactive solutions are currently being performed with temperature control. K-V curves under ambient air conditions are linear with stable SCI throughout the load-relaxation period. However, tests conducted under water result in an initial cracking period with SCI values comparable to ambient air tests, which then gradually transition into stable but significantly lower SCI values of 10-20. The non-linear K-V curves reveal that crack propagation in shales is initially limited by the transport of chemical agents due to their low permeability. Only after the initial cracking do interactions at the crack tip lead to cracking controlled by faster stress corrosion reactions. The decrease of SCI in water indicates higher crack propagation velocity due to faster stress corrosion rate in water than in ambient air. The experimental results are applicable for the prediction of fracture initiation based on KIC, modeling fracture pattern based on SCI, and the estimation of dynamic fracture propagation such as crack growth velocity and crack re-initiation.

The effect of screw tunnels on the biomechanical stability of vertebral body after pedicle screws removal: a finite element analysis.

PubMed

Liu, Jia-Ming; Zhang, Yu; Zhou, Yang; Chen, Xuan-Yin; Huang, Shan-Hu; Hua, Zi-Kai; Liu, Zhi-Li

2017-06-01

Posterior reduction and pedicle screw fixation is a widely used procedure for thoracic and lumbar vertebrae fractures. Usually, the pedicle screws would be removed after the fracture healing and screw tunnels would be left. The aim of this study is to evaluate the effect of screw tunnels on the biomechanical stability of the lumbar vertebral body after pedicle screws removal by finite element analysis (FEA). First, the CT values of the screw tunnels wall in the fractured vertebral bodies were measured in patients whose pedicle screws were removed, and they were then compared with the values of vertebral cortical bone. Second, an adult patient was included and the CT images of the lumbar spine were harvested. Three dimensional finite element models of the L1 vertebra with unilateral or bilateral screw tunnels were created based on the CT images. Different compressive loads were vertically acted on the models. The maximum loads which the models sustained and the distribution of the force in the different parts of the models were recorded and compared with each other. The CT values of the tunnels wall and vertebral cortical bone were 387.126±62.342 and 399.204±53.612, which were not statistically different (P=0.149). The models of three dimensional tetrahedral mesh finite element of normal lumbar 1 vertebra were established with good geometric similarity and realistic appearance. After given the compressive loads, the cortical bone was the first one to reach its ultimate stress. The maximum loads which the bilateral screw tunnels model, unilateral screw tunnel model, and normal vertebral model can sustain were 3.97 Mpa, 3.83 Mpa, and 3.78 Mpa, respectively. For the diameter of the screw tunnels, the model with a diameter of 6.5 mm could sustain the largest load. In addition, the stress distributing on the outside of the cortical bone gradually decreased as the thickness of the tunnel wall increased. Based on the FEA, pedicle screw tunnels would not decrease the biomechanical stability and strength of the vertebral body. A large diameter of screw tunnel and thick tunnel wall were helpful for the biomechanical stability of the vertebral body.

Customized fiber glass posts. Fatigue and fracture resistance.

PubMed

Costa, Rogério Goulart; De Morais, Eduardo Christiano Caregnatto; Campos, Edson Alves; Michel, Milton Domingos; Gonzaga, Carla Castiglia; Correr, Gisele Maria

2012-02-01

To evaluate the root fracture strength of human single-rooted premolars restored with customized fiberglass post-core systems after fatigue simulation. 40 human premolars had their crowns cut and the root length was standardized to 13 mm. The teeth were endodontically treated and embedded in acrylic resin. The specimens were distributed into four groups (n=10) according to the restorative material used: prefabricated fiber post (PFP), PFP+accessory fiber posts (PFPa), PFP+unidirectional fiberglass (PFPf), and unidirectional fiberglass customized post (CP). All posts were luted using resin cement and the cores were built up with a resin composite. The samples were stored for 24 hours at 37 degrees C and 100% relative humidity and then submitted to mechanical cycling. The specimens were then compressive-loaded in a universal testing machine at a crosshead speed of 0.5 mm/minute until fracture. The failure patterns were analyzed and classified. Data was submitted to one-way ANOVA and Tukey's test (alpha = 0.05). The mean values of maximum load (N) were: PFP - 811.4 +/- 124.3; PFPa - 729.2 +/- 157.2; PFPf- 747.5 +/- 204.7; CP - 762.4 +/- 110. Statistical differences were not observed among the groups. All groups showed favorable restorable failures. Fiberglass customized post did not show improved fracture resistance or differences in failure patterns when compared to prefabricated glass fiber posts.

Understanding the etiology of the posteromedial tibial stress fracture.

PubMed

Milgrom, Charles; Burr, David B; Finestone, Aharon S; Voloshin, Arkady

2015-09-01

Previous human in vivo tibial strain measurements from surface strain gauges during vigorous activities were found to be below the threshold value of repetitive cyclical loading at 2500 microstrain in tension necessary to reduce the fatigue life of bone, based on ex vivo studies. Therefore it has been hypothesized that an intermediate bone remodeling response might play a role in the development of tibial stress fractures. In young adults tibial stress fractures are usually oblique, suggesting that they are the result of failure under shear strain. Strains were measured using surface mounted unstacked 45° rosette strain gauges on the posterior aspect of the flat medial cortex just below the tibial midshaft, in a 48year old male subject while performing vertical jumps, staircase jumps and running up and down stadium stairs. Shear strains approaching 5000 microstrain were recorded during stair jumping and vertical standing jumps. Shear strains above 1250 microstrain were recorded during runs up and down stadium steps. Based on predictions from ex vivo studies, stair and vertical jumping tibial shear strain in the test subject was high enough to potentially produce tibial stress fracture subsequent to repetitive cyclic loading without necessarily requiring an intermediate remodeling response to microdamage. Copyright © 2015 Elsevier Inc. All rights reserved.

Percutaneous Dorsal Instrumentation of Vertebral Burst Fractures: Value of Additional Percutaneous Intravertebral Reposition—Cadaver Study

PubMed Central

Krüger, Antonio; Schmuck, Maya; Noriega, David C.; Ruchholtz, Steffen; Baroud, Gamal; Oberkircher, Ludwig

2015-01-01

Purpose. The treatment of vertebral burst fractures is still controversial. The aim of the study is to evaluate the purpose of additional percutaneous intravertebral reduction when combined with dorsal instrumentation. Methods. In this biomechanical cadaver study twenty-eight spine segments (T11-L3) were used (male donors, mean age 64.9 ± 6.5 years). Burst fractures of L1 were generated using a standardised protocol. After fracture all spines were allocated to four similar groups and randomised according to surgical techniques (posterior instrumentation; posterior instrumentation + intravertebral reduction device + cement augmentation; posterior instrumentation + intravertebral reduction device without cement; and intravertebral reduction device + cement augmentation). After treatment, 100000 cycles (100–600 N, 3 Hz) were applied using a servohydraulic loading frame. Results. Overall anatomical restoration was better in all groups where the intravertebral reduction device was used (p < 0.05). In particular, it was possible to restore central endplates (p > 0.05). All techniques decreased narrowing of the spinal canal. After loading, clearance could be maintained in all groups fitted with the intravertebral reduction device. Narrowing increased in the group treated with dorsal instrumentation. Conclusions. For height and anatomical restoration, the combination of an intravertebral reduction device with dorsal instrumentation showed significantly better results than sole dorsal instrumentation. PMID:26137481

The effect of aging on the fracture toughness of esthetic restorative materials.

PubMed

Bagheri, Rafat; Azar, Mohammad R; Tyas, Martin J; Burrow, Michael F

2010-06-01

To compare the fracture toughness (KIc) of tooth-colored restorative materials based on a four-point bending; to assess the effect of distilled water and a resin surface sealant (G-Coat Plus) on the resistance of the materials to fracture. Specimens were prepared from six materials: Quix Fil; Dyract (Dentsply), Freedom (SDI), Fuji VII (GC), Fuji IX (GC); Fuji II LC (GC). Fuji II LC and Fuji IX were tested both with and without applying G-Coat Plus (GC). The specimens were divided into the three groups which were conditioned in distilled water at 37 degrees C for 48 hours, 4 and 8 weeks. The specimens were loaded in a four-point bending test using a universal testing machine. The maximum load to specimen failure was recorded and the fracture toughness calculated. There were significant differences among most of the materials (P < 0.001). Quix Fil had the highest mean KIc value and Fuji VII the lowest. Immersion in distilled water for the resin composite and polyacid-modified resin composites caused a significant decrease in KIc as the time interval increased. For glass-ionomer cements, KIc decreased significantly after 4 weeks, and after 8 weeks immersion slightly increased. G-Coat Plus affected Fuji II LC positively while it had no effect on the Fuji IX.

Contact fatigue mechanisms as a function of crystal aspect ratio in baria-silicate glass ceramics

NASA Astrophysics Data System (ADS)

Suputtamongkol, Kallaya

2003-10-01

Ceramic materials are potentially useful for dental applications because of their esthetic potential and biocompatibility. However, the existence of fatigue damage in ceramics raises considerable concern regarding its effect on the life prediction of dental prostheses. During normal mastication, dental restorations are subjected to repeated loading more than a thousand times per day and relatively high clinical failure rates for ceramic prostheses have been reported. To simulate the intraoral loads, Hertzian indentation loading was used in this study to characterize the fatigue failure mechanisms of ceramic materials using clinically relevant parameters. The baria-silicate system was chosen because of the nearly identical composition between the crystal and the glass matrix. Little or no residual stress is expected from the elastic modulus and thermal expansion mismatches between the two phases. Crystallites with different aspect ratios can also be produced by controlled heat treatment schedules. The objective of this study was to characterize the effect of crystal morphology on the fatigue mechanisms of bariasilicate glass-ceramics under clinically relevant conditions. The results show that the failure of materials with a low toughness such as baria-silicate glass (0.7 MPa•m1/2) and glass-ceramic with an aspect ratio of 3/1 (1.3 MPa•m1/2) initiated from a cone crack developed during cyclic loading for 103 to 105 cycles. The mean strength values of baria-silicate glass and glass-ceramic with an aspect ratio of 3/1 decreased significantly as a result of the presence of a cone crack. Failure of baria-silicate glass-ceramics with an aspect ratio of 8/1 (Kc = 2.1 MPa•m1/2) was initiated from surface flaws caused by either polishing or cyclic loading. The gradual decrease of fracture stress was observed in specimens with an aspect ratio of 8/1 after loading in air for 103 to 10 5 cycles. A reduction of approximately 50% in fracture stress levels was found for specimens with an aspect ratio of 8/1 after loading for 10 5 cycles in deionized water. The mechanisms for cyclic fatigue crack propagation in baria-silicate glass-ceramics are similar to those observed under quasi-static loading conditions. An intergranular fracture path was observed in glass-ceramics with an aspect ratio of 3/1. For an aspect ratio of 8/1, a transgranular fracture mode was dominant.

Elastic-Plastic Fracture Mechanics Analysis of Critical Flaw Size in ARES I-X Flange-to-Skin Welds

NASA Technical Reports Server (NTRS)

Chell, G. Graham; Hudak, Stephen J., Jr.

2008-01-01

NASA's Ares 1 Upper Stage Simulator (USS) is being fabricated from welded A516 steel. In order to insure the structural integrity of these welds it is of interest to calculate the critical initial flaw size (CIFS) to establish rational inspection requirements. The CIFS is in turn dependent on the critical final flaw size (CFS), as well as fatigue flaw growth resulting from transportation, handling and service-induced loading. These calculations were made using linear elastic fracture mechanics (LEFM), which are thought to be conservative because they are based on a lower bound, so called elastic, fracture toughness determined from tests that displayed significant plasticity. Nevertheless, there was still concern that the yield magnitude stresses generated in the flange-to-skin weld by the combination of axial stresses due to axial forces, fit-up stresses, and weld residual stresses, could give rise to significant flaw-tip plasticity, which might render the LEFM results to be non-conservative. The objective of the present study was to employ Elastic Plastic Fracture Mechanics (EPFM) to determine CFS values, and then compare these values to CFS values evaluated using LEFM. CFS values were calculated for twelve cases involving surface and embedded flaws, EPFM analyses with and without plastic shakedown of the stresses, LEFM analyses, and various welding residual stress distributions. For the cases examined, the computed CFS values based on elastic analyses were the smallest in all instances where the failures were predicted to be controlled by the fracture toughness. However, in certain cases, the CFS values predicted by the elastic-plastic analyses were smaller than those predicted by the elastic analyses; in these cases the failure criteria were determined by a breakdown in stress intensity factor validity limits for deep flaws (a greater than 0.90t), rather than by the fracture toughness. Plastic relaxation of stresses accompanying shakedown always increases the calculated CFS values compared to the CFS values determined without shakedown. Thus, it is conservative to ignore shakedown effects.

Stability of radial head and neck fractures: a biomechanical study of six fixation constructs with consideration of three locking plates.

PubMed

Burkhart, Klaus Josef; Mueller, Lars P; Krezdorn, David; Appelmann, Philipp; Prommersberger, Karl J; Sternstein, Werner; Rommens, Pol M

2007-12-01

Open reduction and internal fixation of radial neck fractures can lead to secondary loss of reduction and nonunion due to insufficient stability. Nevertheless, there are only a few biomechanical studies about the stability achieved by different osteosynthesis constructs. Forty-eight formalin-fixed, human proximal radii were divided into 6 groups according to their bone density (measured by dual-energy x-ray absorptiometry). A 2.7-mm gap osteotomy was performed to simulate an unstable radial neck fracture, which was fixed with 3 nonlocking implants: a 2.4-mm T plate, a 2.4-mm blade plate, and 2.0-mm crossed screws, and 3 locking plates: a 2.0-mm LCP T plate, a 2.0-mm 6x2 grid plate, and a 2.0-mm radial head plate. Implants were tested under axial (N/mm) and torsional (Ncm/ degrees ) loads with a servohydraulic materials testing machine. The radial head plate was significantly stiffer than all other implants under axial as well as under torsional loads, with values of 36 N/mm and 13 Ncm/ degrees . The second-stiffest implant was the blade plate, with values of 20 N/mm and 6 Ncm/ degrees . The weakest implants were the 2.0-mm LCP, with values of 6 N/mm and 2 Ncm/ degrees , and the 2.0-mm crossed screws, with values of 18 N/mm and 2 Ncm/ degrees . The 2.4-mm T plate, with values of 14 N/mm and 4 Ncm/ degrees , and the 2.0-mm grid plate, with values of 8 N/mm and 4 Ncm/ degrees came to lie in the midfield. The 2.0-mm angle-stable plates-depending on their design-allow fixation with comparable or even higher stability than the bulky 2.4-mm nonlocking implants and 2.0-mm crossed screws.

Effect of Embedded Piezoelectric Sensors on Fracture Toughness and Fatigue Resistance of Composite Laminates Under Mode I Loading

NASA Technical Reports Server (NTRS)

Murri, Gretchen B.

2006-01-01

Double-cantilevered beam (DCB) specimens of a glass/epoxy composite material with embedded piezoelectric sensors were tested both statically and under fatigue loading to determine the effect of the embedded material on the Mode I fracture toughness and fatigue resistance compared to baseline data without the embedded elements. A material known as LaRC-Macrofiber Composite (LaRC-MFC (TradeMark)), or MFC, was embedded at the midplane of the specimen during the layup. Specimens were manufactured with the embedded MFC material either at the loaded end of the specimen to simulate an initial delamination; or with the MFC material located at the delaminating interface, with a Teflon film at the loaded end to simulate an initial delamination. There were three types of specimens with the embedded material at the delaminating interface: co-cured with no added adhesive; cured with a paste adhesive applied to the embedded element; or cured with a film adhesive added to the embedded material. Tests were conducted with the sensors in both the passive and active states. Results were compared to baseline data for the same material without embedded elements. Interlaminar fracture toughness values (G(sub Ic)) for the passive condition showed little change when the MFC was at the insert end. Passive results varied when the MFC was at the delaminating interface. For the co-cured case and with the paste adhesive, G(sub Ic) decreased compared to the baseline toughness, whereas, for the film adhesive case, G(sub Ic) was significantly greater than the baseline toughness, but the failure was always catastrophic. When the MFC was in the active state, G(sub Ic) was generally lower compared to the passive results. Fatigue tests showed little effect of the embedded material whether it was active or passive compared to baseline values.

Evaluation of bone surrogates for indirect and direct ballistic fractures.

PubMed

Bir, Cynthia; Andrecovich, Chris; DeMaio, Marlene; Dougherty, Paul J

2016-04-01

The mechanism of injury for fractures to long bones has been studied for both direct ballistic loading as well as indirect. However, the majority of these studies have been conducted on both post-mortem human subjects (PMHS) and animal surrogates which have constraints in terms of storage, preparation and testing. The identification of a validated bone surrogate for use in forensic, medical and engineering testing would provide the ability to investigate ballistic loading without these constraints. Two specific bone surrogates, Sawbones and Synbone, were evaluated in comparison to PMHS for both direct and indirect ballistic loading. For the direct loading, the mean velocity to produce fracture was 121 ± 19 m/s for the PMHS, which was statistically different from the Sawbones (140 ± 7 m/s) and Synbone (146 ± 3 m/s). The average distance to fracture in the indirect loading was .70 cm for the PMHS. The Synbone had a statistically similar average distance to fracture (.61 cm, p=0.54) however the Sawbones average distance to fracture was statistically different (.41 cm, p<0.05). Fractures patterns were found to be comparable to the PMHS for tests conducted with Synbones, however the input parameters were slightly varied to produce similar results. The fractures patterns with the Sawbones were not found to be as comparable to the PMHS. An ideal bone surrogate for ballistic testing was not identified and future work is warranted. Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.

Effect of remaining coronal structure on the resistance to fracture of crowned endodontically treated maxillary first premolars.

PubMed

Nissan, Joseph; Barnea, Eitan; Bar Hen, Doron; Assif, David

2008-09-01

Endodontically treated maxillary first premolars present a restorative challenge. The objective of the present study was to assess the resistance to fracture of crowned endodontically treated maxillary first premolars under simulated occlusal load, while preserving various degrees of remaining coronal structure. The study consisted of 50 intact maxillary first premolars with bifurcated roots and similar root diameter and length, randomly divided into 5 equal experimental groups. All dowels were luted with Flexi-Flow titanium-reinforced composite resin cement. TiCore titanium-reinforced composite resin was used to fabricate the core. Complete cast crowns were fabricated and cemented with zinc phosphate cement. Forces at fracture and mode of failure were recorded. Statistically significant differences (P < .05) were found among mean failure forces for all tested groups in their resistance to fracture under load with the Kruskal-Wallias test and among all combinations of the 5 groups (Z = -1.56/-2.34; P > .05) with the Mann-Whitney test. This indicates that crowned maxillary first premolars with varying degrees of remaining coronal structure differ significantly in their resistance to fracture under occlusal load. There was increased protection against fracture under occlusal loads with more remaining tooth structure. Within the limitations of this study, remaining coronal structure influenced the fracture resistance of crowned endodontically treated maxillary first premolars. Preservation of tooth structure is important for its protection against fracture under occlusal loads and may influence the tooth prognosis.

Quantification of Skeletal Blood Flow and Fluoride Metabolism in Rats using PET in a Pre-Clinical Stress Fracture Model

PubMed Central

Tomlinson, Ryan E.; Silva, Matthew J.; Shoghi, Kooresh I.

2013-01-01

Purpose Blood flow is an important factor in bone production and repair, but its role in osteogenesis induced by mechanical loading is unknown. Here, we present techniques for evaluating blood flow and fluoride metabolism in a pre-clinical stress fracture model of osteogenesis in rats. Procedures Bone formation was induced by forelimb compression in adult rats. 15O water and 18F fluoride PET imaging were used to evaluate blood flow and fluoride kinetics 7 days after loading. 15O water was modeled using a one-compartment, two-parameter model, while a two-compartment, three-parameter model was used to model 18F fluoride. Input functions were created from the heart, and a stochastic search algorithm was implemented to provide initial parameter values in conjunction with a Levenberg–Marquardt optimization algorithm. Results Loaded limbs are shown to have a 26% increase in blood flow rate, 113% increase in fluoride flow rate, 133% increase in fluoride flux, and 13% increase in fluoride incorporation into bone as compared to non-loaded limbs (p < 0.05 for all results). Conclusions The results shown here are consistent with previous studies, confirming this technique is suitable for evaluating the vascular response and mineral kinetics of osteogenic mechanical loading. PMID:21785919

Influence of water immersion on the mechanical properties of fiber posts.

PubMed

Komada, Wataru; Inagaki, Tasuku; Ueda, Yoji; Omori, Satoshi; Hosaka, Keiichi; Tagami, Junji; Miura, Hiroyuki

2017-01-01

The purpose of this study was to evaluate the influence of water immersion on the mechanical properties of three kinds of glass fiber posts and the fracture resistance of structures using resin composites with glass fiber posts. Each post was divided into three groups; a control group and two water immersion groups (30 and 90 days). Flexural strength was determined by three-point bending test. Each structure was divided into two groups; a control group and a water immersion group for 30 days. The fracture strength of structures was determined by a static loading test. In the flexural strength, two kinds of post in water immersion groups showed lower values than control groups. In the fracture strength, two kinds of structures in water immersion group showed lower values than control groups. The prefabricated glass fiber posts and structures using resin composites with glass fiber posts were affected by water immersion. Copyright © 2016 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.

Acoustic emission from a growing crack

NASA Technical Reports Server (NTRS)

Jacobs, Laurence J.

1989-01-01

An analytical method is being developed to determine the signature of an acoustic emission waveform from a growing crack and the results of this analysis are compared to experimentally obtained values. Within the assumptions of linear elastic fracture mechanics, a two dimensional model is developed to examine a semi-infinite crack that, after propagating with a constant velocity, suddenly stops. The analytical model employs an integral equation method for the analysis of problems of dynamic fracture mechanics. The experimental procedure uses an interferometric apparatus that makes very localized absolute measurements with very high fidelity and without acoustically loading the specimen.

[Increasing the safety of ceramic femoral heads for hip prostheses].

PubMed

Willmann, G; Pfaff, H G; Richter, H G

1995-12-01

Since 1974 Biolox ceramic femoral ball heads have been used successfully for artificial modular hip joints. The revision rate due to ball head fracture is lower than 0.02%. This is an extremely low value. In this article it is shown how the safety of a ceramic ball head can be improved using the procedures of HIPing, engraving by laser technique, and 100% proof testing. By applying these means the materials properties density, grain size, grain size distribution, and the strength of the ball head, i.e. the fracture load can be improved significantly.

Fracture and contact problems for an elastic wedge

NASA Technical Reports Server (NTRS)

Erdogan, F.; Arin, K.

1974-01-01

The plane elastostatic contact problem for an infinite elastic wedge of arbitrary angle is discussed. The medium is loaded through a frictionless rigid wedge of a given symmetric profile. Using the Mellin transform formulation the mixed boundary value problem is reduced to a singular integral equation with the contact stress as the unknown function. With the application of the results to the fracture of the medium in mind, the main emphasis in the study has been on the investigation of the singular nature of the stress state around the apex of the wedge and on the determination of the contact pressure.

Fracture and contact problems for an elastic wedge

NASA Technical Reports Server (NTRS)

Erdogan, F.; Arin, K.

1976-01-01

The paper deals with the plane elastostatic contact problem for an infinite elastic wedge of arbitrary angle. The medium is loaded through a frictionless rigid wedge of a given symmetric profile. Using the Mellin transform formulation the mixed boundary value problem is reduced to a singular integral equation with the contact stress as the unknown function. With the application of the results to the fracture of the medium in mind, the main emphasis in the study has been on the investigation of the singular nature of the stress state around the apex of the wedge and on the determination of the contact pressure.

Fracture loads and failure modes of customized and non-customized zirconia abutments.

PubMed

Moris, Izabela Cristina Maurício; Chen, Yung-Chung; Faria, Adriana Cláudia Lapria; Ribeiro, Ricardo Faria; Fok, Alex Sui-Lun; Rodrigues, Renata Cristina Silveira

2018-05-05

This study aimed to evaluate the fracture load and pattern of customized and non-customized zirconia abutments with Morse-taper connection. 18 implants were divided into 3 groups according to the abutments used: Zr - with non-customized zirconia abutments; Zrc - with customized zirconia abutments; and Ti - with titanium abutments. To test their load capacity, a universal test machine with a 500-kgf load cell and a 0.5-mm/min speed were used. After, one implant-abutment assembly from each group was analyzed by Scanning Electron Microscopy (SEM). For fractographic analysis, the specimens were transversely sectioned above the threads of the abutment screw in order to examine their fracture surfaces using SEM. A significant difference was noted between the groups (Zr=573.7±11.66N, Zrc=768.0±8.72N and Ti=659.1±7.70N). Also, the zirconia abutments fractured while the titanium abutments deformed plastically. Zrc presented fracture loads significantly higher than Zr (p=0.009). All the zirconia abutments fractured below the implant platform, starting from the area of contact between the abutment and implant and propagating to the internal surface of the abutment. All the zirconia abutments presented complete cleavage in the mechanical test. Fractography detected differences in the position and pattern of fracture between the two groups with zirconia abutments, probably because of the different diameters in the transmucosal region. Customization of zirconia abutments did not affect their fracture loads, which were comparable to that of titanium and much higher than the maximum physiological limit for the anterior region of the maxilla. Copyright © 2018 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

The Failure Envelope Concept Applied To The Bone-Dental Implant System.

PubMed

Korabi, R; Shemtov-Yona, K; Dorogoy, A; Rittel, D

2017-05-17

Dental implants interact with the jawbone through their common interface. While the implant is an inert structure, the jawbone is a living one that reacts to mechanical stimuli. Setting aside mechanical failure considerations of the implant, the bone is the main component to be addressed. With most failure criteria being expressed in terms of stress or strain values, their fulfillment can mean structural flow or fracture. However, in addition to those effects, the bony structure is likely to react biologically to the applied loads by dissolution or remodeling, so that additional (strain-based) criteria must be taken into account. While the literature abounds in studies of particular loading configurations, e.g. angle and value of the applied load to the implant, a general study of the admissible implant loads is still missing. This paper introduces the concept of failure envelopes for the dental implant-jawbone system, thereby defining admissible combinations of vertical and lateral loads for various failure criteria of the jawbone. Those envelopes are compared in terms of conservatism, thereby providing a systematic comparison of the various failure criteria and their determination of the admissible loads.

Less-invasive stabilization of rib fractures by intramedullary fixation: a biomechanical evaluation.

PubMed

Bottlang, Michael; Helzel, Inga; Long, William; Fitzpatrick, Daniel; Madey, Steven

2010-05-01

This study evaluated intramedullary fixation of rib fractures with Kirschner wires and novel ribs splints. We hypothesized that rib splints can provide equivalent fixation strength while avoiding complications associated with Kirschner wires, namely wire migration and cutout. The durability, strength, and failure modes of rib fracture fixation with Kirschner wires and rib splints were evaluated in 22 paired human ribs. First, intact ribs were loaded to failure to determine their strength. After fracture fixation with Kirschner wires and rib splints, fixation constructs were dynamically loaded to 360,000 cycles at five times the respiratory load to determine their durability. Finally, constructs were loaded to failure to determine residual strength and failure modes. All constructs sustained dynamic loading without failure. Dynamic loading caused three times more subsidence in Kirschner wire constructs (1.2 mm +/- 1.4 mm) than in rib splint constructs (0.4 mm +/- 0.2 mm, p = 0.09). After dynamic loading, rib splint constructs remained 48% stronger than Kirschner wire constructs (p = 0.001). Five of 11 Kirschner wire constructs failed catastrophically by cutting through the medial cortex, leading to complete loss of stability and wire migration through the lateral cortex. The remaining six constructs failed by wire bending. Rib splint constructs failed by development of fracture lines along the superior and interior cortices. No splint construct failed catastrophically, and all splint constructs retained functional reduction and fixation. Because of their superior strength and absence of catastrophic failure mode, rib splints can serve as an attractive alternative to Kirschner wires for intramedullary stabilization of rib fractures, especially in the case of posterior rib fractures where access for plating is limited.

Modeling of orthotropic plate fracture under impact load using various strength criteria

NASA Astrophysics Data System (ADS)

Radchenko, Andrey; Krivosheina, Marina; Kobenko, Sergei; Radchenko, Pavel; Grebenyuk, Grigory

2017-01-01

The paper presents the comparative analysis of various tensor multinomial criteria of strength for modeling of orthotropic organic plastic plate fracture under impact load. Ashkenazi, Hoffman and Wu strength criteria were used. They allowed fracture modeling of orthotropic materials with various compressive and tensile strength properties. The modeling of organic plastic fracture was performed numerically within the impact velocity range of 700-1500 m/s.

Accuracy of specimen-specific nonlinear finite element analysis for evaluation of distal radius strength in cadaver material.

PubMed

Matsuura, Yusuke; Kuniyoshi, Kazuki; Suzuki, Takane; Ogawa, Yasufumi; Sukegawa, Koji; Rokkaku, Tomoyuki; Takahashi, Kazuhisa

2014-11-01

Distal radius fracture, which often occurs in the setting of osteoporosis, can lead to permanent deformity and disability. Great effort has been directed toward developing noninvasive methods for evaluating the distal radius strength, with the goal of assessing fracture risk. The aim of this study was to evaluate distal radius strength using a finite element model and to gauge the accuracy of finite element model measurement using cadaver material. Ten wrists were obtained from cadavers with a mean age of 89.5 years at death. CT images of each wrist in an extended position were obtained. CT-based finite element models were prepared with Mechanical Finder software. Fracture on the models was simulated by applying a mechanical load to the palm in a direction parallel to the forearm axis, after which the fracture load and the site at which the fracture began were identified. For comparison, the wrists were fractured using a universal testing machine and the fracture load and the site of fracture were identified. The fracture load was 970.9 N in the finite element model group and 990.0 N in the actual measurement group. The site of the initial fracture was extra-articular to the distal radius in both groups. The finite element model was predictive for distal radius fracture when compared to the actual measurement. In this study, a finite element model for evaluation of distal radius strength was validated and can be used to predict fracture risk. We conclude that a finite element model is useful for the evaluation of distal radius strength. Knowing distal radius strength might avoid distal radius fracture because appropriate antiosteoporotic treatment can be initiated.

Robust QCT/FEA Models of Proximal Femur Stiffness and Fracture Load During a Sideways Fall on the Hip

PubMed Central

Dragomir-Daescu, Dan; Buijs, Jorn Op Den; McEligot, Sean; Dai, Yifei; Entwistle, Rachel C.; Salas, Christina; Melton, L. Joseph; Bennet, Kevin E.; Khosla, Sundeep; Amin, Shreyasee

2013-01-01

Clinical implementation of quantitative computed tomography-based finite element analysis (QCT/FEA) of proximal femur stiffness and strength to assess the likelihood of proximal femur (hip) fractures requires a unified modeling procedure, consistency in predicting bone mechanical properties, and validation with realistic test data that represent typical hip fractures, specifically, a sideways fall on the hip. We, therefore, used two sets (n = 9, each) of cadaveric femora with bone densities varying from normal to osteoporotic to build, refine, and validate a new class of QCT/FEA models for hip fracture under loading conditions that simulate a sideways fall on the hip. Convergence requirements of finite element models of the first set of femora led to the creation of a new meshing strategy and a robust process to model proximal femur geometry and material properties from QCT images. We used a second set of femora to cross-validate the model parameters derived from the first set. Refined models were validated experimentally by fracturing femora using specially designed fixtures, load cells, and high speed video capture. CT image reconstructions of fractured femora were created to classify the fractures. The predicted stiffness (cross-validation R2 = 0.87), fracture load (cross-validation R2 = 0.85), and fracture patterns (83% agreement) correlated well with experimental data. PMID:21052839

On the interfacial fracture of porcelain/zirconia and graded zirconia dental structures.

PubMed

Chai, Herzl; Lee, James J-W; Mieleszko, Adam J; Chu, Stephen J; Zhang, Yu

2014-08-01

Porcelain fused to zirconia (PFZ) restorations are widely used in prosthetic dentistry. However, their susceptibility to fracture remains a practical problem. The failure of PFZ prostheses often involves crack initiation and growth in the porcelain, which may be followed by fracture along the porcelain/zirconia (P/Z) interface. In this work, we characterized the process of fracture in two PFZ systems, as well as a newly developed graded glass-zirconia structure with emphases placed on resistance to interfacial cracking. Thin porcelain layers were fused onto Y-TZP plates with or without the presence of a glass binder. The specimens were loaded in a four-point-bending fixture with the thin porcelain veneer in tension, simulating the lower portion of the connectors and marginal areas of a fixed dental prosthesis (FDP) during occlusal loading. The evolution of damage was observed by a video camera. The fracture was characterized by unstable growth of cracks perpendicular to the P/Z interface (channel cracks) in the porcelain layer, which was followed by stable cracking along the P/Z interface. The interfacial fracture energy GC was determined by a finite-element analysis taking into account stress-shielding effects due to the presence of adjacent channel cracks. The resulting GC was considerably less than commonly reported values for similar systems. Fracture in the graded Y-TZP samples occurred via a single channel crack at a much greater stress than for PFZ. No delamination between the residual glass layer and graded zirconia occurred in any of the tests. Combined with its enhanced resistance to edge chipping and good esthetic quality, graded Y-TZP emerges as a viable material concept for dental restorations. Copyright © 2014 Acta Materialia Inc. All rights reserved.

Fracture behavior of hybrid composite laminates

NASA Technical Reports Server (NTRS)

Kennedy, J. M.

1983-01-01

The tensile fracture behavior of 15 center-notched hybrid laminates was studied. Three basic laminate groups were tested: (1) a baseline group with graphite/epoxy plies, (2) a group with the same stacking sequence but where the zero-deg plies were one or two plies of S-glass or Kevlar, and (3) a group with graphite plies but where the zero-deg plies were sandwiched between layers of perforated Mylar. Specimens were loaded linearly with time; load, far field strain, and crack opening displacement (COD) were monitored. The loading was stopped periodically and the notched region was radiographed to reveal the extent and type of damage (failure progression). Results of the tests showed that the hybrid laminates had higher fracture toughnesses than comparable all-graphite laminates. The higher fracture toughness was due primarily to the larger damage region at the ends of the slit; delamination and splitting lowered the stress concentration in the primary load-carrying plies. A linear elastic fracture analysis, which ignored delamination and splitting, underestimated the fracture toughness. For almost all of the laminates, the tests showed that the fracture toughness increased with crack length. The size of the damage region at the ends of the slit and COD measurements also increased with crack length.

Application of Fracture Mechanics to Specify the Proof Load Factor for Clamp Band Systems of Launch Vehicles

NASA Astrophysics Data System (ADS)

Singaravelu, J.; Sundaresan, S.; Nageswara Rao, B.

2013-04-01

This article presents a methodology for evaluation of the proof load factor (PLF) for clamp band system (CBS) made of M250 Maraging steel following fracture mechanics principles.CBS is most widely used as a structural element and as a separation system. Using Taguchi's design of experiments and the response surface method (RSM) the compact tension specimens were tested to establish an empirical relation for the failure load ( P max) in terms of the ultimate strength, width, thickness, and initial crack length. The test results of P max closely matched with the developed RSM empirical relation. Crack growth rates of the maraging steel in different environments were examined. Fracture strength (σf) of center surface cracks and through-crack tension specimens are evaluated utilizing the fracture toughness ( K IC). Stress induced in merman band at flight loading conditions is evaluated to estimate the higher load factor and PLF. Statistical safety factor and reliability assessments were made for the specified flaw sizes useful in the development of fracture control plan for CBS of launch vehicles.

Patellar fixation protected with a load-sharing cable: a mechanical and clinical study.

PubMed

Perry, C R; McCarthy, J A; Kain, C C; Pearson, R L

1988-01-01

The stability of patellar fracture fixation protected with a load-sharing cable was studied in cadavers. A transverse patellar osteotomy was produced and stabilized with standard patellar fixation with or without a figure-of-eight cable that extends from the proximal pole of the patella to the tibial tubercle. Standard fixation techniques (interfragmentary cancellous screws or modified tension-band wiring) alone failed after significantly fewer cycles of flexion and extension than did the same fixation when supplemented with a load-sharing cable. In the clinical evaluation of the load-sharing cable, 14 consecutive patients with displaced patellar fractures were treated. No immobilization was used and the patients were started on passive and active range of motion and weight-bearing ambulation in the early postoperative period. Thirteen fractures healed uneventfully. The increased stability of patellar fracture fixation protected with a load-sharing cable offers three advantages: (a) adjunctive casting is unnecessary, (b) comminuted fractures can be "pieced" together anatomically with less concern for loss of fixation, and (c) early postoperative passive and active range of motion can be achieved.

Fatigue limit of monolithic Y-TZP three-unit-fixed dental prostheses: Effect of grinding at the gingival zone of the connector.

PubMed

Amaral, Marina; Villefort, Regina F; Melo, Renata Marques; Pereira, Gabriel K R; Zhang, Yu; Valandro, Luiz Felipe; Bottino, Marco Antonio

2017-08-01

To determine the fatigue limits of three-unit monolithic zirconia fixed dental prosthesis (FDPs) before and after grinding of the gingival areas of connectors with diamond burs. FDPs were milled from pre-sintered blocks of zirconia simulating the absence of the first mandibular molar. Half of the specimens were subjected to grinding, simulating clinical adjustment, and all of them were subjected to glazing procedure. Additional specimens were manufactured for roughness analysis. FDPs were adhesively cemented onto glass-fiber reinforced epoxy resin abutments. Fatigue limits and standard deviations were obtained using a staircase fatigue method (n=20, 100,000 loading cycles/5Hz). The initial test load was 70% of the mean load-to-fracture (n=3) and load increments were 5% of the initial test load for both the control and ground specimens. Data were compared by Student's T-test (α≤0.05). Both the control and ground groups exhibited similar values of load-to-fracture and fatigue limits. Neither the surface treatments nor ageing affected the surface roughness of the specimens. The damage induced by grinding with fine-grit diamond bur in the gingival area of the connectors did not decrease the fatigue limit of the three-unit monolithic zirconia FDP. Copyright © 2017 Elsevier Ltd. All rights reserved.

Biomechanical comparison of three different plate configurations for comminuted clavicle midshaft fracture fixation.

PubMed

Uzer, Gokcer; Yildiz, Fatih; Batar, Suat; Bozdag, Ergun; Kuduz, Hacer; Bilsel, Kerem

2017-12-01

The aim of this study was to compare the fixation rigidity of anterior, anterosuperior, and superior plates in the treatment of comminuted midshaft clavicle fractures. Six-hole titanium alloy plates were produced according to anatomic features of fourth-generation artificial clavicle models for anterior (group I; n = 14), anterosuperior (group II; n = 14), and superior (group III; n = 14) fixation. After plate fixation, 5-mm segments were resected from the middle third of each clavicle to create comminuted fracture models. Half the models from each group were tested under rotational forces; the other half were tested under 3-point bending forces. Failure modes, stiffness values, and failure loads were recorded. All models fractured at the level of the distalmost screw during the failure torque, whereas several failure modes were observed in 3-point bending tests. The mean stiffness values of groups I to III were 636 ± 78, 767 ± 72, and 745 ± 214 N ∙ mm/deg (P = .171), respectively, for the torsional tests and 38 ± 5, 20 ± 3, and 13 ± 2 N/mm, respectively, for the bending tests (P < .001 for group I vs. groups II and III; P = .015 for group II vs. group III). The mean failure torque values of groups I to III were 8248 ± 2325, 12,638 ± 1749, and 10,643 ± 1838 N ∙ mm (P = .02 for group I vs. II), respectively, and the mean failure loads were 409 ± 81, 360 ± 122, and 271 ± 87 N, respectively (P = .108). In the surgical treatment of comminuted midshaft clavicle fractures, the fixation strength of anterosuperior plating was greater than that of anterior plating under rotational forces and similar to that of superior plating. Copyright © 2017 Journal of Shoulder and Elbow Surgery Board of Trustees. Published by Elsevier Inc. All rights reserved.

Influence of surface treatment and cyclic loading on the durability of repaired all-ceramic crowns

PubMed Central

ATTIA, Ahmed

2010-01-01

Objective This study investigated the durability of repaired all-ceramic crowns after cyclic loading. Material and methods Eighty In-ceram zirconia crowns were fabricated to restore prepared maxillary premolars. Resin cement was used for cementation of crowns. Palatal cusps were removed to simulate fracture of veneering porcelain and divided into 4 groups (n = 20). Fracture site was treated before repair as follows: roughening with diamond bur, (DB); air abrasion using 50 µm Al2O3, (AA) and silica coating using Cojet system followed by silane application, (SC). Control group (CG) 20 specimens were left without fracture. Palatal cusps were repaired using composite resin. Specimens were stored in water bath at 37°C for one week. Ten specimens of each group were subjected to cyclic loading. Fracture load (N) was recorded for each specimen using a universal testing machine. Two-way analysis of variance (ANOVA) and Tukey honestly significant difference (HSD) test (α=.05) were used for statistical analysis. Results There was statistically significant difference between control and tested groups, (p<0.001). Post Hoc analysis with the Tukey HSD test showed that cyclic loading fatigue significantly decreased means fracture load of control and test groups as follows (CG, 950.4±62.6 / 872.3±87.4, P = 0.0004), (DB, 624.2 ±38 / 425.5± 31.7, P <.001), (AA, 711.5 ±15.5 / 490 ± 25.2, p <0.001) and (SC, 788.7 ± 18.1 / 610.2 ± 25.2, P <.001), while silica coating and silane application significantly increased fracture load of repaired crowns (p<0.05). Conclusion Repair of fractured Inceram zirconia crowns after chairside treatment of the fracture site by silica coating and silane application could improve longevity of repaired In-ceram zirconia crowns. PMID:20485932

Full title: Biomechanical comparison between stainless steel, titanium and carbon-fiber reinforced polyetheretherketone volar locking plates for distal radius fractures.

PubMed

Mugnai, Raffaele; Tarallo, Luigi; Capra, Francesco; Catani, Fabio

2018-05-25

As the popularity of volar locked plate fixation for distal radius fractures has increased, so have the number and variety of implants, including variations in plate design, the size and angle of the screws, the locking screw mechanism, and the material of the plates. carbon-fiber reinforced polyetheretherketone (CFR-PEEK) plate features similar biomechanical properties to metallic plates, representing, therefore, an optimal alternative for the treatment of distal radius fractures. three different materials-composed plates were evaluated: stainless steel volar lateral column (Zimmer); titanium DVR (Hand Innovations); CFR-PEEK DiPHOS-RM (Lima Corporate). Six plates for each type were implanted in sawbones and an extra-articular rectangular osteotomy was created. Three plates for each material were tested for load to failure and bending stiffness in axial compression. Moreover, 3 constructs for each plate were evaluated after dynamically loading for 6000 cycles of fatigue. the mean bending stiffness pre-fatigue was significantly higher for the stainless steel plate. The titanium plate yielded the higher load to failure both pre and post fatigue. After cyclic loading, the bending stiffness increased by a mean of 24% for the stainless steel plate; 33% for the titanium; and 17% for the CFR-PEEK plate. The mean load to failure post-fatigue increased by a mean of 10% for the stainless steel and 14% for CFR-PEEK plates, whereas it decreased (-16%) for the titanium plate. Statistical analysis between groups reported significant values (p <.001) for all comparisons except for Hand Innovations vs. Zimmer bending stiffness post fatigue (p = .197). the significant higher load to failure of the titanium plate, makes it indicated for patients with higher functional requirements or at higher risk of trauma in the post-operative period. The CFR-PEEK plate showed material-specific disadvantages, represented by little tolerance to plastic deformation, and lower load to failure. N/A. Copyright © 2018. Published by Elsevier Masson SAS.

Numerical Evaluation of Mode 1 Stress Intensity Factor as a Function of Material Orientation For BX-265 Foam Insulation Material

NASA Technical Reports Server (NTRS)

Knudsen, Erik; Arakere, Nagaraj K.

2006-01-01

Foam; a cellular material, is found all around us. Bone and cork are examples of biological cell materials. Many forms of man-made foam have found practical applications as insulating materials. NASA uses the BX-265 foam insulation material on the external tank (ET) for the Space Shuttle. This is a type of Spray-on Foam Insulation (SOFI), similar to the material used to insulate attics in residential construction. This foam material is a good insulator and is very lightweight, making it suitable for space applications. Breakup of segments of this foam insulation on the shuttle ET impacting the shuttle thermal protection tiles during liftoff is believed to have caused the space shuttle Columbia failure during re-entry. NASA engineers are very interested in understanding the processes that govern the breakup/fracture of this complex material from the shuttle ET. The foam is anisotropic in nature and the required stress and fracture mechanics analysis must include the effects of the direction dependence on material properties. Material testing at NASA MSFC has indicated that the foam can be modeled as a transversely isotropic material. As a first step toward understanding the fracture mechanics of this material, we present a general theoretical and numerical framework for computing stress intensity factors (SIFs), under mixed-mode loading conditions, taking into account the material anisotropy. We present mode I SIFs for middle tension - M(T) - test specimens, using 3D finite element stress analysis (ANSYS) and FRANC3D fracture analysis software, developed by the Cornel1 Fracture Group. Mode I SIF values are presented for a range of foam material orientations. Also, NASA has recorded the failure load for various M(T) specimens. For a linear analysis, the mode I SIF will scale with the far-field load. This allows us to numerically estimate the mode I fracture toughness for this material. The results represent a quantitative basis for evaluating the strength and fracture properties of anisotropic foam insulation material.

Subcritical fracturing of shales under chemically reactive conditions

NASA Astrophysics Data System (ADS)

Chen, X.; Callahan, O. A.; Eichhubl, P.; Olson, J. E.

2016-12-01

Growth of opening-mode fractures under chemically reactive subsurface conditions is potentially relevant for seal integrity in subsurface CO2 storage and hazardous waste disposal. Using double-torsion load relaxation tests we determine mode-I fracture toughness (KIC), subcritical index (SCI), and the stress-intensity factor vs fracture velocity (K-V) behavior of Marcellus, Woodford, and Mancos shales. Samples are tested under ambient air and aqueous conditions with variable NaCl and KCl concentrations, variable pH, and temperatures of up to 70. Under ambient air condition, KIC determined from double torsion tests is 1.3, 0.6, and 1.1 MPam1/2 for Marcellus, Woodford, and Mancos shales, respectively. SCI under ambient air condition is between 55 and 90 for the shales tested. Tests in aqueous solutions show a significant drop of KIC compared to ambient air condition. For tests in deionized water, KIC reduction is 18.5% for Marcellus and 47.0% for Woodford. The presence of aqueous fluids also results in a reduction of the SCI up to 85% compared to ambient condition. K-V curves generally obey a power-law relation throughout the load-relaxation period. However, aqueous-based tests on samples result in K-V curves deviating from the power-law relation, with the SCI values gradually decreasing with time during the relaxation period. This non-power-law behavior is obvious in Woodford and Mancos, but negligible in Marcellus. We find that the shales interact with the aqueous solution both at the fracture tip and within the rock matrix during subcritical fracturing. For Marcellus shale, water mainly interacts with the fracture tip on both tests due to low matrix permeability and less reactive mineral composition. However, Woodford and Mancos react strongly with water causing significant sample degradation. The competition between degradation and fracture growth results in the time-dependent SCI: at lower fracture velocities, the tip interacts longer with the chemically altered area around the tip; at higher fracture velocities, the fracture propagates through the altered area before significant degradation. Our results display strong weakening effects of chemically reactive fluids on subcritical fracture properties with implications on subsurface storage seal performance.

Influence of restorative material and proximal cavity design on the fracture resistance of MOD inlay restoration.

PubMed

Liu, Xiaozhou; Fok, Alex; Li, Haiyan

2014-03-01

This study aimed to evaluate the effects of the restorative material and cavity design on the facture resistance of inlay restorations under a compressive load using acoustic emission (AE) measurement. Two restorative materials, a composite resin (MZ100, 3M ESPE) and a ceramic (IPS Empress CAD, Ivoclar Vivadent), and two cavity designs, non-proximal box and proximal box, were studied. Thirty-two extracted human third molars were selected and divided into 4 groups. The restorative materials and cavity designs used for the four groups were: (1) composite and non-proximal box; (2) ceramic and non-proximal box; (3) composite and proximal box; (4) ceramic and proximal box. The restored molars were loaded in a MTS machine via a loading head of diameter 10mm. The rate of loading was 0.1mm/min. During loading, an AE system was used to monitor the debonding and fracture of the specimens. The load corresponding to the first AE event, the final maximum load sustained, as well as the total number of AE events recorded were used to evaluate the fracture resistance of the restored teeth. For the initial fracture load, Group 2 (236.15N)Group 2 (1685)>Group 3 (239)>Group 1 (221). The differences from pairwise comparisons in the initial fracture load and final load were mostly insignificant statistically (p>0.05), the only exception being that between Groups 2 and 3 in the initial fracture load (p=0.039). For the total number of AE events, statistically significant differences (p<0.05) were found between all group pairs that involved different materials, with the composite groups giving much fewer AE events than the ceramic groups. Conversely, no statistically significant difference in the AE results was found between groups with the same material, irrespective of the cavity design. For teeth restored with MOD inlays, the use of composite resin as the restorative material may provide higher fracture resistance than using ceramic. Using a proximal box design for the cavity may further improve the fracture resistance of the inlay restoration, although the improvement was not statistically significant under axial compression. Copyright © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Effect of Processing Conditions on Fracture Resistance and Cohesive Laws of Binderfree All-Cellulose Composites

NASA Astrophysics Data System (ADS)

Goutianos, S.; Arévalo, R.; Sørensen, B. F.; Peijs, T.

