Characterization of Cracking Mechanisms of Carbon Anodes Used in Aluminum Industry by Optical Microscopy and Tomography

NASA Astrophysics Data System (ADS)

Amrani, Salah; Kocaefe, Duygu; Kocaefe, Yasar; Bhattacharyay, Dipankar; Bouazara, Mohamed; Morais, Brigitte

2016-10-01

The objective of this work is to understand the different mechanisms of crack formation in dense anodes used in the aluminum industry. The first approach used is based on the qualitative characterization of the surface cracks and the depth of these cracks. The second approach, which constitutes a quantitative characterization, is carried out by determining the distribution of the crack width along its length as well as the percentage of the surface containing cracks. A qualitative analysis of crack formation was also carried out using 3D tomography. It was observed that mixing and forming conditions have a significant effect on crack formation in green anodes. The devolatilization of pitch during baking causes the formation and propagation of cracks in baked anodes in which large particles control the direction of crack propagation.

Surface-crack growth: Models, experiments, and structures; Proceedings of the Symposium, Sparks, NV, Apr. 25, 1988

NASA Technical Reports Server (NTRS)

Reuter, Walter G. (Editor); Underwood, John H. (Editor); Newman, James C., Jr. (Editor)

1990-01-01

The present volume on surface-crack growth modeling, experimental methods, and structures, discusses elastoplastic behavior, the fracture analysis of three-dimensional bodies with surface cracks, optical measurements of free-surface effects on natural surfaces and through cracks, an optical and finite-element investigation of a plastically deformed surface flaw under tension, fracture behavior prediction for rapidly loaded surface-cracked specimens, and surface cracks in thick laminated fiber composite plates. Also discussed are a novel study procedure for crack initiation and growth in thermal fatigue testing, the growth of surface cracks under fatigue and monotonically increasing load, the subcritical growth of a surface flaw, surface crack propagation in notched and unnotched rods, and theoretical and experimental analyses of surface cracks in weldments.

Incidence of apical crack formation and propagation during removal of root canal filling materials with different engine driven nickel-titanium instruments.

PubMed

Özyürek, Taha; Tek, Vildan; Yılmaz, Koray; Uslu, Gülşah

2017-11-01

To determine the incidence of crack formation and propagation in apical root dentin after retreatment procedures performed using ProTaper Universal Retreatment (PTR), Mtwo-R, ProTaper Next (PTN), and Twisted File Adaptive (TFA) systems. The study consisted of 120 extracted mandibular premolars. One millimeter from the apex of each tooth was ground perpendicular to the long axis of the tooth, and the apical surface was polished. Twenty teeth served as the negative control group. One hundred teeth were prepared, obturated, and then divided into 5 retreatment groups. The retreatment procedures were performed using the following files: PTR, Mtwo-R, PTN, TFA, and hand files. After filling material removal, apical enlargement was done using apical size 0.50 mm ProTaper Universal (PTU), Mtwo, PTN, TFA, and hand files. Digital images of the apical root surfaces were recorded before preparation, after preparation, after obturation, after filling removal, and after apical enlargement using a stereomicroscope. The images were then inspected for the presence of new apical cracks and crack propagation. Data were analyzed with χ 2 tests using SPSS 21.0 software. New cracks and crack propagation occurred in all the experimental groups during the retreatment process. Nickel-titanium rotary file systems caused significantly more apical crack formation and propagation than the hand files. The PTU system caused significantly more apical cracks than the other groups after the apical enlargement stage. This study showed that retreatment procedures and apical enlargement after the use of retreatment files can cause crack formation and propagation in apical dentin.

Monitoring of surface-fatigue crack propagation in a welded steel angle structure using guided waves and principal component analysis

NASA Astrophysics Data System (ADS)

Lu, Mingyu; Qu, Yongwei; Lu, Ye; Ye, Lin; Zhou, Limin; Su, Zhongqing

2012-04-01

An experimental study is reported in this paper demonstrating monitoring of surface-fatigue crack propagation in a welded steel angle structure using Lamb waves generated by an active piezoceramic transducer (PZT) network which was freely surface-mounted for each PZT transducer to serve as either actuator or sensor. The fatigue crack was initiated and propagated in welding zone of a steel angle structure by three-point bending fatigue tests. Instead of directly comparing changes between a series of specific signal segments such as S0 and A0 wave modes scattered from fatigue crack tips, a variety of signal statistical parameters representing five different structural status obtained from marginal spectrum in Hilbert-huang transform (HHT), indicating energy progressive distribution along time period in the frequency domain including all wave modes of one wave signal were employed to classify and distinguish different structural conditions due to fatigue crack initiation and propagation with the combination of using principal component analysis (PCA). Results show that PCA based on marginal spectrum is effective and sensitive for monitoring the growth of fatigue crack although the received signals are extremely complicated due to wave scattered from weld, multi-boundaries, notch and fatigue crack. More importantly, this method indicates good potential for identification of integrity status of complicated structures which cause uncertain wave patterns and ambiguous sensor network arrangement.

Fatigue flaw growth and NDI evaluation for preventing through cracks in spacecraft tankage structures

NASA Technical Reports Server (NTRS)

Pettit, D. E.; Hoeppner, D. W.

1972-01-01

A program was conducted to determine the fatigue-crack propagation behavior of parent and welded 2219-T87 aluminum alloy sheet under controlled cyclic stress conditions in room temperature air and 300 F air. Specimens possessing an initial surface defect of controlled dimensions were cycled under constant load amplitude until the propagating fatigue crack penetrated the back surface of the specimen. A series of precracked specimens were prepared to determine optimum penetrant, X-ray, ultrasonic, and eddy current nondestructive inspection procedures.

Propellant Crack Tip Ignition and Propagation under Rapid Pressurization

DTIC Science & Technology

1982-10-01

that the ignition-delay time decreases and the heat flux to the propellant surface increases as the pressurization rate is increased. The decrease in...leading to ignition. The model predicts the experimental obseriation that the ignition delay time decreases as the pressurization rate is increased...pressurization rate on both crack propagation velocity and time variation of crack shape was studied. Experimental results indicated that the crack velocity

Dislocation mechanism based model for stage II fatigue crack propagation rate

NASA Technical Reports Server (NTRS)

Mazumdar, P. K.

1986-01-01

Repeated plastic deformation, which of course depends on dislocation mechanism, at or near the crack tip leads to the fatigue crack propagation. By involving the theory of thermally activated flow and the cumulative plastic strain criterion, an effort is made here to model the stage II fatigue crack propagation rate in terms of the dislocation mechanism. The model, therefore, provides capability to ascertain: (1) the dislocation mechanism (and hence the near crack tip microstructures) assisting the crack growth, (2) the relative resistance of dislocation mechanisms to the crack growth, and (3) the fracture surface characteristics and its interpretation in terms of the dislocation mechanism. The local microstructure predicted for the room temperature crack growth in copper by this model is in good agreement with the experimental results taken from the literature. With regard to the relative stability of such dislocation mechanisms as the cross-slip and the dislocation intersection, the model suggests an enhancement of crack growth rate with an ease of cross-slip which in general promotes dislocation cell formation and is common in material which has high stacking fault energy (produces wavy slips). Cross-slip apparently enhances crack growth rate by promoting slip irreversibility and fracture surface brittleness to a greater degree.

On the variation in crack-opening stresses at different locations in a three-dimensional body

NASA Technical Reports Server (NTRS)

Chermahini, R. G.; Blom, Anders F.

1990-01-01

Crack propagation and closure behavior of thin, and thick middle crack tension specimens under constant amplitude loading were investigated using a three dimensional elastic plastic finite element analysis of fatigue crack propagation and closure. In the thin specimens the crack front closed first on the exterior (free) surface and closed last in the interior during the unloading portion of cyclic loading; a load reduced displacement technique was used to determine crack opening stresses at specified locations in the plate from the displacements calculated after the seven cycle. All the locations were on the plate external surface and were located near the crack tip, behind the crack tip, at the centerline of the crack. With this technique, the opening stresses at the specified points were found to be 0.52, 0.42, and 0.39 times the maximum applied stress.

Generation Mechanism of Nonlinear Rayleigh Surface Waves for Randomly Distributed Surface Micro-Cracks.

PubMed

Ding, Xiangyan; Li, Feilong; Zhao, Youxuan; Xu, Yongmei; Hu, Ning; Cao, Peng; Deng, Mingxi

2018-04-23

This paper investigates the propagation of Rayleigh surface waves in structures with randomly distributed surface micro-cracks using numerical simulations. The results revealed a significant ultrasonic nonlinear effect caused by the surface micro-cracks, which is mainly represented by a second harmonic with even more distinct third/quadruple harmonics. Based on statistical analysis from the numerous results of random micro-crack models, it is clearly found that the acoustic nonlinear parameter increases linearly with micro-crack density, the proportion of surface cracks, the size of micro-crack zone, and the excitation frequency. This study theoretically reveals that nonlinear Rayleigh surface waves are feasible for use in quantitatively identifying the physical characteristics of surface micro-cracks in structures.

Generation Mechanism of Nonlinear Rayleigh Surface Waves for Randomly Distributed Surface Micro-Cracks

PubMed Central

Ding, Xiangyan; Li, Feilong; Xu, Yongmei; Cao, Peng; Deng, Mingxi

2018-01-01

This paper investigates the propagation of Rayleigh surface waves in structures with randomly distributed surface micro-cracks using numerical simulations. The results revealed a significant ultrasonic nonlinear effect caused by the surface micro-cracks, which is mainly represented by a second harmonic with even more distinct third/quadruple harmonics. Based on statistical analysis from the numerous results of random micro-crack models, it is clearly found that the acoustic nonlinear parameter increases linearly with micro-crack density, the proportion of surface cracks, the size of micro-crack zone, and the excitation frequency. This study theoretically reveals that nonlinear Rayleigh surface waves are feasible for use in quantitatively identifying the physical characteristics of surface micro-cracks in structures. PMID:29690580

Fracture on Curved Surfaces

NASA Astrophysics Data System (ADS)

Mitchell, Noah; Koning, Vinzenz; Vitelli, Vincenzo; Irvine, William T. M.

2014-03-01

When an elastic film conforms to a surface with Gaussian curvature, stresses arise in the film. As a result, cracks--typically studied in flat materials--interact with curvature when propagating through the system. Using silicone elastomer sheets that conform to the surface of a Gaussian bump, we find experimental evidence for the deflection of a crack propagating through the material. We interpret our experiments with reference to analytical modeling and simulations of a simplified model system.

Experimental and numerical investigation of crack initiation and propagation in silicon nitride ceramic under rolling and cyclic contact

NASA Astrophysics Data System (ADS)

Raga, Rahul; Khader, Iyas; Zdeněk, Chlup; Kailer, Andreas

2017-05-01

The focus of the work was to investigate crack initiation and propagation mechanisms in silicon nitride undergoing non-conforming hybrid contact under various tribological conditions. In order to understand the prevailing modes of damage in silicon nitride, two distinct model experiments were proposed, namely, rolling contact and cyclic contact experiments. The rolling contact experiment was designed in order to mimic the contact conditions appearing in hybrid bearings at contact pressures ranging from 3 to 6 GPa. On the other hand, cyclic contact experiments with stresses ranging from 4 to 15 GPa under different media were carried out to study damage under localised stresses. In addition, the experimentally observed cracks were implemented in a finite element model to study the stress redistribution and correlate the generated stresses with the corresponding mechanisms. Crack propagation under rolling contact was attributed to two different mechanisms, namely, fatigue induced fracture and lubricant driven crack propagation. The numerical simulations shed light on the tensile stress driven surface and subsurface crack propagation mechanisms. On the other hand, the cyclic contact experiments showed delayed crack formation for lubricated cyclic contact. Ceramographic cross-sectional analysis showed crack patterns similar to Hertzian crack propagation under cyclic contact load.

Fatigue crack propagation in aluminum-lithium alloys

NASA Technical Reports Server (NTRS)

Rao, K. T. V.; Ritchie, R. O.; Piascik, R. S.; Gangloff, R. P.

1989-01-01

The principal mechanisms which govern the fatigue crack propagation resistance of aluminum-lithium alloys are investigated, with emphasis on their behavior in controlled gaseous and aqueous environments. Extensive data describe the growth kinetics of fatigue cracks in ingot metallurgy Al-Li alloys 2090, 2091, 8090, and 8091 and in powder metallurgy alloys exposed to moist air. Results are compared with data for traditional aluminum alloys 2024, 2124, 2618, 7075, and 7150. Crack growth is found to be dominated by shielding from tortuous crack paths and resultant asperity wedging. Beneficial shielding is minimized for small cracks, for high stress ratios, and for certain loading spectra. While water vapor and aqueous chloride environments enhance crack propagation, Al-Li-Cu alloys behave similarly to 2000-series aluminum alloys. Cracking in water vapor is controlled by hydrogen embrittlement, with surface films having little influence on cyclic plasticity.

Incidence of apical crack initiation and propagation during the removal of root canal filling material with ProTaper and Mtwo rotary nickel-titanium retreatment instruments and hand files.

PubMed

Topçuoğlu, Hüseyin Sinan; Düzgün, Salih; Kesim, Bertan; Tuncay, Oznur

2014-07-01

The aim of this study was to determine the incidence of crack initiation and propagation in apical root dentin after retreatment procedures performed by using 2 rotary retreatment systems and hand files with additional instrumentation. Eighty extracted mandibular premolars with single canals were selected. One millimeter from the apex of each tooth was ground perpendicular to the long axis of the tooth, and the apical surface was polished. Twenty teeth served as the control group, and no preparation was performed. The remaining 60 teeth were prepared to size 35 with rotary files and filled with gutta-percha and AH Plus sealer. Specimens were then divided into 3 groups (n = 20), and retreatment procedures were performed with the following devices and techniques: ProTaper Universal retreatment files, Mtwo retreatment files, and hand files. After retreatment, the additional instrumentation was performed by using size 40 ProTaper, Mtwo, and hand files. Digital images of the apical root surface were recorded before preparation, after instrumentation, after filling, after retreatment, and after additional instrumentation. The images were then inspected for the presence of any new apical cracks and propagation. Data were analyzed with the logistic regression and Fisher exact tests. All experimental groups caused crack initiation and propagation after use of retreatment instruments. The ProTaper and Mtwo retreatment groups caused greater crack initiation and propagation than the hand instrument group (P < .05) after retreatment. Additional instrumentation with ProTaper and Mtwo instruments after the use of retreatment instruments caused crack initiation and propagation, whereas hand files caused neither crack initiation nor propagation (P < .05). This study showed that retreatment procedures and additional instrumentation after the use of retreatment files may cause crack initiation and propagation in apical dentin. Copyright © 2014 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

A methodology for the investigation of toughness and crack propagation in mouse bone.

PubMed

Carriero, Alessandra; Zimmermann, Elizabeth A; Shefelbine, Sandra J; Ritchie, Robert O

2014-11-01

Bone fracture is a health concern for those with aged bone and brittle bone diseases. Mouse bone is widely used as a model of human bone, especially to investigate preclinical treatment strategies. However, little is known about the mechanisms of mouse bone fracture and its similarities and differences from fracture in human bone. In this work we present a methodology to investigate the fracture toughness during crack initiation and crack propagation for mouse bone. Mouse femora were dissected, polished on their periosteal surface, notched on the posterior surface at their mid-diaphysis, and tested in three-point bending under displacement control at a rate of 0.1mm/min using an in situ loading stage within an environmental scanning electron microscope. We obtained high-resolution real-time imaging of the crack initiation and propagation in mouse bone. From the images we can measure the crack extension at each step of the crack growth and calculate the toughness of the bone (in terms of stress intensity factor (K) and work to fracture (Wf)) as a function of stable crack length (Δa), thus generating a resistance curve for the mouse bone. The technique presented here provides insight into the evolution of microdamage and the toughening mechanisms that resist crack propagation, which are essential for preclinical development of treatments to enhance bone quality and combat fracture risk. Copyright © 2014 Elsevier Ltd. All rights reserved.

Fractography of a bis-GMA resin.

PubMed

Davis, D M; Waters, N E

1989-07-01

The fracture behavior of a bis-GMA resin was studied by means of the double-torsion test. The fracture parameter measured was the stress-intensity factor. Fracture occurred in either a stick-slip (unstable) or continuous (stable) manner, depending upon the test conditions. When stick-slip propagation occurred, the fracture surfaces showed characteristic crack-arrest lines. The fracture surfaces were examined by use of a reflected-light optical microscope. The stress-intensity factor for crack initiation was found to be related to the size of the crack-arrest line which, in turn, could be related to the Dugdale model for plastic zone size. The evidence supported the concept that the behavior of the crack during propagation was controlled by the amount of plastic deformation occurring at the crack tip.

Visualization and Quantitative Analysis of Crack-Tip Plastic Zone in Pure Nickel

NASA Astrophysics Data System (ADS)

Kelton, Randall; Sola, Jalal Fathi; Meletis, Efstathios I.; Huang, Haiying

2018-05-01

Changes in surface morphology have long been thought to be associated with crack propagation in metallic materials. We have studied areal surface texture changes around crack tips in an attempt to understand the correlations between surface texture changes and crack growth behavior. Detailed profiling of the fatigue sample surface was carried out at short fatigue intervals. An image processing algorithm was developed to calculate the surface texture changes. Quantitative analysis of the crack-tip plastic zone, crack-arrested sites near triple points, and large surface texture changes associated with crack release from arrested locations was carried out. The results indicate that surface texture imaging enables visualization of the development of plastic deformation around a crack tip. Quantitative analysis of the surface texture changes reveals the effects of local microstructures on the crack growth behavior.

Mapping the cyclic plastic zone to elucidate the mechanisms of crack tip deformation in bulk metallic glasses

NASA Astrophysics Data System (ADS)

Scudino, S.; Shahid, R. N.; Escher, B.; Stoica, M.; Li, B. S.; Kruzic, J. J.

2017-02-01

Developing damage-tolerant bulk metallic glasses (BMGs) requires knowledge of the physical mechanisms governing crack propagation. While fractography suggests that fatigue crack propagation occurs in an incremental manner, conclusive evidence of alternating crack tip blunting and resharpening is lacking. By mapping the strain fields in both the monotonic and cyclic plastic zones, it is shown that the characteristic compressive stresses required to resharpen the crack tip are developed in a BMG upon unloading. This result confirms the mechanism of fatigue crack propagation in BMGs. Broader implications of these findings are that the effect of shear banding is rather diffuse and plastic deformation ahead of a stress concentration, such as a crack tip, appears to extend well beyond the extent of visible shear bands on the sample surface.

A local crack-tracking strategy to model three-dimensional crack propagation with embedded methods

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

Annavarapu, Chandrasekhar; Settgast, Randolph R.; Vitali, Efrem

We develop a local, implicit crack tracking approach to propagate embedded failure surfaces in three-dimensions. We build on the global crack-tracking strategy of Oliver et al. (Int J. Numer. Anal. Meth. Geomech., 2004; 28:609–632) that tracks all potential failure surfaces in a problem at once by solving a Laplace equation with anisotropic conductivity. We discuss important modifications to this algorithm with a particular emphasis on the effect of the Dirichlet boundary conditions for the Laplace equation on the resultant crack path. Algorithmic and implementational details of the proposed method are provided. Finally, several three-dimensional benchmark problems are studied and resultsmore » are compared with available literature. Lastly, the results indicate that the proposed method addresses pathological cases, exhibits better behavior in the presence of closely interacting fractures, and provides a viable strategy to robustly evolve embedded failure surfaces in 3D.« less

A local crack-tracking strategy to model three-dimensional crack propagation with embedded methods

DOE PAGES

Annavarapu, Chandrasekhar; Settgast, Randolph R.; Vitali, Efrem; ...

2016-09-29

We develop a local, implicit crack tracking approach to propagate embedded failure surfaces in three-dimensions. We build on the global crack-tracking strategy of Oliver et al. (Int J. Numer. Anal. Meth. Geomech., 2004; 28:609–632) that tracks all potential failure surfaces in a problem at once by solving a Laplace equation with anisotropic conductivity. We discuss important modifications to this algorithm with a particular emphasis on the effect of the Dirichlet boundary conditions for the Laplace equation on the resultant crack path. Algorithmic and implementational details of the proposed method are provided. Finally, several three-dimensional benchmark problems are studied and resultsmore » are compared with available literature. Lastly, the results indicate that the proposed method addresses pathological cases, exhibits better behavior in the presence of closely interacting fractures, and provides a viable strategy to robustly evolve embedded failure surfaces in 3D.« less

Damage, crack growth and fracture characteristics of nuclear grade graphite using the Double Torsion technique

NASA Astrophysics Data System (ADS)

Becker, T. H.; Marrow, T. J.; Tait, R. B.

2011-07-01

The crack initiation and propagation characteristics of two medium grained polygranular graphites, nuclear block graphite (NBG10) and Gilsocarbon (GCMB grade) graphite, have been studied using the Double Torsion (DT) technique. The DT technique allows stable crack propagation and easy crack tip observation of such brittle materials. The linear elastic fracture mechanics (LEFM) methodology of the DT technique was adapted for elastic-plastic fracture mechanics (EPFM) in conjunction with a methodology for directly calculating the J-integral from in-plane displacement fields (JMAN) to account for the non-linearity of graphite deformation. The full field surface displacement measurement techniques of electronic speckle pattern interferometry (ESPI) and digital image correlation (DIC) were used to observe and measure crack initiation and propagation. Significant micro-cracking in the fracture process zone (FPZ) was observed as well as crack bridging in the wake of the crack tip. The R-curve behaviour was measured to determine the critical J-integral for crack propagation in both materials. Micro-cracks tended to nucleate at pores, causing deflection of the crack path. Rising R-curve behaviour was observed, which is attributed to the formation of the FPZ, while crack bridging and distributed micro-cracks are responsible for the increase in fracture resistance. Each contributes around 50% of the irreversible energy dissipation in both graphites.

Two dimensional modeling of elastic wave propagation in solids containing cracks with rough surfaces and friction - Part II: Numerical implementation.

PubMed

Delrue, Steven; Aleshin, Vladislav; Truyaert, Kevin; Bou Matar, Olivier; Van Den Abeele, Koen

2018-01-01

Our study aims at the creation of a numerical toolbox that describes wave propagation in samples containing internal contacts (e.g. cracks, delaminations, debondings, imperfect intergranular joints) of known geometry with postulated contact interaction laws including friction. The code consists of two entities: the contact model and the solid mechanics module. Part I of the paper concerns an in-depth description of a constitutive model for realistic contacts or cracks that takes into account the roughness of the contact faces and the associated effects of friction and hysteresis. In the crack model, three different contact states can be recognized: contact loss, total sliding and partial slip. Normal (clapping) interactions between the crack faces are implemented using a quadratic stress-displacement relation, whereas tangential (friction) interactions were introduced using the Coulomb friction law for the total sliding case, and the Method of Memory Diagrams (MMD) in case of partial slip. In the present part of the paper, we integrate the developed crack model into finite element software in order to simulate elastic wave propagation in a solid material containing internal contacts or cracks. We therefore implemented the comprehensive crack model in MATLAB® and introduced it in the Structural Mechanics Module of COMSOL Multiphysics®. The potential of the approach for ultrasound based inspection of solids with cracks showing acoustic nonlinearity is demonstrated by means of an example of shear wave propagation in an aluminum sample containing a single crack with rough surfaces and friction. Copyright © 2017 Elsevier B.V. All rights reserved.

Effects of friction and high torque on fatigue crack propagation in Mode III

NASA Astrophysics Data System (ADS)

Nayeb-Hashemi, H.; McClintock, F. A.; Ritchie, R. O.

1982-12-01

Turbo-generator and automotive shafts are often subjected to complex histories of high torques. To provide a basis for fatigue life estimation in such components, a study of fatigue crack propagation in Mode III (anti-plane shear) for a mill-annealed AISI 4140 steel (RB88, 590 MN/m2 tensile strength) has been undertaken, using torsionally-loaded, circumferentially-notched cylindrical specimens. As demonstrated previously for higher strength AISI 4340 steel, Mode III cyclic crack growth rates (dc/dN) IIIcan be related to the alternating stress intensity factor ΔKIII for conditions of small-scale yielding. However, to describe crack propagation behavior over an extended range of crack growth rates (˜10-6 to 10-2 mm per cycle), where crack growth proceeds under elastic-plastic and full plastic conditions, no correlation between (dc/dN) III and ΔKIII is possible. Accordingly, a new parameter for torsional crack growth, termed the plastic strain intensity Γ III, is introduced and is shown to provide a unique description of Mode III crack growth behavior for a wide range of testing conditions, provided a mean load reduces friction, abrasion, and interlocking between mating fracture surfaces. The latter effect is found to be dependent upon the mode of applied loading (i.e., the presence of superimposed axial loads) and the crack length and torque level. Mechanistically, high-torque surfaces were transverse, macroscopically flat, and smeared. Lower torques showed additional axial cracks (longitudinal shear cracking) perpendicular to the main transverse surface. A micro-mechanical model for the main radi l Mode III growth, based on the premise that crack advance results from Mode II coalescence of microcracks initiated at inclusions ahead of the main crack front, is extended to high nominal stress levels, and predicts that Mode III fatigue crack propagation rates should be proportional to the range of plastic strain intensity (ΔΓIII if local Mode II growth rates are proportional to the displacements. Such predictions are shown to be in agreement with measured growth rates in AISI {dy4140} steel from 10-6 to 10-2 mm per cycle.

Understanding fast macroscale fracture from microcrack post mortem patterns

PubMed Central

Guerra, Claudia; Scheibert, Julien; Bonamy, Daniel; Dalmas, Davy

2012-01-01

Dynamic crack propagation drives catastrophic solid failures. In many amorphous brittle materials, sufficiently fast crack growth involves small-scale, high-frequency microcracking damage localized near the crack tip. The ultrafast dynamics of microcrack nucleation, growth, and coalescence is inaccessible experimentally and fast crack propagation was therefore studied only as a macroscale average. Here, we overcome this limitation in polymethylmethacrylate, the archetype of brittle amorphous materials: We reconstruct the complete spatiotemporal microcracking dynamics, with micrometer/nanosecond resolution, through post mortem analysis of the fracture surfaces. We find that all individual microcracks propagate at the same low, load-independent velocity. Collectively, the main effect of microcracks is not to slow down fracture by increasing the energy required for crack propagation, as commonly believed, but on the contrary to boost the macroscale velocity through an acceleration factor selected on geometric grounds. Our results emphasize the key role of damage-related internal variables in the selection of macroscale fracture dynamics. PMID:22203962

On the Theory and Numerical Simulation of Cohesive Crack Propagation with Application to Fiber-Reinforced Composites

NASA Technical Reports Server (NTRS)

Rudraraju, Siva Shankar; Garikipati, Krishna; Waas, Anthony M.; Bednarcyk, Brett A.

2013-01-01

The phenomenon of crack propagation is among the predominant modes of failure in many natural and engineering structures, often leading to severe loss of structural integrity and catastrophic failure. Thus, the ability to understand and a priori simulate the evolution of this failure mode has been one of the cornerstones of applied mechanics and structural engineering and is broadly referred to as "fracture mechanics." The work reported herein focuses on extending this understanding, in the context of through-thickness crack propagation in cohesive materials, through the development of a continuum-level multiscale numerical framework, which represents cracks as displacement discontinuities across a surface of zero measure. This report presents the relevant theory, mathematical framework, numerical modeling, and experimental investigations of through-thickness crack propagation in fiber-reinforced composites using the Variational Multiscale Cohesive Method (VMCM) developed by the authors.

Dynamic steady-state analysis of crack propagation in rubber-like solids using an extended finite element method

NASA Astrophysics Data System (ADS)

Kroon, Martin

2012-01-01

In the present study, a computational framework for studying high-speed crack growth in rubber-like solids under conditions of plane stress and steady-state is proposed. Effects of inertia, viscoelasticity and finite strains are included. The main purpose of the study is to examine the contribution of viscoelastic dissipation to the total work of fracture required to propagate a crack in a rubber-like solid. The computational framework builds upon a previous work by the present author (Kroon in Int J Fract 169:49-60, 2011). The model was fully able to predict experimental results in terms of the local surface energy at the crack tip and the total energy release rate at different crack speeds. The predicted distributions of stress and dissipation around the propagating crack tip are presented. The predicted crack tip profiles also agree qualitatively with experimental findings.

Fatigue crack propagation in self-assembling nanocomposites

NASA Astrophysics Data System (ADS)

Klingler, Andreas; Wetzel, Bernd

2016-05-01

Self-assembling block-copolymers allow the easy manufacturing of nanocomposites due to the thermodynamically driven in situ formation of nanosized phases in thermosetting resins during the curing process. Complex mechanical dispersion processes can be avoided. The current study investigates the effect of a block-copolymer on the fatigue crack propagation resistance of a cycloaliphatic amine cured epoxy resin. It was found that a small amount of MAM triblock-copolymer significantly increases the resistance to fatigue crack propagation of epoxy. Crack growth rate and the Paris law exponent for fatigue-crack growth were considerably reduced from m=15.5 of the neat epoxy to m=8.1 of the nanocomposite. To identify the related reinforcing and fracture mechanisms structural analyses of the fractured surfaces were performed by scanning electron microscope. Characteristic features were identified to be deformation, debonding and fracture of the nano-phases as well as crack pinning. However, the highest resistance against fatigue crack propagation was achieved in a bi-continuous microstructure that consisted of an epoxy-rich phase with embedded submicron sized MAM inclusions, and which was surrounded by a block-copolymer-rich phase that showed rupture and plastic deformation.

Diffraction-based study of fatigue crack initiation and propagation in aerospace aluminum alloys

NASA Astrophysics Data System (ADS)

Gupta, Vipul K.

The crack initiation sites and microstructure-sensitive growth of small fatigue cracks are experimentally characterized in two precipitation-hardened aluminum alloys, 7075-T651 and 7050-T7451, stressed in ambient temperature moist-air (warm-humid) and -50°C dry N2 (cold-dry) environmental conditions. Backscattered electron imaging (BSE) and energy dispersive spectroscopy (EDS) of the fracture surfaces showed that Fe-Cu rich constituent particle clusters are the most common initiation sites within both alloys stressed in either environment. The crack growth within each alloy, on average, was observed to be slowed in the cold-dry environment than in the warm-humid environment, but only at longer crack lengths. Although no overwhelming effects of grain boundaries and grain orientations on small-crack growth were observed, crack growth data showed local fluctuations within individual grains. These observations are understood as crack propagation through the underlying substructure at the crack surface and frequent interaction with low/high-angle grain and subgrain boundaries, during cyclic loading, and, are further attributed to periodic changes in crack propagation path and multiple occurrences of crack-branching observed in the current study. SEM-based stereology in combination with electron backscattered diffraction (EBSD) established fatigue crack surface crystallography within the region from ˜1 to 50 mum of crack initiating particle clusters. Fatigue crack facets were parallel to a wide variety of crystallographic planes, with pole orientations distributed broadly across the irreducible stereographic triangle between the {001} and {101}-poles within both warm-humid and cold-dry environments. The results indicate environmentally affected fatigue cracking in both cases, given the similarity between the observed morphology and crystallography with that of a variety of aerospace aluminum alloys cracked in the presence of moist-air. There was no evidence of crystallographic {111} slip-plane cracking typical of the Stage I crack growth mode observed in single crystals and high purity polycrystals of face centered cubic metals, and which has presently been assumed for the present materials within fatigue crack initiation models. Rather, the facets tend to have near-Mode I spatial orientation, which is another indicator of the importance of environmentally affected fatigue damage. The results provide a physical basis to develop microstructurally-based next generation multi-stage fatigue (MSF) models that should include a new crack decohesion criteria based upon environmental fatigue cracking mechanisms. EBSD study of small-cracks in alloy 7050-T7451, stressed in warm-humid environment, showed that crack-path orientation changes and crack-branching occurred at both low/high-angle grain and subgrain boundaries. Single surface trace analysis suggests that the crack-path differs substantially from crystallographic slip-planes. EBSD-based observations of small-crack propagation through subgrain structure, either formed by cyclic plastic strain accumulation or pre-existing (typical of unrecrystallized grain structure in the present materials), suggest that subgrain structure plays a crucial role in small fatigue crack propagation. As mentioned earlier, local fluctuations in small-crack growth rates appear to be caused by frequent interaction with subgrain boundaries, and multiple occurrences of crack-branching and crack-path orientation changes at low/high-angle grain and subgrain boundaries. The aforementioned deviation from low-index {001}/{101}-planes and the occurrence of high-index cracking planes observed by EBSD/Stereology, in this study and others, are interpreted as trans-subgranular decohesion or inter-subgranular cracking, due to trapped hydrogen. In summary, the results provide a firmer experimental foundation for, and clearer understanding of, the mechanisms of environmental fatigue cracking of aluminum alloys, especially the role of inter-subgranular cracking, which had previously been advanced based upon fracture surface observations alone.

Rayleigh wave acoustic emission during crack propagation in steel

NASA Astrophysics Data System (ADS)

Horne, Michael R.

2003-07-01

An investigation was conducted of the existence of seismic surface pulses (SSP) on crack faces in near-failure fatigue. An SSP has components of various modes of wave propagation. The component with the largest amplitude is a Rayleigh surface wave pulse. The possibility that these surface modes have much higher amplitudes than bulk modes of acoustic emission (AE) was illustrated by an idealized thought experiment relating an SSP on a half-space to the response of crack faces to crack extension. A number of aspects of AE monitoring in finite objects were investigated. Attributes of surface wave propagation on the edge of a specimen were found to be easier to monitor than other modes of wave propagation. Wavelet analysis was used to compare the characteristics of brittle AE with other sources. A new testing paradigm was developed to reduce interference from secondary sources of AE and enhance the investigation of AE from critical crack behavior. Unique specimen design features were developed, data acquisition features sought and validated, a dead weight load frame was modified, and data analysis procedures were developed. Criteria based on velocity, frequency content, amplitude and shape were devised to determine if an AE event is an SSP. The tests were designed to mimic load conditions on structures such as bridges and hence investigate the difference between AE generated in field conditions and that of typical laboratory conditions. Varieties of steel, from very ductile to very brittle, were tested. It was concluded that plastic zone formation, considered a secondary source of AE, was found not to interfere with the SSP activity. The SSP was found experimentally to have 2-3 times the amplitude of the bulk wave AE. The lack of sufficient AE did not allow for determination of conclusive changes in the AE as the specimens approached failure. However, it was found that brittle crack extension in fatigue and ductile failure can produce wave propagation resembling the SSP.

Rayleigh wave acoustic emission during crack propagation in steel

NASA Astrophysics Data System (ADS)

Horne, Michael R.

An investigation was conducted of the existence of seismic surface pulses (SSP) on crack faces in near-failure fatigue. An SSP has components of various modes of wave propagation. The component with the largest amplitude is a Rayleigh surface wave pulse. The possibility that these surface modes have much higher amplitudes than bulk modes of acoustic emission (AE) was illustrated by an idealized thought experiment relating an SSP on a half-space to the response of crack faces to crack extension. A number of aspects of AE monitoring in finite objects were investigated. Attributes of surface wave propagation on the edge of a specimen were found to be easier to monitor than other modes of wave propagation. Wavelet analysis was used to compare the characteristics of brittle AE with other sources. A new testing paradigm was developed to reduce interference from secondary sources of AE and enhance the investigation of AE from critical crack behavior. Unique specimen design features were developed, data acquisition features sought and validated, a dead weight load frame was modified, and data analysis procedures were developed. Criteria based on velocity, frequency content, amplitude and shape were devised to determine if an AE event is an SSP. The tests were designed to mimic load conditions on structures such as bridges and hence investigate the difference between AE generated in field conditions and that of typical laboratory conditions. Varieties of steel, from very ductile to very brittle, were tested. It was concluded that plastic zone formation, considered a secondary source of AE, was found not to interfere with the SSP activity. The SSP was found experimentally to have 2-3 times the amplitude of the bulk wave AE. The lack of sufficient AE did not allow for determination of conclusive changes in the AE as the specimens approached failure. However, it was found that brittle crack extension in fatigue and ductile failure can produce wave propagation resembling the SSP.

A Three-Stage Mechanistic Model for Solidification Cracking During Welding of Steel

NASA Astrophysics Data System (ADS)

Aucott, L.; Huang, D.; Dong, H. B.; Wen, S. W.; Marsden, J.; Rack, A.; Cocks, A. C. F.

2018-03-01

A three-stage mechanistic model for solidification cracking during TIG welding of steel is proposed from in situ synchrotron X-ray imaging of solidification cracking and subsequent analysis of fracture surfaces. Stage 1—Nucleation of inter-granular hot cracks: cracks nucleate inter-granularly in sub-surface where maximum volumetric strain is localized and volume fraction of liquid is less than 0.1; the crack nuclei occur at solute-enriched liquid pockets which remain trapped in increasingly impermeable semi-solid skeleton. Stage 2—Coalescence of cracks via inter-granular fracture: as the applied strain increases, cracks coalesce through inter-granular fracture; the coalescence path is preferential to the direction of the heat source and propagates through the grain boundaries to solidifying dendrites. Stage 3—Propagation through inter-dendritic hot tearing: inter-dendritic hot tearing occurs along the boundaries between solidifying columnar dendrites with higher liquid fraction. It is recommended that future solidification cracking criterion shall be based on the application of multiphase mechanics and fracture mechanics to the failure of semi-solid materials.

Laser cutting sandwich structure glass-silicon-glass wafer with laser induced thermal-crack propagation

NASA Astrophysics Data System (ADS)

Cai, Yecheng; Wang, Maolu; Zhang, Hongzhi; Yang, Lijun; Fu, Xihong; Wang, Yang

2017-08-01

Silicon-glass devices are widely used in IC industry, MEMS and solar energy system because of their reliability and simplicity of the manufacturing process. With the trend toward the wafer level chip scale package (WLCSP) technology, the suitable dicing method of silicon-glass bonded structure wafer has become necessary. In this paper, a combined experimental and computational approach is undertaken to investigate the feasibility of cutting the sandwich structure glass-silicon-glass (SGS) wafer with laser induced thermal-crack propagation (LITP) method. A 1064 nm semiconductor laser cutting system with double laser beams which could simultaneously irradiate on the top and bottom of the sandwich structure wafer has been designed. A mathematical model for describing the physical process of the interaction between laser and SGS wafer, which consists of two surface heating sources and two volumetric heating sources, has been established. The temperature stress distribution are simulated by using finite element method (FEM) analysis software ABAQUS. The crack propagation process is analyzed by using the J-integral method. In the FEM model, a stationary planar crack is embedded in the wafer and the J-integral values around the crack front edge are determined using the FEM. A verification experiment under typical parameters is conducted and the crack propagation profile on the fracture surface is examined by the optical microscope and explained from the stress distribution and J-integral value.

Structures associated with strike-slip faults that bound landslide elements

USGS Publications Warehouse

Fleming, R.W.; Johnson, A.M.

1989-01-01

Large landslides are bounded on their flanks and on elements within the landslides by structures analogous to strike-slip faults. We observed the formation of thwse strike-slip faults and associated structures at two large landslides in central Utah during 1983-1985. The strike-slip faults in landslides are nearly vertical but locally may dip a few degrees toward or away from the moving ground. Fault surfaces are slickensided, and striations are subparallel to the ground surface. Displacement along strike-slip faults commonly produces scarps; scarps occur where local relief of the failure surface or ground surface is displaced and becomes adjacent to higher or lower ground, or where the landslide is thickening or thinning as a result of internal deformation. Several types of structures are formed at the ground surface as a strike-slip fault, which is fully developed at some depth below the ground surface, propagates upward in response to displacement. The simplest structure is a tension crack oriented at 45?? clockwise or counterclockwise from the trend of an underlying right- or left-lateral strike-slip fault, respectively. The tension cracks are typically arranged en echelon with the row of cracks parallel to the trace of the underlying strike-slip fault. Another common structure that forms above a developing strike-slip fault is a fault segment. Fault segments are discontinuous strike-slip faults that contain the same sense of slip but are turned clockwise or counterclockwise from a few to perhaps 20?? from the underlying strike-slip fault. The fault segments are slickensided and striated a few centimeters below the ground surface; continued displacement of the landslide causes the fault segments to open and a short tension crack propagates out of one or both ends of the fault segments. These structures, open fault segments containing a short tension crack, are termed compound cracks; and the short tension crack that propagates from the tip of the fault segment is typically oriented 45?? to the trend of the underlying fault. Fault segments are also typically arranged en echelon above the upward-propagating strike-slip fault. Continued displacement of the landslide causes the ground to buckle between the tension crack portions of the compound cracks. Still more displacement produces a thrust fault on one or both limbs of the buckle fold. These compressional structures form at right angles to the short tension cracks at the tips of the fault segments. Thus, the compressional structures are bounded on their ends by one face of a tension crack and detached from underlying material by thrusting or buckling. The tension cracks, fault segments, compound cracks, folds, and thrusts are ephemeral; they are created and destroyed with continuing displacement of the landslide. Ultimately, the structures are replaced by a throughgoing strike-slip fault. At one landslide, we observed the creation and destruction of the ephemeral structures as the landslide enlarged. Displacement of a few centimeters to about a decimeter was sufficient to produce scattered tension cracks and fault segments. Sets of compound cracks with associated folds and thrusts were produced by displacements of up to 1 m, and 1 to 2 m of displacement was required to produce a throughgoing strike-slip fault. The type of first-formed structure above an upward-propagating strike-slip fault is apparently controlled by the rheology of the material. Brittle material such as dry topsoil or the compact surface of a gravel road produces echelon tension cracks and sets of tension cracks and compressional structures, wherein the cracks and compressional structures are normal to each other and 45?? to the strike-slip fault at depth. First-formed structures in more ductile material such as moist cohesive soil are fault segments. In very ductile material such as soft clay and very wet soil in swampy areas, the first-formed structure is a throughgoing strike-slip fault. There are othe

Low temperature spalling of silicon: A crack propagation study

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

Bertoni, Mariana; Uberg Naerland, Tine; Stoddard, Nathan

2017-06-08

Spalling is a promising kerfless method for cutting thin silicon wafers while doubling the yield of a silicon ingot. The main obstacle in this technology is the high total thickness variation of the spalled wafers, often as high as 100% of the wafer thickness. It has been suggested before that a strong correlation exists between low crack velocities and a smooth surface, but this correlation has never been shown during a spalling process in silicon. The reason lies in the challenge associated to measuring such velocities. In this contribution, we present a new approach to assess, in real time, themore » crack velocity as it propagates during a low temperature spalling process. Understanding the relationship between crack velocity and surface roughness during spalling can pave the way to attain full control on the surface quality of the spalled wafer.« less

High-Cycle, Push-Pull Fatigue Fracture Behavior of High-C, Si-Al-Rich Nanostructured Bainite Steel.

PubMed

Zhao, Jing; Ji, Honghong; Wang, Tiansheng

2017-12-29

The high-cycle, push-pull fatigue fracture behavior of high-C, Si-Al-rich nanostructured bainitic steel was studied through the measurement of fatigue limits, a morphology examination and phase composition analysis of the fatigue fracture surface, as well as fractography of the fatigue crack propagation. The results demonstrated that the push-pull fatigue limits at 10⁷ cycles were estimated as 710-889 MPa, for the samples isothermally transformed at the temperature range of 220-260 °C through data extrapolation, measured under the maximum cycle number of 10⁵. Both the interior inclusion and the sample surface constituted the fatigue crack origins. During the fatigue crack propagation, a high amount of secondary cracks were formed in almost parallel arrangements. The apparent plastic deformation occurred in the fracture surface layer, which induced approximately all retained austenite to transform into martensite.

High-Cycle, Push–Pull Fatigue Fracture Behavior of High-C, Si–Al-Rich Nanostructured Bainite Steel

PubMed Central

Zhao, Jing; Ji, Honghong

2017-01-01

The high-cycle, push–pull fatigue fracture behavior of high-C, Si–Al-rich nanostructured bainitic steel was studied through the measurement of fatigue limits, a morphology examination and phase composition analysis of the fatigue fracture surface, as well as fractography of the fatigue crack propagation. The results demonstrated that the push–pull fatigue limits at 107 cycles were estimated as 710–889 MPa, for the samples isothermally transformed at the temperature range of 220–260 °C through data extrapolation, measured under the maximum cycle number of 105. Both the interior inclusion and the sample surface constituted the fatigue crack origins. During the fatigue crack propagation, a high amount of secondary cracks were formed in almost parallel arrangements. The apparent plastic deformation occurred in the fracture surface layer, which induced approximately all retained austenite to transform into martensite. PMID:29286325

Effects of Defects in Laser Additive Manufactured Ti-6Al-4V on Fatigue Properties

NASA Astrophysics Data System (ADS)

Wycisk, Eric; Solbach, Andreas; Siddique, Shafaqat; Herzog, Dirk; Walther, Frank; Emmelmann, Claus

Laser Additive Manufacturing (LAM) enables economical production of complex lightweight structures as well as patient individual implants. Due to these possibilities the additive manufacturing technology gains increasing importance in the aircraft and the medical industry. Yet these industries obtain high quality standards and demand predictability of material properties for static and dynamic load cases. However, especially fatigue and crack propagation properties are not sufficiently determined. Therefore this paper presents an analysis and simulation of crack propagation behavior considering Laser Additive Manufacturing specific defects, such as porosity and surface roughness. For the mechanical characterization of laser additive manufactured titanium alloy Ti-6Al-4V, crack propagation rates are experimentally determined and used for an analytical modeling and simulation of fatigue. Using experimental results from HCF tests and simulated data, the fatigue and crack resistance performance is analyzed considering material specific defects and surface roughness. The accumulated results enable the reliable prediction of the defects influence on fatigue life of laser additive manufactured titanium components.

Microstructure-fatigue crack propagation relationship in TiB{sub 2} particulate reinforced Zn (ZA-8) alloy

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

Hua, W.; Zhang, J.; Wang, Z.

1995-10-01

The relationship between microstructure and propagation behavior of fatigue crack in TiB{sub 2} particulate reinforced ZA-8 Zn alloy and in the corresponding constituent matrix material was studied in three point bending fatigue tests with well-polished and pre-etched specimens. Special attention was paid to the observation of microstructure along the crack path as well as on the fracture surface. Mechanism for the difference in fatigue crack growth behavior of the two materials was investigated. The present results indicate that the addition of reinforcement modified the solidification process of the matrix material leading to a considerable change in the matrix microstructure. Thismore » change in the matrix microstructure and the presence of reinforcing particles considerably affected the fatigue crack propagation behavior in the material.« less

Environmental fatigue of an Al-Li-Cu alloy. Part 3: Modeling of crack tip hydrogen damage

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1992-01-01

Environmental fatigue crack propagation rates and microscopic damage modes in Al-Li-Cu alloy 2090 (Parts 1 and 2) are described by a crack tip process zone model based on hydrogen embrittlement. Da/dN sub ENV equates to discontinuous crack advance over a distance, delta a, determined by dislocation transport of dissolved hydrogen at plastic strains above a critical value; and to the number of load cycles, delta N, required to hydrogenate process zone trap sites that fracture according to a local hydrogen concentration-tensile stress criterion. Transgranular (100) cracking occurs for process zones smaller than the subgrain size, and due to lattice decohesion or hydride formation. Intersubgranular cracking dominates when the process zone encompasses one or more subgrains so that dislocation transport provides hydrogen to strong boundary trapping sites. Multi-sloped log da/dN-log delta K behavior is produced by process zone plastic strain-hydrogen-microstructure interactions, and is determined by the DK dependent rates and proportions of each parallel cracking mode. Absolute values of the exponents and the preexponential coefficients are not predictable; however, fractographic measurements theta sub i coupled with fatigue crack propagation data for alloy 2090 established that the process zone model correctly describes fatigue crack propagation kinetics. Crack surface films hinder hydrogen uptake and reduce da/dN and alter the proportions of each fatigue crack propagation mode.

Fractal characterization of fracture surfaces in concrete

USGS Publications Warehouse

Saouma, V.E.; Barton, C.C.; Gamaleldin, N.A.

1990-01-01

Fractal geometry is used to characterize the roughness of cracked concrete surfaces through a specially built profilometer, and the fractal dimension is subsequently correlated to the fracture toughness and direction of crack propagation. Preliminary results indicate that the fracture surface is indeed fractal over two orders of magnitudes with a dimension of approximately 1.20. ?? 1990.

Effect of Chamber Pressurization Rate on Combustion and Propagation of Solid Propellant Cracks

NASA Astrophysics Data System (ADS)

Yuan, Wei-Lan; Wei, Shen; Yuan, Shu-Shen

2002-01-01

area of the propellant grain satisfies the designed value. But cracks in propellant grain can be generated during manufacture, storage, handing and so on. The cracks can provide additional surface area for combustion. The additional combustion may significantly deviate the performance of the rocket motor from the designed conditions, even lead to explosive catastrophe. Therefore a thorough study on the combustion, propagation and fracture of solid propellant cracks must be conducted. This paper takes an isolated propellant crack as the object and studies the effect of chamber pressurization rate on the combustion, propagation and fracture of the crack by experiment and theoretical calculation. deformable, the burning inside a solid propellant crack is a coupling of solid mechanics and combustion dynamics. In this paper, a theoretical model describing the combustion, propagation and fracture of the crack was formulated and solved numerically. The interaction of structural deformation and combustion process was included in the theoretical model. The conservation equations for compressible fluid flow, the equation of state for perfect gas, the heat conducting equation for the solid-phase, constitutive equation for propellant, J-integral fracture criterion and so on are used in the model. The convective burning inside the crack and the propagation and fracture of the crack were numerically studied by solving the set of nonlinear, inhomogeneous gas-phase governing equations and solid-phase equations. On the other hand, the combustion experiments for propellant specimens with a precut crack were conducted by RTR system. Predicted results are in good agreement with experimental data, which validates the reasonableness of the theoretical model. Both theoretical and experimental results indicate that the chamber pressurization rate has strong effects on the convective burning in the crack, crack fracture initiation and fracture pattern.

Fatigue crack propagation in self-assembling nanocomposites

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

Klingler, Andreas; Wetzel, Bernd

Self-assembling block-copolymers allow the easy manufacturing of nanocomposites due to the thermodynamically driven in situ formation of nanosized phases in thermosetting resins during the curing process. Complex mechanical dispersion processes can be avoided. The current study investigates the effect of a block-copolymer on the fatigue crack propagation resistance of a cycloaliphatic amine cured epoxy resin. It was found that a small amount of MAM triblock-copolymer significantly increases the resistance to fatigue crack propagation of epoxy. Crack growth rate and the Paris law exponent for fatigue-crack growth were considerably reduced from m=15.5 of the neat epoxy to m=8.1 of the nanocomposite.more » To identify the related reinforcing and fracture mechanisms structural analyses of the fractured surfaces were performed by scanning electron microscope. Characteristic features were identified to be deformation, debonding and fracture of the nano-phases as well as crack pinning. However, the highest resistance against fatigue crack propagation was achieved in a bi-continuous microstructure that consisted of an epoxy-rich phase with embedded submicron sized MAM inclusions, and which was surrounded by a block-copolymer-rich phase that showed rupture and plastic deformation.« less

Improving the Fatigue Crack Propagation Resistance and Damage Tolerance of 2524-T3 Alloy with Amorphous Electroless Ni-P Coating

NASA Astrophysics Data System (ADS)

Chen, Lai; Zeng, Diping; Liu, Zhiyi; Bai, Song; Li, Junlin

2018-02-01

The surface microhardness, as well as the fatigue crack propagation (FCP) resistance of 2524-T3 alloy, is improved by producing a 20-μm-thick amorphous electroless Ni-12% P coating on its surface. Compared to the substrate, this deposited EN coating possesses higher strength properties and exhibits a greater ability of accommodating the plastic deformation at the fatigue crack tip, thereby remarkably improving the FCP resistance in near-threshold and early Paris regimes. Regardless of the similar FCP rates in Paris regime (Δ K ≥ 16.2 MPa m0.5), the coated sample exhibits extended Paris regime and enhanced damage tolerance.

Surface crack problems in plates

NASA Technical Reports Server (NTRS)

Joseph, P. F.; Erdogan, F.

1989-01-01

The mode I crack problem in plates under membrane loading and bending is reconsidered. The purpose is to examine certain analytical features of the problem further and to provide some new results. The formulation and the results given by the classical and the Reissner plate theories for through and part-through cracks are compared. For surface cracks the three-dimensional finite element solution is used as the basis of comparison. The solution is obtained and results are given for the crack/contact problem in a plate with a through crack under pure bending and for the crack interaction problem. Also, a procedure is developed to treat the problem of subcritical crack growth and to trace the evolution of the propagating crack.

A method to generate conformal finite-element meshes from 3D measurements of microstructurally small fatigue-crack propagation [A method to generate conformal finite-element meshes from 3D measurements of microstructurally small fatigue-crack propagation: 3D Meshes of Microstructurally Small Crack Growth

DOE PAGES

Spear, Ashley D.; Hochhalter, Jacob D.; Cerrone, Albert R.; ...

2016-04-27

In an effort to reproduce computationally the observed evolution of microstructurally small fatigue cracks (MSFCs), a method is presented for generating conformal, finite-element (FE), volume meshes from 3D measurements of MSFC propagation. The resulting volume meshes contain traction-free surfaces that conform to incrementally measured 3D crack shapes. Grain morphologies measured using near-field high-energy X-ray diffraction microscopy are also represented within the FE volume meshes. Proof-of-concept simulations are performed to demonstrate the utility of the mesh-generation method. The proof-of-concept simulations employ a crystal-plasticity constitutive model and are performed using the conformal FE meshes corresponding to successive crack-growth increments. Although the simulationsmore » for each crack increment are currently independent of one another, they need not be, and transfer of material-state information among successive crack-increment meshes is discussed. The mesh-generation method was developed using post-mortem measurements, yet it is general enough that it can be applied to in-situ measurements of 3D MSFC propagation.« less

A method to generate conformal finite-element meshes from 3D measurements of microstructurally small fatigue-crack propagation [A method to generate conformal finite-element meshes from 3D measurements of microstructurally small fatigue-crack propagation: 3D Meshes of Microstructurally Small Crack Growth

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

Spear, Ashley D.; Hochhalter, Jacob D.; Cerrone, Albert R.

In an effort to reproduce computationally the observed evolution of microstructurally small fatigue cracks (MSFCs), a method is presented for generating conformal, finite-element (FE), volume meshes from 3D measurements of MSFC propagation. The resulting volume meshes contain traction-free surfaces that conform to incrementally measured 3D crack shapes. Grain morphologies measured using near-field high-energy X-ray diffraction microscopy are also represented within the FE volume meshes. Proof-of-concept simulations are performed to demonstrate the utility of the mesh-generation method. The proof-of-concept simulations employ a crystal-plasticity constitutive model and are performed using the conformal FE meshes corresponding to successive crack-growth increments. Although the simulationsmore » for each crack increment are currently independent of one another, they need not be, and transfer of material-state information among successive crack-increment meshes is discussed. The mesh-generation method was developed using post-mortem measurements, yet it is general enough that it can be applied to in-situ measurements of 3D MSFC propagation.« less

Thermally induced stresses in boulders on airless body surfaces, and implications for rock breakdown

NASA Astrophysics Data System (ADS)

Molaro, J. L.; Byrne, S.; Le, J.-L.

2017-09-01

This work investigates the macroscopic thermomechanical behavior of lunar boulders by modeling their response to diurnal thermal forcing. Our results reveal a bimodal, spatiotemporally-complex stress response. During sunrise, stresses occur in the boulders' interiors that are associated with large-scale temperature gradients developed due to overnight cooling. During sunset, stresses occur at the boulders' exteriors due to the cooling and contraction of the surface. Both kinds of stresses are on the order of 10 MPa in 1 m boulders and decrease for smaller diameters, suggesting that larger boulders break down more quickly. Boulders ≤ 30 cm exhibit a weak response to thermal forcing, suggesting a threshold below which crack propagation may not occur. Boulders of any size buried by regolith are shielded from thermal breakdown. As boulders increase in size (>1 m), stresses increase to several 10 s of MPa as the behavior of their surfaces approaches that of an infinite halfspace. As the thermal wave loses contact with the boulder interior, stresses become limited to the near-surface. This suggests that the survival time of a boulder is not only controlled by the amplitude of induced stress, but also by its diameter as compared to the diurnal skin depth. While stresses on the order of 10 MPa are enough to drive crack propagation in terrestrial environments, crack propagation rates in vacuum are not well constrained. We explore the relationship between boulder size, stress, and the direction of crack propagation, and discuss the implications for the relative breakdown rates and estimated lifetimes of boulders on airless body surfaces.

Cracking evolution behaviors of lightweight materials based on in situ synchrotron X-ray tomography: A review

NASA Astrophysics Data System (ADS)

Luo, Y.; Wu, S. C.; Hu, Y. N.; Fu, Y. N.

2018-03-01

Damage accumulation and failure behaviors are crucial concerns during the design and service of a critical component, leading researchers and engineers to thoroughly identifying the crack evolution. Third-generation synchrotron radiation X-ray computed microtomography can be used to detect the inner damage evolution of a large-density material or component. This paper provides a brief review of studying the crack initiation and propagation inside lightweight materials with advanced synchrotron three-dimensional (3D) X-ray imaging, such as aluminum materials. Various damage modes under both static and dynamic loading are elucidated for pure aluminum, aluminum alloy matrix, aluminum alloy metal matrix composite, and aluminum alloy welded joint. For aluminum alloy matrix, metallurgical defects (porosity, void, inclusion, precipitate, etc.) or artificial defects (notch, scratch, pit, etc.) strongly affect the crack initiation and propagation. For aluminum alloy metal matrix composites, the fracture occurs either from the particle debonding or voids at the particle/matrix interface, and the void evolution is closely related with fatigued cycles. For the hybrid laser welded aluminum alloy, fatigue cracks usually initiate from gas pores located at the surface or sub-surface and gradually propagate to a quarter ellipse or a typical semi-ellipse profile.

Further damage induced by water in micro-indentations in phosphate laser glass

NASA Astrophysics Data System (ADS)

Yu, Jiaxin; Jian, Qingyun; Yuan, Weifeng; Gu, Bin; Ji, Fang; Huang, Wen

2014-02-01

Using a microhardness tester, artificial flaws were made by micro-indentation in N31 Nd-doped phosphate laser glass. Indentation fracture toughness, KIC, was estimated as 0.45-0.53 MPa m1/2 from these indentations. The glasses with indentations were then immersed in ultrapure water to investigate further water-induced damage of these indentations. Stress-enhanced hydrolysis leads to the propagations of radial crack, lateral cracks and microcracks in the subsurface. These crack propagations therefore cause deformation in subsurface to form annular reflections regions around the indentations and further material collapse within imprints. After the residual stresses are exhausted, the leaching plays a more dominated role in glass corrosion in the further immersion. After immersion, the material structure slackens around micro-indentation, which decreases the contact stiffness and results in a lower nano-hardness. For the surface far away from flaws, water immersion presents a weak effect on the near-surface mechanical since the matrix leaching in phosphate glass restricts the formation of hydration layer. During first 20 min immersion, due to higher chemical activity and lower fracture toughness, the radial cracks show a faster propagation in phosphate glass compared with that in K9 silicate glass. For further immersion, crack healing occurs in silicate glass but not in phosphate glass. Analysis shows that the formation of hydration layer on crack walls plays an important role in crack healing in glasses.

Oxidation products of INCONEL alloys 600 and 690 in pressurized water reactor environments and their role in intergranular stress corrosion cracking

NASA Astrophysics Data System (ADS)

Ferguson, J. B.; Lopez, Hugo F.

2006-08-01

In this work, thermodynamic arguments for the stability of Ni and Cr compounds developed under pressurized water reactor environments ( P_{H_2 O} and P_{H_2 } ) were experimentally tested. A mechanism is proposed to explain crack initiation and propagation alloy 600 along the grain boundaries, where Cr2O3 has formed from the leaching of Cr from the matrix, leaving behind a porous Ni-rich region. The mechanism is based on the thermodynamic potential for the transformation of a protective NiO surface layer into an amorphous nonprotective Ni(OH)2 gel. This gel would also form along the grain boundaries and when hydrogenated steam reaches the porous Ni-rich regions. Crack initiation is then favored by tensile stressing of the grain boundary regions, which can easily rupture the gelatinous film. The leaching of matrix Cr to form nonprotective CrOOH gel at the crack tip followed by the exposure of fresh porous Ni to the environment could explain crack propagation in INCONEL alloy 600. The proposed crack initiation mechanism is not expected to occur in alloy 690 where a protective Cr2O3 film covers the entire metal surface. However, crack propagation along the grain boundaries in alloy 600 and precracked alloy 690 is expected to be active as hydroxide-forming reactions weaken the boundaries.

Detection and monitoring of shear crack growth using S-P conversion of seismic waves

NASA Astrophysics Data System (ADS)

Modiriasari, A.; Bobet, A.; Pyrak-Nolte, L. J.

2017-12-01

A diagnostic method for monitoring shear crack initiation, propagation, and coalescence in rock is key for the detection of major rupture events, such as slip along a fault. Active ultrasonic monitoring was used in this study to determine the precursory signatures to shear crack initiation in pre-cracked rock. Prismatic specimens of Indiana limestone (203x2101x638x1 mm) with two pre-existing parallel flaws were subjected to uniaxial compression. The flaws were cut through the thickness of the specimen using a scroll saw. The length of the flaws was 19.05 mm and had an inclination angle with respect to the loading direction of 30o. Shear wave transducers were placed on each side of the specimen, with polarization parallel to the loading direction. The shear waves, given the geometry of the flaws, were normally incident to the shear crack forming between the two flaws during loading. Shear crack initiation and propagation was detected on the specimen surface using digital image correlation (DIC), while initiation inside the rock was monitored by measuring full waveforms of the transmitted and reflected shear (S) waves across the specimen. Prior to the detection of a shear crack on the specimen surface using DIC, transmitted S waves were converted to compressional (P) waves. The emergence of converted S-P wave occurs because of the presence of oriented microcracks inside the rock. The microcracks coalesce and form the shear crack observed on the specimen surface. Up to crack coalescence, the amplitude of the converted waves increased with shear crack propagation. However, the amplitude of the transmitted shear waves between the two flaws did not change with shear crack initiation and propagation. This is in agreement with the conversion of elastic waves (P- to S-wave or S- to P-wave) observed by Nakagawa et al., (2000) for normal incident waves. Elastic wave conversions are attributed to the formation of an array of oriented microcracks that dilate under shear stress, which causes energy partitioning into P, S, and P-to-S or S-to-P waves. This finding provides a diagnostic method for detecting shear crack initiation and growth using seismic wave conversions. Acknowledgments: This material is based upon work supported by the National Science Foundation, Geomechanics and Geotechnical Systems Program (award No. CMMI-1162082).

Practical Applications of Fracture Mechanics

DTIC Science & Technology

1980-05-01

Department British AerospaceRoyal Aircraft Establishment Manchester DivisioFarnborough, Hampshire United Kingdom n U nited K ingdom Anit e r Kin1o Francis...Bulkhead (Grange) 385 (6) Crack Propagation Analysis of Flat Stiffened Pa,"is (Schwarmann) 3-88 (7) Crack Propagation Analysis of Pressurized Fusel:. e ...modeling basis for microcrack growth. Major issues as to the applicability of fracture mechanic. e ,"thodoloqy, surface retardation due to machining

Damage Mechanisms and Controlled Crack Propagation in a Hot Pressed Silicon Nitride Ceramic. Ph.D. Thesis - Northwestern Univ., 1993

NASA Technical Reports Server (NTRS)

Calomino, Anthony Martin

1994-01-01

The subcritical growth of cracks from pre-existing flaws in ceramics can severely affect the structural reliability of a material. The ability to directly observe subcritical crack growth and rigorously analyze its influence on fracture behavior is important for an accurate assessment of material performance. A Mode I fracture specimen and loading method has been developed which permits the observation of stable, subcritical crack extension in monolithic and toughened ceramics. The test specimen and procedure has demonstrated its ability to generate and stably propagate sharp, through-thickness cracks in brittle high modulus materials. Crack growth for an aluminum oxide ceramic was observed to be continuously stable throughout testing. Conversely, the fracture behavior of a silicon nitride ceramic exhibited crack growth as a series of subcritical extensions which are interrupted by dynamic propagation. Dynamic initiation and arrest fracture resistance measurements for the silicon nitride averaged 67 and 48 J/sq m, respectively. The dynamic initiation event was observed to be sudden and explosive. Increments of subcritical crack growth contributed to a 40 percent increase in fracture resistance before dynamic initiation. Subcritical crack growth visibly marked the fracture surface with an increase in surface roughness. Increments of subcritical crack growth loosen ceramic material near the fracture surface and the fracture debris is easily removed by a replication technique. Fracture debris is viewed as evidence that both crack bridging and subsurface microcracking may be some of the mechanisms contributing to the increase in fracture resistance. A Statistical Fracture Mechanics model specifically developed to address subcritical crack growth and fracture reliability is used together with a damaged zone of material at the crack tip to model experimental results. A Monte Carlo simulation of the actual experiments was used to establish a set of modeling input parameters. It was demonstrated that a single critical parameter does not characterize the conditions required for dynamic initiation. Experimental measurements for critical crack lengths, and the energy release rates exhibit significant scatter. The resulting output of the model produces good agreement with both the average values and scatter of experimental measurements.