2014-12-01

The fracture properties of all-cellulose composites without matrix were studied using Double Cantilever Beam (DCB) sandwich specimens loaded with pure monotonically increasing bending moments, which give stable crack growth. The experiments were conducted in an environmental scanning electron microscope to a) perform accurate measurements of both the fracture energy for crack initiation and the fracture resistance and b) observe the microscale failure mechanisms especially in the the wake of the crack tip. Since the mechanical behaviour of the all-cellulose composites was non-linear, a general method was first developed to obtain fracture resistance values from the DCB specimens taking into account the non-linear material response. The binderfree all-cellulose composites were prepared by a mechanical refinement process that allows the formation of intramolecular bonds between the cellulose molecules during the drying process. Defibrilation of the raw cellulose material is done in wet medium in a paper-like process. Panels with different refining time were tested and it was found than an increase in fibre fibrillation results in a lower fracture resistance.

Test-Free Fracture Toughness

NASA Technical Reports Server (NTRS)

Minnetyan, Levon; Chamis, Christos C. (Technical Monitor)

2003-01-01

Computational simulation results can give the prediction of damage growth and progression and fracture toughness of composite structures. The experimental data from literature provide environmental effects on the fracture behavior of metallic or fiber composite structures. However, the traditional experimental methods to analyze the influence of the imposed conditions are expensive and time consuming. This research used the CODSTRAN code to model the temperature effects, scaling effects and the loading effects of fiber/braided composite specimens with and without fiber-optic sensors on the damage initiation and energy release rates. The load-displacement relationship and fracture toughness assessment approach is compared with the test results from literature and it is verified that the computational simulation, with the use of established material modeling and finite element modules, adequately tracks the changes of fracture toughness and subsequent fracture propagation for any fiber/braided composite structure due to the change of fiber orientations, presence of large diameter optical fibers, and any loading conditions.

Test-Free Fracture Toughness

NASA Technical Reports Server (NTRS)

Minnetyan, Levon; Chamis, Christos C. (Technical Monitor)

2003-01-01

Computational simulation results can give the prediction of damage growth and progression and fracture toughness of composite structures. The experimental data from literature provide environmental effects on the fracture behavior of metallic or fiber composite structures. However, the traditional experimental methods to analyze the influence of the imposed conditions are expensive and time consuming. This research used the CODSTRAN code to model the temperature effects, scaling effects and the loading effects of fiberbraided composite specimens with and without fiber-optic sensors on the damage initiation and energy release rates. The load-displacement relationship and fracture toughness assessment approach is compared with the test results from literature and it is verified that the computational simulation, with the use of established material modeling and finite element modules, adequately tracks the changes of fracture toughness and subsequent fracture propagation for any fiberbraided composite structure due to the change of fiber orientations, presence of large diameter optical fibers, and any loading conditions.

Controlled shear/tension fixture

DOEpatents

Hsueh, Chun-Hway [Knoxville, TN; Liu, Chain-tsuan [Knoxville, TN; George, Easo P [Knoxville, TN

2012-07-24

A test fixture for simultaneously testing two material test samples is provided. The fixture provides substantially equal shear and tensile stresses in each test specimens. By gradually applying a load force to the fixture only one of the two specimens fractures. Upon fracture of the one specimen, the fixture and the load train lose contact and the second specimen is preserved in a state of upset just prior to fracture. Particular advantages of the fixture are (1) to control the tensile to shear load on the specimen for understanding the effect of these stresses on the deformation behavior of advanced materials, (2) to control the location of fracture for accessing localized material properties including the variation of the mechanical properties and residual stresses across the thickness of advanced materials, (3) to yield a fractured specimen for strength measurement and an unfractured specimen for examining the microstructure just prior to fracture.

Callus remodelling model

NASA Astrophysics Data System (ADS)

Miodowska, Justyna; Bielski, Jan; Kromka-Szydek, Magdalena

2018-01-01

The objective of this paper is to investigate the healing process of the callus using bone remodelling approach. A new mathematical model of bone remodelling is proposed including both underload and overload resorption, as well as equilibrium and bone growth states. The created model is used to predict the stress-stimulated change in the callus density. The permanent and intermittent loading programs are considered. The analyses indicate that obtaining a sufficiently high values of the callus density (and hence the elasticity) modulus is only possible using time-varying load parameters. The model predictions also show that intermittent loading program causes delayed callus healing. Understanding how mechanical conditions influence callus remodelling process may be relevant in the bone fracture treatment and initial bone loading during rehabilitation.

Damage characterization on human femur bone by means of ultrasonics and acoustic emission

NASA Astrophysics Data System (ADS)

Strantza, M.; Polyzos, D.; Louis, O.; Boulpaep, F.; Van Hemelrijck, D.; Aggelis, D. G.

2015-07-01

Human bone tissue is characterized as a material with high brittleness. Due to this nature, visible signs of cracking are not easy to be detected before final failure. The main objective of this work is to investigate if the acoustic emission (AE) technique can offer valuable insight to the fracture process of human femur specimens as in other engineering materials characterization. This study describes the AE activity during fracture of whole femur bones under flexural load. Before fracture, broadband AE sensors were used in order to measure parameters like wave velocity dispersion and attenuation. Waveform parameters like the duration, rise time and average frequency, were also examined relatively to the propagation distance as a preparation for the AE monitoring during fracture. After the ultrasonic study, the samples were partly cast in concrete and fixed as cantilevers. A point load was applied on the femur head, which due to the test geometry resulted in a combination of two different patterns of fracture, bending and torsion. Two AE broadband sensors were placed in different points of the sample, one near the fixing end and the other near the femur head. Preliminary analysis shows that parameters like the number of acquired AE signals and their amplitude are well correlated with the load history. Furthermore, the parameters of rise time and frequency can differentiate the two fracture patterns. Additionally, AE allows the detection of the load at the onset of fracture from the micro-cracking events that occur at the early loading stages, allowing monitoring of the whole fracture process. Parameters that have been used extensively for monitoring and characterization of fracture modes of engineering materials seem to poses characterization power in the case of bone tissue monitoring as well.

Influence of surface finishing on fracture load and failure mode of glass ceramic crowns.

PubMed

Mores, Rafael Tagliari; Borba, Márcia; Corazza, Pedro Henrique; Della Bona, Álvaro; Benetti, Paula

2017-10-01

Ceramic restorations often require adjustments using diamond rotary instruments, which damage the glazed surface. The effect of these adjustments on the fracture behavior of these restorations is unclear. The purpose of this in vitro study was to evaluate the influence of induced surface defects on the fracture load and mode of failure of lithium disilicate-based (LDS) glass ceramic restorations. Premolar crowns were obtained from LDS computer-aided design and computer-aided manufacturing blocks (n=60) and glazed. The crowns were bonded to dentin analog dies and divided into 5 groups (n=12), as follows: glaze; abrasion (diamond rotary instrument 2135); abrasion and reglaze; abrasion and polishing (diamond rotary instrument 2135F, 2135 FF, and polishing devices); and polishing. The topography of the crowns was examined by scanning electron microscopy, and roughness was measured. A compressive load (0.5 mm/min) was applied by a piston to the center of the lingual cusp until fracture. The fracture load was recorded and data were statistically analyzed by ANOVA and the Tukey HSD test (α=.05). Fractured crowns were examined to determine the fracture origin. Polishing and/or reglazing resulted in lower roughness than for the abraded group (P<.05), which did not affect the fracture loads (P=.696). Catastrophic fracture with origin at the intaglio surface was the mode of failure for all the crowns. The experiment design successfully submitted the crowns to a clinical stress state, resulting in a clinically relevant failure. Reglazing or polishing were effective in reducing surface defects. Surface treatments had no effect on the immediate catastrophic failure of LDS crowns. Copyright © 2017 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

Fracture Analysis of Particulate Reinforced Metal Matrix Composites

NASA Technical Reports Server (NTRS)

Min, James B.; Cornie, James A.

2013-01-01

A fracture analysis of highly loaded particulate reinforced composites was performed using laser moire interferometry to measure the displacements within the plastic zone at the tip of an advancing crack. Ten castings were made of five different particulate reinforcement-aluminum alloy combinations. Each casting included net-shape specimens which were used for the evaluation of fracture toughness, tensile properties, and flexure properties resulting in an extensive materials properties data. Measured fracture toughness range from 14.1 MPa for an alumina reinforced 356 aluminum alloy to 23.9 MPa for a silicon carbide reinforced 2214 aluminum alloy. For the combination of these K(sub Ic) values and the measured tensile strengths, the compact tension specimens were too thin to yield true plane strain K(sub Ic) values. All materials exhibited brittle behavior characterized by very small tensile ductility suggesting that successful application of these materials requires that the design stresses be below the elastic limit. Probabilistic design principles similar to those used with ceramics are recommended when using these materials. Such principles would include the use of experimentally determined design allowables. In the absence of thorough testing, a design allowable stress of 60 percent of the measured ultimate tensile stress is recommended.

Fracture of Reduced-Diameter Zirconia Dental Implants Following Repeated Insertion.

PubMed

Karl, Matthias; Scherg, Stefan; Grobecker-Karl, Tanja

Achievement of high insertion torque values indicating good primary stability is a goal during dental implant placement. The objective of this study was to evaluate whether or not two-piece implants made from zirconia ceramic may be damaged as a result of torque application. A total of 10 two-piece zirconia implants were repeatedly inserted into polyurethane foam material with increasing density and decreasing osteotomy size. The insertion torque applied was measured, and implants were checked for fractures by applying the fluorescent penetrant method. Weibull probability of failure was calculated based on the recorded insertion torque values. Catastrophic failures could be seen in five of the implants from two different batches at insertion torques ranging from 46.0 to 70.5 Ncm, while the remaining implants (all belonging to one batch) survived. Weibull probability of failure seems to be low at the manufacturer-recommended maximum insertion torque of 35 Ncm. Chipping fractures at the thread tips as well as tool marks were the only otherwise observed irregularities. While high insertion torques may be desirable for immediate loading protocols, zirconia implants may fracture when manufacturer-recommended insertion torques are exceeded. Evaluating bone quality prior to implant insertion may be useful.

Comparison of Mode II and III Monotonic and Fatigue Delamination Onset Behavior for Carbon/Toughened Epoxy Composites

NASA Technical Reports Server (NTRS)

Li, Jian; OBrien, T. Kevin; Lee, Shaw Ming

1997-01-01

Monotonic and fatigue tests were performed to compare the Mode II and III interlaminar fracture toughness and fatigue delamination onset for Tenax-HTA/R6376 carbon/toughened epoxy composites. The Mode II interlaminar fracture toughness and fatigue delamination onset were characterized using the end-notched flexure (ENF) test while the Mode III interlaminar fracture toughness and fatigue delamination onset were characterized by using the edge crack torsion (ECT) test. Monotonic tests show that the Mode III fracture toughness is higher than the Mode II fracture toughness. Both Mode II and III cyclic loading greatly increases the tendency for a delamination to grow relative to a single monotonically increasing load. Under fatigue loading, the Mode III specimen also has a longer life than the Mode II specimen.

Mechanical behavior and fracture characteristics of off-axis fiber composites. 1: Experimental investigation. [at the Lewis Research Center

NASA Technical Reports Server (NTRS)

Sinclair, J. H.; Chamis, C. C.

1977-01-01

The mechanical behavior, fracture surfaces, and fracture modes of unidirectional high-modulus graphite-fiber/epoxy composites subjected to off-axis tensile loads were investigated experimentally. The investigation included the generation of stress-strain-to-fracture data and scanning electron microscope studies of the fractured surfaces. The results led to the identification of fracture modes and distinct fracture surface characteristics for off-axis tensile loading. The results also led to the formulation of critical for identifying and characterizing these fracture modes and their associated fracture surfaces. The results presented and discussed herein were used in the theoretical investigation and comparisons described in Part 2. These results should also provide a good foundation for identifying, characterizing, and quantifying fracture modes in both off-axis and angle-plied laminates.

Fracture toughness of fiber-reinforced glass ceramic and ceramic matrix composites

NASA Technical Reports Server (NTRS)

Stull, Kevin R.; Parvizi-Majidi, A.

1991-01-01

A fracture mechanics investigation of 2D woven Nicalon SiC/SiC and Nicalon SiC/LAS has been undertaken. An energy approach has been adopted to characterize and quantify the fracture properties of these materials. Chevron-notched bend specimens were tested in an edgewise configuration in which the crack propagated perpendicular to the ply direction. R-curves were obtained from repeated loading and unloading of specimens using several methods of data reduction. Values correconding to the plateau regions of the R-curves were taken as steady-state crack-growth resistance. These ranged from 37 to 63 kJ/sq m for 2D-SiC/LAS and 2.6 to 2.8 kJ/sq m for 2D-SiC/SiC composites.

Aging and loading rate effects on the mechanical behavior of equine bone

NASA Astrophysics Data System (ADS)

Kulin, Robb M.; Jiang, Fengchun; Vecchio, Kenneth S.

2008-06-01

Whether due to a sporting accident, high-speed impact, fall, or other catastrophic event, the majority of clinical bone fractures occur under dynamic loading conditions. However, although extensive research has been performed on the quasi-static fracture and mechanical behavior of bone to date, few high-quality studies on the fracture behavior of bone at high strain rates have been performed. Therefore, many questions remain regarding the material behavior, including not only the loading-rate-dependent response of bone, but also how this response varies with age. In this study, tests were performed on equine femoral bone taken post-mortem from donors 6 months to 28 years of age. Quasi-static and dynamic tests were performed to determine the fracture toughness and compressive mechanical behavior as a function of age at varying loading rates. Fracture paths were then analyzed using scanning confocal and scanning-electron microscopy techniques to assess the role of various microstructural features on toughening mechanisms.

Fracture mechanics; Proceedings of the 22nd National Symposium, Atlanta, GA, June 26-28, 1990. Vols. 1 & 2

NASA Technical Reports Server (NTRS)

Ernst, Hugo A. (Editor); Saxena, Ashok (Editor); Mcdowell, David L. (Editor); Atluri, Satya N. (Editor); Newman, James C., Jr. (Editor); Raju, Ivatury S. (Editor); Epstein, Jonathan S. (Editor)

1992-01-01

Current research on fracture mechanics is reviewed, focusing on ductile fracture; high-temperature and time-dependent fracture; 3D problems; interface fracture; microstructural aspects of fatigue and fracture; and fracture predictions and applications. Particular attention is given to the determination and comparison of crack resistance curves from wide plates and fracture mechanics specimens; a relationship between R-curves in contained and uncontained yield; the creep crack growth behavior of titanium alloy Ti-6242; a crack growth response in three heat resistant materials at elevated temperature; a crack-surface-contact model for determining effective-stress-intensity factors; interfacial dislocations in anisotropic bimaterials; an effect of intergranular crack branching on fracture toughness evaluation; the fracture toughness behavior of exservice chromium-molybdenum steels; the application of fracture mechanics to assess the significance of proof loading; and a load ratio method for estimating crack extension.

Spall fracture and strength of uranium, plutonium and their alloys under shock wave loading

NASA Astrophysics Data System (ADS)

Golubev, Vladimir

2015-06-01

Numerous results on studying the spall fracture phenomenon of uranium, two its alloys with molybdenum and zirconium, plutonium and its alloy with gallium under shock wave loading are presented in the paper. The majority of tests were conducted with the samples in the form of disks 4mm in thickness. They were loaded by the impact of aluminum plates 4mm thick through a copper screen serving as the cover or bottom part of a special container. The initial temperature of samples was changed in the range of -196 - 800 C degree for uranium and 40 - 315 C degree for plutonium. The character of spall failure of materials and the degree of damage for all tested samples were observed on the longitudinal metallographic sections of recovered samples. For a concrete test temperature, the impact velocity was sequentially changed and therefore the loading conditions corresponding to the consecutive transition from microdamage nucleation up to complete macroscopic spall fracture were determined. Numerical calculations of the conditions of shock wave loading and spall fracture of samples were performed in the elastoplastic approach. Several two- and three-dimensional effects of loading were taken into account. Some results obtained under conditions of intensive impulse irradiation and intensive explosive loading are presented too. The rather complete analysis and comparison of obtained results with the data of other researchers on the spall fracture of examined materials were conducted.

Impact Damage and Strain Rate Effects for Toughened Epoxy Composite Structures

NASA Technical Reports Server (NTRS)

Chamis, Christos C.; Minnetyan, Levon

2006-01-01

Structural integrity of composite systems under dynamic impact loading is investigated herein. The GENOA virtual testing software environment is used to implement the effects of dynamic loading on fracture progression and damage tolerance. Combinations of graphite and glass fibers with a toughened epoxy matrix are investigated. The effect of a ceramic coating for the absorption of impact energy is also included. Impact and post impact simulations include verification and prediction of (1) Load and Impact Energy, (2) Impact Damage Size, (3) Maximum Impact Peak Load, (4) Residual Strength, (5) Maximum Displacement, (6) Contribution of Failure Modes to Failure Mechanisms, (7) Prediction of Impact Load Versus Time, and (8) Damage, and Fracture Pattern. A computer model is utilized for the assessment of structural response, progressive fracture, and defect/damage tolerance characteristics. Results show the damage progression sequence and the changes in the structural response characteristics due to dynamic impact. The fundamental premise of computational simulation is that the complete evaluation of composite fracture requires an assessment of ply and subply level damage/fracture processes as the structure is subjected to loads. Simulation results for the graphite/epoxy composite were compared with the impact and tension failure test data, correlation and verification was obtained that included: (1) impact energy, (2) damage size, (3) maximum impact peak load, (4) residual strength, (5) maximum displacement, and (6) failure mechanisms of the composite structure.

Deformation and fracture of explosion-welded Ti/Al plates: A synchrotron-based study

DOE Office of Scientific and Technical Information (OSTI.GOV)

E, J. C.; Huang, J. Y.; Bie, B. X.

Here, explosion-welded Ti/Al plates are characterized with energy dispersive spectroscopy and x-ray computed tomography, and exhibit smooth, well-jointed, interface. We perform dynamic and quasi-static uniaxial tension experiments on Ti/Al with the loading direction either perpendicular or parallel to the Ti/Al interface, using a mini split Hopkinson tension bar and a material testing system in conjunction with time-resolved synchrotron x-ray imaging. X-ray imaging and strain-field mapping reveal different deformation mechanisms responsible for anisotropic bulk-scale responses, including yield strength, ductility and rate sensitivity. Deformation and fracture are achieved predominantly in Al layer for perpendicular loading, but both Ti and Al layers asmore » well as the interface play a role for parallel loading. The rate sensitivity of Ti/Al follows those of the constituent metals. For perpendicular loading, single deformation band develops in Al layer under quasi-static loading, while multiple deformation bands nucleate simultaneously under dynamic loading, leading to a higher dynamic fracture strain. For parallel loading, the interface impedes the growth of deformation and results in increased ductility of Ti/Al under quasi-static loading, while interface fracture occurs under dynamic loading due to the disparity in Poisson's contraction.« less

Deformation and fracture of explosion-welded Ti/Al plates: A synchrotron-based study

DOE PAGES

E, J. C.; Huang, J. Y.; Bie, B. X.; ...

2016-08-02

Here, explosion-welded Ti/Al plates are characterized with energy dispersive spectroscopy and x-ray computed tomography, and exhibit smooth, well-jointed, interface. We perform dynamic and quasi-static uniaxial tension experiments on Ti/Al with the loading direction either perpendicular or parallel to the Ti/Al interface, using a mini split Hopkinson tension bar and a material testing system in conjunction with time-resolved synchrotron x-ray imaging. X-ray imaging and strain-field mapping reveal different deformation mechanisms responsible for anisotropic bulk-scale responses, including yield strength, ductility and rate sensitivity. Deformation and fracture are achieved predominantly in Al layer for perpendicular loading, but both Ti and Al layers asmore » well as the interface play a role for parallel loading. The rate sensitivity of Ti/Al follows those of the constituent metals. For perpendicular loading, single deformation band develops in Al layer under quasi-static loading, while multiple deformation bands nucleate simultaneously under dynamic loading, leading to a higher dynamic fracture strain. For parallel loading, the interface impedes the growth of deformation and results in increased ductility of Ti/Al under quasi-static loading, while interface fracture occurs under dynamic loading due to the disparity in Poisson's contraction.« less

Proximal humerus fractures: a comparative biomechanical analysis of intra and extramedullary implants.

PubMed

Füchtmeier, B; May, R; Hente, R; Maghsudi, M; Völk, M; Hammer, J; Nerlich, M; Prantl, L

2007-08-01

The biomechanical stability of a newly developed humerus nail (Sirustrade mark) for the treatment of fractures of the proximal humerus was analyzed in comparison to established systems. In total, three randomized groups were formed (n = 4 pairs) from 12 matched pairs of human cadaver humeri. All intact bones were mechanically characterized by five subsequent load cycles under bending and torsional loading. The bending moment at the osteotomy was 7.5 N m the torsional moment was 8.3 N m over the hole specimen length. Loading was consistently initiated at the distal epiphysis and the deformation at the distal epiphysis was continuously recorded. Prior to implant reinforcement, a defect of 5 mm was created to simulate an unstable subcapital humerus fracture. For paired comparison, one humerus of each pair was stabilized with the Sirus proximal humerus nail while the counterpart was stabilized by a reference implant. In detail, the following groups were created: Sirus versus Proximal humerus nail (PHN) with spiral blade (group I); Sirus versus PHILOS plate (group II); Sirus versus 4.5 mm AO T-plate (group III). The Sirus nail demonstrated significantly higher stiffness values compared to the reference implants for both bending and torsional loading. The following distal epiphyseal displacements were recorded for a bending moment of 7.5 N m at the osteotomy: Sirus I: 8.8 mm, II: 8.4 mm, III: 7.7 mm (range 6.9-10.9), PHN 21.1 mm (range 15.7-25.2) (P = 0.005), PHILOS plate 27.5 mm (range 21.6-35.8) (P < 0.001), 4.5 AO T-plate 26.3 mm (range 24.3-33.9) (P = 0.01). The rotations corresponding to 8.3 N m torsional moment were: Sirus I: 9.1 degrees , II: 9.3 degrees , III: 10.6 degrees (range 7.5-12.2), PHN 13.5 degrees (range 10.3-15.6) (P = 0.158), PHILOS plate 15.6 degrees (range 13.7-20.8) (P = 0.007), 4.5 AO T-Platte 14.1 degrees (range 11.5-19.7) (P = 0.158). The intramedullary load carriers were biomechanically superior when compared to the plating systems in the fracture model presented here. Supplementary, the Sirus Nail showed higher stiffness values than the PHN. However, the latter are gaining in importance due to the possibility of minimal invasive implantation. Whether this will be associated with functional advantages requires further clinical investigation.

The fractography of casting alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Powell, G.W.

1994-10-01

Several types of casting alloys were fractured using various loading modes (uniaxial tension, bending, impact, and torsion, and cyclic stressing), and the corresponding mechanical properties were determined. The unetched and etched fracture surfaces and the microstructures were examined using conventional techniques. The types of casting alloys that were the subjects f these investigations include gray iron, ductile iron, cast steel, and aluminum-base alloys (A380, A356, and 319). The fractographic studies have yielded these generalizations regarding the topography of the fracture surfaces. In the case of low-ductility alloys such as gray iron and the aluminum-base alloys, the tensile edge of amore » fracture surface produced by a stress system with a strong bending-moment component has a highly irregular contour, whereas the compressive edge of the fracture surface is quite straight and parallel to the bend axis. On the other hand, the periphery of a fracture surface produced by uniaxial tension has a completely irregular contour. The fracture surface produced by cyclic loading of a gray iron does not display any macroscopic evidence (such as a thumb nail) of the loading mode. However, the fracture surface of each of the other casting alloys displays clear, macroscopic evidence of failure induced by fatigue. The aluminum-base alloys fracture completely within the interdendritic region of the microstructure when subjected to monotonic loading by uniaxial tension or bending, whereas a fatigue crack propagates predominantly through the primary crystals of the microstructure.« less

3-D Mixed Mode Delamination Fracture Criteria - An Experimentalist's Perspective

NASA Technical Reports Server (NTRS)

Reeder, James R.

2006-01-01

Many delamination failure criteria based on fracture toughness have been suggested over the past few decades, but most only covered the region containing mode I and mode II components of loading because that is where toughness data existed. With new analysis tools, more 3D analyses are being conducted that capture a mode III component of loading. This has increased the need for a fracture criterion that incorporates mode III loading. The introduction of a pure mode III fracture toughness test has also produced data on which to base a full 3D fracture criterion. In this paper, a new framework for visualizing 3D fracture criteria is introduced. The common 2D power law fracture criterion was evaluated to produce unexpected predictions with the introduction of mode III and did not perform well in the critical high mode I region. Another 2D criterion that has been shown to model a wide range of materials well was used as the basis for a new 3D criterion. The new criterion is based on assumptions that the relationship between mode I and mode III toughness is similar to the relation between mode I and mode II and that a linear interpolation can be used between mode II and mode III. Until mixed-mode data exists with a mode III component of loading, 3D fracture criteria cannot be properly evaluated, but these assumptions seem reasonable.

Mechanical properties and micro-morphology of fiber posts.

PubMed

Zicari, F; Coutinho, E; Scotti, R; Van Meerbeek, B; Naert, I

2013-04-01

To evaluate flexural properties of different fiber posts systems and to morphologically characterize their micro-structure. Six types of translucent fiber posts were selected: RelyX Post (3M ESPE), ParaPost Taper Lux (Colthéne-Whaledent), GC Fiber Post (GC), LuxaPost (DMG), FRC Postec Plus (Ivoclar-Vivadent), D.T. Light-Post (RTD). For each post system and size, ten specimens were subjected to a three-points bending test. Maximum fracture load, flexural strength and flexural modulus were determined using a universal loading device (5848 MicroTester(®), Instron). Besides, for each system, three intact posts of similar dimensions were processed for scanning electron microscopy to morphologically characterize the micro-structure. The following structural characteristics were analyzed: fibers/matrix ratio, density of fibers, diameter of fibers and distribution of fibers. Data were statistically analyzed with ANOVA. Type and diameter of posts were found to significantly affect the fracture load, flexural strength and flexural modulus (p<0.05). Regarding maximum fracture load, it was found to increase with post diameter, in each post system (p<0.001). Regarding flexural strength and flexural modulus, the highest values were recorded for posts with the smallest diameter (p<0.001). Finally, structural characteristics significantly varied among the post systems tested. However, any correlation has been found between flexural strength and structural characteristics. Flexural strength appeared not to be correlated to structural characteristics of fiber posts, but it may rather be affected by mechanical properties of the resin matrix and the interfacial adhesion between fibers and resin matrix. Copyright © 2013. Published by Elsevier Ltd.

Resistance to fracture of teeth instrumented using novel EndoStar E5 rotary versus ProTaper NEXT and WaveOne file systems

PubMed Central

Pawar, Ajinkya M.; Pawar, Mansing G.; Thakur, Bhagyashree; Banga, Kulwinder Singh; Luke, Alexander Maniangat

2018-01-01

Aim: The current study compared the fracture resistance of samples instrumented by two rotary files and a reciprocating file, obturated with gutta-percha and AH Plus. Materials and Methods: A total of 60 freshly extracted mandibular premolar teeth with single roots and single canals were acquired and decoronated at or below the cementoenamel junction. The samples were randomly divided into four groups (n = 15). Group 1 control (noninstrumented/obturated), and for Groups 2–4 root canal instrumentation was done by EndoStar E5 (EE5), ProTaper NEXT (PTN), and WaveOne, respectively. Following instrumentation, the samples were obturated using gutta-percha cones and AH Plus sealer using lateral compaction. A week later, vertical load was applied to the specimen's canal in each group until fracture. The loads required for fracture were recorded and statistically analyzed. Results: The mean loads required to fracture (Newton; N) for the four groups were; 388.54 (±29.93), 310.35 (±26.05), 328.40 (±20.67), and 278.54 (±34.16). The loads exhibited highly significant difference (P < 0.0001; analysis variance). The following Tukey's post hoc test confirmed, both samples in Groups 2 and 3 required similar loads for fracture (P > 0.05) and significantly higher than Group 4 (P < 0.01). Conclusion: The samples instrumented by EE5 and PTN exhibit similar fracture resistance. PMID:29628648

Numerical investigations of rib fracture failure models in different dynamic loading conditions.

PubMed

Wang, Fang; Yang, Jikuang; Miller, Karol; Li, Guibing; Joldes, Grand R; Doyle, Barry; Wittek, Adam

2016-01-01

Rib fracture is one of the most common thoracic injuries in vehicle traffic accidents that can result in fatalities associated with seriously injured internal organs. A failure model is critical when modelling rib fracture to predict such injuries. Different rib failure models have been proposed in prediction of thorax injuries. However, the biofidelity of the fracture failure models when varying the loading conditions and the effects of a rib fracture failure model on prediction of thoracic injuries have been studied only to a limited extent. Therefore, this study aimed to investigate the effects of three rib failure models on prediction of thoracic injuries using a previously validated finite element model of the human thorax. The performance and biofidelity of each rib failure model were first evaluated by modelling rib responses to different loading conditions in two experimental configurations: (1) the three-point bending on the specimen taken from rib and (2) the anterior-posterior dynamic loading to an entire bony part of the rib. Furthermore, the simulation of the rib failure behaviour in the frontal impact to an entire thorax was conducted at varying velocities and the effects of the failure models were analysed with respect to the severity of rib cage damages. Simulation results demonstrated that the responses of the thorax model are similar to the general trends of the rib fracture responses reported in the experimental literature. However, they also indicated that the accuracy of the rib fracture prediction using a given failure model varies for different loading conditions.

Biomechanical comparison of straight DCP and helical plates for fixation of transverse and oblique bone fractures.

PubMed

Aksakal, Bunyamin; Gurger, Murat; Say, Yakup; Yilmaz, Erhan

2014-01-01

Biomechanical comparison of straight DCP and helical plates for fixation of transversal and oblique tibial bone fractures were analyzed and compared to each other by axial compression, bending and torsion tests. An in vitro osteosynthesis of transverse (TF) and oblique bone fracture (OF) fixations have been analysed on fresh sheep tibias by using the DCP and helical compression plates (HP). Statistically significant differences were found for both DCP and helical plate fixations under axial compression, bending and torsional loads. The strength of fixation systems was in favor of DC plating with exception of the TF-HP fixation group under compression loads and torsional moments. The transvers fracture (TF) stability was found to be higher than that found in oblique fracture (OF) fixed by helical plates (HP). However, under torsional testing, compared to conventional plating, the helical plate fixations provided a higher torsional resistance and strength. The maximum stiffness at axial compression loading and maximum torsional strength was achieved in torsional testing for the TF-HP fixations. From in vitro biomechanical analysis, fracture type and plate fixation system groups showed different responses under different loadings. Consequently, current biomechanical analyses may encourage the usage of helical HP fixations in near future during clinical practice for transverse bone fractures.

Influence of core design, production technique, and material selection on fracture behavior of yttria-stabilized tetragonal zirconia polycrystal fixed dental prostheses produced using different multilayer techniques: split-file, over-pressing, and manually built-up veneers.

PubMed

Mahmood, Deyar Jallal Hadi; Linderoth, Ewa H; Wennerberg, Ann; Vult Von Steyern, Per

2016-01-01

To investigate and compare the fracture strength and fracture mode in eleven groups of currently, the most commonly used multilayer three-unit all-ceramic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fixed dental prostheses (FDPs) with respect to the choice of core material, veneering material area, manufacturing technique, design of connectors, and radii of curvature of FDP cores. A total of 110 three-unit Y-TZP FDP cores with one intermediate pontic were made. The FDP cores in groups 1-7 were made with a split-file design, veneered with manually built-up porcelain, computer-aided design-on veneers, and over-pressed veneers. Groups 8-11 consisted of FDPs with a state-of-the-art design, veneered with manually built-up porcelain. All the FDP cores were subjected to simulated aging and finally loaded to fracture. There was a significant difference (P<0.05) between the core designs, but not between the different types of Y-TZP materials. The split-file designs with VITABLOCS(®) (1,806±165 N) and e.max(®) ZirPress (1,854±115 N) and the state-of-the-art design with VITA VM(®) 9 (1,849±150 N) demonstrated the highest mean fracture values. The shape of a split-file designed all-ceramic reconstruction calls for a different dimension protocol, compared to traditionally shaped ones, as the split-file design leads to sharp approximal indentations acting as fractural impressions, thus decreasing the overall strength. The design of a framework is a crucial factor for the load bearing capacity of an all-ceramic FDP. The state-of-the-art design is preferable since the split-file designed cores call for a cross-sectional connector area at least 42% larger, to have the same load bearing capacity as the state-of-the-art designed cores. All veneering materials and techniques tested in the study, split-file, over-press, built-up porcelains, and glass-ceramics are, with a great safety margin, sufficient for clinical use both anteriorly and posteriorly. Analysis of the fracture pattern shows differences between the milled veneers and over-pressed or built-up veneers, where the milled ones show numerically more veneer cracks and the other groups only show complete connector fractures.

Effect of different cements on the biomechanical behavior of teeth restored with cast dowel-and-cores-in vitro and FEA analysis.

PubMed

Soares, Carlos José; Raposo, Luís Henrique Araújo; Soares, Paulo Vinícius; Santos-Filho, Paulo César Freitas; Menezes, Murilo Sousa; Soares, Priscilla Barbosa Ferreira; Magalhães, Denildo

2010-02-01

To test the hypothesis that the type of cement used for fixation of cast dowel-and-cores might influence fracture resistance, fracture mode, and stress distribution of single-rooted teeth restored with this class of metallic dowels. The coronal portion was removed from 40 bovine incisors, leaving a 15 mm root. After endodontic treatment and standardized root canal relief at 10 mm, specimens were embedded in polystyrene resin, and the periodontal ligament was simulated with polyether impression material. The specimens were randomly divided into four groups (n = 10), and restored with Cu-Al cast dowel-and-cores cemented with one of four options: conventional glass ionomer cement (GI); resin-modified glass ionomer cement (GR); dual-cure resin cement (RC); or zinc-phosphate cement (ZP). Sequentially, fracture resistance of the specimens was tested with a tangential load at a 135 degrees angle with a 0.5 mm/min crosshead speed. Data were analyzed using one-way analysis of variance (ANOVA) and the Fisher test. Two-dimensional finite element analysis (2D-FEA) was then performed with representative models of each group simulating a 100 microm cement layer. Results were analyzed based on von Mises stress distribution criteria. The mean fracture resistance values were (in N): RC, 838.2 +/- 135.9; GI, 772.4 +/- 169.8; GR, 613.4 +/- 157.5; ZP, 643.6 +/- 106.7. FEA revealed that RC and GR presented lower stress values than ZP and GI. The higher stress concentration was coincident with more catastrophic failures, and consequently, with lower fracture resistance values. The type of cement influenced fracture resistance, failure mode, and stress distribution on teeth restored with cast dowel-and-cores.

The effect of different torque wrenches on rotational stiffness in compressive femoral nails: a biomechanical study.

PubMed

Karaarslan, A A; Acar, N

2018-02-01

Rotation instability and locking screws failure are common problems. We aimed to determine optimal torque wrench offering maximum rotational stiffness without locking screw failure. We used 10 conventional compression nails, 10 novel compression nails and 10 interlocking nails with 30 composite femurs. We examined rotation stiffness and fracture site compression value by load cell with 3, 6 and 8 Nm torque wrenches using torsion apparatus with a maximum torque moment of 5 Nm in both directions. Rotational stiffness of composite femur-nail constructs was calculated. Rotational stiffness of composite femur-compression nail constructs compressed by 6 Nm torque wrench was 3.27 ± 1.81 Nm/angle (fracture site compression: 1588 N) and 60% more than that compressed with 3 Nm torque wrench (advised previously) with 2.04 ± 0.81 Nm/angle (inter fragmentary compression: 818 N) (P = 0.000). Rotational stiffness of composite-femur-compression nail constructs compressed by 3 Nm torque wrench was 2.04 ± 0.81 Nm/angle (fracture site compression: 818 N) and 277% more than that of interlocking nail with 0.54 ± 0.08 Nm/angle (fracture site compression: 0 N) (P = 0.000). Rotational stiffness and fracture site compression value produced by 3 Nm torque wrench was not satisfactory. To obtain maximum rotational stiffness and fracture site compression value without locking screw failure, 6 Nm torque wrench in compression nails and 8 Nm torque wrench in novel compression nails should be used.

Progressive Fracture of Fiber Composite Builtup Structures

NASA Technical Reports Server (NTRS)

Gotsis, Pascal K.; Chamis, Christos C.; Minnetyan, Levon

1996-01-01

The damage progression and fracture of builtup composite structures was evaluated by using computational simulation to examine the behavior and response of a stiffened composite (0 +/- 45/90)(sub s6) laminate panel subjected to a bending load. The damage initiation, growth, accumulation, progression, and propagation to structural collapse were simulated. An integrated computer code (CODSTRAN) was augmented for the simulation of the progressive damage and fracture of builtup composite structures under mechanical loading. Results showed that damage initiation and progression have a significant effect on the structural response. Also investigated was the influence of different types of bending load on the damage initiation, propagation, and final fracture of the builtup composite panel.

Fracture Strength of Endodontically Treated Teeth with Different Access Cavity Designs.

PubMed

Plotino, Gianluca; Grande, Nicola Maria; Isufi, Almira; Ioppolo, Pietro; Pedullà, Eugenio; Bedini, Rossella; Gambarini, Gianluca; Testarelli, Luca

2017-06-01

The purpose of this study was to compare in vitro the fracture strength of root-filled and restored teeth with traditional endodontic cavity (TEC), conservative endodontic cavity (CEC), or ultraconservative "ninja" endodontic cavity (NEC) access. Extracted human intact maxillary and mandibular premolars and molars were selected and assigned to control (intact teeth), TEC, CEC, or NEC groups (n = 10/group/type). Teeth in the TEC group were prepared following the principles of traditional endodontic cavities. Minimal CECs and NECs were plotted on cone-beam computed tomographic images. Then, teeth were endodontically treated and restored. The 160 specimens were then loaded to fracture in a mechanical material testing machine (LR30 K; Lloyd Instruments Ltd, Fareham, UK). The maximum load at fracture and fracture pattern (restorable or unrestorable) were recorded. Fracture loads were compared statistically, and the data were examined with analysis of variance and the Student-Newman-Keuls test for multiple comparisons. The mean load at fracture for TEC was significantly lower than the one for the CEC, NEC, and control groups for all types of teeth (P < .05), whereas no difference was observed among CEC, NEC, and intact teeth (P > .05). Unrestorable fractures were significantly more frequent in the TEC, CEC, and NEC groups than in the control group in each tooth type (P < .05). Teeth with TEC access showed lower fracture strength than the ones prepared with CEC or NEC. Ultraconservative "ninja" endodontic cavity access did not increase the fracture strength of teeth compared with the ones prepared with CEC. Intact teeth showed more restorable fractures than all the prepared ones. Copyright © 2017 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

In vitro comparison of fracture load of implant-supported, zirconia-based, porcelain- and composite-layered restorations after artificial aging.

PubMed

Komine, Futoshi; Taguchi, Kohei; Fushiki, Ryosuke; Kamio, Shingo; Iwasaki, Taro; Matsumura, Hideo

2014-01-01

This study evaluated fracture load of single-tooth, implant-supported, zirconia-based, porcelain- and indirect composite-layered restorations after artificial aging. Forty-four zirconia-based molar restorations were fabricated on implant abutments and divided into four groups, namely, zirconia-based all-ceramic restorations (ZAC group) and three types of zirconia-based composite-layered restorations (ZIC-P, ZIC-E, and ZIC groups). Before layering an indirect composite material, the zirconia copings in the ZIC-P and ZIC-E groups were primed with Clearfil Photo Bond and Estenia Opaque Primer, respectively. All restorations were cemented on the abutments with glass-ionomer cement and then subjected to thermal cycling and cyclic loading. All specimens survived thermal cycling and cyclic loading. The fracture load of the ZIC-P group (2.72 kN) was not significantly different from that of the ZAC group (3.05 kN). The fracture load of the zirconia-based composite-layered restoration primed with Clearfil Photo Bond (ZIC-P) was comparable to that of the zirconia-based all-ceramic restoration (ZAC) after artificial aging.

Radiographic evaluation of acute distal radius fracture stability: A comparative cadaveric study between a thermo-formable bracing system and traditional fiberglass casting.

PubMed

Santoni, Brandon G; Aira, Jazmine R; Diaz, Miguel A; Kyle Stoops, T; Simon, Peter

2017-08-01

Distal radius fractures are common musculoskeletal injuries and many can be treated non-operatively with cast immobilization. A thermo-formable brace has been developed for management of such fractures, but no data exist regarding its comparative stabilizing efficacy to fiberglass casting. A worst-case distal radius fracture was created in 6 cadaveric forearms. A radiolucent loading fixture was created to apply cantilever bending/compression loads ranging from 4.5N to 66.7N across the simulated fracture in the: (1) non-stabilized, (2) braced; and (3) casted forearms, each forearm serving as its own control. Fracture fragment translations and rotations were measured radiographically using orthogonal radiographs and a 2D-3D, CT-based transformation methodology. Under 4.5N of load in the non-stabilized condition, average sagittal plane rotation and 3D center of mass translation of the fracture fragment were 12.3° and 5.3mm, respectively. At the 4.5N load step, fragment rotation with the brace (avg. 0.0°) and cast (0.1°) reduced sagittal plane rotation compared to the non-stabilized forearm (P<0.001). There were no significant differences in measured sagittal plane fracture fragment rotations or 3D fragment translations between the brace or cast at any of the four load steps (4.5N, 22.2N, 44.5N, and 66.7N, P≥0.138). In this in vitro radiographic study utilizing 6 cadaveric forearms with simulated severe-case, unstable and comminuted distal radius fractures, the thermo-formable brace stabilized the fracture in a manner that was not radiographically or biomechanically different from traditional fiberglass casting. Study results support the use of the thermo-formable brace clinically. Copyright © 2017 Elsevier Ltd. All rights reserved.

Effect of Elevated Temperature and Loading Rate on Delamination Fracture Toughness

NASA Technical Reports Server (NTRS)

Reeder, J. R.; Allen, D. H.; Bradley, W. L.

2003-01-01

The effects of temperature and loading rate on delamination growth were studied. The delamination fracture toughness of IM7/K3B was measured at 149 C, 177 C, and 204 C. At each temperature the tests were performed with a variety of loading rates so that the delamination initiated over the range of time from 0.5 sec to 24 hrs. The double cantilever beam (DCB) test was used to measure fracture toughness. The results showed that the delamination resistance is a complicated function of both time and temperature with the effect of temperature either increasing or decreasing the fracture toughness depending on the time scale. The results also showed that the fracture toughness changed by as much as a factor of three as the time scale changed over the five orders of magnitude tested.

The effect of shape on the fracture of a soft elastic gel subjected to shear load.

PubMed

Kundan, Krishna Kant; Ghatak, Animangsu

2018-02-21

For brittle solids, the fracture energy is the energy required to create a unit area of new surface through the process of division. For crosslinked materials, it is a function of the intrinsic properties like crosslinking density and bond strength of the crosslinks. Here we show that the energy released due to fracture can depend also on the shape of a joint made of this material. Our experiment involves two gel blocks connected via a thin gel disk. The disk is formed into different regular and exotic shapes, but with identical areas of cross-section. When one of the blocks is sheared with respect to the other, the shear load increases with vertical displacement, eventually causing a fracture at a threshold load. The maximum fracture load is different for different disks and among different regularly shaped disks, it is at a maximum for pentagon and hexagon shapes. The fracture energy release rate of the joint depends also on the aspect ratio (height/width) of the shapes. Our experiments also throw light on possible reasons for such a dependence on the shape of the joints.

Improving the mechanical properties of nano-hydroxyapatite

NASA Astrophysics Data System (ADS)

Khanal, Suraj Prasad

Hydroxyapatite (HAp) is an ideal bioactive material that is used in orthopedics. Chemical composition and crystal structure properties of HAp are similar to the natural bone hence it promotes bone growth. However, its mechanical properties of synthetic HAp are not sufficient for major load-bearing bone replacement. The potential of improving the mechanical properties of synthetic hydroxyapatite (HAp) by incorporating carboxyl functionalized single walled carbon nanotubes (CfSWCNT) and polymerized epsilon-caprolactam (nylon) is studied. The fracture toughness, tensile strength, Young's modulus, stiffness and fracture energy were studied for a series of HAp samples with CfSWCNT concentrations varying from 0 to 1.5 wt. % without, and with nylon addition. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC) were used to characterize the samples. The fracture toughness and tensile test was performed under the standard protocol of ASTM D5045 and ASTM D638-02a respectively. Reproducible maximum values of (3.60 +/- 0.3) MPa.m1/2 for fracture toughness and 65.38 MPa for tensile strength were measured for samples containing 1 wt. % CfSWCNT and nylon. The Young's modulus, stiffness and fracture energy of the samples are 10.65 GPa, 1482.12 N/mm, and 644 J/m2 respectively. These values are comparable to those of the cortical bone. Further increase of the CfSWCNT content results to a decreased fracture toughness and tensile strength and formation of a secondary phase.

Fracture resistance of structurally compromised and normal endodontically treated teeth restored with different post systems: An in vitro study

PubMed Central

Mortazavi, Vajihesadat; Fathi, Mohammadhossein; Katiraei, Najmeh; Shahnaseri, Shirin; Badrian, Hamid; Khalighinejad, Navid

2012-01-01

Background: With the aim of developing methods that could increase the fracture resistance of structurally compromised endodontically treated teeth, this study was conducted to compare the effect of three esthetic post systems on the fracture resistance and failure modes of structurally compromised and normal roots. Materials and Methods: Forty five extracted and endodontically treated maxillary central teeth were assigned to 5 experimental groups (n=9). In two groups, the post spaces were prepared with the corresponding drills of the post systems to be restored with double taper light posts (DT.Light-Post) (group DT.N) and zirconia posts (Cosmopost) (group Zr.N). In other 3 groups thin wall canals were simulated to be restored with Double taper Light posts (DT.W), double taper Light posts and Ribbond fibers (DT+R.W) and Zirconia posts (Zr.W). After access cavity restoration and thermocycling, compressive load was applied and the fracture strength values and failure modes were evaluated. Data were analyzed using two-way ANOVA, Tukey and Fisher exact tests (P<0.05). Results: The mean failure loads (N) were 678.56, 638.22, 732.44, 603.44 and 573.67 for groups DT.N, Zr.N, DT.W, DT+R.W and Zr.w respectively. Group DT+R.W exhibited significantly higher resistance to fracture compared to groups Zr.N, DT.W and Zr.w (P<0.05). A significant difference was detected between groups DT.N and Zr.W (P=0.027). Zirconia posts showed significantly higher root fracture compared to fiber posts (P=0.004). Conclusion: The structurally compromised teeth restored with double taper light posts and Ribbond fibers showed the most fracture resistance and their strengths were comparable to those of normal roots restored with double taper light posts. More desirable fracture patterns were observed in teeth restored with fiber posts. PMID:22623936

Analyses of Buckling and Stable Tearing in Thin-Sheet Materials

NASA Technical Reports Server (NTRS)

Seshadri, B. R.; Newman, J. C., Jr.

1998-01-01

This paper was to verify the STAGS (general shell, geometric and material nonlinear) code and the critical crack tip opening angle (CTOA) fracture criterion for predicting stable tearing in cracked panels that fail with severe out of plane buckling. Materials considered ranged from brittle to ductile behavior. Test data used in this study are reported elsewhere. The STAGS code was used to model stable tearing using a critical CTOA value that was determined from a cracked panel that was 'restrained' from buckling. ne analysis methodology was then used to predict the influence of buckling on stable tearing and failure loads. Parameters like crack length to specimen width ratio, crack configuration, thickness, and material tensile properties had a significant influence on the buckling behavior of cracked thin sheet materials. Experimental and predicted results showed a varied buckling response for different crack length to sheet thickness ratios because different buckling modes were activated. Effects of material tensile properties and fracture toughness on buckling response were presented. The STAGS code and the CTOA fracture criterion were able to predict the influence of buckling on stable tearing behavior and failure loads on a variety of materials and crack configurations.

Design for progressive fracture in composite shell structures

NASA Technical Reports Server (NTRS)

Minnetyan, Levon; Murthy, Pappu L. N.

1992-01-01

The load carrying capability and structural behavior of composite shell structures and stiffened curved panels are investigated to provide accurate early design loads. An integrated computer code is utilized for the computational simulation of composite structural degradation under practical loading for realistic design. Damage initiation, growth, accumulation, and propagation to structural fracture are included in the simulation. Progressive fracture investigations providing design insight for several classes of composite shells are presented. Results demonstrate the significance of local defects, interfacial regions, and stress concentrations on the structural durability of composite shells.

Nuclear Graphite - Fracture Behavior and Modeling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Burchell, Timothy D; Battiste, Rick; Strizak, Joe P

2011-01-01

Evidence for the graphite fracture mechanism is reviewed and discussed. The roles of certain microstructural features in the graphite fracture process are reported. The Burchell fracture model is described and its derivation reported. The successful application of the fracture model to uniaxial tensile data from several graphites with widely ranging structure and texture is reported. The extension of the model to multiaxial loading scenarios using two criteria is discussed. Initially, multiaxial strength data for H-451 graphite were modeled using the fracture model and the Principle of Independent Action. The predicted 4th stress quadrant failure envelope was satisfactory but the 1stmore » quadrant predictions were not conservative and thus were unsatisfactory. Multiaxial strength data from the 1st and 4th stress quadrant for NBG-18 graphite are reported. To improve the conservatism of the predicted 1st quadrant failure envelope for NBG-18 the Shetty criterion has been applied to obtain the equivalent critical stress intensity factor, KIc (Equi), for each applied biaxial stress ratio. The equivalent KIc value is used in the Burchell fracture model to predict the failure envelope. The predicted 1st stress quadrant failure envelope is conservative and thus more satisfactory than achieved previously using the fracture model combined with the Principle of Independent Action.« less

Interior Fracture Mechanism Analysis and Fatigue Life Prediction of Surface-Hardened Gear Steel under Axial Loading.

PubMed

Li, Wei; Deng, Hailong; Liu, Pengfei

2016-10-18

The interior defect-induced fracture of surface-hardened metallic materials in the long life region has become a key issue on engineering design. In the present study, the axial loading test with fully reversed condition was performed to examine the fatigue property of a surface-carburized low alloy gear steel in the long life region. Results show that this steel represents the duplex S-N (stress-number of cycles) characteristics without conventional fatigue limit related to 10⁷ cycles. Fatigue cracks are all originated from the interior inclusions in the matrix region due to the inhabitation effect of carburized layer. The inclusion induced fracture with fisheye occurs in the short life region below 5 × 10⁵ cycles, whereas the inclusion induced fracture with fine granular area (FGA) and fisheye occurs in the long life region beyond 10⁶ cycles. The stress intensity factor range at the front of FGA can be regarded as the threshold value controlling stable growth of interior long crack. The evaluated maximum inclusion size in the effective damage volume of specimen is about 27.29 μm. Considering the size relationships between fisheye and FGA, and inclusion, the developed life prediction method involving crack growth can be acceptable on the basis of the good agreement between the predicted and experimental results.

Experimental Study of Slabbing and Rockburst Induced by True-Triaxial Unloading and Local Dynamic Disturbance

NASA Astrophysics Data System (ADS)

Du, Kun; Tao, Ming; Li, Xi-bing; Zhou, Jian

2016-09-01

Slabbing/spalling and rockburst are unconventional types of failure of hard rocks under conditions of unloading and various dynamic loads in environments with high and complex initial stresses. In this study, the failure behaviors of different rock types (granite, red sandstone, and cement mortar) were investigated using a novel testing system coupled to true-triaxial static loads and local dynamic disturbances. An acoustic emission system and a high-speed camera were used to record the real-time fracturing processes. The true-triaxial unloading test results indicate that slabbing occurred in the granite and sandstone, whereas the cement mortar underwent shear failure. Under local dynamically disturbed loading, none of the specimens displayed obvious fracturing at low-amplitude local dynamic loading; however, the degree of rock failure increased as the local dynamic loading amplitude increased. The cement mortar displayed no failure during testing, showing a considerable load-carrying capacity after testing. The sandstone underwent a relatively stable fracturing process, whereas violent rockbursts occurred in the granite specimen. The fracturing process does not appear to depend on the direction of local dynamic loading, and the acoustic emission count rate during rock fragmentation shows that similar crack evolution occurred under the two test scenarios (true-triaxial unloading and local dynamically disturbed loading).

FRACTURE BEHAVIOR OF ALLOY 600, ALLOY 690, EN82H WELDS AND EN52 WELDS IN WATER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mills, W.J., Brown, C.M. and Burke, M.G.

2000-01-11

The cracking resistance of Alloy 600, Alloy 690 and their welds, EN82H and EN52, was characterized by conducting J{sub IC} rising load tests in air and hydrogenated water and cooldown testing in water under constant-displacement conditions. All test materials displayed excellent toughness in air and high temperature water, but Alloy 690 and the two welds were severely embrittled in low temperature water. In 54 C water with 150 cc H{sub 2}/kg H{sub 2}O, J{sub IC} values were reduced by 70% to 95%, relative to their air counterpart. The toughness degradation was associated with a fracture mechanism transition from microvoid coalescencemore » to intergranular fracture. Comparison of the cracking response in water with that for hydrogen-precharged specimens tested in air demonstrated that susceptibility to low temperature crack propagation (LTCP) is due to hydrogen embrittlement of grain boundaries. The effects of water temperature, hydrogen content and loading rate on LTCP were studied. In addition, testing of specimens containing natural weld defects and as-machined notches was performed to determine if low temperature cracking can initiate at these features. Unlike the other materials, Alloy 600 is not susceptible to LTCP as the toughness in 54 C water remained high and a microvoid coalescence mechanism was operative in both air and water. Cooldown testing of EN82H welds under constant-displacement conditions was performed to determine if LTCP data from rising load J{sub IC}/K{sub Pmax} tests predict the onset of LTCP for other load paths. In these tests, bolt-loaded CT specimens were subjected to 288 C water for up to 1 week, cooled to 54 C and held in 54 C hydrogenated water for 1 week. This cycle was repeated up to 6 times. For two of the three welds tested, critical K{sub I} levels for LTCP under constant-displacement conditions were much higher than rising load K{sub Pmax} values. Bolt-loaded specimens from a third weld were found to exhibit LTCP at K{sub I} levels comparable to K{sub Pmax} values. Although work to date indicates that rising load tests either accurately or conservatively predict the critical conditions for LTCP under constant displacement conditions, the potential for LTCP at K{sub I} levels less than K{sub Pmax} has not been fully evaluated. Annealing at 1093 C reduces or eliminates LTCP susceptibility. The microstructure and mechanical properties for susceptible and nonsusceptible EN82H welds were characterized to identify the key material parameters responsible for LTCP in the as-welded condition. The key microstructural feature associated with LTCP appears to be fine Nb- and Ti-rich carbonitrides decorating grain boundaries. In addition, the higher yield strength for the as-fabricated weld also promotes LTCP because it increases stresses and local hydrogen concentrations ahead of a crack.« less

Comparison of fracture resistance of pressable metal ceramic custom implant abutment with a commercially fabricated CAD/CAM zirconia implant abutment.

PubMed

Protopapadaki, Maria; Monaco, Edward A; Kim, Hyeong-Il; Davis, Elaine L

2013-11-01

The predictable nature of the hot pressing ceramic technique has several applications, but no study was identified that evaluated its application to the fabrication of custom implant abutments. The purpose of this study was to compare the fracture resistance of an experimentally designed pressable metal ceramic custom implant abutment (PR) with that of a duplicate zirconia abutment (ZR). Two groups of narrow platform (NP) (Nobel Replace) implant abutment specimens were fabricated (n=10). The experimental abutment (PR) had a metal substructure cast with ceramic alloy (Lodestar) and veneered with leucite pressable glass ceramic (InLine PoM). Each PR abutment was individually scanned and 10 duplicate CAD/CAM ZR abutments were fabricated for the control group. Ceramic crowns (n=20) with the average dimensions of a human lateral incisor were pressed with lithium disilicate glass ceramic (IPS e.max Press) and bonded on the abutments with a resin luting agent (Multilink Automix). The specimens were subjected to thermocycling, cyclic loading, and finally static loading to failure with a computer-controlled Universal Testing Machine. An independent t test (1 sided) determined whether the mean values of the fracture load differed significantly (α=.05) between the 2 groups. No specimen failed during cyclic loading. Upon static loading, the mean (SD) load to failure was significantly higher for the PR group (525.89 [143.547] N) than for the ZR group (413.70 [35.515] N) for internal connection narrow platform bone-level implants (P=.025). Failure was initiated at the screw and internal connection level for both groups. It is possible to fabricate PR abutments that are stronger than ZR abutments for Nobel Biocare internal connection NP bone-level implants. The screw and the internal connection are the weak links for both groups. Copyright © 2013 Editorial Council for the Journal of Prosthetic Dentistry. Published by Mosby, Inc. All rights reserved.

Hydraulic properties of 3D rough-walled fractures during shearing: An experimental study

NASA Astrophysics Data System (ADS)

Yin, Qian; Ma, Guowei; Jing, Hongwen; Wang, Huidong; Su, Haijian; Wang, Yingchao; Liu, Richeng

2017-12-01

This study experimentally analyzed the influence of shear processes on nonlinear flow behavior through 3D rough-walled rock fractures. A high-precision apparatus was developed to perform stress-dependent fluid flow tests of fractured rocks. Then, water flow tests on rough-walled fractures with different mechanical displacements were conducted. At each shear level, the hydraulic pressure ranged from 0 to 0.6 MPa, and the normal load varied from 7 to 35 kN. The results show that (i) the relationship between the volumetric flow rate and hydraulic gradient of rough-walled fractures can be well fit using Forchheimer's law. Notably, both the linear and nonlinear coefficients in Forchheimer's law decrease during shearing; (ii) a sixth-order polynomial function is used to evaluate the transmissivity based on the Reynolds number of fractures during shearing. The transmissivity exhibits a decreasing trend as the Reynolds number increases and an increasing trend as the shear displacement increases; (iii) the critical hydraulic gradient, critical Reynolds number and equivalent hydraulic aperture of the rock fractures all increase as the shear displacement increases. When the shear displacement varies from 0 to 15 mm, the critical hydraulic gradient ranges from 0.3 to 2.2 for a normal load of 7 kN and increases to 1.8-8.6 for a normal load of 35 kN; and (iv) the Forchheimer law results are evaluated by plotting the normalized transmissivity of the fractures during shearing against the Reynolds number. An increase in the normal load shifts the fitted curves downward. Additionally, the Forchheimer coefficient β decreases with the shear displacement but increases with the applied normal load.

Polyaxial Screws in Locked Plating of Tibial Pilon Fractures.

PubMed

Yenna, Zachary C; Bhadra, Arup K; Ojike, Nwakile I; Burden, Robert L; Voor, Michael J; Roberts, Craig S

2015-08-01

This study examined the axial and torsional stiffness of polyaxial locked plating techniques compared with fixed-angle locked plating techniques in a distal tibia pilon fracture model. The effect of using a polyaxial screw to cross the fracture site was examined to determine its ability to control relative fracture site motion. A laboratory experiment was performed to investigate the biomechanical stiffness of distal tibia fracture models repaired with 3.5-mm anterior polyaxial distal tibial plates and locking screws. Sawbones Fourth Generation Composite Tibia models (Pacific Research Laboratories, Inc, Vashon, Washington) were used to model an Orthopaedic Trauma Association 43-A1.3 distal tibia pilon fracture. The polyaxial plates were inserted with 2 central locking screws at a position perpendicular to the cortical surface of the tibia and tested for load as a function of axial displacement and torque as a function of angular displacement. The 2 screws were withdrawn and inserted at an angle 15° from perpendicular, allowing them to span the fracture and insert into the opposing fracture surface. Each tibia was tested again for axial and torsional stiffness. In medial and posterior loading, no statistically significant difference was found between tibiae plated with the polyaxial plate and the central screws placed in the neutral position compared with the central screws placed at a 15° position. In torsional loading, a statistically significant difference was noted, showing greater stiffness in tibiae plated with the polyaxial plate and the central screws placed at a 15° position compared with tibiae plated with the central screws placed at a 0° (or perpendicular) position. This study showed that variable angle constructs show similar stiffness properties between perpendicular and 15° angle insertions in axial loading. The 15° angle construct shows greater stiffness in torsional loading. Copyright 2015, SLACK Incorporated.

Fracture Resistance of Lithium Disilicate Ceramics Bonded to Enamel or Dentin Using Different Resin Cement Types and Film Thicknesses.

PubMed

Rojpaibool, Thitithorn; Leevailoj, Chalermpol

2017-02-01

To investigate the influence of cement film thickness, cement type, and substrate (enamel or dentin) on ceramic fracture resistance. One hundred extracted human third molars were polished to obtain 50 enamel and 50 dentin specimens. The specimens were cemented to 1-mm-thick lithium disilicate ceramic plates with different cement film thicknesses (100 and 300 μm) using metal strips as spacers. The cements used were etch-and-rinse (RelyX Ultimate) and self-adhesive (RelyX U200) resin cements. Compressive load was applied on the ceramic plates using a universal testing machine, and fracture loads were recorded in Newtons (N). Statistical analysis was performed by multiple regression (p < 0.05). Representative specimens were evaluated by scanning electron microscopy to control the cement film thickness. The RelyX Ultimate group with a cement thickness of 100 μm cemented to enamel showed the highest mean fracture load (MFL; 1591 ± 172.59 N). The RelyX Ultimate groups MFLs were significantly higher than the corresponding RelyX U200 groups (p < 0.05), and thinner film cement demonstrated a higher MFL than thicker films (p < 0.05). Bonding to dentin resulted in lower MFL than with enamel (p < 0.001). Higher fracture loads were related to thinner cement film thickness and RelyX Ultimate resin cement. Bonding to dentin resulted in lower fracture loads than bonding to enamel. Reduced resin film thickness could reduce lithium disilicate restoration fracture. Etch-and-rinse resin cements are recommended for cementing on either enamel or dentin, compared with self-adhesive resin cement, for improved fracture resistance. © 2015 by the American College of Prosthodontists.

Load-bearing capacity of various CAD/CAM monolithic molar crowns under recommended occlusal thickness and reduced occlusal thickness conditions

PubMed Central

Choi, Sulki

2017-01-01

PURPOSE The goal of this study was to evaluate the fracture resistances of various monolithic crowns fabricated by computer-aided design and computer-aided manufacturing (CAD/CAM) with different thickness. MATERIALS AND METHODS Test dies were fabricated as mandibular molar forms with occlusal reductions using CAD/CAM. With different occlusal thickness (1.0 or 1.5 mm), a polymer-infiltrated ceramic network (Enamic, EN), and zirconia-reinforced lithium silicate (Suprinity, SU and Celtra-Duo, CD) were used to fabricate molar crowns. Lithium disilicate (e.max CAD, EM) crowns (occlusal: 1.5 mm) were fabricated as control. Seventy crowns (n=10 per group) were bonded to abutments and stored in water for 24 hours. A universal testing machine was used to apply load to crown until fracture. The fractured specimens were examined with a scanning electron microscopy. RESULTS The type of ceramics and the occlusal thickness showed a significant interaction. With a recommended thickness (1.5 mm), the SU revealed the mean load similar to the EM, higher compared with those of the EN and CD. The fracture loads in a reduced thickness (1.0 mm) were similar among the SU, CD, and EN. The mean fracture load of the SU and CD enhanced significantly when the occlusal thickness increased, whereas that of the EN did not. CONCLUSION The fracture loads of monolithic crowns were differently influenced by the changes in occlusal thickness, depending on the type of ceramics. Within the limitations of this study, all the tested crowns withstood the physiological masticatory loads both at the recommended and reduced occlusal thickness. PMID:29279761

Influence of ceramic thickness and ceramic materials on fracture resistance of posterior partial coverage restorations.

PubMed

Bakeman, E M; Rego, N; Chaiyabutr, Y; Kois, J C

2015-01-01

This study evaluated the influence of ceramic thickness and ceramic materials on fracture resistance of posterior partial coverage ceramic restorations. Forty extracted molars were allocated into four groups (n=10) to test for two variables: 1) the thickness of ceramic (1 mm or 2 mm) and 2) the ceramic materials (a lithium disilicate glass-ceramic [IPS e.max] or leucite-reinforced glass ceramic [IPS Empress]). All ceramic restorations were luted with resin cement (Variolink II) on the prepared teeth. These luted specimens were loaded to failure in a universal testing machine, in the compression mode, with a crosshead speed of 1.0 mm/min. The data were analyzed using two-way analysis of variance and the Tukey Honestly Significantly Different multiple comparison test (α =0.05). The fracture resistance revealed a significant effect for materials (p<0.001); however, the thickness of ceramic was not significant (p=0.074), and the interaction between the thickness of ceramic and the materials was not significant (p=0.406). Mean (standard deviation) fracture resistance values were as follows: a 2-mm thickness of a lithium disilicate bonded to tooth structure (2505 [401] N) revealed a significantly higher fracture resistance than did a 1-mm thickness of leucite-reinforced (1569 [452] N) and a 2-mm thickness of leucite-reinforced ceramic bonded to tooth structure (1716 [436] N) (p<0.05). There was no significant difference in fracture resistance values between a lithium disilicate ceramic at 1-mm thickness (2105 [567] N) and at 2-mm thickness. Using a lithium disilicate glass ceramic for partial coverage restoration significantly improved fracture resistance compared to using a leucite-reinforced glass ceramic. The thickness of ceramic had no significant effect on fracture resistance when the ceramics were bonded to the underlying tooth structure.

Effect of increased crown height on stress distribution in short dental implant components and their surrounding bone: A finite element analysis.

PubMed

Bulaqi, Haddad Arabi; Mousavi Mashhadi, Mahmoud; Safari, Hamed; Samandari, Mohammad Mahdi; Geramipanah, Farideh

2015-06-01

Implants in posterior regions of the jaw require short dental implants with long crown heights, leading to increased crown-to-implant ratios and mechanical stress. This can lead to fracture and screw loosening. The purpose of this study was to investigate the dynamic nature and behavior of prosthetic components and preimplant bone and evaluate the effect of increased crown height space (CHS) and crown-to-implant ratio on stress concentrations under external oblique forces. The severely resorbed bone of a posterior mandible site was modeled with Mimics and Catia software. A second mandibular premolar tooth was modeled with CHS values of 8.8, 11.2, 13.6, and 16 mm. A Straumann implant (4.1×8 mm), a directly attached crown, and an abutment screw were modeled with geometric data and designed by using SolidWorks software. Abaqus software was used for the dynamic simulation of screw tightening and the application of an external load to the buccal cusp at a 75.8-degree angle with the occlusal plane. The distribution of screw load and member load at each step was compared, and the stress values were calculated within the dental implant complex and surrounding bone. During tightening, the magnitude and distribution of the preload and clamp load were uniform and equal at the cross section of all CHSs. Under an external load, the screw load decreased and member load increased. An increase in the CHS caused the corresponding distribution to become more nonuniform and increased the maximum compressive and tensile stresses in the preimplant bone. Additionally, the von Mises stress decreased at the abutment screw and increased at the abutment and fixture. Under nonaxial forces, increased CHS does not influence the decrease in screw load or increase in member load. However, it contributes to screw loosening and fatigue fracture by skewing the stress distribution to the transverse section of the implant. Copyright © 2015 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

The Effects of Torsional Preloading on the Torsional Resistance of Nickel-titanium Instruments.

PubMed

Oh, Seung-Hei; Ha, Jung-Hong; Kwak, Sang Won; Ahn, Shin Wook; Lee, WooCheol; Kim, Hyeon-Cheol

2017-01-01

This study evaluated the effect of torsional preloading on the torsional resistance of nickel-titanium (NiTi) endodontic instruments. WaveOne Primary (Dentsply Maillefer, Ballaigues, Switzerland) and ProTaper Universal F2 (Dentsply Maillefer) files were used. The ultimate torsional strength until fracture was determined for each instrument. In the phase 1 experiment, the ProTaper and WaveOne files were loaded to have a maximum load from 2.0 up to 2.7 or 2.8 Ncm, respectively. In the phase 2 experiment, the number of repetitions of preloading for each file was increased from 50 to 200, whereas the preloading torque was fixed at 2.4 Ncm. Using torsionally preloaded specimens from phase 1 and 2, the torsional resistances were calculated to determine the ultimate strength, distortion angle, and toughness. The results were analyzed using 1-way analysis of variance and Duncan post hoc comparison. The fracture surfaces and longitudinal aspect of 5 specimens per group were examined under a scanning electron microscope. All preloaded groups showed significantly higher ultimate strength than the unpreloaded groups (P < .05). There was no significant difference among all groups for distortion angle and toughness. Although WaveOne had no significant difference between the repetition groups for ultimate strength, fracture angle, and toughness, ProTaper had a higher distortion angle and toughness in the 50-repetition group compared with the other repetition groups (P < .05). Scanning electron microscopic examinations of the fractured surface showed typical features of torsional fracture. Torsional preloading within the ultimate values could enhance the torsional strength of NiTi instruments. The total energy until fracture was maintained constantly, regardless of the alloy type. Copyright © 2016 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

Influence of various bonding techniques on the fracture strength of thin CAD/CAM-fabricated occlusal glass-ceramic veneers.

PubMed

Yazigi, Christine; Kern, Matthias; Chaar, Mohamed Sad

2017-11-01

To evaluate the efficiency of immediate dentin sealing and the effects of different bonding protocols on the fracture strength of CAD/CAM occlusal veneers bonded to exposed dentin. Ninety-six extracted maxillary premolars were initially divided into three main groups with 32 specimens each: without immediate dentin sealing, immediate dentin sealing/total etching and immediate dentin sealing/selective etching. Teeth were identically prepared in the dentin to receive occlusal veneers of 0.8mm thickness, milled from lithium disilicate ceramic blocks (IPS e.max CAD). Each main group was later subdivided, according to the pre-cementation surface etching protocol (total/selective), into two subgroups with 16 specimens each. All restorations were adhesively bonded using a resin cement (Variolink Esthetic). Half of the specimens of each subgroup were subjected to thermo-dynamic loading in a chewing simulator with 1,200,000 cycles at 10kg load. The other half and the surviving specimens were subjected to quasi-static loading until failure. Statistical analysis was performed using three-way ANOVA and Tukey's post-hoc tests. All specimens except one survived the artificial aging. A significantly higher fracture strength of restorations (p ≤ 0.001) was obtained when immediate dentin sealing was followed regardless of the etching method with values ranging from a minimum of 1122 ± 336N to a maximum of 1853 ± 333N. Neither the pre-cementation treatment nor the artificial aging had a statistical significant effect on the fracture strength. Immediate dentin sealing protocol is recommended whenever dentin is exposed during the preparation for thin glass-ceramic occlusal veneers. Copyright © 2017 Elsevier Ltd. All rights reserved.

Evaluation of fracture resistance in simulated immature teeth using Resilon and Ribbond as root reinforcements--an in vitro study.

PubMed

Hemalatha, Hiremath; Sandeep, Metgud; Kulkarni, Sadanand; Yakub, Shoeb Sheikh

2009-08-01

To compare the reinforcement and strengthening ability of resilon, gutta-percha, and ribbond in endodontically treated roots of immature teeth. Sixty five freshly extracted human maxillary anterior teeth were prepared with a Peeso no. 6 to simulate immature teeth (Cvek's stage 3 root development). After instrumentation, each root was irrigated with sodium hypochlorite and with ethylene diamino tetra acetic acid to remove the smear layer. To simulate single visit apexification technique a 4-5 mm white Pro Root mineral trioxide aggregate plug was placed apically using schilder carrier. The teeth were divided into three experimental groups and one control group. Group I--control group (root canals instrumented but not filled); Group II--backfilled with thermoplastisized gutta-percha using AH plus sealer; Group III--reinforced with Resilon using epiphany sealer; Group IV--reinforced with Ribbond fibers using Panavia F luting cement. A Universal Testing Machine was used to apply a load, at the level of the lingual cementoenamel junction with a chisel-shaped tip The peak load to fracture was recorded and statistical analysis was completed using student's t-test. Values of peak load to fracture were 1320.8, 1604.88, 1620, and 1851 newtons for Group I to Group IV respectively. The results of student's t-test, revealed no significant difference (P > 0.05,) between Group II and Group III. Comparison between Group IV and Group III and between Group IV and Group II revealed highly significant difference (P > 0.001). Teeth reinforced with Ribbond fibers using Panavia F luting cement showed the highest resistance to fracture. Resilon could not strengthen the roots and showed no statistically significant difference when compared with thermoplastisized gutta-percha in reinforcing immature tooth when tested with universal testing machine in an experimental model of immature tooth.

Performance of Chevron-notch short bar specimen in determining the fracture toughness of silicon nitride and aluminum oxide

NASA Technical Reports Server (NTRS)

Munz, D.; Bubsey, R. T.; Shannon, J. L., Jr.

1980-01-01

Ease of preparation and testing are advantages unique to the chevron-notch specimen used for the determination of the plane strain fracture toughness of extremely brittle materials. During testing, a crack develops at the notch tip and extends stably as the load is increased. For a given specimen and notch configuration, maximum load always occurs at the same relative crack length independent of the material. Fracture toughness is determined from the maximum load with no need for crack length measurement. Chevron notch acuity is relatively unimportant since a crack is produced during specimen loading. In this paper, the authors use their previously determined stress intensity factor relationship for the chevron-notch short bar specimen to examine the performance of that specimen in determining the plane strain fracture toughness of silicon nitride and aluminum oxide.

A discrete element model for damage and fracture of geomaterials under fatigue loading

NASA Astrophysics Data System (ADS)

Gao, Xiaofeng; Koval, Georg; Chazallon, Cyrille

2017-06-01

Failure processes in geomaterials (concrete, asphalt concrete, masonry, etc.) under fatigue loading (repeated moving loads, cycles of temperature, etc.) are responsible for most of the dysfunctions in pavements, brick structures, etc. In the beginning of the lifetime of a structure, the material presents only inner defects (micro cracks, voids, etc.). Due to the effect of the cyclic loading, these small defects tend to grow in size and quantity which damage the material, reducing its stiffness. With a relatively high number of cycles, these growing micro cracks become large cracks, which characterizes the fracture behavior. From a theoretical point of view, both mechanisms are treated differently. Fracture is usually described locally, with the propagation of cracks defined by the energy release rate at the crack tip; damage is usually associated to non-local approaches. In the present work, damage and fracture mechanics are combined in a local discrete element approach.

Relative stability of tension band versus two-cortex screw fixation for treating fifth metatarsal base avulsion fractures.

PubMed

Husain, Z S; DeFronzo, D J

2000-01-01

This study assesses the strength of fixating avulsion fractures of the fifth metatarsal base with a 4.0-mm partially threaded cancellous screw crossing two cortices as compared to tension banding. Our data showed statistically significant fixation strength improvement over tension banding for avulsion fractures (p < 0.02) in both polystyrene foam models and fresh, nonpreserved frozen cadaveric samples. In cadavers, the screw fixations were able to withstand more than three times the load sustained by the tension band fixations. The study utilized the Instron 8500 tensiometer to apply physiologic loads to test the constructs until failure. The displacement and load data at failure show the limitations of both fixations. By increasing the load resistance while maintaining compression, the bicortical cancellous screw fixation created greater stability at the avulsion fracture of the fifth metatarsal base as compared to tension band stabilization.

RSRM nozzle actuator bracket/lug fracture mechanics qualification test

NASA Technical Reports Server (NTRS)

Kelley, Peggy

1993-01-01

This is the final report for the actuator bracket/lug fracture mechanics qualification test. The test plan (CTP-0071) outlined a two-phase test program designed to answer questions about the fracture criticality of the redesigned solid rocket motor (RSRM) nozzle actuator bracket. An analysis conducted using the NASA/FLAGRO fracture mechanics computer program indicated that the actuator bracket might be a fracture critical component. In the NASA/FLAGRO analysis, a simple lug model was used to represent the actuator bracket. It was calculated that the bracket would fracture if subjected to an actuator stall load in the presence of a 0.10 in. corner crack at the actuator attachment hole. The 0.10 in. crack size corresponds to the nondestructive inspection detectability limit for the actuator bracket. The inspection method used is the dye penetrant method. The actuator stall load (103,424 lb) is the maximum load which the actuator bracket is required to withstand during motor operation. This testing was designed to establish the accuracy of the analytical model and to directly determine whether the actuator bracket is capable of meeting fracture mechanics safe-life requirements.

Effect of core thickness differences on post-fatigue indentation fracture resistance of veneered zirconia crowns.

PubMed

Alhasanyah, Abdulrahman; Vaidyanathan, Tritala K; Flinton, Robert J

2013-07-01

Despite the excellent esthetics of veneered zirconia crowns, the incidence of chipping and fracture of veneer porcelain on zirconia crowns has been recognized to be higher than in metal ceramic crowns. The objective of this investigation was to study the effect of selected variations in core thickness on the post-fatigue fracture resistance of veneer porcelain on zirconia crowns. Zirconia crowns for veneering were prepared with three thickness designs of (a) uniform 0.6-mm thick core (group A), (b) extra-thick 1.7 mm occlusal core support (group B), and (c) uniform 1.2-mm thick core (group C). The copings were virtually designed and milled by the CAD/CAM technique. Metal ceramic copings (group D) with the same design as in group C were used as controls. A sample size of N = 20 was used for each group. The copings were veneered with compatible porcelain and fatigue tested under a sinusoidal loading regimen. Loading was done with a 200 N maximum force amplitude under Hertzian axial loading conditions at the center of the crowns using a spherical tungsten carbide indenter. After 100,000 fatigue cycles, the crowns were axially loaded to fracture and maximum load levels before fracture was recorded. One-way ANOVA (P < 0.05) and post hoc Tukey tests (α = 0.05) were used to determine significant differences between means. The mean fracture failure load of group B was not significantly different from that of control group D. In contrast, the mean failure loads of groups A and C were significantly lower than that of control group D. Failure patterns also indicated distinct differences in failure mode distributions. The results suggest that proper occlusal core support improves veneer chipping fracture resistance in zirconia crowns. Extra-thick occlusal core support for porcelain veneer may significantly reduce the veneer chipping and fracture of zirconia crowns. This is suggested as an important consideration in the design of copings for zirconia crowns. © 2013 by the American College of Prosthodontists.

Fracture of Human Femur Tissue Monitored by Acoustic Emission Sensors

PubMed Central

Aggelis, Dimitrios. G.; Strantza, Maria; Louis, Olivia; Boulpaep, Frans; Polyzos, Demosthenes; van Hemelrijck, Danny

2015-01-01

The study describes the acoustic emission (AE) activity during human femur tissue fracture. The specimens were fractured in a bending-torsion loading pattern with concurrent monitoring by two AE sensors. The number of recorded signals correlates well with the applied load providing the onset of micro-fracture at approximately one sixth of the maximum load. Furthermore, waveform frequency content and rise time are related to the different modes of fracture (bending of femur neck or torsion of diaphysis). The importance of the study lies mainly in two disciplines. One is that, although femurs are typically subjects of surgical repair in humans, detailed monitoring of the fracture with AE will enrich the understanding of the process in ways that cannot be achieved using only the mechanical data. Additionally, from the point of view of monitoring techniques, applying sensors used for engineering materials and interpreting the obtained data pose additional difficulties due to the uniqueness of the bone structure. PMID:25763648

Constant-load delayed fracture test of atmospherically corroded high strength steels

NASA Astrophysics Data System (ADS)

Akiyama, Eiji; Matsukado, Katsuhiro; Li, Songjie; Tsuzaki, Kaneaki

2011-07-01

Constant load tests of circumferentially notched round bar specimens of high strength steels after cyclic corrosion test and outdoor exposure have been performed to demonstrate that delayed fracture occurs when the hydrogen content from the environment, H E, exceeds the critical hydrogen content for delayed fracture, H C. During the constant load tests the humidity around the specimen was increased in stepwise manner to increase hydrogen entry. After fracture the specimen was kept at the humidity long enough to homogenize hydrogen in the specimen and to obtain more quantitative hydrogen content by thermal desorption analysis. H E of the fractured specimens was higher than H C, and H E of the specimens not fractured was lower than H C. This result confirms that the balance between H C and H E determines the occurrence of delayed fracture and that hydrogen-content-based evaluation of susceptibility to delayed fracture is reasonable. To certify the increase of H E with increase in humidity, electrochemical hydrogen permeation test was carried out. The hydrogen permeation current density was increased especially at 98%RH. Enhancement of hydrogen entry with increase in CCT number was also shown by the test.

Strength, Fatigue, and Fracture Toughness of Ti-6Al-4V Liner from a Composite Over-Wrapped Pressure Vessel

NASA Technical Reports Server (NTRS)

Salem, Jonathan A.; Lerch, Brad; Thesken, John C.; Sutter, Jim; Russell, Richard

2008-01-01

It was demonstrated by way of experiment that Composite Over-wrapped Pressure Vessel (COPV) Ti-6Al-4V liner material can sustain the expected service loads and cycles. The experiments were performed as part of investigations on the residual life of COPV tanks being used in Space Shuttle Orbiters. Measured properties included tensile strength, compressive strength, reversed loading cycles to simulate liner proof strains, and cyclic fatigue loading to demonstrate the ability to sustain 1000 cycles after liner buckling. The liner material came from a salvaged 40 in. Columbia (orbiter 102) tank (SN029), and tensile strength measurements were made on both boss-transition (thick) and membrane regions (thin). The average measured yield strength was 131 ksi in the boss-transition and membrane regions, in good agreement with measurements made on 1970 s vintage forged plate stock. However, Young s modulus was 17.4+/-0.3 Msi, somewhat higher than typical handbook values (approx.16 Msi). The fracture toughness, as estimated from a failed fatigue specimen, was 74 ksi/sq in, in reasonable agreement with standardized measurements made on 1970 s vintage forged plate stock. Low cycle fatigue of a buckled test specimen implied that as-imprinted liners can sustain over 4000 load cycles.

Porous media fracturing dynamics: stepwise crack advancement and fluid pressure oscillations

NASA Astrophysics Data System (ADS)

Cao, Toan D.; Hussain, Fazle; Schrefler, Bernhard A.

2018-02-01

We present new results explaining why fracturing in saturated porous media is not smooth and continuous but is a distinct stepwise process concomitant with fluid pressure oscillations. All exact solutions and almost all numerical models yield smooth fracture advancement and fluid pressure evolution, while recent experimental results, mainly from the oil industry, observation from geophysics and a very few numerical results for the quasi-static case indeed reveal the stepwise phenomenon. We summarize first these new experiments and these few numerical solutions for the quasi-static case. Both mechanical loading and pressure driven fractures are considered because their behaviours differ in the direction of the pressure jumps. Then we explore stepwise crack tip advancement and pressure fluctuations in dynamic fracturing with a hydro-mechanical model of porous media based on the Hybrid Mixture Theory. Full dynamic analyses of examples dealing with both hydraulic fracturing and mechanical loading are presented. The stepwise fracture advancement is confirmed in the dynamic setting as well as in the pressure fluctuations, but there are substantial differences in the frequency contents of the pressure waves in the two loading cases. Comparison between the quasi-static and fully dynamic solutions reveals that the dynamic response gives much more information such as the type of pressure oscillations and related frequencies and should be applied whenever there is a doubt about inertia forces playing a role - the case in most fracturing events. In the absence of direct relevant dynamic tests on saturated media some experimental results on dynamic fracture in dry materials, a fast hydraulic fracturing test and observations from geophysics confirm qualitatively the obtained results such as the type of pressure oscillations and the substantial difference in the behaviour under the two loading cases.

Type 2 Diabetes and Metformin Influence on Fracture Healing in an Experimental Rat Model.

PubMed

La Fontaine, Javier; Chen, Chris; Hunt, Nathan; Jude, Edward; Lavery, Lawrence

2016-01-01

Persons with diabetes have a greater incidence of fractures compared with persons without diabetes. However, very little published information is available concerning the deleterious effect of late-stage diabetes on osseous structure and bone healing. The purpose of the present study was to evaluate the role of diabetes on fracture healing in a rat femur repair model. Thirty-six lean and diabetic Zucker rats were subdivided into 3 groups: (1) 12 lean rats as the control group; (2) 12 diabetic rats without blood glucose control (DM group); and (3) 12 diabetic rats treated with 300 mg/kg metformin to reduce the blood glucose levels (DM + Met group). Radiographs were taken every week to determine the incidence of bone repair and delayed union. All the rats were killed at 6 weeks after surgery. In both the sham-operated and the fractured and repaired femurs, significant decreases in the fracture-load/weight and marginal decreases in the fracture-load between the lean and DM groups were found. Metformin treatment significantly reduced the blood glucose and body weight 12 days postoperatively. Furthermore, a decrease in the fracture-load and fracture-load/weight in the repaired femurs was found in the DM + Met group. Diabetes impairs bone fracture healing. Metformin treatment reduces the blood glucose and body weight but had an adverse effect on fracture repair in diabetic rats. Further investigations are needed to reveal the mechanisms responsible for the effects of type 2 diabetes mellitus on bone and bone quality and the effect of medications such as metformin might have in diabetic bone in the presence of neuropathy and vascular disease. Copyright © 2016 American College of Foot and Ankle Surgeons. Published by Elsevier Inc. All rights reserved.

Biomechanical Strength of Retrograde Fixation in Proximal Third Scaphoid Fractures.

PubMed

Daly, Charles A; Boden, Allison L; Hutton, William C; Gottschalk, Michael B

2018-04-01

Current techniques for fixation of proximal pole scaphoid fractures utilize antegrade fixation via a dorsal approach endangering the delicate vascular supply of the dorsal scaphoid. Volar and dorsal approaches demonstrate equivalent clinical outcomes in scaphoid wrist fractures, but no study has evaluated the biomechanical strength for fractures of the proximal pole. This study compares biomechanical strength of antegrade and retrograde fixation for fractures of the proximal pole of the scaphoid. A simulated proximal pole scaphoid fracture was produced in 22 matched cadaveric scaphoids, which were then assigned randomly to either antegrade or retrograde fixation with a cannulated headless compression screw. Cyclic loading and load to failure testing were performed and screw length, number of cycles, and maximum load sustained were recorded. There were no significant differences in average screw length (25.5 mm vs 25.6 mm, P = .934), average number of cyclic loading cycles (3738 vs 3847, P = .552), average load to failure (348 N vs 371 N, P = .357), and number of catastrophic failures observed between the antegrade and retrograde fixation groups (3 in each). Practical equivalence between the 2 groups was calculated and the 2 groups were demonstrated to be practically equivalent (upper threshold P = .010). For this model of proximal pole scaphoid wrist fractures, antegrade and retrograde screw configuration have been proven to be equivalent in terms of biomechanical strength. With further clinical study, we hope surgeons will be able to make their decision for fixation technique based on approaches to bone grafting, concern for tenuous blood supply, and surgeon preference without fear of poor biomechanical properties.

Micromechanics based simulation of ductile fracture in structural steels

NASA Astrophysics Data System (ADS)

Yellavajjala, Ravi Kiran

The broader aim of this research is to develop fundamental understanding of ductile fracture process in structural steels, propose robust computational models to quantify the associated damage, and provide numerical tools to simplify the implementation of these computational models into general finite element framework. Mechanical testing on different geometries of test specimens made of ASTM A992 steels is conducted to experimentally characterize the ductile fracture at different stress states under monotonic and ultra-low cycle fatigue (ULCF) loading. Scanning electron microscopy studies of the fractured surfaces is conducted to decipher the underlying microscopic damage mechanisms that cause fracture in ASTM A992 steels. Detailed micromechanical analyses for monotonic and cyclic loading are conducted to understand the influence of stress triaxiality and Lode parameter on the void growth phase of ductile fracture. Based on monotonic analyses, an uncoupled micromechanical void growth model is proposed to predict ductile fracture. This model is then incorporated in to finite element program as a weakly coupled model to simulate the loss of load carrying capacity in the post microvoid coalescence regime for high triaxialities. Based on the cyclic analyses, an uncoupled micromechanics based cyclic void growth model is developed to predict the ULCF life of ASTM A992 steels subjected to high stress triaxialities. Furthermore, a computational fracture locus for ASTM A992 steels is developed and incorporated in to finite element program as an uncoupled ductile fracture model. This model can be used to predict the ductile fracture initiation under monotonic loading in a wide range of triaxiality and Lode parameters. Finally, a coupled microvoid elongation and dilation based continuum damage model is proposed, implemented, calibrated and validated. This model is capable of simulating the local softening caused by the various phases of ductile fracture process under monotonic loading for a wide range of stress states. Novel differentiation procedures based on complex analyses along with existing finite difference methods and automatic differentiation are extended using perturbation techniques to evaluate tensor derivatives. These tensor differentiation techniques are then used to automate nonlinear constitutive models into implicit finite element framework. Finally, the efficiency of these automation procedures is demonstrated using benchmark problems.

Computational characterization of fracture healing under reduced gravity loading conditions.

PubMed

Gadomski, Benjamin C; Lerner, Zachary F; Browning, Raymond C; Easley, Jeremiah T; Palmer, Ross H; Puttlitz, Christian M

2016-07-01

The literature is deficient with regard to how the localized mechanical environment of skeletal tissue is altered during reduced gravitational loading and how these alterations affect fracture healing. Thus, a finite element model of the ovine hindlimb was created to characterize the local mechanical environment responsible for the inhibited fracture healing observed under experimental simulated hypogravity conditions. Following convergence and verification studies, hydrostatic pressure and strain within a diaphyseal fracture of the metatarsus were evaluated for models under both 1 and 0.25 g loading environments and compared to results of a related in vivo study. Results of the study suggest that reductions in hydrostatic pressure and strain of the healing fracture for animals exposed to reduced gravitational loading conditions contributed to an inhibited healing process, with animals exposed to the simulated hypogravity environment subsequently initiating an intramembranous bone formation process rather than the typical endochondral ossification healing process experienced by animals healing in a 1 g gravitational environment. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1206-1215, 2016. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.

Fundamental considerations in dynamic fracture in nuclear materials

NASA Astrophysics Data System (ADS)

Cady, Carl; Eastwood, David; Bourne, Neil; Pei, Ruizhi; Mummery, Paul; Rau, Christoph

2017-06-01

The structural integrity of components used in nuclear power plants is the biggest concern of operators. A diverse range of materials, loading, prior histories and environmental conditions, leads to a complex operating environment. An experimental technique has been developed to characterize brittle materials and using linear elastic fracture mechanics, has given accurate measurements of the fracture toughness of materials. X-ray measurements were used to track the crack front as a function of loading parameters as well as determine the crack surface area as loads increased. This X-ray tomographic study of dynamic fracture in beryllium indicates the onset of damage within the target as load is increased. Similarly, measurements on nuclear graphite were conducted to evaluate the technique. This new, quantitative information obtained using the X-ray techniques has shown application in other materials. These materials exhibited a range of brittle and ductile responses that will test our modelling schemes for fracture. Further visualization of crack front advance and the correlated strain fields that are generated during the experiment for the two distinct deformation processes provide a vital step in validating new multiscale predicative modelling.

Crack propagation and arrest in pressurized containers

NASA Technical Reports Server (NTRS)

Erdogan, F.; Delale, F.; Owczarek, J. A.

1976-01-01

The problem of crack propagation and arrest in a finite volume cylindrical container filled with pressurized gas is considered. It is assumed that the cylinder contains a symmetrically located longitudinal part-through crack with a relatively small net ligament. The net ligament suddenly ruptures initiating the process of fracture propagation and depressurization in the cylinder. Thus the problem is a coupled gas dynamics and solid mechanics problem the exact formulation of which does not seem to be possible. The problem is reduced to a proper initial value problem by introducing a dynamic fracture criterion which relates the crack acceleration to the difference between a load factor and the corresponding strength parameter. The results indicate that generally in gas filled cylinders fracture arrest is not possible unless the material behaves in a ductile manner and the container is relatively long.

Effect of different root canal sealers on fracture strength of simulated immature roots.

PubMed

Ulusoy, Özgür İlke Atasoy; Nayır, Yelda; Darendeliler-Yaman, Sis

2011-10-01

The objective of this study was to compare the effects of different root canal sealers on fracture resistance of simulated immature teeth. One hundred eight roots were divided into 9 groups. The roots were instrumented except the negative controls. Four millimeters of mineral trioxide aggregate (MTA) barriers were placed apically. The roots were backfilled as follows: group 1, AH Plus+gutta-percha; group 2, EndoREZ+gutta-percha; group 3, EndoREZ+Resilon; group 4, Hybrid Root SEAL+gutta-percha; group 5, Hybrid Root SEAL+Resilon; group 6, iRootSP+gutta-percha; group 7, iRootSP+Resilon; group 8, No obturation other than MTA barrier; group 9, No instrumentation, no obturation. A compressive loading was applied at a speed of 1 mm/min. Data were compared with ANOVA and Duncan tests. Group 5 showed the highest resistance to fracture. The fracture values of group 3 were lower than those of the other experimental groups. Hybrid Root SEAL and iRootSP reinforce the simulated immature roots against fracture when used with either gutta-percha or Resilon. Copyright © 2011 Mosby, Inc. All rights reserved.

Anomaly in the dynamic strength of austenitic stainless steel 12Cr19Ni10Ti under shock wave loading

NASA Astrophysics Data System (ADS)

Garkushin, G. V.; Kanel, G. I.; Razorenov, S. V.; Savinykh, A. S.

2017-07-01

Measurement results for the shock wave compression profiles of 12Cr19Ni10Ti steel and its dynamic strength in the strain rate range 105-106 s-1 are presented. The protracted viscous character of the spall fracture is revealed. With the previously obtained data taken into account, the measurement results are described by a polynomial relation, which can be used to construct the fracture kinetics. On the lower boundary of the range, the resistance to spall fracture is close to the value of the true strength of the material under standard low-rate strain conditions; on the upper boundary, the spall strength is more than twice greater than this quantity. An increase in the temperature results in a decrease in both the dynamic limit of elasticity and the spall fracture strength of steel. The most interesting result is the anomaly in the dependence of the spall fracture strength on the duration of the shock wave compression pulse, which is related to the formation of deformation martensite near the growing discontinuities.

Effects of fiber-glass-reinforced composite restorations on fracture resistance and failure mode of endodontically treated molars.

PubMed

Nicola, Scotti; Alberto, Forniglia; Riccardo, Michelotto Tempesta; Allegra, Comba; Massimo, Saratti Carlo; Damiano, Pasqualini; Mario, Alovisi; Elio, Berutti

2016-10-01

The study evaluated the fracture resistance and fracture patterns of endodontically treated mandibular first molars restored with glass-fiber-reinforced direct composite restorations. In total, 60 extracted intact first molars were treated endodontically; a mesio-occluso-distal (MOD) cavity was prepared and specimens were then divided into six groups: sound teeth (G1), no restoration (G2), direct composite restoration (G3), fiber-post-supported direct composite restoration (G4), direct composite reinforced with horizontal mesio-distal glass-fibers (G5), and buccal-palatal glass-fibers (G6). Specimens were subjected to 5000 thermocycles and 20,000 cycles of 45° oblique loading force at 1.3Hz and 50N; they were then loaded until fracture. The maximum fracture loads were recorded in Newtons (N) and data were analyzed with one-way ANOVA and post-hoc Tukey tests (p<0.05). Fractured specimens were analyzed with a scanning electron microscope (SEM). The mean static loads (in Newtons) were: G1, 831.83; G2, 282.86; G3, 364.18; G4, 502.93; G5, 499.26; and G6, 582.22. Fracture resistance did not differ among G4, G5, and G6, but was significantly higher than G3 (p=0.001). All specimens fractured in a catastrophic way. In G6, glass fibers inducted a partial deflection of the fracture, although they were not able to stop crack propagation. For the direct restoration of endodontically treated molars, reinforcement of composite resins with glass-fibers or fiber posts can enhance fracture resistance. The SEM analysis showed a low ability of horizontal glass-fibers to deviate the fracture, but this effect was not sufficient to lead to more favorable fracture patterns above the cement-enamel junction (CEJ). The fracture resistance of endodontically treated molars restored with direct composite restorations seems to be increased by reinforcement with fibers, even if it is insufficient to restore sound molar fracture resistance and cannot avoid vertical fractures. Copyright © 2016 Elsevier Ltd. All rights reserved.

High-rate deformation and spall fracture of hadfield steel under action of high-current nanosecond relativistic electron beam

NASA Astrophysics Data System (ADS)

Gnyusov, S. F.; Rotshtein, V. P.; Polevin, S. D.; Kitsanov, S. A.

2010-09-01

Features of the plastic deformation and dynamic spall fracture of Hadfield steel under conditions of shock wave loading at a straining rate of ˜106 s-1 have been studied. The shock load (˜30 GPa, ˜0.2 μs) was produced by pulses of a SINUS-7 electron accelerator, which generated relativistic electron bunches with an electron energy of up to 1.35 MeV, a duration of 45 ns, and a peak power on the target of 3.4 × 1010 W/cm2. It is established that the spalling proceeds via mixed viscous-brittle intergranular fracture, unlike the cases of quasi-static tensile and impact loading, where viscous transgranular fracture is typical. It is shown that the intergranular character of the spall fracture is caused by the localization of plastic deformation at grain boundaries containing precipitated carbide inclusions.

Immediate effects of modified landing pattern on a probabilistic tibial stress fracture model in runners.