Application of in situ thermography for evaluating the high-cycle and very high-cycle fatigue behaviour of cast aluminium alloy AlSi7Mg (T6).

PubMed

Krewerth, D; Weidner, A; Biermann, H

2013-12-01

The present paper illustrates the application of infrared thermal measurements for the investigation of crack initiation point and crack propagation in the high-cycle and the very high-cycle fatigue range of cast AlSi7Mg alloy (A356). The influence of casting defects, their location, size and amount was studied both by fractography and thermography. Besides internal and surface fatigue crack initiation as a further crack initiation type multiple fatigue crack initiation was observed via in situ thermography which can be well correlated with the results from fractography obtained by SEM investigations. In addition, crack propagation was studied by the development of the temperature measured via thermography. Moreover, the frequency influence on high-cycle fatigue behaviour was investigated. The presented results demonstrate well that the combination of fractography and thermography can give a significant contribution to the knowledge of crack initiation and propagation in the VHCF regime. Copyright © 2013 Elsevier B.V. All rights reserved.

Failure investigations of failed valve plug SS410 steel due to cracking

NASA Astrophysics Data System (ADS)

Kalyankar, V. D.; Deshmukh, D. D.

2017-12-01

Premature and sudden in service failure of a valve plug due to crack formation, applied in power plant has been investigated. The plug was tempered and heat treated, the crack was originated at centre, developed along the axis and propagates radially towards outer surface of plug. The expected life of the component is 10-15 years while, the component had failed just after the installation that is, within 3 months of its service. No corrosion products were observed on the crack interface and on the failed surface; hence, causes of corrosion failure are neglected. This plug of level separator control valve, is welded to the stem by means of plasma-transferred arc welding and as there is no crack observed at the welding zone, the failure due to welding residual stresses are also neglected. The failed component discloses exposed surface of a crack interface that originated from centre and propagates radially. The micro-structural observation, hardness testing, and visual observation are carried out of the specimen prepared from the failed section and base portion. The microstructure from the cracked interface showed severe carbide formation along the grain boundaries. From the microstructural analysis of the failed sample, it is observed that there is a formation of acicular carbides along the grain boundaries due to improper tempering heat treatment.

The noncontinuum crack tip deformation behavior of surface microcracks

NASA Astrophysics Data System (ADS)

Morris, W. L.

1980-07-01

The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.

Role of multiple cusps in tooth fracture.

PubMed

Barani, Amir; Bush, Mark B; Lawn, Brian R

2014-07-01

The role of multiple cusps in the biomechanics of human molar tooth fracture is analysed. A model with four cusps at the bite surface replaces the single dome structure used in previous simulations. Extended finite element modelling, with provision to embed longitudinal cracks into the enamel walls, enables full analysis of crack propagation from initial extension to final failure. The cracks propagate longitudinally around the enamel side walls from starter cracks placed either at the top surface (radial cracks) or from the tooth base (margin cracks). A feature of the crack evolution is its stability, meaning that extension occurs steadily with increasing applied force. Predictions from the model are validated by comparison with experimental data from earlier publications, in which crack development was followed in situ during occlusal loading of extracted human molars. The results show substantial increase in critical forces to produce longitudinal fractures with number of cuspal contacts, indicating a capacity for an individual tooth to spread the load during mastication. It is argued that explicit critical force equations derived in previous studies remain valid, at the least as a means for comparing the capacity for teeth of different dimensions to sustain high bite forces. Copyright © 2014 Elsevier Ltd. All rights reserved.

Effects of Microstructure on Tensile, Charpy Impact, and Crack Tip Opening Displacement Properties of Two API X80 Pipeline Steels

NASA Astrophysics Data System (ADS)

Shin, Sang Yong

2013-06-01

The effects of microstructure on tensile, Charpy impact, and crack tip opening displacement (CTOD) properties of two API X80 pipeline steels were investigated in this study. Two API X80 pipeline steels consisting of acicular ferrite and granular bainite, and a small amount of hard phases such as martensite and secondary phases have elongated grains along the rolling direction, so that they show different mechanical properties as the specimens' directions change. The 90 deg specimens have high tensile strength due to the low stress concentration on the fine hard phases and the high loads for the deformation of the elongated grains. In contrast, the 30 deg specimens have less elongated grains and larger hard phases such as martensite, with the size of about 3 μm, than the 90 deg specimens. Hence, the 30 deg specimens have low tensile strength because of the high stress concentration on the large hard phases and the low loads to deform grains. In the 90 deg specimen, brittle crack propagation surfaces are even since cracks propagate in a straight line along the elongated grain structure. In the 30 deg specimen, however, brittle crack propagation surfaces are uneven, and secondary cracks are observed, because of the zigzag brittle crack propagation path. In the CTOD properties, the 90 deg specimens have maximum forces of higher magnitude than the 30 deg specimens, because of the elongated grain structure. However, CTODs of the 90 deg specimens are lower than those of the 30 deg specimens because of the low plastic deformation areas by the elongated grains in the 90 deg specimens.

Elastic-plastic analysis of a propagating crack under cyclic loading

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Armen, H., Jr.

1974-01-01

Development and application of a two-dimensional finite-element analysis to predict crack-closure and crack-opening stresses during specified histories of cyclic loading. An existing finite-element computer program which accounts for elastic-plastic material behavior under cyclic loading was modified to account for changing boundary conditions - crack growth and intermittent contact of crack surfaces. This program was subsequently used to study the crack-closure behavior under constant-amplitude and simple block-program loading.

Potential applications of paving fabrics to reduce reflective cracking.

DOT National Transportation Integrated Search

2005-02-01

Asphalt concrete overlay on the existing pavement is often used as a cost-saving : surface treatment for deteriorating pavements. A major problem encountered with asphalt : resurfacing is the phenomenon termed reflective cracking, the propagation of ...

Mode I and mixed I/III crack initiation and propagation behavior of V-4Cr-4Ti alloy at 25{degrees}C

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

Li, H.X.; Kurtz, R.J.; Jones, R.H.

1997-04-01

The mode I and mixed-mode I/III fracture behavior of the production-scale heat (No. 832665) of V-4Cr-4Ti has been investigated at 25{degrees}C using compact tension (CT) specimens for a mode I crack and modified CT specimens for a mixed-mode I/III crack. The mode III to mode I load ratio was 0.47. Test specimens were vacuum annealed at 1000{degrees}C for 1 h after final machining. Both mode I and mixed-mode I/III specimens were fatigue cracked prior to J-integral testing. It was noticed that the mixed-mode I/III crack angle decreased from an initial 25 degrees to approximately 23 degrees due to crack planemore » rotation during fatigue cracking. No crack plane rotation occurred in the mode I specimen. The crack initiation and propagation behavior was evaluated by generating J-R curves. Due to the high ductility of this alloy and the limited specimen thickness (6.35 mm), plane strain requirements were not met so valid critical J-integral values were not obtained. However, it was found that the crack initiation and propagation behavior was significantly different between the mode I and the mixed-mode I/III specimens. In the mode I specimen crack initiation did not occur, only extensive crack tip blunting due to plastic deformation. During J-integral testing the mixed-mode crack rotated to an increased crack angle (in contrast to fatigue precracking) by crack blunting. When the crack initiated, the crack angle was about 30 degrees. After crack initiation the crack plane remained at 30 degrees until the test was completed. Mixed-mode crack initiation was difficult, but propagation was easy. The fracture surface of the mixed-mode specimen was characterized by microvoid coalescence.« less

Oxidation of UC: An in situ high temperature environmental scanning electron microscopy study

NASA Astrophysics Data System (ADS)

Gasparrini, Claudia; Podor, Renaud; Horlait, Denis; Rushton, Michael J. D.; Fiquet, Olivier; Lee, William Edward

2017-10-01

In situ HT-ESEM oxidation of sintered UC fragments revealed the morphological changes occurring during the transformation between UC to UO2 and UO2 to U3O8 at 723-848 K and in an atmosphere of 10-100 Pa O2. Two main oxidation pathways were revealed. Oxidation at 723 K in atmospheres ≤25 Pa O2 showed the transformation from UC to UO2+x, as confirmed by post mortem HRTEM analysis. This oxidation pathway was comprised of three steps: (i) an induction period, where only surface UC particles oxidised, (ii) a sample area expansion accompanied by crack formation and propagation, (iii) a stabilisation of the total crack length inferring that crack propagation had stopped. Samples oxidised under 50 Pa O2 at 723 K and at 773-848 K for 10-100 Pa O2 showed an "explosive" oxidation pathway: (i) sample area expansion occurred as soon as oxygen was inserted into the chamber and crack propagation and crack length followed an exponential law; (ii) cracks propagated as a network and the oxide layer fragmented, (iii) an "explosion" occurred causing a popcorn-like transformation, typical for oxidation from UO2 to U3O8. HRTEM characterisation revealed U3O8 preferentially grow in the [001] direction. The explosive growth, triggered by ignition of UC, proceeded as a self-propagating high-temperature synthesis reaction, with a propagation speed of 150-500 ± 50 μm/s.

Micro-indentation fracture behavior of human enamel.

PubMed

Padmanabhan, Sanosh Kunjalukkal; Balakrishnan, Avinash; Chu, Min-Cheol; Kim, Taik Nam; Cho, Seong Jai

2010-01-01

The purpose of this study was to determine the crack resistance behavior (K(R)) of human enamel in relation to its microstructure. Human molar teeth were precision cut, polished and tested using Vickers micro-indentation at different loads ranging from 0.98 to 9.8 N. Five indentation load levels were considered, 20 indentation cracks for each load level were introduced on the surface of the test specimen (10 indentations per tooth) and their variability was evaluated using Weibull statistics and an empirical model. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the crack morphology and propagation mechanisms involved. The results showed that enamel exhibited increasing cracking resistance (K(R)) with increasing load. It was found that the crack propagation mainly depended on the location and the microstructure it encountered. SEM showed the formation of crack bridges and crack deflection near the indentation crack tip. The crack mode was of Palmqvist type even at larger loads of 9.8 N. This was mainly attributed to the large process zone created by the interwoven lamellar rod like microstructure exhibited by the enamel surface. This study shows that there are still considerable prospects for improving dental ceramics and for mimicking the enamel structure developed by nature.

On the response of dynamic cracks to increasing overload

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

Gumbsch, P.

1996-12-01

One of the most interesting questions in the dynamics of brittle fracture is how a running brittle crack responds to an overload, i.e., to a mechanical energy release rate larger than that due to the increase in surface energy of the two cleavage surfaces. To address this question, dynamically running cracks in different crystal lattices are modelled atomistically under the condition of constant energy release rate. Stable crack propagation as well as the onset of crack tip instabilities are studied. It will be shown that small overloads lead to stable crack propagation with steady state velocities which quickly reach themore » terminal velocity of about 0.4 of the Rayleigh wave speed upon increasing the overload. Further increasing the overload does not change the steady state velocity but significantly changes the energy dissipation process towards shock wave emission at the breaking of every single atomic bond. Eventually the perfectly brittle crack becomes unstable, which then leads to dislocation generation and to the production of cleavage steps. The onset of the instability as well as the terminal velocity are related to the non-linearity of the interatomic interaction.« less

Fracture mechanics of shear crack propagation and dissection in the healthy bovine descending aortic media.

PubMed

Haslach, Henry W; Siddiqui, Ahmed; Weerasooriya, Amanda; Nguyen, Ryan; Roshgadol, Jacob; Monforte, Noel; McMahon, Eileen

2018-03-01

This experimental study adopts a fracture mechanics strategy to investigate the mechanical cause of aortic dissection. Inflation of excised healthy bovine aortic rings with a cut longitudinal notch that extends into the media from the intima suggests that an intimal tear may propagate a nearly circumferential-longitudinal rupture surface that is similar to the delamination that occurs in aortic dissection. Radial and 45°-from-radial cut notch orientations, as seen in the thickness surface, produce similar circumferential crack propagation morphologies. Partial cut notches, whose longitudinal length is half the width of the ring, measure the influence of longitudinal material on crack propagation. Such specimens also produce circumferential cracks from the notch root that are visible in the thickness circumferential-radial plane, and often propagate a secondary crack from the base of the notch, visible in the intimal circumferential-longitudinal plane. Inflation of rings with pairs of cut notches demonstrates that a second notch modifies the propagation created in a specimen with a single notch. The circumferential crack propagation is likely a consequence of the laminar medial structure. These fracture surfaces are probably due to non-uniform circumferential shear deformation in the heterogeneous media as the aortic wall expands. The qualitative deformation morphology around the root of the cut notch during inflation is evidence for such shear deformation. The shear apparently results from relative slip in the circumferential direction of collagen fibers. The slip may produce shear in the longitudinal-circumferential plane between medial layers or in the radial-circumferential plane within a medial lamina in an idealized model. Circumferential crack propagation in the media is then a shear mechanical process that might be facilitated by disease of the tissue. An intimal tear of an apparently healthy aortic wall near the aortic arch is life-threatening because it may lead to full rupture or to wall dissection in which delamination of the medial layer extends around most of the aortic circumference. The mechanical events underlying dissection are not definitively established. This experimental fracture mechanics study provides evidence that shear rupture is the main mechanical process underlying aortic dissection. The commonly performed tensile strength tests of aortic tissue are not clinically useful to predict or describe aortic dissection. One implication of the study is that shear tests might produce more fruitful simple assessments of the aortic wall strength. A clinical implication is that when presented with an intimal tear, those who guide care might recommend steps to reduce the shear load on the aorta. Copyright © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Gear crack propagation investigations

NASA Technical Reports Server (NTRS)

Lewicki, David G.; Ballarini, Roberto

1996-01-01

Analytical and experimental studies were performed to investigate the effect of gear rim thickness on crack propagation life. The FRANC (FRacture ANalysis Code) computer program was used to simulate crack propagation. The FRANC program used principles of linear elastic fracture mechanics, finite element modeling, and a unique re-meshing scheme to determine crack tip stress distributions, estimate stress intensity factors, and model crack propagation. Various fatigue crack growth models were used to estimate crack propagation life based on the calculated stress intensity factors. Experimental tests were performed in a gear fatigue rig to validate predicted crack propagation results. Test gears were installed with special crack propagation gages in the tooth fillet region to measure bending fatigue crack growth. Good correlation between predicted and measured crack growth was achieved when the fatigue crack closure concept was introduced into the analysis. As the gear rim thickness decreased, the compressive cyclic stress in the gear tooth fillet region increased. This retarded crack growth and increased the number of crack propagation cycles to failure.

Formation of cycloidal features on Europa.

PubMed

Hoppa, G V; Tufts, B R; Greenberg, R; Geissler, P E

1999-09-17

Cycloidal patterns are widely distributed on the surface of Jupiter's moon Europa. Tensile cracks may have developed such a pattern in response to diurnal variations in tidal stress in Europa's outer ice shell. When the tensile strength of the ice is reached, a crack may occur. Propagating cracks would move across an ever-changing stress field, following a curving path to a place and time where the tensile stress was insufficient to continue the propagation. A few hours later, when the stress at the end of the crack again exceeded the strength, propagation would continue in a new direction. Thus, one arcuate segment of the cycloidal chain would be produced during each day on Europa. For this model to work, the tensile strength of Europa's ice crust must be less than 40 kilopascals, and there must be a thick fluid layer below the ice to allow sufficient tidal amplitude.

Dynamic fracture and hot-spot modeling in energetic composites

NASA Astrophysics Data System (ADS)

Grilli, Nicolò; Duarte, Camilo A.; Koslowski, Marisol

2018-02-01

Defects such as cracks, pores, and particle-matrix interface debonding affect the sensitivity of energetic materials by reducing the time-to-ignition and the threshold pressure to initiate an explosion. Frictional sliding of preexisting cracks is considered to be one of the most important causes of localized heating. Therefore, understanding the dynamic fracture of crystalline energetic materials is of extreme importance to assess the reliability and safety of polymer-bonded explosives. Phase field damage model simulations, based on the regularization of the crack surface as a diffuse delta function, are used to describe crack propagation in cyclotetramethylene-tetranitramine crystals embedded in a Sylgard matrix. A thermal transport model that includes heat generation by friction at crack interfaces is coupled to the solution of crack propagation. 2D and 3D dynamic compression simulations are performed with different boundary velocities and initial distributions of cracks and interface defects to understand their effect on crack propagation and heat generation. It is found that, at an impact velocity of 400 m/s, localized damage at the particle-binder interface is of key importance and that the sample reaches temperatures high enough to create a hot-spot that will lead to ignition. At an impact velocity of 10 m/s, preexisting cracks advanced inside the particle, but the increase of temperature will not cause ignition.

Phase field modeling of crack propagations in fluid-saturated porous media with anisotropic surface energy

NASA Astrophysics Data System (ADS)

Na, S.; Sun, W.; Yoon, H.; Choo, J.

2016-12-01

Directional mechanical properties of layered geomaterials such as shale are important on evaluating the onset and growth of fracture for engineering applications such as hydraulic fracturing, geologic carbon storage, and geothermal recovery. In this study, a continuum phase field modeling is conducted to demonstrate the initiation and pattern of cracks in fluid-saturated porous media. The discontinuity of sharp cracks is formulated using diffusive crack phase field modeling and the anisotropic surface energy is incorporated to account for the directional fracture toughness. In particular, the orientation of bedding in geomaterials with respect to the loading direction is represented by the directional critical energy release rate. Interactions between solid skeleton and fluid are also included to analyze the mechanical behavior of fluid-saturated geologic materials through the coupled hydro-mechanical model. Based on the linear elastic phase field modeling, we also addressed how the plasticity in crack phase field influences the crack patterns by adopting the elasto-plastic model with Drucker-Prager yield criterion. Numerical examples exhibit the features of anisotropic surface energy, the interactions between solid and fluid and the effects of plasticity on crack propagations.Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Investigation of Thermal High Cycle and Low Cycle Fatigue Mechanisms of Thick Thermal Barrier Coatings

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Miller, Robert A.

1998-01-01

Thick thermal barrier coating systems in a diesel engine experience severe thermal low cycle fatigue (LCF) and high cycle fatigue (HCF) during engine operation. In this paper, the mechanisms of fatigue crack initiation and propagation in a ZrO2-8wt.% Y2O3 thermal barrier coating, under simulated engine thermal LCF and HCF conditions, are investigated using a high power CO2 laser. Experiments showed that the combined LCF/HCF tests induced more severe coating surface cracking, microspallation and accelerated crack growth, as compared to the pure LCF test. Lateral crack branching and the ceramic/bond coat interface delaminations were also facilitated by HCF thermal loads, even in the absence of severe interfacial oxidation. Fatigue damages at crack wake surfaces, due to such phenomena as asperity/debris contact induced cracking and splat pull-out bending during cycling, were observed especially for the combined LCF/HCF tests. It is found that the failure associated with LCF is closely related to coating sintering and creep at high temperatures, which induce tensile stresses in the coating after cooling. The failure associated with HCF process, however, is mainly associated with a surface wedging mechanism. The interaction between the LCF, HCF and ceramic coating creep, and the relative importance of LCF and HCF in crack propagation are also discussed based on the experimental evidence.

Distribution of Inclusion-Initiated Fatigue Cracking in Powder Metallurgy Udimet 720 Characterized

NASA Technical Reports Server (NTRS)

Bonacuse, Peter J.; Kantzos, Pete T.; Barrie, Robert; Telesman, Jack; Ghosn, Louis J.; Gabb, Timothy P.

2004-01-01

In the absence of extrinsic surface damage, the fatigue life of metals is often dictated by the distribution of intrinsic defects. In powder metallurgy (PM) alloys, relatively large defects occur rarely enough that a typical characterization with a limited number of small volume fatigue test specimens will not adequately sample inclusion-initiated damage. Counterintuitively, inclusion-initiated failure has a greater impact on the distribution in PM alloy fatigue lives because they tend to have fewer defects than their cast and wrought counterparts. Although the relative paucity of defects in PM alloys leads to higher mean fatigue lives, the distribution in observed lives tends to be broader. In order to study this important failure initiation mechanism without expending an inordinate number of specimens, a study was undertaken at the NASA Glenn Research Center where known populations of artificial inclusions (seeds) were introduced to production powder. Fatigue specimens were machined from forgings produced from the seeded powder. Considerable effort has been expended in characterizing the crack growth rate from inclusion-initiated cracks in seeded PM alloys. A rotating and translating positioning system, with associated software, was devised to map the surface inclusions in low-cycle fatigue (LCF) test bars and to monitor the crack growth from these inclusions. The preceding graph illustrates the measured extension in fatigue cracks from inclusions on a seeded LCF test bar subjected to cyclic loading at a strain range of 0.8 percent and a strain ratio (max/min) of zero. Notice that the observed inclusions fall into three categories: some do not propagate at all (arrest), some propagate with a decreasing crack growth rate, and a few propagate at increasing rates that can be modeled by fracture mechanics. The following graph shows the measured inclusion-initiated crack growth rates from 10 interrupted LCF tests plotted against stress intensities calculated for semi-elliptical cracks with the observed surface lengths. The expected scatter in the crack growth rates for stress intensity ranges near threshold is observed. These data will be used to help determine the distribution in growth rates of cracks emanating from inclusions as well as the proportion of cracks that arrest under various loading conditions.

Use of electrochemical potential noise to detect initiation and propagation of stress corrosion cracks in a 17-4 PH steel

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

Gonzalez-Rodriguez, J.G.; Salinas-Bravo, V.M.; Garcia-Ochoa, E.

1997-09-01

Corrosion potential transients were associated with nucleation and propagation of stress corrosion cracks in a 17-4 precipitation-hardenable (PH) martensitic stainless steel (SS) during slow strain rate tests (SSRT) at 90 C in deaerated sodium chloride (NaCl) solutions, Test solutions included 20 wt% NaCl at pH 3 and 7, similar to normal and faulted steam turbine environments, respectively. Time series were analyzed using the fast Fourier transform method. At the beginning of straining, the consistent noise behavior was perturbed with small potential transients, probably associated with rupture of the surface oxide layer. After yielding, these transients increased in intensity. At maximummore » load, the transients were still higher in intensity and frequency. These potential transients were related to crack nucleation and propagation. When the steel did not fail by stress corrosion cracking (SCC), such transients were found only at the beginning of the test. The power spectra showed some differences in all cases in roll-off slope and voltage magnitude, but these were not reliable tools to monitor the initiation and propagation of stress corrosion cracks.« less

Numerical simulation of evaluation of surface breaking cracks by array-lasers generated narrow-band SAW

NASA Astrophysics Data System (ADS)

Dong, Li-Ming; Ni, Chen-Yin; Shen, Zhong-Hua; Ni, Xiao-Wu

2011-09-01

Most of the factors limiting the extensive application of laser-based ultrasonic for nondestructive evaluation of surface breaking crack are its poor sensitivity, low efficiency relative to conventional contact ultrasonic methods and limit on the dimension of the cracks. For this reason, a new technique that multiplepulse narrow-band ultrasound generated by laser arrays has been proposed. It is found that crack detection dependent on spectrum of narrow-band ultrasound generated by laser arrays can be operated with low amplitude requirements. In this paper, the narrow-band ultrasound generated by pulse laser arrays interacting with surface breaking cracks has been simulated in detail by the finite element method (FEM) according to the thermoelastic theory. The pulsed array lasers were assumed to be transient heat source, and the surface acoustic wave (SAW) which propagating on the top of the plate was computed based on thermoelastic theory. Then the frequency spectrums of both reflected waves by crack and transmission ones through crack were compared with the direct waves. Results demonstrate that multiple-frequency components of the narrow-band ultrasound were varied with change of the depth of surface breaking cracks significantly, which provides the possibility for precise evaluation of surface breaking cracks.

The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

NASA Technical Reports Server (NTRS)

Piascik, Robert S.

2015-01-01

The corrosion fatigue crack growth characteristics of small (greater than 35 micrometers) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500×) crack length measurements in laboratory air and 1% sodium chloride (NaCl) environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.

The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

NASA Technical Reports Server (NTRS)

Piascik, R. S.

2001-01-01

The corrosion fatigue crack growth characteristics of small (less than 35 microns) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500X) crack length measurements in laboratory air and 1% NaCl environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.

The tensile effect on crack formation in single crystal silicon irradiated by intense pulsed ion beam

NASA Astrophysics Data System (ADS)

Liang, Guoying; Shen, Jie; Zhang, Jie; Zhong, Haowen; Cui, Xiaojun; Yan, Sha; Zhang, Xiaofu; Yu, Xiao; Le, Xiaoyun

2017-10-01

Improving antifatigue performance of silicon substrate is very important for the development of semiconductor industry. The cracking behavior of silicon under intense pulsed ion beam irradiation was studied by numerical simulation in order to understand the mechanism of induced surface peeling observed by experimental means. Using molecular dynamics simulation based on Stillinger Weber potential, tensile effect on crack growth and propagation in single crystal silicon was investigated. Simulation results reveal that stress-strain curves of single crystal silicon at a constant strain rate can be divided into three stages, which are not similar to metal stress-strain curves; different tensile load velocities induce difference of single silicon crack formation speed; the layered stress results in crack formation in single crystal silicon. It is concluded that the crack growth and propagation is more sensitive to strain rate, tensile load velocity, stress distribution in single crystal silicon.

Role of Different Kinds of Boundaries Against Cleavage Crack Propagation in Low-Temperature Embrittlement of Low-Carbon Martensitic Steel

NASA Astrophysics Data System (ADS)

Tsuboi, Mizuki; Shibata, Akinobu; Terada, Daisuke; Tsuji, Nobuhiro

2017-07-01

The present paper investigated the relationship between low-temperature embrittlement and microstructure of lath martensite in a low-carbon steel from both microstructural and crystallographic points of view. The fracture surface of the specimen after the miniaturized Charpy impact test at 98 K (-175 °C) mainly consisted of cleavage fracture facets parallel to crystallographic {001} planes of martensite. Through the crystallographic orientation analysis of micro-crack propagation, we found that the boundaries which separated different martensite variants having large misorientation angles of {001} cleavage planes could inhibit crack propagation. It was then concluded that the size of the aggregations of martensite variants belonging to the same Bain deformation group could control the low-temperature embrittlement of martensitic steels.

Subcritical crack growth phenomenon and fractography of barium titanate and barium titanate-based composite

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

Hwang, H.J.; Niihara, Koichi

1997-01-15

Subcritical crack growth (SCG), the propagation of surface and subsurface flaws under subcritical stress, i.e., any stress less than that necessary to catastrophically propagate the flaw, is a general phenomenon frequently observed in ceramics. Recently, electrical devices are miniaturized and used under quite severe atmospheres. Such environments often lead to the initiation and propagation of cracks due to the repeated electrical cycling, stresses by the mismatch in thermal expansion coefficient between devices and other constituents and thermal shock. In this study, the authors fabricated BaTiO{sub 3} and BaTiO{sub 3}-based composites containing nano-sized SiC particulates. The SCG phenomenon and fractography weremore » discussed based on the data obtained from indentation-induced-fracture (IIF) method.« less

Oxidation products of Inconel alloys 600 and 690 in hydrogenated steam environments and their role in stress corrosion cracking

NASA Astrophysics Data System (ADS)

Ferguson, J. Bryce

Inconel Alloys 600 and 690 are used extensively in components of Nuclear Pressurized Water Reactors (PWR) in the primary water loop which consists of H2 supersaturated steam. Alloy 600 has been found to crack intergranularly when exposed to primary water conditions. Alloy 690 was designed as a replacement and is generally regarded as immune to cracking. There is no consensus as to the mechanism which is responsible for cracking or the lack thereof in these alloys. In this work thermodynamic arguments for the stability of Ni and Cr compounds developed under pressurized water reactor environments ( PH2O and PH2 ) were experimentally tested. A mechanism is proposed to explain crack initiation and propagation alloy 600 along the grain boundaries where Cr2O3 has formed from the leaching of Cr from the matrix leaving behind a porous Ni-rich region. The mechanism is based on the thermodynamic potential for the transformation of a protective NiO surface layer into an amorphous non-protective Ni(OH)2 gel. This gel would also form along the grain boundaries and when hydrogenated steam reaches the porous Ni-rich regions. Crack initiation is then favored by tensile stressing of the grain boundary regions which can easily rupture the gelatinous film. The leaching of matrix Cr to form non-protective CrOOH gel at the crack tip followed by the exposure of fresh porous Ni to the environment also explains crack propagation in inconel alloy 600. The proposed crack initiation mechanism is not expected to occur in alloy 690 where a protective Cr2O 3 film covers the entire metal surface. However, crack propagation along the grain boundaries in alloy 600 and pre-cracked alloy 690 is expected to be active as hydroxide-forming reactions weaken the material at the grain boundaries.

Indentation damage and mechanical properties of human enamel and dentin.

PubMed

Xu, H H; Smith, D T; Jahanmir, S; Romberg, E; Kelly, J R; Thompson, V P; Rekow, E D

1998-03-01

Understanding the mechanical properties of human teeth is important to clinical tooth preparation and to the development of "tooth-like" restorative materials. Previous studies have focused on the macroscopic fracture behavior of enamel and dentin. In the present study, we performed indentation studies to understand the microfracture and deformation and the microcrack-microstructure interactions of teeth. It was hypothesized that crack propagation would be influenced by enamel rods and the dentino-enamel junction (DEJ), and the mechanical properties would be influenced by enamel rod orientation and tooth-to-tooth variation. Twenty-eight human third molars were used for the measurement of hardness, fracture toughness, elastic modulus, and energy absorbed during indentation. We examined the effect of enamel rod orientation by propagating cracks in the occlusal surface, and in the axial section in directions parallel and perpendicular to the occlusal surface. The results showed that the cracks in the enamel axial section were significantly longer in the direction perpendicular to the occlusal surface than parallel. The cracks propagating toward the DEJ were always arrested and unable to penetrate dentin. The fracture toughness of enamel was not single-valued but varied by a factor of three as a function of enamel rod orientation. The elastic modulus of enamel showed a significant difference between the occlusal surface and the axial section. It is concluded that the cracks strongly interact with the DEJ and the enamel rods, and that the mechanical properties of teeth are functions of microstructural orientations; hence, single values of properties (e.g., a single toughness value or a single modulus value) should not be used without information on microstructural orientation.

Fatigue crack tip deformation and fatigue crack propagation

NASA Technical Reports Server (NTRS)

Kang, T. S.; Liu, H. W.

1972-01-01

The effects of stress ratio, prestress cycling and plate thickness on the fatigue crack propagation rate are studied on 2024-T351 aluminum alloy. Fatigue crack propagation rate increases with the plate thickness and the stress ratio. Prestress cycling below the static yield strength has no noticeable effect on the fatigue crack propagation rate. However, prestress cycling above the static yield strength causes the material to strain harden and increases the fatigue crack propagation rate. Crack tip deformation is used to study the fatigue crack propagation. The crack tip strains and the crack opening displacements were measured from moire fringe patterns. The moire fringe patterns were obtained by a double exposure technique, using a very high density master grille (13,400 lines per inch).

Environmentally induced crack propagation in Inconel alloys 600 and 690 under hydrogen supersaturated steam

NASA Astrophysics Data System (ADS)

Ali, Mehboob Muzzammil

Intergranular stress corrosion cracking (IGSCC) of Inconel alloys 600 and 690 was investigated by exposing them to 300--400°C in deaerated hydrogen supersaturated steam. Crack growth rates were measured in-situ for the above alloys using modified wedge-opening-loaded (M-WOL) linear elastic fracture specimens under constant displacement conditions. The applied stress intensity factors (K) used varied from 29--90 MPam1/2. An activation energy of 120 kJ/mol was found for crack growth rates as a function of temperature. This activation energy is close to the one corresponding to grain boundary self diffusion of nickel. In addition, it was found that the apparent crack growth rates (da/dt) exhibited a linear dependence with KI, given by (da/dt) = A.KIn, where n = 1 in our case. Microstructurally, crack propagation in both the alloys was predominantly along the grain boundaries. It is suggested that high fugacity hydrogen was generated at the tip of the crack, as a result of the reaction of H2O with Cr2O3 on the fracture surface leading to IGSCC. It was found that the rates of crack propagation in both alloys 600 and 690 are very similar. Moreover, under the applied KIs, both alloys 600 and 690 show a similar tendency to crack intergranularly in a direction perpendicular to the applied stress. Crack branching was also exhibited by both the alloys 600 as well as 690. The difference in the crack growth rates of alloys 600 and 690 was found to be only about 2%, which indicates that the crack growth rates in these alloys is independent of the alloy chromium content and that, possibly very similar crack growth mechanisms are active in both the alloys 600 and 690 under similar conditions of KI and temperature. HREM images at the crack tip of alloy 690 exhibit two distinct regions---a crystalline region, and an adjacent amorphous region, which is likely to be either a hydroxide or an amorphous oxide layer. This layer is expected to result from passivation reactions as the crack surface is exposed to the corrosive environment.

On the interaction of ultrasound with cracks: Applications to fatigue crack growth

NASA Technical Reports Server (NTRS)

Buck, O.; Thompson, R. B.; Rehbein, D. K.

1986-01-01

Partial contact of two rough fatigue crack surfaces leads to transmission and diffraction of an acoustic signal at those contacts. Recent experimental and theoretical efforts to understand and quantify such contact in greater detail are discussed. The objective is to develop an understanding of the closure phenomenon and its application to the interpretation of fatigue data, in particular the R-ratio, spike overload/underload and threshold effects on crack propagation.

Fractography of modern engineering materials: composites and metals

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

Masters, J.E.; Au, J.J.

1987-01-01

The fractographic analysis of fracture surfaces in composites and metals is discussed in reviews and reports of recent theoretical and experimental investigations. Topics addressed include fracture-surface micromorphology in engineering solids, SEM fractography of pure and mixed-mode interlaminar fractures in graphite/epoxy composites, determination of crack propagation directions in graphite/epoxy structures, and the fracture surfaces of irradiated composites. Consideration is given to fractographic feature identification and characterization by digital imaging analysis, fractography of pressure-vessel steel weldments, the micromechanisms of major/minor cycle fatigue crack growth in Inconel 718, and fractographic analysis of hydrogen-assisted cracking in alpha-beta Ti alloys.

Tensile cracking of a brittle conformal coating on a rough substrate

DOE PAGES

Reedy, Jr., E. D.

2016-04-07

This note examines the effect of interfacial roughness on the initiation and growth of channel cracks in a brittle film. A conformal film with cusp-like surface flaws that replicate the substrate roughness is investigated. This type of surface flaw is relatively severe in the sense that stress diverges as the cusp-tip is approached (i.e., there is a power-law stress singularity). For the geometry and range of film properties considered, the analysis suggests that smoothing the substrate could substantially increase the film’s resistance to the formation of the through-the-thickness cracks that precede channel cracking. Furthermore, smoothing the substrate’s surface has amore » relatively modest effect on the film stress needed to propagate a channel crack.« less

Experimental study of crack initiation and propagation in high- and gigacycle fatigue in titanium alloys

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

Bannikov, Mikhail, E-mail: mbannikov@icmm.ru, E-mail: oborin@icmm.ru, E-mail: naimark@icmm.ru; Oborin, Vladimir, E-mail: mbannikov@icmm.ru, E-mail: oborin@icmm.ru, E-mail: naimark@icmm.ru; Naimark, Oleg, E-mail: mbannikov@icmm.ru, E-mail: oborin@icmm.ru, E-mail: naimark@icmm.ru

Fatigue (high- and gigacycle) crack initiation and its propagation in titanium alloys with coarse and fine grain structure are studied by fractography analysis of fracture surface. Fractured specimens were analyzed by interferometer microscope and SEM to improve methods of monitoring of damage accumulation during fatigue test and to verify the models for fatigue crack kinetics. Fatigue strength was estimated for high cycle fatigue regime using the Luong method [1] by “in-situ” infrared scanning of the sample surface for the step-wise loading history for different grain size metals. Fine grain alloys demonstrated higher fatigue resistance for both high cycle fatigue andmore » gigacycle fatigue regimes. Fracture surface analysis for plane and cylindrical samples was carried out using optical and electronic microscopy method. High resolution profilometry (interferometer-profiler New View 5010) data of fracture surface roughness allowed us to estimate scale invariance (the Hurst exponent) and to establish the existence of two characteristic areas of damage localization (different values of the Hurst exponent). Area 1 with diameter ∼300 μm has the pronounced roughness and is associated with damage localization hotspot. Area 2 shows less amplitude roughness, occupies the rest fracture surface and considered as the trace of the fatigue crack path corresponding to the Paris kinetics.« less

Cohesive zone modelling of wafer bonding and fracture: effect of patterning and toughness variations

NASA Astrophysics Data System (ADS)

Kubair, D. V.; Spearing, S. M.

2006-03-01

Direct wafer bonding has increasingly become popular in the manufacture of microelectromechanical systems and semiconductor microelectronics components. The success of the bonding process is controlled by variables such as wafer flatness and surface preparation. In order to understand the effects of these variables, spontaneous planar crack propagation simulations were performed using the spectral scheme in conjunction with a cohesive zone model. The fracture-toughness on the bond interface is varied to simulate the effect of surface roughness (nanotopography) and patterning. Our analysis indicated that the energetics of crack propagation is sensitive to the local surface property variations. The patterned wafers are tougher (well bonded) than the unpatterned ones of the same average fracture-toughness.

3D multiscale crack propagation using the XFEM applied to a gas turbine blade

NASA Astrophysics Data System (ADS)

Holl, Matthias; Rogge, Timo; Loehnert, Stefan; Wriggers, Peter; Rolfes, Raimund

2014-01-01

This work presents a new multiscale technique to investigate advancing cracks in three dimensional space. This fully adaptive multiscale technique is designed to take into account cracks of different length scales efficiently, by enabling fine scale domains locally in regions of interest, i.e. where stress concentrations and high stress gradients occur. Due to crack propagation, these regions change during the simulation process. Cracks are modeled using the extended finite element method, such that an accurate and powerful numerical tool is achieved. Restricting ourselves to linear elastic fracture mechanics, the -integral yields an accurate solution of the stress intensity factors, and with the criterion of maximum hoop stress, a precise direction of growth. If necessary, the on the finest scale computed crack surface is finally transferred to the corresponding scale. In a final step, the model is applied to a quadrature point of a gas turbine blade, to compute crack growth on the microscale of a real structure.

Interaction of Cracks Between Two Adjacent Indents in Glass

NASA Technical Reports Server (NTRS)

Choi, S. R.; Salem, J. A.

1993-01-01

Experimental observations of the interaction behavior of cracks between two adjacent indents were made using an indentation technique in soda-lime glass. It was specifically demonstrated how one indent crack initiates and propagates in the vicinity of another indent crack. Several types of crack interactions were examined by changing the orientation and distance of one indent relative to the other. It was found that the residual stress field produced by elastic/plastic indentation has a significant influence on controlling the mode of crack interaction. The interaction of an indent crack with a free surface was also investigated for glass and ceramic specimens.

Fatigue crack layer propagation in silicon-iron

NASA Technical Reports Server (NTRS)

Birol, Y.; Welsch, G.; Chudnovsky, A.

1986-01-01

Fatigue crack propagation in metal is almost always accompanied by plastic deformation unless conditions strongly favor brittle fracture. The analysis of the plastic zone is crucial to the understanding of crack propagation behavior as it governs the crack growth kinetics. This research was undertaken to study the fatigue crack propagation in a silicon iron alloy. Kinetic and plasticity aspects of fatigue crack propagation in the alloy were obtained, including the characterization of damage evolution.

A preliminary study of crack initiation and growth at stress concentration sites

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Gallagher, J. P.; Hartman, G. A.; Rajendran, A. M.

1982-01-01

Crack initiation and propagation models for notches are examined. The Dowling crack initiation model and the E1 Haddad et al. crack propagation model were chosen for additional study. Existing data was used to make a preliminary evaluation of the crack propagation model. The results indicate that for the crack sizes in the test, the elastic parameter K gave good correlation for the crack growth rate data. Additional testing, directed specifically toward the problem of small cracks initiating and propagating from notches is necessary to make a full evaluation of these initiation and propagation models.

Confocal examination of subsurface cracking in ceramic materials.

PubMed

Etman, Maged K

2009-10-01

The original ceramic surface finish and its microstructure may have an effect on crack propagation. The purpose of this study was to investigate the relation between crack propagation and ceramic microstructure following cyclic fatigue loading, and to qualitatively evaluate and quantitatively measure the surface and subsurface crack depths of three types of ceramic restorations with different microstructures using a Confocal Laser Scanning Microscope (CLSM) and Scanning Electron Microscope (SEM). Twenty (8 x 4 x 2 mm(3)) blocks of AllCeram (AC), experimental ceramic (EC, IPS e.max Press), and Sensation SL (SSL) were prepared, ten glazed and ten polished of each material. Sixty antagonist enamel specimens were made from the labial surfaces of permanent incisors. The ceramic abraders were attached to a wear machine, so that each enamel specimen presented at 45 degrees to the vertical movement of the abraders, and immersed in artificial saliva. Wear was induced for 80K cycles at 60 cycles/min with a load of 40 N and 2-mm horizontal deflection. The specimens were examined for cracks at baseline, 5K, 10K, 20K, 40K, and 80K cycles. Twenty- to 30-microm deep subsurface cracking appeared in SSL, with 8 to 10 microm in AC, and 7 microm close to the margin of the wear facets in glazed EC after 5K cycles. The EC showed no cracks with increasing wear cycles. Seventy-microm deep subsurface cracks were detected in SSL and 45 microm in AC after 80K cycles. Statistically, there was significant difference among the three materials (p < 0.05). Bonferroni multiple comparison of means test confirmed the ANOVA test and showed that there was no statistical difference (p > 0.05) in crack depth within the same ceramic material with different surface finishes. The ceramic materials with different microstructures showed different patterns of subsurface cracking.

The role of local strains from prior cold work on stress corrosion cracking of α-brass in Mattsson's solution

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

Ulaganathan, Jaganathan, E-mail: jagan.ulaganathan@mail.utoronto.ca; Newman, Roger C., E-mail: roger.newman@utoronto.ca

2014-06-01

The dynamic strain rate ahead of a crack tip formed during stress corrosion cracking (SCC) under a static load is assumed to arise from the crack propagation. The strain surrounding the crack tip would be redistributed as the crack grows, thereby having the effect of dynamic strain. Recently, several studies have shown cold work to cause accelerated crack growth rates during SCC, and the slip-dissolution mechanism has been widely applied to account for this via a supposedly increased crack-tip strain rate in cold worked material. While these interpretations consider cold work as a homogeneous effect, dislocations are generated inhomogeneously withinmore » the microstructure during cold work. The presence of grain boundaries results in dislocation pile-ups that cause local strain concentrations. The local strains generated from cold working α-brass by tensile elongation were characterized using electron backscatter diffraction (EBSD). The role of these local strains in SCC was studied by measuring the strain distributions from the same regions of the sample before cold work, after cold work, and after SCC. Though, the cracks did not always initiate or propagate along boundaries with pre-existing local strains from the applied cold work, the local strains surrounding the cracked boundaries had contributions from both the crack propagation and the prior cold work. - Highlights: • Plastic strain localization has a complex relationship with SCC susceptibility. • Surface relief created by cold work creates its own granular strain localization. • Cold work promotes crack growth but several other factors are involved.« less

The Effect of Freezing on the Dynamics of Dike Propagation

NASA Astrophysics Data System (ADS)

Tait, S.; Taisne, B.

2007-12-01

When magma-filled cracks propagate close to the Earth's surface, host rock temperature is well below the magma solidus. Solidification and substantial increase in magma viscosity can occur, are most pronounced near the propagating tip and can slow or arrest the progress of the dike. Quantitative analysis is required to predict whether a given dike will reach the surface to erupt and the duration of the precursor sequence. This challenging physical problem mixes elasticity, fracture mechanics, heat transfer and fluid flow with strong rheologic gradients due to cooling. We describe the propagation behaviour of such a hydraulic fracture using a laboratory experimental system of a crack fed by a constant flux of paraffin wax from a source reservoir propagating through gelatin below the solidus of the wax. The most novel behaviour is an intermittent regime in which cracks periodically stop advancing due to solidification, then swell at constant length while enhancing the elastic deformation in the surrounding solid before propagation resumes. We present a physical model of this system, based on different balances between driving and resistive forces: the former are elastic stress and liquid buoyancy, the latter are fracture resistance at the tip and viscous resistance. The fracture is represented as a head, behind the propagating tip, connected to the source by a narrow tail. Freezing of liquid close to the tip is assumed to enhance fracture resistance according to a cooling law, and propagation is assumed to occur only when the stress exerted by the liquid is enough to overcome fracture resistance. Our theoretical model reproduces intermittent propagation with precise behaviour depending on the controlling stress balances, and provides a tool to analyse natural systems.

Laser surface fusion of plasma sprayed ceramic turbine seals

NASA Technical Reports Server (NTRS)

Wisander, D. W.; Bill, R. C. (Inventor)

1981-01-01

The thermal shock resistance of a ceramic layer is improved. An improved abradable lining that is deposited on a shroud forming a gas path seal in turbomachinery is emphasized. Improved thermal shock resistance of a shroud is effective through the deliberate introduction of 'benign' cracks. These are microcracks which will not propagate appreciably upon exposure to the thermal shock environment in which a turbine seal must function. Laser surface fusion treatment is used to introduce these microcracks. The ceramic surface is laser scanned to form a continuous dense layer. As this cools and solidifies, shrinkage results in the formation of a very fine crack network. The presence of this deliberately introduced fine crack network precludes the formation of a catastrophic crack during thermal shock exposure.

CRACK GROWTH RESPONSE OF ALLOY 690 IN SIMULATED PWR PRIMARY WATER

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

Toloczko, Mychailo B.; Bruemmer, Stephen M.

2009-12-01

The stress corrosion crack growth response of three extruded alloy 690 CRDM tube heats was investigated in several thermomechanical conditions. Extremely low propagation rates (< 1 x 10{sup -9} mm/s) were observed under constant stress intensity factor (K) loading at 325-350 C in the as-received, thermally treated (TT) materials despite using a variety of transitioning techniques. Post-test observation of the crack-growth surfaces revealed only isolated intergranular (IG) cracking. One-dimensional cold rolling to 17% reduction and testing in the S-L orientation did not promote enhanced stress corrosion rates. However, somewhat higher propagation rates were observed in a 30% cold-rolled alloy 690TTmore » specimen tested in the T-L orientation. Cracking of the cold-rolled material was promoted on grain boundaries oriented parallel to the rolling plane with the % IG increasing with the amount of cold rolling.« less

Fracture analysis for a penny-shaped crack problem of a superconducting cylinder in a parallel magnetic field

NASA Astrophysics Data System (ADS)

Gao, S. W.; Feng, W. J.; Fang, X. Q.; Zhang, G. L.

2014-11-01

In this work, the penny-shaped crack problem is investigated for an infinite long superconducting cylinder under electromagnetic forces. The distributions of magnetic flux density in the superconducting cylinder are obtained analytically for both the zero-field cooling (ZFC) and the field cooling (FC) activation processes, where the magnetically impermeable crack surface condition and the Bean model outside the crack region are adopted. Based on the finite element method (FEM), the stress intensity factor (SIF) and energy release rate (ERR) at the crack tips in the process of field descent are further numerically calculated. Numerical results obtained show that according to the maximal energy release rate criterion, the FC process is generally easier to enhance crack initiation and propagation than the ZFC activation process. On the other hand, for the FC activation process, the larger the maximal applied magnetic field, more likely the crack propagates. Additionally, crack size has important and slightly different effects on the crack extension forces for the ZFC and FC cases. Thus, all of the activation processes, the applied field and the diameter of the penny-shaped crack have significant effects on the intensity analysis and design of superconducting materials.

Seeding Cracks Using a Fatigue Tester for Accelerated Gear Tooth Breaking

NASA Technical Reports Server (NTRS)

Nenadic, Nenad G.; Wodenscheck, Joseph A.; Thurston, Michael G.; Lewicki, David G.

2011-01-01

This report describes fatigue-induced seeded cracks in spur gears and compares them to cracks created using a more traditional seeding method, notching. Finite element analysis (FEA) compares the effective compliance of a cracked tooth to the effective compliance of a notched tooth where the crack and the notch are of the same depth. In this analysis, cracks are propagated to the desired depth using FRANC2D and effective compliances are computed in ANSYS. A compliance-based feature for detecting cracks on the fatigue tester is described. The initiated cracks are examined using both nondestructive and destructive methods. The destructive examination reveals variability in the shape of crack surfaces.

Fatigue crack propagation resistance of virgin and highly crosslinked, thermally treated ultra-high molecular weight polyethylene.

PubMed

Gencur, Sara J; Rimnac, Clare M; Kurtz, Steven M

2006-03-01

To prolong the life of total joint replacements, highly crosslinked ultra-high molecular weight polyethylenes (UHMWPEs) have been introduced to improve the wear resistance of the articulating surfaces. However, there are concerns regarding the loss of ductility and potential loss in fatigue crack propagation (FCP) resistance. The objective of this study was to evaluate the effects of gamma radiation-induced crosslinking with two different post-irradiation thermal treatments on the FCP resistance of UHMWPE. Two highly crosslinked and one virgin UHMWPE treatment groups (ram-extruded, orthopedic grade, GUR 1050) were examined. For the two highly crosslinked treatment groups, UHMWPE rods were exposed to 100 kGy and then underwent post-irradiation thermal processing either above the melt temperature or below the melt temperature (2 h-150 degrees C, 110 degrees C). Compact tension specimens were cyclically loaded to failure and the fatigue crack growth rate, da/dN, vs. cyclic stress intensity factor, DeltaK, behavior was determined and compared between groups. Scanning electron microscopy was used to examine fracture surface characteristics. Crosslinking was found to decrease the ability of UHMWPE to resist crack inception and propagation under cyclic loading. The findings also suggested that annealing as a post-irradiation treatment may be somewhat less detrimental to FCP resistance of UHMWPE than remelting. Scanning electron microscopy examination of the fracture surfaces demonstrated that the virgin treatment group failed in a more ductile manner than the two highly crosslinked treatment groups.

A study of fractography in the low-temperature brittle fracture of an 18Cr-18Mn-0.7N austenitic steel

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

Liu, S.C.; Hashida, T.; Takahashi, H.

1998-03-01

The fracture mode and crack propagation behavior of brittle fracture at 77 and 4 K in an 18Cr-18Mn-0.7N austenitic stainless were investigated using optical and scanning electron microscopy. The fracture path was examined by observing the side surface in a partially ruptured specimen. The relationship of the fracture facets to the microstructures were established by observing the fracture surface and the adjacent side surface simultaneously. Three kinds of fracture facets were identified at either temperature. The first is a smooth curved intergranular fracture facet with characteristic parallel lines on it. The second is a fairly planar facet formed by partingmore » along an annealing twin boundary, a real {l_brace}111{r_brace} plane. There are three sets of parallel lines on the facet and the lines in different sets intersect at 60 deg. The third is a lamellar transgranular fracture facet with sets of parallel steps on it. Fracture propagated by the formation of microcracks on a grain boundary, annealing twin boundary, and coalescence of these cracks. The observation suggests that the ease of crack initiation and propagation along the grain boundary and the annealing twin boundary may be the main reason for the low-temperature brittleness of this steel. A mechanism for grain boundary cracking, including annealing twin boundary parting, has been discussed based on the stress concentration induced by impinging planar deformation structures on the grain boundaries.« less

Mechanics of tidally driven fractures in Europa's ice shell and implications for seismic and radar profiling

NASA Astrophysics Data System (ADS)

Lee, S.; Pappalardo, R. T.; Makris, N. C.

2005-12-01

Among Europa's surface features, cycloidal cracks are probably the most important for proving the existence of a subsurface liquid ocean. This is because (1) there is strong evidence that they are caused by tidally induced stress [1], and (2) this stress likely only approaches the ice failure strength if an ocean is present. There are a number of outstanding issues, however, in quantitatively explaining cycloidal cracks. First, current estimates of the pure diurnal tidal stress necessary to cause cycloidal cracks even in the presence of an ocean [1,2] is well below the typical stress known to cause tensile failure in natural terrestrial ice [3]. Second, models of ridge formation suggest that cycloidal cracks penetrate through the entire brittle-ice layer [1,4], but current models limit the depth of tidally induced surface cracks to be less than 100 m even in the presence of an ocean [1,5]. Third, the 3-km/h crack propagation speed determined by [1] is three orders of magnitude lower than the roughly 2-km/s speed at which cracks are known to propagate in ice. Our goal is to quantitatively address these issues in a unified manner. To do this, a fracture mechanics model is developed for the initiation and propagation of a crack through an ice layer of finite thickness in the presence of gravitational overburden and porosity. It is shown that Europa's ice shell may be highly porous and salt-rich. This implies that the strength of Europa's outer ice shell may be sufficiently low to make the crack initiation strengths arrived at by current kinematic models [1,2] highly plausible, even though they are much lower than those typically measured for terrestrial ice. A model is developed for the stress intensity factor at a crack tip in an ice shell with finite thickness, gravitational overburden, and depth-dependent porosity. This leads to the conclusion that cycloids are generated as a sequence of discrete and near instantaneous fracture events, each of which penetrates through the entire brittle layer with horizontal length on the order of the brittle layer thickness. This mechanism yields an apparent propagation speed that is consistent with the 3 km/h crack propagation speed necessary to generate cycloids in current kinematic models [1,2]. An implication of this model is that the level of seismic activity should be higher by orders of magnitude in the presence of an ocean. High correlation is then expected between the level of seismic activity and the tidal period in the presence but not in the absence of an ocean. The cracks associated with cycloids that fully penetrate the brittle layer should be at least 106 times more energetic than the shallow, roughly 100-m deep, surface cracks. We show that this greatly improves the signal-to-noise ratio for the type of seismic profiling discussed in [6] if fully penetrating cracks are used as sources of opportunity. Although Europa's ice is likely highly porous, the size of vacuous pores is likely on the order of a millimeter. Since the pore size is at least three orders of magnitude smaller than the ice-penetrating radar wavelength, our calculations show that porosity-induced scattering should not be significant. [1] Hoppa et al. 1999, Science 285. [2] Crawford et al. 2005, LPSC XXXVI #2042. [3] Weeks and Cox 1984, Ocean Sci. Eng. 9. [4] Pappalardo et al. 1999, J. Geophys. Res. 97. [5] Crawford and Stevenson 1988, Icarus 73. [6] Lee et al. 2003, Icarus 165.

Mode I Failure of Armor Ceramics: Experiments and Modeling

NASA Astrophysics Data System (ADS)

Meredith, Christopher; Leavy, Brian

2017-06-01

The pre-notched edge on impact (EOI) experiment is a technique for benchmarking the damage and fracture of ceramics subjected to projectile impact. A cylindrical projectile impacts the edge of a thin rectangular plate with a pre-notch on the opposite edge. Tension is generated at the notch tip resulting in the initiation and propagation of a mode I crack back toward the impact edge. The crack can be quantitatively measured using an optical method called Digital Gradient Sensing, which measures the crack-tip deformation by simultaneously quantifying two orthogonal surface slopes via measuring small deflections of light rays from a specularly reflective surface around the crack. The deflections in ceramics are small so the high speed camera needs to have a very high pixel count. This work reports on the results from pre-crack EOI experiments of SiC and B4 C plates. The experimental data are quantitatively compared to impact simulations using an advanced continuum damage model. The Kayenta ceramic model in Alegra will be used to compare fracture propagation speeds, bifurcations and inhomogeneous initiation of failure will be compared. This will provide insight into the driving mechanisms required for the macroscale failure modeling of ceramics.

Influence of Initial Inclined Surface Crack on Estimated Residual Fatigue Lifetime of Railway Axle

NASA Astrophysics Data System (ADS)

Náhlík, Luboš; Pokorný, Pavel; Ševčík, Martin; Hutař, Pavel

2016-11-01

Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect on railway axle surface is often required. The contribution deals with an estimation of residual fatigue lifetime of railway axle with initial inclined surface crack. 3D numerical model of inclined semi-elliptical surface crack in railway axle was developed and its curved propagation through the axle was simulated by finite element method. Presence of press-fitted wheel in the vicinity of initial crack was taken into account. A typical loading spectrum of railway axle was considered and residual fatigue lifetime was estimated by NASGRO approach. Material properties of typical axle steel EA4T were considered in numerical calculations and lifetime estimation.

Rupture Dynamics along Thrust Dipping Fault: Inertia Effects due to Free Surface Wave Interactions

NASA Astrophysics Data System (ADS)

Vilotte, J. P.; Scala, A.; Festa, G.

2017-12-01

We numerically investigate the dynamic interaction between free surface and up-dip, in-plane rupture propagation along thrust faults, under linear slip-weakening friction. With reference to shallow along-dip rupture propagation during large subduction earthquakes, we consider here low dip-angle fault configurations with fixed strength excess and depth-increasing initial stress. In this configuration, the rupture undergoes a break of symmetry with slip-induced normal stress perturbations triggered by the interaction with reflected waves from the free surface. We found that both body-waves - behind the crack front - and surface waves - at the crack front - can trigger inertial effects. When waves interact with the rupture before this latter reaches its asymptotic speed, the rupture can accelerate toward the asymptotic speed faster than in the unbounded symmetric case, as a result of these inertial effects. Moreover, wave interaction at the crack front also affects the slip rate generating large ground motion on the hanging wall. Imposing the same initial normal stress, frictional strength and stress drop while varying the static friction coefficient we found that the break of symmetry makes the rupture dynamics dependent on the absolute value of friction. The higher the friction the stronger the inertial effect both in terms of rupture acceleration and slip amount. When the contact condition allows the fault interface to open close to the free surface, the length of the opening zone is shown to depend on the propagation length, the initial normal stress and the static friction coefficient. These new results are shown to agree with analytical results of rupture propagation in bounded media, and open new perspectives for understanding the shallow rupture of large subduction earthquakes and tsunami sources.

Investigation of fretting behaviour in pressure armour layers of flexible pipes

NASA Astrophysics Data System (ADS)

Don Rasika Perera, Solangarachchige

The incidence of fretting damage in the pressure armour wires of flexible pipes used in offshore oil explorations has been investigated. A novel experimental facility which is capable of simulating nub and valley contact conditions of interlocking wire winding with dynamic slip, representative of actual pipe loading, has been developed. The test set-up is equipped with a state of the art data acquisition system and a controller with transducers to measure and control the normal load, slip amplitude and friction force at the contact, in addition to the hoop stress in the wire. Tests were performed with selected loading and the fretted regions were examined using optical microscopy techniques. Results show that the magnitude of contact loading and the slip amplitude have a distinct influence on surface damage. Surface cracks originated from a fretting scar were observed at high contact loads in mixed slip sliding while surface damage predominantly due to wear was observed under gross slip. The position of surface cracks and the wear profile have been related to the contact pressure distribution. The evolution of friction force and surface damage under different slip and normal pressure conditions has been analysed. A fracture mechanics based numerical procedure has been developed to analyse the fretting damage behaviour. A severity parameter is proposed in order to ascertain whether the crack growth is in mode I or mode II cracking. The analysis show the influence of mode II cracking in the early stages of crack growth following which the crack deviates in the mode I direction making mode I the dominant crack propagation mechanism. The crack path determined by the numerical procedure correlates well with the experimental results. A numerical analysis was carried out for the fretting fatigue condition where a cyclic bulk stress superimposes with the friction force. The analysis correlates well with short crack growth behaviour. The analysis confirms that fretting is a significant factor that should be taken into account in design and operation of the pressure armour wires of flexible pipes at high contact pressure if the bulk cyclic load superimposes with the friction force. As predicted by the numerical procedure and further by experimental investigations, the surface cracks initiating on the wire in this condition are self arresting after propagating into a certain depth.

Modified silicon carbide whiskers

DOEpatents

Tiegs, Terry N.; Lindemer, Terrence B.

1991-01-01

Silicon carbide whisker-reinforced ceramic composites are fabricated in a highly reproducible manner by beneficating the surfaces of the silicon carbide whiskers prior to their usage in the ceramic composites. The silicon carbide whiskers which contain considerable concentrations of surface oxides and other impurities which interact with the ceramic composite material to form a chemical bond are significantly reduced so that only a relatively weak chemical bond is formed between the whisker and the ceramic material. Thus, when the whiskers interact with a crack propagating into the composite the crack is diverted or deflected along the whisker-matrix interface due to the weak chemical bonding so as to deter the crack propagation through the composite. The depletion of the oxygen-containing compounds and other impurities on the whisker surfaces and near surface region is effected by heat treating the whiskers in a suitable oxygen sparaging atmosphere at elevated temperatures. Additionally, a sedimentation technique may be utilized to remove whiskers which suffer structural and physical anomalies which render them undesirable for use in the composite. Also, a layer of carbon may be provided on the surface of the whiskers to further inhibit chemical bonding of the whiskers to the ceramic composite material.

Modified silicon carbide whiskers

DOEpatents

Tiegs, T.N.; Lindemer, T.B.

1991-05-21

Silicon carbide whisker-reinforced ceramic composites are fabricated in a highly reproducible manner by beneficating the surfaces of the silicon carbide whiskers prior to their usage in the ceramic composites. The silicon carbide whiskers which contain considerable concentrations of surface oxides and other impurities which interact with the ceramic composite material to form a chemical bond are significantly reduced so that only a relatively weak chemical bond is formed between the whisker and the ceramic material. Thus, when the whiskers interact with a crack propagating into the composite the crack is diverted or deflected along the whisker-matrix interface due to the weak chemical bonding so as to deter the crack propagation through the composite. The depletion of the oxygen-containing compounds and other impurities on the whisker surfaces and near surface region is effected by heat treating the whiskers in a suitable oxygen sparging atmosphere at elevated temperatures. Additionally, a sedimentation technique may be utilized to remove whiskers which suffer structural and physical anomalies which render them undesirable for use in the composite. Also, a layer of carbon may be provided on the surface of the whiskers to further inhibit chemical bonding of the whiskers to the ceramic composite material.

Ceramic composites reinforced with modified silicon carbide whiskers and method for modifying the whiskers

DOEpatents

Tiegs, Terry N.; Lindemer, Terrence B.

1991-01-01

Silicon carbide whisker-reinforced ceramic composites are fabricated in a highly reproducible manner by beneficating the surfaces of the silicon carbide whiskers prior to their usage in the ceramic composites. The silicon carbide whiskers which contain considerable concentrations of surface oxides and other impurities which interact with the ceramic composite material to form a chemical bond are significantly reduced so that only a relatively weak chemical bond is formed between the whisker and the ceramic material. Thus, when the whiskers interact with a crack propagating into the composite the crack is diverted or deflected along the whisker-matrix interface due to the weak chemical bonding so as to deter the crack propagation through the composite. The depletion of the oxygen-containing compounds and other impurities on the whisker surfaces and near surface region is effected by heat treating the whiskers in a suitable oxygen sparaging atmosphere at elevated temperatures. Additionally, a sedimentation technique may be utilized to remove whiskers which suffer structural and physical anomalies which render them undesirable for use in the composite. Also, a layer of carbon may be provided on the surface of the whiskers to further inhibit chemical bonding of the whiskers to the ceramic composite material.

Ceramic composites reinforced with modified silicon carbide whiskers and method for modifying the whiskers

DOEpatents

Tiegs, T.N.; Lindemer, T.B.

1991-02-19

Silicon carbide whisker-reinforced ceramic composites are fabricated in a highly reproducible manner by beneficating the surfaces of the silicon carbide whiskers prior to their usage in the ceramic composites. The silicon carbide whiskers which contain considerable concentrations of surface oxides and other impurities which interact with the ceramic composite material to form a chemical bond are significantly reduced so that only a relatively weak chemical bond is formed between the whisker and the ceramic material. Thus, when the whiskers interact with a crack propagating into the composite the crack is diverted or deflected along the whisker-matrix interface due to the weak chemical bonding so as to deter the crack propagation through the composite. The depletion of the oxygen-containing compounds and other impurities on the whisker surfaces and near surface region is effected by heat treating the whiskers in a suitable oxygen sparging atmosphere at elevated temperatures. Additionally, a sedimentation technique may be utilized to remove whiskers which suffer structural and physical anomalies which render them undesirable for use in the composite. Also, a layer of carbon may be provided on the surface of the whiskers to further inhibit chemical bonding of the whiskers to the ceramic composite material.

Ceramic composites reinforced with modified silicon carbide whiskers

DOEpatents

Tiegs, Terry N.; Lindemer, Terrence B.

1990-01-01

Silicon carbide whisker-reinforced ceramic composites are fabricated in a highly reproducible manner by beneficating the surfaces of the silicon carbide whiskers prior to their usage in the ceramic composites. The silicon carbide whiskers which contain considerable concentrations of surface oxides and other impurities which interact with the ceramic composite material to form a chemical bond are significantly reduced so that only a relatively weak chemical bond is formed between the whisker and the ceramic material. Thus, when the whiskers interact with a crack propagating into the composite the crack is diverted or deflected along the whisker-matrix interface due to the weak chemical bonding so as to deter the crack propagation through the composite. The depletion of the oxygen-containing compounds and other impurities on the whisker surfaces and near surface region is effected by heat treating the whiskers in a suitable oxygen sparaging atmosphere at elevated temperatures. Additionally, a sedimentation technique may be utilized to remove whiskers which suffer structural and physical anomalies which render them undesirable for use in the composite. Also, a layer of carbon may be provided on the surface of the whiskers to further inhibit chemical bonding of the whiskers to the ceramic composite material.