PubMed

Chen, T L; An, W W; Chan, Z Y S; Au, I P H; Zhang, Z H; Cheung, R T H

2016-03-01

Tibial stress fracture is a common injury in runners. This condition has been associated with increased impact loading. Since vertical loading rates are related to the landing pattern, many heelstrike runners attempt to modify their footfalls for a lower risk of tibial stress fracture. Such effect of modified landing pattern remains unknown. This study examined the immediate effects of landing pattern modification on the probability of tibial stress fracture. Fourteen experienced heelstrike runners ran on an instrumented treadmill and they were given augmented feedback for landing pattern switch. We measured their running kinematics and kinetics during different landing patterns. Ankle joint contact force and peak tibial strains were estimated using computational models. We used an established mathematical model to determine the effect of landing pattern on stress fracture probability. Heelstrike runners experienced greater impact loading immediately after landing pattern switch (P<0.004). There was an increase in the longitudinal ankle joint contact force when they landed with forefoot (P=0.003). However, there was no significant difference in both peak tibial strains and the risk of tibial stress fracture in runners with different landing patterns (P>0.986). Immediate transitioning of the landing pattern in heelstrike runners may not offer timely protection against tibial stress fracture, despite a reduction of impact loading. Long-term effects of landing pattern switch remains unknown. Copyright © 2016 Elsevier Ltd. All rights reserved.

Assessment of mechanical properties of human head tissues for trauma modelling.

PubMed

Lozano-Mínguez, Estívaliz; Palomar, Marta; Infante-García, Diego; Rupérez, María José; Giner, Eugenio

2018-05-01

Many discrepancies are found in the literature regarding the damage and constitutive models for head tissues as well as the values of the constants involved in the constitutive equations. Their proper definition is required for consistent numerical model performance when predicting human head behaviour, and hence skull fracture and brain damage. The objective of this research is to perform a critical review of constitutive models and damage indicators describing human head tissue response under impact loading. A 3D finite element human head model has been generated by using computed tomography images, which has been validated through the comparison to experimental data in the literature. The threshold values of the skull and the scalp that lead to fracture have been analysed. We conclude that (1) compact bone properties are critical in skull fracture, (2) the elastic constants of the cerebrospinal fluid affect the intracranial pressure distribution, and (3) the consideration of brain tissue as a nearly incompressible solid with a high (but not complete) water content offers pressure responses consistent with the experimental data. Copyright © 2018 John Wiley & Sons, Ltd.

Strength and fracture mechanism of iron reinforced tricalcium phosphate cermet fabricated by spark plasma sintering.

PubMed

Tkachenko, Serhii; Horynová, Miroslava; Casas-Luna, Mariano; Diaz-de-la-Torre, Sebastian; Dvořák, Karel; Celko, Ladislav; Kaiser, Jozef; Montufar, Edgar B

2018-05-01

The present work studies the microstructure and mechanical performance of tricalcium phosphate (TCP) based cermet toughened by iron particles. A novelty arises by the employment of spark plasma sintering for fabrication of the cermet. Results showed partial transformation of initial alpha TCP matrix to beta phase and the absence of oxidation of iron particles, as well as a lack of chemical reaction between TCP and iron components during sintering. The values of compressive and tensile strength of TCP/Fe cermet were 3.2 and 2.5 times, respectively, greater than those of monolithic TCP. Fracture analysis revealed the simultaneous action of crack-bridging and crack-deflection microstructural toughening mechanisms under compression. In contrast, under tension the reinforcing mechanism was only crack-bridging, being the reason for smaller increment of strength. Elastic properties of the cermet better matched values reported for human cortical bone. Thereby the new TCP/Fe cermet has potential for eventual use as a material for bone fractures fixation under load-bearing conditions. Copyright © 2018 Elsevier Ltd. All rights reserved.

Femur loading in feet-first fall experiments using an anthropomorphic test device.

PubMed

Thompson, Angela; Bertocci, Gina; Smalley, Craig

2018-03-31

Femur fractures are a common orthopedic injury in young children. Falls account for a large portion of accidental femur fractures in young children, but there is also a high prevalence of femur fractures in child abuse, with falls often provided as false histories. Objective information regarding fracture potential in short distance fall scenarios may aid in assessing whether a child's injuries are the result of abuse or an accidental fall. Knowledge of femur loading is the first step towards understanding likelihood of fracture in a fall. Characterize femur loading during feet-first free falls using a surrogate representing a 12-month-old child. The femur and hip joint of a surrogate representing a 12-month-old were modified to improve biofidelity and measure femur loading; 6-axis load cells were integrated into the proximal and distal femur. Femur modification was based upon CT imaging of cadaveric femurs in children 10-14 months of age. Using the modified 12-month-old surrogate, feet-first free falls from 69 cm and 119 cm heights onto padded carpet and linoleum were conducted to assess fall dynamics and determine femur loading. Femur compression, bending moment, shear and torsional moment were measured for each fall. Fall dynamics differed across fall heights, but did not substantially differ by impact surface type. Significant differences were found in all loading conditions across fall heights, while only compression and bending loads differed between carpet and linoleum surfaces. Maximum compression, bending, torsion and shear occurred in 119 cm falls and were 572 N, 23 N-m, 11 N-m and 281 N, respectively. Fall dynamics play an important role in the biomechanical assessment of falls. Fall height was found to influence both fall dynamics and femur loading, while impact surface affected only compression and bending in feet-first falls; fall dynamics did not differ across carpet and linoleum. Improved pediatric thresholds are necessary to predict likelihood of fracture, but morphologically accurate representation of the lower extremity, along with accurate characterization of loading in falls are a crucial first step. Copyright © 2018 Elsevier Ltd and Faculty of Forensic and Legal Medicine. All rights reserved.

Biomechanical evaluation of fracture fixation constructs using a variable-angle locked periprosthetic femur plate system.

PubMed

Hoffmann, Martin F; Burgers, Travis A; Mason, James J; Williams, Bart O; Sietsema, Debra L; Jones, Clifford B

2014-07-01

In the United States there are more than 230,000 total hip replacements annually, and periprosthetic femoral fractures occur in 0.1-4.5% of those patients. The majority of these fractures occur at the tip of the stem (Vancouver type B1). The purpose of this study was to compare the biomechanically stability and strength of three fixation constructs and identify the most desirable construct. Fifteen medium adult synthetic femurs were implanted with a hip prosthesis and were osteotomized in an oblique plane at the level of the implant tip to simulate a Vancouver type B1 periprosthetic fracture. Fractures were fixed with a non-contact bridging periprosthetic proximal femur plate (Zimmer Inc., Warsaw, IN). Three proximal fixation methods were used: Group 1, bicortical screws; Group 2, unicortical screws and one cerclage cable; and Group 3, three cerclage cables. Distally, all groups had bicortical screws. Biomechanical testing was performed using an axial-torsional testing machine in three different loading modalities (axial compression, lateral bending, and torsional/sagittal bending), next in axial cyclic loading to 10,000 cycles, again in the three loading modalities, and finally to failure in torsional/sagittal bending. Group 1 had significantly greater load to failure and was significantly stiffer in torsional/sagittal bending than Groups 2 and 3. After cyclic loading, Group 2 had significantly greater axial stiffness than Groups 1 and 3. There was no difference between the three groups in lateral bending stiffness. The average energy absorbed during cyclic loading was significantly lower in Group 2 than in Groups 1 and 3. Bicortical screw placement achieved the highest load to failure and the highest torsional/sagittal bending stiffness. Additional unicortical screws improved axial stiffness when using cable fixation. Lateral bending was not influenced by differences in proximal fixation. To treat periprosthetic fractures, bicortical screw placement should be attempted to maximize load to failure and torsional/sagittal bending stiffness. Copyright © 2014 Elsevier Ltd. All rights reserved.

Scale dependence of in-situ permeability measurements in the Nankai accretionary prism: The role of fractures

NASA Astrophysics Data System (ADS)

Boutt, David F.; Saffer, Demian; Doan, Mai-Linh; Lin, Weiren; Ito, Takatoshi; Kano, Yasuyuki; Flemings, Peter; McNeill, Lisa C.; Byrne, Timothy; Hayman, Nicholas W.; Moe, Kyaw Thu

2012-04-01

Modeling studies suggest that fluid permeability is an important control on the maintenance and distribution of pore fluid pressures at subduction zones generated through tectonic loading. Yet, to date, few data are available to constrain permeability of these materials, at appropriate scales. During IODP Expedition 319, downhole measurements of permeability within the uppermost accretionary wedge offshore SW Japan were made using a dual-packer device to isolate 1 m sections of borehole at a depth of 1500 m below sea floor. Analyses of pressure transients using numerical models suggest a range of in-situ fluid permeabilities (5E-15-9E-17 m2). These values are significantly higher than those measured on core samples (2E-19 m2). Borehole imagery and cores suggests the presence of multiple open fractures at this depth of measurement. These observations suggest that open permeable natural fractures at modest fracture densities could be important contributors to overall prism permeability structure at these scales.

Inverse problems in heterogeneous and fractured media using peridynamics

DOE PAGES

Turner, Daniel Z.; van Bloemen Waanders, Bart G.; Parks, Michael L.

2015-12-10

The following work presents an adjoint-based methodology for solving inverse problems in heterogeneous and fractured media using state-based peridynamics. We show that the inner product involving the peridynamic operators is self-adjoint. The proposed method is illustrated for several numerical examples with constant and spatially varying material parameters as well as in the context of fractures. We also present a framework for obtaining material parameters by integrating digital image correlation (DIC) with inverse analysis. This framework is demonstrated by evaluating the bulk and shear moduli for a sample of nuclear graphite using digital photographs taken during the experiment. The resulting measuredmore » values correspond well with other results reported in the literature. Lastly, we show that this framework can be used to determine the load state given observed measurements of a crack opening. Furthermore, this type of analysis has many applications in characterizing subsurface stress-state conditions given fracture patterns in cores of geologic material.« less

A unifying strain criterion for fracture of fibrous composite laminates

NASA Technical Reports Server (NTRS)

Poe, C. C., Jr.

1983-01-01

Fibrous composite materials, such as graphite/epoxy, are light, stiff, and strong. They have great potential for reducing weight in aircraft structures. However, for a realization of this potential, designers will have to know the fracture toughness of composite laminates in order to design damage tolerant structures. In connection with the development of an economical testing procedure, there is a great need for a single fracture toughness parameter which can be used to predict the stress-intensity factor (K(Q)) for all laminates of interest to the designer. Poe and Sova (1980) have derived a general fracture toughness parameter (Qc), which is a material constant. It defines the critical level of strains in the principal load-carryng plies. The present investigation is concerned with the calculation of values for the ratio of Qc and the ultimate tensile strain of the fibers. The obtained data indicate that this ratio is reasonably constant for layups which fail largely by self-similar crack extension.

[Intramedullary stabilisation of clavicula fractures].

PubMed

Prokop, A; Schiffer, G; Jubel, A; Chmielnicki, M

2013-10-01

With an incidence of 64/100,000, clavicular shaft fractures are one of the most common fractures. Intramedullary fixation with Prevot nails was initially reported in the late 1990s. This procedure offers minimally invasive stabilization of the fracture, thus enabling immediate mobilization and rapid loading capacity. Using a case study, the positioning and procedure are demonstrated on video. The intramedullary implant accommodates the varying tension loading of the clavicle. This treatment is ideal for clavicular fractures with 2-3 fragments. Compared to patients treated conservatively, operated patients achieve more rapid and improved mobility. Employment disability is shorter, and malunion occurs less frequently. Georg Thieme Verlag KG Stuttgart · New York.

Fracture modes in off-axis fiber composites

NASA Technical Reports Server (NTRS)

Sinclair, J. H.; Chamis, C. C.

1978-01-01

Criteria were developed for identifying, characterizing, and quantifying fracture modes in high-modulus graphite-fiber/resin unidirectional composites subjected to off-axis tensile loading. Procedures are described which use sensitivity analyses and off-axis data to determine the uniaxial strength of fiber composites. It was found that off-axis composites fail by three fracture modes which produce unique fracture surface characteristics. The stress that dominates each fracture mode and the load angle range of its dominance can be identified. Linear composite mechanics is adequate to describe quantitatively the mechanical behavior of off-axis composites. The uniaxial strengths predicted from off-axis data are comparable to these measured in uniaxial tests.

FATIGUE OF DENTAL CERAMICS

PubMed Central

Zhang, Yu; Sailer, Irena; Lawn, Brian R

2013-01-01

Objectives Clinical data on survival rates reveal that all-ceramic dental prostheses are susceptible to fracture from repetitive occlusal loading. The objective of this review is to examine the underlying mechanisms of fatigue in current and future dental ceramics. Data/sources The nature of various fatigue modes is elucidated using fracture test data on ceramic layer specimens from the dental and biomechanics literature. Conclusions Failure modes can change over a lifetime, depending on restoration geometry, loading conditions and material properties. Modes that operate in single-cycle loading may be dominated by alternative modes in multi-cycle loading. While post-mortem examination of failed prostheses can determine the sources of certain fractures, the evolution of these fractures en route to failure remains poorly understood. Whereas it is commonly held that loss of load-bearing capacity of dental ceramics in repetitive loading is attributable to chemically-assisted 'slow crack growth' in the presence of water, we demonstrate the existence of more deleterious fatigue mechanisms, mechanical rather than chemical in nature. Neglecting to account for mechanical fatigue can lead to gross overestimates in predicted survival rates. Clinical significance Strategies for prolonging the clinical lifetimes of ceramic restorations are proposed based on a crack-containment philosophy. PMID:24135295

Fatigue of dental ceramics.

PubMed

Zhang, Yu; Sailer, Irena; Lawn, Brian R

2013-12-01

Clinical data on survival rates reveal that all-ceramic dental prostheses are susceptible to fracture from repetitive occlusal loading. The objective of this review is to examine the underlying mechanisms of fatigue in current and future dental ceramics. The nature of various fatigue modes is elucidated using fracture test data on ceramic layer specimens from the dental and biomechanics literature. Failure modes can change over a lifetime, depending on restoration geometry, loading conditions and material properties. Modes that operate in single-cycle loading may be dominated by alternative modes in multi-cycle loading. While post-mortem examination of failed prostheses can determine the sources of certain fractures, the evolution of these fractures en route to failure remains poorly understood. Whereas it is commonly held that loss of load-bearing capacity of dental ceramics in repetitive loading is attributable to chemically assisted 'slow crack growth' in the presence of water, we demonstrate the existence of more deleterious fatigue mechanisms, mechanical rather than chemical in nature. Neglecting to account for mechanical fatigue can lead to gross overestimates in predicted survival rates. Strategies for prolonging the clinical lifetimes of ceramic restorations are proposed based on a crack-containment philosophy. Copyright © 2013 Elsevier Ltd. All rights reserved.

Fracture and fatigue analysis of functionally graded and homogeneous materials using singular integral equation approach

NASA Astrophysics Data System (ADS)

Zhao, Huaqing

There are two major objectives of this thesis work. One is to study theoretically the fracture and fatigue behavior of both homogeneous and functionally graded materials, with or without crack bridging. The other is to further develop the singular integral equation approach in solving mixed boundary value problems. The newly developed functionally graded materials (FGMs) have attracted considerable research interests as candidate materials for structural applications ranging from aerospace to automobile to manufacturing. From the mechanics viewpoint, the unique feature of FGMs is that their resistance to deformation, fracture and damage varies spatially. In order to guide the microstructure selection and the design and performance assessment of components made of functionally graded materials, in this thesis work, a series of theoretical studies has been carried out on the mode I stress intensity factors and crack opening displacements for FGMs with different combinations of geometry and material under various loading conditions, including: (1) a functionally graded layer under uniform strain, far field pure bending and far field axial loading, (2) a functionally graded coating on an infinite substrate under uniform strain, and (3) a functionally graded coating on a finite substrate under uniform strain, far field pure bending and far field axial loading. In solving crack problems in homogeneous and non-homogeneous materials, a very powerful singular integral equation (SEE) method has been developed since 1960s by Erdogan and associates to solve mixed boundary value problems. However, some of the kernel functions developed earlier are incomplete and possibly erroneous. In this thesis work, mode I fracture problems in a homogeneous strip are reformulated and accurate singular Cauchy type kernels are derived. Very good convergence rates and consistency with standard data are achieved. Other kernel functions are subsequently developed for mode I fracture in functionally graded materials. This work provides a solid foundation for further applications of the singular integral equation approach to fracture and fatigue problems in advanced composites. The concept of crack bridging is a unifying theory for fracture at various length scales, from atomic cleavage to rupture of concrete structures. However, most of the previous studies are limited to small scale bridging analyses although large scale bridging conditions prevail in engineering materials. In this work, a large scale bridging analysis is included within the framework of singular integral equation approach. This allows us to study fracture, fatigue and toughening mechanisms in advanced materials with crack bridging. As an example, the fatigue crack growth of grain bridging ceramics is studied. With the advent of composite materials technology, more complex material microstructures are being introduced, and more mechanics issues such as inhomogeneity and nonlinearity come into play. Improved mathematical and numerical tools need to be developed to allow theoretical modeling of these materials. This thesis work is an attempt to meet these challenges by making contributions to both micromechanics modeling and applied mathematics. It sets the stage for further investigations of a wide range of problems in the deformation and fracture of advanced engineering materials.

Soldier Mobility: Innovations in Load Carriage System Design and Evaluation (la Mobilite du combattant: innovations dans la conception et l’evaluation des gilets d’intervention)

DTIC Science & Technology

2001-05-01

sweating and inappropriate foot care. Metatarsalgia, stress fractures and knee problems are related primarily to load mass and duration of marching...Stress Fracture 0 1 0 1 1 Other 8 3 1 12 12 Total 68 17 17 102 100 aFrom physician’s assistances at fixed medical sites along the march bFrom medical...feet through physical training and road march practice 2. Reduce load mass Stress Fractures Persistent, boney pain 1. Smoking/tobacco cessation 2. Pre

The dynamic failure behavior of tungsten heavy alloys subjected to transverse loads

NASA Astrophysics Data System (ADS)

Tarcza, Kenneth Robert

Tungsten heavy alloys (WHA), a category of particulate composites used in defense applications as kinetic energy penetrators, have been studied for many years. Even so, their dynamic failure behavior is not fully understood and cannot be predicted by numerical models presently in use. In this experimental investigation, a comprehensive understanding of the high-rate transverse-loading fracture behavior of WHA has been developed. Dynamic fracture events spanning a range of strain rates and loading conditions were created via mechanical testing and used to determine the influence of surface condition and microstructure on damage initiation, accumulation, and sample failure under different loading conditions. Using standard scanning electron microscopy metallographic and fractographic techniques, sample surface condition is shown to be extremely influential to the manner in which WHA fails, causing a fundamental change from externally to internally nucleated failures as surface condition is improved. Surface condition is characterized using electron microscopy and surface profilometry. Fracture surface analysis is conducted using electron microscopy, and linear elastic fracture mechanics is used to understand the influence of surface condition, specifically initial flaw size, on sample failure behavior. Loading conditions leading to failure are deduced from numerical modeling and experimental observation. The results highlight parameters and considerations critical to the understanding of dynamic WHA fracture and the development of dynamic WHA failure models.

Effects of different brazing and welding methods on the fracture load of various orthodontic joining configurations.

PubMed

Bock, Jens J; Bailly, Jacqueline; Fuhrmann, Robert A

2009-06-01

The aim of this study was to compare the fracture load of different joints made by conventional brazing, tungston inert gas (TIG) and laser welding. Six standardized joining configurations of spring hard quality orthodontic wire were investigated: end-to-end, round, cross, 3 mm length, 9 mm length and 6.5 mm to orthodontic band. The joints were made by five different methods: brazing with universal silver solder, two TIG and two laser welding devices. The fracture loads were measured with a universal testing machine (Zwick 005). Data were analysed with the Mann-Whitney-Wilcoxon and Kruskal-Wallis tests. The significance level was set at P<0.05). In all cases brazed joints were ruptured at a low level of fracture load (186-407 N). Significant differences between brazing and TIG or laser welding (P<0.05) were found. The highest mean fracture loads were observed for laser welding (826 N). No differences between the various TIG or laser welding devices were demonstrated, although it was not possible to join an orthodontic wire to an orthodontic band using TIG welding. For orthodontic purposes laser and TIG welding are solder free alternatives. TIG welding and laser welding showed similar results. The laser technique is an expensive, but sophisticated and simple method.

Height restoration of osteoporotic vertebral compression fractures using different intravertebral reduction devices: a cadaveric study.

PubMed

Krüger, Antonio; Oberkircher, Ludwig; Figiel, Jens; Floßdorf, Felix; Bolzinger, Florent; Noriega, David C; Ruchholtz, Steffen

2015-05-01

The treatment of osteoporotic vertebral compression fractures using transpedicular cement augmentation has grown significantly during the past two decades. Balloon kyphoplasty was developed to restore vertebral height and improve sagittal alignment. Several studies have shown these theoretical improvements cannot be transferred universally to the clinical setting. The aim of the current study is to evaluate two different procedures used for percutaneous augmentation of vertebral compression fractures with respect to height restoration: balloon kyphoplasty and SpineJack. Twenty-four vertebral bodies of two intact, fresh human cadaveric spines (T6-L5; donor age, 70 years and 60 years; T-score -6.8 points and -6.3 points) were scanned using computed tomography (CT) and dissected into single vertebral bodies. Vertebral wedge compression fractures were created by a material testing machine (Universal testing machine, Instron 5566, Darmstadt, Germany). The axial load was increased continuously until the height of the anterior edge of the vertebral body was reduced by 40% of the initial measured values. After 15 minutes, the load was decreased manually to 100 N. After postfracture CT, the clamped vertebral bodies were placed in a custom-made loading frame with a preload of 100 N. Twelve vertebral bodies were treated using SpineJack (SJ; Vexim, Balma, France), the 12 remaining vertebral bodies were treated with balloon kyphoplasty (BKP; Kyphon, Medtronic, Sunnyvale, CA, USA). The load was maintained during the procedure until the cement set completely. Posttreatment CT was performed. Anterior, central, and posterior height as well as the Beck index were measured prefracture and postfracture as well as after treatment. For anterior height restoration (BKP, 0.14±1.48 mm; SJ, 3.34±1.19 mm), central height restoration (BKP, 0.91±1.04 mm; SJ, 3.24±1.22 mm), and posterior restoration (BKP, 0.37±0.57 mm; SJ, 1.26±1.05), as well as the Beck index (BKP, 0.00±0.06 mm; SJ, 0.10±0.06), the values for the SpineJack group were significantly higher (p<.05) CONCLUSION: The protocols for creating wedge fractures and using the instrumentation under a constant preload of 100 N led to reproducible results and effects. The study showed that height restoration was significantly better in the SpineJack group compared with the balloon kyphoplasty group. The clinical implications include a better restoration of the sagittal balance of the spine and a reduction of the kyphotic deformity, which may relate to clinical outcome and the biological healing process. Copyright © 2015 Elsevier Inc. All rights reserved.

Effect of Pulse Shape on Spall Strength

NASA Astrophysics Data System (ADS)

Smirnov, V. I.; Petrov, Yu. V.

2018-03-01

This paper analyzes the effect of the time-dependent shape of a load pulse on the spall strength of materials. Within the framework of a classical one-dimensional scheme, triangular pulses with signal rise and decay portions and with no signal rise portions considered. Calculation results for the threshold characteristics of fracture for rail steel are given. The possibility of optimization of fracture by selecting a loading time with the use of an introduced characteristic of dynamic strength (pulse fracture capacity) is demonstrated. The study is carried out using a structure-time fracture criterion.

75 FR 3150 - Airworthiness Directives; The Boeing Company Model 747-100, 747-100B, 747-100B SUD, 747-200B, 747...

Federal Register 2010, 2011, 2012, 2013, 2014

2010-01-20

... fractured front spar assembly for strut No. 3, which resulted in the loss of the strut upper link load path... of a fractured front spar assembly for strut No. 3, which resulted in the loss of the strut upper... loss of the strut upper link load path and consequent fracture of the diagonal brace, which could...

Biomechanical analysis of posteromedial tibial plateau split fracture fixation.

PubMed

Zeng, Zhi-Min; Luo, Cong-Feng; Putnis, Sven; Zeng, Bing-Fang

2011-01-01

The purpose of this study was to compare the biomechanical strength of four different fixation methods for a posteromedial tibial plateau split fracture. Twenty-eight tibial plateau fractures were simulated using right-sided synthetic tibiae models. Each fracture model was randomly instrumented with one of the four following constructs, anteroposterior lag-screws, an anteromedial limited contact dynamic compression plate (LC-DCP), a lateral locking plate, or a posterior T-shaped buttress plate. Vertical subsidence of the posteromedial fragment was measured from 500 N to 1500 N during biomechanical testing, the maximum load to failure was also determined. It was found that the posterior T-shaped buttress plate allowed the least subsidence of the posteromedial fragment and produced the highest mean failure load than each of the other three constructs (P=0.00). There was no statistical significant difference between using lag screws or an anteromedial LC-DCP construct for the vertical subsidence at a 1500 N load and the load to failure (P>0.05). This study showed that a posterior-based buttress technique is biomechanically the most stable in-vitro fixation method for posteromedial split tibial plateau fractures, with AP screws and anteromedial-based LC-DCP are not as stable for this type of fracture. Copyright © 2010 Elsevier B.V. All rights reserved.

All-Ceramic Single Crown Restauration of Zirconia Oral Implants and Its Influence on Fracture Resistance: An Investigation in the Artificial Mouth

PubMed Central

Kohal, Ralf-Joachim; Kilian, Jolanta Bernadette; Stampf, Susanne; Spies, Benedikt Christopher

2015-01-01

The aim of the current investigation was to evaluate the fracture resistance of one-piece zirconia oral implants with and without all-ceramic incisor crowns after long-term thermomechanical cycling. A total of 48 implants were evaluated. The groups with crowns (C, 24 samples) and without crowns (N, 24 samples) were subdivided according to the loading protocol, resulting in three groups of 8 samples each: Group “0” was not exposed to cyclic loading, whereas groups “5” and “10” were loaded with 5 and 10 million chewing cycles, respectively. This resulted in 6 different groups: C0/N0, C5/N5 and C10/N10. Subsequently, all 48 implants were statically loaded to fracture and bending moments were calculated. All implants survived the artificial aging. For the static loading the following average bending moments were calculated: C0: 326 Ncm; C5: 339 Ncm; C10: 369 Ncm; N0: 339 Ncm; N5: 398 Ncm and N10: 355 Ncm. To a certain extent, thermomechanical cycling resulted in an increase of fracture resistance which did not prove to be statistically significant. Regarding its fracture resistance, the evaluated ceramic implant system made of Y-TZP seems to be able to resist physiological chewing forces long-term. Restauration with all-ceramic single crowns showed no negative influence on fracture resistance. PMID:28788018

Comparison of Load-Bearing Capacities of 3-Unit Fiber-Reinforced Composite Adhesive Bridges with Different Framework Designs.

PubMed

Tacir, Ibrahim H; Dirihan, Roda S; Polat, Zelal Seyfioglu; Salman, Gizem Ön; Vallittu, Pekka; Lassila, Lippo; Ayna, Emrah

2018-06-28

BACKGROUND The aim of this study was to investigate and compare the load-bearing capacities of three-unit direct resin-bonded fiber-reinforced composite fixed dental prosthesis with different framework designs. MATERIAL AND METHODS Sixty mandibular premolar and molar teeth without caries were collected and direct glass fiber-resin fixed FDPs were divided into 6 groups (n=10). Each group was restored via direct technique with different designs. In Group 1, the inlay-retained bridges formed 2 unidirectional FRC frameworks and pontic-reinforced transversal FRC. In Group 2, the inlay-retained bridges were supported by unidirectional lingual and occlusal FRC frameworks. Group 3, had buccal and lingual unidirectional FRC frameworks without the inlay cavities. Group 4 had reinforced inlay cavities and buccal-lingual FRC with unidirectional FRC frameworks. Group 5, had a circular form of fiber reinforcement around cusps in addition to buccal-lingual FRC frameworks. Group 6 had a circular form of fiber reinforcement around cusps with 2 bidirectional FRC frameworks into inlay cavities. All groups were loaded until final fracture using a universal testing machine at a crosshead speed of 1 mm/min. RESULTS Mean values of the groups were determined with ANOVA and Tukey HSD. When all data were evaluated, Group 6 had the highest load-bearing capacities and revealed significant differences from Group 3 and Group 4. Group 6 had the highest strain (p>0.05). When the fracture patterns were investigated, Group 6 had the durability to sustain fracture propagation within the restoration. CONCLUSIONS The efficiency of fiber reinforcement of the restorations alters not only the amount of fiber, but also the design of the restoration with fibers.

Filling Open Screw Holes in the Area of Metaphyseal Comminution Does Not Affect Fatigue Life of the Synthes Variable Angle Distal Femoral Locking Plate in the AO/OTA 33-A3 Fracture Model.

PubMed

Grau, Luis; Collon, Kevin; Alhandi, Ali; Kaimrajh, David; Varon, Maria; Latta, Loren; Vilella, Fernando

2018-06-01

The aim of this study is to evaluate the biomechanical effect of filling locking variable angle (VA) screw holes at the area of metaphyseal fracture comminution in a Sawbones® (Sawbones USA, Vashon, Washington) model (AO/OTA 33A-3 fracture) using a Synthes VA locking compression plate (LCP) (Depuy Synthes, Warsaw, Indiana). Seven Sawbones® femur models had a Synthes VA-LCP placed as indicated by the manufacturers technique. A 4cm osteotomy was then created to simulate an AO/OTA 33-A3 femoral fracture pattern with metaphyseal comminution. The control group consisted of four constructs in which the open screw holes at the area of comminution were left unfilled; the experimental group consisted of three constructs in which the VA screw holes were filled with locking screws. One of the control constructs was statically loaded to failure at a rate of 5mm/min. A value equal to 75% of the ultimate load to failure was used as the loading force for fatigue testing of 250,000 cycles at 3Hz. Cycles to failure was recorded for each construct and averages were compared between groups. The average number of cycles to failure in the control and experimental groups were 37524±8187 and 43304±23835, respectively (p=0.72). No significant difference was observed with respect to cycles to failure or mechanism of failure between groups. In all constructs in both the control and experimental groups, plate failure reproducibly occurred with cracks through the variable angle holes in the area of bridged comminution. The Synthes VA-LCP in a simulated AO/OTA 33-A3 comminuted metaphyseal femoral fracture fails in a reproducible manner at the area of comminution through the "honeycomb" VA screw holes. Filling open VA screw holes at the site of comminution with locking screws does not increase fatigue life of the Synthes VA-LCP in a simulated AO/OTA 33-A3 distal femoral fracture. Further studies are necessary to determine whether use of this particular plate is contraindicated when bridging distal femoral fractures with metaphyseal comminution.

On the Fracture Toughness and Stable Crack Growth in Shape Memory Alloys Under Combined Thermomechanical Loading

NASA Astrophysics Data System (ADS)

Jape, Sameer Sanjay

Advanced multifunctional materials such as shape memory alloys (SMAs) offer unprecedented improvement over conventional materials when utilized as high power output solid-state actuators in a plethora of engineering applications, viz. aerospace, automotive, oil and gas exploration, etc., replacing complex multi-component assemblies with compact single-piece adaptive components. These potential applications stem from the material's ability to produce large recoverable actuation strains when subjected to combined thermomechanical loads, via a diffusionless solid-to-solid phase transition between high-temperature cubic austenite and low-temperature monoclinic martensite crystalline phases. To ensure reliable design, functioning and durability of SMA-based actuators, it is imperative to develop a thorough scientific knowledge base and understanding about their fracture properties i.e. crack-initiation and growth during thermal actuation, vis-a-vis the phase transformation metrics (i.e. transformation strains, hysteresis, and temperatures, critical stresses for phase transformation, etc.) and microstructural features (grain size, precipitates, and texture). Systematic experimental and analytical investigation of SMA fracture response based on known theories and methodologies is posed with significant challenges due to the inherent complexity in SMA thermomechanical constitutive response arising out of the shape memory and pseudoelastic effects, martensite detwinning and variant reorientation, thermomechanical coupling, and transformation induced plasticity (TRIP). In this study, a numerical analysis is presented that addresses the fundamental need to study fracture in SMAs in the presence of aforementioned complexities. Finite element modeling with an energetics based fracture toughness criterion and SMA thermomechanical behavior with nonlinearities from thermomechanical coupling and TRIP was conducted. A specific analysis of a prototype boundary value fracture problem yielded results similar to those obtained experimentally, viz. stable crack growth with transformation toughening, dependence of failure cycle on bias load and catastrophic failure during cooling, and are explained using classical fracture mechanics theories. Influence of TRIP as a monotonically accumulating irrecoverable plastic strain on the crack-tip mechanical fields in case of stationary and advancing cracks is also investigated using the same computational tools. Thermomechanical coupling in shape memory alloys, which is an important factor when utilized as solid-state actuators manifests itself through the generation and absorption of latent of transformation and leads to non-uniform temperature distribution. The effect of this coupling vis-a-vis the mechanics of static and advancing cracks is also analyzed using the energetics based approach.

Fracto-emission from single fibres of Kevlar

NASA Technical Reports Server (NTRS)

Dickinson, J. T.; Jahan-Latibari, A.; Jensen, L. C.

1985-01-01

Fracto-emission (FE) is the emission of particles (e.g. electrons, ions and photons) during and following fracture. In this paper, we present data on electron emission (EE) and positive ion emission (PIE) from the tensile fracture of Kevlar single fibers. The fibers were initially fractured in pure tension, where a stranded form of fracture was observed, often with multiple peaks spread over several hundred microseconds. The loading condition was then changed by stretching and breaking the fibers over a dull metal edge. With this change in the loading, different forms of fracture were observed, each with distinctive forms of emission curves. When fracture was accompanied by extensive fibril formation, total emission was high and both EE and PIE decay times were long relative to fractures in which little fibril formation occurred. The results of this study suggest that FE has some applicability as a tool for the detection of fracture mechanisms of single fibers.

Fracture behaviors under pure shear loading in bulk metallic glasses

NASA Astrophysics Data System (ADS)

Chen, Cen; Gao, Meng; Wang, Chao; Wang, Wei-Hua; Wang, Tzu-Chiang

2016-12-01

Pure shear fracture test, as a special mechanical means, had been carried out extensively to obtain the critical information for traditional metallic crystalline materials and rocks, such as the intrinsic deformation behavior and fracture mechanism. However, for bulk metallic glasses (BMGs), the pure shear fracture behaviors have not been investigated systematically due to the lack of a suitable test method. Here, we specially introduce a unique antisymmetrical four-point bend shear test method to realize a uniform pure shear stress field and study the pure shear fracture behaviors of two kinds of BMGs, Zr-based and La-based BMGs. All kinds of fracture behaviors, the pure shear fracture strength, fracture angle and fracture surface morphology, are systematically analyzed and compared with those of the conventional compressive and tensile fracture. Our results indicate that both the Zr-based and La-based BMGs follow the same fracture mechanism under pure shear loading, which is significantly different from the situation of some previous research results. Our results might offer new enlightenment on the intrinsic deformation and fracture mechanism of BMGs and other amorphous materials.

Fracture behaviors under pure shear loading in bulk metallic glasses.

PubMed

Chen, Cen; Gao, Meng; Wang, Chao; Wang, Wei-Hua; Wang, Tzu-Chiang

2016-12-23

Pure shear fracture test, as a special mechanical means, had been carried out extensively to obtain the critical information for traditional metallic crystalline materials and rocks, such as the intrinsic deformation behavior and fracture mechanism. However, for bulk metallic glasses (BMGs), the pure shear fracture behaviors have not been investigated systematically due to the lack of a suitable test method. Here, we specially introduce a unique antisymmetrical four-point bend shear test method to realize a uniform pure shear stress field and study the pure shear fracture behaviors of two kinds of BMGs, Zr-based and La-based BMGs. All kinds of fracture behaviors, the pure shear fracture strength, fracture angle and fracture surface morphology, are systematically analyzed and compared with those of the conventional compressive and tensile fracture. Our results indicate that both the Zr-based and La-based BMGs follow the same fracture mechanism under pure shear loading, which is significantly different from the situation of some previous research results. Our results might offer new enlightenment on the intrinsic deformation and fracture mechanism of BMGs and other amorphous materials.

The role of damage-softened material behavior in the fracture of composites and adhesives

NASA Technical Reports Server (NTRS)

Ungsuwarungsri, T.; Knauss, W. G.

1986-01-01

Failure mechanisms of materials under very high strains experienced at and ahead of the crack tip such as formation, growth, and interaction of microvoids in ductile materials, microcracks in brittle solids or crazes in polymers and adhesives are represented by one-dimensional, nonlinear stress-strain relations possessing different ways by which the material loses capacity to carry load up to fracture or total separation. A double cantilever beam (DCB) type specimen is considered. The nonlinear material is confined to a thin strip between the two elastic beams loaded by a wedge. The problem is first modeled as a beam on a nonlinear foundation. The pertinent equation is solved numerically as a two-point boundary value problem for both the stationary and the quasi-stationay propagating crack. A finite element model is then used to model the problem in more detail in order to assess the adequacy of the beam model for the reduction of experimental data to determine in-situ properties of the thin interlayer.

Additive fiber-cerclages in proximal humeral fractures stabilized by locking plates

PubMed Central

Hurschler, Christof; Rech, Louise; Vosshenrich, Rolf; Lill, Helmut

2009-01-01

Background and purpose The effect of additive fiber-cerclages in proximal humeral fractures stabilized by locking plates on fracture stabilization and rotator cuff function is unclear. Here it was assessed in a human cadaver study. Methods 24 paired human shoulder specimens were harvested from median 77-year-old (range 66–85) female donors. An unstable 3-part fracture model with an intact rotator cuff was developed. 1 specimen of each pair received an additive fiber-cerclage of the rotator cuff after plate fixation, and the other one received a plate fixation without an additive fiber-cerclage. Force-controlled hydraulic cylinders were used to simulate physiological rotator cuff tension, while a robot-assisted shoulder simulator performed 4 relevant cases of load: (1) axial loading at 0°, (2) glenohumeral abduction at 60°, (3) internal rotation at 0° abduction, and (4) external rotation at 0° abduction, and imitated hanging arm weight during loading without affecting joint kinematics. A 3-dimensional real-time interfragmentary motion analysis was done in fracture gaps between the greater tuberosity and the head, as well as subcapital. The capacity of the rotator cuff to strain was analyzed with an optical system. Results Interfragmentary motion was similar between the groups with and without fiber-cerclages, in both fracture gaps and in any of the cases of load. Cerclages did not impair the capacity of the rotator cuff to strain. Interpretation Provided that unstable 3-part fractures are reduced and stabilized anatomically by a locking plate, additive fiber-cerclages do not reduce interfragmentary motion. Additive fiber-cerclages may be necessary in locking plate osteosyntheses of multiple-fractured greater tuberosities or lesser tuberosity fractures that cannot be fixed sufficiently by the plate. PMID:19562564

Data Mining Activity for Bone Discipline: Calculating a Factor of Risk for Hip Fracture in Long-Duration Astronauts

NASA Technical Reports Server (NTRS)

Ellman, R.; Sibonga, J. D.; Bouxsein, M. L.

2010-01-01

The factor-of-risk (Phi), defined as the ratio of applied load to bone strength, is a biomechanical approach to hip fracture risk assessment that may be used to identify subjects who are at increased risk for fracture. The purpose of this project was to calculate the factor of risk in long duration astronauts after return from a mission on the International Space Station (ISS), which is typically 6 months in duration. The load applied to the hip was calculated for a sideways fall from standing height based on the individual height and weight of the astronauts. The soft tissue thickness overlying the greater trochanter was measured from the DXA whole body scans and used to estimate attenuation of the impact force provided by soft tissues overlying the hip. Femoral strength was estimated from femoral areal bone mineral density (aBMD) measurements by dual-energy x-ray absorptiometry (DXA), which were performed between 5-32 days of landing. All long-duration NASA astronauts from Expedition 1 to 18 were included in this study, where repeat flyers were treated as separate subjects. Male astronauts (n=20) had a significantly higher factor of risk for hip fracture Phi than females (n=5), with preflight values of 0.83+/-0.11 and 0.36+/-0.07, respectively, but there was no significant difference between preflight and postflight Phi (Figure 1). Femoral aBMD measurements were not found to be significantly different between men and women. Three men and no women exceeded the theoretical fracture threshold of Phi=1 immediately postflight, indicating that they would likely suffer a hip fracture if they were to experience a sideways fall with impact to the greater trochanter. These data suggest that male astronauts may be at greater risk for hip fracture than women following spaceflight, primarily due to relatively less soft tissue thickness and subsequently greater impact force.

Analyzing the rules of fracture and damage, and the characteristics of the acoustic emission signal of a gypsum specimen under uniaxial loading

NASA Astrophysics Data System (ADS)

Chen, Dong; Wang, En-yuan; Li, Nan

2017-08-01

In order to study the mechanism of rock bursts in a mined-out area of a gypsum mine, in this paper acoustic emission testing of the uniaxial compression of gypsum and sandstone samples is carried out. The case of rupture of the specimen is observed, and the load axial deformation curve and acoustic emission parameters are obtained for the whole process of specimen rupture. The similarities and differences between the gypsum and sandstone samples are determined in terms of their mechanical properties, their damage evolution laws and frequency band energy distributions, and the instantaneous energy characteristics of their acoustic emission. The results show that the main fracture morphology of gypsum is ‘eight’-type, and the macroscopic fracture morphology of sandstone is mainly of partial ‘Y’-type and inverted Y-type. The intensity and uniformity of the gypsum and sandstone of the medium are different; because the gypsum is more uniform, it does not show as much variation as sandstone, instead suddenly increasing and decreasing. The maximum value of the damage variable D of gypsum reached 1, but the maximum value of D of the sandstone only reached 0.9. The frequency band of the maximum energy of gypsum and sandstone gradually decreased across the the four stages of rupture, while the maximum energy percentage increased gradually. From the stage where damage gradually increases to the stage of integral fracture of the specimen, the instantaneous energy showed a certain degree of increase. With an increase in the strength of the sample, the maximum energy percentage of the two materials corresponding to each phase gradually increases, and from the stage where damage gradually increases to the stage of integral fracture of the specimen, the value of instantaneous energy obviously increases. The results indicate that gypsum mines will also experience rock bursts, as coal mines do, but the intensity will be different. Therefore, using the three indicators, the frequency band of the maximum energy, the maximum energy percentage, and the maximum instantaneous energy, the rupture of the sample can be predicted, which can be used to improve the accuracy and efficiency of early warning systems for rock bursts in gypsum mines.

A comparison of the fracture resistance of three machinable ceramics after thermal and mechanical fatigue.

PubMed

Yang, Rui; Arola, Dwayne; Han, Zhihui; Zhang, Xiuyin

2014-10-01

Mechanical and thermal fatigue may affect ceramic restorations in the oral environment. The purpose of this study was to determine the influence of thermal and mechanical cycling on the fracture load and fracture patterns of 3 machinable ceramics. Seventy-two human third molar teeth were prepared for bonding ceramic specimens of Sirona CEREC Blocs, IPS e.maxCAD, or inCoris ZI meso blocks. The 24 specimens of each ceramic were divided into 4 groups (n=6), which underwent no preloading (control), thermocycling (5°C-55°C, 2000 cycles), mechanical cycling (10(5) cycles, 100 N), and thermocycling (5°C-55°C, 2000 cycles) plus mechanical cycling (10(5) cycles, 100 N). The specimens were subsequently loaded to failure, and both stereomicroscopy and scanning electron microscopy were used to investigate the fracture patterns. The data were analyzed with 2-way ANOVA and the Fisher exact probability test (α=.05). Mechanical and thermal cycling had a significant influence on the critical load to failure of the 3 ceramics. No significant difference was found between mechanical cycling for 10(5) times and thermocycling for 2000 times within the same ceramic. The specimens of inCoris ZI experienced significantly higher fracture loads for all the groups. The fracture patterns of the 3 machinable ceramics showed that failure mainly occurred at the cement-dentin interface. The effects of combined thermal and mechanical cycling on the fracture load of ceramics were more significant than any individual mode of cyclic fatigue. Overall, the inCoris ZI resisted thermal and mechanical fatigue better than the Sirona CEREC and IPS e.maxCAD. Copyright © 2014 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

Fracture resistance of implant- supported monolithic crowns cemented to zirconia hybrid-abutments: zirconia-based crowns vs. lithium disilicate crowns

PubMed Central

Nawafleh, Noor; Öchsner, Andreas; George, Roy

2018-01-01

PURPOSE The aim of this in vitro study was to investigate the fracture resistance under chewing simulation of implant-supported posterior restorations (crowns cemented to hybrid-abutments) made of different all-ceramic materials. MATERIALS AND METHODS Monolithic zirconia (MZr) and monolithic lithium disilicate (MLD) crowns for mandibular first molar were fabricated using computer-aided design/computer-aided manufacturing technology and then cemented to zirconia hybrid-abutments (Ti-based). Each group was divided into two subgroups (n=10): (A) control group, crowns were subjected to single load to fracture; (B) test group, crowns underwent chewing simulation using multiple loads for 1.2 million cycles at 1.2 Hz with simultaneous thermocycling between 5℃ and 55℃. Data was statistically analyzed with one-way ANOVA and a Post-Hoc test. RESULTS All tested crowns survived chewing simulation resulting in 100% survival rate. However, wear facets were observed on all the crowns at the occlusal contact point. Fracture load of monolithic lithium disilicate crowns was statistically significantly lower than that of monolithic zirconia crowns. Also, fracture load was significantly reduced in both of the all-ceramic materials after exposure to chewing simulation and thermocycling. Crowns of all test groups exhibited cohesive fracture within the monolithic crown structure only, and no abutment fractures or screw loosening were observed. CONCLUSION When supported by implants, monolithic zirconia restorations cemented to hybrid abutments withstand masticatory forces. Also, fatigue loading accompanied by simultaneous thermocycling significantly reduces the strength of both of the all-ceramic materials. Moreover, further research is needed to define potentials, limits, and long-term serviceability of the materials and hybrid abutments. PMID:29503716

Effect of Cortical Screw Diameter on Reduction and Stabilization of Type III Distal Phalanx Fractures: An Equine Cadaveric Study.

PubMed

Kay, Alastair T; Durgam, Sushmitha; Stewart, Matthew; Joslyn, Stephen; Schaeffer, David J; Horn, Gavin; Kesler, Richard; Chew, Peter

2016-11-01

To compare reduction of type III distal phalangeal fractures using 4.5 and 5.5 mm cortical screws placed in lag fashion and an intact hoof capsule model. Cadaveric experimental study. Hooves from 12 adult horses (n=24). Sagittal fractures were created in pairs of distal phalanges after distal interphalangeal joint disarticulation and were reduced with either 4.5 or 5.5 mm cortical screws placed in lag fashion. Contralateral phalanges served as non-reduced controls. Fracture reduction following screw placement was assessed by comparing pre-reduction and post-reduction fracture gap measurements from radiographs using paired t-tests. Effects of incremental loading (0, 135, 270, 540, 800, 1070, and 1335 kg) on fracture gaps in 6 phalanges reduced with 4.5 mm screws and 5 phalanges reduced with 5.5 mm screws were measured from fluoroscopic images and assessed by 2-way ANOVA. Significance was set at P<.05. Type III distal phalanx fractures were reliably created. Only 5.5 mm cortical screws, not 4.5 mm screws, significantly reduced fracture gaps and constrained fracture gap expansion 3 cm distal to the articular surface. Compressive loading closed the fracture gaps at the articular surface in both non-reduced control groups and those reduced with either 5.5 or 4.5 mm screws. The 5.5 mm cortical screws were more effective than 4.5 mm screws in reducing type III distal phalanx fractures and restricting distal fracture gap expansion under load. © Copyright 2016 by The American College of Veterinary Surgeons.

Quasi-Brittle Fracture of Compact Specimens with Sharp Notches and U-Shaped Cuts

NASA Astrophysics Data System (ADS)

Kornev, V. M.; Demeshkin, A. G.

2018-01-01

A two-parameter (coupled) discrete-integral criterion of fracture is proposed. It can be used to construct fracture diagrams for compact specimens with sharp cracks. Curves separating the stress-crack length plane into three domains are plotted. These domains correspond to the absence of fracture, damage accumulation in the pre-fracture region under repeated loading, and specimen fragmentation under monotonic loading. Constants used for the analytical description of fracture diagrams for quasi-brittle materials with cracks are selected with the use of approximation of the classical stress-strain diagrams for the initial material and the critical stress intensity factor. Predictions of the proposed theory are compared with experimental results on fracture of compact specimens with different radii made of polymethylmethacrylate (PMMA) and solid rubber with crack-type effects in the form of U-shaped cuts.

Use of woven glass fibres to reinforce a composite veneer. A fracture resistance and acoustic emission study.

PubMed

Vallittu, P K

2002-05-01

The aim of this study was to investigate the possibility to reinforce the mechanically interlocked veneer of a porcelain-fused-to-metal (PFM) crown by woven glass fibre. A simulated situation to repair a fractured porcelain veneer was used in the experimental test set-up. A brass jig made into the shape of a framework of PFM maxillary central incisor crown with a retentive area at the palatal side of the incisal edge was used. A veneer were made with a restorative hybrid composite on the brass jig (control group). In the test groups, one or two layers of woven polymer pre-impregnated glass fibres (thickness: 0.06 mm/layer) were used by pressing the fibre weaves to the surface of the brass jig. Restorative hybrid composite was applied on the glass fibre weaves. Five veneers were made for all groups and the veneers were not cemented on the test jig. The veneers were loaded from the incisal edge until fracture occurred. The force was measured simultaneously with an acoustic emission analysis (AE) of the fracture propagation. Fracture force values for control veneers were 121 N and for those reinforced with one layer of glass fibres 399 N and for those reinforced with two layers of glass fibres 744 N ANOVA revealed significant difference between the mean values (P=0.003). The AE analysis showed different fracture propagation for the unreinforced and glass fibre reinforced veneers. The results of this study suggests that by placing two layers of woven glass fibres on the retentively shaped metal framework of the PFM crown before applying the restorative composite, considerably higher fracture resistance for the veneer could be obtained.

Time-dependent fracture probability of bilayer, lithium-disilicate-based, glass-ceramic, molar crowns as a function of core/veneer thickness ratio and load orientation.

PubMed

Anusavice, Kenneth J; Jadaan, Osama M; Esquivel-Upshaw, Josephine F

2013-11-01

Recent reports on bilayer ceramic crown prostheses suggest that fractures of the veneering ceramic represent the most common reason for prosthesis failure. The aims of this study were to test the hypotheses that: (1) an increase in core ceramic/veneer ceramic thickness ratio for a crown thickness of 1.6mm reduces the time-dependent fracture probability (Pf) of bilayer crowns with a lithium-disilicate-based glass-ceramic core, and (2) oblique loading, within the central fossa, increases Pf for 1.6-mm-thick crowns compared with vertical loading. Time-dependent fracture probabilities were calculated for 1.6-mm-thick, veneered lithium-disilicate-based glass-ceramic molar crowns as a function of core/veneer thickness ratio and load orientation in the central fossa area. Time-dependent fracture probability analyses were computed by CARES/Life software and finite element analysis, using dynamic fatigue strength data for monolithic discs of a lithium-disilicate glass-ceramic core (Empress 2), and ceramic veneer (Empress 2 Veneer Ceramic). Predicted fracture probabilities (Pf) for centrally loaded 1.6-mm-thick bilayer crowns over periods of 1, 5, and 10 years are 1.2%, 2.7%, and 3.5%, respectively, for a core/veneer thickness ratio of 1.0 (0.8mm/0.8mm), and 2.5%, 5.1%, and 7.0%, respectively, for a core/veneer thickness ratio of 0.33 (0.4mm/1.2mm). CARES/Life results support the proposed crown design and load orientation hypotheses. The application of dynamic fatigue data, finite element stress analysis, and CARES/Life analysis represent an optimal approach to optimize fixed dental prosthesis designs produced from dental ceramics and to predict time-dependent fracture probabilities of ceramic-based fixed dental prostheses that can minimize the risk for clinical failures. Copyright © 2013 Academy of Dental Materials. All rights reserved.

Time-dependent fracture probability of bilayer, lithium-disilicate-based glass-ceramic molar crowns as a function of core/veneer thickness ratio and load orientation

PubMed Central

Anusavice, Kenneth J.; Jadaan, Osama M.; Esquivel–Upshaw, Josephine

2013-01-01

Recent reports on bilayer ceramic crown prostheses suggest that fractures of the veneering ceramic represent the most common reason for prosthesis failure. Objective The aims of this study were to test the hypotheses that: (1) an increase in core ceramic/veneer ceramic thickness ratio for a crown thickness of 1.6 mm reduces the time-dependent fracture probability (Pf) of bilayer crowns with a lithium-disilicate-based glass-ceramic core, and (2) oblique loading, within the central fossa, increases Pf for 1.6-mm-thick crowns compared with vertical loading. Materials and methods Time-dependent fracture probabilities were calculated for 1.6-mm-thick, veneered lithium-disilicate-based glass-ceramic molar crowns as a function of core/veneer thickness ratio and load orientation in the central fossa area. Time-dependent fracture probability analyses were computed by CARES/Life software and finite element analysis, using dynamic fatigue strength data for monolithic discs of a lithium-disilicate glass-ceramic core (Empress 2), and ceramic veneer (Empress 2 Veneer Ceramic). Results Predicted fracture probabilities (Pf) for centrally-loaded 1,6-mm-thick bilayer crowns over periods of 1, 5, and 10 years are 1.2%, 2.7%, and 3.5%, respectively, for a core/veneer thickness ratio of 1.0 (0.8 mm/0.8 mm), and 2.5%, 5.1%, and 7.0%, respectively, for a core/veneer thickness ratio of 0.33 (0.4 mm/1.2 mm). Conclusion CARES/Life results support the proposed crown design and load orientation hypotheses. Significance The application of dynamic fatigue data, finite element stress analysis, and CARES/Life analysis represent an optimal approach to optimize fixed dental prosthesis designs produced from dental ceramics and to predict time-dependent fracture probabilities of ceramic-based fixed dental prostheses that can minimize the risk for clinical failures. PMID:24060349

Comparison of two tension-band fixation materials and techniques in transverse patella fractures: a biomechanical study.

PubMed

Rabalais, R David; Burger, Evalina; Lu, Yun; Mansour, Alfred; Baratta, Richard V

2008-02-01

This study compared the biomechanical properties of 2 tension-band techniques with stainless steel wire and ultra high molecular weight polyethylene (UHMWPE) cable in a patella fracture model. Transverse patella fractures were simulated in 8 cadaver knees and fixated with figure-of-8 and parallel wire configurations in combination with Kirschner wires. Identical configurations were tested with UHMWPE cable. Specimens were mounted to a testing apparatus and the quadriceps was used to extend the knees from 90 degrees to 0 degrees; 4 knees were tested under monotonic loading, and 4 knees were tested under cyclic loading. Under monotonic loading, average fracture gap was 0.50 and 0.57 mm for steel wire and UHMWPE cable, respectively, in the figure-of-8 construct compared with 0.16 and 0.04 mm, respectively, in the parallel wire construct. Under cyclic loading, average fracture gap was 1.45 and 1.66 mm for steel wire and UHMWPE cable, respectively, in the figure-of-8 construct compared with 0.45 and 0.60 mm, respectively, in the parallel wire construct. A statistically significant effect of technique was found, with the parallel wire construct performing better than the figure-of-8 construct in both loading models. There was no effect of material or interaction. In this biomechanical model, parallel wires performed better than the figure-of-8 configuration in both loading regimens, and UHMWPE cable performed similarly to 18-gauge steel wire.

Mechanics of the Delayed Fracture of Viscoelastic Bodies with Cracks: Theory and Experiment (Review)

NASA Astrophysics Data System (ADS)

Kaminsky, A. A.

2014-09-01

Theoretical and experimental studies on the deformation and delayed fracture of viscoelastic bodies due to slow subcritical crack growth are reviewed. The focus of this review is on studies of subcritical growth of cracks with well-developed fracture process zones, the conditions that lead to their critical development, and all stages of slow crack growth from initiation to the onset of catastrophic growth. Models, criteria, and methods used to study the delayed fracture of viscoelastic bodies with through and internal cracks are analyzed. Experimental studies of the fracture process zones in polymers using physical and mechanical methods as well as theoretical studies of these zones using fracture mesomechanics models that take into account the structural and rheological features of polymers are reviewed. Particular attention is given to crack growth in anisotropic media, the effect of the aging of viscoelastic materials on their delayed fracture, safe external loads that do not cause cracks to propagate, the mechanism of multiple-flaw fracture of viscoelastic bodies with several cracks and, especially, processes causing cracks to coalesce into a main crack, which may result in a break of the body. Methods and results of solving two- and three-dimensional problems of the mechanics of delayed fracture of aging and non-aging viscoelastic bodies with cracks under constant and variable external loads, wedging, and biaxial loads are given

EFFECT OF RADIUS OF LOADING NOSE AND SUPPORTS IN SHORT BEAM TEST FIXTURE ON FRACTURE MODE AND INTERLAMINAR SHEAR STRENGTH OF GFRP AT 77 K

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nishimura, A.

2008-03-03

A short beam test is useful to evaluate interlaminar shear strength of glass fiber reinforced plastics, especially for material selection. However, effect of test fixture configuration on interlaminar shear strength has not been clarified. This paper describes dependence of fracture mode and interlaminar shear strength on the fixture radius using the same materials and procedure. In addition, global understanding of the role of the fixture is discussed. When small loading nose and supports are used for the tests, bending fracture or translaminar fracture happens and the interlaminar shear strength would become smaller. By adopting the large radius loading nose andmore » supports (6 mm radius is recommended), it is newly recognized that some stress concentration is able to be reduced, and the interlaminar fracture tends to occur and the other fracture modes will be suppressed. The interlaminar shear strength of 2.5 mm thick GFRP plate of G-10CR is evaluated as 130-150 MPa at 77 K.« less

Fracture Toughness of Thin Plates by the Double-Torsion Test Method

NASA Technical Reports Server (NTRS)

Salem, Jonathan A.; Radovic, Miladin; Lara-Curzio, Edgar; Nelson, George

2006-01-01

Double torsion testing can produce fracture toughness values without crack length measurement that are comparable to those measured via standardized techniques such as the chevron-notch, surface-crack-in-flexure and precracked beam if the appropriate geometry is employed, and the material does not exhibit increasing crack growth resistance. Results to date indicate that 8 < W/d < 80 and L/W > 2 are required if crack length is not considered in stress intensity calculations. At L/W = 2, the normalized crack length should be 0.35 < a/L < 0.65; whereas for L/W = 3, 0.2 < a/L < 0.75 is acceptable. In addition, the load-points need to roll to reduce friction. For an alumina exhibiting increasing crack growth resistance, values corresponding to the plateau of the R-curve were measured. For very thin plates (W/d > 80) nonlinear effects were encountered.

Fracture toughness of Ti-Al3Ti-Al-Al3Ti laminate composites under static and cyclic loading conditions

NASA Astrophysics Data System (ADS)

Patselov, A. M.; Gladkovskii, S. V.; Lavrikov, R. D.; Kamantsev, I. S.

2015-10-01

The static and cyclic fracture toughnesses of a Ti-Al3Ti-Al-Al3Ti laminate composite material containing at most 15 vol % intermetallic compound are studied. Composite specimens are prepared by terminating reaction sintering of titanium and aluminum foils under pressure. The fracture of the titanium layers is quasi-cleavage during cyclic crack growth and is ductile during subsequent static loading.

Three-dimensional finite element analysis and comparison of a new intramedullary fixation with interlocking intramedullary nail.

PubMed

Liu, Chang-cheng; Xing, Wen-zhao; Zhang, Ya-xing; Pan, Zheng-hua; Feng, Wen-ling

2015-03-01

This study was set to introduce a new intramedullary fixation, explore its biomechanical properties, and provide guidance for further biomechanical experiments. With the help of CT scans and finite element modeling software, finite element model was established for a new intramedullary fixation and intramedullary nailing of femoral shaft fractures in a volunteer adult. By finite element analysis software ANSYS 10.0, we conducted 235-2,100 N axial load, 200-1,000 N bending loads and 2-15 Nm torsional loading, respectively, and analyzed maximum stress distribution, size, and displacement of the fracture fragments of the femur and intramedullary nail. During the loading process, the maximum stress of our new intramedullary fixation were within the normal range, and the displacement of the fracture fragments was less than 1 mm. Our new intramedullary fixation exhibited mechanical reliability and unique advantages of anti-rotation, which provides effective supports during fracture recovery.

Hydraulic fracture development in granite during cyclic injection

NASA Astrophysics Data System (ADS)

Diaz, M.; Jung, S. G.; Nam, Y. J.; Yeom, S.; Zhuang, L.; Kim, K. Y.

2017-12-01

The concept of fatigue hydraulic fracturing was introduced by Zang et al. (2013) as an alternative stimulation scheme to mitigate seismicity during hydraulic stimulation. In situ experiments in hard rock, and laboratory tests in granite have shown a decrease in breakdown pressure during cyclic injection. However, little work has been done in relation to the study of fracture evolution with increasing number of injection cycles. This study uses cylindrical granite specimens to observe induced fractures under continuous injection and fracture development during cyclic injection, aided by X-ray CT technology and AE monitoring. The rock specimens have 30 mm in diameter, 48 mm in height, and a 5 mm diameter central borehole drilled along its axis. Each specimen was axially loaded with 10 MPa, and without confining pressure. The first specimen was continuously injected with water at a rate of 50 mm3/s. For the second specimen, the same injection rate was used, but it was stopped multiple times when the pressure reached a value of 4 MPa in order to create cycles. The time during each injection peak was 2 min. The results show how induced fractures are likely to initiate at the borehole wall and between grain mineral boundaries. Also, the fractures increase true length and height with increasing number of cycles, and mineral distribution affected fracture orientation during its development. These observations could shed light into the physics involved behind this process

Dynamic deformation and fracture of single crystal silicon: Fracture modes, damage laws, and anisotropy

DOE PAGES

Huang, J. Y.; E, J. C.; Huang, J. W.; ...

2016-05-25

Impact fracture of single-crystal Si is critical to long-term reliability of electronic devices and solar cells for its wide use as components or substrates in semiconductor industry. Single-crystal Si is loaded along two different crystallographic directions with a split Hopkinson pressure bar integrated with an in situ x-ray imaging and diffraction system. Bulk stress histories are measured, simultaneously with x-ray phase contrast imaging (XPCI) and Laue diffraction. Damage evolution is quantified with grayscale maps from XPCI. Single-crystal Si exhibits pronounced anisotropy in fracture modes, and thus fracture strengths and damage evolution. For loading along [11¯ 0] and viewing along [001],more » (1¯1¯0)[11¯ 0] cleavage is activated and induces horizontal primary cracks followed by perpendicular wing cracks. However, for loading along [011¯] and viewing along [111], random nucleation and growth of shear and tensile-splitting crack networks lead to catastrophic failure of materials with no cleavage. The primary-wing crack mode leads to a lower characteristic fracture strength due to predamage, but a more concentrated strength distribution, i.e., a higher Weibull modulus, compared to the second loading case. Furthermore, the sequential primary cracking, wing cracking and wing-crack coalescence processes result in a gradual increase of damage with time, deviating from theoretical predictions. Particle size and aspect ratios of fragments are discussed with postmortem fragment analysis, which verifies fracture modes observed in XPCI.« less

Effect of ferrule on the fracture resistance of mandibular premolars with prefabricated posts and cores

PubMed Central

2017-01-01

PURPOSE This study evaluated fracture resistance with regard to ferrule lengths and post reinforcement on endodontically treated mandibular premolars incorporating a prefabricated post and resin core. MATERIALS AND METHODS One hundred extracted mandibular premolars were randomly divided into 5 groups (n=20): intact teeth (NR); endodontically treated teeth (ETT) without post (NP); ETT restored with a prefabricated post with ferrule lengths of either 0 mm (F0), 1 mm (F1), or 2 mm (F2). Prepared teeth were restored with metal crowns. A thermal cycling test was performed for 1,000 cycles. Loading was applied at an angle of 135 degrees to the axis of the tooth using a universal testing machine with a crosshead speed of 2.54 mm/min. Fracture loads were analyzed by one-way ANOVA and Tukey HSD test using a statistical program (α=.05). RESULTS There were statistical differences in fracture loads among groups (P<.001). The fracture load of F2 (237.7 ± 83.4) was significantly higher than those of NP (155.6 ± 74.3 N), F0 (98.8 ± 43.3 N), and F1 (152.8 ± 78.5 N) (P=.011, P<.001, and P=.008, respectively). CONCLUSION Fracture resistance of ETT depends on the length of the ferrule, as shown by the significantly increased fracture resistance in the 2 mm ferrule group (F2) compared to the groups with shorter ferrule lengths (F0, F1) and without post (NP). PMID:29142639

Qualitative assessment of bone density at the distal articulating surface of the third metacarpal in Thoroughbred racehorses with and without condylar fracture.

PubMed

Loughridge, A B; Hess, A M; Parkin, T D; Kawcak, C E

2017-03-01

Changes in subchondral bone density, induced by the repetitive cyclical loading of exercise, may potentiate fatigue damage and the risk of fracture. To use computed tomography (CT) to characterise bone density patterns at the articular surface of the third metacarpal bone in racehorses with and without lateral condylar fractures. Case control METHODS: Computed tomographic images of the distal articulating surface of the third metacarpal bone were obtained from Thoroughbred racehorses subjected to euthanasia in the UK. Third metacarpal bones were divided into 3 groups based on lateral condyle status; fractured (FX, n = 42), nonfractured contralateral condyle (NFX, n = 42) and control condyles from horses subjected to euthanasia for reasons unrelated to the third metacarpal bone (control, n = 94). Colour CT images were generated whereby each colour represented a range of pixel values and thus a relative range of bone density. A density value was calculated qualitatively by estimating the percentage of each colour within a specific region. Subchondral bone density was assessed in 6 regions from dorsal to palmar and 1 mm medial and lateral to the centre of the lateral parasagittal groove in NFX and control condyles and 1 mm medial and lateral to the fracture in FX condyles. Bone density was significantly higher in the FX and NFX condyles compared with control condyles for all 6 regions. A significantly higher bone density was observed in FX condyles relative to NFX condyles in the lateral middle and lateral palmar regions. Fractured condyles had increased heterogeneity in density among the 6 regions of interest compared with control and NFX condyles. Adjacent to the fracture, a focal increase in bone density and increased heterogeneity of density were characteristic of limbs with lateral condylar fractures compared with control and NFX condyles. These differences may represent pathological changes in bone density that increase the risk for lateral condylar fractures in racehorses. © 2015 EVJ Ltd.

Fracture resistance of computer-aided design and computer-aided manufacturing ceramic crowns cemented on solid abutments.

PubMed

Stona, Deborah; Burnett, Luiz Henrique; Mota, Eduardo Gonçalves; Spohr, Ana Maria

2015-07-01

Because no information was found in the dental literature regarding the fracture resistance of all-ceramic crowns using CEREC (Sirona) computer-aided design and computer-aided manufacturing (CAD-CAM) system on solid abutments, the authors conducted a study. Sixty synOcta (Straumann) implant replicas and regular neck solid abutments were embedded in acrylic resin and randomly assigned (n = 20 per group). Three types of ceramics were used: feldspathic, CEREC VITABLOCS Mark II (VITA); leucite, IPS Empress CAD (Ivoclar Vivadent); and lithium disilicate, IPS e.max CAD (Ivoclar Vivadent). The crowns were fabricated by the CEREC CAD-CAM system. After receiving glaze, the crowns were cemented with RelyX U200 (3M ESPE) resin cement under load of 1 kilogram. For each ceramic, one-half of the specimens were subjected to the fracture resistance testing in a universal testing machine with a crosshead speed of 1 millimeter per minute, and the other half were subjected to the fractured resistance testing after 1,000,000 cyclic fatigue loading at 100 newtons. According to a 2-way analysis of variance, the interaction between the material and mechanical cycling was significant (P = .0001). According to a Tukey test (α = .05), the fracture resistance findings with or without cyclic fatigue loading were as follows, respectively: CEREC VITABLOCKS Mark II (405 N/454 N) was statistically lower than IPS Empress CAD (1169 N/1240 N) and IPS e.max CAD (1378 N/1025 N) (P < .05). The IPS Empress CAD and IPS e.max CAD did not differ statistically (P > .05). According to a t test, there was no statistical difference in the fracture resistance with and without cyclic fatigue loading for CEREC VITABLOCS Mark II and IPS Empress CAD (P > .05). For IPS e.max CAD, the fracture resistance without cyclic fatigue loading was statistically superior to that obtained with cyclic fatigue loading (P < .05). The IPS Empress CAD and IPS e.max CAD showed higher fracture resistance compared with CEREC VITABLOCS Mark II. The cyclic fatigue loading negatively influenced only IPS e.max CAD. The CEREC VITABLOCS Mark II, IPS Empress CAD, and IPS e.max CAD ceramic crowns cemented on solid abutments showed sufficient resistance to withstand normal chewing forces. Copyright © 2015 American Dental Association. Published by Elsevier Inc. All rights reserved.

Failure in lithium-ion batteries under transverse indentation loading

NASA Astrophysics Data System (ADS)

Chung, Seung Hyun; Tancogne-Dejean, Thomas; Zhu, Juner; Luo, Hailing; Wierzbicki, Tomasz

2018-06-01

Deformation and failure of constrained cells and modules in the battery pack under transverse loading is one of the most common conditions in batteries subjected to mechanical impacts. A combined experimental, numerical and analytical approach was undertaken to reveal the underlying mechanism and develop a new cell failure model. When large format pouch cells were subjected to local indentation all the way to failure, the post-mortem examination of the failure zones beneath the punches indicates a consistent slant fracture surface angle to the battery plane. This type of behavior can be described by the critical fracture plane theory in which fracture is caused by the shear stress modified by the normal stress. The Mohr-Coulomb fracture criterion is then postulated and it is shown how the two material constants can be determined from just one indentation test. The orientation of the fracture plane is invariant with respect to the type of loading and can be considered as a property of the cell stack. In addition, closed-form solutions are derived for the load-displacement relation for both plane-strain and axisymmetric cases. The results are in good agreement with the numerical simulation of the homogenized model and experimentally measured responses.

Plate Versus Intramedullary Nail Fixation of Anterior Tibial Stress Fractures: A Biomechanical Study.

PubMed

Markolf, Keith L; Cheung, Edward; Joshi, Nirav B; Boguszewski, Daniel V; Petrigliano, Frank A; McAllister, David R

2016-06-01

Anterior midtibial stress fractures are an important clinical problem for patients engaged in high-intensity military activities or athletic training activities. When nonoperative treatment has failed, intramedullary (IM) nail and plate fixation are 2 surgical options used to arrest the progression of a fatigue fracture and allow bone healing. A plate will be more effective than an IM nail in preventing the opening of a simulated anterior midtibial stress fracture from tibial bending. Controlled laboratory study. Fresh-frozen human tibias were loaded by applying a pure bending moment in the sagittal plane. Thin transverse saw cuts, 50% and 75% of the depth of the anterior tibial cortex, were created at the midtibia to simulate a fatigue fracture. An extensometer spanning the defect was used to measure the fracture opening displacement (FOD) before and after the application of IM nail and plate fixation constructs. IM nails were tested without locking screws, with a proximal screw only, and with proximal and distal screws. Plates were tested with unlocked bicortical screws (standard compression plate) and locked bicortical screws; both plate constructs were tested with the plate edge placed 1 mm from the anterior tibial crest (anterior location) and 5 mm posterior to the crest. For the 75% saw cut depth, the mean FOD values for all IM nail constructs were 13% to 17% less than those for the saw cut alone; the use of locking screws had no significant effect on the FOD. The mean FOD values for all plate constructs were significantly less than those for all IM nail constructs. The mean FOD values for all plates were 28% to 46% less than those for the saw cut alone. Anterior plate placement significantly decreased mean FOD values for both compression and locked plate constructs, but the mean percentage reductions for locked and unlocked plates were not significantly different from each other for either plate placement. The percentage FOD reductions for all plate constructs and the unlocked IM nail were significantly less with a 50% saw cut depth. Plate fixation was superior to IM nail fixation in limiting the opening of a simulated midtibial stress fracture, and anterior-posterior placement of the plate was an important variable for this construct. Results from these tests can help guide the selection of fixation hardware for patients requiring surgical treatment for a midtibial stress fracture. © 2016 The Author(s).

Fracture load of implant-supported zirconia all-ceramic crowns luted with various cements.

PubMed

Lim, Hyun-Pil; Yoo, Jeong-Min; Park, Sang-Won; Yang, Hong-So

2010-01-01

This study compared the fracture load and failure types of implant-supported zirconia all-ceramic crowns cemented with various luting agents. The ceramic frameworks were fabricated from a presintered yttria-stabilized zirconium dioxide block using computer-aided design/computer-assisted manufacturing technology, and were then veneered with feldspathic porcelain. Three luting agents were used. Composite resin cement (1,560.78 +/- 39.43 N) showed the highest mean fracture load, followed by acrylic/urethane cement (1,116.20 +/- 77.32 N) and zinc oxide eugenol cement (741.21 +/- 41.95 N) (P < .05). The types of failure varied between groups.

Self-designed femoral neck guide pin locator for femoral neck fractures.

PubMed

Xia, Shengli; Wang, Ziping; Wang, Minghui; Wu, Zuming; Wang, Xiuhui

2014-01-01

Closed reduction and fixation with 3 cannulated screws is a widely accepted surgery for the treatment of femoral neck fractures. However, how to obtain optimal screw placement remains unclear. In the current study, the authors designed a guide pin positioning system for femoral neck fracture cannulated screw fixation and examined its application value by comparing it with freehand guide needle positioning and with general guide pin locator positioning provided by equipment manufacturers. The screw reset rate, screw parallelism, triangle area formed by the link line of the entry point of 3 guide pins, and maximum vertical load bearing of the femoral neck after internal fixation were recorded. As expected, the triangle area was largest in the self-designed positioning group, followed by the general positioning group and the freehand positioning group. The difference among the 3 groups was statistically significant (P<.05). Anteroposterior and lateral radiographs showed that the screws were more parallel in the self-designed positioning group and general positioning group compared with the freehand positioning group (P<.05). The screw reset rate in the self-designed positioning group was significantly lower than that in the general positioning group and the freehand positioning group (P<.05). Maximum bearing load among the 3 groups was equivalent, showing no statistically significant difference (P>.05). The authors’ self-designed guide pin positioning system has the potential to accurately insert cannulated screws in femoral neck fractures and may reduce bone loss and unnecessary radiation.

Internal strain analysis of ceramics using scanning laser acoustic microscopy

NASA Technical Reports Server (NTRS)

Kent, Renee M.

1993-01-01

Quantitative studies of material behavior characteristics are essential for predicting the functionality of a material under its operating conditions. A nonintrusive methodology for measuring the in situ strain of small dimeter (to 11 microns) ceramic fibers under uniaxial tensile loading and the local internal strains of ceramics and ceramic composites under flexural loading is introduced. The strain measurements and experimentally observed mechanical behavior are analyzed in terms of the microstructural development and fracture behavior of each test specimen evaluated. Measurement and analysis of Nicalon silicon carbide (SiC) fiber (15 microns diameter) indicate that the mean elastic modulus of the individual fiber is 185.3 GPa. Deviations observed in the experimentally determined elastic modulus values between specimens were attributed to microstructural variations which occur during processing. Corresponding variations in the fracture surface morphology were also observed. The observed local mechanical behavior of a lithium alumino-silicate (LAS) glass ceramic, a LAS/SiC monofilament composite, and a calcium alumino-silicate (CAS)/SiC fully reinforced composite exhibits nonlinearities and apparent hysteresis due to the subcritical mechanical loading. Local hysteresis in the LAS matrices coincided with the occurrence of multiple fracture initiation sites, localized microcracking, and secondary cracking. The observed microcracking phenomenon was attributed to stress relaxation of residual stresses developed during processing, and local interaction of the crack front with the microstructure. The relaxation strain and stress predicted on apparent mechanical hysteresis effects were defined and correlated with the magnitude of the measured fracture stress for each specimen studied. This quantitative correlation indicated a repeatable measure of the stress at which matrix microcracking occurred for stress relief of each material system. Stress relaxation occurred prior to the onset of steady state cracking conditions. The relaxation stress occurred at 18.5 percent of the fracture stress in LAS and 11.0 percent of the yield stress in CAS/SiC. The relaxation stress ratio was dependent upon the dominant fracture mode of the LAS/SiC specimens. Relaxation stress ratios greater than 0.30 were observed for specimens which fractured due to shear at the fiber matrix interface; specimens which fracture due to tensile cracking had relaxation stress ratios less than 0.30. The stress relaxation ratio appeared to be a specific characteristic of the glass ceramic material. The measured stress relaxation for LAS indicated a measure of the inherent residual stresses in the material due to processing and suggested localized toughening mechanisms for brittle material structures.

Screw-blade fixation systems in Pauwels three femoral neck fractures: a biomechanical evaluation.

PubMed

Knobe, Matthias; Altgassen, Simon; Maier, Klaus-Jürgen; Gradl-Dietsch, Gertraud; Kaczmarek, Chris; Nebelung, Sven; Klos, Kajetan; Kim, Bong-Sung; Gueorguiev, Boyko; Horst, Klemens; Buecking, Benjamin

2018-02-01

To reduce mechanical complications after osteosynthesis of femoral neck fractures, improved fixation techniques have been developed including blade or screw-anchor devices. This biomechanical study compares different fixation systems used for treatment of unstable femoral neck fractures with evaluation of failure mode, load to failure, stiffness, femoral head rotation, femoral neck shortening and femoral head migration. Standardized Pauwels type 3 fractures (AO/OTA 31-B2) with comminution were created in 18 biomechanical sawbones using a custom-made sawguide. Fractures were stabilized using either SHS-Screw, SHS-Blade or Rotationally Stable Screw-Anchor (RoSA). Femurs were positioned in 25 degrees adduction and ten degrees posterior flexion and were cyclically loaded with an axial sinusoidal loading pattern of 0.5 Hz, starting with 300 N, with an increase by 300 N every 2000 cycles until bone-implant failure occurred. Mean failure load for the Screw-Anchor fixation (RoSA) was 5100 N (IQR 750 N), 3900 N (IQR 75 N) for SHS-Blade and 3000 N (IQR 675 N; p = 0.002) for SHS-Screw. For SHS-Screw and SHS-Blade we observed fracture displacement with consecutive fracture collapse as the main reason for failure, whereas RoSA mainly showed a cut-out under high loadings. Mean stiffness at 1800 N was 826 (IQR 431) N/mm for SHS-Screw, 1328 (IQR 441) N/mm for SHS-Blade and 1953 (IQR 617) N/mm for RoSA (p = 0.003). With a load of 1800 N (SHS-Screw 12° vs. SHS-Blade 7° vs. RoSA 2°; p = 0.003) and with 2700 N (24° vs. 15° vs. 3°; p = 0.002) the RoSA implants demonstrated a higher rotational stability and had the lowest femoral neck shortening (p = 0.002), compared with the SHS groups. At the 2700 N load point, RoSA systems showed a lower axial (p = 0.019) and cranial (p = 0.031) femoral head migration compared to the SHS-Screw. In our study, the new Screw-Anchor fixation (RoSA) was superior to the comparable SHS implants regarding rotational stability and femoral neck shortening. Failure load, stiffness, femoral head migration, and resistance to fracture displacement were in RoSA implants higher than in SHS-Screws, but without significance in comparison to SHS-Blades.

Fracture Toughness and Elastic Modulus of Epoxy-Based Nanocomposites with Dopamine-Modified Nano-Fillers

PubMed Central

Koh, Kwang Liang; Ji, Xianbai; Lu, Xuehong; Lau, Soo Khim; Chen, Zhong

2017-01-01

This paper examines the effect of surface treatment and filler shape factor on the fracture toughness and elastic modulus of epoxy-based nanocomposite. Two forms of nanofillers, polydopamine-coated montmorillonite clay (D-clay) and polydopamine-coated carbon nanofibres (D-CNF) were investigated. It was found that Young’s modulus increases with increasing D-clay and D-CNF loading. However, the fracture toughness decreases with increased D-clay loading but increases with increased D-CNF loading. Explanations have been provided with the aid of fractographic analysis using electron microscope observations of the crack-filler interactions. Fractographic analysis suggests that although polydopamine provides a strong adhesion between the fillers and the matrix, leading to enhanced elastic stiffness, the enhancement prohibits energy release via secondary cracking, resulting in a decrease in fracture toughness. In contrast, 1D fibre is effective in increasing the energy dissipation during fracture through crack deflection, fibre debonding, fibre break, and pull-out. PMID:28773136

Progressive Fracture of Fiber Composite Thin Shell Structures Under Internal Pressure and Axial Loads

NASA Technical Reports Server (NTRS)

Gotsis, Pascal K.; Chamis, Christos C.; Minnetyan, Levon

1996-01-01

Graphite/epoxy composite thin shell structures were simulated to investigate damage and fracture progression due to internal pressure and axial loading. Defective and defect-free structures (thin cylinders) were examined. The three different laminates examined had fiber orientations of (90/0/+/-0)(sub s), where 0 is 45, 60, and 75 deg. CODSTRAN, an integrated computer code that scales up constituent level properties to the structural level and accounts for all possible failure modes, was used to simulate composite degradation under loading. Damage initiation, growth, accumulation, and propagation to fracture were included in the simulation. Burst pressures for defective and defect-free shells were compared to evaluate damage tolerance. The results showed that damage initiation began with matrix failure whereas damage and/or fracture progression occurred as a result of additional matrix failure and fiber fracture. In both thin cylinder cases examined (defective and defect-free), the optimum layup configuration was (90/0/+/-60)(sub s) because it had the best damage tolerance with respect to the burst pressure.

Response of Metals and Metallic Structures to Dynamic Loading

DTIC Science & Technology

1978-05-01

materials for service by testing under high rates of loading. Impact tests such as the Charpy test, the drop-weight tear test, and the dynamic tear...have clearly shown this for precracked charpy specimens and for the drop-weight tear test. Hence, there is a strong need for additional dynamic...dynamic fracture resistance ( Charpy , dynamic-tear, drop-weight tear test, etc.), normally assures that fracture in dynamically loaded structures is

Injectable gentamicin-loaded thermo-responsive hyaluronic acid derivative prevents infection in a rabbit model.

PubMed

Ter Boo, Gert-Jan A; Arens, Daniel; Metsemakers, Willem-Jan; Zeiter, Stephan; Richards, R Geoff; Grijpma, Dirk W; Eglin, David; Moriarty, T Fintan

2016-10-01

Despite the use of systemic antibiotic prophylaxis, the surgical fixation of open fractures with osteosynthesis implants is associated with high infection rates. Antibiotic-loaded biomaterials (ALBs) are increasingly used in implant surgeries across medical specialties to deliver high concentrations of antibiotics to the surgical site and reduce the risk of implant-associated infection. ALBs which are either less or not restricted in terms of spatial distribution and which may be applied throughout complex wounds could offer improved protection against infection in open fracture care. A thermo-responsive hyaluronic acid derivative (hyaluronic acid-poly(N-isopropylacrylamide) (HApN)) was prepared by a direct amidation reaction between the tetrabutyl ammonium (TBA) salt of hyaluronic acid and amine-terminated poly(N-isopropylacrylamide) (pN). The degree of grafting, and gelation properties of this gel were characterized, and the composition was loaded with gentamicin. The rheological- and release properties of this gentamicin-loaded HApN composition were tested in vitro and its efficacy in preventing infection was tested in a rabbit model of osteosynthesis contaminated with Staphylococcus aureus. The gentamicin-loaded HApN composition was able to prevent bacterial colonization of the implant site as shown by quantitative bacteriology. This finding was supported by histopathological evaluation of the humeri samples where no bacteria were found in the stained sections. In conclusion, this gentamicin-loaded HApN hydrogel effectively prevents infection in a complex wound, simulating a contaminated fracture treated with plating osteosynthesis. Fracture fixation after trauma is associated with high infection rates. Antibiotic loaded biomaterials (ALBs) can provide high local concentrations without systemic side effects. However, the currently available ALBs have limited accessibility to contaminated tissues in open fractures because of predetermined shape. Thus, a novel thermo-responsive hyaluronan based hydrogel with control over gelation temperature is reported. The efficacy of this gentamicin loaded hyaluronan derivative is demonstrated in an in vivo fracture model in the presence of fracture fixation hardware. The bacterial burden is cleared in all of the inoculated rabbits in the presence of the ALB. Thus, the proposed injectable thermo-responsive hyaluronan presents an effective ALB for the prevention of infection. Copyright © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Surface Fractal Analysis for Estimating the Fracture Energy Absorption of Nanoparticle Reinforced Composites

PubMed Central

Pramanik, Brahmananda; Tadepalli, Tezeswi; Mantena, P. Raju

2012-01-01

In this study, the fractal dimensions of failure surfaces of vinyl ester based nanocomposites are estimated using two classical methods, Vertical Section Method (VSM) and Slit Island Method (SIM), based on the processing of 3D digital microscopic images. Self-affine fractal geometry has been observed in the experimentally obtained failure surfaces of graphite platelet reinforced nanocomposites subjected to quasi-static uniaxial tensile and low velocity punch-shear loading. Fracture energy and fracture toughness are estimated analytically from the surface fractal dimensionality. Sensitivity studies show an exponential dependency of fracture energy and fracture toughness on the fractal dimensionality. Contribution of fracture energy to the total energy absorption of these nanoparticle reinforced composites is demonstrated. For the graphite platelet reinforced nanocomposites investigated, surface fractal analysis has depicted the probable ductile or brittle fracture propagation mechanism, depending upon the rate of loading. PMID:28817017

Fracture control methods for space vehicles. Volume 1: Fracture control design methods. [for space shuttle configuration planning

NASA Technical Reports Server (NTRS)

Liu, A. F.

1974-01-01

A systematic approach for applying methods for fracture control in the structural components of space vehicles consists of four major steps. The first step is to define the primary load-carrying structural elements and the type of load, environment, and design stress levels acting upon them. The second step is to identify the potential fracture-critical parts by means of a selection logic flow diagram. The third step is to evaluate the safe-life and fail-safe capabilities of the specified part. The last step in the sequence is to apply the control procedures that will prevent damage to the fracture-critical parts. The fracture control methods discussed include fatigue design and analysis methods, methods for preventing crack-like defects, fracture mechanics analysis methods, and nondestructive evaluation methods. An example problem is presented for evaluation of the safe-crack-growth capability of the space shuttle crew compartment skin structure.

Fracture strength and fatigue resistance of all-ceramic molar crowns manufactured with CAD/CAM technology.

PubMed

Zahran, Mohammed; El-Mowafy, Omar; Tam, Laura; Watson, Philip A; Finer, Yoav

2008-07-01

All-ceramic crowns are subject to fracture during function, especially in the posterior area. The use of yttrium-stabilized zirconium-oxide ceramic as a substructure for all-ceramic crowns to improve fracture resistance is unproven. The aim of this study was to compare fracture strength and fatigue resistance of new zirconium-oxide and feldspathic all-ceramic crowns made with computer-aided design/computer-aided manufacturing (CAD/CAM). An ivorine molar was prepared to receive an all-ceramic crown. Using epoxy resin, 40 replication dies were made of the prepared tooth. Twenty feldspathic all-ceramic crowns (Vita Mark II) (VMII) and 20 zirconium-oxide crown copings (In-Ceram YZ) (YZ) were made using CAD/CAM technique (CEREC-3D). The YZ copings were sintered and veneered manually with a fine-particle ceramic (VM9). All crowns were cemented to their respective dies using resin cement (Panavia F 2.0). Ten crowns in each group were subjected to compressive fatigue loading in a universal testing machine (instron). The other ten crowns from each group were loaded to fracture at a crosshead speed of 1 mm/min. Data were statistically analyzed using independent t-test and Fisher's exact test at alpha= 0.05. There was a significant difference between the survival rates of the two materials during the fatigue test (p < 0.001). All VMII crowns survived without any crack formation, while all YZ crowns fractured (40%) or developed cracks (60%). All the YZ crown fractures occurred within the veneering layer during the fatigue test. There was no significant difference in mean fracture load between the two materials (p= 0.268). Mean fracture loads (standard deviation) in N were: 1459 (492) for YZ crowns and 1272 (109) for VMII crowns. The performance of VMII crowns was superior to YZ crowns in the fatigue test. The premature fractures and cracks of the YZ crowns were attributed to weakness in the YZ veneer layer or in the core/veneer bond.

Simulation of spallation life of metals in relation to operating stresses in the nanosecond loading time range

NASA Astrophysics Data System (ADS)

Makarov, P. V.; Bakeev, R. A.

2015-10-01

Spall fracture of materials is still the only means for investigation of the material life and mechanisms of its fracture in the micro-, nano-, and picosecond time ranges of tensile loading. The phenomenological model based on the concepts of multiscale fracture of materials as nonlinear dynamic systems is shown to satisfactorily describe their life in the given range. The model is employed for the calculation of spallation life.

The Effects of Nuclear Weapons

DTIC Science & Technology

1977-01-01

stage the chamber expanded to form ity and it will have crushed or fractured a spherical cavity 62 feet in radius, much of the rock in the region it...negligible if the flied. A recommended simplification -s to loading is sufficient to fracture the glass. treat the loading a. an impulse, the For asbestos...MULTISTORY, STEEL-FRAME and fracture of beams, failure of col- BU!LDINGS umns, crushing of exterior wall paneis, 5.25 There was apparently only one and

Effects of pulp capping materials on fracture resistance of Class II composite restorations

PubMed Central

Kucukyilmaz, Ebru; Yasa, Bilal; Akcay, Merve; Savas, Selcuk; Kavrik, Fevzi

2015-01-01

Objective: The aim of this study was to investigate the effect of cavity design and the type of pulp capping materials on the fracture resistance of Class II composite restorations. Materials and Methods: Sixty freshly extracted, sound molar teeth were selected for the study. A dovetail cavity on the mesio-occlusal and a slot cavity on disto-occlusal surfaces of each tooth were prepared, and the teeth were divided 4 groups which one of them as a control group. The pulp capping materials (TheraCal LC, Calcimol LC, Dycal) applied on pulpo-axial wall of each cavity, and the restoration was completed with composite resin. The teeth were subjected to a compressive load in a universal mechanical testing machine. The surfaces of the tooth and restoration were examined under a stereomicroscope. The data were analyzed using factorial analysis of variance and Tukey's test. Results: For pulp capping materials, the highest fracture load (931.15 ± 203.81 N) and the lowest fracture load (832.28 ± 245.75 N) were calculated for Control and Dycal group, respectively. However, there were no statistically significant differences among all groups (P > 0.05). The fracture load of the dovetail groups was significantly higher than those of the slot cavity groups (P < 0.05). Conclusion: Dovetail cavity design shows better fracture resistance in Class II composite restorations, independent of used or not used pulp capping materials. PMID:26038653

Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation-induced crosslinking improvements.

PubMed

Peng, Bei; Locascio, Mark; Zapol, Peter; Li, Shuyou; Mielke, Steven L; Schatz, George C; Espinosa, Horacio D

2008-10-01

The excellent mechanical properties of carbon nanotubes are being exploited in a growing number of applications from ballistic armour to nanoelectronics. However, measurements of these properties have not achieved the values predicted by theory due to a combination of artifacts introduced during sample preparation and inadequate measurements. Here we report multiwalled carbon nanotubes with a mean fracture strength >100 GPa, which exceeds earlier observations by a factor of approximately three. These results are in excellent agreement with quantum-mechanical estimates for nanotubes containing only an occasional vacancy defect, and are approximately 80% of the values expected for defect-free tubes. This performance is made possible by omitting chemical treatments from the sample preparation process, thus avoiding the formation of defects. High-resolution imaging was used to directly determine the number of fractured shells and the chirality of the outer shell. Electron irradiation at 200 keV for 10, 100 and 1,800 s led to improvements in the maximum sustainable loads by factors of 2.4, 7.9 and 11.6 compared with non-irradiated samples of similar diameter. This effect is attributed to crosslinking between the shells. Computer simulations also illustrate the effects of various irradiation-induced crosslinking defects on load sharing between the shells.

Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation-induced crosslinking improvements.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peng, B.; Locascio, M.; Zapol, P.

2008-01-01

The excellent mechanical properties of carbon nanotubes are being exploited in a growing number of applications from ballistic armour to nanoelectronics. However, measurements of these properties have not achieved the values predicted by theory due to a combination of artifacts introduced during sample preparation and inadequate measurements. Here we report multiwalled carbon nanotubes with a mean fracture strength >100 GPa, which exceeds earlier observations by a factor of approximately three. These results are in excellent agreement with quantum-mechanical estimates for nanotubes containing only an occasional vacancy defect, and are {approx}80% of the values expected for defect-free tubes. This performance ismore » made possible by omitting chemical treatments from the sample preparation process, thus avoiding the formation of defects. High-resolution imaging was used to directly determine the number of fractured shells and the chirality of the outer shell. Electron irradiation at 200 keV for 10, 100 and 1,800 s led to improvements in the maximum sustainable loads by factors of 2.4, 7.9 and 11.6 compared with non-irradiated samples of similar diameter. This effect is attributed to crosslinking between the shells. Computer simulations also illustrate the effects of various irradiation-induced crosslinking defects on load sharing between the shells.« less

Mechanical torque measurement predicts load to implant cut-out: a biomechanical study investigating DHS anchorage in femoral heads.

PubMed

Suhm, Norbert; Hengg, Clemens; Schwyn, Ronald; Windolf, Markus; Quarz, Volker; Hänni, Markus

2007-08-01

Bone strength plays an important role in implant anchorage. Bone mineral density (BMD) is used as surrogate parameter to quantify bone strength and to predict implant anchorage. BMD can be measured by means of quantitative computer tomography (QCT) or dual energy X-ray absorptiometry (DXA). These noninvasive methods for BMD measurement are not available pre- or intra-operatively. Instead, the surgeon could determine bone strength by direct mechanical measurement. We have evaluated mechanical torque measurement for (A) its capability to quantify local bone strength and (B) its predictive value towards load at implant cut-out. Our experimental study was performed using sixteen paired human cadaver proximal femurs. BMD was determined for all specimens by QCT. The torque to breakaway of the cancellous bone structure (peak torque) was measured by means of a mechanical probe at the exact position of subsequent DHS placement. The fixation strength of the DHS achieved was assessed by cyclic loading in a stepwise protocol beginning with 1,500 N increasing 500 N every 5,000 cycles until 4,000 N. A highly significant correlation of peak torque with BMD (QCT) was found (r = 0.902, r (2) = 0.814, P < 0.001). Peak torque correlated highly significant with the load at implant cut-out (r = 0.795, P < 0.001). All specimens with a measured peak torque below 6.79 Nm failed at the first load level of 1,500 N. The specimens with a peak torque above 8.63 Nm survived until the last load level of 4,000 N. Mechanical peak torque measurement is able to quantify bone strength. In an experimental setup, peak torque identifies those specimens that are likely to fail at low load. In clinical routine, implant migration and cut-out depend on several parameters, which are difficult to control, such as fracture type, fracture reduction achieved, and implant position. The predictive value of peak torque towards cut-out in a clinical set-up therefore has to be carefully validated.

Lateral and posterior dynamic bending of the mid-shaft femur: fracture risk curves for the adult population.

PubMed

Kennedy, Eric A; Hurst, William J; Stitzel, Joel D; Cormier, Joseph M; Hansen, Gail A; Smith, Eric P; Duma, Stefan M

2004-11-01

The purpose of this study was to develop injury risk functions for dynamic bending of the human femur in the lateral-to-medial and posterior-to-anterior loading directions. A total of 45 experiments were performed on human cadaver femurs using a dynamic three-point drop test setup. An impactor of 9.8 kg was dropped from 2.2 m for an impact velocity of 5 m/s. Five-axis load cells measured the impactor and support loads, while an in situ strain gage measured the failure strain and subsequent strain rate. All 45 tests resulted in mid-shaft femur fractures with comminuted wedge and oblique fractures as the most common fracture patterns. In the lateral-to-medial bending tests the reaction loads were 4180 +/- 764 N, and the impactor loads were 4780 +/- 792 N. In the posterior-to-anterior bending tests the reaction loads were 3780 +/- 930 N, and the impactor loads were 4310 +/- 1040 N. The difference between the sum of the reaction forces and the applied load is due to inertial effects. The reaction loads were used to estimate the mid-shaft bending moments at failure since there was insufficient data to include the inertial effects in the calculations. The resulting moments are conservative estimates (lower bounds) of the mid-shaft bending moments at failure and are appropriate for use in the assessment of knee restraints and pedestrian impacts with ATD measurements. Regression analysis was used to identify significant parameters, and parametric survival analysis was used to estimate risk functions. Femur cross-sectional area, area moment of inertia (I), maximum distance to the neutral axis (c), I/c, occupant gender, and occupant mass are shown to be significant predictors of fracture tolerance, while no significant difference is shown for loading direction, bone mineral density, leg aspect and age. Risk functions are presented for femur cross-sectional area and I/c as they offer the highest correlation to peak bending moment. The risk function that utilizes the most highly correlated (R2 = 0.82) and significant (p = 0.0001) variable, cross-sectional area, predicts a 50 percent risk of femur fracture of 240 Nm, 395 Nm, and 562 Nm for equivalent cross-sectional area of the 5(th) percentile female, 50(th) percentile male, and 95(th) percentile male respectively.

Open Screw Placement in a 1.5 mm LCP Over a Fracture Gap Decreases Fatigue Life

PubMed Central

Alwen, Sarah G. J.; Kapatkin, Amy S.; Garcia, Tanya C.; Milgram, Joshua; Stover, Susan M.

2018-01-01

Objective To investigate the influence of plate and screw hole position on the stability of simulated radial fractures stabilized with a 1.5 mm condylar locking compression plate (LCP). Study Design In vitro mechanical testing of paired cadaveric limbs. Sample Population Paired radii (n = 7) stabilized with a 1.5 mm condylar LCP with an open screw hole positioned either proximal to (PG), or over (OG), a simulated small fracture gap. Methods Constructs were cycled in axial compression at a simulated trot load until failure or a maximum of 200,000 cycles. Specimens that sustained 200,000 cycles without failure were then loaded in axial compression in a single cycle to failure. Construct cyclic axial stiffness and gap strain, fatigue life, and residual strength were evaluated and compared between constructs using analysis of variance. Results Of pairs that had a failure during cyclic loading, OG constructs survived fewer cycles (54,700 ± 60,600) than PG (116,800 ± 49,300). OG constructs had significantly lower initial stiffness throughout cyclic loading and higher gap strain range within the first 1,000 cycles than PG constructs. Residual strength variables were not significantly different between constructs, however yield loads occurred at loads only marginally higher than approximated trot loads. Fatigue life decreased with increasing body weight. Conclusion Fracture fixation stability is compromised by an open screw hole directly over a fracture gap compared to the open screw hole being buttressed by bone in the model studied. The 1.5 mm condylar LCP may be insufficient stabilization in dogs with appropriate radial geometry but high body weights. PMID:29876361

Comparison of Femoral Head Rotation and Varus Collapse Between a Single Lag Screw and Integrated Dual Screw Intertrochanteric Hip Fracture Fixation Device Using a Cadaveric Hemi-Pelvis Biomechanical Model.