Dynamic fracture instability of tough bulk metallic glass

NASA Astrophysics Data System (ADS)

Meng, J. X.; Ling, Z.; Jiang, M. Q.; Zhang, H. S.; Dai, L. H.

2008-04-01

We report the observations of a clear fractographic evolution from vein pattern, dimple structure, and then to periodic corrugation structure, followed by microbranching pattern, along the crack propagation direction in the dynamic fracture of a tough Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit.1) bulk metallic glass (BMGs) under high-velocity plate impact. A model based on fracture surface energy dissipation and void growth is proposed to characterize this fracture pattern transition. We find that once the dynamic crack propagation velocity reaches a critical fraction of Rayleigh wave speed, the crack instability occurs; hence, crack microbranching goes ahead. Furthermore, the correlation between the critical velocity of amorphous materials and their intrinsic strength such as Young's modulus is uncovered. The results may shed new insight into dynamic fracture instability for BMGs.

Three-dimensional effects in interfacial crack propagation

NASA Astrophysics Data System (ADS)

Liechti, K. M.; Chai, Y.-S.; Liang, Y.-M.

1992-09-01

The paper describes the use of crack-opening interferometry for examining the variation in normal crack-opening displacements (NCOD) along the front of an interfacial crack in an edge-cracked bimaterial strip under biaxial loading. For the glass/epoxy combination considered here, the crack front was concave in the direction of crack growth, in contrast to previous observations with a glass/polyurethane/glass sandwich specimen and cracks in homogeneous materials. The NCOD were greatest in the interior of the specimen for all mode-mixes considered and the exponents in a power-law fit of NCOD versus distance from the crack front decreased toward the free surface. The exponents varied with mode-mix, suggesting that interfacial crack-front geometries could be similarly affected.

Location, location & size: defects close to surfaces dominate fatigue crack initiation

NASA Astrophysics Data System (ADS)

Serrano-Munoz, Itziar; Buffiere, Jean-Yves; Mokso, Rajmund; Verdu, Catherine; Nadot, Yves

2017-03-01

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards

Location, location &size: defects close to surfaces dominate fatigue crack initiation.

PubMed

Serrano-Munoz, Itziar; Buffiere, Jean-Yves; Mokso, Rajmund; Verdu, Catherine; Nadot, Yves

2017-03-27

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards.

Location, location & size: defects close to surfaces dominate fatigue crack initiation

PubMed Central

Serrano-Munoz, Itziar; Buffiere, Jean-Yves; Mokso, Rajmund; Verdu, Catherine; Nadot, Yves

2017-01-01

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards PMID:28345599

Action of a Local Time-Periodic Load on an Ice Sheet with a Crack

NASA Astrophysics Data System (ADS)

Tkacheva, L. A.

2017-11-01

The problem of vibrations of an ice sheet with a rectilinear crack on the surface of an ideal incompressible fluid of finite depth under the action of a time-periodic local load is solved analytically using the Wiener-Hopf technique. Ice cover is simulated by two thin elastic semi-infinite plates of constant thickness. The thickness of the plates may be different on the opposite sides of the crack. Various boundary conditions on the edges of the plates are considered. For the case of contact of plates of the same thickness, a solution in explicit form is obtained. The asymptotics of the deflection of the plates in the far field is studied. It is shown that in the case of contact of two plates of different thickness, predominant directions of wave propagation at an angle to the crack can be identified in the far field. In the case of contact of plates of the same thickness with free edges and with free overlap, an edge waveguide mode propagating along the crack is excited. It is shown that the edge mode propagates with maximum amplitude if the vertical wall is in contact with the plate. Examples of calculations are given.

Experimental studies of rock fracture behavior related to hydraulic fracture

NASA Astrophysics Data System (ADS)

Ma, Zifeng

The objective of this experimental investigation stems from the uncontrollable of the hydraulic fracture shape in the oil and gas production field. A small-scale laboratory investigation of crack propagation in sandstone was first performed with the objective to simulate the field fracture growth. Test results showed that the fracture resistance increased with crack extension, assuming that there was an interaction between crack faces (bridging, interlocking, and friction). An acoustic emission test was conducted to examine the existence of the interaction by locating AE events and analyzing waveform. Furthermore, the effects of confining stress, loading rate, stress field, and strength heterogeneous on the tortuosity of the fracture surface were experimentally investigated in the study. Finally, a test was designed and conducted to investigate the crack propagation in a stratified media with permeability contrast. Crack was observed to arrested in an interface. The phenomenon of delamination along an interface between layers with permeability contrast was observed. The delamination was proposed to be the cause of crack arrest and crack jump in the saturated stratified materials under confinement test.

New-type steel plate with ultra high crack-arrestability

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

Ishikawa, T.; Nomiyama, Y.; Hagiwara, Y.

1995-12-31

A new-type steel plate has been developed by controlling the microstructure of the surface layers. The surface layer consists of ultra fine grain ferrite microstructure, which provides excellent fracture toughness even at cryogenic temperature. When an unstable brittle crack propagates in the developed steel plate, shear-lips can be easily formed due to the surface layers with ultra fine grain microstructure. Since unstable running crack behavior is strongly affected by side-ligaments (shear-lips), which are associated with extensive plastic deformation, enhanced formation of the shear-lips can improve crack arrestability. This paper describes the developed steel plates of HT500MPa tensile strength class formore » shipbuilding use. Fracture mechanics investigations using large-scale fracture testings (including ultrawide duplex ESSO tests) clarified that the developed steel plates have ultra high crack-arrestability. It was also confirmed that the plates possess sufficient properties, including weldability and workability, for ship building use.« less

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

Silva, F. S.

Functionally graded components exhibit spatial variations of mechanical properties in contrast with, and as an alternative to, purely homogeneous components. A large class of graded materials, however, are in fact mostly homogeneous materials with property variations (chemical or mechanical) restricted to a specific area or layer produced by applying for example a coating or by introducing sub-surface residual stresses. However, it is also possible to obtain graded materials with a smooth transition of mechanical properties along the entire component, for example in a 40 mm component. This is possible, for example, by using centrifugal casting technique or incremental melting andmore » solidification technique. In this paper we will study fully metallic functionally graded components with a smooth gradient, focusing on fatigue crack propagation. Fatigue propagation will be assessed in the direction parallel to the gradation (in different homogeneous layers of the functionally graded component) to assess what would be fatigue crack propagation on the direction perpendicular to the gradation. Fatigue crack growth rate (standard mode I fatigue crack growth) will be correlated to the mode I stress intensity factor range. Other mechanical properties of different layers of the component (Young's modulus) will also be considered in this analysis. The effect of residual stresses along the component gradation on crack propagation will also be taken into account. A qualitative analysis of the effects of some important features, present in functionally graded materials, will be made based on the obtained results.« less

A contactless ultrasonic surface wave approach to characterize distributed cracking damage in concrete.

PubMed

Ham, Suyun; Song, Homin; Oelze, Michael L; Popovics, John S

2017-03-01

We describe an approach that utilizes ultrasonic surface wave backscatter measurements to characterize the volume content of relatively small distributed defects (microcrack networks) in concrete. A simplified weak scattering model is used to demonstrate that the scattered wave field projected in the direction of the surface wave propagation is relatively insensitive to scatterers that are smaller than the propagating wavelength, while the scattered field projected in the opposite direction is more sensitive to sub-wavelength scatterers. Distributed microcracks in the concrete serve as the small scatterers that interact with a propagating surface wave. Data from a finite element simulation were used to demonstrate the viability of the proposed approach, and also to optimize a testing configuration to collect data. Simulations were validated through experimental measurements of ultrasonic backscattered surface waves from test samples of concrete constructed with different concentrations of fiber filler (0.0, 0.3 and 0.6%) to mimic increasing microcrack volume density and then samples with actual cracking induced by controlled thermal cycles. A surface wave was induced in the concrete samples by a 50kHz ultrasonic source operating 10mm above the surface at an angle of incidence of 9°. Silicon-based miniature MEMS acoustic sensors located a few millimeters above the concrete surface both behind and in front of the sender were used to detect leaky ultrasonic surface waves emanating from concrete. A normalized backscattered energy parameter was calculated from the signals. Statistically significant differences in the normalized backscattered energy were observed between concrete samples with varying levels of simulated and actual cracking damage volume. Copyright © 2016 Elsevier B.V. All rights reserved.

Numerical investigation on the prefabricated crack propagation of FV520B stainless steel

NASA Astrophysics Data System (ADS)

Pan, Juyi; Qin, Ming; Chen, Songying

FV520B is a common stainless steel for manufacturing centrifugal compressor impeller and shaft. The internal metal flaw destroys the continuity of the material matrix, resulting in the crack propagation fracture of the component, which seriously reduces the service life of the equipment. In this paper, Abaqus software was used to simulate the prefabricated crack propagation of FV520B specimen with unilateral gap. The results of static crack propagation simulation results show that the maximum value of stress-strain located at the tip of the crack and symmetrical distributed like a butterfly along the prefabricated crack direction, the maximum stress is 1990 MPa and the maximum strain is 9.489 × 10-3. The Mises stress and stress intensity factor KI increases with the increase of the expansion step, the critical value of crack initiation is reached at the 6th extension step. The dynamic crack propagation simulation shows that the crack propagation path is perpendicular to the load loading direction. Similarly, the maximum Mises stress located at the crack tip and is symmetrically distributed along the crack propagation direction. The critical stress range of the crack propagation is 23.3-43.4 MPa. The maximum value of stress-strain curve located at the 8th extension step, that is, the crack initiation point, the maximum stress is 55.22 MPa, and the maximum strain is 2.26 × 10-4. On the crack tip, the stress changed as 32.24-40.16 MPa, the strain is at 1.292 × 10-4-1.897 × 10-4.

Cleavage crystallography of liquid metal embrittled aluminum alloys

NASA Technical Reports Server (NTRS)

Reynolds, A. P.; Stoner, G. E.

1991-01-01

The crystallography of liquid metal-induced transgranular cleavage in six aluminum alloys having a variety of microstructures has been determined via Laue X-ray back reflection. The cleavage crystallography was independent of alloy microstructure, and the cleavage plane was 100-plane oriented in all cases. It was further determined that the cleavage crystallography was not influenced by alloy texture. Examination of the fracture surface indicated that there was not a unique direction of crack propagation. In addition, the existence of 100-plane cleavage on alloy 2024 fracture surfaces was inferred by comparison of secondary cleavage crack intersection geometry on the 2024 surfaces with the geometry of secondary cleavage crack intersections on the test alloys.

Experimental study on fatigue crack propagation rate of RC beam strengthened with carbon fiber laminate

NASA Astrophysics Data System (ADS)

Huang, Peiyan; Liu, Guangwan; Guo, Xinyan; Huang, Man

2008-11-01

The experimental research on fatigue crack propagation rate of reinforced concrete (RC) beams strengthened with carbon fiber laminate (CFL) is carried out by MTS system in this paper. The experimental results show that, the main crack propagation on strengthened beam can be summarized into three phases: 1) fast propagation phase; 2) steady propagation and rest phase; 3) unsteady propagation phase. The phase 2-i.e. steady propagation and rest stage makes up about 95% of fatigue life of the strengthened beam. The propagation rate of the main crack, da/dN, in phase 2 can be described by Paris formula, and the constant C and m can be confirmed by the fatigue crack propagation experiments of the RC beams strengthened with CFL under three-point bending loads.

The Microstructural Evolution of Fatigue Cracks in FCC Metals

NASA Astrophysics Data System (ADS)

Gross, David William

The microstructural evolution during fatigue crack propagation was investigated in a variety of planar and wavy slip FCC metals. The planar materials included Haynes 230, Nitronic 40, and 316 stainless steel, and the wavy materials included pure nickel and pure copper. Three different sets of experiments were performed to fully characterize the microstructural evolution. The first, performed on Haynes 230, mapped the strain field ahead a crack tip using digital image correlation and electron backscatter diffraction techniques. Focused ion beam (FIB) lift-out techniques were then utilized to extract transmission electron microscopy (TEM) samples at specific distances from the crack tip. TEM investigations compared the measured strain to the microstructure. Overall, the strain measured via DIC and EBSD was only weakly correlated to the density of planar slip bands in the microstructure. The second set of experiments concerned the dislocation structure around crack tips. This set of experiments was performed on all the materials. The microstructure at arrested fatigue cracks on the free surface was compared to the microstructure found beneath striations on the fracture surfaces by utilizing FIB micromachining to create site-specific TEM samples. The evolved microstructure depended on the slip type. Strong agreement was found between the crack tip microstructure at the free surface and the fracture surface. In the planar materials, the microstructure in the plastic zone consisted of bands of dislocations or deformation twins, before transitioning to a refined sub-grain microstructure near the crack flank. The sub-grain structure extended 300-500 nm away from the crack flank in all the planar slip materials studied. In contrast, the bulk structure in the wavy slip material consisted of dislocation cells and did not transition to a different microstructure as the crack tip was approached. The strain in wavy slip was highest near the crack tip, as the misorientations between the dislocation cells increased and the cell size decreased as the crack flank was approached. The final set of experiments involved reloading the arrested crack tips in monotonic tension. This was performed on both the Haynes 230 and 316 stainless steel. This technique exposed the fracture surface and location of the arrested crack tip away from the free surface, allowing for a sample to be extracted via FIB micromachining and TEM evaluation of the microstructure. This permitted the crack tip microstructure to be investigated without exposing the microstructure to crack closure or free surface effects. These experiments confirmed what was inferred from the earlier experiments, namely that the banded structure was a product of the crack tip plastic zone and the refined structure was a product of the strain associated with crack advance. Overall the microstructural complexity presented in this work was much higher than would be predicted by current models of fatigue crack propagation. It is recommended that future models attempt to simulate interactions between the dislocations emitted during fatigue crack growth and the pre-existing microstructure to more accurately simulate the processes occurring at the crack tip during crack growth.

Quantity Effect of Radial Cracks on the Cracking Propagation Behavior and the Crack Morphology

PubMed Central

Chen, Jingjing; Xu, Jun; Liu, Bohan; Yao, Xuefeng; Li, Yibing

2014-01-01

In this letter, the quantity effect of radial cracks on the cracking propagation behavior as well as the circular crack generation on the impacted glass plate within the sandwiched glass sheets are experimentally investigated via high-speed photography system. Results show that the radial crack velocity on the backing glass layer decreases with the crack number under the same impact conditions during large quantities of repeated experiments. Thus, the “energy conversion factor” is suggested to elucidate the physical relation between the cracking number and the crack propagation speed. Besides, the number of radial crack also takes the determinative effect in the crack morphology of the impacted glass plate. This study may shed lights on understanding the cracking and propagation mechanism in laminated glass structures and provide useful tool to explore the impact information on the cracking debris. PMID:25048684

Effect of property gradients on enamel fracture in human molar teeth.

PubMed

Barani, Amir; Bush, Mark B; Lawn, Brian R

2012-11-01

A model for the fracture of tooth enamel with graded elastic modulus and toughness is constructed using an extended finite element modeling (XFEM) package. The property gradients are taken from literature data on human molars, with maximum in modulus at the outer enamel surface and in toughness at the inner surface. The tooth is modeled as a brittle shell (enamel) and a compliant interior (dentin), with occlusal loading from a hard, flat contact at the cusp. Longitudinal radial (R) and margin (M) cracks are allowed to extend piecewise along the enamel walls under the action of an incrementally increasing applied load. A simple stratagem is deployed in which fictitious temperature profiles generate the requisite property gradients. The resulting XFEM simulations demonstrate that the crack fronts become more segmented as the property gradients become more pronounced, with enhanced propagation at the outer surface and inhibited propagation at the inner. Whereas the growth history of the cracks is profoundly influenced by the gradients, the ultimate critical loads required to attain full fractures are relatively unaffected. Some implications concerning dentistry are considered. Copyright © 2012 Elsevier Ltd. All rights reserved.

3D ductile crack propagation within a polycrystalline microstructure using XFEM

NASA Astrophysics Data System (ADS)

Beese, Steffen; Loehnert, Stefan; Wriggers, Peter

2018-02-01

In this contribution we present a gradient enhanced damage based method to simulate discrete crack propagation in 3D polycrystalline microstructures. Discrete cracks are represented using the eXtended finite element method. The crack propagation criterion and the crack propagation direction for each point along the crack front line is based on the gradient enhanced damage variable. This approach requires the solution of a coupled problem for the balance of momentum and the additional global equation for the gradient enhanced damage field. To capture the discontinuity of the displacements as well as the gradient enhanced damage along the discrete crack, both fields are enriched using the XFEM in combination with level sets. Knowing the crack front velocity, level set methods are used to compute the updated crack geometry after each crack propagation step. The applied material model is a crystal plasticity model often used for polycrystalline microstructures of metals in combination with the gradient enhanced damage model. Due to the inelastic material behaviour after each discrete crack propagation step a projection of the internal variables from the old to the new crack configuration is required. Since for arbitrary crack geometries ill-conditioning of the equation system may occur due to (near) linear dependencies between standard and enriched degrees of freedom, an XFEM stabilisation technique based on a singular value decomposition of the element stiffness matrix is proposed. The performance of the presented methodology to capture crack propagation in polycrystalline microstructures is demonstrated with a number of numerical examples.

Dynamics of a fluid-driven crack in three dimensions by the finite difference method

NASA Astrophysics Data System (ADS)

Chouet, Bernard

1986-12-01

The finite difference method is applied to the study of the dynamics of a three-dimensional fluid-filled crack excited into resonance by the sudden failure of a small barrier of area ΔS on the crack surface. The impulse response of the crack is examined for various ratios of crack width to crack length and for several values of the crack stiffness C = (b/μ)(L/d), where b is the bulk modulus of the fluid, μ is the rigidity of the solid, and L and d are the crack length and crack thickness, respectively. The motion of the crack is characterized by distinct time scales representing the duration of brittle failure and the periods of acoustic resonance in the lateral and longitudinal dimensions of the source. The rupture has a duration proportional to the area of crack expansion and is the trigger responsible for the excitation of the crack into resonance; the resonant periods are proportional to the crack stiffness and to the width and length of the crack. The crack wave sustaining the resonance is analogous to the tube wave propagating in a fluid-filled borehole. It is dispersive, showing a phase velocity that decreases with increasing wavelength. Its wave speed is always lower than the acoustic velocity of the fluid and shows a strong dependence on the crack stiffness, decreasing as the stiffness increases. The initial motion of the crack surface is an opening, and the radiated far-field compressional wave starts with a strong but brief compression which has a duration proportional to the crack stiffness and size of the rupture area; the amplitude of this pulse increases with the area of rupture but decreases with increasing stiffness. Flow into the newly created cavity triggers a pressure drop in the fluid, which produces a partial collapse of the wall propagated over the crack surface at the speed of the crack wave. The collapse of the crack surface generates a weak long-period component of dilatation following the compressional first motion in the far-field P wave train; the dilatational component is clearer in the signal from stiffer cracks when seen in the direction of the rupture. The energy loss by radiation is stronger for high frequencies, resulting in a progressive enrichment of the crack response in lower frequencies over the duration of resonance. These source characteristics translate into a far-field signature that is marked by a high-frequency content near its onset and dominated by a longer-period component in its coda. The source duration shows a strong dependence on the fluid viscosity and associated viscous damping at the crack wall.

Penny-shaped crack propagation in spallation of Zr-BMGs

NASA Astrophysics Data System (ADS)

Ling, Z.; Huang, X.; Dai, L. H.

2015-09-01

Typical penny-shaped microcracks at their propagating in spallation of Zr-based bulk metallic glass (Zr-BMG) samples were captured by a specially designed plate impact technique. Based on the morphology and stress environment of the microcrack, a damaged zone or propagation zone around the crack tips, similar to the cohesive zone in classical fracture theories, is applied. Especially the scale of such a damaged zone represents a scale of the crack propagation. Its fast propagation would quickly bring a longer crack or cause cracks coalesce to form another longer one. The estimated propagation scales of microcracks are reasonable compared with what occurred in the Zr-BMG samples.

High temperature low cycle fatigue mechanisms for nickel base and a copper base alloy. M.S. Thesis Final Report

NASA Technical Reports Server (NTRS)

Shih, C. I.

1982-01-01

Damage mechanisms were studied in Rene' 95 and NARloy Z, using optical, scanning and transmission in microscopy. In necklace Rene' 95, crack initiation was mainly associated with cracking of surface MC carbides, except for hold time tests at higher strain ranges where initiation was associated more with a grain boundary mechanism. A mixed mode of propagation with a faceted fracture morphology was typical for all cycle characters. The dependence of life on maximum tensile stress can be demonstrated by the data falling onto three lines corresponding to the three tensile hold times, in the life against maximum tensile stress plot. In NARloy Z, crack initiation was always at the grain boundaries. The mode of crack propagation depended on the cycle character. The life decreased with decreasing strain rate and with tensile holds. In terms of damage mode, different life prediction laws may be applicable to different cycle characters.

Microstructural characterization of hydrogen induced cracking in TRIP-assisted steel by EBSD

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

Laureys, A., E-mail: Aurelie.Laureys@UGent.be; Depover, T.; Petrov, R.

2016-02-15

The present work evaluates hydrogen induced cracking by performing an elaborate EBSD (Electron BackScatter Diffraction) study in a steel with transformation induced plasticity (TRIP-assisted steel). This type of steel exhibits a multiphase microstructure which undergoes a deformation induced phase transformation. Additionally, each microstructural constituent displays a different behavior in the presence of hydrogen. The aim of this study is to obtain a better understanding on the mechanisms governing hydrogen induced crack initiation and propagation in the hydrogen saturated multiphase structure. Tensile tests on notched samples combined with in-situ electrochemical hydrogen charging were conducted. The tests were interrupted at stresses justmore » after reaching the tensile strength, i.e. before macroscopic failure of the material. This allowed to study hydrogen induced crack initiation and propagation by SEM (Scanning Electron Microscopy) and EBSD. A correlation was found between the presence of martensite, which is known to be very susceptible to hydrogen embrittlement, and the initiation of hydrogen induced cracks. Initiation seems to occur mostly by martensite decohesion. High strain regions surrounding the hydrogen induced crack tips indicate that further crack propagation may have occurred by the HELP (hydrogen-enhanced localized plasticity) mechanism. Small hydrogen induced cracks located nearby the notch are typically S-shaped and crack propagation was dominantly transgranularly. The second stage of crack propagation consists of stepwise cracking by coalescence of small hydrogen induced cracks. - Highlights: • Hydrogen induced cracking in TRIP-assisted steel is evaluated by EBSD. • Tensile tests were conducted on notched hydrogen saturated samples. • Crack initiation occurs by a H-Enhanced Interface DEcohesion (HEIDE) mechanism. • Crack propagation involves growth and coalescence of small cracks. • Propagation is governed by the characteristics of phases on the crack path.« less

Neural network approach to the inverse problem of the crack-depth determination from ultrasonic backscattering data

NASA Astrophysics Data System (ADS)

Takadoya, M.; Notake, M.; Kitahara, M.; Achenbach, J. D.; Guo, Q. C.; Peterson, M. L.

A neural network approach has been developed to determine the depth of a surface breaking crack in a steel plate from ultrasonic backscattering data. The network is trained by the use of a feedforward three-layered network together with a back-propagation algorithm for error corrections. Synthetic data are employed for network training. The signal used for crack isonification is a mode converted 45 deg transverse wave. The plate with a surface breaking crack is immersed in water, and the crack is insonified from the opposite uncracked side of the plate. A numerical analysis of the backscattered field is carried out based on the elastic wave theory by the use of the boundary element method. The numerical analysis provides synthetic data for the training of the network. The training data have been calculated for cracks with specific increments of the experimental data which are different from the training data.

Analysis of crack propagation as an energy absorption mechanism in metal matrix composites

NASA Technical Reports Server (NTRS)

Adams, D. F.; Murphy, D. P.

1981-01-01

The crack initiation and crack propagation capability was extended to the previously developed generalized plane strain, finite element micromechanics analysis. Also, an axisymmetric analysis was developed, which contains all of the general features of the plane analysis, including elastoplastic material behavior, temperature-dependent material properties, and crack propagation. These analyses were used to generate various example problems demonstrating the inelastic response of, and crack initiation and propagation in, a boron/aluminum composite.

Characterization of fatigue crack growth behavior in LENS fabricated Ti-6Al-4V using high-energy synchrotron x-ray microtomography

DOE PAGES

Sandgren, Hayley R.; Zhai, Yuwei; Lados, Diana A.; ...

2016-09-28

Laser Engineered Net Shaping (LENS) is an additive manufacturing technique that belongs to the ASTM standardized directed energy deposition category. To date, very limited work has been conducted towards understanding the fatigue crack growth behavior of LENS fabricated materials, which hinders the widespread adoption of this technology for high-integrity structural applications. In this study, the propagation of a 20 μm initial crack in LENS fabricated Ti-6Al-4V was captured in-situ, using high-energy synchrotron x-ray microtomography. Fatigue crack growth (FCG) data were then determined from 2D and 3D tomography reconstructions, as well as from fracture surface striation measurements using SEM. The generatedmore » data were compared to those obtained from conventional FCG tests that used compliance and direct current potential drop (DCPD) techniques to measure long and small crack growth. In conclusion, the observed agreement demonstrates that x-ray microtomography and fractographic analysis using SEM can be successfully combined to study the propagation behavior of fatigue cracks.« less

Characterization of fatigue crack growth behavior in LENS fabricated Ti-6Al-4V using high-energy synchrotron x-ray microtomography

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

Sandgren, Hayley R.; Zhai, Yuwei; Lados, Diana A.

Laser Engineered Net Shaping (LENS) is an additive manufacturing technique that belongs to the ASTM standardized directed energy deposition category. To date, very limited work has been conducted towards understanding the fatigue crack growth behavior of LENS fabricated materials, which hinders the widespread adoption of this technology for high-integrity structural applications. In this study, the propagation of a 20 μm initial crack in LENS fabricated Ti-6Al-4V was captured in-situ, using high-energy synchrotron x-ray microtomography. Fatigue crack growth (FCG) data were then determined from 2D and 3D tomography reconstructions, as well as from fracture surface striation measurements using SEM. The generatedmore » data were compared to those obtained from conventional FCG tests that used compliance and direct current potential drop (DCPD) techniques to measure long and small crack growth. In conclusion, the observed agreement demonstrates that x-ray microtomography and fractographic analysis using SEM can be successfully combined to study the propagation behavior of fatigue cracks.« less

Initiation and growth kinetics of solidification cracking during welding of steel

PubMed Central

Aucott, L.; Huang, D.; Dong, H. B.; Wen, S. W.; Marsden, J. A.; Rack, A.; Cocks, A. C. F.

2017-01-01

Solidification cracking is a key phenomenon associated with defect formation during welding. To elucidate the failure mechanisms, solidification cracking during arc welding of steel are investigated in situ with high-speed, high-energy synchrotron X-ray radiography. Damage initiates at relatively low true strain of about 3.1% in the form of micro-cavities at the weld subsurface where peak volumetric strain and triaxiality are localised. The initial micro-cavities, with sizes from 10 × 10−6 m to 27 × 10−6 m, are mostly formed in isolation as revealed by synchrotron X-ray micro-tomography. The growth of micro-cavities is driven by increasing strain induced to the solidifying steel. Cavities grow through coalescence of micro-cavities to form micro-cracks first and then through the propagation of micro-cracks. Cracks propagate from the core of the weld towards the free surface along the solidifying grain boundaries at a speed of 2–3 × 10−3 m s−1. PMID:28074852

Fracture resistance of Ti-5Al-2.5Sn extra-low interstitial castings

NASA Technical Reports Server (NTRS)

Fiftal, C. F.; Bolstad, D. A.; Misra, M. S.

1978-01-01

Fracture toughness and cyclic crack propagation data for Ti-5Al-2.5Sn extra-low interstitial (ELI) castings, 0.51 and 2.54 cm (0.20 and 1.00 in.) thick, at 394, 294, 77, and 20 K (250, 70, -320, and -423 F), are presented. Both surface flaw and compact tension geometries were tested. Comparison is made with other titanium alloys in both wrought and cast forms. Crack propagation resistance is comparable to wrought Ti-5Al-2.5Sn ELI, even with the extremely coarse as-cast grain size encountered.

Fatigue Crack Propagation from Notched Specimens of 304 SS in elevated Temperature Aqueous Environment

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

Wire, G. L.; Mills, W. J.

2002-08-01

Fatigue crack propagation (FCP) rates for 304 stainless steel (304SS) were determined in 24 degree C and 288 degree C air and 288 degree C water using double-edged notch (DEN) specimens of 304 stainless steel (304 SS). Test performed at matched loading conditions in air and water at 288 degree C with 20-6- cc h[sub]2/kg h[sub]2O provided a direct comparison of the relative crack growth rates in air and water over a wide range of crack growth rates. The DEN crack extension ranged from short cracks (0.03-0.25 mm) to long cracks up to 4.06 mm, which are consistent with conventionalmore » deep crack tests. Crack growth rates of 304 SS in water were about 12 times the air rate. This 12X environmental enhancement persisted to crack extensions up to 4.06 mm, far outside the range associated with short crack effects. The large environmental degradation for 304 SS crack growth is consistent with the strong reduction of fatigue life in high hydrogen water. Further, very similar environmental effects w ere reported in fatigue crack growth tests in hydrogen water chemistry (HWC). Most literature data in high hydrogen water show only a mild environmental effect for 304 SS, of order 2.5 times air or less, but the tests were predominantly performed at high cyclic stress intensity or equivalently, high air rates. The environmental effect in low oxygen environments at low stress intensity depends strongly on both the stress ratio, R, and the load rise time, T[sub]r, as recently reported for austenitic stainless steel in BWR water. Fractography was performed for both tests in air and water. At 288 degree C in water, the fracture surfaces were crisply faceted with a crystallographic appearance, and showed striations under high magnification. The cleavage-like facets on the fracture surfaces suggest that hydrogen embrittlement is the primary cause of accelerated cracking.« less

[Mechanism of the dentino-enamel junction on the resist-crack propagation of human teeth by the finite element method].

PubMed

Jingjing, Zheng; Tiezhou, Hou; Hong, Tao; Xueyan, Guo; Cui, Wu

2014-10-01

This study aims to identify the crack tip stress intensity factor of the propagation process, crack propagation path, and the changes in the shape of the crack tip by the finite element method. The finite element model of dentino-enamel junction was established with ANSYS software, and the length of the initial crack in the single edge was set to 0.1 mm. The lower end of the sample was fixed. The tensile load of 1 MPa with frequency of 5 Hz was applied to the upper end. The stress intensity factor, deflection angle, and changes in the shape of the crack tip in the crack propagation were calculated by ANSYS. The stress intensity factor suddenly and continuously decreased in dentino-enamel junction as the crack extended. A large skewed angle appeared, and the stress on crack tip was reduced. The dentino-enamel junction on human teeth may resist crack propagation through stress reduction.

Self-Healing Nanotextured Vascular-like Materials: Mode I Crack Propagation.

PubMed

Lee, Min Wook; Sett, Soumyadip; An, Seongpil; Yoon, Sam S; Yarin, Alexander L

2017-08-16

Here, we investigate crack propagation initiated from an initial notch in a self-healing material. The crack propagation in the core-shell nanofiber mats formed by coelectrospinning and the composites reinforced by them is in focus. All samples are observed from the crack initiation until complete failure. Due to the short-time experiments done on purpose, the resin and cure released from the cores of the core-shell nanofibers could not achieve a complete curing and stop crack growth, especially given the fact that no heating was used. The aim is to elucidate their effect on the rate of crack propagation. The crack propagation speed in polyacrylonitrile (PAN)-resin-cure nanofiber mats (with PAN being the polymer in the shell) was remarkably lower than that in the corresponding monolithic PAN nanofiber mat, down to 10%. The nanofiber mats were also encased in polydimethylsiloxane (PDMS) matrix to form composites. The crack shape and propagation in the composite samples were studied experimentally and analyzed theoretically, and the theoretical results revealed agreement with the experimental data.

Factors affecting stress assisted corrosion cracking of carbon steel under industrial boiler conditions

NASA Astrophysics Data System (ADS)

Yang, Dong

Failure of carbon steel boiler tubes from waterside has been reported in the utility boilers and industrial boilers for a long time. In industrial boilers, most waterside tube cracks are found near heavy attachment welds on the outer surface and are typically blunt, with multiple bulbous features indicating a discontinuous growth. These types of tube failures are typically referred to as stress assisted corrosion (SAC). For recovery boilers in the pulp and paper industry, these failures are particularly important as any water leak inside the furnace can potentially lead to smelt-water explosion. Metal properties, environmental variables, and stress conditions are the major factors influencing SAC crack initation and propagation in carbon steel boiler tubes. Slow strain rate tests (SSRT) were conducted under boiler water conditions to study the effect of temperature, oxygen level, and stress conditions on crack initation and propagation on SA-210 carbon steel samples machined out of boiler tubes. Heat treatments were also performed to develop various grain size and carbon content on carbon steel samples, and SSRTs were conducted on these samples to examine the effect of microstructure features on SAC cracking. Mechanisms of SAC crack initation and propagation were proposed and validated based on interrupted slow strain tests (ISSRT). Water chemistry guidelines are provided to prevent SAC and fracture mechanics model is developed to predict SAC failure on industrial boiler tubes.

Effect of Speed (Centrifugal Load) on Gear Crack Propagation Direction

NASA Technical Reports Server (NTRS)

Lewicki, David G.

2001-01-01

The effect of rotational speed (centrifugal force) on gear crack propagation direction was explored. Gears were analyzed using finite element analysis and linear elastic fracture mechanics. The analysis was validated with crack propagation experiments performed in a spur gear fatigue rig. The effects of speed, rim thickness, and initial crack location on gear crack propagation direction were investigated. Crack paths from the finite element method correlated well with those deduced from gear experiments. For the test gear with a backup ratio (rim thickness divided by tooth height) of nib = 0.5, cracks initiating in the tooth fillet propagated to rim fractures when run at a speed of 10,000 rpm and became tooth fractures for speeds slower than 10,000 rpm for both the experiments and anal sis. From additional analysis, speed had little effect on crack propagation direction except when initial crack locations were near the tooth/rim fracture transition point for a given backup ratio. When at that point, higher speeds tended to promote rim fracture while lower speeds (or neglecting centrifugal force) produced tooth fractures.

Cleaving the Halqeh-ye-nur diamonds: a dynamic fracture analysis.

PubMed

Atkinson, Colin; Martineau, Philip M; Khan, Rizwan U A; Field, John E; Fisher, David; Davies, Nick M; Samartseva, Julia V; Putterman, Seth J; Hird, Jonathan R

2015-03-28

The degree of surface roughness and clarity with which a surface in a brittle material can be formed via fracture is known to be related to the speed of the propagating crack. Cracks traversing a brittle material at low speed produce very smooth surfaces, while those propagating faster create less reflective and rough surfaces (Buehler MJ, Gao H. 2006 Nature 439, 307-310 (doi:10.1038/nature04408)). The elastic wave speeds (c(l)≈18 000 m s(-1), c(s)≈11 750 m s(-1)) in diamond are fast (Willmott GR, Field JE. 2006 Phil. Mag. 86, 4305-4318 (doi:10.1080/14786430500482336)) and present a particular problem in creating smooth surfaces during the cleaving of diamond-a routine operation in the fashioning of diamonds for gemstone purposes--as the waves are reflected from the boundaries of the material and can add a tensile component to the propagating crack tip causing the well-known cleavage steps observed on diamond surfaces (Field JE. 1971 Contemp. Phys. 12, 1-31 (doi:10.1080/00107517108205103); Field JE. 1979 Properties of diamond, 1st edn, Academic Press; Wilks EM. 1958 Phil. Mag. 3, 1074-1080 (doi:10.1080/14786435808237036)). Here we report an analysis of two diamonds, having large dimensions and high aspect ratio, which from a gemological analysis are shown to have been cleaved from the same 200 carat specimen. A methodology for their manufacture is calculated by an analysis of a model problem. This takes into account the effect of multiple reflections from the sample boundaries. It is suggested that the lapidary had an intuitive guide to how to apply the cleavage force in order to control the crack speed. In particular, it is shown that it is likely that this technique caused the fracture to propagate at a lower speed. The sacrifice of a large diamond with the intention of creating thin plates, rather than a faceted gemstone, demonstrates how symbolism and beliefs associated with gemstones have changed over the centuries (Harlow GE. 1998 The nature of diamonds, Cambridge University Press). The scientific insights gained by studying these gemstones suggest a method of producing macroscale atomically flat and stress-free surfaces on other brittle materials.

Cleaving the Halqeh-ye-nur diamonds: a dynamic fracture analysis

PubMed Central

Atkinson, Colin; Martineau, Philip M.; Khan, Rizwan U. A.; Field, John E.; Fisher, David; Davies, Nick M.; Samartseva, Julia V.; Putterman, Seth J.; Hird, Jonathan R.

2015-01-01

The degree of surface roughness and clarity with which a surface in a brittle material can be formed via fracture is known to be related to the speed of the propagating crack. Cracks traversing a brittle material at low speed produce very smooth surfaces, while those propagating faster create less reflective and rough surfaces (Buehler MJ, Gao H. 2006 Nature 439, 307–310 (doi:10.1038/nature04408)). The elastic wave speeds (cl≈18 000 m s−1, cs≈11 750 m s−1) in diamond are fast (Willmott GR, Field JE. 2006 Phil. Mag. 86, 4305–4318 (doi:10.1080/14786430500482336)) and present a particular problem in creating smooth surfaces during the cleaving of diamond—a routine operation in the fashioning of diamonds for gemstone purposes—as the waves are reflected from the boundaries of the material and can add a tensile component to the propagating crack tip causing the well-known cleavage steps observed on diamond surfaces (Field JE. 1971 Contemp. Phys. 12, 1–31 (doi:10.1080/00107517108205103); Field JE. 1979 Properties of diamond, 1st edn, Academic Press; Wilks EM. 1958 Phil. Mag. 3, 1074–1080 (doi:10.1080/14786435808237036)). Here we report an analysis of two diamonds, having large dimensions and high aspect ratio, which from a gemological analysis are shown to have been cleaved from the same 200 carat specimen. A methodology for their manufacture is calculated by an analysis of a model problem. This takes into account the effect of multiple reflections from the sample boundaries. It is suggested that the lapidary had an intuitive guide to how to apply the cleavage force in order to control the crack speed. In particular, it is shown that it is likely that this technique caused the fracture to propagate at a lower speed. The sacrifice of a large diamond with the intention of creating thin plates, rather than a faceted gemstone, demonstrates how symbolism and beliefs associated with gemstones have changed over the centuries (Harlow GE. 1998 The nature of diamonds, Cambridge University Press). The scientific insights gained by studying these gemstones suggest a method of producing macroscale atomically flat and stress-free surfaces on other brittle materials. PMID:25713458

[Monitoring of Crack Propagation in Repaired Structures Based on Characteristics of FBG Sensors Reflecting Spectra].

PubMed

Yuan, Shen-fang; Jin, Xin; Qiu, Lei; Huang, Hong-mei

2015-03-01

In order to improve the security of aircraft repaired structures, a method of crack propagation monitoring in repaired structures is put forward basing on characteristics of Fiber Bragg Grating (FBG) reflecting spectra in this article. With the cyclic loading effecting on repaired structure, cracks propagate, while non-uniform strain field appears nearby the tip of crack which leads to the FBG sensors' reflecting spectra deformations. The crack propagating can be monitored by extracting the characteristics of FBG sensors' reflecting spectral deformations. A finite element model (FEM) of the specimen is established. Meanwhile, the distributions of strains which are under the action of cracks of different angles and lengths are obtained. The characteristics, such as main peak wavelength shift, area of reflecting spectra, second and third peak value and so on, are extracted from the FBGs' reflecting spectral which are calculated by transfer matrix algorithm. An artificial neural network is built to act as the model between the characteristics of the reflecting spectral and the propagation of crack. As a result, the crack propagation of repaired structures is monitored accurately and the error of crack length is less than 0.5 mm, the error of crack angle is less than 5 degree. The accurately monitoring problem of crack propagation of repaired structures is solved by taking use of this method. It has important significance in aircrafts safety improvement and maintenance cost reducing.

Thermally induced stresses in boulders on airless body surfaces: Implications for breakdown

NASA Astrophysics Data System (ADS)

Molaro, Jamie; Byrne, Shane

2016-10-01

We investigate the role of thermally induced rock breakdown in the evolution of airless body surfaces. This process is driven by the propagation of microcracks due to stress caused by changes in temperature. Here we model the thermomechanical response of spherical lunar boulders of varying size to diurnal thermal forcing. Exploring the magnitude and distribution of induced stresses reveals a bimodal response. During sunrise, high stresses occur in the boulders' interiors that are associated with large-scale temperature gradients (developed due to overnight cooling). During sunset, high stresses occur at the boulders' exteriors due to the cooling and contraction of the surface. Both kinds of stresses are on the order of 10 MPa in 1 m boulders and decrease for smaller radii, suggesting that larger boulders break down more quickly. Boulders ≤30 cm exhibit a weak response to thermal forcing, suggesting a boulder-size threshold below which crack propagation may not occur. Boulders of any size buried by regolith are shielded from thermal breakdown.As boulders increase in size (>1 m), stresses increase to several 10s of MPa as the behavior of their surfaces approaches that of an infinite halfspace. The rate of stress-increase is rapid until the boulder reaches ~5 times the skin depth (~4 m) in size. Above this size, stresses only slowly increase as the surface loses thermal contact with the boulder center. Boulders between 3 m and 7 m have less volume of material to erode than larger boulders (> 10 m) but only moderately lower stresses, suggesting they may be preferentially broken down by this process.Stress orientations can yield insight into how breakdown may occur. Interior stresses act on a plane perpendicular to the path of the sun, driving the propagation of surface-parallel cracks and contributing to exfoliation of planar fragments. Exterior stresses act parallel to the boulder surface driving the propagation of surface-perpendicular cracks and contributing to granular disintegration. These two mechanisms likely work together to hasten disaggregation of the near-surface.We will present results for boulder stresses on the Moon and other airless bodies, and discuss implications for breakdown on these surfaces.

Initiation and propagation toughness of delamination crack under an impact load

NASA Astrophysics Data System (ADS)

Kumar, Prashant; Kishore, N. N.

1998-10-01

A combined experimental and finite element method is developed to determine the interlaminar dynamic fracture toughness. An interlaminar crack is propagated at very high speed in a double cantilever beam (DCB) specimen made of two steel strips with a precrack. A special fixture is designed to apply impact load to one cantilever and determine the deflection of the cantilever-end, initiation time and crack propagation history. The experimental results are used as input data in a FE code to calculate J-integral by the gradual release of nodal forces to model the propagation of the interlaminar crack. The initiation fracture toughness and propagation fracture toughness are evaluated for interlaminar crack propagating between 850 and 1785 ms. The initiation and propagation toughness were found to vary between 90-200 Jm 2 and 2-13 Jm 2 respectively. The technique is extended to study initiation and propagation toughness of interlaminar crack in unidirectional FRP laminates. 1998 Elsevier Science Ltd.

Small crack test program for helicopter materials

NASA Technical Reports Server (NTRS)

Annigeri, Bal; Schneider, George

1994-01-01

Crack propagation tests were conducted to determine crack growth behavior in five helicopter materials for surface cracks between 0.005 to 0.020 inches in depth. Constant amplitude tests were conducted at stress ratios R equals 0.1 and 0.5, and emphasis was placed on near threshold data (i.e., 10-8 to 10-6 inches/cycle). Spectrum tests were conducted using a helicopter spectrum. The test specimen was an unnotched tension specimen, and cracks were initiated from a small EDM notch. An optical/video system was used to monitor crack growth. The material for the test specimens was obtained from helicopter part forgings. Testing was conducted at stresses below yield to reflect actual stresses in helicopter parts.

On the role of weak interface in crack blunting process in nanoscale layered composites

NASA Astrophysics Data System (ADS)

Li, Yi; Zhou, Qing; Zhang, Shuang; Huang, Ping; Xu, Kewei; Wang, Fei; Lu, Tianjian

2018-03-01

Heterointerface in a nanoscale metallic layered composite could improve its crack resistance. However, the influence of metallic interface structures on crack propagation has not been well understood at atomic scale. By using the method of molecular dynamics (MD) simulation, the crack propagation behavior in Cu-Nb bilayer is compared with that in Cu-Ni bilayer. We find that the weak Cu-Nb interface plays an important role in hindering crack propagation in two ways: (i) dislocation nucleation at the interface releases stress concentration for the crack to propagate; (ii) the easily sheared weak incoherent interface blunts the crack tip. The results are helpful for understanding the interface structure dependent crack resistance of nanoscale bicrystal interfaces.

Fractographic ceramic failure analysis using the replica technique

PubMed Central

Scherrer, Susanne S.; Quinn, Janet B.; Quinn, George D.; Anselm Wiskott, H. W.

2007-01-01

Objectives To demonstrate the effectiveness of in vivo replicas of fractured ceramic surfaces for descriptive fractography as applied to the analysis of clinical failures. Methods The fracture surface topography of partially failed veneering ceramic of a Procera Alumina molar and an In Ceram Zirconia premolar were examined utilizing gold-coated epoxy poured replicas viewed using scanning electron microscopy. The replicas were inspected for fractographic features such as hackle, wake hackle, twist hackle, compression curl and arrest lines for determination of the direction of crack propagation and location of the origin. Results For both veneering ceramics, replicas provided an excellent reproduction of the fractured surfaces. Fine details including all characteristic fracture features produced by the interaction of the advancing crack with the material's microstructure could be recognized. The observed features are indicators of the local direction of crack propagation and were used to trace the crack's progression back to its initial starting zone (the origin). Drawbacks of replicas such as artifacts (air bubbles) or imperfections resulting from inadequate epoxy pouring were noted but not critical for the overall analysis of the fractured surfaces. Significance The replica technique proved to be easy to use and allowed an excellent reproduction of failed ceramic surfaces. It should be applied before attempting to remove any failed part remaining in situ as the fracture surface may be damaged during this procedure. These two case studies are intended as an introduction for the clinical researcher in using qualitative (descriptive) fractography as a tool for understanding fracture processes in brittle restorative materials and, secondarily, to draw conclusions as to possible design inadequacies in failed restorations. PMID:17270267

Fatigue Crack Topography.

DTIC Science & Technology

1984-01-01

nominal cycle frequency of 15 Hz. Buckling of the specimens during compression loading was prevented by felt-lined aluminium alloy antibuckling guides...evaluating ciack initiation time and crack propagation, prgram I was used for performing the major fatigue test with the aircraft structure. In...direction of the notch to prevent scratches in the through-the-thickness direction. Prior to testing, the notch surfaces were lightly etched to reveal

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.

Stone orientation affects the mechanism of failure in artificial kidney stones subject to shock waves

NASA Astrophysics Data System (ADS)

van Cauwelaert, Javier; Cleveland, Robin O.

2003-10-01

Micro computed tomography (CT) imaging was used to follow the progressive development of cracks in artificial kidney stones. The artificial stones were made from U30 cement with a cylindrical shape (6.5 mm diameter and 8.5 mm long). The stones were held within a polypropylene vial in one of three orientations: vertical, horizontal, and angled at 45 deg. The stones were treated with an electromagnetic lithotripter and the initiation and growth of cracks was observed using microCT. The images show that the orientation of the stones with respect to the shock changes the dominant mechanism for fragmentation. Vertical stones developed a spall-like crack near the distal surface, which propagated from the surface to the interior of the stone. Initiation of a secondary spall-like crack was observed proximal to the first crack. Little surface damage was observed. Horizontal stones presented pitting in the proximal surface and erosion in lateral faces, indicating the action of cavitation. Angled stones presented both spall-like fracture in either the leading or the distal corners and surface damage (pitting) in the proximal surface. Experiments are being performed to follow the development of cracks in human kidney stones. [Work supported by the Whitaker Foundation.

Investigation of the effects of manufacturing variations and materials on fatigue crack detection methods in gear teeth

NASA Technical Reports Server (NTRS)

Wheitner, Jeffrey A.; Houser, Donald R.

1994-01-01

The fatigue life of a gear tooth can be thought of as the sum of the number of cycles required to initiate a crack, N(sub i), plus the number of cycles required to propagate the crack to such a length that fracture occurs, N(sub p). The factors that govern crack initiation are thought to be related to localized stress or strain at a point, while propagation of a fatigue crack is a function of the crack tip parameters such as crack shape, stress state, and stress intensity factor. During a test there is no clear transition between initiation and propagation. The mechanisms of initiation and propagation are quite different and modeling them separately produces a higher degree of accuracy, but then the question that continually arises is 'what is a crack?' The total life prediction in a fracture mechanics model presently hinges on the assumption of an initial crack length, and this length can significantly affect the total life prediction. The size of the initial crack is generally taken to be in the range of 0.01 in. to 0.2 in. Several researchers have used various techniques to determine the beginning of the crack propagation stage. Barhorst showed the relationship between dynamic stiffness changes and crack propagation. Acoustic emissions, which are stress waves produced by the sudden movement of stressed materials, have also been successfully used to monitor the growth of cracks in tensile and fatigue specimens. The purpose of this research is to determine whether acoustic emissions can be used to define the beginning of crack propagation in a gear using a single-tooth bending fatigue test.

Concrete Cracking Prediction Including the Filling Proportion of Strand Corrosion Products.

PubMed

Wang, Lei; Dai, Lizhao; Zhang, Xuhui; Zhang, Jianren

2016-12-23

The filling of strand corrosion products during concrete crack propagation is investigated experimentally in the present paper. The effects of stirrups on the filling of corrosion products and concrete cracking are clarified. A prediction model of crack width is developed incorporating the filling proportion of corrosion products and the twisting shape of the strand. Experimental data on cracking angle, crack width, and corrosion loss obtained from accelerated corrosion tests of concrete beams are presented. The proposed model is verified by experimental data. Results show that the filling extent of corrosion products varies with crack propagation. The rust filling extent increases with the propagating crack until a critical width. Beyond the critical width, the rust-filling extent remains stable. Using stirrups can decrease the critical crack width. Stirrups can restrict crack propagation and reduce the rust filling. The tangent of the cracking angle increases with increasing corrosion loss. The prediction of corrosion-induced crack is sensitive to the rust-filling extent.

Concrete Cracking Prediction Including the Filling Proportion of Strand Corrosion Products

PubMed Central

Wang, Lei; Dai, Lizhao; Zhang, Xuhui; Zhang, Jianren

2016-01-01

The filling of strand corrosion products during concrete crack propagation is investigated experimentally in the present paper. The effects of stirrups on the filling of corrosion products and concrete cracking are clarified. A prediction model of crack width is developed incorporating the filling proportion of corrosion products and the twisting shape of the strand. Experimental data on cracking angle, crack width, and corrosion loss obtained from accelerated corrosion tests of concrete beams are presented. The proposed model is verified by experimental data. Results show that the filling extent of corrosion products varies with crack propagation. The rust filling extent increases with the propagating crack until a critical width. Beyond the critical width, the rust-filling extent remains stable. Using stirrups can decrease the critical crack width. Stirrups can restrict crack propagation and reduce the rust filling. The tangent of the cracking angle increases with increasing corrosion loss. The prediction of corrosion-induced crack is sensitive to the rust-filling extent. PMID:28772367

Microstructural characterization of Charpy-impact-tested nanostructured bainite

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

Tsai, Y.T.; Chang, H.T.; Huang, B.M.

2015-09-15

In this work, a possible cause of the extraordinary low impact toughness of nanostructured bainite has been investigated. The microstructure of nanostructured bainite consisted chiefly of carbide-free bainitic ferrite with retained austenite films. X-ray diffractometry (XRD) measurement indicated that no retained austenite existed in the fractured surface of the Charpy-impact-tested specimens. Fractographs showed that cracks propagated mainly along bainitic ferrite platelet boundaries. The change in microstructure after impact loading was verified by transmission electron microscopy (TEM) observations, confirming that retained austenite was completely transformed to strain-induced martensite during the Charpy impact test. However, the zone affected by strained-induced martensite wasmore » found to be extremely shallow, only to a depth of several micrometers from the fracture surface. It is appropriately concluded that upon impact, as the crack forms and propagates, strain-induced martensitic transformation immediately occurs ahead of the advancing crack tip. The successive martensitic transformation profoundly facilitates the crack propagation, resulting in the extremely low impact toughness of nanostructured bainite. Retained austenite, in contrast to its well-known beneficial role, has a deteriorating effect on toughness during the course of Charpy impact. - Highlights: • The microstructure of nanostructured bainite consisted of nano-sized bainitic ferrite subunits with retained austenite films. • Special sample preparations for SEM, XRD and TEM were made, and the strain-affected structures have been explored. • Retained austenite films were found to transform into martensite after impact loading, as evidenced by XRD and TEM results. • The zone of strain-induced martensite was found to extend to only several micrometers from the fracture surface. • The poor Charpy impact toughness is associated with the fracture of martensite at a high strain rate during impact loading.« less

Crack propagation at stresses below the fatigue limit.

NASA Technical Reports Server (NTRS)

Holden, F. C.; Hyler, W. S.; Marschall, C. W.

1967-01-01

Crack propagation for stainless steel and Ti alloy at stresses below fatigue limit, noting of alternating stress cycles crack propagation for stainless steel and Ti alloy at stresses below fatigue limit, noting role of alternating stress cycles

Use of Carbon Nano-Fiber Foams as Strain Gauges to Detect Crack Propagation

DTIC Science & Technology

2015-06-01

FIBER FOAMS AS STRAIN GAUGES TO DETECT CRACK PROPAGATION by Ervin N. Mercado June 2015 Thesis Advisor: Claudia C. Luhrs Co-Advisor...AND DATES COVERED Master’s Thesis 4. TITLE AND SUBTITLE USE OF CARBON NANO-FIBER FOAMS AS STRAIN GAUGES TO DETECT CRACK PROPAGATION 5. FUNDING...using carbon nanofiber foams as strain gauge material to detect crack propagation in aluminum structures. We produced the tridimensional carbon

Method of fabricating an abradable gas path seal

NASA Technical Reports Server (NTRS)

Bill, R. C.; Wisander, D. W. (Inventor)

1984-01-01

The thermal shock resistance of a ceramic layer is improved. The invention is particularly directed to an improved abradable lining that is deposited on shroud forming a gas path in turbomachinery. Improved thermal shock resistance of a shroud is effected through the deliberate introduction of benign cracks. These are microcracks which will not propagate appreciably upon exposure to the thermal shock environment in which a turbine seal must function. Laser surface fusion treatment is used to introduce these microcracks. The ceramic surface is laser scanned to form a continuous dense layer. As this layer cools and solidifies, shrinkage results in the formation of a very fine crack network. The presence of this deliberately introduced fine crack network precludes the formation of a catastrophic crack during thermal shock exposure.

Stress corrosion cracking of titanium alloys

NASA Technical Reports Server (NTRS)

May, R. C.; Beck, F. H.; Fontana, M. G.

1971-01-01

Experiments were conducted to study (1) the basic electrochemical behavior of titanium in acid chloride solutions and (2) the response of the metal to dynamic straining in the same evironment. The aim of this group of experiments was to simulate, as nearly as possible, the actual conditions which exist at the tip of a crack. One of the foremost theories proposed to explain the propagation of stress corrosion cracks is a hydrogen embrittlement theory involving the precipitation of embrittling titanium hydrides inside the metal near the crack tip. An initial survey of the basic electrochemical literature indicated that surface hydrides play a critical role in the electrochemistry of titanium in acid solutions. A comprehensive analysis of the effect of surface films, particularly hydrides, on the electrochemical behavior of titanium in these solution is presented.

Effect of micromorphology at the fatigue crack tip on the crack growth in electron beam welded Ti-6Al-4V joint

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

Tao, Junhui; Hu, Shubing, E-mail: 187352581@qq.com

In this paper, we describe experiments on welded joints of Ti-6Al-4V alloy specimens exhibiting fatigue characteristics in the base metal (BM), hot affected zone (HAZ) and fuse zone (FZ). The effect of micromorphology on crack propagation at the tip of the fatigue crack in joints formed by electron beam welding was investigated using an optical microscope, transmission electron microscope and other methodologies. The results demonstrated that the fatigue crack originated in and propagated along α/β boundaries in the BM. In the HAZ, the fatigue crack occurred at the boundary between martensite laths, and propagated through most irregular-equiaxed α phases andmore » a few martensite laths. In the FZ, the fatigue crack originated at the boundaries between the fine crushing phases among martensite laths, and propagated along a majority of α/β boundaries and several narrow martensite laths. The electron beam welded joint of Ti-6Al-4V alloy showed instances of zigzag fatigue cracks that increased in degree from lowest in the HAZ, moderate in the FZ to greatest in the BM. Conversely, fatigue crack growth rate (FCGR) was greatest in the HAZ, less in the FZ and slowest in the BM. - Highlights: •Ti-6Al-4V welded joint exhibits different fatigue characteristics. •The fatigue crack propagates along α/β boundaries in the BM. •The fatigue crack propagates through α phases and martensite laths in the HAZ. •The fatigue crack propagates along α/β boundaries and martensite laths in the FZ. •Fatigue crack growth rate is fastest in the HAZ, less in the FZ, slowest in the BM.« less

The use of COD and plastic instability in crack propagation and arrest in shells

NASA Technical Reports Server (NTRS)

Erdogan, F.; Ratwani, M.

1974-01-01

The initiation, growth, and possible arrest of fracture in cylindrical shells containing initial defects are dealt with. For those defects which may be approximated by a part-through semi-elliptic surface crack which is sufficiently shallow so that part of the net ligament in the plane of the crack is still elastic, the existing flat plate solution is modified to take into account the shell curvature effect as well as the effect of the thickness and the small scale plastic deformations. The problem of large defects is then considered under the assumptions that the defect may be approximated by a relatively deep meridional part-through surface crack and the net ligament through the shell wall is fully yielded. The results given are based on an 8th order bending theory of shallow shells using a conventional plastic strip model to account for the plastic deformations around the crack border.

A comparison of the fatigue behavior of cast Ti-7.5Mo with c.p. titanium, Ti-6Al-4V and Ti-13Nb-13Zr alloys.

PubMed

Lin, Chia-Wei; Ju, Chien-Ping; Chern Lin, Jiin-Huey

2005-06-01

The purpose of the present study is to compare the high-cycle fatigue behavior of newly developed Ti-7.5Mo alloy with that of c.p. Ti, Ti-13Nb-13Zr and Ti-6Al-4V alloys in their as-cast state. Experimental results indicate that Ti-6Al-4V and c.p. Ti have higher stress-controlled fatigue resistance but lower strain-controlled fatigue resistance than Ti-7.5Mo and Ti-13Nb-13Zr. Among four materials Ti-7.5Mo demonstrates the best strain-controlled fatigue performance. The fracture surfaces of the present materials are comprised of three morphologically distinct zones: crack initiation zone, crack propagation zone, and the final-stage overload zone. The fatigue cracks almost always initiate from casting-induced surface/subsurface pores. A river pattern is observed in the propagation zone. In the overload zone dimples are typically observed. Three factors most significantly affecting the fatigue performance of the present materials are the presence of the casting-induced surface/subsurface pores; the location of the pores; and the inherent mechanical properties of the materials.

Numerical simulations of electric potential field for alternating current potential drop associated with surface cracks in low-alloy steel nuclear material

NASA Astrophysics Data System (ADS)

Yeh, Chun-Ping; Huang, Jiunn-Yuan

2018-04-01

Low-alloy steels used as structural materials in nuclear power plants are subjected to cyclic stresses during power plant operations. As a result, cracks may develop and propagate through the material. The alternating current potential drop technique is used to measure the lengths of cracks in metallic components. The depth of the penetration of the alternating current is assumed to be small compared to the crack length. This assumption allows the adoption of the unfolding technique to simplify the problem to a surface Laplacian field. The numerical modelling of the electric potential and current density distribution prediction model for a compact tension specimen and the unfolded crack model are presented in this paper. The goal of this work is to conduct numerical simulations to reduce deviations occurring in the crack length measurements. Numerical simulations were conducted on AISI 4340 low-alloy steel with different crack lengths to evaluate the electric potential distribution. From the simulated results, an optimised position for voltage measurements in the crack region was proposed.

Research on anti crack mechanism of bionic coupling brake disc

NASA Astrophysics Data System (ADS)

Shi, Lifeng; Yang, Xiao; Zheng, Lingnan; Wu, Can; Ni, Jing

2017-09-01

According to the biological function of fatigue resistance possessed by biology, this study designed a Bionic Coupling Brake Disc (BCBD) which can inhibit crack propagation as the result of improving fatigue property. Thermal stress field of brake disc was calculated under emergency working condition, and circumferential and radial stress field which lead to fatigue failure of brake disc were investigated simultaneously. Results showed that the maximum temperature of surface reached 890°C and the maximum residual tensile stress was 207 Mpa when the initial velocity of vehicle was 200 km/h. Based on the theory of elastic plastic fracture mechanics, the crack opening displacement and the crack front J integrals of the BCBD and traditional brake disc (TBD) with pre-cracking were calculated, and the strength of crack front was compared. Results revealed the growth behavior of fatigue crack located on surface of brake disc, and proved the anti fatigue resistance of BCBD was better, and the strength of crack resistance of BCBD was much stronger than that of TBD. This simulation research provided significant references for optimization and manufacturing of BCBD.

Safety assessment of Cracked K-joint Structure Based on Fracture Mechanics

NASA Astrophysics Data System (ADS)

Wang, Xin; Pengyu, Yan; Jianwei, Du; Fuhai, Cai

2017-05-01

The K-joint is the main bearing structure of lattice jib crane. During frequent operation of the crane, surface cracks often occur at its weld toe, and then continue to expand until failure. The safety of the weak structure K-joint of the crane jib can be evaluated by BS7910 failure assessment standard in order to improve its utilization. The finite element model of K-joint structure with cracks is established, and its mechanical properties is analyzed by ABAQUS software, the results show that the crack depth has a great influence on the bearing capacity of the structure compared with the crack length. It is assumed that the K-joint with the semi-elliptical surface crack under the action of the tension propagate stably under the condition that the c/a (ratio of short axis to long axis of ellipse) is about 0.3. The safety assessment of K-joint with different lengths crack is presented according to the 2A failure assessment diagram of BS7910, and the critical crack of K-joint under different loads can be obtained.

The Molecular Volcano Revisited: Determination of Crack Propagation and Distribution During the Crystallization of Nanoscale Amorphous Solid Water Films.

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

May, Robert A.; Smith, R. Scott; Kay, Bruce D.

2012-02-02

Temperature programmed desorption (TPD) is utilized to determine the length distribution of cracks formed through amorphous solid water (ASW) during crystallization. This distribution is determined by monitoring how the thickness of an ASW overlayer alters desorption of an underlayer of O2. As deposited the ASW overlayer prevents desorption of O2. During crystallization, cracks form through the ASW overlayer and open a path to vacuum which allows O2 to escape in a rapid episodic release known as the 'molecular volcano'. Sufficiently thick ASW overlayers further trap O2 resulting in a second O2 desorption peak commensurate with desorption of the last ofmore » the ASW overlayer. The evolution of this trapping peak with overlayer thickness is the basis for determining the distribution of crystallization induced cracks through the ASW. Reflection adsorption infrared spectroscopy (RAIRS) and TPD of multicomponent parfait structures of ASW, O2 and Kr indicate that a preponderance of these cracks propagate down from the outer surface of the ASW.« less

Modeling Of Spontaneous Multiscale Roughening And Branching of Ruptures Propagating On A Slip-Weakening Frictional Fault

NASA Astrophysics Data System (ADS)

Elbanna, A. E.

2013-12-01

Numerous field and experimental observations suggest that faults surfaces are rough at multiple scales and tend to produce a wide range of branch sizes ranging from micro-branching to large scale secondary faults. The development and evolution of fault roughness and branching is believed to play an important role in rupture dynamics and energy partitioning. Previous work by several groups has succeeded in determining conditions under which a main rupture may branch into a secondary fault. Recently, there great progress has been made in investigating rupture propagation on rough faults with and without off-fault plasticity. Nonetheless, in most of these models the heterogeneity, whether the roughness profile or the secondary faults orientation, was built into the system from the beginning and consequently the final outcome depends strongly on the initial conditions. Here we introduce an adaptive mesh technique for modeling mode-II crack propagation on slip weakening frictional interfaces. We use a Finite Element Framework with random mesh topology that adapts to crack dynamics through element splitting and sequential insertion of frictional interfaces dictated by the failure criterion. This allows the crack path to explore non-planar paths and develop the roughness profile that is most compatible with the dynamical constraints. It also enables crack branching at different scales. We quantify energy dissipation due to the roughening process and small scale branching. We compare the results of our model to a reference case for propagation on a planar fault. We show that the small scale processes of roughening and branching influence many characteristics of the rupture propagation including the energy partitioning, rupture speed and peak slip rates. We also estimate the fracture energy required for propagating a crack on a planar fault that will be required to produce comparable results. We anticipate that this modeling approach provides an attractive methodology that complements the current efforts in modeling off-fault plasticity and damage.