PubMed

Santoni, Brandon G; Nayak, Aniruddh N; Cooper, Seth A; Smithson, Ian R; Cox, Jacob L; Marberry, Scott T; Sanders, Roy W

2016-04-01

This study compared the stabilizing effect of 2 intertrochanteric (IT) fracture fixation devices in a cadaveric hemi-pelvis biomechanical model. Eleven pairs of cadaveric osteopenic female hemi-pelves with intact hip joint and capsular ligaments were used. An unstable IT fracture (OTA 31-A2) was created in each specimen and stabilized with a single lag screw device (Gamma 3) or an integrated dual screw (IDS) device (InterTAN). The hemi-pelves were inverted, coupled to a biaxial apparatus and subjected to 13.5 k cycles of loading (3 months) using controlled, oscillating pelvic rotation (0-90 degrees) plus cyclic axial femoral loading at a 2:1 body weight (BW) ratio. Femoral head rotation and varus collapse were monitored optoelectonically. For specimens surviving 3 months of loading, additional loading was performed in 0.25 × BW/250 cycle increments to a maximum of 4 × BW or failure. Femoral head rotation with IDS fixation was significantly less than the single lag screw construct after 3 months of simulated loading (P = 0.016). Maximum femoral head rotation at the end of 4 × BW loading was 7× less for the IDS construct (P = 0.006). Varus collapse was significantly less with the IDS construct over the entire loading cycle (P = 0.021). In this worst-case model of an osteopenic, unstable, IT fracture, the IDS construct, likely owing to its larger surface area, noncylindrical profile, and fracture compression, provided significantly greater stability and resistance to femoral head rotation and varus collapse.

Experimental and Numerical Studies on Development of Fracture Process Zone (FPZ) in Rocks under Cyclic and Static Loadings

NASA Astrophysics Data System (ADS)

Ghamgosar, M.; Erarslan, N.

2016-03-01

The development of fracture process zones (FPZ) in the Cracked Chevron Notched Brazilian Disc (CCNBD) monsonite and Brisbane tuff specimens was investigated to evaluate the mechanical behaviour of brittle rocks under static and various cyclic loadings. An FPZ is a region that involves different types of damage around the pre-existing and/or stress-induced crack tips in engineering materials. This highly damaged area includes micro- and meso-cracks, which emerge prior to the main fracture growth or extension and ultimately coalescence to macrofractures, leading to the failure. The experiments and numerical simulations were designed for this study to investigate the following features of FPZ in rocks: (1) ligament connections and (2) microcracking and its coalescence in FPZ. A Computed Tomography (CT) scan technique was also used to investigate the FPZ behaviour in selected rock specimens. The CT scan results showed that the fracturing velocity is entirely dependent on the appropriate amount of fracture energy absorbed in rock specimens due to the change of frequency and amplitudes of the dynamic loading. Extended Finite Element Method (XFEM) was used to compute the displacements, tensile stress distribution and plastic energy dissipation around the propagating crack tip in FPZ. One of the most important observations, the shape of FPZ and its extension around the crack tip, was made using numerical and experimental results, which supported the CT scan results. When the static rupture and the cyclic rupture were compared, the main differences are twofold: (1) the number of fragments produced is much greater under cyclic loading than under static loading, and (2) intergranular cracks are formed due to particle breakage under cyclic loading compared with smooth and bright cracks along cleavage planes under static loading.

Growth characteristics of a plane crack subjected to three-dimensional loading. [based on stress intensity factors

NASA Technical Reports Server (NTRS)

Hartranft, R. J.; Sih, G. C.

1973-01-01

The closed form expressions for the stress intensity factors due to concentrated forces applied to the surfaces of a half plane crack in an infinite body are used to generate solutions for distributed loads in this geometry. The stress intensity factors for uniformly distributed loads applied over a rectangular portion of the crack surface are given in closed form. An example of non-uniformly distributed loads which can be treated numerically is also included. In particular, combinations of normal and shear stresses on the crack which simulate the case of loading at an angle to the crack front are considered. The resulting stress intensity factors are combined with the strain energy density fracture criterion for the purpose of predicting the most likely direction of crack propagation. The critical value of the energy density factor can then be used for determining the allowable load on a specimen with a crack front not perpendicular to the tensile axis.

The effect of different posts on fracture strength of roots with vertical fracture and re-attached fragments.

PubMed

Ozcopur, B; Akman, S; Eskitascioglu, G; Belli, S

2010-08-01

The aim of this in vitro study was to test the effect of different post systems on fracture strength of roots with re-attached fragments. Root canals of eighty extracted single-rooted human teeth were instrumented (ProFile) and randomly divided into two groups. The roots in the first group were vertically cracked, and the fragments were re-attached using Super Bond C&B (Sun Medical, Tokya, Japan). The roots in the second group were kept sound. Obturation of the roots was performed with MetaSEAL (Sun Medical) and gutta-percha. Post spaces were prepared, and the roots were restored with one of the followings: UniCore (Ultradent), Everstick (Stick Tech), Ribbond (Ribbond), ParaPost (Coltene/Whaledent) (n = 10). Four mm high build-ups were created (Clearfil DC Bond Core; Kuraray, Tokyo, Japan). Compressive loading of the samples was performed after 24 h (1 mm min(-1)). Mean load necessary to fracture each sample was recorded (Newton) and statistically analysed (One-way anova, t-tests). ParaPost showed the highest fracture strength among the roots with re-attached fragments (P < 0.05). UniCore and ParaPost systems showed similar fracture strength in the sound roots (P > 0.05). Re-attached fragments significantly reduced the fracture strength of roots in UniCore group (P = 0.000). Ribbond post showed mostly repairable fractures. Metal post (ParaPost) showed the highest fracture strength in the roots with re-attached fragments; however, fracture pattern was 41% non-repairable. Re-attached fragments significantly reduced the fracture strength of the roots in UniCore group. Prefabricated posts showed similar fracture strength in the sound roots. Customized post systems EverStick and Ribbond showed mostly repairable failure after loading in sound roots or roots with re-attached fragments.

Fracture toughness, tensile and stress corrosion cracking properties of Alloy 600, Alloy 690 and their welds in water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, C.M.; Mills, W.J.

1996-10-01

The fracture toughness and tensile properties of Alloy 600, Alloy 690 and their welds, EN82H and EN52, were characterized in 54 and 338 C water with an elevated hydrogen content. Results were compared with air data to evaluate the effect of low and high temperature water on mechanical properties. In addition, the stress corrosion cracking (SCC) behavior of EN82H and EN52 welds was evaluated in 360 C water with high hydrogen. Elastic-plastic J{sub c} fracture toughness testing revealed that the fracture resistance of all test materials was exceptionally high in 54 and 338 C air and 338 C water, demonstratingmore » that fracture properties were essentially unaffected by the high temperature water environment. In 54 C water, however, J{sub c} values for EN82H and EN52 welds were reduced by an order of magnitude, and Alloy 690 showed a five-fold decrease in J{sub c}. Tensile properties for all test materials were essentially unaffected by the water environment, except for the total elongation for EN82H welds which was significantly reduced in 54 C water. At a strain rate of 5 x 10{sup 6} sec{sup {minus}1} in low temperature water, there appears to be sufficient time for environmental interactions to restrict ductility in EN82H welds. Constant-load testing of precracked weld specimens in 360 C water resulted in extensive intergranular SCC in EN82H welds loaded to K{sub I} levels between 35 and 55 MPa {radical}m, whereas no SCC occurred in EN52 welds under comparable test conditions.« less

Mechanical Failure of Endocrowns Manufactured with Different Ceramic Materials: An In Vitro Biomechanical Study.

PubMed

Aktas, Guliz; Yerlikaya, Hatice; Akca, Kivanc

2018-04-01

To evaluate the effect of different silica-based ceramic materials on the mechanical failure behavior of endocrowns used in the restoration of endodontically treated mandibular molar teeth. Thirty-six intact mandibular molar teeth extracted because of a loss of periodontal support received root canal treatment. The teeth were prepared with a central cavity to support the endocrowns, replacing the occlusal surface with mesial-lingual-distal walls. Data acquisition of the prepared tooth surfaces was carried out digitally with a powder-free intraoral scanner. Restoration designs were completed on manufactured restorations from three silicate ceramics: alumina-silicate (control), zirconia-reinforced (Zr-R), and polymer-infiltrated (P-I). Following adhesive cementation, endocrowns were subjected to thermal aging, and then, each specimen was obliquely loaded to record the fracture strength and define the mechanical failure. For the failure definition, the fracture type characteristics were identified, and further analytic measurements were made on the fractured tooth and ceramic structure. Load-to-fracture failure did not differ significantly, and the calculated mean values were 1035.08 N, 1058.33 N, and 1025.00 N for control, Zr-R, and P-I groups, respectively; however, the stiffness of the restoration-tooth complex was significantly higher than that in both test groups. No statistically significant correlation was established in paired comparisons of the failure strength, restorative stiffness, and fractured tooth distance parameters. The failure mode for teeth restored with zirconia-reinforced glass ceramics was identified as non-restorable. The resin interface in the control and P-I groups presented similar adhesive failure behavior. Mechanical failure of endocrown restorations does not significantly differ for silica-based ceramics modified either with zirconia or polymer. © 2016 by the American College of Prosthodontists.

Fracture-permeability behavior of shale

DOE Office of Scientific and Technical Information (OSTI.GOV)

Carey, J. William; Lei, Zhou; Rougier, Esteban

The fracture-permeability behavior of Utica shale, an important play for shale gas and oil, was investigated using a triaxial coreflood device and X-ray tomography in combination with finite-discrete element modeling (FDEM). Fractures generated in both compression and in a direct-shear configuration allowed permeability to be measured across the faces of cylindrical core. Shale with bedding planes perpendicular to direct-shear loading developed complex fracture networks and peak permeability of 30 mD that fell to 5 mD under hydrostatic conditions. Shale with bedding planes parallel to shear loading developed simple fractures with peak permeability as high as 900 mD. In addition tomore » the large anisotropy in fracture permeability, the amount of deformation required to initiate fractures was greater for perpendicular layering (about 1% versus 0.4%), and in both cases activation of existing fractures are more likely sources of permeability in shale gas plays or damaged caprock in CO₂ sequestration because of the significant deformation required to form new fracture networks. FDEM numerical simulations were able to replicate the main features of the fracturing processes while showing the importance of fluid penetration into fractures as well as layering in determining fracture patterns.« less

Fracture-permeability behavior of shale

DOE PAGES

Carey, J. William; Lei, Zhou; Rougier, Esteban; ...

2015-05-08

The fracture-permeability behavior of Utica shale, an important play for shale gas and oil, was investigated using a triaxial coreflood device and X-ray tomography in combination with finite-discrete element modeling (FDEM). Fractures generated in both compression and in a direct-shear configuration allowed permeability to be measured across the faces of cylindrical core. Shale with bedding planes perpendicular to direct-shear loading developed complex fracture networks and peak permeability of 30 mD that fell to 5 mD under hydrostatic conditions. Shale with bedding planes parallel to shear loading developed simple fractures with peak permeability as high as 900 mD. In addition tomore » the large anisotropy in fracture permeability, the amount of deformation required to initiate fractures was greater for perpendicular layering (about 1% versus 0.4%), and in both cases activation of existing fractures are more likely sources of permeability in shale gas plays or damaged caprock in CO₂ sequestration because of the significant deformation required to form new fracture networks. FDEM numerical simulations were able to replicate the main features of the fracturing processes while showing the importance of fluid penetration into fractures as well as layering in determining fracture patterns.« less

Dynamic mechanical characterization of aluminum: analysis of strain-rate-dependent behavior

NASA Astrophysics Data System (ADS)

Rahmat, Meysam

2018-05-01

A significant number of materials show different mechanical behavior under dynamic loads compared to quasi-static (Salvado et al. in Prog. Mater. Sci. 88:186-231, 2017). Therefore, a comprehensive study of material dynamic behavior is essential for applications in which dynamic loads are dominant (Li et al. in J. Mater. Process. Technol. 255:373-386, 2018). In this work, aluminum 6061-T6, as an example of ductile alloys with numerous applications including in the aerospace industry, has been studied under quasi-static and dynamic tensile tests with strain rates of up to 156 s^{-1}. Dogbone specimens were designed, instrumented and tested with a high speed servo-hydraulic load frame, and the results were validated with the literature. It was observed that at a strain rate of 156 s^{-1} the yield and ultimate strength increased by 31% and 33% from their quasi-static values, respectively. Moreover, the failure elongation and fracture energy per unit volume also increased by 18% and 52%, respectively. A Johnson-Cook model was used to capture the behavior of the material at different strain rates, and a modified version of this model was presented to enhance the capabilities of the original model, especially in predicting material properties close to the failure point. Finally, the fracture surfaces of specimens tested under quasi-static and dynamic loads were compared and conclusions about the differences were drawn.

Study of the variations of fall induced hip fracture risk between right and left femurs using CT-based FEA.

PubMed

Faisal, Tanvir R; Luo, Yunhua

2017-10-03

Hip fracture of elderly people-suffering from osteoporosis-is a severe public health concern, which can be reduced by providing a prior assessment of hip fracture risk. Image-based finite element analysis (FEA) has been considered an effective computational tool to assess the hip fracture risk. Considering the femoral neck region is the weakest, fracture risk indicators (FRI) are evaluated for both single-legged stance and sideways fall configurations and are compared between left and right femurs of each subject. Quantitative Computed Tomography (QCT) scan datasets of thirty anonymous patients' left and right femora have been considered for the FE models, which have been simulated with an equal magnitude of load applied to the aforementioned configurations. The requirement of bilateral hip assessment in predicting the fracture risk has been explored in this study. Comparing the sideways fall and single-legged stance, the FRI varies by 64 to 74% at the superior aspects and by 14 to 19% at the inferior surfaces of both the femora. The results of this in vivo analysis clearly substantiate that the fracture is expected to initiate at the superior surface of femoral neck region if a patient falls from his/her standing height. The distributions of FRI between the femurs vary considerably, and the variability is significant at the superior aspects. The p value (= 0.02) obtained from paired sample t-Test yields p value ≤ 0.05, which shows the evidence of variability of the FRI distribution between left and right femurs. Moreover, the comparison of FRIs between the left and right femur of men and women shows that women are more susceptible to hip fracture than men. The results and statistical variation clearly signify a need for bilateral hip scanning in predicting hip fracture risk, which is clinically conducted, at present, based on one hip chosen randomly and may lead to inaccurate fracture prediction. This study, although preliminary, may play a crucial role in assessing the hip fractures of the geriatric population and thereby, reducing the cost of treatment by taking predictive measure.

A comparison of resistance to fracture among four commercially available forms of hydroxyapatite cement.

PubMed

Miller, Lee; Guerra, Aldo Benjamin; Bidros, Rafi Sirop; Trahan, Christopher; Baratta, Richard; Metzinger, Stephen Eric

2005-07-01

Hydroxyapatite cement is a relatively new biomaterial that has found widespread use in craniomaxillofacial surgery. Despite its common usage, complication rates as high as 32% have been reported. When failed implants are removed, implant fracture has been cited as a potential cause of failure. The purpose of this study was to evaluate resistance to fracture among 4 commercially available hydroxyapatite cement formulations. The materials tested included Norian Craniofacial Repair System (carbonated apatite cement) (AO North America, Devon, PA), Norian CRS Fast Set Putty (carbonated apatite cement) (AO North America), BoneSource (hydroxyapatite cement) (Stryker Leibinger, Portage, MI), and Mimix (hydroxyapatite cement) (Walter Lorenz Surgical, Inc, Jacksonville, FL). To ensure consistency, all materials were embedded in acrylic wells. Each material was placed into a well 2.54 cm in diameter and 0.953 cm in thickness. The materials were prepared per manufacturer specifications. All materials were incubated at 37.0 degrees C, in 6% CO2, 100% humidity for 36 hours. Using the Bionix MTS Test System, a 12-mm-diameter probe applied incremental force to the center of the disk at a rate of 0.1 mm per second. The transmitted force was measured using a Bionix MTS Axial-Torsional Load Transducer for each disk. The force which resulted in fracture was recorded for each material. Ten disks of each material were processed by this method, for a total of 40 disks. The significance of resistance to fracture for the 4 compounds was analyzed using 1-way analysis of variance with post hoc Scheffe method. Mean fracture force with related P values was plotted for direct comparison of group outcomes. Material type contributed significantly to variance in fracture force for the biomaterials studied. Norian CRS required the greatest mean fracture force (1385 N, SD+/-292 N), followed by Norian CRS Fast Set Putty (1143 N, SD+/-193 N). Mimix required a mean fracture force of 740 N, SD+/-79 N. BoneSource required a mean fracture force of 558 N, SD+/-150 N. Mimix and BoneSource required significantly less force for fracture when compared with Norian CRS and Fast Set Putty (P<0.01). Comparisons of fracture load resistance between 4 commonly used bone substitute materials have not been previously reported. Increasing biomaterial strength may reduce complications resulting from reinjury to cranioplasty sites. In this model, Norian CRS and Norian CRS Fast Set Putty demonstrated a significantly greater resistance to fracture when compared with BoneSource and Mimix.

NASA/FLAGRO - FATIGUE CRACK GROWTH COMPUTER PROGRAM

NASA Technical Reports Server (NTRS)

Forman, R. G.

1994-01-01

Structural flaws and cracks may grow under fatigue inducing loads and, upon reaching a critical size, cause structural failure to occur. The growth of these flaws and cracks may occur at load levels well below the ultimate load bearing capability of the structure. The Fatigue Crack Growth Computer Program, NASA/FLAGRO, was developed as an aid in predicting the growth of pre-existing flaws and cracks in structural components of space systems. The earlier version of the program, FLAGRO4, was the primary analysis tool used by Rockwell International and the Shuttle subcontractors for fracture control analysis on the Space Shuttle. NASA/FLAGRO is an enhanced version of the program and incorporates state-of-the-art improvements in both fracture mechanics and computer technology. NASA/FLAGRO provides the fracture mechanics analyst with a computerized method of evaluating the "safe crack growth life" capabilities of structural components. NASA/FLAGRO could also be used to evaluate the damage tolerance aspects of a given structural design. The propagation of an existing crack is governed by the stress field in the vicinity of the crack tip. The stress intensity factor is defined in terms of the relationship between the stress field magnitude and the crack size. The propagation of the crack becomes catastrophic when the local stress intensity factor reaches the fracture toughness of the material. NASA/FLAGRO predicts crack growth using a two-dimensional model which predicts growth independently in two directions based on the calculation of stress intensity factors. The analyst can choose to use either a crack growth rate equation or a nonlinear interpolation routine based on tabular data. The growth rate equation is a modified Forman equation which can be converted to a Paris or Walker equation by substituting different values into the exponent. This equation provides accuracy and versatility and can be fit to data using standard least squares methods. Stress-intensity factor numerical values can be computed for making comparisons or checks of solutions. NASA/FLAGRO can check for failure of a part-through crack in the mode of a through crack when net ligament yielding occurs. NASA/FLAGRO has a number of special subroutines and files which provide enhanced capabilities and easy entry of data. These include crack case solutions, cyclic load spectrums, nondestructive examination initial flaw sizes, table interpolation, and material properties. The materials properties files are divided into two types, a user defined file and a fixed file. Data is entered and stored in the user defined file during program execution, while the fixed file contains already coded-in property value data for many different materials. Prompted input from CRT terminals consists of initial crack definition (which can be defined automatically), rate solution type, flaw type and geometry, material properties (if they are not in the built-in tables of material data), load spectrum data (if not included in the loads spectrum file), and design limit stress levels. NASA/FLAGRO output includes an echo of the input with any error or warning messages, the final crack size, whether or not critical crack size has been reached for the specified stress level, and a life history profile of the crack propagation. NASA/FLAGRO is modularly designed to facilitate revisions and operation on minicomputers. The program was implemented on a DEC VAX 11/780 with the VMS operating system. NASA/FLAGRO is written in FORTRAN77 and has a memory requirement of 1.4 MB. The program was developed in 1986.

Damage tolerance and arrest characteristics of pressurized graphite/epoxy tape cylinders

NASA Technical Reports Server (NTRS)

Ranniger, Claudia U.; Lagace, Paul A.; Graves, Michael J.

1993-01-01

An investigation of the damage tolerance and damage arrest characteristics of internally-pressurized graphite/epoxy tape cylinders with axial notches was conducted. An existing failure prediction methodology, developed and verified for quasi-isotropic graphite/epoxy fabric cylinders, was investigated for applicability to general tape layups. In addition, the effect of external circumferential stiffening bands on the direction of fracture path propagation and possible damage arrest was examined. Quasi-isotropic (90/0/plus or minus 45)s and structurally anisotropic (plus or minus 45/0)s and (plus or minus 45/90)s coupons and cylinders were constructed from AS4/3501-6 graphite/epoxy tape. Notched and unnotched coupons were tested in tension and the data correlated using the equation of Mar and Lin. Cylinders with through-thickness axial slits were pressurized to failure achieving a far-field two-to-one biaxial stress state. Experimental failure pressures of the (90/0/plus or minus 45)s cylinders agreed with predicted values for all cases but the specimen with the smallest slit. However, the failure pressures of the structurally anisotropic cylinders, (plus or minus 45/0)s and (plus or minus 45/90)s, were above the values predicted utilizing the predictive methodology in all cases. Possible factors neglected by the predictive methodology include structural coupling in the laminates and axial loading of the cylindrical specimens. Furthermore, applicability of the predictive methodology depends on the similarity of initial fracture modes in the coupon specimens and the cylinder specimens of the same laminate type. The existence of splitting which may be exacerbated by the axial loading in the cylinders, shows that this condition is not always met. The circumferential stiffeners were generally able to redirect fracture propagation from longitudinal to circumferential. A quantitative assessment for stiffener effectiveness in containing the fracture, based on cylinder radius, slit size, and bending stiffnesses of the laminates, is proposed.

Postoperative weight bearing and patient reported outcomes at one year following tibial plateau fractures.

PubMed

Thewlis, Dominic; Fraysse, Francois; Callary, Stuart A; Verghese, Viju Daniel; Jones, Claire F; Findlay, David M; Atkins, Gerald J; Rickman, Mark; Solomon, Lucian B

2017-07-01

Tibial plateau fractures are complex and the current evidence for postoperative rehabilitation is weak, especially related to the recommended postoperative weight bearing. The primary aim of this study was to investigate if loading in the first 12 weeks of recovery is associated with patient reported outcome measures at 26 and 52 weeks postoperative. We hypothesized that there would be no association between loading and patient reported outcome measures. Seventeen patients, with a minimum of 52-week follow-up following fragment-specific open reduction and internal fixation for tibial plateau fracture, were selected for this retrospective analysis. Postoperatively, patients were advised to load their limb to a maximum of 20kg during the first 6 weeks. Loading data were collected during walking using force platforms. A ratio of limb loading (affected to unaffected) was calculated at 2, 6 and 12 weeks postoperative. Knee Injury and Osteoarthritis Scores were collected at 6, 12, 26 and 52 weeks postoperative. The association between loading ratios and patient reported outcomes were investigated. Compliance with weight bearing recommendations and changes in the patient reported outcome measures are described. Fracture reduction and migration were assessed on plain radiographs. No fractures demonstrated any measurable postoperative migration at 52 weeks. Significant improvements were seen in all patient reported outcome measures over the first 52 weeks, despite poor adherence to postoperative weight bearing restrictions. There were no associations between weight bearing ratio and patient reported outcomes at 52 weeks postoperative. Significant associations were identified between the loading ratio at 2 weeks and knee-related quality of life at six months (R 2 =0.392), and between the loading ratio at 6 weeks combined with injury severity and knee-related quality of life at 26 weeks (R 2 =0.441). In summary, weight bearing as tolerated does not negatively affect the results of tibial plateau fracture and may therefore be safe for postoperative management. These findings should be taken in context of the sample size, which was not sufficient for sub-group analysis to investigate the role of impaction grafting. Copyright © 2017 Elsevier Ltd. All rights reserved.

A methodology to condition distorted acoustic emission signals to identify fracture timing from human cadaver spine impact tests.

PubMed

Arun, Mike W J; Yoganandan, Narayan; Stemper, Brian D; Pintar, Frank A

2014-12-01

While studies have used acoustic sensors to determine fracture initiation time in biomechanical studies, a systematic procedure is not established to process acoustic signals. The objective of the study was to develop a methodology to condition distorted acoustic emission data using signal processing techniques to identify fracture initiation time. The methodology was developed from testing a human cadaver lumbar spine column. Acoustic sensors were glued to all vertebrae, high-rate impact loading was applied, load-time histories were recorded (load cell), and fracture was documented using CT. Compression fracture occurred to L1 while other vertebrae were intact. FFT of raw voltage-time traces were used to determine an optimum frequency range associated with high decibel levels. Signals were bandpass filtered in this range. Bursting pattern was found in the fractured vertebra while signals from other vertebrae were silent. Bursting time was associated with time of fracture initiation. Force at fracture was determined using this time and force-time data. The methodology is independent of selecting parameters a priori such as fixing a voltage level(s), bandpass frequency and/or using force-time signal, and allows determination of force based on time identified during signal processing. The methodology can be used for different body regions in cadaver experiments. Copyright © 2014 Elsevier Ltd. All rights reserved.

Fracture strength of all-ceramic lithium disilicate and porcelain-fused-to-metal bridges for molar replacement after dynamic loading.

PubMed

Chitmongkolsuk, Somsak; Heydecke, Guido; Stappert, Christian; Strub, Joerg R

2002-03-01

The replacement of missing posterior teeth using all-ceramic bridges remains a challenge. This study compares the fracture resistance of all-ceramic 3-unit bridges for the replacement of first molars to conventional porcelain-fused-to-metal bridges. Human premolars and molars were used to create two test groups and one control group of 16 specimens each. To simulate clinical parameters, the specimens were exposed to cyclic fatigue loading in an artificial mouth with simultaneous thermocycling. All samples were thereafter exposed to fracture strength testing. Porcelain-fused-to-metal bridges showed significantly higher fracture strengths than all-ceramic bridges. However, the fracture strength of the all-ceramic bridges was higher than peak physiological chewing forces.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chang, Shih-Jung

Dynamic strength of the High Flux Isotope Reactor (HFIR) vessel to resist hypothetical accidents is analyzed by using the method of fracture mechanics. Vessel critical stresses are estimated by applying dynamic pressure pulses of a range of magnitudes and pulse-durations. The pulses versus time functions are assumed to be step functions. The probability of vessel fracture is then calculated by assuming a distribution of possible surface cracks of different crack depths. The probability distribution function for the crack depths is based on the form that is recommended by the Marshall report. The toughness of the vessel steel used in themore » analysis is based on the projected and embrittled value after 10 effective full power years from 1986. From the study made by Cheverton, Merkle and Nanstad, the weakest point on the vessel for fracture evaluation is known to be located within the region surrounding the tangential beam tube HB3. The increase in the probability of fracture is obtained as an extension of the result from that report for the regular operating condition to include conditions of higher dynamic pressures due to accident loadings. The increase in the probability of vessel fracture is plotted for a range of hoop stresses to indicate the vessel strength against hypothetical accident conditions.« less

Biomechanical Evaluation of Standard Versus Extended Proximal Fixation Olecranon Plates for Fixation of Olecranon Fractures.

PubMed

Boden, Allison L; Daly, Charles A; Dalwadi, Poonam P; Boden, Stephanie A; Hutton, William C; Muppavarapu, Raghuveer C; Gottschalk, Michael B

2018-01-01

Small olecranon fractures present a significant challenge for fixation, which has resulted in development of plates with proximal extension. Olecranon-specific plates with proximal extensions are widely thought to offer superior fixation of small proximal fragments but have distinct disadvantages: larger dissection, increased hardware prominence, and the increased possibility of impingement. Previous biomechanical studies of olecranon fracture fixation have compared methods of fracture fixation, but to date there have been no studies defining olecranon plate fixation strength for standard versus extended olecranon plates. The purpose of this study is to evaluate the biomechanical utility of the extended plate for treatment of olecranon fractures. Sixteen matched pairs of fresh-frozen human cadaveric elbows were used. Of the 16, 8 matched pairs received a transverse osteotomy including 25% and 8 including 50% of the articular surface on the proximal fragment. One elbow from each pair was randomly assigned to a standard-length plate, and the other elbow in the pair received the extended-length plate, for fixation of the fracture. The ulnae were cyclically loaded and subsequently loaded to failure, with ultimate load, number of cycles, and gap formation recorded. There was no statistically significant difference between the standard and extended fixation plates in simple transverse fractures at either 25% or 50% from the proximal most portion of the articular surface of the olecranon. Standard fixation plates are sufficient for the fixation of small transverse fractures, but caution should be utilized particularly with comminution and nontransverse fracture patterns.

Analysis of crack propagation in human long bone by using finite element modeling

NASA Astrophysics Data System (ADS)

Salim, Mohammad Shahril; Salleh, Ahmad Faizal; Daud, Ruslizam

2017-12-01

The aim of this research is to present a numerical modeling of crack for human long bone specifically on femur shaft bone under mode I loading condition. Two - dimensional model (2D) of long bone was developed based on past research study. The finite element analysis and construction of the model are done using Mechanical APDL (ANSYS) v14.0 software. The research was conducted mainly based on two conditions that were at different crack lengths and different loading forces for male and female. In order to evaluate the stress intensity factor (KI) of the femur shaft of long bone, this research employed finite element method to predict the brittle fracture loading by using three-point bending test. The result of numerical test found that the crack was formed when the crack length reached 0.0022 m where KI values are proportional with the crack's length. Also, various loading forces in range of 400 N to 1000 N were applied in an attempt to study their effect on stress intensity factor and it was found that the female dimension has higher KI values compared to male. It was also observed that K values found by this method have good agreement with theoretical results based on previous research.

A method for predicting the fatigue life of pre-corroded 2024-T3 aluminum from breaking load tests

NASA Astrophysics Data System (ADS)

Gruenberg, Karl Martin

Characterization of material properties is necessary for design purposes and has been a topic of research for many years. Over the last several decades, much progress has been made in identifying metrics to describe fracture mechanics properties and developing procedures to measure the appropriate values. However, in the context of design, there has not been as much success in quantifying the susceptibility of a material to corrosion damage and its subsequent impact on material behavior in the framework of fracture mechanics. A natural next step in understanding the effects of corrosion damage was to develop a link between standard material test procedures and fatigue life in the presence of corrosion. Simply stated, the goal of this investigation was to formulate a cheaper and quicker method for assessing the consequences of corrosion on remaining fatigue life. For this study, breaking load specimens and fatigue specimens of a single nominal gage (0.063″) of aluminum alloy 2024-T3 were exposed to three levels of corrosion. The breaking load specimens were taken from three different material lots, and the fatigue tests were carried out at three stress levels. All failed specimens, both breaking load and fatigue specimens, were examined to characterize the damage state(s) and failure mechanism(s). Correlations between breaking load results and fatigue life results in the presence of corrosion damage were developed using a fracture mechanics foundation and the observed mechanisms of failure. Where breaking load tests showed a decrease in strength due to increased corrosion exposure, the corresponding set of fatigue tests showed a decrease in life. And where breaking load tests from different specimen orientations exhibited similar levels of strength, the corresponding set of fatigue specimens showed similar lives. The spread from shortest to longest fatigue lives among the different corrosion conditions decreased at the higher stress levels. Life predictions based on breaking load data were generally shorter than the experimental lives by an average of 20%. The life prediction methodology developed from this investigation is a very valuable tool for the purpose of assessing material substitution for aircraft designers, alloy differentiation for manufacturers, or inspection intervals and aircraft retirement schedules for aircraft in service.

Comparison of load-bearing capacity of direct resin-bonded fiber-reinforced composite FPDs with four framework designs.

PubMed

Xie, Qiufei; Lassila, Lippo V J; Vallittu, Pekka K

2007-07-01

This in vitro study was aimed to compare the fracture resistance of directly fabricated inlay-retained fiber-reinforced composite (FRC) fixed partial dentures (FPDs) with four types of framework designs. Forty-eight directly fabricated inlay retained FPDs were made of FRC and particulate resin composite (everStick/Tetric flow and Ceram). Extracted human mandibular first premolars and first molars were as abutments. The following framework designs were tested: in the Group A (control group), the framework was made of two prepregs of unidirectional glass FRC; the Group B, two prepregs in pontic portion were covered with one layer of multidirectional fiber veil FRC; the Group C, the FRC prepregs were covered in pontic portion with four short unidirectional FRC pieces along the main prepregs; in Group D, one short unidirectional FRC prepregs were placed on the main prepregs in 90 degrees angle to the main framework. After thermal cycling, FPDs of each group (n=12) were randomly divided into two subgroups (n=6). Fracture test was performed at the universal testing machine (1mm/min) where FPDs were loaded from the occlusal direction to the occlusal fossa or to the buccal cusp. Failure patterns were observed with stereomicroscope. Median and 25%/75% percentile values were calculated and nonparametric analysis was performed. Compared with three other framework designs, the FPDs in Group D showed the highest resistance when loading to the occlusal fossa, with maximum load of 2,353.8N (25%/75%: 2,155.5/2,500.0) (p=0.000, 0.000, and 0.005 for compared with Group A, B, and C). The same group showed also higher resistance when loaded to the buccal cusp (1,416.3N (1,409.2/1,480.8)) if compared to the FPDs of the Group A and Group C (p=0.044, 0.010). In general the FPDs showed higher resistant to loading at the occlusal fossa (p<0.05). This in vitro study showed that inlay-retained FRC FPD constructed with direct technique provided high fracture resistance. The framework design that provided support for the veneering composite of the pontic contributed to the highest load-bearing capacity even when loaded to the buccal cusp.

Fracture Strength of Single-Crystal Silicon Carbide Microspecimens at Room and Elevated Temperature

NASA Technical Reports Server (NTRS)

Nemeth, Noel N.; Sharpe, William N., Jr.; Beheim, Glenn M.; Evans, Laura J.; Jadaan, Osama M.

2007-01-01

Three shapes of tensile specimens were tested--curved with a very low stress concentration factor and straight with either a circular hole or an elliptical hole. The nominal thickness was 125 micron with a net section 100 micron wide; the overall length of these microspecimens was 3.1 mm. They were fabricated by an improved version of deep reactive ion etching, which produced specimens with smooth sidewalls and cross-sections having a slightly trapezoidal shape that was exaggerated inside the holes. The novel test setup used a vertical load train extending into a resistance furnace. The specimens had wedge-shaped ends which fit into ceramic grips. The fixed grip was mounted on a ceramic post, and the movable grip was connected to a load cell and actuator outside the furnace with a ceramic-encased nichrome wire. The same arrangement was used for tests at 24 and at 1000 C. The strengths of the curved specimens for two batches of material (made with slightly different processes) were 0.66+/-0.12 GPa and 0.45+/-0.20 GPa respectively at 24 C with identical values at 1000 C. The fracture strengths of the circular-hole and elliptical-hole specimens (computed from the stress concentration factors and measured loads at failure) were approximately 1.2 GPa with slight decreases at the higher temperature. Fractographic examinations showed failures initiating on the surface--primarily at corners. Weibull predictions of fracture strengths for the hole specimens based on the properties of the curved specimens were reasonably effective for the circular holes, but not for the elliptical holes.

Fracture Response Enhancement Of Aluminum Using In-Situ Selective Reinforcement

NASA Technical Reports Server (NTRS)

Abada, Christopher H.; Farley, Gary L.; Hyer, Michael W.

2006-01-01

A computer-based parametric study of the effect of reinforcement architectures on fracture response of aluminum compact-tension (CT) specimens is performed. Eleven different reinforcement architectures consisting of rectangular and triangular cross-section reinforcements were evaluated. Reinforced specimens produced between 13 and 28 percent higher fracture load than achieved with the unreinforced case. Reinforcements with blunt leading edges (rectangular reinforcements) exhibited superior performance relative to the triangular reinforcements with sharp leading edges. Relative to the rectangular reinforcements, the most important architectural feature was reinforcement thickness. At failure, the reinforcements carried between 58 and 85 percent of the load applied to the specimen, suggesting that there is considerable load transfer between the base material and the reinforcement.

Viscoplastic crack initiation and propagation in crosslinked UHMWPE from clinically relevant notches up to 0.5mm radius.

PubMed

Sirimamilla, P Abhiram; Rimnac, Clare M; Furmanski, Jevan

2018-01-01

Highly crosslinked UHMWPE is now the material of choice for hard-on-soft bearing couples in total joint replacements. However, the fracture resistance of the polymer remains a design concern for increased longevity of the components in vivo. Fracture research utilizing the traditional linear elastic fracture mechanics (LEFM) or elastic plastic fracture mechanics (EPFM) approach has not yielded a definite failure criterion for UHMWPE. Therefore, an advanced viscous fracture model has been applied to various notched compact tension specimen geometries to estimate the fracture resistance of the polymer. Two generic crosslinked UHMWPE formulations (remelted 65kGy and remelted 100kGy) were analyzed in this study using notched test specimens with three different notch radii under static loading conditions. The results suggest that the viscous fracture model can be applied to crosslinked UHMWPE and a single value of critical energy governs crack initiation and propagation in the material. To our knowledge, this is one of the first studies to implement a mechanistic approach to study crack initiation and propagation in UHMWPE for a range of clinically relevant stress-concentration geometries. It is believed that a combination of structural analysis of components and material parameter quantification is a path to effective failure prediction in UHMWPE total joint replacement components, though additional testing is needed to verify the rigor of this approach. Copyright © 2017 Elsevier Ltd. All rights reserved.

Progressive Fracture of Fiber Composite Build-Up Structures

NASA Technical Reports Server (NTRS)

Gotsis, Pascal K.; Chamis, C. C.; Minnetyan, Levon

1997-01-01

Damage progression and fracture of built-up composite structures is evaluated by using computational simulation. The objective is to examine the behavior and response of a stiffened composite (0/ +/- 45/90)(sub s6) laminate panel by simulating the damage initiation, growth, accumulation, progression and propagation to structural collapse. An integrated computer code, CODSTRAN, was augmented for the simulation of the progressive damage and fracture of built-up composite structures under mechanical loading. Results show that damage initiation and progression have significant effect on the structural response. Influence of the type of loading is investigated on the damage initiation, propagation and final fracture of the build-up composite panel.

Progressive Fracture of Fiber Composite Build-Up Structures

NASA Technical Reports Server (NTRS)

Minnetyan, Levon; Gotsis, Pascal K.; Chamis, C. C.

1997-01-01

Damage progression and fracture of built-up composite structures is evaluated by using computational simulation. The objective is to examine the behavior and response of a stiffened composite (0 +/-45/90)(sub s6) laminate panel by simulating the damage initiation, growth, accumulation, progression and propagation to structural collapse. An integrated computer code CODSTRAN was augmented for the simulation of the progressive damage and fracture of built-up composite structures under mechanical loading. Results show that damage initiation and progression to have significant effect on the structural response. Influence of the type of loading is investigated on the damage initiation, propagation and final fracture of the build-up composite panel.

A Flexible Method for Producing F.E.M. Analysis of Bone Using Open-Source Software

NASA Technical Reports Server (NTRS)

Boppana, Abhishektha; Sefcik, Ryan; Myers, Jerry G.; Lewandowski, Beth

2016-01-01

Individuals who experience decreases in load-bearing bone densities can be subject to a higher risk of bone fracture during daily activity. Astronauts may lose up to nine percent of their load-bearing bone density for every month they spend in space [1]. Because of this, specialized countermeasures reduce percent loss in bone density and reduce fracture risk upon returning to Earth. Astronauts will typically not be at risk for fracture during spaceflight, because of the lesser loads experienced in microgravity conditions. However, once back on Earth, astronauts have an increased risk for bone fracture as a result of weakened bone and return to 1G conditions [2]. It is therefore important to understand the significance of any bone density loss in addition to developing exercises in an attempt to limit losses in bone strength. NASA seeks to develop a deeper understanding of fracture risk through the development of a computational bone strength model to assess the bone fracture risk of astronauts pre-flight and post-flight. This study addresses the several key processes needed to develop such strength analyses using medical image processing and finite element modeling.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parab, Niranjan D.; Hudspeth, Matthew; Claus, Ben

Granular materials are widely used to resist impact and blast. Under these dynamic loadings, the constituent particles in the granular system fracture. To study the fracture mechanisms in brittle particles under dynamic compressive loading, a high speed X-ray phase contrast imaging setup was synchronized with a Kolsky bar apparatus. Controlled compressive loading was applied on two contacting particles using the Kolsky bar apparatus and fracture process was captured using the high speed X-ray imaging setup. Five different particles were investigated: soda-lime glass, polycrystalline silica (silicon dioxide), polycrystalline silicon, barium titanate glass, and yttrium stabilized zirconia. For both soda lime glassmore » and polycrystalline silica particles, one of the particles fragmented explosively, thus breaking into many small pieces. For Silicon and barium titanate glass particles, a finite number of cracks were observed in one of the particles causing it to fracture. For yttrium stabilized zirconia particles, a single meridonial crack developed in one of the particles, breaking it into two parts.« less

Control of the permeability of fractures in geothermal rocks

NASA Astrophysics Data System (ADS)

Faoro, Igor

This thesis comprises three journal articles that will be submitted for publication (Journal of Geophysical Research-Solid Earth). Their respective titles are: "Undrained through Drained Evolution of Permeability in Dual Permeability Media" by Igor Faoro, Derek Elsworth and Chris Marone, "Evolution of Stiffness and Permeability in Fractures Subject to Thermally-and Mechanically-Activated Dissolution" by Igor Faoro, Derek Elsworth Chris Marone; "Linking permeability and mechanical damage for basalt from Mt. Etna volcano (Italy)" by Igor Faoro, Sergio Vinciguerra, Chris Marone and Derek Elsworth. Undrained through Drained Evolution of Permeability in Dual Permeability Media: temporary permeability changes of fractured aquifers subject to earthquakes have been observed and recorded worldwide, but their comprehension still remains a complex issue. In this study we report on flow-through fracture experiments on cracked westerly cores that reproduce, at laboratory scale, those (steps like) permeability changes that have been recorded when earthquakes occur. In particular our experiments show that under specific test boundary conditions, rapid increments of pore pressure induce transient variations of flow rate of the fracture whose peak magnitudes decrease as the variations of the effective stresses increase. We identify that the observed hydraulic behavior of the fracture is due to two principal mechanisms of origin; respectively mechanical (shortening of core) and poro-elastic (radial diffusion of the pore fluid into the matrix of the sample) whose interaction cause respectively an instantaneous opening and then a progressive closure of the fracture. Evolution of Stiffness and Permeability in Fractures Subject to Thermally-and Mechanically-Activated Dissolution: we report the results of radial flow-through experiments conducted on heated samples of Westerly granite. These experiments are performed to examine the influence of thermally and mechanically activated dissolution on the mechanical (stiffness) and transport (stress-permeability) characteristics of fractures. The sample is thermally stressed to 80 °C and measurements of the constrained axial stress acting on the sample and of the flow rate of the fracture are recorded with time. Net efflux of dissolved mineral mass is also measured periodically to provide a record of rates of net mass removal. During the experiment the fracture permeability shows high sensitivity to the changing conditions of stress and temperature but no significant permanent variation of permeability have been recorded once the thermal cycle ends. Linking permeability and mechanical damage for basalt from Mt. Etna volcano (Italy): volcanic edifices, such as Mt. Etna volcano (Italy), are affected from repeated episodes of pressurization due to magma emplacement from deep reservoirs to shallow depths. This mechanism pressurizes the large aquifers within the edifice and increases the level of crack damage within the rocks of the edifice over extended periods of times. In order to improve our understanding of the complex coupling between circulating fluids and the development of crack damage we performed flow-through tests using cylindrical cores of Etna Basalt (Etna, Italy) cyclically loaded either by constant increments of the principal stress: sigma1 (deviatoric condition), or by increments of the effective confining pressure: sigma1 = sigma 2 = sigma3 (isostatic conditions). Under hydrostatic stresses, the permeability values of the intact sample decrease linearly with the increments of pressure and range between 5.2*10-17 m2 and 1.5*10-17m2. At deviatoric stresses (up to 60 MPa) the permeability from the initial value of 5*10-17 m2 slightly decays to the minimum value of 2*10 -17 m2 observed when the axial deviatoric stresses range between 40 MPa and 60 MPa. For higher deviatoric stresses, increases to 10-16 m2 are then observed up to the peak stress at 92 MPa. After failure the permeability persisted steady at the value of 8*10-16 m2 for the whole duration of the test, independently from the applied stress. We interpreted the decrease observed as due to the progressive closure of the voids space, as the axial load is incremented.

Variation of brine compositions resulting from flow from matrix or fracture permeability, investigated by high pressure laboratory experiments

NASA Astrophysics Data System (ADS)

Poszwa, A. C.; Coleman, M. L.; Pouya, A.; Ader, M.; Bounenni, A.

2003-04-01

Planning oil production from a chalk reservoir oilfield is difficult because the matrix usually has low permeability despite its high porosity. Most oil is thought to come from fracture porosity but the matrix contribution should increase as compaction occurs during production. To better understand the respective contributions from matrix and fracture, we studied the geochemical characteristics of fluids using high-pressure brine flow experiments on chalk cores. During the experiment axial load was changed relative to confining pressure to induce fractures and to close them again. We used chlorine stable isotope variations to study fluid pathway, because chlorine is a chemically conservative element in sedimentary systems and its isotopes fractionate only with physical processes like diffusion or adsorption that could occur mainly in the chalk matrix. A first experiment was performed on a very porous chalk from Henley (on-shore UK) and using a low-salinity brine. Large variations of brine Cl isotope composition were observed (from -0.56 to +0.08 per mil). The variations were correlated positively with the brine flux through the chalk and the permeability of the rock, both parameters controlled by the rock fracturing. A second experiment used brine with salinity similar to that of seawater. In this case, chemical and isotopic variations were not significant. From the beginning, the chalk structure seems to have been destroyed very quickly (induced fracture porosity collapsed) possibly because of the fluid nature, so that whatever pressure was applied, the permeability did not change significantly. Using Valhall reservoir chalk (offshore Norwegian North Sea) and fluid half the salinity of seawater in a third experiment, we obtained a large range of permeabilities. Brine isotopic trends were very similar on average to those of the first experiment even though variations were smaller (Cl isotopes from -0.09 to +0.29 per mil) and not significantly correlated simply to permeability values. The highest isotopic values were in brine flowed through chalk when the permeability was high and fractures opened; the lowest values were in brine flowed through the chalk when its permeability was reduced by closing fractures and increasing the relative contribution from matrix flow where diffusion processes fractionated chlorine isotopes. From this work it seems that the relative contributions from fracture and matrix permeability in reservoirs can be estimated from the geochemical compositions of brines that flowed from them.

Fracture behavior of large-scale thin-sheet aluminum alloy

NASA Technical Reports Server (NTRS)

Dewit, Roland; Fields, Richard J.; Mordfin, Leonard; Low, Samuel R.; Harne, Donald

1994-01-01

A series of fracture tests on large-scale, pre-cracked, aluminum alloy panels is being carried out to examine and to characterize the process by which cracks propagate and link up in this material. Extended grips and test fixtures were specially designed to enable the panel specimens to be loaded in tension, in a 1780-kN-capacity universal testing machine. Twelve panel specimens, each consisting of a single sheet of bare 2024-T3 aluminum alloy, 3988 mm high, 2286 mm wide, and 1.016 mm thick are being fabricated with simulated through-cracks oriented horizontally at mid-height. Using existing information, a test matrix has been set up that explores regions of failure that are controlled by fracture mechanics, with additional tests near the boundary between plastic collapse and fracture. In addition, a variety of multiple site damage (MSD) configurations have been included to distinguish between various proposed linkage mechanisms. All tests but one use anti-buckling guides. At this writing seven specimens have been tested. Three were fabricated with a single central crack, three others had multiple cracks on each side of the central crack, and one had a single crack but no anti-buckling guides. Each fracture event was recorded on film, video, computer, magnetic tape, and occasionally optical microscopy. The visual showed the crack tip with a load meter in the field of view, using motion picture film for one tip and SVHS video tape for the other. The computer recorded the output of the testing machine load cell, the stroke, and twelve strain gages at 1.5 second intervals. A wideband FM magnetic tape recorder was used to record data from the same sources. The data were analyzed by two different procedures: (1) the plastic zone model based on the residual strength diagram; and (2) the R-curve. The first three tests were used to determine the basic material properties, and these results were then used in the analysis of the two subsequent tests with MSD cracks. There is good agreement between measured values and results obtained from the model.