Crack Propagation Calculations for Optical Fibers under Static Bending and Tensile Loads Using Continuum Damage Mechanics

PubMed Central

Chen, Yunxia; Cui, Yuxuan; Gong, Wenjun

2017-01-01

Static fatigue behavior is the main failure mode of optical fibers applied in sensors. In this paper, a computational framework based on continuum damage mechanics (CDM) is presented to calculate the crack propagation process and failure time of optical fibers subjected to static bending and tensile loads. For this purpose, the static fatigue crack propagation in the glass core of the optical fiber is studied. Combining a finite element method (FEM), we use the continuum damage mechanics for the glass core to calculate the crack propagation path and corresponding failure time. In addition, three factors including bending radius, tensile force and optical fiber diameter are investigated to find their impacts on the crack propagation process and failure time of the optical fiber under concerned situations. Finally, experiments are conducted and the results verify the correctness of the simulation calculation. It is believed that the proposed method could give a straightforward description of the crack propagation path in the inner glass core. Additionally, the predicted crack propagation time of the optical fiber with different factors can provide effective suggestions for improving the long-term usage of optical fibers. PMID:29140284

[The effect of notch's angle and depth on crack propagation of zirconia ceramics].

PubMed

Chen, Qingya; Chen, Xinmin

2012-10-01

This paper is aimed to study the effect of notch's angle and depth on crack propagation of zirconia ceramics. We fabricated cuboid-shaped zirconia ceramics samples with the standard sizes of 4. 4 mm x 2. 2 mm x 18 mm for the experiments, divided the samples into 6 groups, and prepared notches on these samples with different angles and depth. We placed the samples with loads until they were broke, and observe the fracture curve of each sample. We then drew coordinates and described the points of the fracture curve under a microscope, and made curve fitting by the software-Origin. When the notch angle beta = 90 degrees, the crack propagation is pure type I; when beta = 60 degrees, the crack propagation is mainly type I; and when beta = 30 degrees, the crack propagation is a compound of type I and type III. With the increasing of the notch depth, the effect of notch angles on crack propagation increases. In addition, Notch angle is a very important fracture mechanics parameter for crack propagation of zirconia ceramics. With the increasing of notch depth, the impact of notch angle increases.

Crack Propagation Calculations for Optical Fibers under Static Bending and Tensile Loads Using Continuum Damage Mechanics.

PubMed

Chen, Yunxia; Cui, Yuxuan; Gong, Wenjun

2017-11-15

Static fatigue behavior is the main failure mode of optical fibers applied in sensors. In this paper, a computational framework based on continuum damage mechanics (CDM) is presented to calculate the crack propagation process and failure time of optical fibers subjected to static bending and tensile loads. For this purpose, the static fatigue crack propagation in the glass core of the optical fiber is studied. Combining a finite element method (FEM), we use the continuum damage mechanics for the glass core to calculate the crack propagation path and corresponding failure time. In addition, three factors including bending radius, tensile force and optical fiber diameter are investigated to find their impacts on the crack propagation process and failure time of the optical fiber under concerned situations. Finally, experiments are conducted and the results verify the correctness of the simulation calculation. It is believed that the proposed method could give a straightforward description of the crack propagation path in the inner glass core. Additionally, the predicted crack propagation time of the optical fiber with different factors can provide effective suggestions for improving the long-term usage of optical fibers.

Crack propagation of brittle rock under high geostress

NASA Astrophysics Data System (ADS)

Liu, Ning; Chu, Weijiang; Chen, Pingzhi

2018-03-01

Based on fracture mechanics and numerical methods, the characteristics and failure criterions of wall rock cracks including initiation, propagation, and coalescence are analyzed systematically under different conditions. In order to consider the interaction among cracks, adopt the sliding model of multi-cracks to simulate the splitting failure of rock in axial compress. The reinforcement of bolts and shotcrete supporting to rock mass can control the cracks propagation well. Adopt both theory analysis and simulation method to study the mechanism of controlling the propagation. The best fixed angle of bolts is calculated. Then use ansys to simulate the crack arrest function of bolt to crack. Analyze the influence of different factors on stress intensity factor. The method offer more scientific and rational criterion to evaluate the splitting failure of underground engineering under high geostress.

The plane elasticity problem for a crack near the curved surface

NASA Astrophysics Data System (ADS)

Lebedeva, M. V.

2018-05-01

The unconventional approach to the plane elasticity problem for a crack near the curved surface is presented. The solution of the problem is considered in the form of the sum of solutions of two auxiliary problems. The first one describes the plane with a crack, whose surfaces are loaded by some unknown self-balanced force p(x). The second problem is dealing with the semi-infinite region with the boundary conditions equal to the difference of boundary conditions of the problem to be sought and the solution of the first problem on the region border. The unknown function p(x) is supposed to be approximated with the sufficient level of accuracy by N order polynomial with complex coefficients. This paper is aimed to determine the critical loads causing the spontaneous growth of cracks. The angles of propagation of the stationary cracks located in the region with a ledge or a cut are found. The influence of length of a crack on the bearing ability of an elastic body with the curved surface is investigated. The effect of a form of the concentrator and orientation of a crack to the fracture load subject to the different combinations of forces acting both on a surface of a crack and at infinity is analysed. The results of this research can be applied for calculation of the durability of thin-walled vessels of pressure, e.g., chemical reactors, in order to ensure their ecological safety.

Effect of strain wave shape on low-cycle fatigue crack propagation of SUS 304 stainless steel at elevated temperatures

NASA Astrophysics Data System (ADS)

Okazaki, Masakazu; Hattori, Ichiro; Shiraiwa, Fujio; Koizumi, Takashi

1983-08-01

Effect of strain wave shape on strain-controlled low-cycle fatigue crack propagation of SUS 304 stainless steel was investigated at 600 and 700 °C. It was found that the rate of crack propagation in a cycle-dependent region was successfully correlated with the range of cyclic J-integral, Δ Jf, regardless of the strain wave shape, frequency, and test temperature. It was also shown that the rate of crack propagation gradually increased from cycle-dependent curve to time-dependent one with decreasing frequency and slow-fast strain wave shape, and that one of the factors governing the rate of crack propagation in such a region was the ratio of the range of creep J-integral to that of total J-integral, Δ J c/Δ JT. Based on the results thus obtained, an interaction damage rule proposed semi-empirically was interpreted, with regard to crack propagation. Furthermore, fatigue crack initiation mechanism in slow-fast strain wave shape was studied, and it was shown that grain boundary sliding took an important role in it.

Basaltic Dike Propagation at Yucca Mountain, Nevada, USA

NASA Astrophysics Data System (ADS)

Gaffney, E. S.; Damjanac, B.; Warpinski, N. R.

2004-12-01

We describe simulations of the propagation of basaltic dikes using a 2-dimensional, incompressible hydrofracture code including the effects of the free surface with specific application to potential interactions of rising magma with a nuclear waste repository at Yucca Mountain, Nevada. As the leading edge of the dike approaches the free surface, confinement at the crack tip is reduced and the tip accelerates relative to the magma front. In the absence of either excess confining stress or excess gas pressure in the tip cavity, this leads to an increase of crack-tip velocity by more than an order of magnitude. By casting the results in nondimensional form, they can be applied to a wide variety of intrusive situations. When applied to an alkali basalt intrusion at the proposed high-level nuclear waste repository at Yucca Mountain, the results provide for a description of the subsurface phenomena. For magma rising at 1 m/s and dikes wider than about 0.5 m, the tip of the fissure would already have breached the surface by the time magma arrived at the nominal 300-m repository depth. An approximation of the effect of magma expansion on dike propagation is used to show that removing the restriction of an incompressible magma would result in even greater crack-tip acceleration as the dike approached the surface. A second analysis with a distinct element code indicates that a dike could penetrate the repository even during the first 2000 years after closure during which time heating from radioactive decay of waste would raise the minimum horizontal compressive stress above the vertical stress for about 80 m above and below the repository horizon. Rather than sill formation, the analysis indicates that increased pressure and dike width below the repository cause the crack tip to penetrate the horizon, but much more slowly than under in situ stress conditions. The analysis did not address the effects of either anisotropic joints or heat loss on this result.

Micro-mechanisms of Surface Defects Induced on Aluminum Alloys during Plastic Deformation at Elevated Temperatures

NASA Astrophysics Data System (ADS)

Gali, Olufisayo A.

Near-surface deformed layers developed on aluminum alloys significantly influence the corrosion and tribological behavior as well as reduce the surface quality of the rolled aluminum. The evolution of the near-surface microstructures induced on magnesium containing aluminum alloys during thermomechanical processing has been investigated with the aim generating an understanding of the influence of individual forming parameters on its evolution and examine the microstructure of the roll coating induced on the mating steel roll through material transfer during rolling. The micro-mechanisms related to the various features of near-surface microstructure developed during tribological conditions of the simulated hot rolling process were identified. Thermomechanical processing experiments were performed with the aid of hot rolling (operating temperature: 550 to 460 °C, 4, 10 and 20 rolling pass schedules) and hot forming (operating temperature: 350 to 545 °C, strain rate: 4 x 10-2 s-1) tribo-simulators. The surface, near-surface features and material transfer induced during the elevated temperature plastic deformation were examined and characterized employing optical interferometry, SEM/EDS, FIB and TEM. Near-surface features characterized on the rolled aluminum alloys included; cracks, fractured intermetallic particles, aluminum nano-particles, oxide decorated grain boundaries, rolled-in oxides, shingles and blisters. These features were related to various individual rolling parameters which included, the work roll roughness, which induced the formation of shingles, rolling marks and were responsible for the redistribution of surface oxide and the enhancements of the depth of the near-surface damage. The enhanced stresses and strains experienced during rolling were related to the formation and propagation of cracks, the nanocrystalline structure of the near-surface layers and aluminum nano-particles. The mechanism of the evolution of the near-surface microstructure were determined to include grain boundary sliding which induced the cracks at the surface and subsurface of the alloy, magnesium diffusion to free surfaces, crack propagation from shear stresses and the shear strains inducing the nanocrystalline grain structure, the formation of shingles by the shear deformation of micro-wedges induced by the work roll grooves, and the deformation of this oxide covered micro-wedges inducing the rolled-in oxides. Magnesium diffusion to free surfaces was identified as inducing crack healing due to the formation of MgO within cracks and was responsible for the oxide decorated grain boundaries. An examination of the roll coating revealed a complex layered microstructure that was induced through tribo-chemical and mechanical entrapment mechanisms. The microstructure of the roll coating suggested that the work roll material and the rolled aluminum alloy were essential in determining its composition and structure. Subsequent hot forming processes revealed the rich oxide-layer of the near-surface microstructure was beneficial for reducing the coefficient of friction during tribological contact with the steel die. Damage to the microstructure include cracks induced from grain boundary sliding of near-surface grains and the formation of oxide fibres within cracks of the near-surface deformed layers.

Research on a Lamb Wave and Particle Filter-Based On-Line Crack Propagation Prognosis Method.

PubMed

Chen, Jian; Yuan, Shenfang; Qiu, Lei; Cai, Jian; Yang, Weibo

2016-03-03

Prognostics and health management techniques have drawn widespread attention due to their ability to facilitate maintenance activities based on need. On-line prognosis of fatigue crack propagation can offer information for optimizing operation and maintenance strategies in real-time. This paper proposes a Lamb wave-particle filter (LW-PF)-based method for on-line prognosis of fatigue crack propagation which takes advantages of the possibility of on-line monitoring to evaluate the actual crack length and uses a particle filter to deal with the crack evolution and monitoring uncertainties. The piezoelectric transducers (PZTs)-based active Lamb wave method is adopted for on-line crack monitoring. The state space model relating to crack propagation is established by the data-driven and finite element methods. Fatigue experiments performed on hole-edge crack specimens have validated the advantages of the proposed method.

Statistical investigation of fatigue crack initiation and growth around chamfered rivet holes in Alclad 2024 T3 as affected by corrosion

NASA Technical Reports Server (NTRS)

Fadragas, M. I.; Fine, M. E.; Moran, B.

1994-01-01

In panel specimens with rivet holes cracks initiate in the blunted knife edge of the chamfered rivet hole and propagate inward as well as along the hole. The fatigue lifetime to dominant crack information was defined as the number of cycles, N500 micrometer, to formation of a 500 micrometer long crack. Statistical data on N500 micrometer and on crack propagation after N500 micrometer were obtained for a large number of uncorroded specimens and specimens corroded in an ASTM B 117 salt spray. Considerable variation in N500 micrometer and crack propagation behavior was observed from specimen to specimen of the same nominal geometry with chamfered rivet holes increased the probability for both early formation and later formation of a propagating 500 micrometer fatigue crack. The growth of fatigue cracks after 500 micrometer size was little affected by prior salt spray.

Pellet Cladding Mechanical Interaction Modeling Using the Extended Finite Element Method

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

Spencer, Benjamin W.; Jiang, Wen; Dolbow, John E.

As a brittle material, the ceramic UO2 used as light water reactor fuel experiences significant fracturing throughout its life, beginning with the first rise to power of fresh fuel. This has multiple effects on the thermal and mechanical response of the fuel/cladding system. One such effect that is particularly important is that when there is mechanical contact between the fuel and cladding, cracks that extending from the outer surface of the fuel into the volume of the fuel cause elevated stresses in the adjacent cladding, which can potentially lead to cladding failure. Modeling the thermal and mechanical response of themore » cladding in the vicinity of these surface-breaking cracks in the fuel can provide important insights into this behavior to help avoid operating conditions that could lead to cladding failure. Such modeling has traditionally been done in the context of finite-element-based fuel performance analysis by modifying the fuel mesh to introduce discrete cracks. While this approach is effective in capturing the important behavior at the fuel/cladding interface, there are multiple drawbacks to explicitly incorporating the cracks in the finite element mesh. Because the cracks are incorporated in the original mesh, the mesh must be modified for cracks of specified location and depth, so it is difficult to account for crack propagation and the formation of new cracks at other locations. The extended finite element method (XFEM) has emerged in recent years as a powerful method to represent arbitrary, evolving, discrete discontinuities within the context of the finite element method. Development work is underway by the authors to implement XFEM in the BISON fuel performance code, and this capability has previously been demonstrated in simulations of fracture propagation in ceramic nuclear fuel. These preliminary demonstrations have included only the fuel, and excluded the cladding for simplicity. This paper presents initial results of efforts to apply XFEM to model stress concentrations induced by fuel fractures at the fuel/cladding interface during pellet cladding mechanical interaction (PCMI). This is accomplished by enhancing the thermal and mechanical contact enforcement algorithms employed by BISON to permit their use in conjunction with XFEM. The results from this methodology are demonstrated to be equivalent to those from using meshed discrete cracks. While the results of the two methods are equivalent for the case of a stationary crack, it is demonstrated that XFEM provides the additional flexibility of allowing arbitrary crack initiation and propagation during the analysis, and minimizes model setup effort for cases with stationary cracks.« less

Lifetime prediction for the subsurface crack propagation using three-dimensional dynamic FEA model

NASA Astrophysics Data System (ADS)

Yin, Yuan; Chen, Yun-Xia; Liu, Le

2017-03-01

The subsurface crack propagation is one of the major interests for gear system research. The subsurface crack propagation lifetime is the number of cycles remaining for a spall to appear, which can be obtained through either stress intensity factor or accumulated plastic strain analysis. In this paper, the heavy loads are applied to the gear system. When choosing stress intensity factor, the high compressive stress suppresses Mode I stress intensities and severely reduces Mode II stress intensities in the heavily loaded lubricated contacts. Such that, the accumulated plastic strain is selected to calculate the subsurface crack propagation lifetime from the three-dimensional FEA model through ANSYS Workbench transient analysis. The three-dimensional gear FEA dynamic model with the subsurface crack is built through dividing the gears into several small elements. The calculation of the total cycles of the elements is proposed based on the time-varying accumulated plastic strain, which then will be used to calculate the subsurface crack propagation lifetime. During this process, the demonstration from a subsurface crack to a spall can be uncovered. In addition, different sizes of the elements around the subsurface crack are compared in this paper. The influences of the frictional coefficient and external torque on the crack propagation lifetime are also discussed. The results show that the lifetime of crack propagation decreases significantly when the external load T increasing from 100 N m to 150 N m. Given from the distributions of the accumulated plastic strain, the lifetime shares no significant difference when the frictional coefficient f ranging in 0.04-0.06.

Crack layer theory

NASA Technical Reports Server (NTRS)

Chudnovsky, A.

1984-01-01

A damage parameter is introduced in addition to conventional parameters of continuum mechanics and consider a crack surrounded by an array of microdefects within the continuum mechanics framework. A system consisting of the main crack and surrounding damage is called crack layer (CL). Crack layer propagation is an irreversible process. The general framework of the thermodynamics of irreversible processes are employed to identify the driving forces (causes) and to derive the constitutive equation of CL propagation, that is, the relationship between the rates of the crack growth and damage dissemination from one side and the conjugated thermodynamic forces from another. The proposed law of CL propagation is in good agreement with the experimental data on fatigue CL propagation in various materials. The theory also elaborates material toughness characterization.

Crack layer theory

NASA Technical Reports Server (NTRS)

Chudnovsky, A.

1987-01-01

A damage parameter is introduced in addition to conventional parameters of continuum mechanics and consider a crack surrounded by an array of microdefects within the continuum mechanics framework. A system consisting of the main crack and surrounding damage is called crack layer (CL). Crack layer propagation is an irreversible process. The general framework of the thermodynamics of irreversible processes are employed to identify the driving forces (causes) and to derive the constitutive equation of CL propagation, that is, the relationship between the rates of the crack growth and damage dissemination from one side and the conjugated thermodynamic forces from another. The proposed law of CL propagation is in good agreement with the experimental data on fatigue CL propagation in various materials. The theory also elaborates material toughness characterization.

Finite Element Model Characterization Of Nano-Composite Thermal And Environmental Barrier Coatings

NASA Technical Reports Server (NTRS)

Yamada, Yoshiki; Zhu, Dongming

2011-01-01

Thermal and environmental barrier coatings have been applied for protecting Si based ceramic matrix composite components from high temperature environment in advanced gas turbine engines. It has been found that the delamination and lifetime of T/EBC systems generally depend on the initiation and propagation of surface cracks induced by the axial mechanical load in addition to severe thermal loads. In order to prevent T/EBC systems from surface cracking and subsequent delamination due to mechanical and thermal stresses, T/EBC systems reinforced with nano-composite architectures have showed promise to improve mechanical properties and provide a potential crack shielding mechanism such as crack bridging. In this study, a finite element model (FEM) was established to understand the potential beneficial effects of nano-composites systems such as SiC nanotube-reinforced oxide T/EBC systems.

Periodic cracks and temperature-dependent stress in Mo/Si multilayers on Si substrates

NASA Astrophysics Data System (ADS)

Kravchenko, Grygoriy; Tran, Hai T.; Volinsky, Alex A.

2018-07-01

This work examines formation of the peculiar periodic crack patterns observed in the thermally loaded Mo/Si multilayers. Using the substrate curvature measurements, the macroscopic film stress evolution during thermal cycling was investigated. Then high-speed microscopic observation of crack propagation in the annealed Mo/Si multilayers was presented providing experimental evidence of the mechanism underlying formation of the periodic crack patterns. The origin of the peculiar periodic crack patterns was determined. They were observed to form by the slow crack propagation under quasi-static conditions as a result of the interaction between the channelling crack propagation and the advance of the delamination front.

Subcritical growth of natural hydraulic fractures

NASA Astrophysics Data System (ADS)

Garagash, D.

2014-12-01

Joints are the most common example of brittle tensile failure in the crust. Their genesis at depth is linked to the natural hydraulic fracturing, which requires pore fluid pressure in excess of the minimum in situ stress [Pollard and Aidyn, JSG1988]. Depending on the geological setting, high pore pressure can result form burial compaction of interbedded strata, diagenesis, or tectonics. Common to these loading scenarios is slow build-up of pore pressure over a geological timescale, until conditions for initiation of crack growth are met on favorably oriented/sized flaws. The flaws can vary in size from grain-size cracks in igneous rocks to a fossil-size flaws in clastic rock, and once activated, are inferred to propagate mostly subcritically [Segall JGR 1984; Olson JGR 1993]. Despite many observational studies of natural hydraulic fractures, the modeling attempts appear to be few [Renshaw and Harvey JGR 1994]. Here, we use boundary integral formulation for the pore fluid inflow from the permeable rock into a propagating joint [Berchenko et al. IJRMMS 1997] coupled with the criteria for subcritical propagation assisted by the environmental effects of pore fluid at the crack tip to solve for the evolution of a penny-shape joint, which, in interbedded rock, may eventually evolve to short-blade geometry (propagation confined to a bed). Initial growth is exceedingly slow, paced by the stress corrosion reaction kinetics at the crack tip. During this stage the crack is fully-drained (i.e. the fluid pressure in the crack is equilibrated with the ambient pore pressure). This "slow" stage is followed by a rapid acceleration, driven by the increase of the mechanical stress intensity factor with the crack length, towards the terminal joint velocity. We provide an analytical expression for the latter as a function of the rock diffusivity, net pressure loading at the initiation (or flaw lengthscale), and parameters describing resistance to fracture growth. Due to a much slower rate of the crack volume expansion of short-blade joints compared to that of penny-shape joints, the former would propagate much faster than the latter under otherwise identical conditions. Finally, we speculate about possible relation of the predicted patterns of joint development with morphology of joint fracture surfaces observed in sedimentary rock.

In situ AFM investigation of slow crack propagation mechanisms in a glassy polymer

NASA Astrophysics Data System (ADS)

George, M.; Nziakou, Y.; Goerke, S.; Genix, A.-C.; Bresson, B.; Roux, S.; Delacroix, H.; Halary, J.-L.; Ciccotti, M.

2018-03-01

A novel experimental technique based on in situ AFM monitoring of the mechanisms of damage and the strain fields associated to the slow steady-state propagation of a fracture in glassy polymers is presented. This micron-scale investigation is complemented by optical measurements of the sample deformation up to the millimetric macroscopic scale of the sample in order to assess the proper crack driving conditions. These multi-scale observations provide important insights towards the modeling of the fracture toughness of glassy polymers and its relationship with the macromolecular structure and non-linear rheological properties. This novel technique is first tested on a standard PMMA thermoplastic in order to both evaluate its performance and the richness of this new kind of observations. Although the fracture propagation in PMMA is well known to proceed through crazing in the bulk of the samples, our observations provide a clear description and quantitative evaluation of a change of fracture mechanism towards shear yielding fracture accompanied by local necking close to the free surface of the sample, which can be explained by the local change of stress triaxiality. Moreover, this primary surface necking mechanism is shown to be accompanied by a network of secondary grooves that can be related to surface crazes propagating towards the interior of the sample. This overall scenario is validated by post-mortem fractographic investigations by scanning electron microscopy.

Self-similar crack-generation effects in the fracture process in brittle materials

NASA Astrophysics Data System (ADS)

Hilarov, V. L.

1998-07-01

Using acoustic-emission data banks we have computed time and space correlation functions for the purpose of investigation of crack-propagation self-similarity during the fracture process in brittle materials. It is shown that the whole fracture process may be represented as a two-stage process. In the first stage, the crack propagation is uniform and uncorrelated in space, having a time spectral density of the white-noise type and a correlation fractal dimension approximately equal to that of 3D Euclidean space. In the second stage, this fractal dimension decreases significantly, reaching the value of 2.2-2.4, characteristic for the fracture surfaces, while the time spectral density exhibits a significant low-frequency increase becoming of 0965-0393/6/4/002/img1-noise type. The resulting fractal shows no multifractal behaviour, appearing to be a single fractal.

Effect of Mechanical Heterogeneity on the Crack Driving Force of a Reactor Pressure Vessel Outlet Nozzle DMW Joint

NASA Astrophysics Data System (ADS)

Lingyan, Zhao; Yinghao, Cui; He, Xue

2017-12-01

The welding mechanical heterogeneity, load complexity, material and geometrical structure makes it very difficult to assess the structural integrity of dissimilar metal weld (DMW) joints. Based on a numerical simulated approach of the continuous change of material mechanical property in the buttering layer, a reactor pressure vessel (RPV) outlet nozzle DMW joint with service loads is studied, effect of mechanical heterogeneity on the stress-strain field and stress triaxiality at the semi-elliptical surface crack front are discussed. The analyses show that once the crack extends into the high hardness zone of Alloy 182 buttering, the strain decreases sharply, the strain gradient increases and the crack propagation slows down. The influence of strength mismatch on the stress triaxiality at the shallow crack front is greater than that at the deep crack front. The interaction between strength mismatch and crack depth directly affects the crack growth direction.

Experimental study of thermodynamics propagation fatigue crack in metals

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

Vshivkov, A., E-mail: vshivkov.a@icmm.ru; Iziumova, A., E-mail: fedorova@icmm.ru; Plekhov, O., E-mail: poa@icmm.ru

This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE samples. The investigation of the fatigue crack propagation was carried out on flat samples with stress concentrators. The stress concentrator was three central holes. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at themore » crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip.« less

Crack Initiation and Growth in Rigid Polymeric Closed-Cell Foam Cryogenic Applications

NASA Technical Reports Server (NTRS)

Sayyah, Tarek; Steeve, Brian; Wells, Doug

2006-01-01

Cryogenic vessels, such as the Space Shuttle External Tank, are often insulated with closed-cell foam because of its low thermal conductivity. The coefficient of thermal expansion mismatch between the foam and metallic substrate places the foam under a biaxial tension gradient through the foam thickness. The total foam thickness affects the slope of the stress gradient and is considered a significant contributor to the initiation of subsurface cracks. Rigid polymeric foams are brittle in nature and any subsurface cracks tend to propagate a finite distance toward the surface. This presentation investigates the relationship between foam thickness and crack initiation and subsequent crack growth, using linear elastic fracture mechanics, in a rigid polymeric closed-cell foam through analysis and comparison with experimental results.

[Fractographic analysis of clinically failed anterior all ceramic crowns].

PubMed

DU, Qian; Zhou, Min-bo; Zhang, Xin-ping; Zhao, Ke

2012-04-01

To identify the site of crack initiation and propagation path of clinically failed all ceramic crowns by fractographic analysis. Three clinically failed anterior IPS Empress II crowns and two anterior In-Ceram alumina crowns were retrieved. Fracture surfaces were examined using both optical stereo and scanning electron microscopy. Fractographic theory and fracture mechanics principles were applied to disclose the damage characteristics and fracture mode. All the crowns failed by cohesive failure within the veneer on the labial surface. Critical crack originated at the incisal contact area and propagated gingivally. Porosity was found within the veneer because of slurry preparation and the sintering of veneer powder. Cohesive failure within the veneer is the main failure mode of all ceramic crown. Veneer becomes vulnerable when flaws are present. To reduce the chances of chipping, multi-point occlusal contacts are recommended, and layering and sintering technique of veneering layer should also be improved.

Elevated Temperature Crack Propagation

NASA Technical Reports Server (NTRS)

Orange, Thomas W.

1994-01-01

This paper is a summary of two NASA contracts on high temperature fatigue crack propagation in metals. The first evaluated the ability of fairly simple nonlinear fracture parameters to correlate crack propagation. Hastelloy-X specimens were tested under isothermal and thermomechanical cycling at temperatures up to 980 degrees C (1800 degrees F). The most successful correlating parameter was the crack tip opening displacement derived from the J-integral. The second evaluated the ability of several path-independent integrals to correlate crack propagation behavior. Inconel 718 specimens were tested under isothermal, thermomechanical, temperature gradient, and creep conditions at temperatures up to 650 degrees C (1200 degrees F). The integrals formulated by Blackburn and by Kishimoto correlated the data reasonably well under all test conditions.

Fatigue and Creep Crack Propagation behaviour of Alloy 617 in the Annealed and Aged Conditions

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

Julian K. Benz; Richard N. Wright

2013-10-01

The crack propagation behaviour of Alloy 617 was studied under various conditions. Elevated temperature fatigue and creep-fatigue crack growth experiments were conducted at 650 and 800 degrees C under constant stress intensity (triangle K) conditions and triangular or trapezoidal waveforms at various frequencies on as-received, aged, and carburized material. Environmental conditions included both laboratory air and characteristic VHTR impure helium. As-received Alloy 617 displayed an increase in the crack growth rate (da/dN) as the frequency was decreased in air which indicated a time-dependent contribution component in fatigue crack propagation. Material aged at 650°C did not display any influence on themore » fatigue crack growth rates nor the increasing trend of crack growth rate with decreasing frequency even though significant microstructural evolution, including y’ (Ni3Al) after short times, occurred during aging. In contrast, carburized Alloy 617 showed an increase in crack growth rates at all frequencies tested compared to the material in the standard annealed condition. Crack growth studies under quasi-constant K (i.e. creep) conditions were also completed at 650 degrees C and a stress intensity of K = 40 MPa9 (square root)m. The results indicate that crack growth is primarily intergranular and increased creep crack growth rates exist in the impure helium environment when compared to the results in laboratory air. Furthermore, the propagation rates (da/dt) continually increased for the duration of the creep crack growth either due to material aging or evolution of a crack tip creep zone. Finally, fatigue crack propagation tests at 800 degrees C on annealed Alloy 617 indicated that crack propagation rates were higher in air than impure helium at the largest frequencies and lowest stress intensities. The rates in helium, however, eventually surpass the rates in air as the frequency is reduced and the stress intensity is decreased which was not observed at 650 degrees C.« less

Peak Stress Intensity Factor Governs Crack Propagation Velocity In Crosslinked UHMWPE

PubMed Central

Sirimamilla, P. Abhiram; Furmanski, Jevan; Rimnac, Clare

2013-01-01

Ultra high molecular weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint replacement components. However, these bearing materials can fail as a result of in vivo static and cyclic loads. Crack propagation behavior in this material has been considered using the Paris relationship which relates fatigue crack growth rate, da/dN (mm/cycle) versus the stress intensity factor range, ΔK (Kmax-Kmin, MPa√m). However, recent work suggests that the crack propagation velocity of conventional UHMWPE is driven by the peak stress intensity (Kmax), not ΔK. The hypothesis of this study is that the crack propagation velocity of highly crosslinked and remelted UHMWPE is also driven by the peak stress intensity, Kmax, during cyclic loading, rather than by ΔK. To test this hypothesis, two highly crosslinked (65 kGy and 100 kGy) and remelted UHMWPE materials were examined. Frequency, waveform and R-ratio were varied between test conditions to determine the governing factor for fatigue crack propagation. It was found that the crack propagation velocity in crosslinked UHMWPE is also driven by Kmax and not ΔK, and is dependent on loading waveform and frequency in a predictable quasi-static manner. The current study supports that crack growth in crosslinked UHMWPE materials, even under cyclic loading conditions, can be described by a relationship between the velocity of crack growth, da/dt and the peak stress intensity, Kmax. The findings suggest that stable crack propagation can occur as a result of static loading only and this should be taken into consideration in design of UHMWPE total joint replacement components. PMID:23165898

Crack deflection: Implications for the growth of long and short fatigue cracks

NASA Astrophysics Data System (ADS)

Suresh, S.

1983-11-01

The influences of crack deflection on the growth rates of nominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of the local mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.

Detection of multiple thin surface cracks using vibrothermography with low-power piezoceramic-based ultrasonic actuator—a numerical study with experimental verification

NASA Astrophysics Data System (ADS)

Parvasi, Seyed Mohammad; Xu, Changhang; Kong, Qingzhao; Song, Gangbing

2016-05-01

Ultrasonic vibrations in cracked structures generate heat at the location of defects mainly due to frictional rubbing and viscoelastic losses at the defects. Vibrothermography is an effective nondestructive evaluation method which uses infrared imaging (IR) techniques to locate defects such as cracks and delaminations by detecting the heat generated at the defects. In this paper a coupled thermo-electro-mechanical analysis with the use of implicit finite element method was used to simulate a low power (10 W) piezoceramic-based ultrasonic actuator and the corresponding heat generation in a metallic plate with multiple surface cracks. Numerical results show that the finite element software Abaqus can be used to simultaneously model the electrical properties of the actuator, the ultrasonic waves propagating within the plate, as well as the thermal properties of the plate. Obtained numerical results demonstrate the ability of these low power transducers in detecting multiple cracks in the simulated aluminum plate. The validity of the numerical simulations was verified through experimental studies on a physical aluminum plate with multiple surface cracks while the same low power piezoceramic stack actuator was used to excite the plate and generate heat at the cracks. An excellent qualitative agreement exists between the experimental results and the numerical simulation’s results.

Remote detection of stress corrosion cracking: Surface composition and crack detection

NASA Astrophysics Data System (ADS)

Lissenden, Cliff J.; Jovanovic, Igor; Motta, Arthur T.; Xiao, Xuan; Le Berre, Samuel; Fobar, David; Cho, Hwanjeong; Choi, Sungho

2018-04-01

Chloride induced stress corrosion cracking (SCC) of austenitic stainless steel is a potential issue in long term dry storage of spent nuclear fuel canisters. In order for SCC to occur there must be a corrosive environment, a susceptible material, and a driving force. Because it is likely that the material in the heat affected zone (HAZ) of welded stainless steel structures has been sensitized as a result of chromium depletion at the grain boundaries and a thermal residual stress driving force is likely present if solution annealing is not performed, two issues are critical. Is the environment corrosive, i.e., are chlorides present in solution on the surface? And then, are there cracks that could propagate? Remote detection of chlorides on the surface can be accomplished by laser induced breakdown spectroscopy (LIBS), while cracks can be detected by shear horizontal guided waves generated by electromagnetic acoustic transducers (EMATs). Both are noncontact methods that are amenable to robotic delivery systems and harsh environments. The sensitivity to chlorine on stainless steel of a LIBS system that employs optical fiber for pulse delivery is demonstrated. Likewise, the ability of the EMAT system to detect cracks of a prescribed size and orientation is shown. These results show the potential for remote detection of Cl and cracks in dry storage spent fuel canisters.

Consideration of Moving Tooth Load in Gear Crack Propagation Predictions

NASA Technical Reports Server (NTRS)

Lewicki, David G.; Handschuh, Robert F.; Spievak, Lisa E.; Wawrzynek, Paul A.; Ingraffea, Anthony R.

2001-01-01

Robust gear designs consider not only crack initiation, but crack propagation trajectories for a fail-safe design. In actual gear operation, the magnitude as well as the position of the force changes as the gear rotates through the mesh. A study to determine the effect of moving gear tooth load on crack propagation predictions was performed. Two-dimensional analysis of an involute spur gear and three-dimensional analysis of a spiral-bevel pinion gear using the finite element method and boundary element method were studied and compared to experiments. A modified theory for predicting gear crack propagation paths based on the criteria of Erdogan and Sih was investigated. Crack simulation based on calculated stress intensity factors and mixed mode crack angle prediction techniques using a simple static analysis in which the tooth load was located at the highest point of single tooth contact was validated. For three-dimensional analysis, however, the analysis was valid only as long as the crack did not approach the contact region on the tooth.

Crack propagation in aluminum sheets reinforced with boron-epoxy

NASA Technical Reports Server (NTRS)

Roderick, G. L.

1979-01-01

An analysis was developed to predict both the crack growth and debond growth in a reinforced system. The analysis was based on the use of complex variable Green's functions for cracked, isotropic sheets and uncracked, orthotropic sheets to calculate inplane and interlaminar stresses, stress intensities, and strain-energy-release rates. An iterative solution was developed that used the stress intensities and strain-energy-release rates to predict crack and debond growths, respectively, on a cycle-by-cycle basis. A parametric study was made of the effects of boron-epoxy composite reinforcement on crack propagation in aluminum sheets. Results show that the size of the debond area has a significant effect on the crack propagation in the aluminum. For small debond areas, the crack propagation rate is reduced significantly, but these small debonds have a strong tendency to enlarge. Debond growth is most likely to occur in reinforced systems that have a cracked metal sheet reinforced with a relatively thin composite sheet.

Effect of Compressive Mode I on the Mixed Mode I/II Fatigue Crack Growth Rate of 42CrMo4

NASA Astrophysics Data System (ADS)

Heirani, Hasan; Farhangdoost, Khalil

2018-01-01

Subsurface cracks in mechanical contact loading components are subjected to mixed mode I/II, so it is necessary to evaluate the fatigue behavior of materials under mixed mode loading. For this purpose, fatigue crack propagation tests are performed with compact tension shear specimens for several stress intensity factor (SIF) ratios of mode I and mode II. The effect of compressive mode I loading on mixed mode I/II crack growth rate and fracture surface is investigated. Tests are carried out for the pure mode I, pure mode II, and two different mixed mode loading angles. On the basis of the experimental results, mixed mode crack growth rate parameters are proposed according to Tanaka and Richard with Paris' law. Results show neither Richard's nor Tanaka's equivalent SIFs are very useful because these SIFs depend strongly on the loading angle, but Richard's equivalent SIF formula is more suitable than Tanaka's formula. The compressive mode I causes the crack closure, and the friction force between the crack surfaces resists against the crack growth. In compressive loading with 45° angle, d a/d N increases as K eq decreases.

Study of the thermal effect on silicon surface induced by ion beam from plasma focus device

NASA Astrophysics Data System (ADS)

Ahmad, Z.; Ahmad, M.; Al-Hawat, Sh.; Akel, M.

2017-04-01

Structural modifications in form of ripples and cracks are induced by nitrogen ions from plasma focus on silicon surface. The investigation of such structures reveals correlation between ripples and cracks formation in peripheral region of the melt spot. The reason of such correlation and structure formation is explained as result of thermal effect. Melting and resolidification of the center of irradiated area occur within one micro second of time. This is supported by a numerical simulation used to investigate the thermal effect induced by the plasma focus ion beams on the silicon surface. This simulation provides information about the temperature profile as well as the dynamic of the thermal propagation in depth and lateral directions. In accordance with the experimental observations, that ripples are formed in latter stage after the arrival of last ion, the simulation shows that the thermal relaxation takes place in few microseconds after the end of the ion beam arrival. Additionally, the dependency of thermal propagation and relaxation on the distance of the silicon surface from the anode is presented.

The deformation of the front of a 3D interface crack propagating quasistatically in a medium with random fracture properties

NASA Astrophysics Data System (ADS)

Pindra, Nadjime; Lazarus, Véronique; Leblond, Jean-Baptiste

One studies the evolution in time of the deformation of the front of a semi-infinite 3D interface crack propagating quasistatically in an infinite heterogeneous elastic body. The fracture properties are assumed to be lower on the interface than in the materials so that crack propagation is channelled along the interface, and to vary randomly within the crack plane. The work is based on earlier formulae which provide the first-order change of the stress intensity factors along the front of a semi-infinite interface crack arising from some small but otherwise arbitrary in-plane perturbation of this front. The main object of study is the long-time behavior of various statistical measures of the deformation of the crack front. Special attention is paid to the influences of the mismatch of elastic properties, the type of propagation law (fatigue or brittle fracture) and the stable or unstable character of 2D crack propagation (depending on the loading) upon the development of this deformation.

Investigation of Low-Cycle Bending Fatigue of AISI 9310 Steel Spur Gears

NASA Technical Reports Server (NTRS)

Handschuh, Robert F.; Krantz, Timothy L.; Lerch, Bradley A.; Burke, Christopher S.

2007-01-01

An investigation of the low-cycle bending fatigue of spur gears made from AISI 9310 gear steel was completed. Tests were conducted using the single-tooth bending method to achieve crack initiation and propagation. Tests were conducted on spur gears in a fatigue test machine using a dedicated gear test fixture. Test loads were applied at the highest point of single tooth contact. Gear bending stresses for a given testing load were calculated using a linear-elastic finite element model. Test data were accumulated from 1/4 cycle to several thousand cycles depending on the test stress level. The relationship of stress and cycles for crack initiation was found to be semi-logarithmic. The relationship of stress and cycles for crack propagation was found to be linear. For the range of loads investigated, the crack propagation phase is related to the level of load being applied. Very high loads have comparable crack initiation and propagation times whereas lower loads can have a much smaller number of cycles for crack propagation cycles as compared to crack initiation.

Investigation of Low-Cycle Bending Fatigue of AISI 9310 Steel Spur Gears

NASA Technical Reports Server (NTRS)

Handschuh, Robert F.; Krantz, Timothy L.; Lerch, Bradley A.; Burke, Christopher S.

2007-01-01

An investigation of the low-cycle bending fatigue of spur gears made from AISI 9310 gear steel was completed. Tests were conducted using the single-tooth bending method to achieve crack initiation and propagation. Tests were conducted on spur gears in a fatigue test machine using a dedicated gear test fixture. Test loads were applied at the highest point of single tooth contact. Gear bending stresses for a given testing load were calculated using a linear-elastic finite element model. Test data were accumulated from 1/4 cycle to several thousand cycles depending on the test stress level. The relationship of stress and cycles for crack initiation was found to be semilogarithmic. The relationship of stress and cycles for crack propagation was found to be linear. For the range of loads investigated, the crack propagation phase is related to the level of load being applied. Very high loads have comparable crack initiation and propagation times whereas lower loads can have a much smaller number of cycles for crack propagation cycles as compared to crack initiation.

An Introduction to Navy Corrosion Problems. A Guideline for Designers and Engineers of Naval Ordnance and Hardware.

DTIC Science & Technology

1986-03-31

titanium, stainless steel , and copper alloys . During SCC, the alloy surface remains essentially unattacked while insidious crack propagation through...strength steels Water I High strength aluminum Chloride solutions, Appears to be due I alloys organic solvents moisture Copper alloys - Ammoniacal solutions... Precipitation hardened martensitic stainless steels , above 1240 MPa, have exhibited cracking in salt-spray and when fully immersed in aqueous media (23

Influence of High Cycle Thermal Loads on Thermal Fatigue Behavior of Thick Thermal Barrier Coatings

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Miller, Robert A.

1997-01-01

Thick thermal barrier coating systems in a diesel engine experience severe thermal Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) during engine operation. In the present study, the mechanisms of fatigue crack initiation and propagation, as well as of coating failure, under thermal loads which simulate engine conditions, are investigated using a high power CO2 laser. In general, surface vertical cracks initiate early and grow continuously under LCF and HCF cyclic stresses. It is found that in the absence of interfacial oxidation, the failure associated with LCF is closely related to coating sintering and creep at high temperatures, which induce tensile stresses in the coating after cooling. Experiments show that the HCF cycles are very damaging to the coating systems. The combined LCF and HCF tests produced more severe coating surface cracking, microspallation and accelerated crack growth, as compared to the pure LCF test. It is suggested that the HCF component cannot only accelerate the surface crack initiation, but also interact with the LCF by contributing to the crack growth at high temperatures. The increased LCF stress intensity at the crack tip due to the HCF component enhances the subsequent LCF crack growth. Conversely, since a faster HCF crack growth rate will be expected with lower effective compressive stresses in the coating, the LCF cycles also facilitate the HCF crack growth at high temperatures by stress relaxation process. A surface wedging model has been proposed to account for the HCF crack growth in the coating system. This mechanism predicts that HCF damage effect increases with increasing temperature swing, the thermal expansion coefficient and the elastic modulus of the ceramic coating, as well as the HCF interacting depth. A good agreement has been found between the analysis and experimental evidence.

Bridge-indentation precracking of glass bars

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

Lue, J.L.; Scattergood, R.O.

1993-07-01

Bridge indentation precracking was first reported by Sadahiro et al., and has been used by a number of subsequent investigators. The procedure involves placing a half-penny starter crack, formed by Vickers indentation, at the center of the bottom surface of a rectangular bar specimen. The bar is loaded between pusher and anvil platens. The bridge span S produces tensile bending stress on the bottom surface of the bar, which contains the starter crack. With increasing load, the crack grows stably outward and inward, and at a certain load the crack will propagate unstably (pop-in) to a straight, through-section crack ofmore » length c. The method is very convenient for producing sharp, through-section precracks in ceramic bars without the need for saw cutting or fatigue cycling. However, the effects of bridge geometry and friction on the precracking results will be presented here which point to an important influence of the friction between the specimen and anvil platens.« less

Corrosion initiation and propagation behavior of corrosion resistant concrete reinforcing materials

NASA Astrophysics Data System (ADS)

Hurley, Michael F.

The life of a concrete structure exposed to deicing compounds or seawater is often limited by chloride induced corrosion of the steel reinforcement. In this study, the key material attributes that affect the corrosion initiation and propagation periods were studied. These included material composition, surface condition, ageing time, propagation behavior during active corrosion, morphology of attack, and type of corrosion products generated by each rebar material. The threshold chloride concentrations for solid 316LN stainless steel, 316L stainless steel clad over carbon steel, 2101 LDX, MMFX-2, and carbon steel rebar were investigated using electrochemical techniques in saturated calcium hydroxide solutions. Surface preparation, test method, duration of period exposed to a passivating condition prior to introduction of chloride, and presence of cladding defects all affected the threshold chloride concentration obtained. A model was implemented to predict the extension of time until corrosion initiation would be expected. 8 years was the predicted time to corrosion initiation for carbon steel. However, model results confirmed that use of 316LN may increase the time until onset of corrosion to 100 years or more. To assess the potential benefits afforded by new corrosion resistant rebar alloys from a corrosion resistance standpoint the corrosion propagation behavior and other factors that might affect the risk of corrosion-induced concrete cracking must also be considered. Radial pit growth was found to be ohmically controlled but repassivation occurred more readily at high potentials in the case of 316LN and 2101 stainless steels. The discovery of ohmically controlled propagation enabled transformation of propagation rates from simulated concrete pore solution to less conductive concrete by accounting for resistance changes in the surrounding medium. The corrosion propagation behavior as well as the morphology of attack directly affects the propensity for concrete cracking. Experimental results were used in conjunction with an existing model to predict the time until concrete cracking occurs for new rebar materials. The results suggest that corrosion resistant materials offer a significant extension to the corrosion propagation stage over carbon steel, even in very aggressive conditions because small, localized anodes develop when initiated.

Characterization and Prediction of Cracks in Coated Materials: Direction and Length of Crack Propagation in Bimaterials

PubMed Central

Azari, Z.; Pappalettere, C.

2015-01-01

The behaviour of materials is governed by the surrounding environment. The contact area between the material and the surrounding environment is the likely spot where different forms of degradation, particularly rust, may be generated. A rust prevention treatment, like bluing, inhibitors, humidity control, coatings, and galvanization, will be necessary. The galvanization process aims to protect the surface of the material by depositing a layer of metallic zinc by either hot-dip galvanizing or electroplating. In the hot-dip galvanizing process, a metallic bond between steel and metallic zinc is obtained by immersing the steel in a zinc bath at a temperature of around 460°C. Although the hot-dip galvanizing procedure is recognized to be one of the most effective techniques to combat corrosion, cracks can arise in the intermetallic δ layer. These cracks can affect the life of the coated material and decrease the lifetime service of the entire structure. In the present paper the mechanical response of hot-dip galvanized steel submitted to mechanical loading condition is investigated. Experimental tests were performed and corroborative numerical and analytical methods were then applied in order to describe both the mechanical behaviour and the processes of crack/cracks propagation in a bimaterial as zinc-coated material. PMID:27347531

Characterization and Prediction of Cracks in Coated Materials: Direction and Length of Crack Propagation in Bimaterials.

PubMed

Pruncu, C I; Azari, Z; Casavola, C; Pappalettere, C

2015-01-01

The behaviour of materials is governed by the surrounding environment. The contact area between the material and the surrounding environment is the likely spot where different forms of degradation, particularly rust, may be generated. A rust prevention treatment, like bluing, inhibitors, humidity control, coatings, and galvanization, will be necessary. The galvanization process aims to protect the surface of the material by depositing a layer of metallic zinc by either hot-dip galvanizing or electroplating. In the hot-dip galvanizing process, a metallic bond between steel and metallic zinc is obtained by immersing the steel in a zinc bath at a temperature of around 460°C. Although the hot-dip galvanizing procedure is recognized to be one of the most effective techniques to combat corrosion, cracks can arise in the intermetallic δ layer. These cracks can affect the life of the coated material and decrease the lifetime service of the entire structure. In the present paper the mechanical response of hot-dip galvanized steel submitted to mechanical loading condition is investigated. Experimental tests were performed and corroborative numerical and analytical methods were then applied in order to describe both the mechanical behaviour and the processes of crack/cracks propagation in a bimaterial as zinc-coated material.

Effect of service exposure on fatigue crack propagation of Inconel 718 turbine disc material at elevated temperatures

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

Jeong, Dae-Ho; Choi, Myung-Je; Goto, Masahiro

In this study, the fatigue crack propagation behavior of Inconel 718 turbine disc with different service times from 0 to 4229 h was investigated at 738 and 823 K. No notable change in microstructural features, other than the increase in grain size, was observed with increasing service time. With increasing service time from 0 to 4229 h, the fatigue crack propagation rates tended to increase, while the ΔK{sub th} value decreased, in low ΔK regime and lower Paris' regime at both testing temperatures. The fractographic observation using a scanning electron microscope suggested that the elevated temperature fatigue crack propagation mechanismmore » of Inconel 718 changed from crystallographic cleavage mechanism to striation mechanism in the low ΔK regime, depending on the grain size. The fatigue crack propagation mechanism is proposed for the crack propagating through small and large grains in the low ΔK regime, and the fatigue crack propagation behavior of Inconel 718 with different service times at elevated temperatures is discussed. - Highlights: • The specimens were prepared from the Inconel 718 turbine disc used for 0 to 4229 h. • FCP rates were measured at 738 and 823 K. • The ΔK{sub th} values decreased with increasing service time. • The FCP behavior showed a strong correlation with the grain size of used turbine disc.« less

Corrosion Effects on the Fatigue Crack Propagation of Giga-Grade Steel and its Heat Affected Zone in pH Buffer Solutions for Automotive Application

NASA Astrophysics Data System (ADS)

Lee, H. S.

2018-03-01

Corrosion fatigue crack propagation test was conducted of giga-grade steel and its heat affected zone in pH buffer solutions, and the results were compared with model predictions. Pure corrosion effect on fatigue crack propagation, particularly, in corrosive environment was evaluated by means of the modified Forman equation. As shown in results, the average corrosion rate determined from the ratio of pure corrosion induced crack length to entire crack length under a cycle load were 0.11 and 0.37 for base metal and heat affected zone, respectively, with load ratio of 0.5, frequency of 0.5 and pH 10.0 environment. These results demonstrate new interpretation methodology for corrosion fatigue crack propagation enabling the pure corrosion effects on the behavior to be determined.

Thermal stress weathering and the spalling of Antarctic rocks

NASA Astrophysics Data System (ADS)

Lamp, J. L.; Marchant, D. R.; Mackay, S. L.; Head, J. W.

2017-01-01

Using in situ field measurements, laboratory analyses, and numerical modeling, we test the potential efficacy of thermal stress weathering in the flaking of millimeter-thick alteration rinds observed on cobbles and boulders of Ferrar Dolerite on Mullins Glacier, McMurdo Dry Valleys (MDV). In particular, we examine whether low-magnitude stresses, arising from temperature variations over time, result in thermal fatigue weathering, yielding slow crack propagation along existing cracks and ultimate flake detachment. Our field results show that during summer months clasts of Ferrar Dolerite experience large-temperature gradients across partially detached alteration rinds (>4.7°C mm-1) and abrupt fluctuations in surface temperature (up to 12°C min-1); the latter are likely due to the combined effects of changing solar irradiation and cooling from episodic winds. The results of our thermal stress model, coupled with subcritical crack growth theory, suggest that thermal stresses induced at the base of thin alteration rinds 2 mm thick, common on rocks exposed for 105 years, may be sufficient to cause existing cracks to propagate under present-day meteorological forcing, eventually leading to rind detachment. The increase in porosity observed within alteration rinds relative to unaltered rock interiors, as well as predicted decreases in rind strength based on allied weathering studies, likely facilitates thermal stress crack propagation through a reduction of fracture toughness. We conclude that thermal stress weathering may be an active, though undervalued, weathering process in hyperarid, terrestrial polar deserts such as the stable upland region of the MDV.

Predicting the 3D fatigue crack growth rate of small cracks using multimodal data via Bayesian networks: In-situ experiments and crystal plasticity simulations

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

Rovinelli, Andrea; Sangid, Michael D.; Proudhon, Henry

Small crack propagation accounts for most of the fatigue life of engineering structures subject to high cycle fatigue loading conditions. Determining the fatigue crack growth rate of small cracks propagating into polycrystalline engineering alloys is critical to improving fatigue life predictions, thus lowering cost and increasing safety. In this work, cycle-by-cycle data of a small crack propagating in a beta metastable titanium alloy is available via phase and diffraction contrast tomography. Crystal plasticity simulations are used to supplement experimental data regarding the micromechanical fields ahead of the crack tip. Experimental and numerical results are combined into a multimodal dataset andmore » sampled utilizing a non-local data mining procedure. Furthermore, to capture the propensity of body-centered cubic metals to deform according to the pencil-glide model, a non-local driving force is postulated. The proposed driving force serves as the basis to construct a data-driven probabilistic crack propagation framework using Bayesian networks as building blocks. The spatial correlation between the postulated driving force and experimental observations is obtained by analyzing the results of the proposed framework. Results show that the above correlation increases proportionally to the distance from the crack front until the edge of the plastic zone. Moreover, the predictions of the propagation framework show good agreement with experimental observations. Finally, we studied the interaction of a small crack with grain boundaries (GBs) utilizing various slip transmission criteria, revealing the tendency of a crack to cross a GB by propagating along the slip directions minimizing the residual Burgers vector within the GB.« less

Predicting the 3D fatigue crack growth rate of small cracks using multimodal data via Bayesian networks: In-situ experiments and crystal plasticity simulations

NASA Astrophysics Data System (ADS)

Rovinelli, Andrea; Sangid, Michael D.; Proudhon, Henry; Guilhem, Yoann; Lebensohn, Ricardo A.; Ludwig, Wolfgang

2018-06-01

Small crack propagation accounts for most of the fatigue life of engineering structures subject to high cycle fatigue loading conditions. Determining the fatigue crack growth rate of small cracks propagating into polycrystalline engineering alloys is critical to improving fatigue life predictions, thus lowering cost and increasing safety. In this work, cycle-by-cycle data of a small crack propagating in a beta metastable titanium alloy is available via phase and diffraction contrast tomography. Crystal plasticity simulations are used to supplement experimental data regarding the micromechanical fields ahead of the crack tip. Experimental and numerical results are combined into a multimodal dataset and sampled utilizing a non-local data mining procedure. Furthermore, to capture the propensity of body-centered cubic metals to deform according to the pencil-glide model, a non-local driving force is postulated. The proposed driving force serves as the basis to construct a data-driven probabilistic crack propagation framework using Bayesian networks as building blocks. The spatial correlation between the postulated driving force and experimental observations is obtained by analyzing the results of the proposed framework. Results show that the above correlation increases proportionally to the distance from the crack front until the edge of the plastic zone. Moreover, the predictions of the propagation framework show good agreement with experimental observations. Finally, we studied the interaction of a small crack with grain boundaries (GBs) utilizing various slip transmission criteria, revealing the tendency of a crack to cross a GB by propagating along the slip directions minimizing the residual Burgers vector within the GB.

Predicting the 3D fatigue crack growth rate of small cracks using multimodal data via Bayesian networks: In-situ experiments and crystal plasticity simulations

DOE PAGES

Rovinelli, Andrea; Sangid, Michael D.; Proudhon, Henry; ...

2018-03-11

Small crack propagation accounts for most of the fatigue life of engineering structures subject to high cycle fatigue loading conditions. Determining the fatigue crack growth rate of small cracks propagating into polycrystalline engineering alloys is critical to improving fatigue life predictions, thus lowering cost and increasing safety. In this work, cycle-by-cycle data of a small crack propagating in a beta metastable titanium alloy is available via phase and diffraction contrast tomography. Crystal plasticity simulations are used to supplement experimental data regarding the micromechanical fields ahead of the crack tip. Experimental and numerical results are combined into a multimodal dataset andmore » sampled utilizing a non-local data mining procedure. Furthermore, to capture the propensity of body-centered cubic metals to deform according to the pencil-glide model, a non-local driving force is postulated. The proposed driving force serves as the basis to construct a data-driven probabilistic crack propagation framework using Bayesian networks as building blocks. The spatial correlation between the postulated driving force and experimental observations is obtained by analyzing the results of the proposed framework. Results show that the above correlation increases proportionally to the distance from the crack front until the edge of the plastic zone. Moreover, the predictions of the propagation framework show good agreement with experimental observations. Finally, we studied the interaction of a small crack with grain boundaries (GBs) utilizing various slip transmission criteria, revealing the tendency of a crack to cross a GB by propagating along the slip directions minimizing the residual Burgers vector within the GB.« less

In situ SEM observation of microscale strain fields around a crack tip in polycrystalline molybdenum

NASA Astrophysics Data System (ADS)

Li, J. J.; Li, W. C.; Jin, Y. J.; Wang, L. F.; Zhao, C. W.; Xing, Y. M.; Lang, F. C.; Yan, L.; Yang, S. T.

2016-06-01

In situ scanning electron microscopy was employed to investigate the crack initiation and propagation in polycrystalline molybdenum under uniaxial tensile load at room temperature. The microscale grid pattern was fabricated using the sputtering deposition technology on the specimen surface covered with a fine square mesh copper grid. The microscale strain fields around the crack tip were measured by geometric phase analysis technique and compared with the theoretical solutions based on the linear elastic fracture mechanics theory. The results showed that as the displacement increases, the crack propagated mainly perpendicular to the tensile direction during the fracture process of molybdenum. The normal strain ɛ xx and shear strain ɛ xy are relatively small, and the normal strain ɛ yy holds a dominant position in the deformation fields and plays a key role in the whole fracture process of molybdenum. With the increase in displacement, the ɛ yy increases rapidly and the two lobes grow significantly but maintain the same shape and orientation. The experimental ɛ yy is in agreement with the theoretical solution. Along the x-axis in front of the crack tip, there is minor discrepancy between the experimental ɛ yy and theoretical ɛ yy within 25 μm from the crack tip, but the agreement between them is very good far from the crack tip (>25 μm).

Crack identification for reinforced concrete using PZT based smart rebar active sensing diagnostic network

NASA Astrophysics Data System (ADS)

Song, N. N.; Wu, F.

2016-04-01

An active sensing diagnostic system using PZT based smart rebar for SHM of RC structure has been currently under investigation. Previous test results showed that the system could detect the de-bond of concrete from reinforcement, and the diagnostic signals were increased exponentially with the de-bonding size. Previous study also showed that the smart rebar could function well like regular reinforcement to undertake tension stresses. In this study, a smart rebar network has been used to detect the crack damage of concrete based on guided waves. Experimental test has been carried out for the study. In the test, concrete beams with 2 reinforcements have been built. 8 sets of PZT elements were mounted onto the reinforcement bars in an optimized way to form an active sensing diagnostic system. A 90 kHz 5-cycle Hanning-windowed tone burst was used as input. Multiple cracks have been generated on the concrete structures. Through the guided bulk waves propagating in the structures from actuators and sensors mounted from different bars, crack damage could be detected clearly. Cases for both single and multiple cracks were tested. Different crack depths from the surface and different crack numbers have been studied. Test result shows that the amplitude of sensor output signals is deceased linearly with a propagating crack, and is decreased exponentially with increased crack numbers. From the study, the active sensing diagnostic system using PZT based smart rebar network shows a promising way to provide concrete crack damage information through the "talk" among sensors.

Fatigue crack propagation behavior of stainless steel welds

NASA Astrophysics Data System (ADS)

Kusko, Chad S.

The fatigue crack propagation behavior of austenitic and duplex stainless steel base and weld metals has been investigated using various fatigue crack growth test procedures, ferrite measurement techniques, light optical microscopy, stereomicroscopy, scanning electron microscopy, and optical profilometry. The compliance offset method has been incorporated to measure crack closure during testing in order to determine a stress ratio at which such closure is overcome. Based on this method, an empirically determined stress ratio of 0.60 has been shown to be very successful in overcoming crack closure for all da/dN for gas metal arc and laser welds. This empirically-determined stress ratio of 0.60 has been applied to testing of stainless steel base metal and weld metal to understand the influence of microstructure. Regarding the base metal investigation, for 316L and AL6XN base metals, grain size and grain plus twin size have been shown to influence resulting crack growth behavior. The cyclic plastic zone size model has been applied to accurately model crack growth behavior for austenitic stainless steels when the average grain plus twin size is considered. Additionally, the effect of the tortuous crack paths observed for the larger grain size base metals can be explained by a literature model for crack deflection. Constant Delta K testing has been used to characterize the crack growth behavior across various regions of the gas metal arc and laser welds at the empirically determined stress ratio of 0.60. Despite an extensive range of stainless steel weld metal FN and delta-ferrite morphologies, neither delta-ferrite morphology significantly influence the room temperature crack growth behavior. However, variations in weld metal da/dN can be explained by local surface roughness resulting from large columnar grains and tortuous crack paths in the weld metal.

Fatigue crack propagation in additively manufactured porous biomaterials.

PubMed

Hedayati, R; Amin Yavari, S; Zadpoor, A A

2017-07-01

Additively manufactured porous titanium implants, in addition to preserving the excellent biocompatible properties of titanium, have very small stiffness values comparable to those of natural bones. Although usually loaded in compression, biomedical implants can also be under tensional, shear, and bending loads which leads to crack initiation and propagation in their critical points. In this study, the static and fatigue crack propagation in additively manufactured porous biomaterials with porosities between 66% and 84% is investigated using compact-tension (CT) samples. The samples were made using selective laser melting from Ti-6Al-4V and were loaded in tension (in static study) and tension-tension (in fatigue study) loadings. The results showed that displacement accumulation diagram obtained for different CT samples under cyclic loading had several similarities with the corresponding diagrams obtained for cylindrical samples under compression-compression cyclic loadings (in particular, it showed a two-stage behavior). For a load level equaling 50% of the yield load, both the CT specimens studied here and the cylindrical samples we had tested under compression-compression cyclic loading elsewhere exhibited similar fatigue lives of around 10 4 cycles. The test results also showed that for the same load level of 0.5F y , the lower density porous structures demonstrate relatively longer lives than the higher-density ones. This is because the high bending stresses in high-density porous structures gives rise to local Mode-I crack opening in the rough external surface of the struts which leads to quicker formation and propagation of the cracks. Under both the static and cyclic loading, all the samples showed crack pathways which were not parallel to but made 45 ° angles with respect to the notch direction. This is due to the fact that in the rhombic dodecahedron unit cell, the weakest struts are located in 45 ° direction with respect to the notch direction. Copyright © 2017 Elsevier B.V. All rights reserved.

Correlation between average frequency and RA value (rise time/amplitude) for crack classification of reinforced concrete beam using acoustic emission technique

NASA Astrophysics Data System (ADS)

Noorsuhada, M. N.; Abdul Hakeem, Z.; Soffian Noor, M. S.; Noor Syafeekha, M. S.; Azmi, I.

2017-12-01

Health monitoring of structures during their service life become a vital thing as it provides crucial information regarding the performance and condition of the structures. Acoustic emission (AE) is one of the non-destructive techniques (NDTs) that could be used to monitor the performance of the structures. Reinforced concrete (RC) beam associated with AE monitoring was monotonically loaded to failure under three-point loading. Correlation between average frequency and RA value (rise time / amplitude) was computed. The relationship was established to classify the crack types that propagated in the RC beam. The crack was classified as tensile crack and shear crack. It was found that the relationship is well matched with the actual crack pattern that appeared on the beam surface. Hence, this relationship is useful for prediction of the crack occurrence in the beam and its performance can be determined.

Analysis of mixed-mode crack propagation using the boundary integral method

NASA Technical Reports Server (NTRS)

Mendelson, A.; Ghosn, L. J.

1986-01-01

Crack propagation in a rotating inner raceway of a high speed roller bearing is analyzed using the boundary integral equation method. The method consists of an edge crack in a plate under tension, upon which varying Hertzian stress fields are superimposed. A computer program for the boundary integral equation method was written using quadratic elements to determine the stress and displacement fields for discrete roller positions. Mode I and Mode II stress intensity factors and crack extension forces G sub 00 (energy release rate due to tensile opening mode) and G sub r0 (energy release rate due to shear displacement mode) were computed. These calculations permit determination of that crack growth angle for which the change in the crack extension forces is maximum. The crack driving force was found to be the alternating mixed-mode loading that occurs with each passage of the most heavily loaded roller. The crack is predicted to propagate in a step-like fashion alternating between radial and inclined segments, and this pattern was observed experimentally. The maximum changes DeltaG sub 00 and DeltaG sub r0 of the crack extension forces are found to be good measures of the crack propagation rate and direction.

Microstructural analysis of the 2195 aluminum-lithium alloy welds

NASA Technical Reports Server (NTRS)

Talia, George E.

1993-01-01

The principal objective of this research was to explain a tendency of 2195 Al-Li alloy to crack at elevated temperature during welding. Therefore, a study was made on the effect of welding and thermal treatment on the microstructure of Al-Li Alloy 2195. The critical roles of precipitates, boundaries, phases, and other features of the microstructure were inferred from the crack propagation paths and the morphology of fracture surface of the alloy with different microstructures. Particular emphasis was placed on the microstructures generated by the welding process and the mechanisms of crack propagation in such structures. Variation of the welding parameters and thermal treatments were used to alter the micro/macro structures, and they were characterized by optical and scanning electron microscopy. A theoretical model is proposed to explain changes in the microstructure of welded material. This model proposes a chemical reaction in which gases from the air (i.e., nitrogen) release hydrogen inside the alloy. Such a reaction could generate large internal stresses capable to induce porosity and crack-like delamination in the material.