Energy release rate analysis on the interface cracks of enamel-cement-bracket fracture using virtual crack closure technique

NASA Astrophysics Data System (ADS)

Samshuri, S. F.; Daud, R.; Rojan, M. A.; Mat, F.; Basaruddin, K. S.; Hassan, R.

2017-10-01

This paper presents the energy method to evaluate fracture behavior of enamel-cement-bracket system based on cement thickness. Finite element (FE) model of enamel-cement-bracket was constructed by using ANSYS Parametric Design Language (APDL). Three different thickness were used in this study, 0.05, 0.2, and 0.271 mm which assigned as thin, medium and thick for both enamel-cement and cement bracket interface cracks. Virtual crack closure technique (VCCT) was implemented as a simulation method to calculated energy release rate (ERR). Simulation results were obtained for each thickness are discussed by using Griffith’s energy balance approach. ERR for thin thickness are found to be the lowest compared to medium and thick. Peak value of ERR also showed a significant different between medium and thick thickness. Therefore, weakest bonding occurred at low cement thickness because less load required to produce enough energy to detach the bracket. For medium and thick thickness, both increased rapidly in energy value at about the mid-point of the enamel-cement interface. This behavior occurred because of the increasing in mechanical and surface energy when the cracks are increasing. However, result for thick thickness are higher at mid-point compared to thin thickness. In conclusion, fracture behavior of enamel cracking process for medium most likely the safest to avoid enamel fracture and withstand bracket debonding.

Evaluation of Stress Corrosion Cracking Susceptibility Using Fracture Mechanics Techniques, Part 1. [environmental tests of aluminum alloys, stainless steels, and titanium alloys

NASA Technical Reports Server (NTRS)

Sprowls, D. O.; Shumaker, M. B.; Walsh, J. D.; Coursen, J. W.

1973-01-01

Stress corrosion cracking (SSC) tests were performed on 13 aluminum alloys, 13 precipitation hardening stainless steels, and two titanium 6Al-4V alloy forgings to compare fracture mechanics techniques with the conventional smooth specimen procedures. Commercially fabricated plate and rolled or forged bars 2 to 2.5-in. thick were tested. Exposures were conducted outdoors in a seacoast atmosphere and in an inland industrial atmosphere to relate the accelerated tests with service type environments. With the fracture mechanics technique tests were made chiefly on bolt loaded fatigue precracked compact tension specimens of the type used for plane-strain fracture toughness tests. Additional tests of the aluminum alloy were performed on ring loaded compact tension specimens and on bolt loaded double cantilever beams. For the smooth specimen procedure 0.125-in. dia. tensile specimens were loaded axially in constant deformation type frames. For both aluminum and steel alloys comparative SCC growth rates obtained from tests of precracked specimens provide an additional useful characterization of the SCC behavior of an alloy.

Delamination onset in polymeric composite laminates under thermal and mechanical loads

NASA Technical Reports Server (NTRS)

Martin, Roderick H.

1991-01-01

A fracture mechanics damage methodology to predict edge delamination is described. The methodology accounts for residual thermal stresses, cyclic thermal stresses, and cyclic mechanical stresses. The modeling is based on the classical lamination theory and a sublaminate theory. The prediction methodology determines the strain energy release rate, G, at the edge of a laminate and compares it with the fatigue and fracture toughness of the composite. To verify the methodology, isothermal static tests at 23, 125, and 175 C and tension-tension fatigue tests at 23 and 175 C were conducted on laminates. The material system used was a carbon/bismaleimide, IM7/5260. Two quasi-isotropic layups were used. Also, 24 ply unidirectional double cantilever beam specimens were tested to determine the fatigue and fracture toughness of the composite at different temperatures. Raising the temperature had the effect of increasing the value of G at the edge for these layups and also to lower the fatigue and fracture toughness of the composite. The static stress to edge delamination was not affected by temperature but the number of cycles to edge delamination decreased.

Moving mesh finite element simulation for phase-field modeling of brittle fracture and convergence of Newton's iteration

NASA Astrophysics Data System (ADS)

Zhang, Fei; Huang, Weizhang; Li, Xianping; Zhang, Shicheng

2018-03-01

A moving mesh finite element method is studied for the numerical solution of a phase-field model for brittle fracture. The moving mesh partial differential equation approach is employed to dynamically track crack propagation. Meanwhile, the decomposition of the strain tensor into tensile and compressive components is essential for the success of the phase-field modeling of brittle fracture but results in a non-smooth elastic energy and stronger nonlinearity in the governing equation. This makes the governing equation much more difficult to solve and, in particular, Newton's iteration often fails to converge. Three regularization methods are proposed to smooth out the decomposition of the strain tensor. Numerical examples of fracture propagation under quasi-static load demonstrate that all of the methods can effectively improve the convergence of Newton's iteration for relatively small values of the regularization parameter but without compromising the accuracy of the numerical solution. They also show that the moving mesh finite element method is able to adaptively concentrate the mesh elements around propagating cracks and handle multiple and complex crack systems.

Biomechanical comparison of cemented versus non-cemented anterior screw fixation in type II odontoid fractures in the elderly: a cadaveric study.

PubMed

Rehousek, Petr; Jenner, Edward; Holton, James; Czyz, Marcin; Capek, Lukas; Henys, Petr; Kulvajtova, Marketa; Krbec, Martin; Skala-Rosenbaum, Jiri

2018-05-18

Odontoid process fractures are the most common injuries of the cervical spine in the elderly. Anterior screw stabilization of type II odontoid process fractures improves survival and function in these patients but may be complicated by failure of fixation. The present study aimed to determine whether cement augmentation of a standard anterior screw provides biomechanically superior fixation of type II odontoid fractures in comparison with a non-cemented standard screw. Twenty human cadaveric C2 vertebrae from elderly donors (mean age 83 years) were obtained. Anderson and D'Alonzo type IIa odontoid fracture was created by transverse osteotomy, and fluoroscopy-guided anterior screw fixation was performed. The specimens were divided into two matched groups. The cemented group (n=10) had radiopaque high viscosity polymethylmethacrylate cement injected via Jamshidi needle into the base of the odontoid process. The other group was not augmented. A V-shaped punch was used for loading the odontoid in an anteroposterior direction until failure. The failure state was defined as screw cutout or 5% force decrease. Mean failure load and bending stiffness were calculated. The mean failure load for the cemented group was 352±12 N compared with 168±23 N for the non-cemented group (p<.001). The mean initial stiffness of the non-cemented group was 153±19 N/mm compared with 195±29 N/mm for the cemented group (p<.001) CONCLUSIONS: Cement augmentation of an anterior standard screw fixation of type II odontoid process fractures in elderly patients significantly increased load to failure under anteroposterior load in comparison with non-augmented fixation. This may be a valuable technique to reduce failure of fixation. Copyright © 2018 Elsevier Inc. All rights reserved.

Effect of porcelain and enamel thickness on porcelain veneer failure loads in vitro.

PubMed

Ge, Chunling; Green, Chad C; Sederstrom, Dalene; McLaren, Edward A; White, Shane N

2014-05-01

Bonded porcelain veneers are widely used esthetic restorations. Although high success and survival rates have been reported, failures occur. Fracture is the most common failure mode. Fractures range from incomplete cracks to the catastrophic. Minimally invasive or thin partial veneers have gained popularity. The aim of this study was to measure the influences of porcelain veneer thickness and enamel substrate thickness on the loads needed to cause the initial fracture and catastrophic failure of porcelain veneers. Model discoid porcelain veneer specimens of varying thickness were bonded to the flattened facial surfaces of incisors, artificially aged, and loaded to failure with a small sphere. Individual fracture events were identified and analyzed statistically and fractographically. Fracture events included initial Hertzian cracks, intermediate radial cracks, and catastrophic gross failure. Increased porcelain, enamel, and their combined thickness had like effects in substantially raising resistance to catastrophic failure but also slightly decreased resistance to initial Hertzian cracking. Fractographic and numerical data demonstrated that porcelain and tooth enamel behaved in a remarkably similar manner. As porcelain thickness, enamel thickness, and their combined thickness increased, the loads needed to produce initial fracture and catastrophic failure rose substantially. Porcelain veneers withstood considerable damage before catastrophic failure. Increased enamel thickness, increased porcelain thickness, and increased combined enamel and porcelain thickness all profoundly raised the failure loads necessary to cause catastrophic failure. Enamel and feldspathic porcelain behaved in a like manner. Surface contact damage occurred initially. Final catastrophic failure followed flexural radial cracking. Bonded porcelain veneers were highly damage tolerant. Copyright © 2014 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

Fracture Resistance of Endodontically Treated Teeth Restored with Biodentine, Resin Modified GIC and Hybrid Composite Resin as a Core Material.

PubMed

Subash, Dayalan; Shoba, Krishnamma; Aman, Shibu; Bharkavi, Srinivasan Kumar Indu; Nimmi, Vijayan; Abhilash, Radhakrishnan

2017-09-01

The restoration of a severely damaged tooth usually needs a post and core as a part of treatment procedure to provide a corono - radicular stabilization. Biodentine is a class of dental material which possess high mechanical properties with excellent biocompatibility and bioactive behaviour. The sealing ability coupled with optimum physical properties could make Biodentine an excellent option as a core material. The aim of the study was to determine the fracture resistance of Biodentine as a core material in comparison with resin modified glass ionomer and composite resin. Freshly extracted 30 human permanent maxillary central incisors were selected. After endodontic treatment followed by post space preparation and luting of Glass fibre post (Reforpost, Angelus), the samples were divided in to three groups based on the type of core material. The core build-up used in Group I was Biodentine (Septodont, France), Group II was Resin-Modified Glass Ionomer Cement (GC, Japan) and Group III was Hybrid Composite Resin (TeEconom plus, Ivoclar vivadent). The specimens were subjected to fracture toughness using Universal testing machine (1474, Zwick/Roell, Germany) and results were compared using One-way analysis of variance with Tukey's Post hoc test. The results showed that there was significant difference between groups in terms of fracture load. Also, composite resin exhibited highest mean fracture load (1039.9 N), whereas teeth restored with Biodentine demonstrated the lowest mean fracture load (176.66 N). Resin modified glass ionomer exhibited intermediate fracture load (612.07 N). The primary mode of failure in Group I and Group II was favourable (100%) while unfavourable fracture was seen in Group III (30%). Biodentine, does not satisfy the requirements to be used as an ideal core material. The uses of RMGIC's as a core build-up material should be limited to non-stress bearing areas. Composite resin is still the best core build-up material owing to its high fracture resistance and bonding to tooth.

Finite element analysis of intramedullary nailing and double locking plate for treating extra-articular proximal tibial fractures.

PubMed

Chen, Fancheng; Huang, Xiaowei; Ya, Yingsun; Ma, Fenfen; Qian, Zhi; Shi, Jifei; Guo, Shuolei; Yu, Baoqing

2018-01-16

Proximal tibia fractures are one of the most familiar fractures. Surgical approaches are usually needed for anatomical reduction. However, no single treatment method has been widely established as the standard care. Our present study aims to compare the stress and stability of intramedullary nails (IMN) fixation and double locking plate (DLP) fixation in the treatment of extra-articular proximal tibial fractures. A three-dimensional (3D) finite element model of the extra-articular proximal tibial fracture, whose 2-cm bone gap began 7 cm from the tibial plateau articular surface, was created fixed by different fixation implants. The axial compressive load on an adult knee during single-limb stance was imitated by an axial force of 2500 N with a distribution of 60% to the medial compartment, while the distal end was fixed effectively. The equivalent von Mises stress and displacement of the model was used as the output measures for analysis. The maximal equivalent von Mises stress value of the system in the IMN model was 293.23 MPa, which was higher comparing against that in the DLP fixation model (147.04 MPa). And the mean stress of the model in the IMN model (9.25 MPa) was higher than that of the DLP fixation system in terms of equivalent von Mises stress (EVMS) (P < 0.0001). The maximal value of displacement (sum) in the IMN system was 8.82 mm, which was lower than that in the DLP fixation system (9.48 mm). This study demonstrated that the stability provided by the locking plate fixation system was superior to the intramedullary nails fixation system and served as an alternative fixation for the extra-articular proximal tibial fractures of young patients.

Fatigue pre-cracking and fracture toughness in polycrystalline tungsten and molybdenum

NASA Astrophysics Data System (ADS)

Taguchi, Katsuya; Nakadate, Kazuhito; Matsuo, Satoru; Tokunaga, Kazutoshi; Kurishita, Hiroaki

2018-01-01

Fatigue pre-cracking performance and fracture toughness in polycrystalline tungsten (W) and molybdenum (Mo) have been investigated in relation to grain boundary (GB) configuration with respect to the crack advance direction. Sub-sized, single edge notched bend (SENB) specimens with three different orientations, R-L (ASTM notation) for a forged Mo rod and L-S and T-S for a rolled W plate, were pre-cracked in two steps: fully uniaxial compression fatigue loading to provoke crack initiation and its stable growth from the notch root, and subsequent 3-point bend (3PB) fatigue loading to extend the crack. The latter step intends to minimize the influence of the residual tensile stresses generated during compression fatigue by moving the crack tip away from the plastic zone. It is shown that fatigue pre-cracking performance, especially pre-crack extension behavior, is significantly affected by the specimen orientation. The R-L orientation, giving the easiest cracking path, permitted crack extension completely beyond the plastic zone, while the L-S and T-S orientations with the thickness cracking direction of the rolled plate sustained the crack lengths around or possibly within the plastic zone size due to difficulty in crack advance through an aligned grain structure. Room temperature fracture toughness tests revealed that the 3PB fatigued specimens exhibited appreciably higher fracture toughness by about 30% for R-L, 40% for L-S and 60% for T-S than the specimens of each orientation pre-cracked by compression fatigue only. This indicates that 3PB fatigue provides the crack tip front out of the residual tensile stress zone by crack extension or leads to reduction in the residual stresses at the crack tip front. Strong dependence of fracture toughness on GB configuration was evident. The obtained fracture toughness values are compared with those in the literature and its strong GB configuration dependence is discussed in connection with the appearance of pop-in.

FRACTURE STRENGTH OF FLARED BOVINE ROOTS RESTORED WITH DIFFERENT INTRARADICULAR POSTS

PubMed Central

Clavijo, Victor Grover Rene; Reis, José Maurício dos Santos Nunes; Kabbach, William; Silva, André Luis Faria e; de Oliveira, Osmir Batista; de Andrade, Marcelo Ferrarezi

2009-01-01

Objective: The aim of this study was to evaluate the fracture strength and failure mode of flared bovine roots restored with different intraradicular posts. Material and Methods: Fifty bovine incisors with similar dimensions were selected and their roots were flared until 1.0 mm of dentin wall remained. Next, the roots were allocated into five groups (n=10): GI-cast metal post-and-core; GII-fiber posts plus accessory fiber posts; GIII- direct anatomic post; GIV- indirect anatomic post and GV- control (specimens without intraradicular post). A polyether impression material was used to simulate the periodontal ligament. After periodontal ligament simulation, the specimens were subjected to a compressive load at a crosshead speed of 0.5 mm/min in a servo-hydraulic testing machine (MTS 810) applied at 135° to the long axis of the tooth until failure. The data (N) were subjected to ANOVA and Tukey's post-hoc test (α=0.05). Results: GI and GIV presented higher fracture strength (p<0.05) than GII. GIII presented intermediate values without statistically significant differences (p>0.05) from GI, GII and GIV. Control specimens (GV) produced the lowest fracture strength mean values (p<0.05). Despite obtaining the highest mean value, GI presented 100% of unfavorable failures. GII presented 20% of unfavorable failures. GIII, GIV and GV presented only favorable failures. Conclusions: Although further in vitro and in vivo studies are necessary, the results of this study showed that the use of direct and indirect anatomic posts in flared roots could be an alternative to cast metal post-and-core. PMID:20027429

Radiographic morphometry and densitometry predict strength of cadaveric proximal humeri more reliably than age and DXA scan density.

PubMed

Skedros, John G; Knight, Alex N; Pitts, Todd C; O'Rourke, Peter J; Burkhead, Wayne Z

2016-02-01

Methods are needed for identifying poorer quality cadaver proximal humeri to ensure that they are not disproportionately segregated into experimental groups for fracture studies. We hypothesized that measurements made from radiographs of cadaveric proximal humeri are stronger predictors of fracture strength than chronological age or bone density values derived from dual-energy x-ray absorptiometry (DXA) scans. Thirty-three proximal humeri (range: 39-78 years) were analyzed for: (1) bone mineral density (BMD, g/cm(2)) using DXA, (2) bulk density (g/cm(3)) using DXA and volume displacement, (3) regional bone density in millimeters of aluminum (mmAl) using radiographs, and (4) regional mean (medial+lateral) cortical thickness and cortical index (CI) using radiographs. The bones were then fractured simulating a fall. Strongest correlations with ultimate fracture load (UFL) were: mean cortical thickness at two diaphyseal locations (r = 0.71; p < 0.001), and mean mmAl in the humeral head (r = 0.70; p < 0.001). Weaker correlations were found between UFL and DXA-BMD (r = 0.60), bulk density (r = 0.43), CI (r = 0.61), and age (r = -0.65) (p values <0.01). Analyses between UFL and the product of any two characteristics showed six combinations with r-values >0.80, but none included DXA-derived density, CI, or age. Radiographic morphometric and densitometric measurements from radiographs are therefore stronger predictors of UFL than age, CI, or DXA-derived density measurements. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.

Successful short-segment instrumentation and fusion for thoracolumbar spine fractures: a consecutive 41/2-year series.

PubMed

Parker, J W; Lane, J R; Karaikovic, E E; Gaines, R W

2000-05-01

A retrospective review of all the surgically managed spinal fractures at the University of Missouri Medical Center during the 41/2-year period from January 1989 to July 1993 was performed. Of the 51 surgically managed patients, 46 were instrumented by short-segment technique (attachment of one level above the fracture to one level below the fracture). The other 5 patients in this consecutive series had multiple trauma. These patients were included in the review because this was a consecutive series. However, they were grouped separately because they were instrumented by long-segment technique because of their multiple organ system injuries. The choice of the anterior or posterior approach for short-segment instrumentation was based on the Load-Sharing Classification published in a 1994 issue of Spine. The purpose of this review was to demonstrate that grading comminution by use of the Load-Sharing Classification for approach selection and the choice of patients with isolated fractures who are cooperative with spinal bracing for 4 months provide the keys to successful short-segment treatment of isolated spinal fractures. The current literature implies that the use of pedicle screws for short-segment instrumentation of spinal fracture is dangerous and inappropriate because of the high screw fracture rate. Charts, operative notes, preoperative and postoperative radiographs, computed tomography scans, and follow-up records of all patients were reviewed carefully from the time of surgery until final follow-up assessment. The Load-Sharing Classification had been used prospectively for all patients before their surgery to determine the approach for short-segment instrumentation. Denis' Pain Scale and Work Scales were obtained during follow-up evaluation for all patients. All patients were observed over 40 months except for 1 patient who died of unrelated causes after 35 months. The mean follow-up period was 66 months (51/2 years). No patient was lost to follow-up evaluation. Prospective application of the Load-Sharing Classification to the patients' injury and restriction of the short-segment approach to cooperative patients with isolated spinal fractures (excluding multisystem trauma patients) allowed 45 of 46 patients instrumented by the short-segment technique to proceed to successful healing in virtual anatomic alignment. The Load-Sharing Classification is a straightforward way to describe the amount of bony comminution in a spinal fracture. When applied to patients with isolated spine fractures who are cooperative with 3 to 4 months of spinal bracing, it can help the surgeon select short-segment pedicle-screw-based fixation using the posterior approach for less comminuted injuries and the anterior approach for those more comminuted. The choice of which fracture-dislocations should be strut grafted anteriorly and which need only posterior short-segment pedicle-screw-based instrumentation also can be made using the Load-Sharing Classification.

Influence of the geometry of curved artificial canals on the fracture of rotary nickel-titanium instruments subjected to cyclic fatigue tests.

PubMed

Lopes, Hélio P; Vieira, Márcia V B; Elias, Carlos N; Gonçalves, Lucio S; Siqueira, José F; Moreira, Edson J L; Vieira, Victor T L; Souza, Letícia C

2013-05-01

This study evaluated the influence of different features of canal curvature geometry on the number of cycles to fracture of a rotary nickel-titanium endodontic instrument subjected to a cyclic fatigue test. BioRaCe BR4C instruments (FKG Dentaire, La Chaux-de Fonds, Switzerland) were tested in 4 grooves simulating curved metallic artificial canals, each one measuring 1.5 mm in width, 20 mm in total length, and 3.5 mm in depth with a U-shaped bottom. The parameters of curvature including the radius and arc lengths and the position of the arc differed in the 4 canal designs. Fractured surfaces and helical shafts of the separated instruments were analyzed by scanning electron microscopy. The Student's t test showed that a significantly lower number of cycles to fracture values were observed for instruments tested in canals with the smallest radius, the longest arc, and the arc located in the middle portion of the canal. Scanning electron microscopic analysis of the fracture surfaces revealed morphologic characteristics of ductile fracture. Plastic deformation was not observed in the helical shaft of the fractured instruments. Curvature geometry including the radius and arc lengths and the position of the arc along the root canal influence the number of cycles to fracture of rotary nickel-titanium instruments subjected to flexural load. Copyright © 2013 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

Workshop on Numerical Analysis of Human and Surrogate Response to Accelerative Loading

DTIC Science & Technology

2014-05-01

calcaneal fractures with high rates of amputation and poor outcome [1].The first body part in contact with the loading boundary is the heel fat pad...Laboratory • “Material Properties of the Human Heel Fat Pad Across Loading Rates” by Grigoris Grigoriadis, Nic Newell, Spyros Masouros, and Anthony Bull...and Micro/Sub-structural Details on the Fracture Response of Human Cortical Bones” by Tusit Weerasooriya, ARL • “Recent Developments in a

The Influence of Physical Forces on Progenitor Cell Migration, Proliferation and Differentiation in Fracture Repair

DTIC Science & Technology

2007-11-01

accelerated healing patterns in the loaded specimens. Periosteal callus formation appears more robust with more chondrocytes present in loaded... periosteal callus formation on one side of the fracture gap. It is hypothesized that callus development may occur first on the medial side of the femoral...Figure 10: Comparison of periosteal callus formation (trichrome stain) between a loaded limb at section levels 1 (a), 3 (b), and 5 (c), and

Strength and fracture of uranium, plutonium and several their alloys under shock wave loading

NASA Astrophysics Data System (ADS)

Golubev, V. K.

2012-08-01

Results on studying the spall fracture of uranium, plutonium and several their alloys under shock wave loading are presented in the paper. The problems of influence of initial temperature in a range of - 196 - 800∘C and loading time on the spall strength and failure character of uranium and two its alloys with molybdenum and both molybdenum and zirconium were studied. The results for plutonium and its alloy with gallium were obtained at a normal temperature and in a temperature range of 40-315∘C, respectively. The majority of tests were conducted with the samples in the form of disks 4 mm in thickness. They were loaded by the impact of aluminum plates 4 mm thick through a copper screen 12 mm thick serving as the cover or bottom part of a special container. The character of spall failure of materials and the damage degree of samples were observed on the longitudinal metallographic sections of recovered samples. For a concrete test temperature, the impact velocity was sequentially changed and therefore the loading conditions corresponding to the consecutive transition from microdamage nucleation up to complete macroscopic spall fracture were determined. The conditions of shock wave loading were calculated using an elastic-plastic computer program. The comparison of obtained results with the data of other researchers on the spall fracture of examined materials was conducted.

Fracture analysis of randomized implant-supported fixed dental prostheses

PubMed Central

Esquivel-Upshaw, Josephine F.; Mehler, Alex; Clark, Arthur E.; Neal, Dan; Anusavice, Kenneth J.

2014-01-01

Objective Fractures of posterior fixed dental all-ceramic prostheses can be caused by one or more factors including prosthesis design, flaw distribution, direction and magnitude of occlusal loading, and nature of supporting infrastructure (tooth root/implant), and presence of adjacent teeth. This clinical study of implant-supported, all-ceramic fixed dental prostheses, determined the effects of (1) presence of a tooth distal to the most distal retainer; (2) prosthesis loading either along the non-load bearing or load bearing areas; (3) presence of excursive contacts or maximum intercuspation contacts in the prosthesis; and (4) magnitude of bite force on the occurrence of veneer ceramic fracture. Methods 89 implant-supported FDPs were randomized as either a three-unit posterior metal-ceramic (Au-Pd-Ag alloy and InLine POM, Ivoclar, Vivadent) FDP or a ceramic-ceramic (ZirCAD and ZirPress, Ivoclar, Vivadent) FDP. Two implants (Osseospeed, Dentsply) and custom abutments (Atlantis, Dentsply) supported these FDPs, which were cemented with resin cement (RelyX Universal Cement). Baseline photographs were made with markings of teeth from maximum intercuspation (MI) and excursive function. Patients were recalled at 6 months and 1 to 3 years. Fractures were observed, their locations recorded, and images compared with baseline photographs of occlusal contacts. Conclusion No significant relationship exists between the occurrence of fracture and: (1) the magnitude of bite force; (2) a tooth distal to the most distal retainer; and (3) contacts in load-bearing or non-load-bearing areas. However, there was a significantly higher likelihood of fracture in areas with MI contacts only. Clinical Significance This clinical study demonstrates that there is a need to evaluate occlusion differently with implant-supported prostheses than with natural tooth supported prostheses because of the absence of a periodontal ligament. Implant supported prostheses should have minimal occlusion and lighter contacts than ones supported by natural dentition. PMID:25016139

Effects of artificial aging conditions on yttria-stabilized zirconia implant abutments.

PubMed

Basílio, Mariana de Almeida; Cardoso, Kátia Vieira; Antonio, Selma Gutierrez; Rizkalla, Amin Sami; Santos Junior, Gildo Coelho; Arioli Filho, João Neudenir

2016-08-01

Most ceramic abutments are fabricated from yttria-stabilized tetragonal zirconia (Y-TZP). However, Y-TZP undergoes hydrothermal degradation, a process that is not well understood. The purpose of this in vitro study was to assess the effects of artificial aging conditions on the fracture load, phase stability, and surface microstructure of a Y-TZP abutment. Thirty-two prefabricated Y-TZP abutments were screwed and tightened down to external hexagon implants and divided into 4 groups (n = 8): C, control; MC, mechanical cycling (1×10(6) cycles; 10 Hz); AUT, autoclaving (134°C; 5 hours; 0.2 MPa); and TC, thermal cycling (10(4) cycles; 5°/55°C). A single-load-to-fracture test was performed at a crosshead speed of 0.5 mm/min to assess the assembly's resistance to fracture (ISO Norm 14801). X-ray diffraction (XRD) analysis was applied to observe and quantify the tetragonal-monoclinic (t-m) phase transformation. Representative abutments were examined with high-resolution scanning electron microscopy (SEM) to observe the surface characteristics of the abutments. Load-to-fracture test results (N) were compared by ANOVA and Tukey test (α=.05). XRD measurements revealed the monoclinic phase in some abutments after each aging condition. All the aging conditions reduced the fracture load significantly (P<.001). Mechanical cycling reduced the fracture load more than autoclaving (P=.034). No differences were found in the process of surface degradation among the groups; however, the SEM detected grinding-induced surface flaws and microcracks. The resistance to fracture and the phase stability of Y-TZP implant abutments were susceptible to hydrothermal and mechanical conditions. The surface microstructure of Y-TZP abutments did not change after aging conditions. Copyright © 2016 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

The effect of filler loading and morphology on the mechanical properties of contemporary composites.

PubMed

Kim, Kyo-Han; Ong, Joo L; Okuno, Osamu

2002-06-01

Little information exists regarding the filler morphology and loading of composites with respect to their effects on selected mechanical properties and fracture toughness. The objectives of this study were to: (1) classify commercial composites according to filler morphology, (2) evaluate the influence of filler morphology on filler loading, and (3) evaluate the effect of filler morphology and loading on the hardness, flexural strength, flexural modulus, and fracture toughness of contemporary composites. Field emission scanning electron microscopy/energy dispersive spectroscopy was used to classify 3 specimens from each of 14 commercial composites into 4 groups according to filler morphology. The specimens (each 5 x 2.5 x 15 mm) were derived from the fractured remnants after the fracture toughness test. Filler weight content was determined by the standard ash method, and the volume content was calculated using the weight percentage and density of the filler and matrix components. Microhardness was measured with a Vickers hardness tester, and flexural strength and modulus were measured with a universal testing machine. A 3-point bending test (ASTM E-399) was used to determine the fracture toughness of each composite. Data were compared with analysis of variance followed by Duncan's multiple range test, both at the P<.05 level of significance. The composites were classified into 4 categories according to filler morphology: prepolymerized, irregular-shaped, both prepolymerized and irregular-shaped, and round particles. Filler loading was influenced by filler morphology. Composites containing prepolymerized filler particles had the lowest filler content (25% to 51% of filler volume), whereas composites containing round particles had the highest filler content (59% to 60% of filler volume). The mechanical properties of the composites were related to their filler content. Composites with the highest filler by volume exhibited the highest flexural strength (120 to 129 MPa), flexural modulus (12 to 15 GPa), and hardness (101 to 117 VHN). Fracture toughness was also affected by filler volume, but maximum toughness was found at a threshold level of approximately 55% filler volume. Within the limitations of this study, the commercial composites tested could be classified by their filler morphology. This property influenced filler loading. Both filler morphology and filler loading influenced flexural strength, flexural modulus, hardness, and fracture toughness.

Effects of proof loads and combined mode loadings on fracture and flaw growth characteristics of aerospace alloys

NASA Technical Reports Server (NTRS)

Shah, R. C.

1974-01-01

This experimental program was undertaken to determine the effects of (1) combined tensile and bending loadings, (2) combined tensile and shear loadings, and (3) proof overloads on fracture and flaw growth characteristics of aerospace alloys. Tests were performed on four alloys: 2219-T87 aluminum, 5Al-2.5Sn (ELl) titanium, 6Al-4V beta STA titanium and high strength 4340 steel. Tests were conducted in room air, gaseous nitrogen at -200F (144K), liquid nitrogen and liquid hydrogen. Flat center cracked and surface flawed specimens, cracked tube specimens, circumferentially notched round bar and surface flawed cylindrical specimens were tested. The three-dimensional photoelastic technique of stress freezing and slicing was used to determine stress intensity factors for surface flawed cylindrical specimens subjected to tension or torsion. Results showed that proof load/temperature histories used in the tests have a small beneficial effect or no effect on subsequent fracture strength and flaw growth rates.

Crack growth induced by thermal-mechanical loading

NASA Astrophysics Data System (ADS)

John, R.; Hartman, G. A.; Gallagher, J. P.

1992-06-01

Advanced aerospace structures are often subjected to combined thermal and mechanical loads. The fracture-mechanics behavior of the structures may be altered by the thermal state existing around the crack. Hence, design of critical structural elements requires the knowledge of stress-intensity factors under both thermal and mechanical loads. This paper describes the development of an experimental technique to verify the thermal-stress-intensity factor generated by a temperature gradient around the crack. Thin plate specimens of a model material (AISI-SAE 1095 steel) were used for the heat transfer and thermal-mechanical fracture tests. Rapid thermal loading was achieved using high-intensity focused infrared spot heaters. These heaters were also used to generate controlled temperature rates for heat-transfer verification tests. The experimental results indicate that thermal loads can generate stress-intensity factors large enough to induce crack growth. The proposed thermal-stress-intensity factors appear to have the same effect as the conventional mechanical-stress-intensity factors with respect to fracture.

Fracture resistance of the implant-abutment connection in implants with internal hex and internal conical connections under oblique compressive loading: an in vitro study.

PubMed

Coppedê, Abílio Ricciardi; Bersani, Edmilson; de Mattos, Maria da Gloria Chiarello; Rodrigues, Renata Cristina Silveira; Sartori, Ivete Aparecida de Mattias; Ribeiro, Ricardo Faria

2009-01-01

The objective of this study was to verify if differences in the design of internal hex (IH) and internal conical (IC) connection implant systems influence fracture resistance under oblique compressive forces. Twenty implant-abutment assemblies were utilized: 10 with IH connections and 10 with IC connections. Maximum deformation force for IC implants (90.58 +/- 6.72 kgf) was statistically higher than that for IH implants (83.73 +/- 4.94 kgf) (P = .0182). Fracture force for the IH implants was 79.86 +/- 4.77 kgf. None of the IC implants fractured. The friction-locking mechanics and the solid design of the IC abutments provided greater resistance to deformation and fracture under oblique compressive loading when compared to the IH abutments.

Coupled Fracture and Flow in Shale in Hydraulic Fracturing

NASA Astrophysics Data System (ADS)

Carey, J. W.; Mori, H.; Viswanathan, H.

2014-12-01

Production of hydrocarbon from shale requires creation and maintenance of fracture permeability in an otherwise impermeable shale matrix. In this study, we use a combination of triaxial coreflood experiments and x-ray tomography characterization to investigate the fracture-permeability behavior of Utica shale at in situ reservoir conditions (25-50 oC and 35-120 bars). Initially impermeable shale core was placed between flat anvils (compression) or between split anvils (pure shear) and loaded until failure in the triaxial device. Permeability was monitored continuously during this process. Significant deformation (>1%) was required to generate a transmissive fracture system. Permeability generally peaked at the point of a distinct failure event and then dropped by a factor of 2-6 when the system returned to hydrostatic failure. Permeability was very small in compression experiments (< 1 mD), possibly because of limited fracture connectivity through the anvils. In pure share experiments, shale with bedding planes perpendicular to shear loading developed complex fracture networks with narrow apertures and peak permeability of 30 mD. Shale with bedding planes parallel to shear loading developed simple fractures with large apertures and a peak permeability as high as 1 D. Fracture systems held at static conditions for periods of several hours showed little change in effective permeability at hydrostatic conditions as high as 140 bars. However, permeability of fractured systems was a function of hydrostatic pressure, declining in a pseudo-linear, exponential fashion as pressure increased. We also observed that permeability decreased with increasing fluid flow rate indicating that flow did not follow Darcy's Law, possibly due to non-laminar flow conditions, and conformed to Forscheimer's law. The coupled deformation and flow behavior of Utica shale, particularly the large deformation required to initiate flow, indicates the probable importance of activation of existing fractures in hydraulic fracturing and that these fractures can have adequate permeability for the production of hydrocarbon.

Fracture toughness of dentin/resin-composite adhesive interfaces.

PubMed

Tam, L E; Pilliar, R M

1993-05-01

The reliability and validity of tensile and shear bond strength determinations of dentin-bonded interfaces have been questioned. The fracture toughness value (KIC) reflects the ability of a material to resist crack initiation and unstable propagation. When applied to an adhesive interface, it should account for both interfacial bond strength and inherent defects at or near the interface, and should therefore be more appropriate for characterization of interface fracture resistance. This study introduced a fracture toughness test for the assessment of dentin/resin-composite bonded interfaces. The miniature short-rod specimen geometry was used for fracture toughness testing. Each specimen contained a tooth slice, sectioned from a bovine incisor, to form the bonded interface. The fracture toughness of an enamel-bonded interface was assessed in addition to the dentin-bonded interfaces. Tensile bond strength specimens were also prepared from the dentin surfaces of the cut bovine incisors. A minimum of ten specimens was fabricated for each group of materials tested. After the specimens were aged for 24 h in distilled water at 37 degrees C, the specimens were loaded to failure in an Instron universal testing machine. There were significant differences (p < 0.05) between the dental adhesives tested. Generally, both the fracture toughness and tensile bond strength measurements were highest for AllBond 2, intermediate for 3M MultiPurpose, and lowest for Scotchbond 2. Scanning electron microscopy of the fractured specimen halves confirmed that crack propagation occurred along the bond interface during the fracture toughness test. It was therefore concluded that the mini-short-rod fracture toughness test provided a valid method for characterization of the fracture resistance of the dentin-resin composite interface.

Incorporating ligament laxity in a finite element model for the upper cervical spine.

PubMed

Lasswell, Timothy L; Cronin, Duane S; Medley, John B; Rasoulinejad, Parham

2017-11-01

Predicting physiological range of motion (ROM) using a finite element (FE) model of the upper cervical spine requires the incorporation of ligament laxity. The effect of ligament laxity can be observed only on a macro level of joint motion and is lost once ligaments have been dissected and preconditioned for experimental testing. As a result, although ligament laxity values are recognized to exist, specific values are not directly available in the literature for use in FE models. The purpose of the current study is to propose an optimization process that can be used to determine a set of ligament laxity values for upper cervical spine FE models. Furthermore, an FE model that includes ligament laxity is applied, and the resulting ROM values are compared with experimental data for physiological ROM, as well as experimental data for the increase in ROM when a Type II odontoid fracture is introduced. The upper cervical spine FE model was adapted from a 50th percentile male full-body model developed with the Global Human Body Models Consortium (GHBMC). FE modeling was performed in LS-DYNA and LS-OPT (Livermore Software Technology Group) was used for ligament laxity optimization. Ordinate-based curve matching was used to minimize the mean squared error (MSE) between computed load-rotation curves and experimental load-rotation curves under flexion, extension, and axial rotation with pure moment loads from 0 to 3.5 Nm. Lateral bending was excluded from the optimization because the upper cervical spine was considered to be primarily responsible for flexion, extension, and axial rotation. Based on recommendations from the literature, four varying inputs representing laxity in select ligaments were optimized to minimize the MSE. Funding was provided by the Natural Sciences and Engineering Research Council of Canada as well as GHMBC. The present study was funded by the Natural Sciences and Engineering Research Council of Canada to support the work of one graduate student. There are no conflicts of interest to be reported. The MSE was reduced to 0.28 in the FE model with optimized ligament laxity compared with an MSE 0f 4.16 in the FE model without laxity. In all load cases, incorporating ligament laxity improved the agreement between the ROM of the FE model and the ROM of the experimental data. The ROM for axial rotation and extension was within one standard deviation of the experimental data. The ROM for flexion and lateral bending was outside one standard deviation of the experimental data, but a compromise was required to use one set of ligament laxity values to achieve a best fit to all load cases. Atlanto-occipital motion was compared as a ratio to overall ROM, and only in extension did the inclusion of ligament laxity not improve the agreement. After a Type II odontoid fracture was incorporated into the model, the increase in ROM was consistent with experimental data from the literature. The optimization approach used in this study provided values for ligament laxities that, when incorporated into the FE model, generally improved the ROM response when compared with experimental data. Successfully modeling a Type II odontoid fracture showcased the robustness of the FE model, which can now be used in future biomechanics studies. Copyright © 2017 Elsevier Inc. All rights reserved.

Rock fracture processes in chemically reactive environments

NASA Astrophysics Data System (ADS)

Eichhubl, P.

2015-12-01

Rock fracture is traditionally viewed as a brittle process involving damage nucleation and growth in a zone ahead of a larger fracture, resulting in fracture propagation once a threshold loading stress is exceeded. It is now increasingly recognized that coupled chemical-mechanical processes influence fracture growth in wide range of subsurface conditions that include igneous, metamorphic, and geothermal systems, and diagenetically reactive sedimentary systems with possible applications to hydrocarbon extraction and CO2 sequestration. Fracture processes aided or driven by chemical change can affect the onset of fracture, fracture shape and branching characteristics, and fracture network geometry, thus influencing mechanical strength and flow properties of rock systems. We are investigating two fundamental modes of chemical-mechanical interactions associated with fracture growth: 1. Fracture propagation may be aided by chemical dissolution or hydration reactions at the fracture tip allowing fracture propagation under subcritical stress loading conditions. We are evaluating effects of environmental conditions on critical (fracture toughness KIc) and subcritical (subcritical index) fracture properties using double torsion fracture mechanics tests on shale and sandstone. Depending on rock composition, the presence of reactive aqueous fluids can increase or decrease KIc and/or subcritical index. 2. Fracture may be concurrent with distributed dissolution-precipitation reactions in the hostrock beyond the immediate vicinity of the fracture tip. Reconstructing the fracture opening history recorded in crack-seal fracture cement of deeply buried sandstone we find that fracture length growth and fracture opening can be decoupled, with a phase of initial length growth followed by a phase of dominant fracture opening. This suggests that mechanical crack-tip failure processes, possibly aided by chemical crack-tip weakening, and distributed solution-precipitation creep in the hostrock can independently affect fracture opening displacement and thus fracture aperture profiles and aperture distribution.

NASA-Langley Research Center's participation in a round-robin comparison between some current crack-propagation prediction methods

NASA Technical Reports Server (NTRS)

Hudson, C. M.; Lewis, P. E.

1979-01-01

A round-robin study was conducted which evaluated and compared different methods currently in practice for predicting crack growth in surface-cracked specimens. This report describes the prediction methods used by the Fracture Mechanics Engineering Section, at NASA-Langley Research Center, and presents a comparison between predicted crack growth and crack growth observed in laboratory experiments. For tests at higher stress levels, the correlation between predicted and experimentally determined crack growth was generally quite good. For tests at lower stress levels, the predicted number of cycles to reach a given crack length was consistently higher than the experimentally determined number of cycles. This consistent overestimation of the number of cycles could have resulted from a lack of definition of crack-growth data at low values of the stress intensity range. Generally, the predicted critical flaw sizes were smaller than the experimentally determined critical flaw sizes. This underestimation probably resulted from using plane-strain fracture toughness values to predict failure rather than the more appropriate values based on maximum load.

Three-dimensional fracture instability of a displacement-weakening planar interface under locally peaked nonuniform loading

NASA Astrophysics Data System (ADS)

Uenishi, Koji

2018-06-01

We consider stability of fracture on a three-dimensional planar interface subjected to a loading stress that is locally peaked spatially, the level of which increases quasi-statically in time. Similar to the earlier study on the two-dimensional case (Uenishi and Rice, 2003; Rice and Uenishi, 2010), as the loading stress increases, a crack, or a region of displacement discontinuity (opening gap in tension or slip for shear fracture), develops on the interface where the stress is presumed to decrease according to a displacement-weakening constitutive relation. Upon reaching the instability point at which no further quasi-static solution for the extension of the crack on the interface exists, dynamic fracture follows. For the investigation of this instability point, we employ a dimensional analysis as well as an energy approach that gives a Rayleigh-Ritz approximation for the dependence of crack size and maximum displacement discontinuity on the level and quadratic shape of the loading stress distribution. We show that, if the linear displacement-weakening law is applied and the crack may be assumed of an elliptical form, the critical crack size at instability is independent of the curvature of the loading stress distribution and it is of the same order for all two- and three-dimensional cases.

Characterizing trabecular bone structure for assessing vertebral fracture risk on volumetric quantitative computed tomography

NASA Astrophysics Data System (ADS)

Nagarajan, Mahesh B.; Checefsky, Walter A.; Abidin, Anas Z.; Tsai, Halley; Wang, Xixi; Hobbs, Susan K.; Bauer, Jan S.; Baum, Thomas; Wismüller, Axel

2015-03-01

While the proximal femur is preferred for measuring bone mineral density (BMD) in fracture risk estimation, the introduction of volumetric quantitative computed tomography has revealed stronger associations between BMD and spinal fracture status. In this study, we propose to capture properties of trabecular bone structure in spinal vertebrae with advanced second-order statistical features for purposes of fracture risk assessment. For this purpose, axial multi-detector CT (MDCT) images were acquired from 28 spinal vertebrae specimens using a whole-body 256-row CT scanner with a dedicated calibration phantom. A semi-automated method was used to annotate the trabecular compartment in the central vertebral slice with a circular region of interest (ROI) to exclude cortical bone; pixels within were converted to values indicative of BMD. Six second-order statistical features derived from gray-level co-occurrence matrices (GLCM) and the mean BMD within the ROI were then extracted and used in conjunction with a generalized radial basis functions (GRBF) neural network to predict the failure load of the specimens; true failure load was measured through biomechanical testing. Prediction performance was evaluated with a root-mean-square error (RMSE) metric. The best prediction performance was observed with GLCM feature `correlation' (RMSE = 1.02 ± 0.18), which significantly outperformed all other GLCM features (p < 0.01). GLCM feature correlation also significantly outperformed MDCTmeasured mean BMD (RMSE = 1.11 ± 0.17) (p< 10-4). These results suggest that biomechanical strength prediction in spinal vertebrae can be significantly improved through characterization of trabecular bone structure with GLCM-derived texture features.

Comparison of the quasi-static method and the dynamic method for simulating fracture processes in concrete

NASA Astrophysics Data System (ADS)

Liu, J. X.; Deng, S. C.; Liang, N. G.

2008-02-01

Concrete is heterogeneous and usually described as a three-phase material, where matrix, aggregate and interface are distinguished. To take this heterogeneity into consideration, the Generalized Beam (GB) lattice model is adopted. The GB lattice model is much more computationally efficient than the beam lattice model. Numerical procedures of both quasi-static method and dynamic method are developed to simulate fracture processes in uniaxial tensile tests conducted on a concrete panel. Cases of different loading rates are compared with the quasi-static case. It is found that the inertia effect due to load increasing becomes less important and can be ignored with the loading rate decreasing, but the inertia effect due to unstable crack propagation remains considerable no matter how low the loading rate is. Therefore, an unrealistic result will be obtained if a fracture process including unstable cracking is simulated by the quasi-static procedure.

Combined bending and thermal fatigue of high-temperature metal-matrix composites - Computational simulation

NASA Technical Reports Server (NTRS)

Gotsis, Pascal K.; Chamis, Christos C.

1992-01-01

The nonlinear behavior of a high-temperature metal-matrix composite (HT-MMC) was simulated by using the metal matrix composite analyzer (METCAN) computer code. The simulation started with the fabrication process, proceeded to thermomechanical cyclic loading, and ended with the application of a monotonic load. Classical laminate theory and composite micromechanics and macromechanics are used in METCAN, along with a multifactor interaction model for the constituents behavior. The simulation of the stress-strain behavior from the macromechanical and the micromechanical points of view, as well as the initiation and final failure of the constituents and the plies in the composite, were examined in detail. It was shown that, when the fibers and the matrix were perfectly bonded, the fracture started in the matrix and then propagated with increasing load to the fibers. After the fibers fractured, the composite lost its capacity to carry additional load and fractured.

Combined thermal and bending fatigue of high-temperature metal-matrix composites: Computational simulation

NASA Technical Reports Server (NTRS)

Gotsis, Pascal K.

1991-01-01

The nonlinear behavior of a high-temperature metal-matrix composite (HT-MMC) was simulated by using the metal matrix composite analyzer (METCAN) computer code. The simulation started with the fabrication process, proceeded to thermomechanical cyclic loading, and ended with the application of a monotonic load. Classical laminate theory and composite micromechanics and macromechanics are used in METCAN, along with a multifactor interaction model for the constituents behavior. The simulation of the stress-strain behavior from the macromechanical and the micromechanical points of view, as well as the initiation and final failure of the constituents and the plies in the composite, were examined in detail. It was shown that, when the fibers and the matrix were perfectly bonded, the fracture started in the matrix and then propagated with increasing load to the fibers. After the fibers fractured, the composite lost its capacity to carry additional load and fractured.

Analysis of mechanical strength to fixing the femoral neck fracture in synthetic bone type Asnis

PubMed Central

Freitas, Anderson; Lula, Welder Fernandes; de Oliveira, Jonathan Sampaio; Maciel, Rafael Almeida; Souto, Diogo Ranier de Macedo; Godinho, Patrick Fernandes

2014-01-01

OBJECTIVE: To analyze the results of biomechanical assays of fixation of Pauwels type III femoral neck fracture in synthetic bone, using 7.5mm cannulated screws in inverted triangle formation, in relation to the control group. METHODS: Ten synthetic bones were used, from a domestic brand, divided into two groups: test and control. In the test group, a 70° tilt osteotomy of the femoral neck was fixated using three cannulated screws in inverted triangle formation. The resistance of this fixation and its rotational deviation were analyzed at 5mm displacement (phase 1) and 10mm displacement (phase 2). The control group was tested in its integrity until the fracture of the femoral neck occurred. The Mann-Whitney test was used for group analysis and comparison. RESULTS: The values in the test group in phase 1, in samples 1-5, showed a mean of 579N and SD =77N. Rotational deviations showed a mean of 3.33°, SD = 2.63°. In phase 2, the mean was 696N and SD =106N. The values of the maximum load in the control group had a mean of 1329N and SD=177N. CONCLUSION: The analysis of mechanical strength between the groups determined a statistically significant lower value in the test group. Level of Evidence III, Control Case. PMID:25246851

Analysis of propagation mechanisms of stimulation-induced fractures in rocks

NASA Astrophysics Data System (ADS)

Krause, Michael; Renner, Joerg

2016-04-01

Effectivity of geothermal energy production depends crucially on the heat exchange between the penetrated hot rock and the circulating water. Hydraulic stimulation of rocks at depth intends to create a network of fractures that constitutes a large area for exchange. Two endmembers of stimulation products are typically considered, tensile hydro-fractures that propagate in direction of the largest principal stress and pre-existing faults that are sheared when fluid pressure reduces the effective normal stress acting on them. The understanding of the propagation mechanisms of fractures under in-situ conditions is still incomplete despite intensive research over the last decades. Wing-cracking has been suggested as a mechanism of fracture extension from pre-existent faults with finite length that are induced to shear. The initiation and extension of the wings is believed to be in tensile mode. Open questions concern the variability of the nominal material property controlling tensile fracture initiation and extension, the mode I facture toughness KIC, with in-situ conditions, e.g., its mean-stress dependence. We investigated the fracture-propagation mechanism in different rocks (sandstones and granites) under varying conditions mimicking those representative for geothermal systems. To determine KIC-values we performed 3-point bending experiments. We varied the confining pressure, the piston velocity, and the position of the chevron notch relative to the loading configuration. Additional triaxial experiments at a range of confining pressures were performed to study wing crack propagation from artificial flaws whose geometrical characteristics, i.e., length, width, and orientation relative to the axial load are varied. We monitored acoustic emissions to constrain the spacio-temporal evolution of the fracturing. We found a significant effect of the length of the artificial flaw and the confining pressure on wing-crack initiation but did not observe a systematic dependence of wing-crack initiation on the orientation of the initial flaw in the range of tested angles. In fact, wings do not develop for artificial flaws shorter than 3 mm. The force required to initiate wing cracking increases with increasing confining pressure as does the apparent fracture toughness. So called ``anti-wing cracks'' were observed too, probably an artifact of the geometrical constraints imposed on the sample in a conventional triaxial compression test.