Identification and prioritization of rail squat defects in the field using rail magnetisation technology

NASA Astrophysics Data System (ADS)

Kaewunruen, Sakdirat

2015-04-01

Inevitably, rail squats and studs are continuing to be a serious problem for railway organisations around the world in the 21st century. They are typically classified as the growth of any cracks that have grown longitudinally through the subsurface and some of the cracks propagating to the bottom of rails transversely, and have branched from initial longitudinal cracks with a depression of rail surface. The horizontal crack, which results in a depression of rail surface, induces increased maintenance level, more frequent monitoring, compromised rail testing (as the crack shields the signal echoes), and possible broken rails. This paper presents field investigations using a magnetised-rail testing device developed by MRX Technologies to identify and prioritise the rail squats. Most of the in situ squats were found on the high rail of the transition (variable-radius curved track), which is associated with rolling contact fatigue (RCF). This investigation highlights the field performance of the MRX's surface crack detection technology in comparison with the traditional ultrasonic method and detailed walking inspection. Visually, it was found in the field that the size of the RCF squats varies from very small to moderate. The predicted crack data were obtained by scanning the magnitised rails. The comparison of the actual crack depths (ultrasonic) and the predicted crack depths (MRX device) shows: • A possible correlation for small RCF/ squat cracks. • Poor interpretation of larger defects and welds. The field assessment also suggests some practical issues required for further development, including the detection of rail spalling, deep transverse crack, welding, and so on.

Time-dependent corrosion fatique crack propagation in 7000 series aluminum alloys. M.S. Thesis

NASA Technical Reports Server (NTRS)

Mason, Mark E.

1995-01-01

The goal of this research is to characterize environmentally assisted subcritical crack growth for the susceptible short-longitudinal orientation of aluminum alloy 7075-T651, immersed in acidified and inhibited NaCl solution. This work is necessary in order to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as NASA-FLAGRO (NASGRO). This effort concentrates on determining relevant inputs to a superposition model in order to more accurately model environmental fatigue crack propagation.

Corrosion-Fatigue Crack Growth in Plates: A Model Based on the Paris Law

PubMed Central

Toribio, Jesús; Matos, Juan-Carlos; González, Beatriz

2017-01-01

In this paper, a Paris law-based model is presented whereby crack propagation occurs under cyclic loading in air (fatigue) and in an aggressive environment (corrosion-fatigue) for the case of corner cracks (with a wide range of aspect ratios in the matter of the initial cracks) in finite-thickness plates of 316L austenitic stainless steel subjected to tension, bending, or combined (tension + bending) loading. Results show that the cracks tend during their growth towards a preferential propagation path, exhibiting aspect ratios slightly lower than unity only for the case of very shallow cracks, and diminishing as the crack grows (increasing the relative crack depth)—more intensely in the case of bending than in the case of tension (the mixed loading tension/bending representing an intermediate case). In addition, the crack aspect ratios during fatigue propagation evolution are lower in fatigue (in air) than in corrosion-fatigue (in aggressive environment). PMID:28772798

Recent developments in analysis of crack propagation and fracture of practical materials. [stress analysis in aircraft structures

NASA Technical Reports Server (NTRS)

Hardrath, H. F.; Newman, J. C., Jr.; Elber, W.; Poe, C. C., Jr.

1978-01-01

The limitations of linear elastic fracture mechanics in aircraft design and in the study of fatigue crack propagation in aircraft structures are discussed. NASA-Langley research to extend the capabilities of fracture mechanics to predict the maximum load that can be carried by a cracked part and to deal with aircraft design problems are reported. Achievements include: (1) improved stress intensity solutions for laboratory specimens; (2) fracture criterion for practical materials; (3) crack propagation predictions that account for mean stress and high maximum stress effects; (4) crack propagation predictions for variable amplitude loading; and (5) the prediction of crack growth and residual stress in built-up structural assemblies. These capabilities are incorporated into a first generation computerized analysis that allows for damage tolerance and tradeoffs with other disciplines to produce efficient designs that meet current airworthiness requirements.

Role of sulphur atoms on stress relaxation and crack propagation in monolayer MoS2

NASA Astrophysics Data System (ADS)

Wang, Baoming; Islam, Zahabul; Zhang, Kehao; Wang, Ke; Robinson, Joshua; Haque, Aman

2017-09-01

We present in-situ transmission electron microscopy of crack propagation in a freestanding monolayer MoS2 and molecular dynamic analysis of the underlying mechanisms. Chemical vapor deposited monolayer MoS2 was transferred from sapphire substrate using interfacial etching for defect and contamination minimization. Atomic resolution imaging shows crack tip atoms sustaining 14.5% strain before bond breaking, while the stress field decays at unprecedented rate of 2.15 GPa Å-1. Crack propagation is seen mostly in the zig-zag direction in both model and experiment, suggesting that the mechanics of fracture is not brittle. Our computational model captures the mechanics of the experimental observations on crack propagation in MoS2. While molybdenum atoms carry most of the mechanical load, we show that the sliding motion of weakly bonded sulphur atoms mediate crack tip stress relaxation, which helps the tip sustain very high, localized stress levels.

Comparative Study of Vibration Condition Indicators for Detecting Cracks in Spur Gears

NASA Technical Reports Server (NTRS)

Nanadic, Nenad; Ardis, Paul; Hood, Adrian; Thurston, Michael; Ghoshal, Anindya; Lewicki, David

2013-01-01

This paper reports the results of an empirical study on the tooth breakage failure mode in spur gears. Of four dominant gear failure modes (breakage, wear, pitting, and scoring), tooth breakage is the most precipitous and often leads to catastrophic failures. The cracks were initiated using a fatigue tester and a custom-designed single-tooth bending fixture to simulate over-load conditions, instead of traditional notching using wire electrical discharge machining (EDM). The cracks were then propagated on a dynamometer. The ground truth of damage level during crack propagation was monitored with crack-propagation sensors. Ten crack propagations have been performed to compare the existing condition indicators (CIs) with respect to their: ability to detect a crack, ability to assess the damage, and sensitivity to sensor placement. Of more than thirty computed CIs, this paper compares five commonly used: raw RMS, FM0, NA4, raw kurtosis, and NP4. The performance of combined CIs was also investigated, using linear, logistic, and boosted regression trees based feature fusion.

Small-crack effects in high-strength aluminum alloys

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Wu, X. R.; Venneri, S. L.; Li, C. G.

1994-01-01

The National Aeronautics and Space Administration and the Chinese Aeronautical Establishment participated in a Fatigue and Fracture Mechanics Cooperative Program. The program objectives were to identify and characterize crack initiation and growth of small cracks (10 microns to 2 mm long) in commonly used US and PRC aluminum alloys, to improve fracture mechanics analyses of surface- and corner-crack configurations, and to develop improved life-prediction methods. Fatigue and small-crack tests were performed on single-edgenotch tension (SENT) specimens and large-crack tests were conducted on center-crack tension specimens for constant-amplitude (stress ratios of -1, 0, and 0.5) and Mini-TWIST spectrum loading. The plastic replica method was used to monitor the initiation and growth of small fatigue cracks at the semicircular notch. Crack growth results from each laboratory on 7075-T6 bare and LC9cs clad aluminum alloys agreed well and showed that fatigue life was mostly crack propagation from a material defect (inclusion particles or void) or from the cladding layer. Finite-element and weight-function methods were used to determine stress intensity factors for surface and corner cracks in the SENT specimens. Equations were then developed and used in a crack growth and crack-closure model to correlate small- and large-crack data and to make life predictions for various load histories. The cooperative program produced useful experimental data and efficient analysis methods for improving life predictions. The results should ultimately improve aircraft structural reliability and safety.

Transverse Isotropy of Phyllite Under Brazilian Tests: Laboratory Testing and Numerical Simulations

NASA Astrophysics Data System (ADS)

Xu, Guowen; He, Chuan; Chen, Ziquan; Su, Ang

2018-04-01

Phyllite is a low-grade, metamorphic rock with well-developed foliation. We characterized the fracture pattern and failure strength of phyllite specimens under Brazilian tests. The specimens were obtained from the Zhegu mountain tunnel in China and had different foliation-loading angles, namely 0°, 15°, 30°, 45°, 60°, 75° and 90°. The processes for the initiation and propagation of macro-cracks were recorded using high-speed photography. The evolution of micro-cracks was analyzed based on the results of acoustic emission (AE) tests. The failure process of the specimens during the Brazilian tests was simulated with a new numerical approach based on the particle discrete element method. The influence of foliation strength and the microstructure of the rock matrix were also studied numerically. The experimental results showed that the failure strength of the specimens was related to their fracture patterns and the areas of their fracture surfaces. The initial cracking point of the specimens appeared at the upper or lower loading position, and the cracks propagated to the boundaries of the specimens along or across foliation. The temporal distributions of the AE counts and AE energy of the specimens were affected predominantly by the fracture pattern, and we divided these distributions into two modes: the peak mode and the uniformly distributed mode. The numerical results indicated that the fracture surface was roughly parallel to the loading direction and that the surface was located in the central part of the disk specimens for rocks with loose structure (low coordination number or large crack density) or with strong foliation, i.e., foliation with high shear strength. The failure pattern and trends of variation in failure strength as a function of foliation-loading angles varied with the ratio of cohesion to the tensile strength of foliation, the crack density, and the coordination number.

Early detection of materials degradation

NASA Astrophysics Data System (ADS)

Meyendorf, Norbert

2017-02-01

Lightweight components for transportation and aerospace applications are designed for an estimated lifecycle, taking expected mechanical and environmental loads into account. The main reason for catastrophic failure of components within the expected lifecycle are material inhomogeneities, like pores and inclusions as origin for fatigue cracks, that have not been detected by NDE. However, material degradation by designed or unexpected loading conditions or environmental impacts can accelerate the crack initiation or growth. Conventional NDE methods are usually able to detect cracks that are formed at the end of the degradation process, but methods for early detection of fatigue, creep, and corrosion are still a matter of research. For conventional materials ultrasonic, electromagnetic, or thermographic methods have been demonstrated as promising. Other approaches are focused to surface damage by using optical methods or characterization of the residual surface stresses that can significantly affect the creation of fatigue cracks. For conventional metallic materials, material models for nucleation and propagation of damage have been successfully applied for several years. Material microstructure/property relations are well established and the effect of loading conditions on the component life can be simulated. For advanced materials, for example carbon matrix composites or ceramic matrix composites, the processes of nucleation and propagation of damage is still not fully understood. For these materials NDE methods can not only be used for the periodic inspections, but can significantly contribute to the material scientific knowledge to understand and model the behavior of composite materials.

Subcritical crack propagation due to chemical rock weakening: macroscale chemo-plasticity and chemo-elasticity modeling

NASA Astrophysics Data System (ADS)

Hueckel, T.; Hu, M.

2015-12-01

Crack propagation in a subcritically stressed rock subject to chemically aggressive environment is analyzed and numerically simulated. Chemically induced weakening is often encountered in hydraulic fracturing of low-permeability oil/gas reservoirs and heat reservoirs, during storage of CO2 and nuclear waste corroding canisters, and other circumstances when rock matrix acidizing is involved. Upon acidizing, mineral mass dissolution is substantially enhanced weakening the rock and causing crack propagation and eventually permeability changes in the medium. The crack process zone is modeled mathematically via a chemo-plastic coupling and chemo-elastic coupling model. In plasticity a two-way coupling is postulated between mineral dissolution and a yield limit of rock matrix. The rate of dissolution is described by a rate law, but the mineral mass removal per unit volume is also a function of a variable internal specific surface area, which is in turn affected by the micro-cracking (treated as a plastic strain). The behavior of the rock matrix is modeled as rigid-plastic adding a chemical softening capacity to Cam-Clay model. Adopting the Extended Johnson's approximation of processes around the crack tip, the evolution of the stress field and deformation as a function of the chemically enhanced rock damage is modeled in a simplified way. In addition, chemical reactive transport is made dependent on plastic strain representing micro-cracking. Depending on mechanical and chemical boundary conditions, the area of enhanced chemical softening is near or somewhat away from the crack tip.In elasticity, chemo-mechanical effect is postulated via a chemical volumetric shrinkage strain proportional to mass removal variable, conceived analogously to thermal expansion. Two versions are considered: of constant coefficient of shrinkage and a variable one, coupled to deviatoric strain. Airy Potential approach used for linear elasticity is extended considering an extra term, which is uncoupled or coupled to strain. The later case requires iterations with solution of reactive transport equation. A decrease of stress intensity factor with time of reaction is well reproduced.

Control of serpentinisation rate by reaction-induced cracking

NASA Astrophysics Data System (ADS)

Malvoisin, Benjamin; Brantut, Nicolas; Kaczmarek, Mary-Alix

2017-10-01

Serpentinisation of mantle rocks requires the generation and maintenance of transport pathways for water. The solid volume increase during serpentinisation can lead to stress build-up and trigger cracking, which ease fluid penetration into the rock. The quantitative effect of this reaction-induced cracking mechanism on reactive surface generation is poorly constrained, thus hampering our ability to predict serpentinisation rate in geological environments. Here we use a combined approach with numerical modelling and observations in natural samples to provide estimates of serpentinisation rate at mid-ocean ridges. We develop a micromechanical model to quantify the propagation of serpentinisation-induced cracks in olivine. The maximum crystallisation pressure deduced from thermodynamic calculations reaches several hundreds of megapascals but does not necessary lead to crack propagation if the olivine grain is subjected to high compressive stresses. The micromechanical model is then coupled to a simple geometrical model to predict reactive surface area formation during grain splitting, and thus bulk reaction rate. Our model reproduces quantitatively experimental kinetic data and the typical mesh texture formed during serpentinisation. We also compare the model results with olivine grain size distribution data obtained on natural serpentinised peridotites from the Marum ophiolite and the Papuan ultramafic belt (Papua New Guinea). The natural serpentinised peridotites show an increase of the number of olivine grains for a decrease of the mean grain size by one order of magnitude as reaction progresses from 5 to 40%. These results are in agreement with our model predictions, suggesting that reaction-induced cracking controls the serpentinisation rate. We use our model to estimate that, at mid-ocean ridges, serpentinisation occurs up to 12 km depth and reaction-induced cracking reduces the characteristic time of serpentinisation by one order of magnitude, down to values comprised between 10 and 1000 yr. The increase of effective pressure with depth also prevents cracking, which positions the peak in serpentinisation rate at shallower depths, 4 km above previous predictions.

Crack propagation life of detail fractures in rails

DOT National Transportation Integrated Search

1988-10-01

The results of a comprehensive study of the crack propagation behavior of detail fractures in railroad rails are presented. The study includes full-scale crack growth experiments in a test track under simulated heavy freight train service, similar fi...

Monitoring Concrete Deterioration Due to Reinforcement Corrosion by Integrating Acoustic Emission and FBG Strain Measurements

PubMed Central

Li, Weijie; Xu, Changhang; Ho, Siu Chun Michael; Wang, Bo; Song, Gangbing

2017-01-01

Corrosion of concrete reinforcement members has been recognized as a predominant structural deterioration mechanism for steel reinforced concrete structures. Many corrosion detection techniques have been developed for reinforced concrete structures, but a dependable one is more than desired. Acoustic emission technique and fiber optic sensing have emerged as new tools in the field of structural health monitoring. In this paper, we present the results of an experimental investigation on corrosion monitoring of a steel reinforced mortar block through combined acoustic emission and fiber Bragg grating strain measurement. Constant current was applied to the mortar block in order to induce accelerated corrosion. The monitoring process has two aspects: corrosion initiation and crack propagation. Propagation of cracks can be captured through corresponding acoustic emission whereas the mortar expansion due to the generation of corrosion products will be monitored by fiber Bragg grating strain sensors. The results demonstrate that the acoustic emission sources comes from three different types, namely, evolution of hydrogen bubbles, generation of corrosion products and crack propagation. Their corresponding properties are also discussed. The results also show a good correlation between acoustic emission activity and expansive strain measured on the specimen surface. PMID:28327510

Monitoring Concrete Deterioration Due to Reinforcement Corrosion by Integrating Acoustic Emission and FBG Strain Measurements.

PubMed

Li, Weijie; Xu, Changhang; Ho, Siu Chun Michael; Wang, Bo; Song, Gangbing

2017-03-22

Corrosion of concrete reinforcement members has been recognized as a predominant structural deterioration mechanism for steel reinforced concrete structures. Many corrosion detection techniques have been developed for reinforced concrete structures, but a dependable one is more than desired. Acoustic emission technique and fiber optic sensing have emerged as new tools in the field of structural health monitoring. In this paper, we present the results of an experimental investigation on corrosion monitoring of a steel reinforced mortar block through combined acoustic emission and fiber Bragg grating strain measurement. Constant current was applied to the mortar block in order to induce accelerated corrosion. The monitoring process has two aspects: corrosion initiation and crack propagation. Propagation of cracks can be captured through corresponding acoustic emission whereas the mortar expansion due to the generation of corrosion products will be monitored by fiber Bragg grating strain sensors. The results demonstrate that the acoustic emission sources comes from three different types, namely, evolution of hydrogen bubbles, generation of corrosion products and crack propagation. Their corresponding properties are also discussed. The results also show a good correlation between acoustic emission activity and expansive strain measured on the specimen surface.

Fatigue creep damage at the cement-bone interface: an experimental and a micro-mechanical finite element study

PubMed Central

Waanders, Daan; Janssen, Dennis; Miller, Mark A.; Mann, Kenneth A.; Verdonschot, Nico

2009-01-01

The goal of this study was to quantify the micromechanics of the cement-bone interface under tensile fatigue loading using finite element analysis (FEA) and to understand the underlying mechanisms that play a role in the fatigue behavior of this interface. Laboratory cement-bone specimens were subjected to a tensile fatigue load, while local displacements and crack growth on the specimen's surface were monitored. FEA models were created from these specimens based upon micro-computed tomography data. To accurately model interfacial gaps at the interface between the bone and cement, a custom-written erosion algorithm was applied to the bone model. A fatigue load was simulated in the FEA models while monitoring the local displacements and crack propagation. The results showed the FEA models were able to capture the general experimental creep damage behavior and creep stages of the interface. Consistent with the experiments, the majority of the deformation took place at the contact interface. Additionally, the FEA models predicted fatigue crack patterns similar to experimental findings. Experimental surface cracks correlated moderately with FEA surface cracks (r2=0.43), but did not correlate with the simulated crack volume fraction (r2=0.06). Although there was no relationship between experimental surface cracks and experimental creep damage displacement (r2=0.07), there was a strong relationship between the FEA crack volume fraction and the FEA creep damage displacement (r2=0.76). This study shows the additional value of FEA of the cement-bone interface relative to experimental studies and can therefore be used to optimize its mechanical properties. PMID:19682690

Capillary fracture of soft gels.

PubMed

Bostwick, Joshua B; Daniels, Karen E

2013-10-01

A liquid droplet resting on a soft gel substrate can deform that substrate to the point of material failure, whereby fractures develop on the gel surface that propagate outwards from the contact line in a starburst pattern. In this paper, we characterize (i) the initiation process, in which the number of arms in the starburst is controlled by the ratio of the surface tension contrast to the gel's elastic modulus, and (ii) the propagation dynamics showing that once fractures are initiated they propagate with a universal power law L[proportional]t(3/4). We develop a model for crack initiation by treating the gel as a linear elastic solid and computing the deformations within the substrate from the liquid-solid wetting forces. The elastic solution shows that both the location and the magnitude of the wetting forces are critical in providing a quantitative prediction for the number of fractures and, hence, an interpretation of the initiation of capillary fractures. This solution also reveals that the depth of the gel is an important factor in the fracture process, as it can help mitigate large surface tractions; this finding is confirmed with experiments. We then develop a model for crack propagation by considering the transport of an inviscid fluid into the fracture tip of an incompressible material and find that a simple energy-conservation argument can explain the observed material-independent power law. We compare predictions for both linear elastic and neo-Hookean solids, finding that the latter better explains the observed exponent.

Study of the Effects of Metallurgical Factors on the Growth of Fatigue Microcracks.

DTIC Science & Technology

1987-11-25

polycrystalline) yield stress. 8. The resulting model, predicated on the notion of orientation-dependent microplastic grains, predicts quantitatively the entire...Figure 5. Predicted crack growth curves for small cracks propagating from a microplastic grain into elastic-plastic, contiguous grains; Ao is defined as...or the crack tip opening *displacement, 6. Figure 5. Predicted crack growth curves for small cracks propagating from a microplastic grain into

Mathematical modeling of the crack growth in linear elastic isotropic materials by conventional fracture mechanics approaches and by molecular dynamics method: crack propagation direction angle under mixed mode loading

NASA Astrophysics Data System (ADS)

Stepanova, Larisa; Bronnikov, Sergej

2018-03-01

The crack growth directional angles in the isotropic linear elastic plane with the central crack under mixed-mode loading conditions for the full range of the mixity parameter are found. Two fracture criteria of traditional linear fracture mechanics (maximum tangential stress and minimum strain energy density criteria) are used. Atomistic simulations of the central crack growth process in an infinite plane medium under mixed-mode loading using Large-scale Molecular Massively Parallel Simulator (LAMMPS), a classical molecular dynamics code, are performed. The inter-atomic potential used in this investigation is Embedded Atom Method (EAM) potential. The plane specimens with initial central crack were subjected to Mixed-Mode loadings. The simulation cell contains 400000 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 diapason of temperatures (from 0.1 К to 800 К) are obtained and analyzed. It is shown that the crack propagation direction angles obtained by molecular dynamics method 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.

Comparative Study on Prediction Effects of Short Fatigue Crack Propagation Rate by Two Different Calculation Methods

NASA Astrophysics Data System (ADS)

Yang, Bing; Liao, Zhen; Qin, Yahang; Wu, Yayun; Liang, Sai; Xiao, Shoune; Yang, Guangwu; Zhu, Tao

2017-05-01

To describe the complicated nonlinear process of the fatigue short crack evolution behavior, especially the change of the crack propagation rate, two different calculation methods are applied. The dominant effective short fatigue crack propagation rates are calculated based on the replica fatigue short crack test with nine smooth funnel-shaped specimens and the observation of the replica films according to the effective short fatigue cracks principle. Due to the fast decay and the nonlinear approximation ability of wavelet analysis, the self-learning ability of neural network, and the macroscopic searching and global optimization of genetic algorithm, the genetic wavelet neural network can reflect the implicit complex nonlinear relationship when considering multi-influencing factors synthetically. The effective short fatigue cracks and the dominant effective short fatigue crack are simulated and compared by the Genetic Wavelet Neural Network. The simulation results show that Genetic Wavelet Neural Network is a rational and available method for studying the evolution behavior of fatigue short crack propagation rate. Meanwhile, a traditional data fitting method for a short crack growth model is also utilized for fitting the test data. It is reasonable and applicable for predicting the growth rate. Finally, the reason for the difference between the prediction effects by these two methods is interpreted.

Characterization of Cracking and Crack Growth Properties of the C5A Aircraft Tie-Box Forging

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Smith, Stephen W.; Newman, John A.; Willard, Scott A.

2003-01-01

Detailed destructive examinations were conducted to characterize the integrity and material properties of two aluminum alloy (7075-T6) horizontal stabilizer tie box forgings removed.from US. Air Force C5A and C5B transport aircraft. The C5B tie box forging was,found to contain no evidence of cracking. Thirteen cracks were found in the CSA,forging. All but one of the cracks observed in the C5A component were located along the top cap region (one crack was located in the bottom cap region). The cracks in the C5A component initiated at fastener holes and propagated along a highly tunneled intergranular crack path. The tunneled crack growth configuration is a likelv result of surface compressive stress produced during peening of the .forging suijace. The tie box forging ,fatigue crack growth, fracture and stress corrosion cracking (SCC) properties were characterized. Reported herein are the results of laboratory air ,fatigue crack growth tests and 95% relative humidity SCC tests conducted using specimens machined from the C5A ,forging. SCC test results revealed that the C5A ,forging material was susceptible to intergranular environmental assisted cracking: the C5A forging material exhibited a SCC crack-tip stress-intensity factor threshold of less than 6 MPadn. Fracture toughness tests revealed that the C5A forging material exhibited a fracture toughness that was 25% less than the C5B forging. The C5A forging exhibited rapid laboratory air fatigue crack growth rates having a threshold crack-tip stress-intensity factor range of less than 0.8 MPa sup m. Detailed fractographic examinations revealed that the ,fatigue crack intergranular growth crack path was similar to the cracking observed in the C5A tie box forging. Because both fatigue crack propagation and SCC exhibit similar intergranular crack path behavior, the damage mechanism resulting in multi-site cracking of tie box forgings cannot be determined unless local cyclic stresses can be quantified.

Intrinsic fatigue crack propagation in aluminum-lithium alloys - The effect of gaseous environments

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1989-01-01

Gaseous environmental effects on intrinsic fatigue crack growth are significant for the Al-Li-Cu alloy 2090, peak aged. For both moderate Delta K-low R and low Delta K-high R regimes, crack growth rates decrease according to the environment order: purified water vapor, moist air, helium and oxygen. Gaseous environmental effects are pronounced near threshold and are not closure dominated. Here, embrittlement by low levels of H2O (ppm) supports hydrogen embrittlement and suggests that molecular transport controlled cracking, established for high Delta K-low R, is modified near threshold. Localized crack tip reaction sites or high R crack opening shape may enable the strong, environmental effect at low levels of Delta K. Similar crack growth in He and O2 eliminates the contribution of surface films to fatigue damage in alloy 2090. While 2090 and 7075 exhibit similar environmental trends, the Al-Li-Cu alloy is more resistant to intrinsic corrosion fatigue crack growth.

Analysis of Fatigue Crack Paths in Cold Drawn Pearlitic Steel

PubMed Central

Toribio, Jesús; González, Beatriz; Matos, Juan-Carlos

2015-01-01

In this paper, a fracto-metallographic analysis was performed on the cracked specimens of cold drawn pearlitic steel subjected to fatigue tests. Fatigue cracks are transcollonial and exhibit a preference for fracturing pearlitic lamellae, with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and frequent local deflections that create microstructural roughness. At the micro-level, the cold drawn pearlitic steel exhibits higher micro-roughness than the hot rolled bar (this is a consequence of the manufacturing process by cold drawing), so that the actual fractured surface in the cold drawn wire is greater than that in the hot rolled bar, due to the fact that the crack deflection events are more frequent and with higher angle in the former (the heavily drawn prestressing steel wire). These findings show the relevant role on the manufacturing process by cold drawing in the fatigue crack propagation in pearlitic steel. PMID:28793647

Analysis of Fatigue Crack Paths in Cold Drawn Pearlitic Steel.

PubMed

Toribio, Jesús; González, Beatriz; Matos, Juan-Carlos

2015-11-04

In this paper, a fracto-metallographic analysis was performed on the cracked specimens of cold drawn pearlitic steel subjected to fatigue tests. Fatigue cracks are transcollonial and exhibit a preference for fracturing pearlitic lamellae, with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and frequent local deflections that create microstructural roughness. At the micro-level, the cold drawn pearlitic steel exhibits higher micro-roughness than the hot rolled bar (this is a consequence of the manufacturing process by cold drawing), so that the actual fractured surface in the cold drawn wire is greater than that in the hot rolled bar, due to the fact that the crack deflection events are more frequent and with higher angle in the former (the heavily drawn prestressing steel wire). These findings show the relevant role on the manufacturing process by cold drawing in the fatigue crack propagation in pearlitic steel.

Effects of the Electron Beam Welding Process on the Microstructure, Tensile, Fatigue and Fracture Properties of Nickel Alloy Nimonic 80A

NASA Astrophysics Data System (ADS)

Zhang, H.; Huang, Chongxiang; Guan, Zhongwei; Li, Jiukai; Liu, Yongjie; Chen, Ronghua; Wang, Qingyuan

2018-01-01

The purpose of this study was to evaluate rotary bending high-cycle fatigue properties and crack growth of Nimonic 80A-based metal and electron beam-welded joints. All the tests were performed at room temperature. Fracture surfaces under high-cycle fatigue and fatigue crack growth were observed by scanning electron microscopy. Microstructure, hardness and tensile properties were also evaluated in order to understand the effects on the fatigue results obtained. It was found that the tensile properties, hardness and high-cycle fatigue properties of the welded joint are lower than the base metal. The fracture surface of the high-cycle fatigue shows that fatigue crack initiated from the surface under the high stress amplitude and from the subsurface under the low stress amplitude. The effect of the welding process on the statistical fatigue data was studied with a special focus on probabilistic life prediction and probabilistic lifetime limits. The fatigue crack growth rate versus stress intensity factor range data were obtained from the fatigue crack growth tests. From the results, it was evident that the fatigue crack growth rates of the welded are higher than the base metal. The mechanisms and fracture modes of fatigue crack growth of welded specimens were found to be related to the stress intensity factor range ΔK. In addition, the effective fatigue crack propagation thresholds and mismatch of welded joints were described and discussed.

Influence of material ductility and crack surface roughness on fracture instability

NASA Astrophysics Data System (ADS)

Khezrzadeh, Hamed; Wnuk, Michael P.; Yavari, Arash

2011-10-01

This paper presents a stability analysis for fractal cracks. First, the Westergaard stress functions are proposed for semi-infinite and finite smooth cracks embedded in the stress fields associated with the corresponding self-affine fractal cracks. These new stress functions satisfy all the required boundary conditions and according to Wnuk and Yavari's (2003 Eng. Fract. Mech. 70 1659-74) embedded crack model they are used to derive the stress and displacement fields generated around a fractal crack. These results are then used in conjunction with the final stretch criterion to study the quasi-static stable crack extension, which in ductile materials precedes the global failure. The material resistance curves are determined by solving certain nonlinear differential equations and then employed in predicting the stress levels at the onset of stable crack growth and at the critical point, where a transition to the catastrophic failure occurs. It is shown that the incorporation of the fractal geometry into the crack model, i.e. accounting for the roughness of the crack surfaces, results in (1) higher threshold levels of the material resistance to crack propagation and (2) higher levels of the critical stresses associated with the onset of catastrophic fracture. While the process of quasi-static stable crack growth (SCG) is viewed as a sequence of local instability states, the terminal instability attained at the end of this process is identified with the global instability. The phenomenon of SCG can be used as an early warning sign in fracture detection and prevention.

Towards Large-Scale, Non-Destructive Inspection of Concrete Bridges

NASA Astrophysics Data System (ADS)

Mahmoud, A.; Shah, A. H.; Popplewell, N.

2005-04-01

It is estimated that the rehabilitation of deteriorating engineering infrastructure in the harsh North American environment could cost billions of dollars. Bridges are key infrastructure components for surface transportation. Steel-free and fibre-reinforced concrete is used increasingly nowadays to circumvent the vulnerability of steel rebar to corrosion. Existing steel-free and fibre-reinforced bridges may experience extensive surface-breaking cracks that need to be characterized without incurring further damage. In the present study, a method that uses Lamb elastic wave propagation to non-destructively characterize cracks in plain as well as fibre-reinforced concrete is investigated both numerically and experimentally. Numerical and experimental data are corroborated with good agreement.

Brittle Fracture Mechanics of Snow : In Situ Testing and Distinct Element Modeling

NASA Astrophysics Data System (ADS)

Faillettaz, J.; Daudon, D.; Louchet, F.

A snow slab avalanche release usually results from the rupture of the snow cover at the interface between an upper layer (slab) and an underlying substrate. Amazingly, the models proposed so far to predict this kind of rupture were only based on continuum mechanics, as they did not take into account the existing cracks or cohesion defects at the interface between the two layers, and their possible unstable propagation that eventually triggers the avalanche. This is why the present work, essentially devoted to human triggered avalanches, is based instead on Griffith's fracture approach, widely used in modelling brittle fracture of materials. The possible rupture scenario involves a propagation in a shear mode of a "basal crack" nucleated and gradually grown at the interface by the skier's weight, followed by a mode I opening and propagation of a "crown crack" at the top of the sheared zone. Different avalanche sizes are predicted according whether the basal crack propagation reaches or not the Griffith's instabil- ity size before crown crack opening (Louchet 2000). Accurate predictions therefore require a precise knowledge of snow toughness values in both modes. A theoretical estimation of toughness considering snow as an ice foam was proposed by Kirchner and Michot (2000), but the question of whether these results may be extended to an assembly of sintered grains is still open. A mode I toughness measurement of snow was also published for the first time by Kirchner and Michot on samples gathered in the Vosges range. In the present work, we developed an experimental set similar to Michot's, in order to measure mode I toughness: a vertical crack of increasing size is gradually machined from the top surface in an horizontal snow beam until failure takes place under its own weight. The toughness value is computed from the snow weight and the crack length at the onset of rapid crack propagation. A similar device was designed for mode II testing, but is still under development. The experimental cam- paign carried out in the Alps during the 2000-2001 winter on homogeneous sintered snow with a density of 200 kg/m3 (typical of a snow slab) gave results of the same or- der of magnitude as Michot's. A numerical modeling of these toughness experiments was performed using a distinct element code, considering snow as a cohesive granu- lar material. Both crack propagation and rupture patterns are in close agreement with experiments. References: Kirchner, Michot, Suzuki 2000 Fracture thoughness of snow in tension 1 Philisophical Magazine A, vol 80,N5, p1265-1272. Louchet 2001,A transition in dry snow slab avalanche triggering modes, Annales de glaciologie, vol 32,Symphosium on Snow, Avalanches and Impact of the Frest Cover, Innsbruck,Austria,22-26 may 2000, p2285-289 2

Temperature dependence of liquid metal embrittlement susceptibility of a modified 9Cr-1Mo steel under low cycle fatigue in lead-bismuth eutectic at 160-450 °C

NASA Astrophysics Data System (ADS)

Gong, Xing; Marmy, Pierre; Qin, Ling; Verlinden, Bert; Wevers, Martine; Seefeldt, Marc

2016-01-01

Low cycle fatigue properties of a 9Cr-1Mo ferritic-martensitic steel (T91) have been tested in a low oxygen concentration (LOC) lead-bismuth eutectic (LBE) environment and in vacuum at 160-450 °C. The results show a clear fatigue endurance "trough" in LOC LBE, while no such a strong temperature dependence of the fatigue endurance is observed when the steel is tested in vacuum. The fractographic observations by means of scanning electron microscopy (SEM) show that ductile microdimples are prevalent on the fracture surfaces of the specimens tested in vacuum, whereas the fracture surfaces produced in LOC LBE at all the temperatures are characterized by quasi-cleavage. Interestingly, using electron backscatter diffraction (EBSD), martensitic laths close to the fatigue crack walls or to the fracture surfaces of the specimens tested in vacuum are found to have transformed into very fine equiaxed subgrains. Nevertheless, such microstructural modifications do not happen to the specimens tested in LOC LBE at 160-450 °C. These interesting microstructural distinctions indicate that liquid metal embrittlement (LME) is able to occur throughout the fatigue crack propagation phase in the full range of the temperatures investigated, i.e. LME is not very sensitive to temperature during the fatigue crack propagation.

Mechanical and fracture behavior of veneer-framework composites for all-ceramic dental bridges.

PubMed

Studart, André R; Filser, Frank; Kocher, Peter; Lüthy, Heinz; Gauckler, Ludwig J

2007-01-01

High-strength ceramics are required in dental posterior restorations in order to withstand the excessive tensile stresses that occur during mastication. The aim of this study was to investigate the fracture behavior and the fast-fracture mechanical strength of three veneer-framework composites (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) for all-ceramic dental bridges. The load bearing capacity of the veneer-framework composites were evaluated using a bending mechanical apparatus. The stress distribution through the rectangular-shaped layered samples was assessed using simple beam calculations and used to estimate the fracture strength of the veneer layer. Optical microscopy of fractured specimens was employed to determine the origin of cracks and the fracture mode. Under fast fracture conditions, cracks were observed to initiate on, or close to, the veneer outer surface and propagate towards the inner framework material. Crack deflection occurred at the veneer-framework interface of composites containing a tough framework material (TZP/Cercon S and Inceram-Zirconia/Vita VM7), as opposed to the straight propagation observed in the case of weaker frameworks (Empress 2/IPS Eris). The mechanical strength of dental composites containing a weak framework (K(IC)<3 MPam(1/2)) is ultimately determined by the low fracture strength of the veneer layer, since no crack arresting occurs at the veneer-framework interface. Therefore, high-toughness ceramics (K(IC)>5 MPam(1/2)) should be used as framework materials of posterior all-ceramic bridges, so that cracks propagating from the veneer layer do not lead to a premature failure of the prosthesis.

Fracture and failure: Analyses, mechanisms and applications; Proceedings of the Symposium, Los Angeles, CA, March 17-20, 1980

NASA Technical Reports Server (NTRS)

Tung, P. P. (Editor); Agrawal, S. P.; Kumar, A.; Katcher, M.

1981-01-01

Papers are presented on the application of fracture mechanics to spacecraft design, fracture control applications on the Space Shuttle reaction control thrusters, and an assessment of fatigue crack growth rate relationships for metallic airframe materials. Also considered are fracture mechanisms and microstructural relationships in Ni-base alloy systems, the use of surface deformation markings to determine crack propagation directions, case histories of metallurgical failures in the electronics industry, and a failure analysis of silica phenolic nozzle liners.

Comparison of Visual and Acoustic Emission Observations in a Four Point Bending Experiment on Barre Granite

NASA Astrophysics Data System (ADS)

Li, Bing Qiuyi; Einstein, Herbert H.

2017-09-01

We present an experimental study in which a pre-notched specimen of Barre Granite was subjected to four point bending under crack mouth opening displacement control. The experimental observations consisted of load-displacement measurements, acoustic emissions, and photography on a macroscopic ( cm) as well as microscopic ( μm) scale. These observations were compared and analysed to better understand process zone development and crack propagation. Load-displacement data showed that the load reaches its maximum at crack initiation, and the machine input work is constant while the crack propagates. AE moment magnitudes between Mw = -6 to -10 were observed, and focal mechanisms consisted of both shear and tensile components. During process zone development, AE formed a large cloud of events located near the notch tip and then tended to occur away from the notch tip as the crack propagated. Image analysis at the microscopic scale showed that microcracks formed and coalesced during process zone development; specifically, the microcracks initiated in tension and then propagated as a series of en-echelon cracks. In general, the synthesis of the three observations showed that a wider bulb of activity at lower energy tended to occur during process zone development, while crack propagation tended to be more spatially concentrated and contained higher energy.

Hydrogen-Assisted Crack Propagation in Austenitic Stainless Steel Fusion Welds

NASA Astrophysics Data System (ADS)

Somerday, B. P.; Dadfarnia, M.; Balch, D. K.; Nibur, K. A.; Cadden, C. H.; Sofronis, P.

2009-10-01

The objective of this study was to characterize hydrogen-assisted crack propagation in gas-tungsten arc (GTA) welds of the nitrogen-strengthened, austenitic stainless steel 21Cr-6Ni-9Mn (21-6-9), using fracture mechanics methods. The fracture initiation toughness and crack growth resistance curves were measured using fracture mechanics specimens that were thermally precharged with 230 wppm (1.3 at. pct) hydrogen. The fracture initiation toughness and slope of the crack growth resistance curve for the hydrogen-precharged weld were reduced by as much as 60 and 90 pct, respectively, relative to the noncharged weld. A physical model for hydrogen-assisted crack propagation in the welds was formulated from microscopy evidence and finite-element modeling. Hydrogen-assisted crack propagation proceeded by a sequence of microcrack formation at the weld ferrite, intense shear deformation in the ligaments separating microcracks, and then fracture of the ligaments. One salient role of hydrogen in the crack propagation process was promoting microcrack formation at austenite/ferrite interfaces and within the ferrite. In addition, hydrogen may have facilitated intense shear deformation in the ligaments separating microcracks. The intense shear deformation could be related to the development of a nonuniform distribution of hydrogen trapped at dislocations between microcracks, which in turn created a gradient in the local flow stress.

Simulation of Crack Propagation in Engine Rotating Components under Variable Amplitude Loading

NASA Technical Reports Server (NTRS)

Bonacuse, P. J.; Ghosn, L. J.; Telesman, J.; Calomino, A. M.; Kantzos, P.

1998-01-01

The crack propagation life of tested specimens has been repeatedly shown to strongly depend on the loading history. Overloads and extended stress holds at temperature can either retard or accelerate the crack growth rate. Therefore, to accurately predict the crack propagation life of an actual component, it is essential to approximate the true loading history. In military rotorcraft engine applications, the loading profile (stress amplitudes, temperature, and number of excursions) can vary significantly depending on the type of mission flown. To accurately assess the durability of a fleet of engines, the crack propagation life distribution of a specific component should account for the variability in the missions performed (proportion of missions flown and sequence). In this report, analytical and experimental studies are described that calibrate/validate the crack propagation prediction capability ]or a disk alloy under variable amplitude loading. A crack closure based model was adopted to analytically predict the load interaction effects. Furthermore, a methodology has been developed to realistically simulate the actual mission mix loading on a fleet of engines over their lifetime. A sequence of missions is randomly selected and the number of repeats of each mission in the sequence is determined assuming a Poisson distributed random variable with a given mean occurrence rate. Multiple realizations of random mission histories are generated in this manner and are used to produce stress, temperature, and time points for fracture mechanics calculations. The result is a cumulative distribution of crack propagation lives for a given, life limiting, component location. This information can be used to determine a safe retirement life or inspection interval for the given location.

Regulated fracture in tooth enamel: a nanotechnological strategy from nature.

PubMed

Ghadimi, Elnaz; Eimar, Hazem; Song, Jun; Marelli, Benedetto; Ciobanu, Ovidiu; Abdallah, Mohamed-Nur; Stähli, Christoph; Nazhat, Showan N; Vali, Hojatollah; Tamimi, Faleh

2014-07-18

Tooth enamel is a very brittle material; however it has the ability to sustain cracks without suffering catastrophic failure throughout the lifetime of mechanical function. We propose that the nanostructure of enamel can play a significant role in defining its unique mechanical properties. Accordingly we analyzed the nanostructure and chemical composition of a group of teeth, and correlated it with the crack resistance of the same teeth. Here we show how the dimensions of apatite nanocrystals in enamel can affect its resistance to crack propagation. We conclude that the aspect ratio of apatite nanocrystals in enamel determines its resistance to crack propagation. According to this finding, we proposed a new model based on the Hall-Petch theory that accurately predicts crack propagation in enamel. Our new biomechanical model of enamel is the first model that can successfully explain the observed variations in the behavior of crack propagation of tooth enamel among different humans. Copyright © 2014 Elsevier Ltd. All rights reserved.

Velocity mode transition of dynamic crack propagation in hyperviscoelastic materials: A continuum model study

PubMed Central

Kubo, Atsushi; Umeno, Yoshitaka

2017-01-01

Experiments of crack propagation in rubbers have shown that a discontinuous jump of crack propagation velocity can occur as energy release rate increases, which is known as the “mode transition” phenomenon. Although it is believed that the mode transition is strongly related to the mechanical properties, the nature of the mode transition had not been revealed. In this study, dynamic crack propagation on an elastomer was investigated using the finite element method (FEM) with a hyperviscoelastic material model. A series of pure shear test was carried out numerically with FEM simulations and crack velocities were measured under various values of tensile strain. As a result, our FEM simulations successfully reproduced the mode transition. The success of realising the mode transition phenomenon by a simple FEM model, which was achieved for the first time ever, helped to explain that the phenomenon occurs owing to a characteristic non-monotonic temporal development of principal stress near the crack tip. PMID:28186205

Velocity mode transition of dynamic crack propagation in hyperviscoelastic materials: A continuum model study

NASA Astrophysics Data System (ADS)

Kubo, Atsushi; Umeno, Yoshitaka

2017-02-01

Experiments of crack propagation in rubbers have shown that a discontinuous jump of crack propagation velocity can occur as energy release rate increases, which is known as the “mode transition” phenomenon. Although it is believed that the mode transition is strongly related to the mechanical properties, the nature of the mode transition had not been revealed. In this study, dynamic crack propagation on an elastomer was investigated using the finite element method (FEM) with a hyperviscoelastic material model. A series of pure shear test was carried out numerically with FEM simulations and crack velocities were measured under various values of tensile strain. As a result, our FEM simulations successfully reproduced the mode transition. The success of realising the mode transition phenomenon by a simple FEM model, which was achieved for the first time ever, helped to explain that the phenomenon occurs owing to a characteristic non-monotonic temporal development of principal stress near the crack tip.

Velocity mode transition of dynamic crack propagation in hyperviscoelastic materials: A continuum model study.

PubMed

Kubo, Atsushi; Umeno, Yoshitaka

2017-02-10

Experiments of crack propagation in rubbers have shown that a discontinuous jump of crack propagation velocity can occur as energy release rate increases, which is known as the "mode transition" phenomenon. Although it is believed that the mode transition is strongly related to the mechanical properties, the nature of the mode transition had not been revealed. In this study, dynamic crack propagation on an elastomer was investigated using the finite element method (FEM) with a hyperviscoelastic material model. A series of pure shear test was carried out numerically with FEM simulations and crack velocities were measured under various values of tensile strain. As a result, our FEM simulations successfully reproduced the mode transition. The success of realising the mode transition phenomenon by a simple FEM model, which was achieved for the first time ever, helped to explain that the phenomenon occurs owing to a characteristic non-monotonic temporal development of principal stress near the crack tip.

Direct observation of the residual plastic deformation caused by a single tensile overload

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

Bichler, C.; Pippan, R.

1999-07-01

The fatigue crack growth behavior following single tensile overloads at high stress intensity ranges in a cold-rolled austenitic steel has been studied experimentally. After tensile overloads, fatigue cracks initially accelerate, followed by significant retardation, before the growth rates return to their baseline level. The initial acceleration was attributed to an immediate reduction in near-tip closure. Scanning electron micrography and stereophotogrammetric reconstruction of the fracture surface were applied to study the residual plastic deformation caused by a single tensile overload in the mid-thickness of the specimen. The measured residual opening displacement of the crack as a function of the overload ismore » presented and compared with simple estimations. Also, free specimen surface observations of the residual plastic deformation and crack growth rate were performed. In the midsection of the specimens the striation spacing-length, i.e., the microscopic growth rates, were measured before and after the applied overload. It will be shown that the measured plasticity-induced wedges from the single overload and the observed propagation behavior support the significance of the concept of crack closure.« less

Analysis of fatigue, fatique-crack propagation, and fracture data. [design of metallic aerospace structural components

NASA Technical Reports Server (NTRS)

Jaske, C. E.; Feddersen, C. E.; Davies, K. B.; Rice, R. C.

1973-01-01

Analytical methods have been developed for consolidation of fatigue, fatigue-crack propagation, and fracture data for use in design of metallic aerospace structural components. To evaluate these methods, a comprehensive file of data on 2024 and 7075 aluminums, Ti-6A1-4V, and 300M and D6Ac steels was established. Data were obtained from both published literature and unpublished reports furnished by aerospace companies. Fatigue and fatigue-crack-propagation analyses were restricted to information obtained from constant-amplitude load or strain cycling of specimens in air at room temperature. Fracture toughness data were from tests of center-cracked tension panels, part-through crack specimens, and compact-tension specimens.

Corrosion, stress corrosion cracking, and electrochemistry of the iron and nickel base alloys in caustic environments. Progress report, 1 March 1977--28 February 1978

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

Staehle, R.W.; Agrawal, A.K.

1978-01-01

The straining electrode technique was used to evaluate the stress corrosion cracking (SCC) susceptibility of AISI 304 stainless steel in 20N NaOH solution, and of Inconel 600 Alloy and Incoloy 800 Alloy in boiling 17.5N NaOH solution. The crack propagation rate estimated from the straining experiments correlated well with the previous constant load experiments. It was found that the straining electrode technique is a useful method for estimating, through short term experiments, parameters like crack propagation rate, crack morphology, and repassivation rate, as a function of the electrode potential. The role of alloying elements on the crack propagation rate inmore » the above alloys are also discussed.« less

Small Crack Growth and Fatigue Life Predictions for High-Strength Aluminium Alloys. Part 1; Experimental and Fracture Mechanics Analysis

NASA Technical Reports Server (NTRS)

Wu, X. R.; Newman, J. C.; Zhao, W.; Swain, M. H.; Ding, C. F.; Phillips, E. P.

1998-01-01

The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests A,ere conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and A weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plastisity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions, Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminum alloys was developed and demonstrated using the crack-closure model.

Photomultiplier tube failure under hydrostatic pressure in future neutrino detectors

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

Chambliss, K.; Diwan, M.; Simos, N.

Failure of photomultiplier tubes (PMTs) under hydrostatic pressure is a concern in neutrino detection, specifically, in the proposed Long-Baseline Neutrino Experiment project. Controlled hydrostatic implosion tests were performed on prototypic PMT bulbs of 10-inch diameter and recorded using high speed filming techniques to capture failures in detail. These high-speed videos were analyzed frame-by-frame in order to identify the origin of a crack, measure the progression of individual crack along the surface of the bulb as it propagates through the glass, and estimate crack velocity. Crack velocity was calculated for each individual crack, and an average velocity was determined for allmore » measurable cracks on each bulb. Overall, 32 cracks were measured in 9 different bulbs tested. Finite element modeling (FEM) of crack formation and growth in prototypic PMT shows stress concentration near the middle section of the PMT bulbs that correlates well with our crack velocity measurements in that section. The FEM model predicts a crack velocity value that is close to the terminal crack velocity reported. Our measurements also reveal significantly reduced crack velocities compared to terminal crack velocities measured in glasses using fracture mechanics testing and reported in literature.« less

Photomultiplier tube failure under hydrostatic pressure in future neutrino detectors

DOE PAGES

Chambliss, K.; Diwan, M.; Simos, N.; ...

2014-10-09

Failure of photomultiplier tubes (PMTs) under hydrostatic pressure is a concern in neutrino detection, specifically, in the proposed Long-Baseline Neutrino Experiment project. Controlled hydrostatic implosion tests were performed on prototypic PMT bulbs of 10-inch diameter and recorded using high speed filming techniques to capture failures in detail. These high-speed videos were analyzed frame-by-frame in order to identify the origin of a crack, measure the progression of individual crack along the surface of the bulb as it propagates through the glass, and estimate crack velocity. Crack velocity was calculated for each individual crack, and an average velocity was determined for allmore » measurable cracks on each bulb. Overall, 32 cracks were measured in 9 different bulbs tested. Finite element modeling (FEM) of crack formation and growth in prototypic PMT shows stress concentration near the middle section of the PMT bulbs that correlates well with our crack velocity measurements in that section. The FEM model predicts a crack velocity value that is close to the terminal crack velocity reported. Our measurements also reveal significantly reduced crack velocities compared to terminal crack velocities measured in glasses using fracture mechanics testing and reported in literature.« less

Stress corrosion-controlled rates of mode I fracture propagation in calcareous bedrock

NASA Astrophysics Data System (ADS)

Voigtlaender, Anne; Leith, Kerry; Krautblatter, Michael

2014-05-01

Surface bedrock on natural rock slopes is subject to constant and cyclic environmental stresses (wind, water, wave, ice, seismic or gravitational). Studies indicate that these stresses range up to several hundred kPa, generally too low to cause macroscopic changes in intact rock, although clear evidence of fracture generation, crack propagation and weathering of bedrock illustrates the effect of environmental stresses at the Earth's surface. We suggest that material degradation and its extent, is likely to be controlled by the rate of stress corrosion cracking (SCC). Stress corrosion is a fluid-material reaction, where fluids preferentially react with strained atomic bonds at the tip of developing fractures. Stress corrosion in ferrous and siliceous materials is often accepted as the fracture propagation and degradation rate-controlling process where materials are subject to stresses and fluids. Although evidence for chemical weathering in propagating bedrock fractures is clear in natural environments, the physical system and quantification of stress corrosion in natural rocks is yet to be addressed. Here, we present preliminary data on the relationship between stresses at levels commonly present on natural rock slopes, and material damage resulting from stress corrosion under constant or cyclic tensile loading. We undertake single notch three-point bending tests (SNBT) on fresh calcareous bedrock specimens (1100x100x100mm) over a two-month period. Two beams containing an artificial notch are stressed to 75% of their ultimate strength, and a constant supply of weak acid is applied at the notch tip to enhance chemical reactions. A third, unloaded, beam is also exposed to weak acid in order to elucidate the contribution of stress corrosion cracking to the material degradation. Stresses at the tip of propagating cracks affect the kinetics of the chemical reaction in the specimen exposed to both loading and corrosion, leading to an increase in degradation, and greater stress relaxation. These changes in material properties are monitored using strain gauges, acoustic emission sensors, changes in P-wave velocity, and records of time to failure where appropriate. Our preliminary studies indicate changes in material properties are concentrated in the region of predicted tensile stress intensification. Reactions seem to favourably occur at the stressed bonds around the crack tip. The rate of chemical dissolution and further propagation of the fracture at the notch tip appears to be enhanced by the level of stress applied to the specimen. This provides the foundation for a suite of repeated experiments in which we plan to test corrosion-controlled rates of degradation across a range of loading conditions. The improved understanding into micro-mechanical controls, will contribute to the assessment of rock fall production rates and erosion processes in natural environments as well as natural building stones.

Plates and shells containing a surface crack under general loading conditions

NASA Technical Reports Server (NTRS)

Joseph, Paul F.; Erdogan, Fazil

1987-01-01

Various through and part-through crack problems in plates and shells are considered. The line-spring model of Rice and Levy is generalized to the skew-symmetric case to solve surface crack problems involving mixed-mode, coplanar crack growth. Compliance functions are introduced which are valid for crack depth to thickness ratios at least up to .95. This includes expressions for tension and bending as well as expressions for in-plane shear, out-of-plane shear, and twisting. Transverse shear deformation is taken into account in the plate and shell theories and this effect is shown to be important in comparing stress intensity factors obtained from the plate theory with three-dimensional solutions. Stress intensity factors for cylinders obtained by the line-spring model also compare well with three-dimensional solution. By using the line-spring approach, stress intensity factors can be obtained for the through crack and for part-through crack of any crack front shape, without recalculation integrals that take up the bulk of the computer time. Therefore, parameter studies involving crack length, crack depth, shell type, and shell curvature are made in some detail. The results will be useful in brittle fracture and in fatigue crack propagation studies. All problems considered are of the mixed boundary value type and are reducted to strongly singular integral equations which make use of the finite-part integrals of Hadamard. The equations are solved numerically in a manner that is very efficient.

Past Performance analysis of HPOTP bearings

NASA Technical Reports Server (NTRS)

Bhat, B. N.; Dolan, F. J.

1982-01-01

The past performance analysis conducted on three High Pressure Oxygen Turbopump (HPOTP) bearings from the Space Shuttle Main Engine is presented. Metallurgical analysis of failed bearing balls and races, and wear track and crack configuration analyses were carried out. In addition, one bearing was tested in laboratory at very high axial loads. The results showed that the cracks were surface initiated and propagated into subsurface locations at relatively small angles. Subsurface cracks were much more extensive than was appeared on the surface. The location of major cracks in the races corresponded to high radial loads rather than high axial loads. There was evidence to suggest that the inner races were heated to elevated temperatures. A failure scenario was developed based on the above findings. According to this scenario the HPOTP bearings are heated by a combination of high loads and high coefficient of friction (poor lubrication). Different methods of extending the HPOTP bearing life are also discussed. These include reduction of axial loads, improvements in bearing design, lubrication and cooling, and use of improved bearing materials.

Molecular dynamics simulation of propagating cracks

NASA Technical Reports Server (NTRS)

Mullins, M.

1982-01-01

Steady state crack propagation is investigated numerically using a model consisting of 236 free atoms in two (010) planes of bcc alpha iron. The continuum region is modeled using the finite element method with 175 nodes and 288 elements. The model shows clear (010) plane fracture to the edge of the discrete region at moderate loads. Analysis of the results obtained indicates that models of this type can provide realistic simulation of steady state crack propagation.

Prediction of Fatigue Crack Growth in Gas Turbine Engine Blades Using Acoustic Emission

PubMed Central

Zhang, Zhiheng; Yang, Guoan; Hu, Kun

2018-01-01

Fatigue failure is the main type of failure that occurs in gas turbine engine blades and an online monitoring method for detecting fatigue cracks in blades is urgently needed. Therefore, in this present study, we propose the use of acoustic emission (AE) monitoring for the online identification of the blade status. Experiments on fatigue crack propagation based on the AE monitoring of gas turbine engine blades and TC11 titanium alloy plates were conducted. The relationship between the cumulative AE hits and the fatigue crack length was established, before a method of using the AE parameters to determine the crack propagation stage was proposed. A method for predicting the degree of crack propagation and residual fatigue life based on the AE energy was obtained. The results provide a new method for the online monitoring of cracks in the gas turbine engine blade. PMID:29693556

Path (un)predictability of two interacting cracks in polycarbonate sheets using Digital Image Correlation.

PubMed

Koivisto, J; Dalbe, M-J; Alava, M J; Santucci, S

2016-08-31

Crack propagation is tracked here with Digital Image Correlation analysis in the test case of two cracks propagating in opposite directions in polycarbonate, a material with high ductility and a large Fracture Process Zone (FPZ). Depending on the initial distances between the two crack tips, one may observe different complex crack paths with in particular a regime where the two cracks repel each other prior to being attracted. We show by strain field analysis how this can be understood according to the principle of local symmetry: the propagation is to the direction where the local shear - mode KII in fracture mechanics language - is zero. Thus the interactions exhibited by the cracks arise from symmetry, from the initial geometry, and from the material properties which induce the FPZ. This complexity makes any long-range prediction of the path(s) impossible.

Prediction of Fatigue Crack Growth in Gas Turbine Engine Blades Using Acoustic Emission.

PubMed

Zhang, Zhiheng; Yang, Guoan; Hu, Kun

2018-04-25

Fatigue failure is the main type of failure that occurs in gas turbine engine blades and an online monitoring method for detecting fatigue cracks in blades is urgently needed. Therefore, in this present study, we propose the use of acoustic emission (AE) monitoring for the online identification of the blade status. Experiments on fatigue crack propagation based on the AE monitoring of gas turbine engine blades and TC11 titanium alloy plates were conducted. The relationship between the cumulative AE hits and the fatigue crack length was established, before a method of using the AE parameters to determine the crack propagation stage was proposed. A method for predicting the degree of crack propagation and residual fatigue life based on the AE energy was obtained. The results provide a new method for the online monitoring of cracks in the gas turbine engine blade.

A Continuum-Atomistic Analysis of Transgranular Crack Propagation in Aluminum

NASA Technical Reports Server (NTRS)

Yamakov, V.; Saether, E.; Glaessgen, E.

2009-01-01

A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [111] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes.

Axial crack propagation and arrest in pressurized fuselage

NASA Technical Reports Server (NTRS)

Kosai, M.; Shimamoto, A.; Yu, C.-T.; Walker, S. I.; Kobayashi, A. S.; Tan, P.

1994-01-01

The crack arrest capability of a tear strap in a pressurized precracked fuselage was studied through instrumented axial rupture tests of small scale models of an idealized fuselage. Upon pressurization, rapid crack propagation initiated at an axial through crack along the stringer and immediately kinked due to the mixed modes 1 and 2 state caused by the one-sided opening of the crack flap. The diagonally running crack further turned at the tear straps. Dynamic finite element analysis of the rupturing cylinder showed that the crack kinked and also ran straight in the presence of a mixed mode state according to a modified two-parameter crack kinking criterion.

Crack networks in damaged glass

NASA Astrophysics Data System (ADS)

Mallet, Celine; Fortin, Jerome; Gueguen, Yves

2013-04-01

We investigate how cracks develop and propagate in synthetic glass samples. Cracks are introduced in glass by a thermal shock of 300oC. Crack network is documented from optical and electronic microscopy on these samples that have been submitted to a thermal shock only. Samples are cylinder of 80 mm length and 40 mm diameter. Sections were cut along the cylinder axis and perpendicular to it. Using SEM, crack lengths and apertures can be measured. Optical microscopy allows to get the crack distribution over the entire sample. The sample average crack length is 3 mm. The average aperture is 6 ± 3μm. There is however a clear difference between the sample core, where the crack network has approximatively a transverse isotrope symmetry and the outer ring, where cracks are smaller and more numerous. By measuring before and after the thermal treatment the radial P and S wave velocities in room conditions, we can determine the total crack density which is 0.24. Thermally cracked samples, as described above, were submitted to creep tests. Constant axial stress and lateral stress were applied. Several experiments were performed at different stress values. Samples are saturated for 48 hours (to get an homogeneous pore fluid distribution), the axial stress is increased up to 80% of the sample strength. Stress step tests were performed in order to get creep data. The evolution of strain (axial and radial strain) is measured using strain gages, gap sensors (for the global axial strain) and pore volume change (for the volumetric strain). Creep data are interpreted as evidence of sub-critical crack growth in the cracked glass samples. The above microstructural observations are used, together with a crack propagation model, to account for the creep behavior. Assuming that (i) the observed volumetric strain rate is due to crack propagation and (ii) crack aspect ratio is constant we calculate the creep rate. We obtain some value on the crack propagation during a 24 hours of constant stress test. At each of these test, crack propagate of 0.3 to 0.4 mm. From the initial average crack length of 3 mm, the crack reach the size of 5.8 mm at the end of a complete creep test (with 8 constant stress step of 24 hours).

Investigation of wing crack formation with a combined phase-field and experimental approach

NASA Astrophysics Data System (ADS)

Lee, Sanghyun; Reber, Jacqueline E.; Hayman, Nicholas W.; Wheeler, Mary F.

2016-08-01

Fractures that propagate off of weak slip planes are known as wing cracks and often play important roles in both tectonic deformation and fluid flow across reservoir seals. Previous numerical models have produced the basic kinematics of wing crack openings but generally have not been able to capture fracture geometries seen in nature. Here we present both a phase-field modeling approach and a physical experiment using gelatin for a wing crack formation. By treating the fracture surfaces as diffusive zones instead of as discontinuities, the phase-field model does not require consideration of unpredictable rock properties or stress inhomogeneities around crack tips. It is shown by benchmarking the models with physical experiments that the numerical assumptions in the phase-field approach do not affect the final model predictions of wing crack nucleation and growth. With this study, we demonstrate that it is feasible to implement the formation of wing cracks in large scale phase-field reservoir models.

Self-healing of cracks formed in Silicon-Aluminum anodes electrochemically cycled at high lithiation rates

NASA Astrophysics Data System (ADS)

Bhattacharya, Sandeep; Alpas, Ahmet T.

2016-10-01

Lithiation-induced volume changes in Si result in fracture and fragmentation of Si anodes in Li-ion batteries. This paper reports the self-healing behaviour of cracks observed in micron-sized Si particles dispersed in a ductile Al matrix of a Si-Al electrode electrochemically cycled vs. Li/Li+ using a high lithiation rate of 15.6 C. Cross-sectional high-resolution transmission electron microscopy and Raman spectroscopy revealed that an amorphous layer with a depth up to ∼100 nm was formed at the surface of Si particles. In-situ optical microscopy performed during electrochemical experiments revealed development of cracks in Si particles as the voltage decreased to 0.02 V during lithiation. Self-healing of cracks in Si particles occurred in two steps: i) arresting of the crack growth at the Si/Al interface as the surrounding Al matrix had a higher fracture toughness and thus acted as a barrier to crack propagation, and ii) closure of cracks due to compressive stresses applied to the crack faces by the amorphous zones formed on each side of the crack paths.

Fatigue disbonding analysis of wide composite panels by means of Lamb waves

NASA Astrophysics Data System (ADS)

Michalcová, Lenka; Rechcígel, Lukáš; Bělský, Petr; Kucharský, Pavel

2018-03-01

Guided wave-based monitoring of composite structures plays an important role in the area of structural health monitoring (SHM) of aerospace structures. Adhesively bonded joints have not yet fulfilled current airworthiness requirements; hence, assemblies of carbon fibre-reinforced parts still require mechanical fasteners, and a verified SHM method with reliable disbonding/delamination detection and propagation assessment is needed. This study investigated the disbonding/delamination propagation in adhesively bonded panels using Lamb waves during fatigue tests. Analyses focused on the proper frequency and mode selection, sensor placement and selection of parameter sensitive to the growth of disbonding areas. Piezoelectric transducers placed across the bonded area were used as actuators and sensors. Lamb wave propagation was investigated considering the actual shape of the crack front and the mode of the crack propagation. The actual cracked area was determined by ultrasonic A-scans. A correlation between the crack propagation rate and the A0 mode velocity was found.

Surface Studies of Ultra Strength Drilling Steel after Corrosion Fatigue in Simulated Sour Environment

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

M. Ziomek-Moroz; J.A. Hawk; R. Thodla

2012-05-06

The Unites States predicted 60% growth in energy demand by 2030 makes oil and natural gas primary target fuels for energy generation. The fact that the peak of oil production from shallow wells (< 5000 m) is about to be reached, thereby pushing the oil and natural gas industry into deeper wells. However, drilling to depths greater than 5000 m requires increasing the strength-to weight ratio of the drill pipe materials. Grade UD-165 is one of the ultra- high yield strength carbon steels developed for ultra deep drilling (UDD) activities. Drilling UDD wells exposes the drill pipes to Cl{sup -},more » HCO{sub 3}{sup -}/CO{sub 3}{sup 2-}, and H{sub 2}S-containig corrosive environments (i.e., sour environments) at higher pressures and temperatures compared to those found in conventional wells. Because of the lack of synergism within the service environment, operational stresses can result in catastrophic brittle failures characteristic for environmentally assisted cracking (EAC). Approximately 75% of all drill string failures are caused by fatigue or corrosion fatigue. Since there is no literature data on the corrosion fatigue performance of UD-165 in sour environments, research was initiated to better clarify the fatigue crack growth (FCGR) behavior of this alloy in UDD environments. The FCGR behavior of ultra-strength carbon steel, grade UD-165, was investigated by monitoring crack growth rate in deaerated 5%NaCl solution buffered with NaHCO{sub 3}/Na{sub 2}CO{sub 3} and in contact with H{sub 2}S. The partial pressure of H{sub 2}S (p{sub H2S}) was 0.83 kPa and pH of the solution was adjusted by NaOH to 12. The fatigue experiments were performed at 20 and 85 C in an autoclave with surface investigations augmented by scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy. In this study, research focused on surface analyses supported by the fatigue crack growth rate measurements. Fig. 1 shows an SEM micrograph of the crack that propagated from the notch in the solution at 20 C. Accumulation of the corrosion products is visible along the crack. The EDX chemical analysis near the crack tip found iron, sulfur and oxygen in the passive layer. The surface of the sample after the fatigue test in the sour environment at 85{sup o}, Fig. 2, C looks different from that fatigued surface at 20 C. The crack propagates across the passive film that covers the surface fairly uniformly. Some spallation of the passive film is observed near the notch. The EDX chemical analysis of the passive film near the crack tip identified mainly iron, carbon and oxygen. It appears that temperature plays a very important role in formation of the passive film. This may be associated with different solubility of H{sub 2}S in the solution, which will be further studied.« less

A thermodynamic analysis of propagating subcritical cracks with cohesive zones

NASA Technical Reports Server (NTRS)

Allen, David H.

1993-01-01

The results of the so-called energetic approach to fracture with particular attention to the issue of energy dissipation due to crack propagation are applied to the case of a crack with cohesive zone. The thermodynamic admissibility of subcritical crack growth (SCG) is discussed together with some hypotheses that lead to the derivation of SCG laws. A two-phase cohesive zone model for discontinuous crack growth is presented and its thermodynamics analyzed, followed by an example of its possible application.

Experimental Investigation and Finite Element Analysis on Fatigue Behavior of Aluminum Alloy 7050 Single-Lap Joints

NASA Astrophysics Data System (ADS)

Zhou, Bing; Cui, Hao; Liu, Haibo; Li, Yang; Liu, Gaofeng; Li, Shujun; Zhang, Shangzhou

2018-03-01

The fatigue behavior of single-lap four-riveted aluminum alloy 7050 joints was investigated by using high-frequency fatigue test and scanning electron microscope (SEM). Stress distributions obtained by finite element (FE) analysis help explain the fatigue performance. The fatigue test results showed that the fatigue lives of the joints depend on cold expansion and applied cyclic loads. FE analysis and fractography indicated that the improved fatigue lives can be attributed to the reduction in maximum stress and evolution of fatigue damage at the critical location. The beneficial effects of strengthening techniques result in tearing ridges or lamellar structure on fracture surface, decrease in fatigue striations spacing, delay of fatigue crack initiation, crack deflection in fatigue crack propagation and plasticity-induced crack closure.

Investigation on Characteristic Variation of the FBG Spectrum with Crack Propagation in Aluminum Plate Structures

PubMed Central

Jin, Bo; Zhang, Weifang; Zhang, Meng; Ren, Feifei; Dai, Wei; Wang, Yanrong

2017-01-01

In order to monitor the crack tip propagation of aluminum alloy, this study investigates the variation of the spectrum characteristics of a fiber Bragg grating (FBG), combined with an analysis of the spectrum simulation. The results identify the location of the subordinate peak as significantly associated with the strain distribution along the grating, corresponding to the different plastic zones ahead of the crack tip with various crack lengths. FBG sensors could observe monotonic and cyclic plastic zones ahead of the crack tip, with the quadratic strain distribution along the grating at the crack tip-FBG distance of 1.2 and 0.7 mm, respectively. FBG sensors could examine the process zones ahead of the crack tip with the cubic strain distribution along the grating at the crack tip-FBG distance of 0.5 mm. The spectrum oscillation occurs as the crack approaches the FBG where the highly heterogeneous strain is distributed. Another idea is to use a finite element method (FEM), together with a T-matrix method, to analyze the reflection intensity spectra of FBG sensors for various crack sizes. The described crack propagation detection system may apply in structural health monitoring. PMID:28772949

Investigation on Characteristic Variation of the FBG Spectrum with Crack Propagation in Aluminum Plate Structures.

PubMed

Jin, Bo; Zhang, Weifang; Zhang, Meng; Ren, Feifei; Dai, Wei; Wang, Yanrong

2017-05-27

In order to monitor the crack tip propagation of aluminum alloy, this study investigates the variation of the spectrum characteristics of a fiber Bragg grating (FBG), combined with an analysis of the spectrum simulation. The results identify the location of the subordinate peak as significantly associated with the strain distribution along the grating, corresponding to the different plastic zones ahead of the crack tip with various crack lengths. FBG sensors could observe monotonic and cyclic plastic zones ahead of the crack tip, with the quadratic strain distribution along the grating at the crack tip-FBG distance of 1.2 and 0.7 mm, respectively. FBG sensors could examine the process zones ahead of the crack tip with the cubic strain distribution along the grating at the crack tip-FBG distance of 0.5 mm. The spectrum oscillation occurs as the crack approaches the FBG where the highly heterogeneous strain is distributed. Another idea is to use a finite element method (FEM), together with a T -matrix method, to analyze the reflection intensity spectra of FBG sensors for various crack sizes. The described crack propagation detection system may apply in structural health monitoring.

Biaxial fatigue crack propagation behavior of perfluorosulfonic-acid membranes

NASA Astrophysics Data System (ADS)

Lin, Qiang; Shi, Shouwen; Wang, Lei; Chen, Xu; Chen, Gang

2018-04-01

Perfluorosulfonic-acid membranes have long been used as the typical electrolyte for polymer-electrolyte fuel cells, which not only transport proton and water but also serve as barriers to prevent reactants mixing. However, too often the structural integrity of perfluorosulfonic-acid membranes is impaired by membrane thinning or cracks/pinholes formation induced by mechanical and chemical degradations. Despite the increasing number of studies that report crack formation, such as crack size and shape, the underlying mechanism and driving forces have not been well explored. In this paper, the fatigue crack propagation behaviors of Nafion membranes subjected to biaxial loading conditions have been investigated. In particular, the fatigue crack growth rates of flat cracks in responses to different loading conditions are compared, and the impact of transverse stress on fatigue crack growth rate is clarified. In addition, the crack paths for slant cracks under both uniaxial and biaxial loading conditions are discussed, which are similar in geometry to those found after accelerated stress testing of fuel cells. The directions of initial crack propagation are calculated theoretically and compared with experimental observations, which are in good agreement. The findings reported here lays the foundation for understanding of mechanical failure of membranes.

Irwin's conjecture: Crack shape adaptability in transversely isotropic solids

NASA Astrophysics Data System (ADS)

Laubie, Hadrien; Ulm, Franz-Josef

2014-08-01

The planar crack propagation problem of a flat elliptical crack embedded in a brittle elastic anisotropic solid is investigated. We introduce the concept of crack shape adaptability: the ability of three-dimensional planar cracks to shape with the mechanical properties of a cracked body. A criterion based on the principle of maximum dissipation is suggested in order to determine the most stable elliptical shape. This criterion is applied to the specific case of vertical cracks in transversely isotropic solids. It is shown that contrary to the isotropic case, the circular shape (i.e. penny-shaped cracks) is not the most stable one. Upon propagation, the crack first grows non-self-similarly before it reaches a stable shape. This stable shape can be approximated by an ellipse of an aspect ratio that varies with the degree of elastic anisotropy. By way of example, we apply the so-derived crack shape adaptability criterion to shale materials. For this class of materials it is shown that once the stable shape is reached, the crack propagates at a higher rate in the horizontal direction than in the vertical direction. We also comment on the possible implications of these findings for hydraulic fracturing operations.

Effect of initial contact surface condition on the friction and wear properties of bearing steel in cyclic reciprocating sliding contact

NASA Astrophysics Data System (ADS)

Tanaka, Y.; Endo, M.; Moriyama, S.

2017-05-01

Delamination failure is one of the most important engineering problems. This failure can frequently be detrimental to rolling contact machine elements such as bearings, gear wheels, etc. This phenomenon, called rolling contact fatigue, has a close relationship not only with opening-mode but also with shear-mode fatigue crack growth. The crack face interference is known to significantly affect the shear-mode fatigue crack propagation and its threshold behavior. Quantitative investigation on friction and wear at fatigue crack faces in the material is essentially impossible. Previously, thus, a novel ring-on-ring test by making use of fatigue testing machine was proposed to simulate a cyclic reciprocating sliding contact of crack surfaces. However, this test procedure had some problems. For instance, in order to achieve the uniform contact at the start of test, the rubbing of specimens must be conducted in advance. By this treatment, the specimen surfaces were already damaged before the test. In this study, an improvement of experimental method was made to perform the test using the damage-free specimens. The friction and wear properties for heat-treated high carbon-chromium bearing steel were investigated with this new method and the results were compared to the results obtained by using the initially damaged specimens.

Effects of Changing Stress Amplitude on the Rate of Fatigue-Crack Propagation in Two Aluminum Alloys

NASA Technical Reports Server (NTRS)

Hudson, C. Michael; Hardrath, Herbert F.

1961-01-01

A series of fatigue tests with specimens subjected to constant amplitude and two-step axial loads were conducted on 12-inch-wide sheet specimens of 2024-T3 and 7075-T6 aluminum alloy to study the effects of a change in stress level on fatigue-crack propagation. Comparison of the results of the tests in which the specimens were tested at first a high and then a low stress level with those of the constant-stress- amplitude tests indicated that crack propagation was generally delayed after the transition to the lower stress level. In the tests in which the specimens were tested at first a low and then a high stress level, crack propagation continued at the expected rate after the change in stress levels.

Environment enhanced fatigue crack propagation in metals: Inputs to fracture mechanics life prediction models

NASA Technical Reports Server (NTRS)

Gangloff, Richard P.; Kim, Sang-Shik

1993-01-01

This report is a critical review of both environment-enhanced fatigue crack propagation data and the predictive capabilities of crack growth rate models. This information provides the necessary foundation for incorporating environmental effects in NASA FLAGRO and will better enable predictions of aerospace component fatigue lives. The review presents extensive literature data on 'stress corrosion cracking and corrosion fatigue.' The linear elastic fracture mechanics approach, based on stress intensity range (Delta(K)) similitude with microscopic crack propagation threshold and growth rates, provides a basis for these data. Results are presented showing enhanced growth rates for gases (viz., H2 and H2O) and electrolytes (e.g. NaCl and H2O) in aerospace alloys including: C-Mn and heat treated alloy steels, aluminum alloys, nickel-based superalloys, and titanium alloys. Environment causes purely time-dependent accelerated fatigue crack growth above the monotonic load cracking threshold (KIEAC) and promotes cycle-time dependent cracking below (KIEAC). These phenomenon are discussed in terms of hydrogen embrittlement, dissolution, and film rupture crack tip damage mechanisms.

Application of Viscoelastic Fracture Model and Non-uniform Crack Initiation at Clinically Relevant Notches in Crosslinked UHMWPE

PubMed Central

Sirimamilla, P. Abhiram; Furmanski, Jevan; Rimnac, Clare M.

2012-01-01

The mechanism of crack initiation from a clinically relevant notch is not well-understood for crosslinked ultra high molecular weight polyethylene (UHMWPE) used in total joint replacement components. Static mode driving forces, rather than the cyclic mode conditions typically associated with fatigue processes, have been shown to drive crack propagation in this material. Thus, in this study, crack initiation in a notched specimen under a static load was investigated. A video microscope was used to monitor the notch surface of the specimen and crack initiation time was measured from the video by identifying the onset of crack initiation at the notch. Crack initiation was considered using a viscoelastic fracture theory. It was found that the mechanism of crack initiation involved both single layer and a distributed multi-layer phenomenon and that multi-layer crack initiation delayed the crack initiation time for all loading conditions examined. The findings of this study support that the viscoelastic fracture theory governs fracture mechanics in crosslinked UHMWPE. The findings also support that crack initiation from a notch in UHMWPE is a more complex phenomenon than treated by traditional fracture theories for polymers. PMID:23127638

Changes in bone microstructure and toughness during the healing process of long bones

NASA Astrophysics Data System (ADS)

Ishimoto, T.; Nakano, T.; Umakoshi, Y.; Tabata, Y.

2009-05-01

It is of great importance to understand how bone defects regain the microstructure and mechanical function of bone and how the microstructure affects the mechanical function during the bone healing process. In the present study on long bone defects, we investigated the relationship between the recovery process of fracture toughness and biological apatite (BAp)/collagen (Col) alignment as an index of the bone microstructure to clarify the bone toughening mechanisms. A 5-mm defect introduced in the rabbit ulna was allowed to heal naturally and a three-point bending test was conducted on the regenerated site to assess bone toughness. The bone toughness was quite low at the early stage of bone regeneration but increased during the postoperative period. The change in toughness agreed well with the characteristics of the fracture surface morphology, which reflected the history of the crack propagation. SEM and microbeam X-ray diffraction analyses indicated that the toughness was dominated by the degree and orientation of the preferred BAp/Col alignment, i.e. bundles aligned perpendicular to the crack propagation clearly contributed to the bone toughening owing to extra energy consumption for resistance to crack propagation. In conclusion, regenerated bone improves fracture toughness by reconstructing the preferred BAp/Col alignment along the bone longitudinal axis during the healing process of long bones.

Fatigue testing of a NiTi rotary instrument. Part 2: Fractographic analysis.

PubMed

Cheung, G S P; Darvell, B W

2007-08-01

To examine the topographic features of the fracture surface of a NiTi instrument after fatigue failure, and to correlate the measurements of some features with the cyclic load. A total of 212 ProFile rotary instruments were subjected to a rotational-bending test at various curvatures until broken. The fracture surface of all fragments was examined by SEM to identify the crack origins. The crack radius, i.e. extent of the fatigue-crack growth towards the centroid of the cross-section, was also measured, and correlated with the strain amplitude for each instrument. All fracture surfaces revealed the presence of one or more crack origins, a region occupied by microscopic striations, and an area with microscopic dimples. The number of specimens showing multiple crack origins was significantly greater in the group fatigued under water than in air (P < 0.05). A linear relationship between the reciprocal of the square root of the crack radius and the strain amplitude was discernible (P < 0.001), the slopes of which were not significantly different for instruments fatigued in air and water. The fractographic appearance of NiTi engine-files that had failed because of fatigue is typical of that for other metals. The fatigue behaviour of NiTi instruments is adversely affected by water, not only for the low-cycle fatigue life, but also the number of crack origins. There appears to be a critical extent of crack propagation for various strain amplitudes leading to final rupture (akin to the Griffith's criterion for brittle materials).

Thermal fatigue behaviour for a 316 L type steel

NASA Astrophysics Data System (ADS)

Fissolo, A.; Marini, B.; Nais, G.; Wident, P.

1996-10-01

This paper deals with initiation and growth of cracks produced by thermal fatigue loadings on 316 L steel, which is a reference material for the first wall of the next fusion reactor ITER. Two types of facilities have been built. As for true components, thermal cycles have been repeatedly applied on the surface of the specimen. The first is mainly concerned with initiation, which is detected with a light microscope. The second allows one to determine the propagation of a single crack. Crack initiation is analyzed using the French RCC-MR code procedure, and the strain-controlled isothermal fatigue curves. To predict crack growth, a model previously proposed by Haigh and Skelton is applied. This is based on determination of effective stress intensity factors, which takes into account both plastic strain and crack closure phenomena. It is shown that estimations obtained with such methodologies are in good agreement with experimental data.

Fracture study of windshield glass panes

NASA Technical Reports Server (NTRS)

Yeh, H. Y.

1987-01-01

The major stresses which cause crack propagation in windshield glass panes are induced by bending moments which result from the pressure differentials across the panes. Hence the stress intensity factors for the finite plate with the semi-elliptical surface flaw and edge crack under the bending moments are examined. The results show that the crack growth will be upperbound if it is computed by using the stress intensity factor for the finite plate with the edge crack subjected to pure bending moments. Furthermore, if the ratio of crack depth to plate thickness, a/t, is within 0.3, the stress intensity factor can be conservatively assumed to be constant at the value of a/t equal to zero. A simplified equation to predict the structural life of glass panes is derived based on constant stress intensity factor. The accuracy of structural life is mainly dependent on how close the empirical parameter, m, can be estimated.

Flaw growth of 7075, 7475, 7050 and 7049 aluminum alloy plate in stress corrosion environments: 4-year marine atmosphere results

NASA Technical Reports Server (NTRS)

Hasse, K. R.; Dorward, R. C.

1981-01-01

After nearly 53 months of exposure to marine atmosphere, crack growth in SL DCB specimens from 7075, 7475, 7050, and 7049-T7X plate has slowed to the arbitrary 10 to the -10 power m/sec used to define threshold stress intensity. Because some specimens appear to be approaching crack arrest, the importance of self-loading from corrosion product wedging as a significant driving force for crack propagation in overaged materials is questioned. Crack length-time data were analyzed using a computer curve fitting program which minimized the effects of normal data scatter, and provided a clearer picture of material performance. Precracked specimen data are supported by the results of smooth specimen tests. Transgranular stress corrosion cracking was observed in TL DCB specimens from all four alloys. This process is extremely slow and is characterized by a striated surface morphology.

Influence of microstructure on the fretting resistance of Al-Cu-Li alloys

NASA Astrophysics Data System (ADS)

Delacroix, Jessica; Cazottes, Sophie; Daniélou, Armelle; Fouvry, Siegfried; Buffiere, Jean-Yves

The resistance of two Al-Cu-Li alloys (2050 and 2196) to fretting has been investigated. For each material two heat treatments have been studied (T8 and low temperature ageing). Fretting tests with a cylinder-plane configuration have been performed in the partial slip regime. The results obtained show that the low temperature temper gives a better resistance to fretting crack initiation and propagation than the T8 temper for both alloys. The 3D shape of the fretting cracks has been observed by high resolution synchrotron X-ray tomography. Multiple initiation sites were observed below the contact. In their early stages of development, the fretting cracks grow approximately radially within the material leading to thumb nail cracks which eventually merge laterally. The difference in fretting resistance is analysed with respect to the 3D fracture surface of the fretting cracks in relation with the alloys precipitation state.

Estimation of Crack Initiation and Propagation Thresholds of Confined Brittle Coal Specimens Based on Energy Dissipation Theory

NASA Astrophysics Data System (ADS)

Ning, Jianguo; Wang, Jun; Jiang, Jinquan; Hu, Shanchao; Jiang, Lishuai; Liu, Xuesheng

2018-01-01

A new energy-dissipation method to identify crack initiation and propagation thresholds is introduced. Conventional and cyclic loading-unloading triaxial compression tests and acoustic emission experiments were performed for coal specimens from a 980-m deep mine with different confining pressures of 10, 15, 20, 25, 30, and 35 MPa. Stress-strain relations, acoustic emission patterns, and energy evolution characteristics obtained during the triaxial compression tests were analyzed. The majority of the input energy stored in the coal specimens took the form of elastic strain energy. After the elastic-deformation stage, part of the input energy was consumed by stable crack propagation. However, with an increase in stress levels, unstable crack propagation commenced, and the energy dissipation and coal damage were accelerated. The variation in the pre-peak energy-dissipation ratio was consistent with the coal damage. This new method demonstrates that the crack initiation threshold was proportional to the peak stress ( σ p) for ratios that ranged from 0.4351 to 0.4753 σ p, and the crack damage threshold ranged from 0.8087 to 0.8677 σ p.

Evaluating the irradiation effects on the elastic properties of miniature monolithic SiC tubular specimens

NASA Astrophysics Data System (ADS)

Singh, G.; Koyanagi, T.; Petrie, C.; Terrani, K.; Katoh, Y.

2018-02-01

The initial results of a post-irradiation examination study conducted on CVD SiC tubular specimens irradiated under a high radial heat flux are presented herein. The elastic moduli were found to decrease more than that estimated based on previous studies. The significant decreases in modulus are attributed to the cracks present in the specimens. The stresses in the specimens, calculated through finite element analyses, were found to be greater than the expected strength of irradiated specimens, indicating that the irradiation-induced stresses caused these cracks. The optical microscopy images and predicted stress distributions indicate that the cracks initiated at the inner surface and propagated outward.

Fabrication and testing of prestressed composite rotor blade spar specimens

NASA Technical Reports Server (NTRS)

Gleich, D.

1974-01-01

Prestressed composite spar specimens were fabricated and evaluated by crack propagation and ballistic penetration tests. The crack propagation tests on flawed specimens showed that the prestressed composite spar construction significantly suppresses crack growth. Damage from three high velocity 30 caliber projectile hits was confined to three small holes in the ballistic test specimen. No fragmentation or crack propagation was observed indicating good ballistic damage resistance. Rotor attachment approaches and improved structural performance configurations were identified. Design theory was verified by tests. The prestressed composite spar configuration consisted of a compressively prestressed high strength ARDEFORM 301 stainless steel liner overwrapped with pretensioned S-994 fiberglass.

Mechanical behaviour of metallic thin films on polymeric substrates and the effect of ion beam assistance on crack propagation

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

George, M.; Coupeau, C.; Colin, J.

2005-01-10

The mechanisms of crack propagation in metallic films on polymeric substrates have been studied through in situ atomic force microscopy observations of thin films under tensile stresses and finite element stress calculations. Two series of films - ones deposited with ion beam assistance, the others without - have been investigated. The observations and stress calculations show that ion beam assistance can change drastically the propagation of cracks in coated materials: by improving the adhesion film/substrate, it slows down the delamination process, but in the same time enhances the cracks growth in the thickness of the material.

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.

Oxidation- and Creep-Enhanced Fatigue of Haynes 188 Alloy-Oxide Scale System Under Simulated Pulse Detonation Engine Conditions

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Fox, Dennis S.; Miller, Robert A.

2002-01-01

The development of the pulse detonation engine (PDE) requires robust design of the engine components that are capable of enduring harsh detonation environments. In this study, a high cycle thermal fatigue test rig was developed for evaluating candidate PDE combustor materials using a CO2 laser. The high cycle thermal fatigue behavior of Haynes 188 alloy was investigated under an enhanced pulsed laser test condition of 30 Hz cycle frequency (33 ms pulse period, and 10 ms pulse width including 0.2 ms pulse spike). The temperature swings generated by the laser pulses near the specimen surface were characterized by using one-dimensional finite difference modeling combined with experimental measurements. The temperature swings resulted in significant thermal cyclic stresses in the oxide scale/alloy system, and induced extensive surface cracking. Striations of various sizes were observed at the cracked surfaces and oxide/alloy interfaces under the cyclic stresses. The test results indicated that oxidation and creep-enhanced fatigue at the oxide scale/alloy interface was an important mechanism for the surface crack initiation and propagation under the simulated PDE condition.

Scattering of Lamb waves by cracks in a composite graphite fiber-reinforced epoxy plate

NASA Technical Reports Server (NTRS)

Bratton, Robert; Datta, Subhendu K.; Shah, Arvind

1990-01-01

Recent investigations of space construction techniques have explored the used of composite materials in the construction of space stations and platforms. These composites offer superior strength to weight ratio and are thermally stable. For example, a composite material being considered is laminates of graphite fibers in an epoxy matrix. The overall effective elastic constants of such a medium can be calculated from fiber and matrix properties by using an effective modulus theory as shown in Datta, el. al. The investigation of propagation and scattering of elastic waves in composite materials is necessary in order to develop an ability to characterize cracks and predict the reliability of composite structures. The objective of this investigation is the characterization of a surface breaking crack by ultrasonic techniques. In particular, the use of Lamb waves for this purpose is studied here. The Lamb waves travel through the plate, encountering a crack, and scatter. Of interest is the modeling of the scattered wave in terms of the Lamb wave modes. The direct problem of propagation and scattering of Lamb waves by a surface breaking crack has been analyzed. This would permit an experimentalist to characterize the crack by comparing the measured response to the analytical model. The plate is assumed to be infinite in the x and y directions with a constant thickness in the z direction. The top and bottom surfaces are traction free. Solving the governing wave equations and using the stress-free boundary conditions results in the dispersion equation. This equation yields the guided modes in the homogeneous plate. The theoretical model is a hybrid method that combines analytical and finite elements techniques to describe the scattered displacements. A finite region containing the defects is discretized by finite elements. Outside the local region, the far field solution is expressed as a Fourier summation of the guided modes obtained from the dispersion equation. Continuity of tractions and displacements at the boundaries of the two regions provides the necessary equations to determine the expansion coefficients and the nodal displacements. In the hybrid method used here these defects can be of arbitrary shapes as well as inclusions of different materials.

Fatigue behavior and encrustation characteristics of nanocrystalline metals

NASA Astrophysics Data System (ADS)

Lai, Li-Chung

The nanocrstalline (NC) metals have been reported to have high mechanical performance owing to it's small grain interior and a large volume fraction of grain boundary (GB) atoms. Small grain leads to the forbidden dislocation activities in grain interior while GB activities become dominant due to a higher volume fraction of GB atoms. Regarding the fatigue response to nanocrstalline metals, it has been reported that decreasing grain led to both significantly improvement on the fatigue-endurance limit and deleterious effect on the resistance to subcritical fatigue crack propagation. The increases endurance limit has been attributed to the greater resistance to fatigue crack initiation at near-surface regions. On the other hand, the less resistance to fatigue crack growth were resulted from less tortuous fatigue crack profiles supported by the deflection/closure theory. However, it has never been studied the influence of proceeding and pre-existing defects on the fatigue performance considering the difference response of NC structure from than coarse grain (CG) structure. In the present work, the influence of electrical discharge machining (EDM) and surface defects on the fatigue behavior of both conventional cold-rolled CG and electro-deposited (ED) NC Ni were investigated. The experimental results revealed considerable influence by EDM on the fatigue strength of NC Ni, while it has little or no affect on that for CG Ni. Specifically, EDM led to a 50 to 75% reduction in fatigue strength for NC Ni despite a relatively small depth of EDM affected material (˜ 1% of width). Rationale for this effect can be attributed to grain growth, microcracks, and a higher sulfur content at the GBs in the EDM affected zone. In addition, the pre-existing surface defects that appear to be due to impurity segregation near the electro-deposition substrate significantly reduced the fatigue resistance of ED NC Ni. In order to understand the fatigued behavior in NC Ni, crack tip grain structures were investigated using transmission electron microscope (TEM). Crack tip grain growth was observed at early state of crack propagation with low stress intensity factor (K ˜ 6 MPa m 1/2). As K increased, the size of grain growth zone increased exponentially in width and crack propagation behavior transmitted from interganular to transgranular. It appears that this transmission is associated with grain growth. The coalesced grains due to grain rotation/GB diffusion created larger paths for more extended dislocation movement. Dislocation activities become less forbidden and the dislocation-slip mechanism can be dominant leading to a more plastically transgranular fracture. In addition to fatigue study of ED NC Ni, encrustation on ED NC Ti was investigated. The use of materials for medical applications in the urinary tract is hampered by the formation of calcium-based crystalline deposits, generally referred to as encrustation, that act as precursors to urinary stones. Anecdotal evidence suggests that titanium can possess encrustation-resistant properties in vivo and may be useful in urologic applications. To test the utility of coating surfaces with nanostructured titanium, several forms of materials were submersed in artificial urine with saturating concentrations of calcium for a period of 14 days. The specimens were then analyzed using scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) to determine the extent of encrustation on the surface of the various samples. Our observations indicate that nanostructured titanium offers superior resistance to encrustation when compared to polyurethane, polyvinyl chloride or conventional coarser grained titanium. Further studies investigating the use of nanostructured titanium in urologic applications are warranted.

Development of an Improved Crack Propagation Model for Corrosion-Induced Cover Cracking in RC Structures

NASA Astrophysics Data System (ADS)

Hilyati, S.; Nizam, Z. M.; Zurisman, M. A. A.; Azhar, A. T. S.

2017-06-01

During the last two decades, reinforced concrete (RC) has been extensively used in most of the world as one of the common construction material due to its advantages and durability. However, RC structures exposed to marine environments are subjected to chloride attack. Chlorides from seawater penetrate into RC structures are not only causing severe corrosion problems but also affect the durability and serviceability of such structures. This paper investigates the influence of transverse reinforcement and spacing of reinforcing bars on concrete cover cracking of two-way RC slab specimens using accelerated corrosion tests. The experimental program involved the testing of four RC slab specimens and was generally designed to observe the crack width and the time of crack to propagate. An improved model for predicting the timing of crack propagation based on the experimental data was then developed.

Micromechanical predictions of crack propagation and fracture energy in a single fiber boron/aluminum model composite

NASA Technical Reports Server (NTRS)

Adams, D. F.; Mahishi, J. M.

1982-01-01

The axisymmetric finite element model and associated computer program developed for the analysis of crack propagation in a composite consisting of a single broken fiber in an annular sheath of matrix material was extended to include a constant displacement boundary condition during an increment of crack propagation. The constant displacement condition permits the growth of a stable crack, as opposed to the catastropic failure in an earlier version. The finite element model was refined to respond more accurately to the high stresses and steep stress gradients near the broken fiber end. The accuracy and effectiveness of the conventional constant strain axisymmetric element for crack problems was established by solving the classical problem of a penny-shaped crack in a thick cylindrical rod under axial tension. The stress intensity factors predicted by the present finite element model are compared with existing continuum results.

Environmental fatigue of an Al-Li-Cu alloy. Part 1: Intrinsic crack propagation kinetics in hydrogenous environments

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1991-01-01

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant delta K and K(sub max) loading. The da/dN are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 percent NaCl with anodic polarization, pure water vapor, moist air, and NaCl with cathodic polarization. While da/dN depends on delta K(sup 4.0) for the inert gases, water vapor and chloride induced multiple power-laws, and a transition growth rate 'plateau'. Environmental effects are strongest at low delta K. Crack tip damage is ascribed to hydrogen embrittlement because of the following: (1) accelerated da/dN due to part-per-million levels of H2O without condensation; (2) impeded molecular flow model predictions of the measured water vapor pressure dependence of da/dN as affected by mean crack opening; (3) the lack of an effect of film-forming O2; (4) the likelihood for crack tip hydrogen production in NaCl, and (5) the environmental and delta K-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3, and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.

Ultrasonic vibration double scratch morphology and scratching force of BK7 glass

NASA Astrophysics Data System (ADS)

Wang, Chu; Wang, Hongxiang; Liu, Junliang; Gao, Shi

2018-03-01

In this paper, the damage morphology and scratching force of BK7 glass components were analyzed by ultrasonic vibration double-scratch test. The results showed that there was surface damage caused by plastic flow and brittle fracture during the scratching process, and the scratching depth and the distance between the two scratches had effect on the propagation and overlapping of lateral cracks. When the scratching depth was small, the jagged scratch was produced on the surface, and accompanied by a small amount of tiny pieces of debris off. With the increase in scratching depth, the lateral cracks caused by scratching overlapped and expended to form a mesh sheet, and then fell off from the surface. When the scratching distance was small, the interaction of the cracks caused large slice of material to fall off. With the increase in scratching distance, the area between the two scratches was not easy to occur the overlapping of the lateral cracks. In addition, with the increase of the scratching depth, the scratching force showed a gradual increase trend, and the scratching force of the second scratch would increase with the scratching distance.

Failure analysis of an aluminum alloy material framework component induced by casting defects

NASA Astrophysics Data System (ADS)

Li, Bo; Hu, Weiye

2017-09-01

Failure analysis on a fractured radome framework component was carried out through visual observations, metallographic examination using optical microscope, fractog-raphy inspections using scanning electron microscope and chemical composition analysis. The failed frame was made of casting Al-Si7-Mg0.4 aluminum alloy. It had suffered a former vi-bration performance tests. It was indicated that the fractures were attributed to fatigue cracks which were induced by casting porosities at the outer surfaces of frame. Failure analysis was carefully conducted for the semi-penetrating crack appearing on the framework. According to the fractography inspected by scanning electron microscope, it was indicated that numerous casting porosities at the outer surface of the framework played the role of multiple fracture sources due to some applied stresses. Optical microstructure observations suggested that the dendrite-shaped casting porosities largely contributed to the crack-initiation. The groove-shaped structure at roots of spatial convex-bodies on the edge of casting porosities supplied the preferred paths of the crack-propagation. Besides, the brittle silicon eutectic particles distrib-uting along grain boundaries induced the intergranular fracture mode in the region of the over-load final fracture surface.

Effect of a Diffusion Zone on Fatigue Crack Propagation in Layered FGMs

NASA Astrophysics Data System (ADS)

Hauber, Brett; Brockman, Robert; Paulino, Glaucio

2008-02-01

Research into functionally graded materials (FGMs) has led to advances in our ability to analyze cracks. However, two prominent aspects remain relatively unexplored: 1) development and validation of modeling methods for fatigue crack propagation in FGMs, and 2) experimental validation of stress intensity models in engineered materials such as two phase monolithic and graded materials. This work addresses some of these problems for a limited set of conditions, material systems (e.g., Ti/TiB), and material gradients. Numerical analyses are conducted for single edge notch bend (SENB) specimens. Stress intensity factors are computed using the specialized finite element code I-Franc (Illinois Fracture Analysis Code), which is tailored for both homogeneous and graded materials, as well as Franc2DL and ABAQUS. Crack extension is considered by means of specified crack increments, together with fatigue evaluations to predict crack propagation life. Results will be used to determine linear material gradient parameters that are significant for prediction of fatigue crack growth behavior.

The release of trapped gases from amorphous solid water films. I. "Top-down" crystallization-induced crack propagation probed using the molecular volcano.

PubMed

May, R Alan; Smith, R Scott; Kay, Bruce D

2013-03-14

In this (Paper I) and the companion paper (Paper II; R. May, R. Smith, and B. Kay, J. Chem. Phys. 138, 104502 (2013)), we investigate the mechanisms for the release of trapped gases from underneath amorphous solid water (ASW) films. In prior work, we reported the episodic release of trapped gases in concert with the crystallization of ASW, a phenomenon that we termed the "molecular volcano." The observed abrupt desorption is due to the formation of cracks that span the film to form a connected pathway for release. In this paper, we utilize the "molecular volcano" desorption peak to characterize the formation of crystallization-induced cracks. We find that the crack length distribution is independent of the trapped gas (Ar, Kr, Xe, CH4, N2, O2, or CO). Selective placement of the inert gas layer is used to show that cracks form near the top of the film and propagate downward into the film. Isothermal experiments reveal that, after some induction time, cracks propagate linearly in time with an Arrhenius dependent velocity corresponding to an activation energy of 54 kJ∕mol. This value is consistent with the crystallization growth rates reported by others and establishes a direct connection between crystallization growth rate and the crack propagation rate. A two-step model in which nucleation and crystallization occurs in an induction zone near the top of the film followed by the propagation of a crystallization∕crack front into the film is in good agreement with the temperature programmed desorption results.

The Release of Trapped Gases from Amorphous Solid Water Films: I. “Top-Down” Crystallization-Induced Crack Propagation Probed using the Molecular Volcano

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

May, Robert A.; Smith, R. Scott; Kay, Bruce D.

In this (Paper I) and the companion paper (Paper II) we investigate the mechanisms for the release of trapped gases from underneath of amorphous solid water (ASW) films. In prior work, we reported the episodic release of trapped gases in concert with the crystallization ASW, a phenomenon that we termed the "molecular volcano". The observed abrupt desorption is due to the formation of cracks that span the film to form a connected pathway for release. In this paper we utilize the "molecular volcano" desorption peak to characterize the formation of crystallization-induced cracks. We find that the crack length and distributionmore » are independent of the trapped gas (Ar, Kr, Xe, CH4, N2, O2 or CO). Selective placement of the inert gas layer is used to show that cracks form near the top of the film and propagate downward into the film. Isothermal experiments reveal that, after some induction time, cracks propagate linearly in time with an Arrhenius dependent velocity corresponding to an activation energy of 54 kJ/mol. This value is consistent with the crystallization growth rate reported by others and establishes a direct connection between crystallization growth rate and the crack propagation rate. A two-step model in which nucleation and crystallization occurs in an induction zone near the top of the film followed by the propagation of a crystallization/crack front into the film is in good agreement with the temperature programmed desorption results.« less

Cracks growth behaviors of commercial pure titanium under nanosecond laser irradiation for formation of nanostructure-covered microstructures (with sub-5-μm)

NASA Astrophysics Data System (ADS)

Pan, A. F.; Wang, W. J.; Mei, X. S.; Zheng, B. X.; Yan, Z. X.

2016-11-01

This study reported on the formation of sub-5-μm microstructures covered on titanium by cracks growth under 10-ns laser radiation at the wavelength of 532 nm and its induced light modification for production of nanostructures. The electric field intensity and laser power density absorbed by commercial pure titanium were computed to investigate the self-trapping introduced by cracks and the effect of surface morphology on laser propagation characteristics. It is found that nanostructures can form at the surface with the curvature radius below 20 μm. Meanwhile, variable laser fluences were applied to explore the evolution of cracks on commercial pure titanium with or without melt as spot overlap number increased. Experimental study was first performed at the peak laser fluence of 1.063 J/cm2 to investigate the microstructures induced only by cracks growth. The results demonstrated that angular microstructures with size between 1.68 μm and 4.74 μm was obtained and no nanostructure covered. Then, at the peak laser fluence of 2.126 J/cm2, there were some nanostructures covered on the melt-induced curved microstructured surface. However, surface molten material submerged in the most of cracks at the spot overlap number of 744, where the old cracks disappeared. The results indicated that there was too much molten material and melting time at the peak laser fluence of 2.126 J/cm2, which was not suitable for obtainment of perfect micro-nano structures. On this basis, peak laser fluence was reduced down to 1.595 J/cm2 and the sharp sub-5 μm microstructures with nanostructures covered was obtained at spot overlap number of 3720.

Fracture mechanics criteria for turbine engine hot section components

NASA Technical Reports Server (NTRS)

Meyers, G. J.

1982-01-01

The application of several fracture mechanics data correlation parameters to predicting the crack propagation life of turbine engine hot section components was evaluated. An engine survey was conducted to determine the locations where conventional fracture mechanics approaches may not be adequate to characterize cracking behavior. Both linear and nonlinear fracture mechanics analyses of a cracked annular combustor liner configuration were performed. Isothermal and variable temperature crack propagation tests were performed on Hastelloy X combustor liner material. The crack growth data was reduced using the stress intensity factor, the strain intensity factor, the J integral, crack opening displacement, and Tomkins' model. The parameter which showed the most effectiveness in correlation high temperature and variable temperature Hastelloy X crack growth data was crack opening displacement.

Crack tip field and fatigue crack growth in general yielding and low cycle fatigue

NASA Technical Reports Server (NTRS)

Minzhong, Z.; Liu, H. W.

1984-01-01

Fatigue life consists of crack nucleation and crack propagation periods. Fatigue crack nucleation period is shorter relative to the propagation period at higher stresses. Crack nucleation period of low cycle fatigue might even be shortened by material and fabrication defects and by environmental attack. In these cases, fatigue life is largely crack propagation period. The characteristic crack tip field was studied by the finite element method, and the crack tip field is related to the far field parameters: the deformation work density, and the product of applied stress and applied strain. The cyclic carck growth rates in specimens in general yielding as measured by Solomon are analyzed in terms of J-integral. A generalized crack behavior in terms of delta is developed. The relations between J and the far field parameters and the relation for the general cyclic crack growth behavior are used to analyze fatigue lives of specimens under general-yielding cyclic-load. Fatigue life is related to the applied stress and strain ranges, the deformation work density, crack nucleus size, fracture toughness, fatigue crack growth threshold, Young's modulus, and the cyclic yield stress and strain. The fatigue lives of two aluminum alloys correlate well with the deformation work density as depicted by the derived theory. The general relation is reduced to Coffin-Manson low cycle fatigue law in the high strain region.

Dynamic crack propagation in a 2D elastic body: The out-of-plane case

NASA Astrophysics Data System (ADS)

Nicaise, Serge; Sandig, Anna-Margarete

2007-05-01

Already in 1920 Griffith has formulated an energy balance criterion for quasistatic crack propagation in brittle elastic materials. Nowadays, a generalized energy balance law is used in mechanics [F. Erdogan, Crack propagation theories, in: H. Liebowitz (Ed.), Fracture, vol. 2, Academic Press, New York, 1968, pp. 498-586; L.B. Freund, Dynamic Fracture Mechanics, Cambridge Univ. Press, Cambridge, 1990; D. Gross, Bruchmechanik, Springer-Verlag, Berlin, 1996] in order to predict how a running crack will grow. We discuss this situation in a rigorous mathematical way for the out-of-plane state. This model is described by two coupled equations in the reference configuration: a two-dimensional scalar wave equation for the displacement fields in a cracked bounded domain and an ordinary differential equation for the crack position derived from the energy balance law. We handle both equations separately, assuming at first that the crack position is known. Then the weak and strong solvability of the wave equation will be studied and the crack tip singularities will be derived under the assumption that the crack is straight and moves tangentially. Using the energy balance law and the crack tip behavior of the displacement fields we finally arrive at an ordinary differential equation for the motion of the crack tip.

The Rehbinder effect in iron during giga-cycle fatigue loading

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

Bannikov, M. V., E-mail: mbannikov@icmm.ru; Naimark, O. B.

The influence of the adsorptive strength reduction effect (the Rehbinder effect) on the fatigue life of pure iron under the giga-cycle loading regime was investigated. Specimens were loaded by an ultrasonic testing machine with a frequency of 20 kHz in air and in contact with eutectic alloy of gallium with tin and indium. A significant (by several orders of magnitude) worsening of the life-time of iron in contact with a molten metal as compared with tests in air was established. The liquid metal penetrates into the material to a depth of 200 μm to the center of a fatigue crack. Themore » mechanism of the fatigue crack initiation in the giga-cycle regime of loading in contact with a surfactant is differing: the crack is formed on the surface of the specimen rather than within it as is the case for air. Based on the electron and optical microscopy data for the fracture surface, it can be concluded that exactly the change in the crack initiation mechanism reduces the fatigue life of iron in contact with a liquid metal because the initiated crack propagates regardless of the surfactant.« less

Microstructural Features Controlling the Variability in Low-Cycle Fatigue Properties of Alloy Inconel 718DA at Intermediate Temperature

NASA Astrophysics Data System (ADS)

Texier, Damien; Gómez, Ana Casanova; Pierret, Stéphane; Franchet, Jean-Michel; Pollock, Tresa M.; Villechaise, Patrick; Cormier, Jonathan

2016-03-01

The low-cycle fatigue behavior of two direct-aged versions of the nickel-based superalloy Inconel 718 (IN718DA) was examined in the low-strain amplitude regime at intermediate temperature. High variability in fatigue life was observed, and abnormally short lifetimes were systematically observed to be due to crack initiation at (sub)-surface non-metallic inclusions. However, crack initiation within (sub)-surface non-metallic inclusions did not necessarily lead to short fatigue life. The macro- to micro-mechanical mechanisms of deformation and damage have been examined by means of detailed microstructural characterization, tensile and fatigue mechanical tests, and in situ tensile testing. The initial stages of crack micro-propagation from cracked non-metallic particles into the surrounding metallic matrix occupies a large fraction of the fatigue life and requires extensive local plastic straining in the matrix adjacent to the cracked inclusions. Differences in microstructure that influence local plastic straining, i.e., the δ-phase content and the grain size, coupled with the presence of non-metallic inclusions at the high end of the size distribution contribute strongly to the fatigue life variability.

The Rehbinder effect in iron during giga-cycle fatigue loading

NASA Astrophysics Data System (ADS)

Bannikov, M. V.; Naimark, O. B.

2015-10-01

The influence of the adsorptive strength reduction effect (the Rehbinder effect) on the fatigue life of pure iron under the giga-cycle loading regime was investigated. Specimens were loaded by an ultrasonic testing machine with a frequency of 20 kHz in air and in contact with eutectic alloy of gallium with tin and indium. A significant (by several orders of magnitude) worsening of the life-time of iron in contact with a molten metal as compared with tests in air was established. The liquid metal penetrates into the material to a depth of 200 μm to the center of a fatigue crack. The mechanism of the fatigue crack initiation in the giga-cycle regime of loading in contact with a surfactant is differing: the crack is formed on the surface of the specimen rather than within it as is the case for air. Based on the electron and optical microscopy data for the fracture surface, it can be concluded that exactly the change in the crack initiation mechanism reduces the fatigue life of iron in contact with a liquid metal because the initiated crack propagates regardless of the surfactant.

The Statistical Nature of Fatigue Crack Propagation

DTIC Science & Technology

1977-03-01

LEVEL x - V AFFDL-TRt-T843 r THE STATISTICAL NATURE OF b FATIGUE CRACK PROPAGATION D. A. VIRKLER B. M. HILLBERR Y LL= P. K. GOEL C* SCHOOL...function of crack length was best represented by the three-parameter log-normal distribution. Six growth rate calculation methods were investigated and the...dN, which varied moderately as a function of crack length, replicate a vs. N data were predicted This predicted data reproduced the mean behavior but

In Situ SEM Observations of Fracture Behavior of Laser Welded-Brazed Al/Steel Dissimilar Joint

NASA Astrophysics Data System (ADS)

Xia, Hongbo; Tan, Caiwang; Li, Liqun; Ma, Ninshu

2018-03-01

Laser welding-brazing of 6061-T6 aluminum alloy to DP590 dual-phase steel with Al-Si12 flux-cored filler wire was performed. The microstructure at the brazing interface was characterized. Fracture behavior was observed and analyzed by in situ scanning electron microscope. The microstructure of the brazing interface showed that inhomogeneous intermetallic compounds formed along the thickness direction, which had a great influence on the crack initiation and propagation. In the top region, the reaction layer at the interface consisted of scattered needle-like Fe(Al,Si)3 and serration-shaped Fe1.8Al7.2Si. In the middle region, the compound at the interface was only serration-shaped Fe1.8Al7.2Si. In the bottom region, the interface was composed of lamellar-shaped Fe1.8Al7.2Si. The cracks were first detected in the bottom region and propagated from bottom to top along the interface. At the bottom region, the crack initiated and propagated along the Fe1.8Al7.2Si/weld seam interface during the in situ tensile test. When the crack propagated into the middle region, a deflection of crack propagation appeared. The crack first propagated along the steel/Fe1.8Al7.2Si interface and then moved along the weld seam until the failure of the joint. The tensile strength of the joint was 146.5 MPa. Some micro-cracks were detected at Fe(Al,Si)3 and the interface between the steel substrate and Fe(Al,Si)3 in the top region while the interface was still connected.

Effect of micromorphology of cortical bone tissue on crack propagation under dynamic loading

NASA Astrophysics Data System (ADS)

Wang, Mayao; Gao, Xing; Abdel-Wahab, Adel; Li, Simin; Zimmermann, Elizabeth A.; Riedel, Christoph; Busse, Björn; Silberschmidt, Vadim V.

2015-09-01

Structural integrity of bone tissue plays an important role in daily activities of humans. However, traumatic incidents such as sports injuries, collisions and falls can cause bone fracture, servere pain and mobility loss. In addition, ageing and degenerative bone diseases such as osteoporosis can increase the risk of fracture [1]. As a composite-like material, a cortical bone tissue is capable of tolerating moderate fracture/cracks without complete failure. The key to this is its heterogeneously distributed microstructural constituents providing both intrinsic and extrinsic toughening mechanisms. At micro-scale level, cortical bone can be considered as a four-phase composite material consisting of osteons, Haversian canals, cement lines and interstitial matrix. These microstructural constituents can directly affect local distributions of stresses and strains, and, hence, crack initiation and propagation. Therefore, understanding the effect of micromorphology of cortical bone on crack initiation and propagation, especially under dynamic loading regimes is of great importance for fracture risk evaluation. In this study, random microstructures of a cortical bone tissue were modelled with finite elements for four groups: healthy (control), young age, osteoporosis and bisphosphonate-treated, based on osteonal morphometric parameters measured from microscopic images for these groups. The developed models were loaded under the same dynamic loading conditions, representing a direct impact incident, resulting in progressive crack propagation. An extended finite-element method (X-FEM) was implemented to realize solution-dependent crack propagation within the microstructured cortical bone tissues. The obtained simulation results demonstrate significant differences due to micromorphology of cortical bone, in terms of crack propagation characteristics for different groups, with the young group showing highest fracture resistance and the senior group the lowest.

Universality of periodicity as revealed from interlayer-mediated cracks

NASA Astrophysics Data System (ADS)

Cho, Myung Rae; Jung, Jong Hyun; Seo, Min Key; Cho, Sung Un; Kim, Young Duck; Lee, Jae Hyun; Kim, Yong Seung; Kim, Pilkwang; Hone, James; Ihm, Jisoon; Park, Yun Daniel

2017-03-01

A crack and its propagation is a challenging multiscale materials phenomenon of broad interest, from nanoscience to exogeology. Particularly in fracture mechanics, periodicities are of high scientific interest. However, a full understanding of this phenomenon across various physical scales is lacking. Here, we demonstrate periodic interlayer-mediated thin film crack propagation and discuss the governing conditions resulting in their periodicity as being universal. We show strong confinement of thin film cracks and arbitrary steering of their propagation by inserting a predefined thin interlayer, composed of either a polymer, metal, or even atomically thin graphene, between the substrate and the brittle thin film. The thin interlayer-mediated controllability arises from local modification of the effective mechanical properties of the crack medium. Numerical calculations incorporating basic fracture mechanics principles well model our experimental results. We believe that previous studies of periodic cracks in SiN films, self-de-bonding sol-gel films, and even drying colloidal films, along with this study, share the same physical origins but with differing physical boundary conditions. This finding provides a simple analogy for various periodic crack systems that exist in nature, not only for thin film cracks but also for cracks ranging in scale.

Generating Fatigue Crack Growth Thresholds with Constant Amplitude Loads

NASA Technical Reports Server (NTRS)

Forth, Scott C.; Newman, James C., J.; Forman, Royce G.

2002-01-01

The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. Some experimental procedures tend to induce load history effects that result in remote crack closure from plasticity. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor, K, will increase, as will the crack growth rate, da/dN. A fatigue crack growth threshold test procedure is developed and experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R.

Progressive failure of sheeted rock slopes: the 2009–2010 Rhombus Wall rock falls in Yosemite Valley, California, USA

USGS Publications Warehouse

Stock, Greg M.; Martel, Stephen J.; Collins, Brian D.; Harp, Edwin L.

2012-01-01

Progressive rock-fall failures in natural rock slopes are common in many environments, but often elude detailed quantitative documentation and analysis. Here we present high-resolution photography, video, and laser scanning data that document spatial and temporal patterns of a 15-month-long sequence of at least 14 rock falls from the Rhombus Wall, a sheeted granitic cliff in Yosemite Valley, California. The rock-fall sequence began on 26 August 2009 with a small failure at the tip of an overhanging rock slab. Several hours later, a series of five rock falls totaling 736 m3progressed upward along a sheeting joint behind the overhanging slab. Over the next 3 weeks, audible cracking occurred on the Rhombus Wall, suggesting crack propagation, while visual monitoring revealed opening of a sheeting joint adjacent to the previous failure surface. On 14 September 2009 a 110 m3 slab detached along this sheeting joint. Additional rock falls between 30 August and 20 November 2010, totaling 187 m3, radiated outward from the initial failure area along cliff (sub)parallel sheeting joints. We suggest that these progressive failures might have been related to stress redistributions accompanying propagation of sheeting joints behind the cliff face. Mechanical analyses indicate that tensile stresses should occur perpendicular to the cliff face and open sheeting joints, and that sheeting joints should propagate parallel to a cliff face from areas of stress concentrations. The analyses also account for how sheeting joints can propagate to lengths many times greater than their depths behind cliff faces. We posit that as a region of failure spreads across a cliff face, stress concentrations along its margin will spread with it, promoting further crack propagation and rock falls.

Consolidation of fatigue and fatigue-crack-propagation data for design use

NASA Technical Reports Server (NTRS)

Rice, R. C.; Davies, K. B.; Jaske, C. E.; Feddersen, C. E.

1975-01-01

Analytical methods developed for consolidation of fatigue and fatigue-crack-propagation data for use in design of metallic aerospace structural components are evaluated. A comprehensive file of data on 2024 and 7075 aluminums, Ti-6Al-4V alloy, and 300M steel was established by obtaining information from both published literature and reports furnished by aerospace companies. Analyses are restricted to information obtained from constant-amplitude load or strain cycling of specimens in air at room temperature. Both fatigue and fatigue-crack-propagation data are analyzed on a statistical basis using a least-squares regression approach. For fatigue, an equivalent strain parameter is used to account for mean stress or stress ratio effects and is treated as the independent variable; cyclic fatigue life is considered to be the dependent variable. An effective stress-intensity factor is used to account for the effect of load ratio on fatigue-crack-propagation and treated as the independent variable. In this latter case, crack-growth rate is considered to be the dependent variable. A two term power function is used to relate equivalent strain to fatigue life, and an arc-hyperbolic-tangent function is used to relate effective stress intensity to crack-growth rate.

Transmission and reflection of the fundamental Lamb modes in a metallic plate with a semi-infinite horizontal crack.

PubMed

Ramadas, C; Hood, Avinash; Khan, Irfan; Balasubramaniam, Krishnan; Joshi, M

2013-03-01

Numerical simulations were carried out to quantify the reflection and transmission characteristics of the fundamental Lamb modes propagating in aluminium sub-plates, which are formed due to a semi-infinite horizontal crack. It was observed that, a Lamb mode propagating in a sub-plate when incident at the edge of a crack, undergoes reflection and transmits through the main plate, as well as the other sub-plate. The mode transmitted through the sub-plate has been termed the 'Turning Lamb Mode' (TLM). Furthermore, a mode converted mode also propagates along with the TLM. This mode has been termed the 'Mode Converted Turning Lamb Mode' (MCTLM). Reflection and transmission characteristics of the fundamental Lamb modes in aluminium sub-plates were studied at frequencies 150 kHz, 175 kHz, and 200 kHz. Experiments conducted to validate the observations made in numerical simulations, confirmed that the transmission and reflection characteristics depend on the thickness ratio. From this study it is surmised that when a Lamb mode propagates through a plate containing horizontal crack, the TLM and the MCTLM start propagating from one sub-plate to the other at the rear edge of the crack and amplitude of these modes depends on the location of the crack across the plate thickness. Copyright © 2012 Elsevier B.V. All rights reserved.

Recrystallization-Induced Surface Cracks of Carbon Ions Irradiated 6H-SiC after Annealing.

PubMed

Ye, Chao; Ran, Guang; Zhou, Wei; Shen, Qiang; Feng, Qijie; Lin, Jianxin

2017-10-25

Single crystal 6H-SiC wafers with 4° off-axis [0001] orientation were irradiated with carbon ions and then annealed at 900 °C for different time periods. The microstructure and surface morphology of these samples were investigated by grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Ion irradiation induced SiC amorphization, but the surface was smooth and did not have special structures. During the annealing process, the amorphous SiC was recrystallized to form columnar crystals that had a large amount of twin structures. The longer the annealing time was, the greater the amount of recrystallized SiC would be. The recrystallization volume fraction was accorded with the law of the Johnson-Mehl-Avrami equation. The surface morphology consisted of tiny pieces with an average width of approximately 30 nm in the annealed SiC. The volume shrinkage of irradiated SiC layer and the anisotropy of newly born crystals during annealing process produced internal stress and then induced not only a large number of dislocation walls in the non-irradiated layer but also the initiation and propagation of the cracks. The direction of dislocation walls was perpendicular to the growth direction of the columnar crystal. The longer the annealing time was, the larger the length and width of the formed crack would be. A quantitative model of the crack growth was provided to calculate the length and width of the cracks at a given annealing time.

Review of the Effects of Microstructure on Fatigue in Aluminum Alloys. Ph.D. Thesis - Cincinnati Univ.

NASA Technical Reports Server (NTRS)

Telesman, J.

1984-01-01

Literature survey was conducted to determine the effects of different microstructural features and different load histories on fatigue crack initiation and propagation of aluminum alloys. Comparison of microstructure and monotonic and cyclic properties between powder metallurgy (P/M) and ingot metallurgy (I/M) alloys is presented. The two alloys that are representative of each process on which the comparison is focused are X7091 and 7050. Included is a detailed description of the microstructure produced through the P/M and I/M proesses. The effect of each pertinent microstructural feature on monotonic and cyclic properties, such as yield strength, toughness, crack initiation and propagation is discussed. Also discussed are the proposed mechanisms for crack initiation and propagation, as well as the effects of aggressive environments on these cyclic properties. The effects of variable amplitude loadin on fatigue crack propagation and the various models proposed to predict load interaction effects are discussed.

Development of Standard Methods of Testing and Analyzing Fatigue Crack Growth Rate Data

DTIC Science & Technology

1978-05-01

nitrogen cooled cryostat; high temperature tests were conducted using resistance heating tapes . An automatic controller maintained test temperatures...Cracking," Int. J. Fracture, Vol. 9, 1973, pp. 63-74. 87. P. Paris and F. Erdogan , "A Critical Analysis of Crack Propagation Laws," Trans. ASME, Ser. D: J...requirements of Sec. 7.2 and Appendix B. 200 REFERENCES 1. P. C. Paris and F. Erdogan , "A Critical Analysis of Crack Propagation Laws", Trans. ASME, Ser. D: 3

Finite-element analysis of dynamic fracture

NASA Technical Reports Server (NTRS)

Aberson, J. A.; Anderson, J. M.; King, W. W.

1976-01-01

Applications of the finite element method to the two dimensional elastodynamics of cracked structures are presented. Stress intensity factors are computed for two problems involving stationary cracks. The first serves as a vehicle for discussing lumped-mass and consistent-mass characterizations of inertia. In the second problem, the behavior of a photoelastic dynamic tear test specimen is determined for the time prior to crack propagation. Some results of a finite element simulation of rapid crack propagation in an infinite body are discussed.

Application of Self Nulling Eddy Current Probe Technique to the Detection of Fatigue Crack Initiation and Control of Test Procedures

NASA Technical Reports Server (NTRS)

Namkung, M.; Nath, S.; Wincheski, B.; Fulton, J. P.

1994-01-01

A major part of fracture mechanics is concerned with studying the initiation and propagation of fatigue cracks. This typically requires constant monitoring of crack growth during fatigue cycles and the knowledge of the precise location of the crack tip at any given time. One technique currently available for measuring fatigue crack length is the Potential Drop method. The method, however, may be inaccurate if the direction of crack growth deviates considerably from what was assumed initially or the curvature of the crack becomes significant. Another popular approach is to optically view the crack using a high magnification microscope, but this entails a person constantly monitoring it. The present proposed technique uses an automated scheme, in order to eliminate the need for a person to constantly monitor the experiment. Another technique under development elsewhere is to digitize an optical image of the test specimen surface and then apply a pattern recognition algorithm to locate the crack tip. A previous publication showed that the self nulling eddy current probe successfully tracked a simulated crack in an aluminum sample. This was the impetus to develop an online real time crack monitoring system. An automated system has been developed which includes a two axis scanner mounted on the tensile testing machine, the probe and its instrumentation and a personal computer (PC) to communicate and control all the parameters. The system software controls the testing parameters as well as monitoring the fatigue crack as it propagates. This paper will discuss the experimental setup in detail and demonstrate its capabilities. A three dimensional finite element model is utilized to model the magnetic field distribution due to the probe and how the probe voltage changes as it scans the crack. Experimental data of the probe for different samples under zero load, static load and high cycle fatigue load will be discussed. The final section summarizes the major accomplishments of the present work, the elements of the future R&D needs and the advantages and disadvantages of using this system in the laboratory and field.

Gear fatigue crack prognosis using embedded model, gear dynamic model and fracture mechanics

NASA Astrophysics Data System (ADS)

Li, C. James; Lee, Hyungdae

2005-07-01

This paper presents a model-based method that predicts remaining useful life of a gear with a fatigue crack. The method consists of an embedded model to identify gear meshing stiffness from measured gear torsional vibration, an inverse method to estimate crack size from the estimated meshing stiffness; a gear dynamic model to simulate gear meshing dynamics and determine the dynamic load on the cracked tooth; and a fast crack propagation model to forecast the remaining useful life based on the estimated crack size and dynamic load. The fast crack propagation model was established to avoid repeated calculations of FEM and facilitate field deployment of the proposed method. Experimental studies were conducted to validate and demonstrate the feasibility of the proposed method for prognosis of a cracked gear.

Laser Peening Effects on Friction Stir Welding

NASA Technical Reports Server (NTRS)

Hatamleh, Omar

2011-01-01

Friction Stir Welding (FSW) is a welding technique that uses frictional heating combined with forging pressure to produce high strength bonds. It is attractive for aerospace applications. Although residual stresses in FSW are generally lower when compared to conventional fusion welds, recent work has shown that significant tensile residual stresses can be present in the weld after fabrication. Therefore, laser shock peening was investigated as a means of moderating the tensile residual stresses produced during welding. This slide presentation reviews the effect of Laser Peening on the weld, in tensile strength, strain, surface roughness, microhardness, surface wear/friction, and fatigue crack growth rates. The study concluded that the laser peening process can result in considerable improvement to crack initiaion, propagation and mechanical properties in FSW.

On Generating Fatigue Crack Growth Thresholds

NASA Technical Reports Server (NTRS)

Forth, Scott C.; Newman, James, Jr.; Forman, Royce G.

2003-01-01

The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. These experimental procedures can induce load history effects that result in crack closure. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake or blunt at the crack tip, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor range, Delta K, will increase, as will the crack growth rate. da/dN. A fatigue crack growth threshold test procedure is experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R. The authors have chosen to study a ductile aluminum alloy where the plastic deformations generated during testing may be of the magnitude to impact the crack opening.

Thermal Fatigue Testing of ZrO2-Y2O3 Thermal Barrier Coating Systems using a High Power CO2 Laser

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Miller, Robert A.

1997-01-01

In the present study, the mechanisms of fatigue crack initiation and propagation, and of coating failure, under thermal loads that simulate diesel engine conditions, are investigated. The surface cracks initiate early and grow continuously under thermal Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) stresses. It is found that, in the absence of interfacial oxidation, the failure associated with LCF is closely related to coating sintering and creep at high temperatures. Significant LCF and HCF interactions have been observed in the thermal fatigue tests. The fatigue crack growth rate in the ceramic coating strongly depends on the characteristic HCF cycle number, N*(sub NCF), which is defined as the number of HCF cycles per LCF cycle. The crack growth rate is increased from 0.36 microns/LCF cycle for a pure LCF test to 2.8 microns/LCF cycle for a combined LCF and HCF test at N*(sub NCF) about 20,000. A surface wedging model has been proposed to account for the HCF crack growth in the coating systems. This mechanism predicts that HCF damage effect increases with increasing surface temperature swing, the thermal expansion coefficient and the elastic modulus of the ceramic coating, as well as with the HCF interacting depth. A good agreement has been found between the analysis and experimental evidence.

Effects of NaCl, pH, and Potential on the Static Creep Behavior of AA1100

NASA Astrophysics Data System (ADS)

Wan, Quanhe; Quesnel, David J.

2013-03-01

The creep rates of AA1100 are measured during exposure to a variety of aggressive environments. NaCl solutions of various concentrations have no influence on the steady-state creep behavior, producing creep rates comparable to those measured in lab air at room temperature. However, after an initial incubation period of steady strain rate, a dramatic increase of strain rate is observed on exposure to HCl solutions and NaOH solutions, as well as during cathodic polarization of specimens in NaCl solutions. Creep strain produces a continuous deformation and elongation of the sample surface that is comparable to slow strain rates at crack tips thought to control the kinetics of crack growth during stress corrosion cracking (SCC). In this experiment, we separate the strain and surface deformation from the complex geometry of the crack tip to better understand the processes at work. Based on this concept, two possible explanations for the environmental influences on creep strain rates are discussed relating to the anodic dissolution of the free surface and hydrogen influences on deformation mechanisms. Consistencies of pH dependence between corrosion creep and SCC at low pH prove a creep-involved SCC mechanism, while the discrepancies between corrosion creep behavior and previous SCC results at high pH indicate a rate-limit step change in the crack propagation of the SCC process.

Corrosion fatigue crack propagation in metals

NASA Technical Reports Server (NTRS)

Gangloff, Richard P.