Effect of Reinforcement Architecture on Fracture of Selectively Reinforced Metallic Compact Tension Specimens

NASA Technical Reports Server (NTRS)

Abada, Christopher H.; Farley, Gary L.; Hyer, Michael W.

2006-01-01

A computer-based parametric study of the effect of reinforcement architectures on fracture response of aluminum compact-tension (CT) specimens is performed. Eleven different reinforcement architectures consisting of rectangular and triangular cross-section reinforcements were evaluated. Reinforced specimens produced between 13 and 28 percent higher fracture load than achieved with the non-reinforced case. Reinforcements with blunt leading edges (rectangular reinforcements) exhibited superior performance relative to the triangular reinforcements with sharp leading edges. Relative to the rectangular reinforcements, the most important architectural feature was reinforcement thickness. At failure, the reinforcements carried between 58 and 85 percent of the load applied to the specimen, suggesting that there is considerable load transfer between the base material and the reinforcement.

Load-Unload Response Ratio (LURR), Accelerating Moment/Energy Release (AM/ER) and State Vector Saltation as Precursors to Failure of Rock Specimens

NASA Astrophysics Data System (ADS)

Yin, Xiang-Chu; Yu, Huai-Zhong; Kukshenko, Victor; Xu, Zhao-Yong; Wu, Zhishen; Li, Min; Peng, Keyin; Elizarov, Surgey; Li, Qi

2004-12-01

In order to verify some precursors such as LURR (Load/Unload Response Ratio) and AER (Accelerating Energy Release) before large earthquakes or macro-fracture in heterogeneous brittle media, four acoustic emission experiments involving large rock specimens under tri-axial stress, have been conducted. The specimens were loaded in two ways: monotonous or cycling. The experimental results confirm that LURR and AER are precursors of macro-fracture in brittle media. A new measure called the state vector has been proposed to describe the damage evolution of loaded rock specimens.

Fracture Resistance and Mode of Failure of Ceramic versus Titanium Implant Abutments and Single Implant-Supported Restorations.

PubMed

Sghaireen, Mohd G

2015-06-01

The material of choice for implant-supported restorations is affected by esthetic requirements and type of abutment. This study compares the fracture resistance of different types of implant abutments and implant-supported restorations and their mode of failure. Forty-five Oraltronics Pitt-Easy implants (Oraltronics Dental Implant Technology GmbH, Bremen, Germany) (4 mm diameter, 10 mm length) were embedded in clear autopolymerizing acrylic resin. The implants were randomly divided into three groups, A, B and C, of 15 implants each. In group A, titanium abutments and metal-ceramic crowns were used. In group B, zirconia ceramic abutments and In-Ceram Alumina crowns were used. In group C, zirconia ceramic abutments and IPS Empress Esthetic crowns were used. Specimens were tested to failure by applying load at 130° from horizontal plane using an Instron Universal Testing Machine. Subsequently, the mode of failure of each specimen was identified. Fracture resistance was significantly different between groups (p < .05). The highest fracture loads were associated with metal-ceramic crowns supported by titanium abutments (p = .000). IPS Empress crowns supported by zirconia abutments had the lowest fracture loads (p = .000). Fracture modes of metal-ceramic crowns supported by titanium abutments included screw fracture and screw bending. Fracture of both crown and abutment was the dominant mode of failure of In-Ceram/IPS Empress crowns supported by zirconia abutments. Metal-ceramic crowns supported by titanium abutments were more resistant to fracture than In-Ceram crowns supported by zirconia abutments, which in turn were more resistant to fracture than IPS Empress crowns supported by zirconia abutments. In addition, failure modes of restorations supported by zirconia abutments were more catastrophic than those for restorations supported by titanium abutments. © 2013 Wiley Periodicals, Inc.

Dynamic Torsional and Cyclic Fracture Behavior of ProFile Rotary Instruments at Continuous or Reciprocating Rotation as Visualized with High-speed Digital Video Imaging.

PubMed

Tokita, Daisuke; Ebihara, Arata; Miyara, Kana; Okiji, Takashi

2017-08-01

This study examined the dynamic fracture behavior of nickel-titanium rotary instruments in torsional or cyclic loading at continuous or reciprocating rotation by means of high-speed digital video imaging. The ProFile instruments (size 30, 0.06 taper; Dentsply Maillefer, Ballaigues, Switzerland) were categorized into 4 groups (n = 7 in each group) as follows: torsional/continuous (TC), torsional/reciprocating (TR), cyclic/continuous (CC), and cyclic/reciprocating (CR). Torsional loading was performed by rotating the instruments by holding the tip with a vise. For cyclic loading, a custom-made device with a 38° curvature was used. Dynamic fracture behavior was observed with a high-speed camera. The time to fracture was recorded, and the fractured surface was examined with scanning electron microscopy. The TC group initially exhibited necking of the file followed by the development of an initial crack line. The TR group demonstrated opening and closing of a crack according to its rotation in the cutting and noncutting directions, respectively. The CC group separated without any detectable signs of deformation. In the CR group, initial crack formation was recognized in 5 of 7 samples. The reciprocating rotation exhibited a longer time to fracture in both torsional and cyclic fatigue testing (P < .05). The scanning electron microscopic images showed a severely deformed surface in the TR group. The dynamic fracture behavior of NiTi rotary instruments, as visualized with high-speed digital video imaging, varied between the different modes of rotation and different fatigue testing. Reciprocating rotation induced a slower crack propagation and conferred higher fatigue resistance than continuous rotation in both torsional and cyclic loads. Copyright © 2017 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

Fracture behavior of single-structure fiber-reinforced composite restorations

PubMed Central

Nagata, Kohji; Garoushi, Sufyan K.; Vallittu, Pekka K.; Wakabayashi, Noriyuki; Takahashi, Hidekazu; Lassila, Lippo V. J.

2016-01-01

Abstract Objective: The applications of single-structure fiber-reinforced composite (FRC) in restorative dentistry have not been well reported. This study aimed to clarify the static mechanical properties of anterior crown restorations prepared using two types of single-structure FRC. Materials and methods : An experimental crown restoration was designed for an upper anterior incisor. The restorations were made from IPS Empress CAD for CEREC (Emp), IPS e.max® CAD (eMx), experimental single-structure all-FRC (a-FRC), Filtek™ Supreme XTE (XTE), and commercially available single-structure short-FRC (everX Posterior™) (n = 8 for each material) (s-FRC). The a-FRC restorations were prepared from an experimental FRC blank using a computer-aided design and manufacturing (CAD/CAM) device. A fracture test was performed to assess the fracture load, toughness, and failure mode. The fracture loads were vertically applied on the restorations. The surface micromorphology of the FRC restorations was observed by scanning electron microscopy (SEM). The data were analyzed by analysis of variance (p = .05) followed by Tukey's test. Results : s-FRC showed the highest mean fracture load (1145.0 ± 89.6 N) and toughness (26.2 ± 5.8 Ncm) among all the groups tested. With regard to the micromorphology of the prosthetic surface, local crushing of the fiberglass was observed in s-FRC, whereas chopped fiberglass was observed in a-FRC. Conclusions : The restorations made of short-FRC showed a higher load-bearing capacity than those made of the experimental all-FRC blanks for CAD/CAM. The brittle-like fractures were exhibited in the recent dental esthetic materials, while local crushing fractures were shown for single-structure FRC restorations. PMID:28642921

Newly designed anterolateral and posterolateral locking anatomic plates for lateral tibial plateau fractures: a finite element study.

PubMed

Chen, Pengbo; Lu, Hua; Shen, Hao; Wang, Wei; Ni, Binbin; Chen, Jishizhan

2017-02-23

Lateral column tibial plateau fracture fixation with a locking screw plate has higher mechanical stability than other fixation methods. The objectives of the present study were to introduce two newly designed locking anatomic plates for lateral tibial plateau fracture and to demonstrate their characteristics of the fixation complexes under the axial loads. Three different 3D finite element models of the lateral tibial plateau fracture with the bone plates were created. Various axial forces (100, 500, 1000, and 1500 N) were applied to simulate the axial compressive load on an adult knee during daily life. The equivalent maps of displacement and stress were output, and relative displacement was calculated along the fracture lines. The displacement and stresses in the fixation complexes increased with the axial force. The equivalent displacement or stress map of each fixation under different axial forces showed similar distributing characteristics. The motion characteristics of the three models differed, and the max-shear stress of trabecula increased with the axial load. These two novel plates could fix lateral tibial plateau fractures involving anterolateral and posterolateral fragments. Motions after open reduction and stable internal fixation should be advised to decrease the risk of trabecular microfracture. The relative displacement of the posterolateral fragments is different when using anterolateral plate and posterolateral plate, which should be considered in choosing the implants for different posterolateral plateau fractures.

Biomechanical Tolerance of Calcaneal Fractures

PubMed Central

Yoganandan, Narayan; Pintar, Frank A.; Gennarelli, Thomas A.; Seipel, Robert; Marks, Richard

1999-01-01

Biomechanical studies have been conducted in the past to understand the mechanisms of injury to the foot-ankle complex. However, statistically based tolerance criteria for calcaneal complex injuries are lacking. Consequently, this research was designed to derive a probability distribution that represents human calcaneal tolerance under impact loading such as those encountered in vehicular collisions. Information for deriving the distribution was obtained by experiments on unembalmed human cadaver lower extremities. Briefly, the protocol included the following. The knee joint was disarticulated such that the entire lower extremity distal to the knee joint remained intact. The proximal tibia was fixed in polymethylmethacrylate. The specimens were aligned and impact loading was applied using mini-sled pendulum equipment. The pendulum impactor dynamically loaded the plantar aspect of the foot once. Following the test, specimens were palpated and radiographs in multiple planes were obtained. Injuries were classified into no fracture, and extra-and intra-articular fractures of the calcaneus. There were 14 cases of no injury and 12 cases of calcaneal fracture. The fracture forces (mean: 7802 N) were significantly different (p<0.01) from the forces in the no injury (mean: 4144 N) group. The probability of calcaneal fracture determined using logistic regression indicated that a force of 6.2 kN corresponds to 50 percent probability of calcaneal fracture. The derived probability distribution is useful in the design of dummies and vehicular surfaces.

Biomechanical Assessment of Locked Plating for the Fixation of Patella Fractures.

PubMed

Wurm, Simone; Augat, Peter; Bühren, Volker

2015-09-01

To analyze the mechanical stability of locked plating in comparison with tension-band wiring for the fixation of fractures of the patella. Biomechanical tests were performed on artificial foam patella specimens comparing an angular stable plate and monocortical screws with tension-band wiring. Tests were performed under combined tension and bending until failure simulating physiological loading of the tibia during walking. Tension-band wiring failed at 66% of the failure load of plating (1052 N, P = 0.002) and had 5 times larger fracture gap displacements (P = 0.002). Based on the biomechanical advantages, locked plating of the patella may constitute a reasonable alternative in the treatment of patella fractures.

Computational Simulation of Composite Structural Fatigue

NASA Technical Reports Server (NTRS)

Minnetyan, Levon; Chamis, Christos C. (Technical Monitor)

2005-01-01

Progressive damage and fracture of composite structures subjected to monotonically increasing static, tension-tension cyclic, pressurization, and flexural cyclic loading are evaluated via computational simulation. Constituent material properties, stress and strain limits are scaled up to the structure level to evaluate the overall damage and fracture propagation for composites. Damage initiation, growth, accumulation, and propagation to fracture due to monotonically increasing static and cyclic loads are included in the simulations. Results show the number of cycles to failure at different temperatures and the damage progression sequence during different degradation stages. A procedure is outlined for use of computational simulation data in the assessment of damage tolerance, determination of sensitive parameters affecting fracture, and interpretation of results with insight for design decisions.

Computational Simulation of Composite Structural Fatigue

NASA Technical Reports Server (NTRS)

Minnetyan, Levon

2004-01-01

Progressive damage and fracture of composite structures subjected to monotonically increasing static, tension-tension cyclic, pressurization, and flexural cyclic loading are evaluated via computational simulation. Constituent material properties, stress and strain limits are scaled up to the structure level to evaluate the overall damage and fracture propagation for composites. Damage initiation, growth, accumulation, and propagation to fracture due to monotonically increasing static and cyclic loads are included in the simulations. Results show the number of cycles to failure at different temperatures and the damage progression sequence during different degradation stages. A procedure is outlined for use of computational simulation data in the assessment of damage tolerance, determination of sensitive parameters affecting fracture, and interpretation of results with insight for design decisions.

An exploratory investigation of cumulative shock fatigue.

NASA Technical Reports Server (NTRS)

Simonson, D.; Byrne, J. G.

1972-01-01

A simple device for producing cumulative shock loading in solids is described. The device uses a ballistic-impact-driven projectile to introduce high-stress waves into a solid. The impact time and load amplitude can be varied to produce fracture in one or several impacts in PMMA rods. The wavefront approached a square wave shape. Materials other than PMMA were loaded to failure to demonstrate the versatility of the device. Fracture morphologies observed with optical and scanning-electron microscopy are described.

Indenter flaw geometry and fracture toughness estimates for a glass-ceramic

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shetty, D.K.; Duckworth, W.H.; Rosenfield, A.R.

1985-10-01

Shapes of cracks associated with Vickers indenter flaws in a glass-ceramic were assessed by stepwise polishing and measuring surface traces as a function of depth. The cracks were of the Palmqvist type even at 200-N indentation load. The load dependence of crack lengths and fracture toughness estimates were examined in terms of relations proposed for Palmqvist and half-penny cracks. Estimates based on the half-penny crack analogy were in closer agreement with bulk fracture toughness measurements despite the Palmqvist nature of the cracks.

The Development of a new Numerical Modelling Approach for Naturally Fractured Rock Masses

NASA Astrophysics Data System (ADS)

Pine, R. J.; Coggan, J. S.; Flynn, Z. N.; Elmo, D.

2006-11-01

An approach for modelling fractured rock masses has been developed which has two main objectives: to maximise the quality of representation of the geometry of existing rock jointing and to use this within a loading model which takes full account of this style of jointing. Initially the work has been applied to the modelling of mine pillars and data from the Middleton Mine in the UK has been used as a case example. However, the general approach is applicable to all aspects of rock mass behaviour including the stress conditions found in hangingwalls, tunnels, block caving, and slopes. The rock mass fracture representation was based on a combination of explicit mapping of rock faces and the synthesis of this data into a three-dimensional model, based on the use of the FracMan computer model suite. Two-dimensional cross sections from this model were imported into the finite element computer model, ELFEN, for loading simulation. The ELFEN constitutive model for fracture simulation includes the Rotating Crack, and Rankine material models, in which fracturing is controlled by tensile strength and fracture energy parameters. For tension/compression stress states, the model is complemented with a capped Mohr-Coulomb criterion in which the softening response is coupled to the tensile model. Fracturing due to dilation is accommodated by introducing an explicit coupling between the inelastic strain accrued by the Mohr-Coulomb yield surface and the anisotropic degradation of the mutually orthogonal tensile yield surfaces of the rotating crack model. Pillars have been simulated with widths of 2.8, 7 and 14 m and a height of 7 m (the Middleton Mine pillars are typically 14 m wide and 7 m high). The evolution of the pillar failure under progressive loading through fracture extension and creation of new fractures is presented, and pillar capacities and stiffnesses are compared with empirical models. The agreement between the models is promising and the new model provides useful insights into the influence of pre-existing fractures. Further work is needed to consider the effects of three-dimensional loading and other boundary condition problems.

Biomechanical comparison of straight and helical compression plates for fixation of transverse and oblique bone fractures: Modeling and experiments.

PubMed

Sezek, Sinan; Aksakal, Bunyamin; Gürger, Murat; Malkoc, Melih; Say, Y

2016-08-12

Total deformation and stability of straight and helical compression plates were studied by means of the finite element method (FEM) and in vitro biomechanical experiments. Fixations of transverse (TF) and oblique (45°) bone (OF) fractures have been analyzed on sheep tibias by designing the straight compression (SP) and Helical Compression Plate (HP) models. The effects of axial compression, bending and torsion loads on both plating systems were analyzed in terms of total displacements. Numerical models and experimental models suggested that under compression loadings, bone fracture gap closures for both fracture types were found to be in the favor of helical plate designs. The helical plate (HP) fixations provided maximum torsional resistance compared to the (SP) fixations. The fracture gap closure and stability of helical plate fixation for transverse fractures was determined to be higher than that found for the oblique fractures. The comparison of average compression stress, bending and torsion moments showed that the FEM and experimental results are in good agreement and such designs are likely to have a positive impact in future bone fracture fixation designs.

Thoracolumbar spine loading associated with kinematics of the young and the elderly during activities of daily living.

PubMed

Ignasiak, Dominika; Rüeger, Andrea; Sperr, Ramona; Ferguson, Stephen J

2018-03-21

Excessive mechanical loading of the spine is a critical factor in vertebral fracture initiation. Most vertebral fractures develop spontaneously or due to mild trauma, as physiological loads during activities of daily living might exceed the failure load of osteoporotic vertebra. Spinal loading patterns are affected by vertebral kinematics, which differ between elderly and young individuals. In this study, the effects of age-related changes in spine kinematics on thoracolumbar spinal segmental loading during dynamic activities of daily living were investigated using combined experimental and modeling approach. Forty-four healthy volunteers were recruited into two age groups: young (N = 23, age = 27.1 ± 3.8) and elderly (N = 21, age = 70.1 ± 3.9). The spinal curvature was assessed with a skin-surface device and the kinematics of the spine and lower extremities were recorded during daily living tasks (flexion-extension and stand-sit-stand) with a motion capture system. The obtained data were used as input for a musculoskeletal model with a detailed thoracolumbar spine representation. To isolate the effect of kinematics on predicted loads, other model properties were kept constant. Inverse dynamics simulations were performed in the AnyBody Modeling System to estimate corresponding spinal loads. The maximum compressive loads predicted for the elderly motion patterns were lower than those of the young for L2/L3 and L3/L4 lumbar levels during flexion and for upper thoracic levels during stand-to-sit (T1/T2-T8/T9) and sit-to-stand (T3/T4-T6/T7). However, the maximum loads predicted for the lower thoracic levels (T9/T10-L1/L2), a common site of vertebral fractures, were similar compared to the young. Nevertheless, these loads acting on the vertebrae of reduced bone quality might contribute to a higher fracture risk for the elderly. Copyright © 2017 Elsevier Ltd. All rights reserved.

The load separation technique in the elastic-plastic fracture analysis of two- and three-dimensional geometries

NASA Technical Reports Server (NTRS)

Sharobeam, Monir H.

1994-01-01

Load separation is the representation of the load in the test records of geometries containing cracks as a multiplication of two separate functions: a crack geometry function and a material deformation function. Load separation is demonstrated in the test records of several two-dimensional geometries such as compact tension geometry, single edge notched bend geometry, and center cracked tension geometry and three-dimensional geometries such as semi-elliptical surface crack. The role of load separation in the evaluation of the fracture parameter J-integral and the associated factor eta for two-dimensional geometries is discussed. The paper also discusses the theoretical basis and the procedure for using load separation as a simplified yet accurate approach for plastic J evaluation in semi-elliptical surface crack which is a three-dimensional geometry. The experimental evaluation of J, and particularly J(sub pl), for three-dimensional geometries is very challenging. A few approaches have been developed in this regard and they are either complex or very approximate. The paper also presents the load separation as a mean to identify the blunting and crack growth regions in the experimental test records of precracked specimens. Finally, load separation as a methodology in elastic-plastic fracture mechanics is presented.

Evolution of Friction and Permeability in a Propped Fracture under Shear

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Fengshou; Fang, Yi; Elsworth, Derek

We explore the evolution of friction and permeability of a propped fracture under shear. We examine the effects of normal stress, proppant thickness, proppant size, and fracture wall texture on the frictional and transport response of proppant packs confined between planar fracture surfaces. The proppant-absent and proppant-filled fractures show different frictional strength. For fractures with proppants, the frictional response is mainly controlled by the normal stress and proppant thickness. The depth of shearing-concurrent striations on fracture surfaces suggests that the magnitude of proppant embedment is controlled by the applied normal stress. Under high normal stress, the reduced friction implies thatmore » shear slip is more likely to occur on propped fractures in deeper reservoirs. The increase in the number of proppant layers, from monolayer to triple layers, significantly increases the friction of the propped fracture due to the interlocking of the particles and jamming. Permeability of the propped fracture is mainly controlled by the magnitude of the normal stress, the proppant thickness, and the proppant grain size. Permeability of the propped fracture decreases during shearing due to proppant particle crushing and related clogging. Proppants are prone to crushing if the shear loading evolves concurrently with the normal loading.« less

Evolution of Friction and Permeability in a Propped Fracture under Shear

DOE PAGES

Zhang, Fengshou; Fang, Yi; Elsworth, Derek; ...

2017-12-04

We explore the evolution of friction and permeability of a propped fracture under shear. We examine the effects of normal stress, proppant thickness, proppant size, and fracture wall texture on the frictional and transport response of proppant packs confined between planar fracture surfaces. The proppant-absent and proppant-filled fractures show different frictional strength. For fractures with proppants, the frictional response is mainly controlled by the normal stress and proppant thickness. The depth of shearing-concurrent striations on fracture surfaces suggests that the magnitude of proppant embedment is controlled by the applied normal stress. Under high normal stress, the reduced friction implies thatmore » shear slip is more likely to occur on propped fractures in deeper reservoirs. The increase in the number of proppant layers, from monolayer to triple layers, significantly increases the friction of the propped fracture due to the interlocking of the particles and jamming. Permeability of the propped fracture is mainly controlled by the magnitude of the normal stress, the proppant thickness, and the proppant grain size. Permeability of the propped fracture decreases during shearing due to proppant particle crushing and related clogging. Proppants are prone to crushing if the shear loading evolves concurrently with the normal loading.« less

Innovations in the management of hip fractures.

PubMed

Teasdall, Robert D; Webb, Lawrence X

2003-08-01

Hip fractures include fractures of the head, neck, intertrochanteric, and subtrochanteric regions. Head fractures commonly accompany dislocations. Neck fractures and intertrochanteric fractures occur with greatest frequency in elderly patients with a low bone mineral density and are produced by low-energy mechanisms. Subtrochanteric fractures occur in a predominantly strong cortical osseous region that is exposed to large compressive stresses. Implants used to address these fractures must accommodate significant loads while the fractures consolidate. Complications secondary to hip fractures produce significant morbidity and include infection, nonunion, malunion, decubitus ulcers, fat emboli, deep venous thrombosis, pulmonary embolus, pneumonia, myocardial infarction, stroke, and death.

Probabilistic Risk Assessment for Astronaut Post Flight Bone Fracture

NASA Technical Reports Server (NTRS)

Lewandowski, Beth; Myers, Jerry; Licata, Angelo

2015-01-01

Introduction: Space flight potentially reduces the loading that bone can resist before fracture. This reduction in bone integrity may result from a combination of factors, the most common reported as reduction in astronaut BMD. Although evaluating the condition of bones continues to be a critical aspect of understanding space flight fracture risk, defining the loading regime, whether on earth, in microgravity, or in reduced gravity on a planetary surface, remains a significant component of estimating the fracture risks to astronauts. This presentation summarizes the concepts, development, and application of NASA's Bone Fracture Risk Module (BFxRM) to understanding pre-, post, and in mission astronaut bone fracture risk. The overview includes an assessment of contributing factors utilized in the BFxRM and illustrates how new information, such as biomechanics of space suit design or better understanding of post flight activities may influence astronaut fracture risk. Opportunities for the bone mineral research community to contribute to future model development are also discussed. Methods: To investigate the conditions in which spaceflight induced changes to bone plays a critical role in post-flight fracture probability, we implement a modified version of the NASA Bone Fracture Risk Model (BFxRM). Modifications included incorporation of variations in physiological characteristics, post-flight recovery rate, and variations in lateral fall conditions within the probabilistic simulation parameter space. The modeled fracture probability estimates for different loading scenarios at preflight and at 0 and 365 days post-flight time periods are compared. Results: For simple lateral side falls, mean post-flight fracture probability is elevated over mean preflight fracture probability due to spaceflight induced BMD loss and is not fully recovered at 365 days post-flight. In the case of more energetic falls, such as from elevated heights or with the addition of lateral movement, the contribution of space flight quality changes is much less clear, indicating more granular assessments, such as Finite Element modeling, may be needed to further assess the risks in these scenarios.

Evaluation of load at fracture of Procera AllCeram copings using different luting cements.

PubMed

Al-Makramani, Bandar M A; Razak, A A A; Abu-Hassan, M I

2008-02-01

The current study investigated the effect of different luting agents on the fracture resistance of Procera AllCeram copings. Six master dies were duplicated from the prepared maxillary first premolar tooth using nonprecious metal alloy (Wiron 99). Thirty copings (Procera AllCeram) of 0.6-mm thickness were manufactured. Three types of luting media were used: zinc phosphate cement (Elite), glass ionomer cement (Fuji I), and dual-cured composite resin cement (Panavia F). Ten copings were cemented with each type. Two master dies were used for each group, and each of them was used to lute five copings. All groups were cemented according to manufacturer's instructions and received a static load of 5 kg during cementation. After 24 hours of distilled water storage at 37 degrees C, the copings were vertically compressed using a universal testing machine at a crosshead speed of 1 mm/min. ANOVA revealed significant differences in the load at fracture among the three groups (p < 0.001). The fracture strength results showed that the mean fracture strength of zinc phosphate cement (Elite), glass ionomer cement (Fuji I), and resin luting cement (Panavia F) were 1091.9 N, 784.8 N, and 1953.5 N, respectively. Different luting agents have an influence on the fracture resistance of Procera AllCeram copings.

Time-dependent stress concentration and microcrack nucleation in TiAl

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoo, M.H.

1995-07-01

Localized stress evolution associated with the interaction of slip or twinning with an interface is treated by means of a superposition of the {open_quotes}internal loading{close_quotes} of a crystalline subsystem by dynamic dislocation pile-up and the stress relaxation by climb of interfacial dislocations. The peak value of a stress concentration factor depends on both the angular function that includes the effect of mode mixity and the ratio of characteristic times for stress relaxation and internal loading. The available experimental data on orientation and strain-rate dependences of interfacial fracture mode in polysynthetically twinned TiAl crystals are discussed in view of the theoreticalmore » concepts presented in this paper.« less

Microscopic Characterization of Tensile and Shear Fracturing in Progressive Failure in Marble

NASA Astrophysics Data System (ADS)

Cheng, Yi; Wong, Louis Ngai Yuen

2018-01-01

Compression-induced tensile and shear fractures were reported to be the two fundamental fracture types in rock fracturing tests. This study investigates such tensile and shear fracturing process in marble specimens containing two different flaw configurations. Observations first reveal that the development of a tensile fracture is distinct from shear fracture with respect to their nucleation, propagation, and eventual formation in macroscale. Second, transgranular cracks and grain-scale spallings become increasingly abundant in shear fractures as loading increases, which is almost not observed in tensile fractures. Third, one or some dominant extensional microcracks are commonly observed in the center of tensile fractures, while such development of microcracks is almost absent in shear fractures. Microcracks are generally of a length comparable to grain size and distribute uniformly within the damage zone of the shear fracture. Fourth, the width of densely damaged zone in the shear fracture is nearly 10 times of that in the tensile fracture. Quantitative measurement on microcrack density suggests that (1) microcrack density in tensile and shear fractures display distinct characteristics with increasing loading, (2) transgranular crack density in the shear fracture decreases logarithmically with the distance away from the shear fracture center, and (3) whatever the fracture type, the anisotropy can only be observed for transgranular cracks with a large density, which partially explains why microcrack anisotropy usually tends to be unobvious until approaching peak stress in specimens undergoing brittle failure. Microcracking characteristics observed in this work likely shed light to some phenomena and conclusions generalized in seismological studies.

Stress fractures: pathophysiology, clinical presentation, imaging features, and treatment options.

PubMed

Matcuk, George R; Mahanty, Scott R; Skalski, Matthew R; Patel, Dakshesh B; White, Eric A; Gottsegen, Christopher J

2016-08-01

Stress fracture, in its most inclusive description, includes both fatigue and insufficiency fracture. Fatigue fractures, sometimes equated with the term "stress fractures," are most common in runners and other athletes and typically occur in the lower extremities. These fractures are the result of abnormal, cyclical loading on normal bone leading to local cortical resorption and fracture. Insufficiency fractures are common in elderly populations, secondary to osteoporosis, and are typically located in and around the pelvis. They are a result of normal or traumatic loading on abnormal bone. Subchondral insufficiency fractures of the hip or knee may cause acute pain that may present in the emergency setting. Medial tibial stress syndrome is a type of stress injury of the tibia related to activity and is a clinical syndrome encompassing a range of injuries from stress edema to frank-displaced fracture. Atypical subtrochanteric femoral fracture associated with long-term bisphosphonate therapy is also a recently discovered entity that needs early recognition to prevent progression to a complete fracture. Imaging recommendations for evaluation of stress fractures include initial plain radiographs followed, if necessary, by magnetic resonance imaging (MRI), which is preferred over computed tomography (CT) and bone scintigraphy. Radiographs are the first-line modality and may reveal linear sclerosis and periosteal reaction prior to the development of a frank fracture. MRI is highly sensitive with findings ranging from periosteal edema to bone marrow and intracortical signal abnormality. Additionally, a brief description of relevant clinical management of stress fractures is included.

Microfracture and changes in energy absorption to fracture of young vertebral cancellous bone following physiological fatigue loading.

PubMed

Lu, W W; Luk, K D K; Cheung, K C M; Gui-Xing, Qiu; Shen, J X; Yuen, L; Ouyang, J; Leong, J C Y

2004-06-01

Fifty-five human thoracolumbar vertebrae were randomly fatigue loaded and analyzed. The purpose of this study was to explore the relationship between fatigue loading, trabecular microfracture, and energy absorption to fracture in human cadaveric thoracolumbar vertebrae. Although trabecular microfractures are found in vivo and have been produced by fatigue loading in vitro, the effect of the level of physiologic fatigue loading on microfracture and energy absorption has not been investigated. Fifty-five human thoracolumbar vertebrae (T11-L4) were randomly divided into 5 groups: 1) control (no loading, n = 6); 2) axial compression to yield (n = 7); and 3-5) 20,000 cycles of fatigue loading at 2 Hz (each n = 14). The level of fatigue loading was determined as a proportion of the yield load of Group 2 as follows: 10% (Group 3), 20% (Group 4), and 30% (Group 5). Half of the specimens in groups 3 to 5 were used for radiographic and histomorphometric analysis to determine microfracture density and distribution, whereas the other half were tested to determine the energy absorption to yield failure. No radiographic evidence of gross fracture was found in any of the groups following fatigue loading. A mean 7.5% increase in stiffness was found in specimens subject to cyclic loading at 10% of yield stress (Group 3). Fatigue at 20% (Group 4) and 30% of yield stress (Group 5) caused significantly higher (P < 0.05) increases in mean stiffness of 23.6% and 24.2%, respectively. Microfracture density increased from 0.46/mm in Group 3 to 0.66/mm in Group 4 and 0.94/mm in Group 5 (P < 0.05). The energy absorbed to failure decreased from 21.9 J in Group 3 to 18.1 J and 19.6 J in Groups 4 and 5, respectively (P < 0.05). Fatigue loading at physiologic levels produced microfractures that are not detectable by radiography. Increased fatigue load results in an increase in microfracture density and decrease energy absorbed to fracture, indicating a reduced resistance to further fatigue loading.

A Biomechanical Analysis of 2 Constructs for Metacarpal Spiral Fracture Fixation in a Cadaver Model: 2 Large Screws Versus 3 Small Screws.

PubMed

Eu-Jin Cheah, Andre; Behn, Anthony W; Comer, Garet; Yao, Jeffrey

2017-12-01

Surgeons confronted with a long spiral metacarpal fracture may choose to fix it solely with lagged screws. A biomechanical analysis of a metacarpal spiral fracture model was performed to determine whether 3 1.5-mm screws or 2 2.0-mm screws provided more stability during bending and torsional loading. Second and third metacarpals were harvested from 12 matched pairs of fresh-frozen cadaveric hands and spiral fractures were created. One specimen from each matched pair was fixed with 2 2.0-mm lagged screws whereas the other was fixed with 3 1.5-mm lagged screws. Nine pairs underwent combined cyclic cantilever bending and axial compressive loading followed by loading to failure. Nine additional pairs were subjected to cyclic external rotation while under a constant axial compressive load and were subsequently externally rotated to failure under a constant axial compressive load. Paired t tests were used to compare cyclic creep, stiffness, displacement, rotation, and peak load levels. Average failure torque for all specimens was 7.2 ± 1.7 Nm. In cyclic torsional testing, the group with 2 screws exhibited significantly less rotational creep than the one with 3 screws. A single specimen in the group with 2 screws failed before cyclic bending tests were completed. No other significant differences were found between test groups during torsional or bending tests. Both constructs were biomechanically similar except that the construct with 2 screws displayed significantly less loosening during torsional cyclic loading, although the difference was small and may not be clinically meaningful. Because we found no obvious biomechanical advantage to using 3 1.5-mm lagged screws to fix long spiral metacarpal fractures, the time efficiency and decreased implant costs of using 2-2.0 mm lagged screws may be preferred. Copyright © 2017 American Society for Surgery of the Hand. Published by Elsevier Inc. All rights reserved.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, J. Y.; E, J. C.; Huang, J. W.

Impact fracture of single-crystal Si is critical to long-term reliability of electronic devices and solar cells for its wide use as components or substrates in semiconductor industry. Single-crystal Si is loaded along two different crystallographic directions with a split Hopkinson pressure bar integrated with an in situ x-ray imaging and diffraction system. Bulk stress histories are measured, simultaneously with x-ray phase contrast imaging (XPCI) and Laue diffraction. Damage evolution is quantified with grayscale maps from XPCI. Single-crystal Si exhibits pronounced anisotropy in fracture modes, and thus fracture strengths and damage evolution. For loading along [11¯ 0] and viewing along [001],more » (1¯1¯0)[11¯ 0] cleavage is activated and induces horizontal primary cracks followed by perpendicular wing cracks. However, for loading along [011¯] and viewing along [111], random nucleation and growth of shear and tensile-splitting crack networks lead to catastrophic failure of materials with no cleavage. The primary-wing crack mode leads to a lower characteristic fracture strength due to predamage, but a more concentrated strength distribution, i.e., a higher Weibull modulus, compared to the second loading case. Furthermore, the sequential primary cracking, wing cracking and wing-crack coalescence processes result in a gradual increase of damage with time, deviating from theoretical predictions. Particle size and aspect ratios of fragments are discussed with postmortem fragment analysis, which verifies fracture modes observed in XPCI.« less

Investigation of the effect of vacuum environment on the fatigue and fracture behavior of 7075-T6.

NASA Technical Reports Server (NTRS)

Hudson, C. M.

1972-01-01

Axial-load fatigue-life, fatigue-crack propagation, and fracture-toughness experiments were conducted on sheet specimens made of 7075-T6 aluminum alloy. These experiments were conducted at air pressures ranging from 101 kN/sq m to 7 micronewtons/sq m to determine the effect of air pressure on fatigue behavior. Analysis of the results from the fatigue-life experiments indicated that for a given stress level, the lower the air pressure was the longer the fatigue life. At a pressure of 7 micronewtons/sq m, fatigue lives were 15 to 30 times longer than at 101 kN/sq m. Analysis of the results from the fatigue-crack-growth experiments indicates that at low values of stress-intensity range, the fatigue-crack-growth rates were approximately twice as high at atmospheric pressure as in vacuum. However, at higher values of stress-intensity range, the fatigue-crack-growth rates were nominally the same in vacuum and at atmospheric pressure.

Scanning electron microscope fractography in failure analysis of steels

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wouters, R.; Froyen, L.

1996-04-01

For many failure cases, macroscopic examination of the fracture surface permits discrimination of fatigue fractures from overload fractures. For clarifying fatigue fractures, the practical significance of microfractography is limited to an investigation of the crack initiation areas. Scanning electron microscopy is successfully used in tracing local material abnormalities that act as fatigue crack initiators. The task for the scanning electron microscope, however, is much more substantial in failure analysis of overload fractures, especially for steels. By revealing specific fractographic characteristics, complemented by information about the material and the loading conditions, scanning electron microscopy provides a strong indication of the probablemore » cause of failure. A complete dimple fracture is indicative of acceptable bulk material properties; overloading, by subdimensioning or excessive external loading, has to be verified. The presence of cleavage fracture makes the material properties questionable if external conditions causing embrittlement are absent. Intergranular brittle fracture requires verification of grain-boundary weakening conditions--a sensitized structure, whether or not combined with a local stress state or a specific environment. The role of scanning electron microscopy in failure analysis is illustrated by case histories of the aforementioned fracture types.« less

Characterization of Mechanical Properties of Nuclear Graphite Using Subsize Specimens and Reusing Tested Specimens

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ji Hyun, Yoon; Byun, Thak Sang; Strizak, Joe P

2011-01-01

The mechanical properties of NBG-18 nuclear grade graphite have been characterized using small specimen test techniques and statistical treatment on the test results. New fracture strength and toughness test techniques were developed to use subsize cylindrical specimens with glued heads and to reuse their broken halves. Three sets of subsize cylindrical specimens with the different diameters of 4 mm, 8 mm, and 12 mm were tested to obtain tensile fracture strength. The longer piece of the broken halves was cracked from side surfaces and tested under three-point bend loading to obtain fracture toughness. Both the strength and fracture toughness datamore » were analyzed using Weibull distribution models focusing on size effect. The mean fracture strength decreased from 22.9 MPa to 21.5 MPa as the diameter increased from 4 mm to 12 mm, and the mean strength of 15.9 mm diameter standard specimen, 20.9 MPa, was on the extended trend line. These fracture strength data indicate that in the given diameter range the size effect is not significant and much smaller than that predicted by the Weibull statistics-based model. Further, no noticeable size effect existed in the fracture toughness data, whose mean values were in a narrow range of 1.21 1.26 MPa. The Weibull moduli measured for fracture strength and fracture toughness datasets were around 10. It is therefore believed that the small or negligible size effect enables to use the subsize specimens and that the new fracture toughness test method to reuse the broken specimens to help minimize irradiation space and radioactive waste.« less

[BIOMECHANICAL COMPARATIVE STUDY ON FOUR INTERNAL FIXATIONS FOR ACETABULAR FRACTURES IN QUADRILATERAL AREA].

PubMed

Wang, Lei; Wu, Xiaobo; Qi, Wei; Wang, Yongbin; He, Quanjie; Xu, Fengsong; Liu, Hongyang

2015-10-01

To compare the biomechanical difference of 4 kinds of internal fixations for acetabular fracture in quadrilateral area. The transverse fracture models were created in 16 hemipelves specimens from 8 adult males, and were randomly divided into 4 groups according to different internal fixation methods (n = 4): infrapectineal buttress reconstruction plate (group A), infrapectineal buttress locking reconstruction plate (group B), reconstruction plate combined with trans-plate quadrilateral screws (group C), and anterior reconstruction plate-lag screw (group D). Then the horizontal displacement, longitudinal displacement of fractures, and axial stiffness were measured and counted to compare the stability after continuous vertical loading. Under the same loading, the horizontal and longitudinal displacements of groups A, B, C, and D were decreased gradually; when the loading reached 1 800 N, the longitudinal displacement of group A was more than 3.00 mm, indicating the failure criterion, while the axial stiffness increased gradually. Under 200 N loading, there was no significant difference (P > 0.05) in horizontal displacement, longitudinal displacement, and axial stiffness among 4 groups. When the loading reached 600-1 800 N, significant differences were found in horizontal displacement, longitudinal displacement, and axial stiffness among 4 groups (P < 0.05) except the horizontal displacement between groups C and D (P > 0.05). For acetabular fracture in the quadrilateral area, anterior reconstruction plate-lag screw for internal fixation has highest stability, followed by reconstruction plate combined with trans-plate quadrilateral screws, and they are better than infrapectineal buttress reconstruction plate and infrapectineal buttress locking reconstruction plate.

Plastic damage induced fracture behaviors of dental ceramic layer structures subjected to monotonic load.

PubMed

Wang, Raorao; Lu, Chenglin; Arola, Dwayne; Zhang, Dongsheng

2013-08-01

The aim of this study was to compare failure modes and fracture strength of ceramic structures using a combination of experimental and numerical methods. Twelve specimens with flat layer structures were fabricated from two types of ceramic systems (IPS e.max ceram/e.max press-CP and Vita VM9/Lava zirconia-VZ) and subjected to monotonic load to fracture with a tungsten carbide sphere. Digital image correlation (DIC) and fractography technology were used to analyze fracture behaviors of specimens. Numerical simulation was also applied to analyze the stress distribution in these two types of dental ceramics. Quasi-plastic damage occurred beneath the indenter in porcelain in all cases. In general, the fracture strength of VZ specimens was greater than that of CP specimens. The crack initiation loads of VZ and CP were determined as 958 ± 50 N and 724 ± 36 N, respectively. Cracks were induced by plastic damage and were subsequently driven by tensile stress at the elastic/plastic boundary and extended downward toward to the veneer/core interface from the observation of DIC at the specimen surface. Cracks penetrated into e.max press core, which led to a serious bulk fracture in CP crowns, while in VZ specimens, cracks were deflected and extended along the porcelain/zirconia core interface without penetration into the zirconia core. The rupture loads for VZ and CP ceramics were determined as 1150 ± 170 N and 857 ± 66 N, respectively. Quasi-plastic deformation (damage) is responsible for crack initiation within porcelain in both types of crowns. Due to the intrinsic mechanical properties, the fracture behaviors of these two types of ceramics are different. The zirconia core with high strength and high elastic modulus has better resistance to fracture than the e.max core. © 2013 by the American College of Prosthodontists.

Fracture resistance of metal-free composite crowns-effects of fiber reinforcement, thermal cycling, and cementation technique.

PubMed

Lehmann, Franziska; Eickemeyer, Grit; Rammelsberg, Peter

2004-09-01

The improved mechanical properties of contemporary composites have resulted in their extensive use for the restoration of posterior teeth. However, the influence of fiber reinforcement, cementation technique, and physical stress on the fracture resistance of metal-free crowns is unknown. This in vitro study evaluated the effect of fiber reinforcement, physical stress, and cementation methods on the fracture resistance of posterior metal-free Sinfony crowns. Ninety-six extracted human third molars received a standardized tooth preparation: 0.5-mm chamfer preparation and occlusal reduction of 1.3 to 1.5 mm. Sinfony (nonreinforced crowns, n=48) and Sinfony-Vectris (reinforced crowns, n=48) crowns restoring original tooth contour were prepared. Twenty-four specimens of each crown type were cemented, using either glass ionomer cement (GIC) or resin cement. Thirty-two crowns (one third) were stored in humidity for 48 hours. Another third was exposed to 10,000 thermal cycles (TC) between 5 degrees C and 55 degrees C. The remaining third was treated with thermal cycling and mechanical loading (TCML), consisting of 1.2 million axial loads of 50 N. The artificial crowns were then vertically loaded with a steel sphere until failure occurred. Significant differences in fracture resistance (N) between experimental groups were assessed by nonparametric Mann-Whitney U-test (alpha=.05). Fifty percent of the Sinfony and Sinfony-Vectris crowns cemented with glass ionomer cement loosened after thermal cycling. Thermal cycling resulted in a significant reduction in the mean fracture resistance for Sinfony crowns cemented with GIC, from 2037 N to 1282 N (P=.004). Additional fatigue produced no further effects. Fiber reinforcement significantly increased fracture resistance, from 1555 N to 2326 N (P=.001). The minimal fracture resistance was above 600 N for all combinations of material, cement and loading. Fracture resistance of metal-free Sinfony crowns was significantly increased by fiber reinforcement. Adhesive cementation may be recommended to avoid cementation failure.

Method of determining elastic and plastic mechanical properties of ceramic materials using spherical indenters

DOEpatents

Adler, Thomas A.

1996-01-01

The invention pertains a method of determining elastic and plastic mechanical properties of ceramics, intermetallics, metals, plastics and other hard, brittle materials which fracture prior to plastically deforming when loads are applied. Elastic and plastic mechanical properties of ceramic materials are determined using spherical indenters. The method is most useful for measuring and calculating the plastic and elastic deformation of hard, brittle materials with low values of elastic modulus to hardness.

Selecting boundary conditions in physiological strain analysis of the femur: Balanced loads, inertia relief method and follower load.

PubMed

Heyland, Mark; Trepczynski, Adam; Duda, Georg N; Zehn, Manfred; Schaser, Klaus-Dieter; Märdian, Sven

2015-12-01

Selection of boundary constraints may influence amount and distribution of loads. The purpose of this study is to analyze the potential of inertia relief and follower load to maintain the effects of musculoskeletal loads even under large deflections in patient specific finite element models of intact or fractured bone compared to empiric boundary constraints which have been shown to lead to physiological displacements and surface strains. The goal is to elucidate the use of boundary conditions in strain analyses of bones. Finite element models of the intact femur and a model of clinically relevant fracture stabilization by locking plate fixation were analyzed with normal walking loading conditions for different boundary conditions, specifically re-balanced loading, inertia relief and follower load. Peak principal cortex surface strains for different boundary conditions are consistent (maximum deviation 13.7%) except for inertia relief without force balancing (maximum deviation 108.4%). Influence of follower load on displacements increases with higher deflection in fracture model (from 3% to 7% for force balanced model). For load balanced models, follower load had only minor influence, though the effect increases strongly with higher deflection. Conventional constraints of fixed nodes in space should be carefully reconsidered because their type and position are challenging to justify and for their potential to introduce relevant non-physiological reaction forces. Inertia relief provides an alternative method which yields physiological strain results. Copyright © 2015 IPEM. Published by Elsevier Ltd. All rights reserved.

Biomechanical comparison of 3.0 mm headless compression screw and 3.5 mm cortical bone screw in a canine humeral condylar fracture model.

PubMed

Gonsalves, Mishka N; Jankovits, Daniel A; Huber, Michael L; Strom, Adam M; Garcia, Tanya C; Stover, Susan M

2016-09-20

To compare the biomechanical properties of simulated humeral condylar fractures reduced with one of two screw fixation methods: 3.0 mm headless compression screw (HCS) or 3.5 mm cortical bone screw (CBS) placed in lag fashion. Bilateral humeri were collected from nine canine cadavers. Standardized osteotomies were stabilized with 3.0 mm HCS in one limb and 3.5 mm CBS in the contralateral limb. Condylar fragments were loaded to walk, trot, and failure loads while measuring construct properties and condylar fragment motion. The 3.5 mm CBS-stabilized constructs were 36% stiffer than 3.0 mm HCS-stabilized constructs, but differences were not apparent in quality of fracture reduction nor in yield loads, which exceeded expected physiological loads during rehabilitation. Small residual fragment displacements were not different between CBS and HCS screws. Small fragment rotation was not significantly different between screws, but was weakly correlated with moment arm length (R² = 0.25). A CBS screw placed in lag fashion provides stiffer fixation than an HCS screw, although both screws provide similar anatomical reduction and yield strength to condylar fracture fixation in adult canine humeri.

Fracture Toughness and Fatigue Crack Growth Behavior of As-Cast High-Entropy Alloys

NASA Astrophysics Data System (ADS)

Seifi, Mohsen; Li, Dongyue; Yong, Zhang; Liaw, Peter K.; Lewandowski, John J.

2015-08-01

The fracture toughness and fatigue crack growth behavior of two as-vacuum arc cast high-entropy alloys (HEAs) (Al0.2CrFeNiTi0.2 and AlCrFeNi2Cu) were determined. A microstructure examination of both HEA alloys revealed a two-phase structure consisting of body-centered cubic (bcc) and face-centered cubic (fcc) phases. The notched and fatigue precracked toughness values were in the range of those reported in the literature for two-phase alloys but significantly less than recent reports on a single phase fcc-HEA that was deformation processed. Fatigue crack growth experiments revealed high fatigue thresholds that decreased significantly with an increase in load ratio, while Paris law slopes exhibited metallic-like behavior at low R with significant increases at high R. Fracture surface examinations revealed combinations of brittle and ductile/dimpled regions at overload, with some evidence of fatigue striations in the Paris law regime.