1990-01-01

This review assesses fracture mechanics data and mechanistic models for corrosion fatigue crack propagation in structural alloys exposed to ambient temperature gases and electrolytes. Extensive stress intensity-crack growth rate data exist for ferrous, aluminum and nickel based alloys in a variety of environments. Interactive variables (viz., stress intensity range, mean stress, alloy composition and microstructure, loading frequency, temperature, gas pressure and electrode potential) strongly affect crack growth kinetics and complicate fatigue control. Mechanistic models to predict crack growth rates were formulated by coupling crack tip mechanics with occluded crack chemistry, and from both the hydrogen embrittlement and anodic dissolution/film rupture perspectives. Research is required to better define: (1) environmental effects near threshold and on crack closure; (2) damage tolerant life prediction codes and the validity of similitude; (3) the behavior of microcrack; (4) probes and improved models of crack tip damage; and (5) the cracking performance of advanced alloys and composites.

Effect of glide path and apical preparation size on the incidence of apical crack during the canal preparation using Reciproc, WaveOne, and ProTaper Next systems in curved root canals: A stereomicroscope study.

PubMed

Topçuoğlu, Hüseyin Sinan; Düzgün, Salih; Akpek, Firdevs; Topçuoğlu, Gamze

2016-11-01

This study evaluated the effect of creating a glide path and apical preparation size on the incidence of apical cracks during canal preparation in mandibular molar teeth with curved canals. One hundred and forty extracted teeth were used. The teeth were randomly assigned to one control group or six experimental groups (n = 20 per group) for canal preparation. No preparation was performed on teeth in the control group. In three of the six experimental groups, a glide path was not created; a glide path was created on the curved mesial canals of all teeth in the remaining three experimental groups. All teeth in experimental groups were then instrumented with the following systems: Reciproc, WaveOne (WO), and ProTaper Next (PTN). Digital images of the apical root surfaces of these teeth were recorded before preparation, after instrumentation with size 25 files, and after instrumentation with size 40 files. The images were then inspected for the presence of any new apical cracks and propagation. There was no significant difference between the experimental groups during canal preparation using size 25 files (p > 0.05). Reciproc and WO caused more new apical cracks than did PTN during canal preparation using size 40 files (p < 0.05). However, canal preparation using size 40 files did not cause propagation of existing cracks (p > 0.05). Performing a glide path prior to canal preparation did not change the incidence of apical crack during preparation. Additionally, increasing apical preparation size may increase the incidence of apical crack during canal preparation. SCANNING 38:585-590, 2016. © 2016 Wiley Periodicals, Inc. © Wiley Periodicals, Inc.

Low Cycle Fatigue Properties of Extruded Mg10GdxNd Alloys

NASA Astrophysics Data System (ADS)

Tober, Gerhard; Maier, Petra; Müller, Sören; Hort, Norbert

The Rare Earth (RE) containing magnesium alloys Mg10Gd and Mg10Gd1Nd show after extrusion very good low cycle fatigue (LCF) properties. Considering extruded AZ31 as a possible benchmark alloy, life times as a function of LCF stress values are similar to the alloys investigated in this study. Mechanical properties determined in tension and compression show smaller values for both RE containing alloys. Therefore the LCF behavior is analyzed by the stress-strain hysteresis evaluation resulting in cyclic creep and plastic hardening or softening. LCF tests were strain controlled with amplitude of 0.5 % and 0.8 % at a frequency of 5Hz. The fracture surfaces are examined by SEM, where the area of crack propagation and overload were of main interest. Micrographs of longitudinal cross sections reveal twinning along the region of crack propagation. The correlation between the amount of twins and the number of cycles is discussed.

Three-dimensional elastic-plastic finite-element analysis of fatigue crack propagation

NASA Technical Reports Server (NTRS)

Goglia, G. L.; Chermahini, R. G.

1985-01-01

Fatigue cracks are a major problem in designing structures subjected to cyclic loading. Cracks frequently occur in structures such as aircraft and spacecraft. The inspection intervals of many aircraft structures are based on crack-propagation lives. Therefore, improved prediction of propagation lives under flight-load conditions (variable-amplitude loading) are needed to provide more realistic design criteria for these structures. The main thrust was to develop a three-dimensional, nonlinear, elastic-plastic, finite element program capable of extending a crack and changing boundary conditions for the model under consideration. The finite-element model is composed of 8-noded (linear-strain) isoparametric elements. In the analysis, the material is assumed to be elastic-perfectly plastic. The cycle stress-strain curve for the material is shown Zienkiewicz's initial-stress method, von Mises's yield criterion, and Drucker's normality condition under small-strain assumptions are used to account for plasticity. The three-dimensional analysis is capable of extending the crack and changing boundary conditions under cyclic loading.

Forecasting Frost Damage: Follow the Water

NASA Astrophysics Data System (ADS)

Rempel, A. W.

2015-12-01

Frost damage takes place when the pressure exerted against pore walls exceeds the cohesive strength of water-infiltrated rock and causes cracks to extend. Elegant theoretical treatments supported by meticulous field and laboratory observations have combined to unravel the basic mechanical and thermodynamic controls in idealized systems. Frost damage is most vigorous when conditions are cold enough that the net pressure exerted against the pore walls can cause crack extension, yet warm enough to enable the flow that supplies further ice growth in the newly opened space. This insight is applied here to develop practical geomorphic process laws for the effects of frost damage at the larger scales that are relevant for describing the evolution of landscapes. To this end, a direct connection is made between the intensity of frost damage and the porosity increase that results from gradients in water flux under conditions that are cold enough for ice-rock interactions to propagate cracks. This implies that the annual temperature variation at the ground surface can be combined with considerations of heat and mass transport to derive rigorous forecasts of the potential for frost damage that are tied to the increases in water mass that accompany solidification in porous rock. As an example, the image shows the depth-integrated porosity change λ promoted by crack growth at temperatures colder than -ΔTc over an annual cycle for different choices of mean annual temperature MAT and surface amplitude A (assuming a thermal diffusivity of 1 mm2/s and a power-law relationship between permeability and undercooling with exponent α=4, such that a base value of 10-14m2 is reached at a reference undercooling of 0.1 ºC). The abrupt onset in cracking once MAT decreases below a threshold is produced by the requirement that undercooling surpass ΔTc in order to generate sufficient pressures to propagate cracks. The eventual reduction and gradual tail in λ at colder MAT is produced by the clogging of the permeable pathways needed to supply additional water to propmote crack growth.

Fatigue crack propagation path across the dentinoenamel junction complex in human teeth.

PubMed

Dong, X D; Ruse, N D

2003-07-01

The human tooth structures should be understood clearly to improve clinically used restorative materials. The dentinoenamel junction (DEJ) plays a key role in resisting crack propagation in teeth. The aim of this study was to determine the fracture toughness of the enamel-DEJ-dentin complex and to investigate the influence of the DEJ on the fatigue crack propagation path across it by characterizing fatigue-fractured enamel-DEJ-dentin complexes using optical and scanning electron microscopy. The results of this study showed that the fracture toughness of the enamel-DEJ-dentin complex was 1.50 +/- 0.28 Mpa x m(1/2). Based on the results of this investigation, it was concluded that the DEJ complex played a critical role in resisting crack propagation from enamel into dentin. The DEJ complex is, approximately, a 100 to 150 microm broad region at the interface between enamel and dentin. The toughening mechanism of the DEJ complex may be explained by the fact that crack paths were deflected as cracks propagated across it. Understanding the mechanism of crack deflection could help in improving dentin-composite as well as ceramic-cement interfacial qualities with the aim to decrease the risk of clinical failure of restorations. Both can be viewed as being composed from a layer of material of high strength and hardness bonded to a softer but tougher substratum (dentin). The bonding agent or the luting cement layer may play the critical role of the DEJ in improving the strength of these restorations in clinical situations. Copyright 2003 Wiley Periodicals, Inc.

Crack propagation analysis and fatigue life prediction for structural alloy steel based on metal magnetic memory testing

NASA Astrophysics Data System (ADS)

Ni, Chen; Hua, Lin; Wang, Xiaokai

2018-09-01

To monitor the crack propagation and predict the fatigue life of ferromagnetic material, the metal magnetic memory (MMM) testing was carried out to the single edge notched specimen made from structural alloy steel under three-point bending fatigue experiment in this paper. The variation of magnetic memory signal Hp (y) in process of fatigue crack propagation was investigated. The gradient K of Hp (y) was investigated and compared with the stress of specimen obtained by finite element analysis. It indicated that the gradient K can qualitatively reflect the distribution and variation of stress. The maximum gradient Kmax and crack size showed a good linear relationship, which indicated that the crack propagation can be estimated by MMM testing. Furthermore, the damage model represented by magnetic memory characteristic was created and a fatigue life prediction method was developed. The fatigue life can be evaluated by the relationship between damage parameter and normalized life. The method was also verified by another specimen. Because of MMM testing, it provided a new approach for predicting fatigue life.

Microstructural effects on the room and elevated temperature low cycle fatigue behavior of Waspaloy. M.S. Thesis Final Report

NASA Technical Reports Server (NTRS)

Lerch, B. A.

1982-01-01

Longitudinal specimens of Waspaloy containing either coarse grains with small gamma or fine grains with large gamma were tested in air at a frequency of 0.33 Hz or 0.50 Hz. The coarse grained structures exhibited planar slip on (III) planes and precipitate shearing at all temperatures. Cracks initiated by a Stage 1 mechanism and propagated by a striation forming mechanism. At 700 C and 800 C, cleavage and intergranular cracking were observed. Testing at 500 C, 700 C, and 800 C caused precipitation of grain boundary carbides. At 700 C, carbides precipitated on slip bands. The fine grained structures exhibited planar slip on (111) planes. Dislocations looped the large gamma precipitates. This structure led to stress saturation and propagation was observed. Increasing temperatures resulted in increased specimen oxidation for both heat treatments. Slip band and grain boundary oxidation were observed. At 800 C, oxidized grain boundaries were cracked by intersecting slip bands which resulted in intergranular failure. The fine specimens had crack initiation later in the fatigue life, but with more rapid propagation crack propagation.

Calculation of contraction stresses in dental composites by analysis of crack propagation in the matrix surrounding a cavity.

PubMed

Yamamoto, Takatsugu; Ferracane, Jack L; Sakaguchi, Ronald L; Swain, Michael V

2009-04-01

Polymerization contraction of dental composite produces a stress field in the bonded surrounding substrate that may be capable of propagating cracks from pre-existing flaws. The objectives of this study were to assess the extent of crack propagation from flaws in the surrounding ceramic substrate caused by composite contraction stresses, and to propose a method to calculate the contraction stress in the ceramic using indentation fracture. Initial cracks were introduced with a Vickers indenter near a cylindrical hole drilled into a glass-ceramic simulating enamel. Lengths of the radial indentation cracks were measured. Three composites having different contraction stresses were cured within the hole using one- or two-step light-activation methods and the crack lengths were measured. The contraction stress in the ceramic was calculated from the crack length and the fracture toughness of the glass-ceramic. Interfacial gaps between the composite and the ceramic were expressed as the ratio of the gap length to the hole perimeter, as well as the maximum gap width. All groups revealed crack propagation and the formation of contraction gaps. The calculated contraction stresses ranged from 4.2 MPa to 7.0 MPa. There was no correlation between the stress values and the contraction gaps. This method for calculating the stresses produced by composites is a relatively simple technique requiring a conventional hardness tester. The method can investigate two clinical phenomena that may occur during the placement of composite restorations, i.e. simulated enamel cracking near the margins and the formation of contraction gaps.

Effects of External Hydrogen on Hydrogen Transportation and Distribution Around the Fatigue Crack Tip in Type 304 Stainless Steel

NASA Astrophysics Data System (ADS)

Chen, Xingyang; Zhou, Chengshuang; Cai, Xiao; Zheng, Jinyang; Zhang, Lin

2017-10-01

The effects of external hydrogen on hydrogen transportation and distribution around the fatigue crack tip in type 304 stainless steel were investigated by using hydrogen microprint technique (HMT) and thermal desorption spectrometry. HMT results show that some silver particles induced by hydrogen release are located near the fatigue crack and more silver particles are concentrated around the crack tip, which indicates that hydrogen accumulates in the vicinity of the crack tip during the crack growth in hydrogen gas environment. Along with the crack propagation, strain-induced α' martensite forms around the crack tip and promotes hydrogen invasion into the matrix, which will cause the crack initiation and propagation at the austenite/ α' martensite interface. In addition, the hydrogen content in the vicinity of the crack tip is higher than that at the crack edge far away from the crack tip, which is related to the stress state and strain-induced α' martensite.

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.

Modeling crack propagation in polycrystalline microstructure using variational multiscale method

DOE PAGES

Sun, Shang; Sundararaghavan, Veera

2016-01-01

Crack propagation in a polycrystalline microstructure is analyzed using a novel multiscale model. The model includes an explicit microstructural representation at critical regions (stress concentrators such as notches and cracks) and a reduced order model that statistically captures the microstructure at regions far away from stress concentrations. Crack propagation is modeled in these critical regions using the variational multiscale method. In this approach, a discontinuous displacement field is added to elements that exceed the critical values of normal or tangential tractions during loading. Compared to traditional cohesive zone modeling approaches, the method does not require the use of any specialmore » interface elements in the microstructure and thus can model arbitrary crack paths. As a result, the capability of the method in predicting both intergranular and transgranular failure modes in an elastoplastic polycrystal is demonstrated under tensile and three-point bending loads.« less

Residual strength and crack propagation tests on C-130 airplane center wings with service-imposed fatigue damage

NASA Technical Reports Server (NTRS)

Snider, H. L.; Reeder, F. L.; Dirkin, W. J.

1972-01-01

Fourteen C-130 airplane center wings, each containing service-imposed fatigue damage resulting from 4000 to 13,000 accumulated flight hours, were tested to determine their fatigue crack propagation and static residual strength characteristics. Eight wings were subjected to a two-step constant amplitude fatigue test prior to static testing. Cracks up to 30 inches long were generated in these tests. Residual static strengths of these wings ranged from 56 to 87 percent of limit load. The remaining six wings containing cracks up to 4 inches long were statically tested as received from field service. Residual static strengths of these wings ranged from 98 to 117 percent of limit load. Damage-tolerant structural design features such as fastener holes, stringers, doublers around door cutouts, and spanwise panel splices proved to be effective in retarding crack propagation.

Measurements of radiated elastic wave energy from dynamic tensile cracks

NASA Technical Reports Server (NTRS)

Boler, Frances M.

1990-01-01

The role of fracture-velocity, microstructure, and fracture-energy barriers in elastic wave radiation during a dynamic fracture was investigated in experiments in which dynamic tensile cracks of two fracture cofigurations of double cantilever beam geometry were propagating in glass samples. The first, referred to as primary fracture, consisted of fractures of intact glass specimens; the second configuration, referred to as secondary fracture, consisted of a refracture of primary fracture specimens which were rebonded with an intermittent pattern of adhesive to produce variations in fracture surface energy along the crack path. For primary fracture cases, measurable elastic waves were generated in 31 percent of the 16 fracture events observed; the condition for radiation of measurable waves appears to be a local abrupt change in the fracture path direction, such as occurs when the fracture intersects a surface flaw. For secondary fractures, 100 percent of events showed measurable elastic waves; in these fractures, the ratio of radiated elastic wave energy in the measured component to fracture surface energy was 10 times greater than for primary fracture.

Structural Phase Transformation in Strained Monolayer MoWSe2 Alloy.

PubMed

Apte, Amey; Kochat, Vidya; Rajak, Pankaj; Krishnamoorthy, Aravind; Manimunda, Praveena; Hachtel, Jordan A; Idrobo, Juan Carlos; Syed Amanulla, Syed Asif; Vashishta, Priya; Nakano, Aiichiro; Kalia, Rajiv K; Tiwary, Chandra Sekhar; Ajayan, Pulickel M

2018-04-24

Two-dimensional (2D) materials exhibit different mechanical properties from their bulk counterparts owing to their monolayer atomic thickness. Here, we have examined the mechanical behavior of 2D molybdenum tungsten diselenide (MoWSe 2 ) precipitation alloy grown using chemical vapor deposition and composed of numerous nanoscopic MoSe 2 and WSe 2 regions. Applying a bending strain blue-shifted the MoSe 2 and WSe 2 A 1g Raman modes with the stress concentrated near the precipitate interfaces predominantly affecting the WSe 2 modes. In situ local Raman measurements suggested that the crack propagated primarily thorough MoSe 2 -rich regions in the monolayer alloy. Molecular dynamics (MD) simulations were performed to study crack propagation in an MoSe 2 monolayer containing nanoscopic WSe 2 regions akin to the experiment. Raman spectra calculated from MD trajectories of crack propagation confirmed the emergence of intermediate peaks in the strained monolayer alloy, mirroring experimental results. The simulations revealed that the stress buildup around the crack tip caused an irreversible structural transformation from the 2H to 1T phase both in the MoSe 2 matrix and WSe 2 patches. This was corroborated by high-angle annular dark-field images. Crack branching and subsequent healing of a crack branch were also observed in WSe 2 , indicating the increased toughness and crack propagation resistance of the alloyed 2D MoWSe 2 over the unalloyed counterparts.

Damage tolerance assessment handbook. Volume 1 : introduction, fracture mechanics, fatigue crack propagation

DOT National Transportation Integrated Search

1999-02-01

The handbook is presented in two volumes. This volume, Volume I, introduces the damage tolerance concept with an historical perspective followed by the fundamentals of fracture mechanics and fatigue crack propagation. Various fracture criteria and cr...

In Situ Acoustic Monitoring of Thermal Spray Process Using High-Frequency Impulse Measurements

NASA Astrophysics Data System (ADS)

Tillmann, Wolfgang; Walther, Frank; Luo, Weifeng; Haack, Matthias; Nellesen, Jens; Knyazeva, Marina

2018-01-01

In order to guarantee their protective function, thermal spray coatings must be free from cracks, which expose the substrate surface to, e.g., corrosive media. Cracks in thermal spray coatings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology and coating defects were inspected using light microscopy on metallographic cross sections. Additionally, the resulting crack patterns were imaged in 3D by means of x-ray microtomography.

Fatigue and fracture assessment of cracks in steel elements using acoustic emission

NASA Astrophysics Data System (ADS)

Nemati, Navid; Metrovich, Brian; Nanni, Antonio

2011-04-01

Single edge notches provide a very well defined load and fatigue crack size and shape environment for estimation of the stress intensity factor K, which is not found in welded elements. ASTM SE(T) specimens do not appear to provide ideal boundary conditions for proper recording of acoustic wave propagation and crack growth behavior observed in steel bridges, but do provide standard fatigue crack growth rate data. A modified versions of the SE(T) specimen has been examined to provide small scale specimens with improved acoustic emission(AE) characteristics while still maintaining accuracy of fatigue crack growth rate (da/dN) versus stress intensity factor (ΔK). The specimens intend to represent a steel beam flange subjected to pure tension, with a surface crack growing transverse to a uniform stress field. Fatigue test is conducted at low R ratio. Analytical and numerical studies of stress intensity factor are developed for single edge notch test specimens consistent with the experimental program. ABAQUS finite element software is utilized for stress analysis of crack tips. Analytical, experimental and numerical analysis were compared to assess the abilities of AE to capture a growing crack.

Failure of disordered materials as a depinning transition

NASA Astrophysics Data System (ADS)

Ponson, Laurent

2010-03-01

Crack propagation is the fundamental process leading to material failure. However, its dynamics is far from being fully understood. In this work, we investigate both experimentally and theoretically the far-from-equilibrium propagation of a crack within a disordered brittle material. At first, we focus on the average dynamics of a crack, and study the variations of its growth velocity v with respect to the external driving force G [1]. Carefully measured on a brittle rock, these variations are shown to display two regimes: above a given threshold Gc, the velocity evolves as a power law v ˜(G- Gc)^0.8, while at low driving force, its variations are well described by a sub-critical creep law, characteristic of a thermally activated crack propagation. Extending the continuum theory of Fracture Mechanics to inhomogeneous media, we show that this behavior is reminiscent of a dynamical critical transition: critical failure occurs when the driving force is sufficiently large to depin the crack front from the material heterogeneities. Another way to reveal such a transition is to investigate the fluctuations of crack velocity [2]. Considering a crack at the heterogeneous interface between two elastic solids, we predict that its propagation occurs through sudden jumps, with power law distributed sizes and durations. These predictions compare quantitatively well with recent direct observations of interfacial crack propagation [3]. Such an interpretation of material failure opens new perspectives in the field of Engineering and Applied Science that will be finally discussed. [4pt] [1] L. Ponson, Depinning transition in failure of inhomogeneous brittle materials, Phys. Rev. Lett. 103, 055501 (2009). [0pt] [2] D. Bonamy, S. Santucci and L. Ponson, Crackling dynamics in material failure as a signature of a self-organized dynamic phase transition, Phys. Rev. Lett. 101, 045501 (2008). [0pt] [3] K.J. Måløy, S. Santucci, J. Schmittbuhl and R. Toussaint, Local waiting time fluctuations along a randomly pinned crack front, Phys. Rev. Lett. 96, 045501 (2006).

Surface morphology and subsurface damaged layer of various glasses machined by 193-nm ArF excimer laser

NASA Astrophysics Data System (ADS)

Liao, Yunn-shiuan; Chen, Ying-Tung; Chao, Choung-Lii; Liu, Yih-Ming

2005-01-01

Owing to the high bonding energy, most of the glasses are removed by photo-thermal rather than photo-chemical effect when they are ablated by the 193 or 248nm excimer lasers. Typically, the machined surface is covered by re-deposited debris and the sub-surface, sometimes surface as well, is scattered with micro-cracks introduced by thermal stress generated during the process. This study aimed to investigate the nature and extent of the surface morphology and sub-surface damaged (SSD) layer induced by the laser ablation. The effects of laser parameters such as fluence, shot number and repetition rate on the morphology and SSD were discussed. An ArF excimer laser (193 nm) was used in the present study to machine glasses such as soda-lime, Zerodur and BK-7. It is found that the melt ejection and debris deposition tend to pile up higher and become denser in structure under a higher energy density, repetition rate and shot number. There are thermal stress induced lateral cracks when the debris covered top layer is etched away. Higher fluence and repetition rate tend to generate more lateral and median cracks which propagate into the substrate. The changes of mechanical properties of the SSD layer were also investigated.

Generation mechanism of nonlinear ultrasonic Lamb waves in thin plates with randomly distributed micro-cracks.

PubMed

Zhao, Youxuan; Li, Feilong; Cao, Peng; Liu, Yaolu; Zhang, Jianyu; Fu, Shaoyun; Zhang, Jun; Hu, Ning

2017-08-01

Since the identification of micro-cracks in engineering materials is very valuable in understanding the initial and slight changes in mechanical properties of materials under complex working environments, numerical simulations on the propagation of the low frequency S 0 Lamb wave in thin plates with randomly distributed micro-cracks were performed to study the behavior of nonlinear Lamb waves. The results showed that while the influence of the randomly distributed micro-cracks on the phase velocity of the low frequency S 0 fundamental waves could be neglected, significant ultrasonic nonlinear effects caused by the randomly distributed micro-cracks was discovered, which mainly presented as a second harmonic generation. By using a Monte Carlo simulation method, we found that the acoustic nonlinear parameter increased linearly with the micro-crack density and the size of micro-crack zone, and it was also related to the excitation frequency and friction coefficient of the micro-crack surfaces. In addition, it was found that the nonlinear effect of waves reflected by the micro-cracks was more noticeable than that of the transmitted waves. This study theoretically reveals that the low frequency S 0 mode of Lamb waves can be used as the fundamental waves to quantitatively identify micro-cracks in thin plates. Copyright © 2017 Elsevier B.V. All rights reserved.

Near-IR imaging of cracks in teeth

NASA Astrophysics Data System (ADS)

Fried, William A.; Simon, Jacob C.; Lucas, Seth; Chan, Kenneth H.; Darling, Cynthia L.; Staninec, Michal; Fried, Daniel

2014-02-01

Dental enamel is highly transparent at near-IR wavelengths and several studies have shown that these wavelengths are well suited for optical transillumination for the detection and imaging of tooth decay. We hypothesize that these wavelengths are also well suited for imaging cracks in teeth. Extracted teeth with suspected cracks were imaged at several wavelengths in the near-IR from 1300-1700-nm. Extracted teeth were also examined with optical coherence tomography to confirm the existence of suspected cracks. Several teeth of volunteers were also imaged in vivo at 1300-nm to demonstrate clinical potential. In addition we induced cracks in teeth using a carbon dioxide laser and imaged crack formation and propagation in real time using near-IR transillumination. Cracks were clearly visible using near-IR imaging at 1300-nm in both in vitro and in vivo images. Cracks and fractures also interfered with light propagation in the tooth aiding in crack identification and assessment of depth and severity.

Numerical Assessment of Rockbursting.

DTIC Science & Technology

1987-05-27

static equilibrium, nonlinear elasticity, strain-softening • material , unstable propagation of pre-existing cracks , and finally - surface...structure of LINOS, which is common to most of the large finite element codes, the library of element and material subroutines can be easily expanded... material model subroutines , are tested by comparing finite element results with analytical or numerical results derived for hypo-elastic and

Material heterogeneity in cancellous bone promotes deformation recovery after mechanical failure.

PubMed

Torres, Ashley M; Matheny, Jonathan B; Keaveny, Tony M; Taylor, David; Rimnac, Clare M; Hernandez, Christopher J

2016-03-15

Many natural structures use a foam core and solid outer shell to achieve high strength and stiffness with relatively small amounts of mass. Biological foams, however, must also resist crack growth. The process of crack propagation within the struts of a foam is not well understood and is complicated by the foam microstructure. We demonstrate that in cancellous bone, the foam-like component of whole bones, damage propagation during cyclic loading is dictated not by local tissue stresses but by heterogeneity of material properties associated with increased ductility of strut surfaces. The increase in surface ductility is unexpected because it is the opposite pattern generated by surface treatments to increase fatigue life in man-made materials, which often result in reduced surface ductility. We show that the more ductile surfaces of cancellous bone are a result of reduced accumulation of advanced glycation end products compared with the strut interior. Damage is therefore likely to accumulate in strut centers making cancellous bone more tolerant of stress concentrations at strut surfaces. Hence, the structure is able to recover more deformation after failure and return to a closer approximation of its original shape. Increased recovery of deformation is a passive mechanism seen in biology for setting a broken bone that allows for a better approximation of initial shape during healing processes and is likely the most important mechanical function. Our findings suggest a previously unidentified biomimetic design strategy in which tissue level material heterogeneity in foams can be used to improve deformation recovery after failure.

The origins of Asteroidal rock disaggregation: Interplay of thermal fatigue and microstructure

NASA Astrophysics Data System (ADS)

Hazeli, Kavan; El Mir, Charles; Papanikolaou, Stefanos; Delbo, Marco; Ramesh, K. T.

2018-04-01

The distributions of size and chemical composition in regolith on airless bodies provide clues to the evolution of the solar system. Recently, the regolith on asteroid (25143) Itokawa, visited by the JAXA Hayabusa spacecraft, was observed to contain millimeter to centimeter sized particles. Itokawa boulders commonly display well-rounded profiles and surface textures that appear inconsistent with mechanical fragmentation during meteorite impact; the rounded profiles have been hypothesized to arise from rolling and movement on the surface as a consequence of seismic shaking. This investigation provides a possible explanation of these observations by exploring the primary crack propagation mechanism during thermal fatigue of a chondrite. Herein, we present the evolution of the full-field strains on the surface as a function of temperature and microstructure, and examine the crack growth during thermal cycling. Our experimental results demonstrate that thermal-fatigue-driven fracture occurs under these conditions. The results suggest that the primary fatigue crack path preferentially follows the interfaces between monominerals, leaving the minerals themselves intact after fragmentation. These observations are explained through a microstructure-based finite element model that is quantitatively compared with our experimental results. These results on the interactions of thermal fatigue cracking with the microstructure may ultimately allow us to distinguish between thermally induced fragments and impact products.

Flexural edge waves generated by steady-state propagation of a loaded rectilinear crack in an elastically supported thin plate

NASA Astrophysics Data System (ADS)

Nobili, Andrea; Radi, Enrico; Lanzoni, Luca

2017-08-01

The problem of a rectilinear crack propagating at constant speed in an elastically supported thin plate and acted upon by an equally moving load is considered. The full-field solution is obtained and the spotlight is set on flexural edge wave generation. Below the critical speed for the appearance of travelling waves, a threshold speed is met which marks the transformation of decaying edge waves into edge waves propagating along the crack and dying away from it. Yet, besides these, and for any propagation speed, a pair of localized edge waves, which rapidly decay behind the crack tip, is also shown to exist. These waves are characterized by a novel dispersion relation and fade off from the crack line in an oscillatory manner, whence they play an important role in the far field behaviour. Dynamic stress intensity factors are obtained and, for speed close to the critical speed, they show a resonant behaviour which expresses the most efficient way to channel external work into the crack. Indeed, this behaviour is justified through energy considerations regarding the work of the applied load and the energy release rate. Results might be useful in a wide array of applications, ranging from fracturing and machining to acoustic emission and defect detection.

Flexural edge waves generated by steady-state propagation of a loaded rectilinear crack in an elastically supported thin plate.

PubMed

Nobili, Andrea; Radi, Enrico; Lanzoni, Luca

2017-08-01

The problem of a rectilinear crack propagating at constant speed in an elastically supported thin plate and acted upon by an equally moving load is considered. The full-field solution is obtained and the spotlight is set on flexural edge wave generation. Below the critical speed for the appearance of travelling waves, a threshold speed is met which marks the transformation of decaying edge waves into edge waves propagating along the crack and dying away from it. Yet, besides these, and for any propagation speed, a pair of localized edge waves, which rapidly decay behind the crack tip, is also shown to exist. These waves are characterized by a novel dispersion relation and fade off from the crack line in an oscillatory manner, whence they play an important role in the far field behaviour. Dynamic stress intensity factors are obtained and, for speed close to the critical speed, they show a resonant behaviour which expresses the most efficient way to channel external work into the crack. Indeed, this behaviour is justified through energy considerations regarding the work of the applied load and the energy release rate. Results might be useful in a wide array of applications, ranging from fracturing and machining to acoustic emission and defect detection.

A numerical method for determination of source time functions for general three-dimensional rupture propagation

NASA Technical Reports Server (NTRS)

Das, S.

1979-01-01

A method to determine the displacement and the stress on the crack plane for a three-dimensional shear crack of arbitrary shape propagating in an infinite, homogeneous medium which is linearly elastic everywhere off the crack plane is presented. The main idea of the method is to use a representation theorem in which the displacement at any given point on the crack plane is written as an integral of the traction over the whole crack plane. As a test of the accuracy of the numerical technique, the results are compared with known solutions for two simple cases.

Simulation of extreme ground water flow in the fractal crack structure of Earth's crust - impact on catastrophic floods

NASA Astrophysics Data System (ADS)

Bukharov, Dmitriy; Aleksey, Kucherik; Tatyana, Trifonova

2014-05-01

Recently, the contribution of groundwater in catastrophic floods is the question under discussion [1,2]. The principal problem in such an approach - to analyze the transportation ways for groundwater in dynamics, and especially - the reasons of exit it on land surface. The crackness, being a characteristic property for all rocks, should be associated with the process in respect of unified dynamic system as a river water basin is, taking into account fundamental phenomena of the 3D-crack network development/modification (up to faults) as a transport groundwater system [3]. 2. In the system of fractal cracks (connected with the main channel for groundwater) the formation of extreme flow is possible, i.e. a devastating case occurs by instantaneous flash mechanism. The development of such a process is related to two factors. First, within the main channel of propagation of the groundwater when a motion is turbulent. In accordance with the theory of Kolmogorov [4], we assume that such a turbulence is isotropic. The fact means that both velocity and pressure fields in the water flow have pulsations related to the non-linear energy transfer between the vortices. This approach allows us to determine both that a maximum possible size of the vortices defined by characteristic dimensions of the underground channel and another - a minimum size of their due to process of dissipation. Energy transfer in the eddies formed near a border, is a complex nonlinear process, which we described by using a modernized Prandtl semi-empirical model [5]. Second, the mechanism of groundwater propagation in the system of cracks extending from the main underground channel is described in the frames of the fractal geometry methods [6]. The approach allows to determine the degree of similarity in the crack system, i.e. the ratio of mean diameters and lengths of cracks/faults for each step of decomposition. The fact results in integrated quantitative characteristics of 3D-network in all, by fractal dimension. Formation of fractal cracks (in coupling of fault length and it number) ensures an optimal traveling network for propagation of water, but changes in external conditions can lead to the formation of hydroblow with extreme water flow formation on surface, i.e. a flash event arise. 3. The proposed approach allows to carry out the modeling in different spatial scales, to determine the features of hydrodynamic processes for generate extreme water flow, when it is going out on the land surface, and results in catastrophic water phenomenon development. 1. Trifonova T.A., Arakelian M.M., Arakelian S.M. European Geosciences Union General Assembly 2013, Vienna, Austria, 2013. http://www.egu2013.eu ; 2. Arakelian S.M., Trifonova T.A., Arakelian M.M. IGU Kyoto Regional Conference (KRC), Kyoto, Japan, 2013, www.igu-kyoto2013.org. 3.Trifonova T. A.. // Izv. RAS, series on geography, 2008, No.1, pp.28-36. 4. Kolmogorov A.N. //Bulletin of Soviet Academy of Science, 1941. V. 30, No.4. pp. 299-303. 5.Volynov M.A. // Fundamental research No.10. 2013, pp. 1676-1688. 6. Mandelbrot B.B. // Institute of computer research ISBN 5-93972-108-7 (2002).

Self-healing slip pulses in dynamic rupture models due to velocity-dependent strength

USGS Publications Warehouse

Beeler, N.M.; Tullis, T.E.

1996-01-01

Seismological observations of short slip duration on faults (short rise time on seismograms) during earthquakes are not consistent with conventional crack models of dynamic rupture and fault slip. In these models, the leading edge of rupture stops only when a strong region is encountered, and slip at an interior point ceases only when waves from the stopped edge of slip propagate back to that point. In contrast, some seismological evidence suggests that the duration of slip is too short for waves to propagate from the nearest edge of the ruptured surface, perhaps even if the distance used is an asperity size instead of the entire rupture dimension. What controls slip duration, if not dimensions of the fault or of asperities? In this study, dynamic earthquake rupture and slip are represented by a propagating shear crack. For all propagating shear cracks, slip velocity is highest near the rupture front, and at a small distance behind the rupture front, the slip velocity decreases. As pointed out by Heaton (1990), if the crack obeys a negative slip-rate-dependent strength relation, the lower slip velocity behind the rupture front will lead to strengthening that further reduces the velocity, and under certain circumstances, healing of slip can occur. The boundary element method of Hamano (1974) is used in a program adapted from Andrews (1985) for numerical simulations of mode II rupture with two different velocity-dependent strength functions. For the first function, after a slip-weakening displacement, the crack follows an exponential velocity-weakening relation. The characteristic velocity V0 of the exponential determines the magnitude of the velocity-dependence at dynamic velocities. The velocity-dependence at high velocity is essentially zero when V0 is small and the resulting slip velocity distribution is similar to slip weakening. If V0 is larger, rupture propagation initially resembles slip-weakening, but spontaneous healing occurs behind the rupture front. The rise time and rupture propagation velocity depend on the choice of constitutive parameters. The second strength function is a natural log velocity-dependent form similar to constitutive laws that fit experimental rock friction data at lower velocities. Slip pulses also arise with this function. For a reasonable choice of constitutive parameters, slip pulses with this function do not propagate at speeds greater than the Raleighwave velocity. The calculated slip pulses are similar in many aspects to seismic observations of short rise time. In all cases of self-healing slip pulses, the residual stress increases with distance behind the trailing edge of the pulse so that the final stress drop is much less than the dynamic stress drop, in agreement with the model of Brune (1976) and some recent seismological observations of rupture.

Dynamics of Nanoscale Grain-Boundary Decohesion in Aluminum by Molecular-Dynamics Simulation

NASA Technical Reports Server (NTRS)

Yamakov, V.; Saether, E.; Phillips, D. R.; Glaessegen, E. H.

2007-01-01

The dynamics and energetics of intergranular crack growth along a flat grain boundary in aluminum is studied by a molecular-dynamics simulation model for crack propagation under steady-state conditions. Using the ability of the molecular-dynamics simulation to identify atoms involved in different atomistic mechanisms, it was possible to identify the energy contribution of different processes taking place during crack growth. The energy contributions were divided as: elastic energy, defined as the potential energy of the atoms in fcc crystallographic state; and plastically stored energy, the energy of stacking faults and twin boundaries; grain-boundary and surface energy. In addition, monitoring the amount of heat exchange with the molecular-dynamics thermostat gives the energy dissipated as heat in the system. The energetic analysis indicates that the majority of energy in a fast growing crack is dissipated as heat. This dissipation increases linearly at low speed, and faster than linear at speeds approaching 1/3 the Rayleigh wave speed when the crack tip becomes dynamically unstable producing periodic dislocation bursts until the crack is blunted.

An electro-mechanical impedance model of a cracked composite beam with adhesively bonded piezoelectric patches

NASA Astrophysics Data System (ADS)

Yan, Wei; Cai, J. B.; Chen, W. Q.

2011-01-01

A model of a laminated composite beam including multiple non-propagating part-through surface cracks as well as installed PZT transducers is presented based on the method of reverberation-ray matrix (MRRM) in this paper. Toward determining the local flexibility characteristics induced by the individual cracks, the concept of the massless rotational spring is applied. A Timoshenko beam theory is then used to simulate the behavior of the composite beam with open cracks. As a result, transverse shear and rotatory inertia effects are included in the model. Only one-dimensional axial vibration of the PZT wafer is considered and the imperfect interfacial bonding between PZT patches and the host beam is further investigated based on a Kelvin-type viscoelastic model. Then, an accurate electro-mechanical impedance (EMI) model can be established for crack detection in laminated beams. In this model, the effects of various parameters such as the ply-angle, fibre volume fraction, crack depth and position on the EMI signatures are highlighted. Furthermore, comparison with existent numerical results is presented to validate the present analysis.

Applications of infrared thermography for nondestructive testing of fatigue cracks in steel bridges

NASA Astrophysics Data System (ADS)

Sakagami, Takahide; Izumi, Yui; Kobayashi, Yoshihiro; Mizokami, Yoshiaki; Kawabata, Sunao

2014-05-01

In recent years, fatigue crack propagations in aged steel bridge which may lead to catastrophic structural failures have become a serious problem. For large-scale steel structures such as orthotropic steel decks in highway bridges, nondestructive inspection of deteriorations and fatigue damages are indispensable for securing their safety and for estimating their remaining strength. As conventional NDT techniques for steel bridges, visual testing, magnetic particle testing and ultrasonic testing have been commonly employed. However, these techniques are time- and labor- consuming techniques, because special equipment is required for inspection, such as scaffolding or a truck mount aerial work platform. In this paper, a new thermography NDT technique, which is based on temperature gap appeared on the surface of structural members due to thermal insulation effect of the crack, is developed for detection of fatigue cracks. The practicability of the developed technique is demonstrated by the field experiments for highway steel bridges in service. Detectable crack size and factors such as measurement time, season or spatial resolution which influence crack detectability are investigated.

Fractography, NDE, and fracture mechanics applications in failure analysis studies

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

Morin, C.R.; Shipley, R.J.; Wilkinson, J.A.

1994-10-01

While identification of the precise mode of a failure can lead logically to the underlying cause, a thorough failure investigation requires much more than just the identification of a specific metallurgical mechanism, for example, fatigue, creep, stress corrosion cracking, etc. Failures involving fracture provide good illustrations of this concept. An initial step in characterizing fracture surfaces is often the identification of an origin or origins. However, the analysis should not stop there. If the origin is associated with a discontinuity, the manner in which it was formed must also be addressed. The stresses that would have existed at the originmore » must be determined and compared with material properties to determine whether or not a crack should have initiated and propagated during normal operation. Many critical components are inspected throughout their lives by nondestructive methods. When a crack progresses to failure, its nondetection at earlier inspections must also be understood. Careful study of the fracture surface combined with crack growth analysis based on fracture mechanics can provide an estimate of the crack length at the times of previous inspections. An important issue often overlooked in such studies is how processing of parts during manufacture or rework affects the probability of detection of such cracks. The ultimate goal is to understand thoroughly the progression of the failure, to understand the root cause(s), and to design appropriate corrective action(s) to minimize recurrence.« less

Strain corrosion cracking in rpm sewer piping

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

Hopkins, S.W.; Wachob, H.F.; Duffner, D.H.

1993-12-31

Long term, aggressive environmental exposure can result in localized failure of large diameter, glass reinforced plastic mortar (RPM) piping. In order to evaluate the performance of the liner and glass reinforced matrix polyester resin, accelerated strain corrosion tests were performed on samples of RPM piping that had already experienced almost 15 years of service. To assess the sensitivity of RPM pipe to acidic environments and to correlate the fractography of the laboratory produced failures with the excavated crack, short segments of 8-inch and 48-inch diameter piping were statically loaded to produce various known surface strains. After preloading the specimens tomore » fixed strain levels, these samples were then exposed to sulfuric acid solutions having pH values of 2.7 and 4.7 and monitored as a function of time until failure. The resulting lifetimes were related to initial surface strains and showed a decreasing logarithmic relationship. Fractographic examination of the excavated crack revealed the typical strain corrosion fractography of glass fibers after almost a 1000 hour exposure at 1.3 % strain; similar fractographic observations were obtained from failed laboratory samples. At shorter times, failure appeared to be overload in nature and exhibited little, if any, timedependent fracture features. Fractographic examination of the excavated crack strongly indicated that the crack had been present for a significant time. The extremely aggressive environment had totally dissolved the exposed glass reinforcement. Based on the laboratory strain corrosion performance, the nature of the contained cracking, and fractography of the failed surface, cracking of the excavated RPM pipe was believed to be the result of an early overload failure that subsequently propagated slowly via strain corrosion in an extremely aggressive environment.« less

Evaluation of fracture toughness of ZrO 2 and Si 3N 4 engineering ceramics following CO 2 and fibre laser surface treatment

NASA Astrophysics Data System (ADS)

Shukla, P. P.; Lawrence, J.

2011-02-01

The fracture toughness property ( K1C) of Si 3N 4 and ZrO 2 engineering ceramics was investigated by means of CO 2 and a fibre laser surface treatment. Near surface modifications in the hardness were investigated by employing the Vickers indentation method. Crack lengths and the crack geometry were then measured by using the optical microscopy. A co-ordinate measuring machine was used to investigate the diamond indentations and to measure the lengths of the cracks. Thereafter, computational and analytical methods were employed to determine the K1C. An increase in the K1C of both ceramics was found by the CO 2 and the fibre laser surface treatment in comparison to the as-received surfaces. The K1C of the CO 2 laser radiated surface of the Si 3N 4 was over 3% higher in comparison to that of the fibre laser treated surface. This was by softening of the near surface layer of the Si 3N 4 which comprised of lowering of hardness, which in turn increased the crack resistance. The effects were not similar in ZrO 2 ceramic to that of the Si 3N 4 as the fibre laser radiation in this case had produced an increase of 34% compared to that of the CO 2 laser radiation. This occurred due to propagation of lower crack resulting from the Vickers indentation test during the fibre laser surface treatment which inherently affected the end K1C through an induced compressive stress layer. The K1C modification of the two ceramics treated by the CO 2 and the fibre laser was also believed to be influenced by the different laser wavelength and its absorption co-efficient, the beam delivery system as well as the differences in the brightness of the two lasers used.

Three-dimensional welding residual stresses evaluation based on the eigenstrain methodology via X-ray measurements at the surface

NASA Astrophysics Data System (ADS)

Ogawa, Masaru

2014-12-01

In order to assure structural integrity for operating welded structures, it is necessary to evaluate crack growth rate and crack propagation direction for each observed crack non-destructively. Here, three dimensional (3D) welding residual stresses must be evaluated to predict crack propagation. Today, X-ray diffraction is used and the ultrasonic method has been proposed as non-destructive method to measure residual stresses. However, it is impossible to determine residual stress distributions in the thickness direction. Although residual stresses through a depth of several tens of millimeters can be evaluated non-destructively by neutron diffraction, it cannot be used as an on-site measurement technique. This is because neutron diffraction is only available in special irradiation facilities. Author pays attention to the bead flush method based on the eigenstrain methodology. In this method, 3D welding residual stresses are calculated by an elastic Finite Element Method (FEM) analysis from eigenstrains which are evaluated by an inverse analysis from released strains by strain gauges in the removal of the reinforcement of the weld. Here, the removal of the excess metal can be regarded as non-destructive treatment because toe of weld which may become crack starters can be eliminated. The effectiveness of the method has been proven for welded plates and pipes even with relatively lower bead height. In actual measurements, stress evaluation accuracy becomes poorer because measured values of strain gauges are affected by processing strains on the machined surface. In the previous studies, the author has developed the bead flush method that is free from the influence of the affecting strains by using residual strains on surface by X-ray diffraction. However, stress evaluation accuracy is not good enough because of relatively poor measurement accuracy of X-ray diffraction. In this study, a method to improve the estimation accuracy of residual stresses in this method is formulated, and it is shown numerically that inner welding residual stresses can be estimated accurately from the residual strains measured by X-ray diffraction.

Phase-contrast x-ray imaging of microstructure and fatigue-crack propagation in single-crystal nickel-base superalloys

NASA Astrophysics Data System (ADS)

Husseini, Naji Sami

Single-crystal nickel-base superalloys are ubiquitous in demanding turbine-blade applications, and they owe their remarkable resilience to their dendritic, hierarchical microstructure and complex composition. During normal operations, they endure rapid low-stress vibrations that may initiate fatigue cracks. This failure mode in the very high-cycle regime is poorly understood, in part due to inadequate testing and diagnostic equipment. Phase-contrast imaging with coherent synchrotron x rays, however, is an emergent technique ideally suited for dynamic processes such as crack initiation and propagation. A specially designed portable ultrasonic-fatigue apparatus, coupled with x-ray radiography, allows real-time, in situ imaging while simulating service conditions. Three contrast mechanisms - absorption, diffraction, and phase contrast - span the immense breadth of microstructural features in superalloys. Absorption contrast is sensitive to composition and crack displacements, and diffraction contrast illuminates dislocation aggregates and crystallographic misorientations. Phase contrast enhances electron-density gradients and is particularly useful for fatigue-crack studies, sensitive to internal crack tips and openings less than one micrometer. Superalloy samples were imaged without external stresses to study microstructure and mosaicity. Maps of rhenium and tungsten concentrations revealed strong segregation to the center of dendrites, as manifested by absorption contrast. Though nominally single crystals, dendrites were misoriented from the bulk by a few degrees, as revealed by diffraction contrast. For dynamic studies of cyclic fatigue, superalloys were mounted in the portable ultrasonic-fatigue apparatus, subjected to a mean tensile stress of ˜50-150 MPa, and cycled in tension to initiate and propagate fatigue cracks. Radiographs were recorded every thousand cycles over the multimillion-cycle lifetime to measure micron-scale crack growth. Crack openings were very small, as determined by absorption and phase contrast, and suggested multiple fracture modes for propagation along {111} planes at room temperature, which was verified by finite element analysis. With increasing temperature, cracks became Mode I (perpendicular to the loading axis) in character and more sensitive to the microstructure. Advancing plastic zones ahead of crack tips altered the crystallographic quality, from which diffraction contrast anticipated initiation and propagation. These studies demonstrate the extreme sensitivity of x-ray radiography for detailed studies of superalloys and crack growth processes.

Real-time direct and diffraction X-ray imaging of irregular silicon wafer breakage.

PubMed

Rack, Alexander; Scheel, Mario; Danilewsky, Andreas N

2016-03-01

Fracture and breakage of single crystals, particularly of silicon wafers, are multi-scale problems: the crack tip starts propagating on an atomic scale with the breaking of chemical bonds, forms crack fronts through the crystal on the micrometre scale and ends macroscopically in catastrophic wafer shattering. Total wafer breakage is a severe problem for the semiconductor industry, not only during handling but also during temperature treatments, leading to million-dollar costs per annum in a device production line. Knowledge of the relevant dynamics governing perfect cleavage along the {111} or {110} faces, and of the deflection into higher indexed {hkl} faces of higher energy, is scarce due to the high velocity of the process. Imaging techniques are commonly limited to depicting only the state of a wafer before the crack and in the final state. This paper presents, for the first time, in situ high-speed crack propagation under thermal stress, imaged simultaneously in direct transmission and diffraction X-ray imaging. It shows how the propagating crack tip and the related strain field can be tracked in the phase-contrast and diffracted images, respectively. Movies with a time resolution of microseconds per frame reveal that the strain and crack tip do not propagate continuously or at a constant speed. Jumps in the crack tip position indicate pinning of the crack tip for about 1-2 ms followed by jumps faster than 2-6 m s(-1), leading to a macroscopically observed average velocity of 0.028-0.055 m s(-1). The presented results also give a proof of concept that the described X-ray technique is compatible with studying ultra-fast cracks up to the speed of sound.

Ductile flow by water-assisted cataclasis

NASA Astrophysics Data System (ADS)

den Brok, Bas

2003-04-01

In the presence of water otherwise brittle materials may deform macroscopically ductile by water-assisted cataclastic creep. This is possible as long as (i) solubility is high enough, so that stress-corrosion can occur, and (ii) local stress is low enough, to that fracturing remains subcritical. Water-assisted cataclastic creep (WACC) may play an important role in the middle and lower continental crust where mineral solubilities are high and stresses low. WACC is a poorly understood deformation process. Experiments were performed on very soluble brittle salts (Na-chlorate; K-alum) to study microstructure development by WACC. The experiments were carried out at room temperature and atmospheric pressure in a small see-through vessel. In this way the cataclastic deformation process could be studied "in-situ" under the microscope. Crystals were loaded in the presence of saturated salt solution. It appeared that originally straight mineral surfaces were instable when kept under stress. Grooves (or channels) slowly developed in the surface by local dissolution. These grooves behave like so-called Grinfeld instabilities. They develop because the energy of a grooved surface under stress is lower than the energy of a straight surface under stress. The grooves may deepen and turn into subcritical cracks when local stress further increases. These cracks propagate slowly. They propagate parallel to sigma1 but also at an angle and even perpendicular to sigma1, often following crystallographically controlled directions. The fractures mostly change direction while propagating, locally making turns of more than 180 degrees. Irregular fracture fragments thus develop. The fractures may migrate sideways (as with grain bounday migration) probably by solution-redeposition driven by differences in stress between both sides of the fracture. Thus the shape of the fragments changes. The size of the fracture fragments seems to be controlled by the distance of the grooves, which decreases with increasing stress.

Recrystallization-Induced Surface Cracks of Carbon Ions Irradiated 6H-SiC after Annealing

PubMed Central

Ye, Chao; Ran, Guang; Zhou, Wei; Shen, Qiang; Feng, Qijie; Lin, Jianxin

2017-01-01

Single crystal 6H-SiC wafers with 4° off-axis [0001] orientation were irradiated with carbon ions and then annealed at 900 °C for different time periods. The microstructure and surface morphology of these samples were investigated by grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Ion irradiation induced SiC amorphization, but the surface was smooth and did not have special structures. During the annealing process, the amorphous SiC was recrystallized to form columnar crystals that had a large amount of twin structures. The longer the annealing time was, the greater the amount of recrystallized SiC would be. The recrystallization volume fraction was accorded with the law of the Johnson–Mehl–Avrami equation. The surface morphology consisted of tiny pieces with an average width of approximately 30 nm in the annealed SiC. The volume shrinkage of irradiated SiC layer and the anisotropy of newly born crystals during annealing process produced internal stress and then induced not only a large number of dislocation walls in the non-irradiated layer but also the initiation and propagation of the cracks. The direction of dislocation walls was perpendicular to the growth direction of the columnar crystal. The longer the annealing time was, the larger the length and width of the formed crack would be. A quantitative model of the crack growth was provided to calculate the length and width of the cracks at a given annealing time. PMID:29068408

Fundamental Investigation of Fatigue Crack Growth Retardation in Aluminum Alloys

DTIC Science & Technology

1976-09-01

Fatigue Crack Propagation in 2024 -T3 Aluminum Alloy , " ASTM STP 536, p. 115, 1973. 9. J. Schijve, " Effect of Load Sequences...Hertzberg, " Effect of Multiple Over- loads on Fatigue Crack Propagation in 2024 -T3 Aluminum Alloy , " ASTM STP-536, p. 115, 1973. 9. J. Schijve... Effect of Thickness on Retardation Behavior of 7075 and 2024 Aluminum Alloys .......... 185 vi LIST OF ILLUSTRATIONS FIGURE PAGE 1 SEN

Effect of amorphous lamella on the crack propagation behavior of crystalline Mg/amorphous Mg-Al nanocomposites

NASA Astrophysics Data System (ADS)

Hai-Yang, Song; Yu-Long, Li

2016-02-01

The effects of amorphous lamella on the crack propagation behavior in crystalline/amorphous (C/A) Mg/Mg-Al nanocomposites under tensile loading are investigated using the molecular dynamics simulation method. The sample with an initial crack of orientation [0001] is considered here. For the nano-monocrystal Mg, the crack growth exhibits brittle cleavage. However, for the C/A Mg/Mg-Al nanocomposites, the ‘double hump’ behavior can be observed in all the stress-strain curves regardless of the amorphous lamella thickness. The results indicate that the amorphous lamella plays a critical role in the crack deformation, and it can effectively resist the crack propagation. The above mentioned crack deformation behaviors are also disclosed and analyzed in the present work. The results here provide a strategy for designing the high-performance hexagonal-close-packed metal and alloy materials. Project supported by the National Natural Science Foundation of China (Grant Nos. 11372256 and 11572259), the 111 Project (Grant No. B07050), the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-12-1046), and the Program for New Scientific and Technological Star of Shaanxi Province, China (Grant No. 2012KJXX-39).

Corrosion fatigue studies on a bulk glassy Zr-based alloy under three-point bending

NASA Astrophysics Data System (ADS)

Grell, Daniel; Wilkin, Yannic; Gostin, Petre F.; Gebert, Annett; Kerscher, Eberhard

2016-12-01

Corrosion fatigue (CF) tests were carried out on bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 (Vitreloy 105) samples under load-controlled three-point bending conditions with a load ratio of R = 0.1 in 0.01 M Na2SO4 + 0.01 M NaCl electrolyte. During cyclic testing, the bar-shaped specimens were polarized in situ at constant potentials and the current was monitored. Three different anodic potentials within the interval between the pitting potential EP and the repassivation potential ER, and three different load amplitudes were applied. In some cases, in situ microscopic observations revealed the formation of black corrosion products in the vicinity of the crack tip during anodic polarization. Fractographic analysis revealed a clear distinction between two modes of crack growth characterized by smooth dissolution induced regions on the one hand and slim fast fracture areas on the other hand. Both alternating features contributed to a broad striated corrosion fatigue fracture surface. Moreover, further fatigue tests were carried out under free corrosion conditions yielding additional information on crack initiation and crack propagation period by means of the open circuit potential (OCP) changes. Thereby, a slight increase in OCP was detected after rupture of the passive layer due to bare metal exposed to the electrolyte. The electrochemical response increased continuously according to stable crack propagation until fracture occurred. Finally, the fracture surfaces of the corrosion fatigue samples were investigated by energy dispersive X-ray with the objective of analyzing the elemental distribution after anodic dissolution. Interestingly, anodic polarization at a near repassivation potential of -50 mV vs. SCE (Saturated Calomel Electrode, E = 0.241 V vs. SHE, Standard Hydrogen Electrode) led to favorable effects on the fatigue lifetime. In conclusion, all results are conflated to a corrosion fatigue model for bulk glassy Vitreloy 105 under anodic polarization in chloride-containing electrolyte and compared to the previously proposed stress corrosion mechanisms under similar conditions.

Investigation of deformation mechanisms of staggered nanocomposites using molecular dynamics

NASA Astrophysics Data System (ADS)

Mathiazhagan, S.; Anup, S.

2016-08-01

Biological materials with nanostructure of regularly or stair-wise staggered arrangements of hard platelets reinforced in a soft protein matrix have superior mechanical properties. Applications of these nanostructures to ceramic matrix composites could enhance their toughness. Using molecular dynamics simulations, mechanical behaviour of the bio-inspired nanocomposites is studied. Regularly staggered model shows better flow behaviour compared to stair-wise staggered model due to the symmetrical crack propagation along the interface. Though higher stiffness and strength are obtained for stair-wise staggered models, rapid crack propagation reduces the toughness. Arresting this crack propagation could lead to superior mechanical properties in stair-wise staggered models.

Fatigue crack propagation behavior of ultrahigh molecular weight polyethylene.

PubMed

Connelly, G M; Rimnac, C M; Wright, T M; Hertzberg, R W; Manson, J A

1984-01-01

The relative fatigue crack propagation resistance of plain and carbon fiber-reinforced ultrahigh molecular weight polyethylene (UHMWPE) was determined from cyclic loading tests performed on compact tension specimens machined from the tibial components of total knee prostheses. Both materials were characterized by dynamic mechanical spectroscopy, X-ray diffraction, and differential scanning calorimetry. The cyclic tests used loading in laboratory air at 5 Hz using a sinusoidal wave form. Dynamic mechanical spectroscopy showed that the reinforced UHMWPE had a higher elastic storage modulus than the plain UHMWPE, whereas X-ray diffraction and differential scanning calorimetry showed that the percent crystallinity and degree of order in the crystalline regions were similar for the two materials. Fatigue crack propagation in both materials proved to be very sensitive to small changes in the applied cyclic stress intensity range. A 10% increase in stress intensity resulted in approximately an order of magnitude increase in fatigue crack growth rate. The fatigue crack propagation resistance of the reinforced UHMWPE was found to be significantly worse than that of the plain UHMWPE. This result was attributed to poor bonding between the carbon fibers and the UHMWPE matrix and the ductile nature of the matrix itself.

Crack Extension and Possibility of Debonding in Encapsulation-Based Self-Healing Materials.

PubMed

Li, Wenting; Jiang, Zhengwu; Yang, Zhenghong

2017-05-27

The breakage of capsules upon crack propagation is crucial for achieving crack healing in encapsulation-based self-healing materials. A mesomechanical model was developed in this study to simulate the process of crack propagation in a matrix and the potential of debonding. The model used the extended finite element method (XFEM) combined with a cohesive zone model (CZM) in a two-dimensional (2D) configuration. The configuration consisted of an infinite matrix with an embedded crack and a capsule nearby, all subjected to a uniaxial remote tensile load. A parametric study was performed to investigate the effect of geometry, elastic parameters and fracture properties on the fracture response of the system. The results indicated that the effect of the capsule wall on the fracture behavior of the matrix is insignificant for t c / R c ≤ 0.05. The matrix strength influenced the ultimate crack length, while the Young's modulus ratio E c / E m only affected the rate of crack propagation. The potential for capsule breakage or debonding was dependent on the comparative strength between capsule and interface (S c /S int ), provided the crack could reach the capsule. The critical value of S c ,cr /S int,cr was obtained using this model for materials design.

Tensile strength of composite sheets with unidirectional stringers and crack-like damage

NASA Technical Reports Server (NTRS)

Poe, C. C., Jr.

1984-01-01

The damage tolerance characteristics of metal tension panels with riveted and bonded stringers are well known. The stringers arrest unstable cracks and retard propagation of fatigue cracks. Residual strengths and fatigue lives are considerably greater than those of unstiffened or integrally stiffened sheets. The damage tolerance of composite sheets with bonded composite stringers loaded in tension was determined. Cracks in composites do not readily propagate in fatigue, at least not through fibers. Moreover, the residual strength of notched composites is sometimes even increased by fatigue loading. Therefore, the residual strength aspect of damage tolerance, and not fatigue crack propagation, was investigated. About 50 graphite/epoxy composite panels were made with two sheet layups and several stringer configurations. Crack-like slots were cut in the middle of the panels to simulate damage. The panels were instrumented and monotonically loaded in tension to failure. The tests indicate that the composite panels have considerable damage tolerance, much like metal panels. The stringers arrested cracks that ran from the crack-like slots, and the residual strengths were considerably greater than those of unstiffened composite sheets. A stress intensity factor analysis was developed to predict the failing strains of the stiffened panels. Using the analysis, a single design curve was produced for composite sheets with bonded stringers of any configuration.

Remote monitoring and prognosis of fatigue cracking in steel bridges with acoustic emission

NASA Astrophysics Data System (ADS)

Yu, Jianguo Peter; Ziehl, Paul; Pollock, Adrian

2011-04-01

Acoustic emission (AE) monitoring is desirable to nondestructively detect fatigue damage in steel bridges. Investigations of the relationship between AE signals and crack growth behavior are of paramount importance prior to the widespread application of passive piezoelectric sensing for monitoring of fatigue crack propagation in steel bridges. Tests have been performed to detect AE from fatigue cracks in A572G50 steel. Noise induced AE signals were filtered based on friction emission tests, loading pattern, and a combined approach involving Swansong II filters and investigation of waveforms. The filtering methods based on friction emission tests and load pattern are of interest to the field evaluation using sparse datasets. The combined approach is suitable for data filtering and interpretation of actual field tests. The pattern recognition program NOESIS (Envirocoustics) was utilized for the evaluation of AE data quality. AE parameters are associated with crack length, crack growth rate, maximum stress intensity and stress intensity range. It is shown that AE hits, counts, absolute energy, and signal strength are able to provide warnings at the critical cracking level where cracking progresses from stage II (stable propagation) to stage III (unstable propagation which may result in failure). Absolute energy rate and signal strength rate may be better than count rate to assess the remaining fatigue life of inservice steel bridges.

Isolated Mesoporous Microstructures Prepared by Stress Localization-Induced Crack Manipulation.

PubMed

Wooh, Sanghyuk; Lee, Soojin; Lee, Yunchan; Ryu, Ji Ho; Lee, Won Bo; Yoon, Hyunsik; Char, Kookheon

2016-09-22

Cracks observed in brittle materials are mostly regarded as defects or failures. However, they can be a valuable tool when implemented in a controlled way. Here, we introduce a strategy to control the crack propagation of mesoporous micropatterns (prisms and pyramids), which leads to the isolation of well-defined microstructures. Mesoporous micropatterns were fabricated by the soft imprinting technique with wet TiO 2 nanoparticle (NP) pastes, followed by sintering to remove organic components. Since the volume of the paste significantly shrinks during the sintering step, stress is localized at the edge of micropatterns, in good agreement with finite element method simulations, creating well-defined cracks and their propagation. It was demonstrated that the degree of stress localization is determined by the thickness of residual layers, NP size, and heating rate. After controlled crack propagation and delamination of microparticles from the substrates, mesoporous microwires and microparticles were successfully produced and functionalized from the isolated mesoporous prisms and pyramids. The method proposed in this study for controlled crack manipulation and delamination opens a door for straightforward and economical fabrication of well-defined mesoporous microparticles.

Analysis of crack propagation in roller bearings using the boundary integral equation method - A mixed-mode loading problem

NASA Technical Reports Server (NTRS)

Ghosn, L. J.

1988-01-01

Crack propagation in a rotating inner raceway of a high-speed roller bearing is analyzed using the boundary integral method. The model consists of an edge plate under plane strain condition upon which varying Hertzian stress fields are superimposed. A multidomain boundary integral equation using quadratic elements was written to determine the stress intensity factors KI and KII at the crack tip for various roller positions. The multidomain formulation allows the two faces of the crack to be modeled in two different subregions, making it possible to analyze crack closure when the roller is positioned on or close to the crack line. KI and KII stress intensity factors along any direction were computed. These calculations permit determination of crack growth direction along which the average KI times the alternating KI is maximum.

Degradation Mechanisms of Electrochemically Cycled Graphite Anodes in Lithium-ion Cells

NASA Astrophysics Data System (ADS)

Bhattacharya, Sandeep

This research is aimed at developing advanced characterization methods for studying the surface and subsurface damage in Li-ion battery anodes made of polycrystalline graphite and identifying the degradation mechanisms that cause loss of electrochemical capacity. Understanding microstructural aspects of the graphite electrode degradation mechanisms during charging and discharging of Li-ion batteries is of key importance in order to design durable anodes with high capacity. An in-situ system was constructed using an electrochemical cell with an observation window, a large depth-of-field digital microscope and a micro-Raman spectrometer. It was revealed that electrode damage by removal of the surface graphite fragments of 5-10 mum size is the most intense during the first cycle that led to a drastic capacity drop. Once a solid electrolyte interphase (SEI) layer covered the electrode surface, the rate of graphite particle loss decreased. Yet, a gradual loss of capacity continued by the formation of interlayer cracks adjacent to SEI/graphite interfaces. Deposition of co-intercalation compounds, LiC6, Li2CO3 and Li2O, near the crack tips caused partial closure of propagating graphite cracks during cycling and reduced the crack growth rate. Bridging of crack faces by delaminated graphite layers also retarded crack propagation. The microstructure of the SEI layer, formed by electrochemical reduction of the ethylene carbonate based electrolyte, consisted of ˜5-20 nm sized crystalline domains (containing Li2CO3, Li2O 2 and nano-sized graphite fragments) dispersed in an amorphous matrix. During the SEI formation, two regimes of Li-ion diffusion were identified at the electrode/electrolyte interface depending on the applied voltage scan rate (dV/dt). A low Li-ion diffusion coefficient ( DLi+) at dV/dt < 0.05 mVs-1 produced a tubular SEI that uniformly covered the graphite surface and prevented damage at 25°C. At 60°C, a high D Li+ formed a Li2CO3-enriched SEI and ensued a 28% increase in the battery capacity at 25°C. On correlating the microscopic information to the electrochemical performance, novel Li2CO3-coated electrodes were fabricated that were durable. The SEI formed on pre-treated electrodes reduced the strain in the graphite lattice from 0.4% (for uncoated electrodes) to 0.1%, facilitated Li-ion diffusion and hence improved the capacity retention of Li-ion batteries during long-term cycling.

Effects of Outlets on Cracking Risk and Integral Stability of Super-High Arch Dams

PubMed Central

Hu, Hang

2014-01-01

In this paper, case study on outlet cracking is first conducted for the Goupitan and Xiaowan arch dams. A nonlinear FEM method is then implemented to study effects of the outlets on integral stability of the Xiluodu arch dam under two loading conditions, i.e., normal loading and overloading conditions. On the basis of the case study and the numerical modelling, the outlet cracking mechanism, risk, and corresponding reinforcement measures are discussed. Furthermore, the numerical simulation reveals that (1) under the normal loading conditions, the optimal distribution of the outlets will contribute to the tensile stress release in the local zone of the dam stream surface and decrease the outlet cracking risk during the operation period. (2) Under the overloading conditions, the cracks initiate around the outlets, then propagate along the horizontal direction, and finally coalesce with those in adjacent outlets, where the yield zone of the dam has a shape of butterfly. Throughout this study, a dam outlet cracking risk control and reinforcement principle is proposed to optimize the outlet design, select the appropriate concrete material, strengthen the temperature control during construction period, design reasonable impounding scheme, and repair the cracks according to their classification. PMID:25152907

Nanoscale Stress-Corrosion of Geomaterials in Aqueous Solutions

NASA Astrophysics Data System (ADS)

Criscenti, L. J.; Rimsza, J. M.; Matteo, E. N.; Jones, R. E.

2017-12-01

Predicting subcritical crack propagation in low-permeability geo-materials is an unsolved problem crucial to assessing shale caprocks at CO2 sequestration sites, and controlling fracturing for gas and oil extraction. Experiments indicate that chemical reactions at fluid-material interfaces play a major role in subcritical crack growth by weakening the material and altering crack nucleation and growth rates. However, understanding subsurface fracture has been hindered by a lack of understanding of the mechanisms relating chemical environment to mechanical outcome, and a lack of capability directly linking atomistic insight to macroscale observables. We are using both molecular simulation and experiment to develop an atomistic-level understanding of the chemical-mechanical coupling that controls subcritical crack propagation. We are investigating fracture of isotropic silica glass in different environments (air, distilled water, and Na+-rich solutions) and will extend our research to include clay minerals in shales. Molecular simulations are performed with ReaxFF, a reactive force field that allows for explicit modeling of bond breaking and formation processes during crack propagation. A coarse-graining method produces calculated fracture toughness values from the atomistic data. We are performing double cleavage drilled compression (DCDC) experiments in aqueous environmental chambers and monitoring crack propagation with either a confocal or atomic force microscope. Our results show that silica fracture toughness decreases as the environment changes from air to distilled water to Na+-rich solutions. These results suggest that our newly developed computational and experimental techniques can be used to investigate the impact of fluid composition on crack growth in geo-materials and that we will be able to use these methods to understand coupled chemo-mechanical processes and predict crack propagation in shale minerals. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

Physically Based Failure Criteria for Transverse Matrix Cracking

NASA Technical Reports Server (NTRS)

Davila, Carlos G.; Camanho, Pedro P.

2003-01-01

A criterion for matrix failure of laminated composite plies in transverse tension and in-plane shear is developed by examining the mechanics of transverse matrix crack growth. Matrix cracks are assumed to initiate from manufacturing defects and can propagate within planes parallel to the fiber direction and normal to the ply mid-plane. Fracture mechanics models of cracks in unidirectional laminates, embedded plies and outer plies are developed to determine the onset and direction of propagation for unstable crack growth. The models for each ply configuration relate ply thickness and ply toughness to the corresponding in-situ ply strength. Calculated results for several materials are shown to correlate well with experimental results.

Effect of anisotropy and texture on the low cycle fatigue behavior of Inconel 718 processed via electron beam melting

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

Kirka, Michael M.; Greeley, Duncan A.; Hawkins, Charles S.

Here in this study, the impact of texture (columnar/equiax grain structure) and influence of material orientation on the low cycle fatigue (LCF) behavior of hot isostatic pressed (HIP) and heat-treated Inconel 718 fabricated through electron beam melting (EBM) is investigated. Material was tested both parallel and perpendicular (transverse) to the build direction. In all instances, the EBM HIP and heat-treated Inconel 718 performed similarly or exceeded the LCF life of wrought Inconel 718 plate and bar stock under fully reversed strain-controlled loading at 650 °C. Amongst the textures, the columnar grains oriented parallel to the build direction exhibited the highestmore » life on average compared to the transverse columnar and equiax EBM material. Further, in relation to the reference wrought material the parallel columnar grain material exhibited a greater life. While a negligible life difference was observed in the equiax grained material between the two orientations, a consistently lower accumulated inelastic strain was measured for the material loaded parallel to the build direction than the transverse orientation. Failure of the parallel columnar material occurred in a transgranular manner with cracks emanating from the surface whereas the transverse columnar material failed in a intergranular manner, with crack growth occurring through repeated rupture of oxide at the crack-tip. Finally, in the case of the equiax material, an influence of material orientation was not observed on the failure mechanism with crack propagation occurring through a combination of debonded/cracked carbides and void formation along twin boundaries resulting in a mixture of intergranular and transgranular crack propagation.« less

Effect of anisotropy and texture on the low cycle fatigue behavior of Inconel 718 processed via electron beam melting

DOE PAGES

Kirka, Michael M.; Greeley, Duncan A.; Hawkins, Charles S.; ...

2017-09-11

Here in this study, the impact of texture (columnar/equiax grain structure) and influence of material orientation on the low cycle fatigue (LCF) behavior of hot isostatic pressed (HIP) and heat-treated Inconel 718 fabricated through electron beam melting (EBM) is investigated. Material was tested both parallel and perpendicular (transverse) to the build direction. In all instances, the EBM HIP and heat-treated Inconel 718 performed similarly or exceeded the LCF life of wrought Inconel 718 plate and bar stock under fully reversed strain-controlled loading at 650 °C. Amongst the textures, the columnar grains oriented parallel to the build direction exhibited the highestmore » life on average compared to the transverse columnar and equiax EBM material. Further, in relation to the reference wrought material the parallel columnar grain material exhibited a greater life. While a negligible life difference was observed in the equiax grained material between the two orientations, a consistently lower accumulated inelastic strain was measured for the material loaded parallel to the build direction than the transverse orientation. Failure of the parallel columnar material occurred in a transgranular manner with cracks emanating from the surface whereas the transverse columnar material failed in a intergranular manner, with crack growth occurring through repeated rupture of oxide at the crack-tip. Finally, in the case of the equiax material, an influence of material orientation was not observed on the failure mechanism with crack propagation occurring through a combination of debonded/cracked carbides and void formation along twin boundaries resulting in a mixture of intergranular and transgranular crack propagation.« less

Overload effect and fatigue crack propagation in amorphous metallic alloys

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

Chaki, T.K.; Li, J.C.M.

1984-07-01

Fatigue crack propagation in amorphous metals has an overload effect which usually increases with the number of overload cycles. The variation of overload effect with delta K is explained by the size of the plastic zone which depends on delta K. A comparison of the spacing between striations and da/dN shows that the crack jumps a step about every hundred cycles. The featureless region is probably due to shear fracture along a shear band during overload. Both crack tip blunting and branching occur during the application of overload. Work hardening is not a necessary factor for the overloading effect.

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.

Delamination initiated by a defect

NASA Astrophysics Data System (ADS)

Biel, A.; Toftegaard, H.

2016-07-01

Composite materials in wind turbines are mainly joined with adhesives. Adhesive joining is preferable since it distributes the stresses over a larger area. This study shows how a defect can influence the fracture behaviour of adhesively joined composite. Repeated experiments are performed using double cantilever beam specimens loaded with bending moments. The specimens consist of two 8 mm thick GFRP-laminates which are joined by a 3 mm thick epoxy adhesive. A thin foil close to one of the laminates is used to start the crack. For some of the specimens a defect is created by an initial load-unload operation. During this operation, a clamp is used in order to prevent crack propagation in the main direction. For the specimens without defect, the crack propagates in the middle of the adhesive layer. For the specimens with defect, the crack directly deviates into the laminate. After about 25 mm propagation in the laminate, the crack returns to the adhesive. Compared to the adhesive the fracture energy for the laminate is significantly higher.

Damage properties simulations of self-healing composites.

PubMed

Chen, Cheng; Ji, Hongwei; Wang, Huaiwen

2013-10-01

Self-healing materials are inspired by biological systems in which damage triggers an autonomic healing response. The damage properties of a self-healing polymer composite were investigated by numerical simulation in this paper. Unit cell models with single-edge centered crack and single-edge off-centered crack were employed to investigate the damage initiation and crack evolution by the extended finite element method (XFEM) modeling. The effect of microcapsule's Young's modulus on composites was investigated. Result indicates the microcapsule's Young's modulus has little effect on the unit cell's carrying capacity. It was found that during the crack propagation process, its direction is attracted toward the microcapsules, which makes it helpful for the microcapsules to be ruptured by the propagating crack fronts resulting in release of the healing agent into the cracks by capillary action.

Fractography and mechanisms of environmentally enhanced fatigue crack propagation of a reactor pressure vessel steel

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

Torronen, K.; Kemppainen, M.

1981-10-01

This paper describes the findings and interpretations of the fractographic survey conducted for the International Cyclic Crack Growth Rate (ICCGR) cooperative group round-robin specimens. Specimens of A533B pressure vessel steel were tested at several laboratories in the United States and elsewhere with the same nominal test parameters. A rather wide scatter of the results was found. A fractographic and metallographic survey was carried out in order to clarify the scatter and to evaluate the micromechanism of the crack growth. The fractographic findings are reported in detail and correlated to the crack growth behavior. A hydrogen-assisted crack propagation mechanism based onmore » the fractography is proposed and applied to the observed crack growth behavior.« less

Mitigation strategies for reflective cracking in pavements : [research project capsule].

DOT National Transportation Integrated Search

2013-12-01

Refl ection cracks are caused by discontinuities (cracks or joints) in underlying layers, : which propagate through hot-mix asphalt (HMA) overlay due to continuous movement : at the crack prompted by thermal expansion and traffi c loading. If the new...

Structural integrity test and assessment.

NASA Technical Reports Server (NTRS)

Suggs, F.; Poe, R.; Sannicandro, R.

1972-01-01

The feasibility of using an ultrasonic system on board the Space Shuttle Orbiter to facilitate structural evaluation and assessment was studied. Two factors are considered that could limit the capability of an ultrasonic system: (1) the effect of structure configuration and (2) the noise generated during vehicle launch. Results of the study indicate that although the structural configuration has direct bearing on sound propagation, strategic location of transducers will still permit flaw detection. The ultrasonic response data show that a severe acoustic environment does not interfere significantly with either propagation and reflection of surface waves or detection of crack-like flaws in the structure.

Influence of mean stress on fatigue strength of ferritic-pearlite ductile cast iron with small defects

NASA Astrophysics Data System (ADS)

Deguchi, T.; Kim, H. J.; Ikeda, T.; Yanase, K.

2017-05-01

Because of their excellent mechanical properties, low cost and good workability, the application of ductile cast iron has been increased in various industries such as the automotive, construction and rail industries. For safety designing of the ductile cast iron component, it is necessary to understand the effect of stress ratio, R, on fatigue limit of ductile cast iron in the presence of small defects. Correspondingly in this study, rotating bending fatigue tests at R = -1 and tension-compression fatigue tests at R = -1 and 0.1 were performed by using a ferritic-pearlitic ductile cast iron. To study the effects of small defects, we introduced a small drilled hole at surface of a specimen. The diameter and depth of a drilled hole were 50, 200 and 500 μm, respectively. The non-propagating cracks emanating from graphite particles and holes edge were observed at fatigue limit, irrespective of the value of stress ratio. From the microscopic observation of crack propagation behavior, it can be concluded that the fatigue limit is determined by the threshold condition for propagation of a small crack. It was found that the effect of stress ratio on the fatigue limit of ductile cast iron with small defects can be successfully predicted based on \\sqrt {area} parameter model. Furthermore, a use of the tensile strength, σ B, instead of the Vickers hardness, HV, is effective for fatigue limit prediction.

Fatigue of DIN 1.4914 martensitic stainless steel in a hydrogen environment

NASA Astrophysics Data System (ADS)

Shakib, J. I.; Ullmaier, H.; Little, E. A.; Faulkner, R. G.; Schmilz, W.; Chung, T. E.

1994-09-01

Fatigue tests at room temperature in vacuum, air and hydrogen have been carried out on specimens of DIN 1.4914 martensitic stainless steel in load-controlled, push-pull type experiments. Fatigue lifetimes in hydrogen are significantly lower than in both vacuum and air and the degradation is enhanced by lowering the test frequency or introducing hold times into the tension half-cycle. Fractographic examinations reveal hydrogen embrittlement effects in the form of internal cracking between fatigue striations together with surface modifications, particularly at low stress amplitudes. It is suggested that gaseous hydrogen can influence both fatigue crack initiation and propagation events in martensitic steels.

Structural Durability of Damaged Metallic Panel Repaired with Composite Patches

NASA Technical Reports Server (NTRS)

Minnetyan, Levon; Chamis, Christos C.

1997-01-01

Structural durability/damage tolerance characteristics of an aluminum tension specimen possessing a short crack and repaired by applying a fiber composite surface patch is investigated via computational simulation. The composite patch is made of graphite/epoxy plies with various layups. An integrated computer code that accounts for all possible failure modes is utilized for the simulation of combined fiber-composite/aluminum structural degradation under loading. Damage initiation, growth, accumulation, and propagation to structural fracture are included in the simulation. Results show the structural degradation stages due to tensile loading and illustrate the use of computational simulation for the investigation of a composite patch repaired cracked metallic panel.

Evolution of Residual-Strain Distribution through an Overload-Induced Retardation Period during Fatigue Crack Growth

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

Lee, S. Y.; Sun, Yinan; An, Ke

2010-01-01

Neutron diffraction was employed to investigate the crack-growth retardation phenomenon after a single tensile overload by mapping both one-dimensional and two-dimensional residual-strain distributions around the crack tip in a series of compact-tension specimens representing various crack-growth stages through an overload-induced retardation period. The results clearly show a large compressive residual-strain field near the crack tip immediately after the overload. As the fatigue crack propagates through the overload-induced plastic zone, the compressive residual strains are gradually relaxed, and a new compressive residual-strain field is developed around the propagating crack tip, illustrating that the subsequent fatigue-induced plastic zone grows out of themore » large plastic zone caused by the overloading. The relationship between the overload-induced plastic zone and subsequent fatigue-induced plastic zone, and its influence on the residual-strain distributions in the perturbed plastic zone are discussed.« less

Cost effective prevention of reflective cracking of composite pavement : research project capsule.

DOT National Transportation Integrated Search

2008-12-01

Reflection cracks are caused by discontinuities (cracks or joints) in underlying layers, which : propagate through a hot-mix asphalt (HMA) overlay due to continuous movement at the crack : prompted by thermal and traffic loading. If the new overlay i...

Ice-coupled wave propagation across an abrupt change in ice rigidity, density, or thickness

NASA Astrophysics Data System (ADS)

Barrett, Murray D.; Squire, Vernon A.

1996-09-01

The model of Fox and Squire [1990, 1991, 1994], which discusses the oblique propagation of surface gravity waves from the open sea into an ice sheet of constant thickness and properties, is augmented to include propagation across an abrupt transition of properties within a continuous ice sheet or across two dissimilar ice sheets that abut one another but are free to move independently. Rigidity, thickness, and/or density may change across the transition, allowing, for example, the modeling of ice-coupled waves into, across, and out of refrozen leads and polynyas, across cracks, and through coherent pressure ridges. Reflection and transmission behavior is reported for various changes in properties under both types of transition conditions.

The initiation and persistence of cracks in Enceladus' ice shell

NASA Astrophysics Data System (ADS)

Rudolph, M. L.; Jordan, J.; Manga, M.; Hawkins, E. K.; Grannan, A. M.; Reinhard, A.; Farough, A.; Mittal, T.; Hernandez, J. A.

2016-12-01

The eruption of water from a global ocean underlying Enceladus' ice shell requires; i. a mechanism to create stresses sufficient to produce cracks that reach the ocean, ii. that the ascent of water through the crack must be fast enough to keep the crack from freezing. We develop models for the evolution of stresses in the ice shell and overpressure in the ocean, the propagation of cracks into the ice shell, and the melting of ice caused by the eruption of water through the cracks. We show that modest cooling of Enceladus' interior can produce extensional stresses in the ice shell sufficient to overcome the tensile strength of ice. We show that the resultant ice shell cracks can penetrate to depths greater than 10 km. Cracks of 10 km are required to reach the interior oceans of Enceladus in the polar regions. After crack formation, we show that the present eruption rate is sufficient to keep cracks from freezing below the water-table, at which water boils and subsequently erupts. The ascent of warm water from Enceladus' ocean widens the cracks and thins the ice shell in the South Polar Terrain (SPT). Model predictions show that a crack with the minimum, sufficient heat flow to persist without freezing, would thin the surrounding ice shell by about a factor of two. This calculation for heat flow is consistent with observed heat fluxes at the surface and recent inferences of the ice shell thickness in the SPT based on the shape and gravity of Enceladus.

Material heterogeneity in cancellous bone promotes deformation recovery after mechanical failure

PubMed Central

Torres, Ashley M.; Matheny, Jonathan B.; Keaveny, Tony M.; Taylor, David; Rimnac, Clare M.; Hernandez, Christopher J.

2016-01-01

Many natural structures use a foam core and solid outer shell to achieve high strength and stiffness with relatively small amounts of mass. Biological foams, however, must also resist crack growth. The process of crack propagation within the struts of a foam is not well understood and is complicated by the foam microstructure. We demonstrate that in cancellous bone, the foam-like component of whole bones, damage propagation during cyclic loading is dictated not by local tissue stresses but by heterogeneity of material properties associated with increased ductility of strut surfaces. The increase in surface ductility is unexpected because it is the opposite pattern generated by surface treatments to increase fatigue life in man-made materials, which often result in reduced surface ductility. We show that the more ductile surfaces of cancellous bone are a result of reduced accumulation of advanced glycation end products compared with the strut interior. Damage is therefore likely to accumulate in strut centers making cancellous bone more tolerant of stress concentrations at strut surfaces. Hence, the structure is able to recover more deformation after failure and return to a closer approximation of its original shape. Increased recovery of deformation is a passive mechanism seen in biology for setting a broken bone that allows for a better approximation of initial shape during healing processes and is likely the most important mechanical function. Our findings suggest a previously unidentified biomimetic design strategy in which tissue level material heterogeneity in foams can be used to improve deformation recovery after failure. PMID:26929343

Summary of available data for estimating chloride-induced SCC crack growth rates for 304/316 stainless steel.

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

Bryan, Charles R.; Enos, David

The majority of existing dry storage systems used for spent nuclear fuel (SNF) consist of a welded 304 stainless steel container placed within a passively-ventilated concrete or steel overpack. More recently fielded systems are constructed with dual certified 304/304L and in some cases, 316 or 316L. In service, atmospheric salts, a portion of which will be chloride bearing, will be deposited on the surface of these containers. Initially, the stainless steel canister surface temperatures will be high (exceeding the boiling point of water in many cases) due to decay heat from the SNF. As the SNF cools over time, themore » container surface will also cool, and deposited salts will deliquesce to form potentially corrosive chloride-rich brines. Because austenitic stainless steels are prone to chloride-induced stress corrosion cracking (CISCC), the concern has been raised that SCC may significantly impact long-term canister performance. While the susceptibility of austenitic stainless steels to CISCC in the general sense is well known, the behavior of SCC cracks (i.e., initiation and propagation behavior) under the aforementioned atmospheric conditions is poorly understood.« less

Failure analysis of ceramic clinical cases using qualitative fractography.

PubMed

Scherrer, Susanne S; Quinn, Janet B; Quinn, George D; Kelly, J Robert

2006-01-01

To educate dental academic staff and clinicians on the application of descriptive (qualitative) fractography for analyses of clinical and laboratory failures of brittle materials such as glass and ceramic. The fracture surface topography of failed glass, glass fiber-reinforced composite, and ceramic restorations (Procera, Cerestore, In-Ceram, porcelain-fused-to-metal) was examined utilizing a scanning electron microscope. Replicas and original failed parts were scrutinized for classic fractographic features such as hackle, wake hackle, twist hackle, arrest lines, and mirrors. Failed surfaces of the veneering porcelain of ceramic and porcelain-fused-to-metal crowns exhibited hackle, wake hackle, twist hackle, arrest lines, and compression curl, which were produced by the interaction of the advancing crack with the microstructure of the material. Fracture surfaces of glass and glass fiber-reinforced composite showed additional features, such as velocity hackle and mirrors. The observed features were good indicators of the local direction of crack propagation and were used to trace the crack back to an initial starting area (the origin). Examples of failure analysis in this study are intended to guide the researcher in using qualitative (descriptive) fractography as a tool for understanding the failure process in brittle restorative materials and also for assessing possible design inadequacies.

Molecular-dynamics Simulation-based Cohesive Zone Representation of Intergranular Fracture Processes in Aluminum

NASA Technical Reports Server (NTRS)

Yamakov, Vesselin I.; Saether, Erik; Phillips, Dawn R.; Glaessgen, Edward H.

2006-01-01

A traction-displacement relationship that may be embedded into a cohesive zone model for microscale problems of intergranular fracture is extracted from atomistic molecular-dynamics simulations. A molecular-dynamics model for crack propagation under steady-state conditions is developed to analyze intergranular fracture along a flat 99 [1 1 0] symmetric tilt grain boundary in aluminum. Under hydrostatic tensile load, the simulation reveals asymmetric crack propagation in the two opposite directions along the grain boundary. In one direction, the crack propagates in a brittle manner by cleavage with very little or no dislocation emission, and in the other direction, the propagation is ductile through the mechanism of deformation twinning. This behavior is consistent with the Rice criterion for cleavage vs. dislocation blunting transition at the crack tip. The preference for twinning to dislocation slip is in agreement with the predictions of the Tadmor and Hai criterion. A comparison with finite element calculations shows that while the stress field around the brittle crack tip follows the expected elastic solution for the given boundary conditions of the model, the stress field around the twinning crack tip has a strong plastic contribution. Through the definition of a Cohesive-Zone-Volume-Element an atomistic analog to a continuum cohesive zone model element - the results from the molecular-dynamics simulation are recast to obtain an average continuum traction-displacement relationship to represent cohesive zone interaction along a characteristic length of the grain boundary interface for the cases of ductile and brittle decohesion. Keywords: Crack-tip plasticity; Cohesive zone model; Grain boundary decohesion; Intergranular fracture; Molecular-dynamics simulation

Avoidance of stress corrosion susceptibility in high strength aluminum alloys by control of grain boundary and matrix microstructure

NASA Technical Reports Server (NTRS)

Adler, P.; Deiasi, R.

1974-01-01

The relation of microstructure to the mechanical strength and stress corrosion resistance of highest strength and overaged tempers of BAR and 7050 aluminum alloys was investigated. Comparison is made with previously studied 7075 aluminum alloy. Optical microscopy, transmission electron microscopy, and differential scanning calorimetry were used to characterize the grain morphology, matrix microstructure, and grain boundary microstructure of these tempers. Grain boundary interparticle spacing was significant to stress corrosion crack propagation for all three alloys; increasing interparticle spacing led to increased resistance to crack propagation. In addition, the fire grain size in Bar and 7050 appears to enhance crack propagation. The highest strength temper of 7050 has a comparatively high resistance to crack initiation. Overall stress corrosion behavior is dependent on environment pH, and evaluation over a range of pH is recommended.

An energy-consistent fracture model for ferroelectrics

NASA Astrophysics Data System (ADS)

Miao, Hongchen; Li, Faxin

2017-02-01

The fracture behavior of ferroelectrics has been intensively studied in recent decades, though currently a widely accepted fracture mechanism is still lacking. In this work, enlightened by previous experimental observations that crack propagation in ferroelectrics is always accompanied by domain switching, we propose a micromechanical model in which both crack propagation and domain switching are controlled by energy-based criteria. Both electric energy and mechanical energy can induce domain switching, while only mechanical energy can drive crack propagation. Furthermore, constrained domain switching is considered in this model, leading to the gradient domain switching zone near the crack tip. Analysis results show that stress-induced ferroelastic switching always has a toughening effect as the mechanical energy release rate serves as the driving force for both fracture and domain switching. In comparison, the electric-field-induced switching may have either a toughening or detoughening effect. The proposed model can qualitatively agree with the existing experimental results.

Mode I Cohesive Law Characterization of Through-Crack Propagation in a Multidirectional Laminate

NASA Technical Reports Server (NTRS)

Bergan, Andrew C.; Davila, Carlos G.; Leone, Frank A.; Awerbuch, Jonathan; Tan, Tein-Min

2014-01-01

A method is proposed and assessed for the experimental characterization of through-the-thickness crack propagation in multidirectional composite laminates with a cohesive law. The fracture toughness and crack opening displacement are measured and used to determine a cohesive law. Two methods of computing fracture toughness are assessed and compared. While previously proposed cohesive characterizations based on the R-curve exhibit size effects, the proposed approach results in a cohesive law that is a material property. The compact tension specimen configuration is used to propagate damage while load and full-field displacements are recorded. These measurements are used to compute the fracture toughness and crack opening displacement from which the cohesive law is characterized. The experimental results show that a steady-state fracture toughness is not reached. However, the proposed method extrapolates to steady-state and is demonstrated capable of predicting the structural behavior of geometrically-scaled specimens.

A modified moment-fitted integration scheme for X-FEM applications with history-dependent material data

NASA Astrophysics Data System (ADS)

Zhang, Ziyu; Jiang, Wen; Dolbow, John E.; Spencer, Benjamin W.

2018-01-01

We present a strategy for the numerical integration of partial elements with the eXtended finite element method (X-FEM). The new strategy is specifically designed for problems with propagating cracks through a bulk material that exhibits inelasticity. Following a standard approach with the X-FEM, as the crack propagates new partial elements are created. We examine quadrature rules that have sufficient accuracy to calculate stiffness matrices regardless of the orientation of the crack with respect to the element. This permits the number of integration points within elements to remain constant as a crack propagates, and for state data to be easily transferred between successive discretizations. In order to maintain weights that are strictly positive, we propose an approach that blends moment-fitted weights with volume-fraction based weights. To demonstrate the efficacy of this simple approach, we present results from numerical tests and examples with both elastic and plastic material response.

Approximate stresses in 2-D flat elastic contact fretting problems

NASA Astrophysics Data System (ADS)

Urban, Michael Rene

Fatigue results from the cyclic loading of a solid body. If the body subject to fatigue is in contact with another body and relative sliding motion occurs between these two bodies, then rubbing surface damage can accelerate fatigue failure. The acceleration of fatigue failure is especially important if the relative motion between the two bodies results in surface damage without excessive surface removal via wear. The situation just described is referred to as fretting fatigue. Understanding of fretting fatigue is greatly enhanced if the stress state associated with fretting can be characterized. For Hertzian contact, this can readily be done. Unfortunately, simple stress formulae are not available for flat body contact. The primary result of the present research is the development of a new, reasonably accurate, approximate closed form expression for 2-dimensional contact stresses which has been verified using finite element modeling. This expression is also combined with fracture mechanics to provide a simple method of determining when a crack is long enough to no longer be affected by the contact stress field. Lower bounds on fatigue life can then easily be calculated using fracture mechanics. This closed form expression can also be used to calculate crack propagation within the contact stress field. The problem of determining the cycles required to generate an initial crack and what to choose as an initial crack size is unresolved as it is in non-fretting fatigue.

Identification of corrosion and damage mechanisms by using scanning electron microscopy and energy-dispersive X-ray microanalysis: contribution to failure analysis case histories

NASA Astrophysics Data System (ADS)

Pantazopoulos, G.; Vazdirvanidis, A.

2014-03-01

Emphasis is placed on the evaluation of corrosion failures of copper and machineable brass alloys during service. Typical corrosion failures of the presented case histories mainly focussed on stress corrosion cracking and dezincification that acted as the major degradation mechanisms in components used in piping and water supply systems. SEM assessment, coupled with EDS spectroscopy, revealed the main cracking modes together with the root-source(s) that are responsible for the damage initiation and evolution. In addition, fracture surface observations contributed to the identification of the incurred fracture mechanisms and potential environmental issues that stimulated crack initiation and propagation. Very frequently, the detection of chlorides among the corrosion products served as a suggestive evidence of the influence of working environment on passive layer destabilisation and metal dissolution.

Investigation of Mechanical Properties and Fracture Simulation of Solution-Treated AA 5754

NASA Astrophysics Data System (ADS)

Kumar, Pankaj; Singh, Akhilendra

2017-10-01

In this work, mechanical properties and fracture toughness of as-received and solution-treated aluminum alloy 5754 (AA 5754) are experimentally evaluated. Solution heat treatment of the alloy is performed at 530 °C for 2 h, and then, quenching is done in water. Yield strength, ultimate tensile strength, impact toughness, hardness, fatigue life, brittle fracture toughness (K_{Ic} ) and ductile fracture toughness (J_{Ic} ) are evaluated for as-received and solution-treated alloy. Extended finite element method has been used for the simulation of tensile and fracture behavior of material. Heaviside function and asymptotic crack tip enrichment functions are used for modelling of the crack in the geometry. Ramberg-Osgood material model coupled with fracture energy is used to simulate the crack propagation. Fracture surfaces obtained from various mechanical tests are characterized by scanning electron microscopy.

The Effect of High Concentration and Small Size of Nanodiamonds on the Strength of Interface and Fracture Properties in Epoxy Nanocomposite

PubMed Central

Haleem, Yasir A.; Song, Pin; Liu, Daobin; Wang, Changda; Gan, Wei; Saleem, Muhammad Farooq; Song, Li

2016-01-01

The concentration and small size of nanodiamonds (NDs) plays a crucial role in the mechanical performance of epoxy-based nanocomposites by modifying the interface strength. Herein, we systemically analyzed the relation between the high concentration and small size of ND and the fracture properties of its epoxy-based nanocomposites. It was observed that there is a two-fold increase in fracture toughness and a three-fold increase in fracture energy. Rationally, functionalized-NDs (F-NDs) showed a much better performance for the nanocomposite than pristine NDs (P-NDs) because of additional functional groups on its surface. The F-ND/epoxy nanocomposites exhibited rougher surface in contrast with the P-ND/epoxy, indicating the presence of a strong interface. We found that the interfaces in F-ND/epoxy nanocomposites at high concentrations of NDs overlap by making a web, which can efficiently hinder further crack propagation. In addition, the de-bonding in P-ND/epoxy nanocomposites occurred at the interface with the appearance of plastic voids or semi-naked particles, whereas the de-bonding for F-ND/epoxy nanocomposites happened within the epoxy molecular network instead of the interface. Because of the strong interface in F-ND/epoxy nanocomposites, at high concentrations the de-bonding within the epoxy molecular network may lead to subsequent cracks, parallel to the parent crack, via crack splitting which results in a fiber-like structure on the fracture surface. The plastic void growth, crack deflection and subsequent crack growth were correlated to higher values of fracture toughness and fracture energy in F-ND/epoxy nanocomposites. PMID:28773628

The Effect of High Concentration and Small Size of Nanodiamonds on the Strength of Interface and Fracture Properties in Epoxy Nanocomposite.

PubMed

Haleem, Yasir A; Song, Pin; Liu, Daobin; Wang, Changda; Gan, Wei; Saleem, Muhammad Farooq; Song, Li

2016-06-23

The concentration and small size of nanodiamonds (NDs) plays a crucial role in the mechanical performance of epoxy-based nanocomposites by modifying the interface strength. Herein, we systemically analyzed the relation between the high concentration and small size of ND and the fracture properties of its epoxy-based nanocomposites. It was observed that there is a two-fold increase in fracture toughness and a three-fold increase in fracture energy. Rationally, functionalized-NDs (F-NDs) showed a much better performance for the nanocomposite than pristine NDs (P-NDs) because of additional functional groups on its surface. The F-ND/epoxy nanocomposites exhibited rougher surface in contrast with the P-ND/epoxy, indicating the presence of a strong interface. We found that the interfaces in F-ND/epoxy nanocomposites at high concentrations of NDs overlap by making a web, which can efficiently hinder further crack propagation. In addition, the de-bonding in P-ND/epoxy nanocomposites occurred at the interface with the appearance of plastic voids or semi-naked particles, whereas the de-bonding for F-ND/epoxy nanocomposites happened within the epoxy molecular network instead of the interface. Because of the strong interface in F-ND/epoxy nanocomposites, at high concentrations the de-bonding within the epoxy molecular network may lead to subsequent cracks, parallel to the parent crack, via crack splitting which results in a fiber-like structure on the fracture surface. The plastic void growth, crack deflection and subsequent crack growth were correlated to higher values of fracture toughness and fracture energy in F-ND/epoxy nanocomposites.

CT Identification and Fractal Characterization of 3-D Propagation and Distribution of Hydrofracturing Cracks in Low-Permeability Heterogeneous Rocks

NASA Astrophysics Data System (ADS)

Liu, Peng; Ju, Yang; Gao, Feng; Ranjith, Pathegama G.; Zhang, Qianbing

2018-03-01

Understanding and characterization of the three-dimensional (3-D) propagation and distribution of hydrofracturing cracks in heterogeneous rock are key for enhancing the stimulation of low-permeability petroleum reservoirs. In this study, we investigated the propagation and distribution characteristics of hydrofracturing cracks, by conducting true triaxial hydrofracturing tests and computed tomography on artificial heterogeneous rock specimens. Silica sand, Portland cement, and aedelforsite were mixed to create artificial heterogeneous rock specimens using the data of mineral compositions, coarse gravel distribution, and mechanical properties that were measured from the natural heterogeneous glutenite cores. To probe the effects of material heterogeneity on hydrofracturing cracks, the artificial homogenous specimens were created using the identical matrix compositions of the heterogeneous rock specimens and then fractured for comparison. The effects of horizontal geostress ratio on the 3-D growth and distribution of cracks during hydrofracturing were examined. A fractal-based method was proposed to characterize the complexity of fractures and the efficiency of hydrofracturing stimulation of heterogeneous media. The material heterogeneity and horizontal geostress ratio were found to significantly influence the 3-D morphology, growth, and distribution of hydrofracturing cracks. A horizontal geostress ratio of 1.7 appears to be the upper limit for the occurrence of multiple cracks, and higher ratios cause a single crack perpendicular to the minimum horizontal geostress component. The fracturing efficiency is associated with not only the fractured volume but also the complexity of the crack network.

In-plane time-harmonic elastic wave motion and resonance phenomena in a layered phononic crystal with periodic cracks.

PubMed

Golub, Mikhail V; Zhang, Chuanzeng

2015-01-01

This paper presents an elastodynamic analysis of two-dimensional time-harmonic elastic wave propagation in periodically multilayered elastic composites, which are also frequently referred to as one-dimensional phononic crystals, with a periodic array of strip-like interior or interface cracks. The transfer matrix method and the boundary integral equation method in conjunction with the Bloch-Floquet theorem are applied to compute the elastic wave fields in the layered periodic composites. The effects of the crack size, spacing, and location, as well as the incidence angle and the type of incident elastic waves on the wave propagation characteristics in the composite structure are investigated in details. In particular, the band-gaps, the localization and the resonances of elastic waves are revealed by numerical examples. In order to understand better the wave propagation phenomena in layered phononic crystals with distributed cracks, the energy flow vector of Umov and the corresponding energy streamlines are visualized and analyzed. The numerical results demonstrate that large energy vortices obstruct elastic wave propagation in layered phononic crystals at resonance frequencies. They occur before the cracks reflecting most of the energy transmitted by the incoming wave and disappear when the problem parameters are shifted from the resonant ones.

Estimation and Simulation of Slow Crack Growth Parameters from Constant Stress Rate Data

NASA Technical Reports Server (NTRS)

Salem, Jonathan A.; Weaver, Aaron S.

2003-01-01

Closed form, approximate functions for estimating the variances and degrees-of-freedom associated with the slow crack growth parameters n, D, B, and A(sup *) as measured using constant stress rate ('dynamic fatigue') testing were derived by using propagation of errors. Estimates made with the resulting functions and slow crack growth data for a sapphire window were compared to the results of Monte Carlo simulations. The functions for estimation of the variances of the parameters were derived both with and without logarithmic transformation of the initial slow crack growth equations. The transformation was performed to make the functions both more linear and more normal. Comparison of the Monte Carlo results and the closed form expressions derived with propagation of errors indicated that linearization is not required for good estimates of the variances of parameters n and D by the propagation of errors method. However, good estimates variances of the parameters B and A(sup *) could only be made when the starting slow crack growth equation was transformed and the coefficients of variation of the input parameters were not too large. This was partially a result of the skewered distributions of B and A(sup *). Parametric variation of the input parameters was used to determine an acceptable range for using closed form approximate equations derived from propagation of errors.

Understanding breakage in curly hair.

PubMed

Camacho-Bragado, G A; Balooch, G; Dixon-Parks, F; Porter, C; Bryant, H

2015-07-01

In 2005, the L'Oréal Institute for hair and skin research carried out a multiethnic study to investigate hair breakage in women residing in the U.S.A. In this study it was reported that a large percentage (96%) of the African-American respondents experience breakage. A combination of structural differences and grooming-induced stresses seem to contribute to the higher breakage incidence in the African-American group as the chemical composition of African-American hair is not significantly different from other ethnic groups. Some authors have proposed that the repeated elongation, torsion and flexion actions may affect the components of the hair fibre. However, considering the different properties of cuticle and cortex, one would expect a different wearing mechanism of each, leading to the ultimate failure of hair. Knowing in detail how each part of the structure fails can potentially lead to better ways to protect the hair from physical insults. To investigate crack propagation and fracture mechanisms in African-American hair. Virgin hair of excellent quality was collected, with informed consent, from a female African-American volunteer. A series of controlled mechanical stresses was applied to 10-mm hair sections using a high-resolution mechanical stage (20 mN) up to the fracture of the fibre. The surface was monitored using scanning electron microscopy imaging during the stress application. X-ray tomographic microscopy images were acquired and quantified to detect changes in energy absorption as a function of applied stress that could be linked to increase in crack density. Analysis of the mechanical response of hair combined with the two imaging techniques led us to propose the following mechanism of hair breakage: cuticle sliding; failure of the cuticle-cortex interface; nucleation of intercellular cracks and growth of cracks at the cuticle-cortex junction; and propagation of intercellular cracks towards the surface of the hair and final breakage when these cracks merge at the cuticular junction. The combination of scanning electron microscopy and X-ray tomography provided new information about the fracture of hair. Mechanical damage from grooming and some environmental factors accumulate in hair creating internal cracks that eventually result in breakage at unpredictable sites and therefore a continuous care regimen for the hair throughout the life cycle of the fibres is recommended. © 2015 The Authors BJD © 2015 British Association of Dermatologists.

Three-dimensional study of grain boundary engineering effects on intergranular stress corrosion cracking of 316 stainless steel in high temperature water

NASA Astrophysics Data System (ADS)

Liu, Tingguang; Xia, Shuang; Bai, Qin; Zhou, Bangxin; Zhang, Lefu; Lu, Yonghao; Shoji, Tetsuo

2018-01-01

The intergranular cracks and grain boundary (GB) network of a GB-engineered 316 stainless steel after stress corrosion cracking (SCC) test in high temperature high pressure water of reactor environment were investigated by two-dimensional and three-dimensional (3D) characterization in order to expose the mechanism that GB-engineering mitigates intergranular SCC. The 3D microstructure shown that the essential characteristic of the GB-engineered microstructure is formation of many large twin-boundaries as a result of multiple-twinning, which results in the formation of large grain-clusters. The large grain-clusters played a key role to the improvement of intergranular SCC resistance by GB-engineering. The main intergranular cracks propagated in a zigzag along the outer boundaries of these large grain-clusters because all inner boundaries of the grain-clusters were twin-boundaries (∑3) or twin-related boundaries (∑3n) which had much lower susceptibility to SCC than random boundaries. These large grain-clusters had tree-ring-shaped topology structure and very complex morphology. They got tangled so that difficult to be separated during SCC, resulting in some large crack-bridges retained in the crack surface.

Slow and fast motion of cracks in inelastic solids. Part 1: Slow growth of cracks in a rate sensitive tresca solid. Part 2: Dynamic crack represented by the Dugdale model

NASA Technical Reports Server (NTRS)

Wnuk, M. P.; Sih, G. C.

1972-01-01

An extension is proposed of the classical theory of fracture to viscoelastic and elastic-plastic materials in which the plasticity effects are confined to a narrow band encompassing the crack front. It is suggested that the Griffith-Irwin criterion of fracture, which requires that the energy release rate computed for a given boundary value problem equals the critical threshold, ought to be replaced by a differential equation governing the slow growth of a crack prior to the onset of rapid propagation. A new term which enters the equation of motion in the dissipative media is proportional to the energy lost within the end sections of the crack, and thus reflects the extent of inelastic behavior of a solid. A concept of apparent surface energy is introduced to account for the geometry dependent and the rate dependent phenomena which influence toughness of an inelastic solid. Three hypotheses regarding the condition for fracture in the subcritical range of load are compared. These are: (1) constant fracture energy (Cherepanov), (2) constant opening displacement at instability (Morozov) and (3) final stretch criterion (Wnuk).

A Review on Strengthening Steel Beams Using FRP under Fatigue

PubMed Central

Jumaat, Mohd Zamin; Ramli Sulong, N. H.

2014-01-01

In recent decades, the application of fibre-reinforced polymer (FRP) composites for strengthening structural elements has become an efficient option to meet the increased cyclic loads or repair due to corrosion or fatigue cracking. Hence, the objective of this study is to explore the existing FRP reinforcing techniques to care for fatigue damaged structural steel elements. This study covers the surface treatment techniques, adhesive curing, and support conditions under cyclic loading including fatigue performance, crack propagation, and failure modes with finite element (FE) simulation of the steel bridge girders and structural elements. FRP strengthening composites delay initial cracking, reduce the crack growth rate, extend the fatigue life, and decrease the stiffness decay with residual deflection. Prestressed carbon fibre-reinforced polymer (CFRP) is the best strengthening option. End anchorage prevents debonding of the CRRP strips at the beam ends by reducing the local interfacial shear and peel stresses. Hybrid-joint, nanoadhesive, and carbon-flex can also be attractive for strengthening systems. PMID:25243221

A review on strengthening steel beams using FRP under fatigue.

PubMed

Kamruzzaman, Mohamed; Jumaat, Mohd Zamin; Sulong, N H Ramli; Islam, A B M Saiful

2014-01-01

In recent decades, the application of fibre-reinforced polymer (FRP) composites for strengthening structural elements has become an efficient option to meet the increased cyclic loads or repair due to corrosion or fatigue cracking. Hence, the objective of this study is to explore the existing FRP reinforcing techniques to care for fatigue damaged structural steel elements. This study covers the surface treatment techniques, adhesive curing, and support conditions under cyclic loading including fatigue performance, crack propagation, and failure modes with finite element (FE) simulation of the steel bridge girders and structural elements. FRP strengthening composites delay initial cracking, reduce the crack growth rate, extend the fatigue life, and decrease the stiffness decay with residual deflection. Prestressed carbon fibre-reinforced polymer (CFRP) is the best strengthening option. End anchorage prevents debonding of the CRRP strips at the beam ends by reducing the local interfacial shear and peel stresses. Hybrid-joint, nanoadhesive, and carbon-flex can also be attractive for strengthening systems.

Effects of Thermo-Mechanical Treatments on Deformation Behavior and IGSCC Susceptibility of Stainless Steels in Pwr Primary Water Chemistry

NASA Astrophysics Data System (ADS)

Nouraei, S.; Tice, D. R.; Mottershead, K. J.; Wright, D. M.

Field experience of 300 series stainless steels in the primary circuit of PWR plant has been good. Stress Corrosion Cracking of components has been infrequent and mainly associated with contamination by impurities/oxygen in occluded locations. However, some instances of failures have been observed which cannot necessarily be attributed to deviations in the water chemistry. These failures appear to be associated with the presence of cold-work produced by surface finishing and/or by welding-induced shrinkage. Recent data indicate that some heats of SS show an increased susceptibility to SCC; relatively high crack growth rates were observed even when the crack growth direction is orthogonal to the cold-work direction. SCC of cold-worked SS in PWR coolant is therefore determined by a complex interaction of material composition, microstructure, prior cold-work and heat treatment. This paper will focus on the interactions between these parameters on crack propagation in simulated PWR conditions.

Reliability Factors for Electronic Components in a Storage Environment

DTIC Science & Technology

1977-09-01

Other moisture-induced failure mechanisms include crack propagation in brittle materials such as ceramic seals, glass passivation layers, nitride ...for Aluminum -Gold .... .............. ... 80 4-5 Fatigue S-N Curve Typical for Most Metals and Polymers . 85 4-6 Comparison of Surface Damageý Within...8 Aluminum -Silicon Phase Diagram ...... ............... 96 5-1 Evaluation of Gases from Microcircuit Package .... ....... 121 6-1 Plot of Resistivity

The Influence of Surface Roughness on Biocompatibility and Fatigue Life of Titanium Based Alloys

NASA Astrophysics Data System (ADS)

Major, S.; Cyrus, P.; Hubálovská, M.

2017-02-01

This article deals with the effect of treatment on the mechanical properties ofbiocompatible alloys. In the case of implants, it is desirable to ensure good biocompatibility. Generally, the environment in the body is very aggressive and implants can quickly degrade due the corrosion. The process of corrosion leads to the release of harmful particles into the body. Other reasons for rejection of the implants, is their coverage bacterial plaque. Another reason for the rejection of the implant may be a smooth surface. In some cases, the tissue does not adhere to the smooth surface of the implant, in this regionsoccurs an accumulation of body fluids. This problem can be solved with a rough surface. From the viewpoint of fatigue resistance, the rough surface containing grooves and holes has a negative influence on the fatigue resistance against mechanical loading. The rough surface can be produced by machining or asymmetric deposition of particles of oxides, nitrides or other particles on surface. In this work the formation and propagation of fatigue cracks in the material with granular surface is analysed. The formation and growth of fatigue crack originated from granular surface is simulated. Also, experimental studies were carried out.

Time-Dependent Fatigue Crack Propagation Behavior of Two Solid-Solution-Strengthened Ni-Based Superalloys—INCONEL 617 and HAYNES 230

NASA Astrophysics Data System (ADS)

Ma, Longzhou; Roy, Shawoon K.; Hasan, Muhammad H.; Pal, Joydeep; Chatterjee, Sudin

2012-02-01

The fatigue crack propagation (FCP) as well as the sustained loading crack growth (SLCG) behavior of two solid-solution-strengthened Ni-based superalloys, INCONEL 617 (Special Metals Corporation Family of Companies) and HAYNES 230 (Haynes International, Inc., Kokomo, IN), were studied at increased temperatures in laboratory air under a constant stress-intensity-factor ( K) condition. The crack propagation tests were conducted using a baseline cyclic triangular waveform with a frequency of 1/3 Hz. Various hold times were imposed at the maximum load of a fatigue cycle to study the hold time effect. The results show that a linear elastic fracture mechanics (LEFM) parameter, stress intensity factor ( K), is sufficient to describe the FCP and SLCG behavior at the testing temperatures ranging from 873 K to 1073 K (600 °C to 800 °C). As observed in the precipitation-strengthened superalloys, both INCONEL 617 and HAYNES 230 exhibited the time-dependent FCP, steady SLCG behavior, and existence of a damage zone ahead of crack tip. A thermodynamic equation was adapted to correlate the SLCG rates to determine thermal activation energy. The fracture modes associated with crack propagation behavior were discussed, and the mechanism of time-dependent FCP as well as SLCG was identified. Compared with INCONEL 617, the lower crack propagation rates of HAYNES 230 under the time-dependent condition were ascribed to the different fracture mode and the presence of numerous W-rich M6C-type and Cr-rich M23C6-type carbides. Toward the end, a phenomenological model was employed to correlate the FCP rates at cycle/time-dependent FCP domain. All the results suggest that an environmental factor, the stress assisted grain boundary oxygen embrittlement (SAGBOE) mechanism, is mainly responsible for the accelerated time-dependent FCP rates of INCONEL 617 and HAYNES 230.

Field experiments to determine wave propagation principles and mechanical properties of snow

NASA Astrophysics Data System (ADS)

Simioni, Stephan; Gebhard, Felix; Dual, Jürg; Schweizer, Jürg

2017-04-01

To understand the release of snow avalanches by explosions one needs to know how acoustic waves travel above and within the snowpack. Hitherto, wave propagation was investigated in the laboratory with small samples or in the field in the shock wave region. We developed a measurement system and layout to derive wave attenuation in snow, wave speeds and elastic moduli on small-scale (1-2 m) field experiments to close the gap between the lab scale (0.1 m) and the scale of artificial release (10-100 m). We used solid explosives and hammer blows to create the load and accelerometers to measure the resulting wave within the snowpack. The strong attenuation we observed indicates that we measured the second longitudinal wave which propagates through the pore space. The wave speeds, however, corresponded to the speeds of the first longitudinal wave within the ice skeleton. The elastic moduli were high on the order of several tens of MPa for lower densities (150 kg m-3) and agreed well with earlier lab studies, in particular for the higher densities 250-400 kg m-3). However, the scatter was rather large as expected for in-situ experiments in the layered snow cover. In addition, we measured accelerations during propagation saw test experiments. The propagation of cracks during this type of snow instability test has mainly been studied by analysing the bending of the slab (due to the saw cut) using particle tracking velocimetry. We used the accelerometers to measure crack propagation speeds. The wave speeds were slightly higher for most experiments than reported previously. Furthermore, in some experiments, we encountered to different wave types with one propagating at a higher speed. This finding may be interpreted as the actual crack propagation and the settling of the weak layer (collapse wave). Our results show that field measurements of propagation properties are feasible and that crack propagation as observed during propagation saw tests may involve different processes that need to be further investigated.

A Linearized Model for Wave Propagation through Coupled Volcanic Conduit-crack Systems Filled with Multiphase Magma

NASA Astrophysics Data System (ADS)

Liang, C.; Dunham, E. M.; OReilly, O. J.; Karlstrom, L.

2015-12-01

Both the oscillation of magma in volcanic conduits and resonance of fluid-filled cracks (dikes and sills) are appealing explanations for very long period signals recorded at many active volcanoes. While these processes have been studied in isolation, real volcanic systems involve interconnected networks of conduits and cracks. The overall objective of our work is to develop a model of wave propagation and ultimately eruptive fluid dynamics through this coupled system. Here, we present a linearized model for wave propagation through a conduit with multiple cracks branching off of it. The fluid is compressible and viscous, and is comprised of a mixture of liquid melt and gas bubbles. Nonequilibrium bubble growth and resorption (BGR) is quantified by introducing a time scale for mass exchange between phases, following the treatment in Karlstrom and Dunham (2015). We start by deriving the dispersion relation for crack waves travelling along the multiphase-magma-filled crack embedded in an elastic solid. Dissipation arises from magma viscosity, nonequilibrium BGR, and radiation of seismic waves into the solid. We next introduce coupling conditions between the conduit and crack, expressing conservation of mass and the balance of forces across the junction. Waves in the conduit, like those in the crack, are influenced by nonequilibrium BGR, but the deformability of the surrounding solid is far less important than for cracks. Solution of the coupled system of equations provides the evolution of pressure and fluid velocity within the conduit-crack system. The system has various resonant modes that are sensitive to fluid properties and to the geometry of the conduit and cracks. Numerical modeling of seismic waves in the solid allows us to generate synthetic seismograms.

Stress corrosion cracking of duplex stainless steels in caustic solutions

NASA Astrophysics Data System (ADS)

Bhattacharya, Ananya

Duplex stainless steels (DSS) with roughly equal amount of austenite and ferrite phases are being used in industries such as petrochemical, nuclear, pulp and paper mills, de-salination plants, marine environments, and others. However, many DSS grades have been reported to undergo corrosion and stress corrosion cracking in some aggressive environments such as chlorides and sulfide-containing caustic solutions. Although stress corrosion cracking of duplex stainless steels in chloride solution has been investigated and well documented in the literature but the SCC mechanisms for DSS in caustic solutions were not known. Microstructural changes during fabrication processes affect the overall SCC susceptibility of these steels in caustic solutions. Other environmental factors, like pH of the solution, temperature, and resulting electrochemical potential also influence the SCC susceptibility of duplex stainless steels. In this study, the role of material and environmental parameters on corrosion and stress corrosion cracking of duplex stainless steels in caustic solutions were investigated. Changes in the DSS microstructure by different annealing and aging treatments were characterized in terms of changes in the ratio of austenite and ferrite phases, phase morphology and intermetallic precipitation using optical micrography, SEM, EDS, XRD, nano-indentation and microhardness methods. These samples were then tested for general and localized corrosion susceptibility and SCC to understand the underlying mechanisms of crack initiation and propagation in DSS in the above-mentioned environments. Results showed that the austenite phase in the DSS is more susceptible to crack initiation and propagation in caustic solutions, which is different from that in the low pH chloride environment where the ferrite phase is the more susceptible phase. This study also showed that microstructural changes in duplex stainless steels due to different heat treatments could affect their SCC susceptibility. Annealed and water quenched specimens were found to be immune to SCC in caustic environment. Aging treatment at 800°C gave rise to sigma and chi precipitates in the DSS. However, these sigma and chi precipitates, known to initiate cracking in DSS in chloride environment did not cause any cracking of DSS in caustic solutions. Aging of DSS at 475°C had resulted in '475°C embrittlement' and caused cracks to initiate in the ferrite phase. This was in contrast to the cracks initiating in the austenite phase in the as-received DSS. Alloy composition and microstructure of DSS as well as solution composition (dissolved ionic species) was also found to affect the electrochemical behavior and passivation of DSS which in turn plays a major role in stress corrosion crack initiation and propagation. Corrosion rates and SCC susceptibility of DSS was found to increase with addition of sulfide to caustic solutions. Corrosion films on DSS, characterized using XRD and X-ray photoelectron spectroscopy, indicated that the metal sulfide compounds were formed along with oxides at the metal surface in the presence of sulfide containing caustic environments. These metal sulfide containing passive films are unstable and hence breaks down under mechanical straining, leading to SCC initiations. The overall results from this study helped in understanding the mechanism of SCC in caustic solutions. Favorable slip systems in the austenite phase of DSS favors slip-induced local film damage thereby initiating a stress corrosion crack. Repeated film repassivation and breaking, followed by crack tip dissolution results in crack propagation in the austenite phase of DSS alloys. Result from this study will have a significant impact in terms of identifying the alloy compositions, fabrication processes, microstructures, and environmental conditions that may be avoided to mitigate corrosion and stress corrosion cracking of DSS in caustic solutions.

A High-Sensitivity Flexible Eddy Current Array Sensor for Crack Monitoring of Welded Structures under Varying Environment.

PubMed

Chen, Tao; He, Yuting; Du, Jinqiang

2018-06-01

This paper develops a high-sensitivity flexible eddy current array (HS-FECA) sensor for crack monitoring of welded structures under varying environment. Firstly, effects of stress, temperature and crack on output signals of the traditional flexible eddy current array (FECA) sensor were investigated by experiments that show both stress and temperature have great influences on the crack monitoring performance of the sensor. A 3-D finite element model was established using Comsol AC/DC module to analyze the perturbation effects of crack on eddy currents and output signals of the sensor, which showed perturbation effect of cracks on eddy currents is reduced by the current loop when crack propagates. Then, the HS-FECA sensor was proposed to boost the sensitivity to cracks. Simulation results show that perturbation effect of cracks on eddy currents excited by the HS-FECA sensor gradually grows stronger when the crack propagates, resulting in much higher sensitivity to cracks. Experimental result further shows that the sensitivity of the new sensor is at least 19 times that of the original one. In addition, both stress and temperature variations have little effect on signals of the new sensor.

Ballistic Impact of Braided Composites with a Soft Projectile

NASA Technical Reports Server (NTRS)

Roberts, Gary D.; Pereira, J. Michael; Revilock, Duane M., Jr.; Binienda, Wieslaw K.; Xie, Ming; Braley, Mike

2002-01-01

Impact tests using a soft gelatin projectile were performed to identify failure modes that occur at high strain energy density during impact loading. Failure modes were identified for aluminum plates and for composites plates and half-rings made from triaxial carbon fiber braid having a 0/+/- 60deg architecture. For aluminum plates, a large hole formed as a result of crack propagation from the initiation site at the center of the plate. For composite plates, fiber tensile failure occurred in the back ply at the center of the plate. Cracks then propagated from this site along the +/-60deg fiber directions until triangular flaps opened to form a hole. For composite half-rings fabricated with 0deg fibers aligned circumferentially, fiber tensile failure also occurred in the back ply. Cracks first propagated from this site perpendicular the 0deg fibers. The cracks then turned to follow the +/-60deg fibers and 0deg fibers until rectangular flaps opened to form a hole. Damage in the composites was localized near the impact site, while cracks in the aluminum extended to the boundaries.

The fracture behaviour of dental enamel.

PubMed

Bechtle, Sabine; Habelitz, Stefan; Klocke, Arndt; Fett, Theo; Schneider, Gerold A

2010-01-01

Enamel is the hardest tissue in the human body covering the crowns of teeth. Whereas the underlying dental material dentin is very well characterized in terms of mechanical and fracture properties, available data for enamel are quite limited and are apart from the most recent investigation mainly based on indentation studies. Within the current study, stable crack-growth experiments in bovine enamel have been performed, to measure fracture resistance curves for enamel. Single edge notched bending specimens (SENB) prepared out of bovine incisors were tested in 3-point bending and subsequently analysed using optical and environmental scanning electron microscopy. Cracks propagated primarily within the protein-rich rod sheaths and crack propagation occurred under an inclined angle to initial notch direction not only due to enamel rod and hydroxyapatite crystallite orientation but potentially also due to protein shearing. Determined mode I fracture resistance curves ranged from 0.8-1.5 MPa*m(1/2) at the beginning of crack propagation up to 4.4 MPa*m(1/2) at 500 microm crack extension; corresponding mode II values ranged from 0.3 to 1.5 MPa*m(1/2).

Mechanical Properties of Photovoltaic Silicon in Relation to Wafer Breakage

NASA Astrophysics Data System (ADS)

Kulshreshtha, Prashant Kumar

This thesis focuses on the fundamental understanding of stress-modified crack-propagation in photovoltaic (PV) silicon in relation to the critical issue of PV silicon "wafer breakage". The interactions between a propagating crack and impurities/defects/residual stresses have been evaluated for consequential fracture path in a thin PV Si wafer. To investigate the mechanism of brittle fracture in silicon, the phase transformations induced by elastic energy released at a propagating crack-tip have been evaluated by locally stressing the diamond cubic Si lattice using a rigid Berkovich nanoindenter tip (radius ≈50 nm). Unique pressure induced phase transformations and hardness variations have been then related to the distribution of precipitates (O, Cu, Fe etc.), and the local stresses in the wafer. This research demonstrates for the first time the "ductile-like fracture" in almost circular crack path that significantly deviates from its energetically favorable crystallographic [110](111) system. These large diameter (≈ 200 mm) Si wafers were sliced to less than 180 microm thickness from a Czochralski (CZ) ingot that was grown at faster than normal growth rates. The vacancy (vSi) driven precipitation of oxygen at enhanced thermal gradients in the wafer core develops large localized stresses (upto 100 MPa) which we evaluated using Raman spectral analysis. Additional micro-FTIR mapping and microscopic etch pit measurements in the wafer core have related the observed crack path deviations to the presence of concentric ring-like distributions of oxygen precipitates (OPs). To replicate these "real-world" breakage scenarios and provide better insight on crack-propagation, several new and innovative tools/devices/methods have been developed in this study. An accurate quantitative profiling of local stress, phase changes and load-carrying ability of Si lattice has been performed in the vicinity of the controlled micro-cracks created using micro-indentations to represent the surface/edge micro-cracks (i.e. sources of crack initiation). The low load (<10mN) nanoindentations using Hysitron Triboindenter RTM have been applied to estimate the zone of crack-propagation related plastic deformation and amorphization around the radial or the lateral cracks. The gradual reduction in hardness due to local stress field and phase change around the crack has been established using electron back scattered diffraction (EBSD), atomic force microscopy (AFM) and Raman spectroscopy, respectively, at nano- and micro-scale. The load (P) vs. displacement (h) curves depict characteristic phase transformation events (eg. elbow or pop-out) depending on the sign of residual stress in the silicon lattice. The formation of Si-XII/III phases (elastic phases) in large volumes during indentation of compressed Si lattice have been discussed as an option to eliminate the edge micro-cracks formed during wafer sawing by ductile flow. The stress gradient at an interface, which can be a grain-boundary (GB), twin or a interface between silicon and precipitate, has been evaluated for crack path modification. An direct-silicon-bonded (DSB) based ideal [110]/[100] interface has been examined to study the effect of crystallographic orientation variation across a planar silicon 2D boundary. Using constant source diffusion/annealing process, Fe and Cu impurities have been incorporated in model [110]/[100]GB to provide equivalence to a real decorated multi-crystalline grain boundary. We found that Fe precipitates harden the undecorated GB structure, whereas Cu precipitates introduce dislocation-induced plasticity to soften it. Aluminum Schottky diodes have been evaporated on the DSB samples to sensitively detect the instantaneous current response from the phase-transformed Si under nanoindenter tip. The impact of metallic impurity and their precipitates on characteristic phase transformations (i.e. pop-in or pop-out) demonstrate that scattered distribution of large Cu-precipitates (upto 50 nm) compresses Si-lattice to facilitate Si-XII/III formations, i.e. high pressure ductile phases. Sweeping voltage measurements at a given load determine that Si lattice has to be stressed beyond 1 mN to complete the Si-I (semiconducting) to Si-II (ohmic) phase changes. Above 1 mN load DSB sample has a varistor-like behavior due to higher grain-boundary resistance from interfacial states. The precipitate defect structure stimulated stresses at the bulk Si lattice or grain boundary modify the rate of elastic energy release at the crack-tip and associated phase change and hardness values in response to external loading. The systematic approach in this thesis elucidates that the interfacial surface area between Si-lattice and precipitate plays pivotal role in defining extent of stresses in the silicon, i.e. smaller precipitates in higher densities are severe than few larger volume precipitates. The finding of high-pressure ductile phase formation during loading of compressed silicon structure has been suggested to PV industry as a prospective candidate for reducing the wafer breakage and allowing larger handling stresses.

Tracking and Motion Analysis of Crack Propagations in Crystals for Molecular Dynamics

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

Tsap, L V; Duchaineau, M; Goldgof, D B

2001-05-14

This paper presents a quantitative analysis for a discovery in molecular dynamics. Recent simulations have shown that velocities of crack propagations in crystals under certain conditions can become supersonic, which is contrary to classical physics. In this research, they present a framework for tracking and motion analysis of crack propagations in crystals. It includes line segment extraction based on Canny edge maps, feature selection based on physical properties, and subsequent tracking of primary and secondary wavefronts. This tracking is completely automated; it runs in real time on three 834-image sequences using forty 250 MHZ processors. Results supporting physical observations aremore » presented in terms of both feature tracking and velocity analysis.« less

Gear Crack Propagation Path Studies-- Guidelines Developed for Ultrasafe Design

NASA Technical Reports Server (NTRS)

Lewicki, David G.

2002-01-01

Effective gear designs balance strength, durability, reliability, size, weight, and cost. However, unexpected gear failures may occur even with adequate gear tooth design. To design an extremely safe system, the designer must ask and address the question "What happens when a failure occurs?" With regard to gear-tooth bending fatigue, tooth or rim fractures may occur. For aircraft, a crack that propagated through a rim would be catastrophic, leading to the disengagement of a rotor or propeller, the loss of an aircraft, and possible fatalities. This failure mode should be avoided. However, a crack that propagated through a tooth might or might not be catastrophic, depending on the design and operating conditions. Also, early warning of this failure mode might be possible because of advances in modern diagnostic systems. An analysis was performed at the NASA Glenn Research Center to develop design guidelines to prevent catastrophic rim fracture failure modes in the event of gear-tooth bending fatigue. The finite element method was used with principles of linear elastic fracture mechanics. Crack propagation paths were predicted for a variety of gear tooth and rim configurations. The effects of rim and web thicknesses, initial crack locations, and gear-tooth geometry factors such as diametral pitch, number of teeth, pitch radius, and tooth pressure angle were considered. Design maps of tooth and rim fracture modes, including the effects of gear geometry, applied load, crack size, and material properties were developed. The occurrence of rim fractures significantly increased as the backup ratio (rim thickness divided by tooth height) decreased. The occurrence of rim fractures also increased as the initial crack location was moved down the root of the tooth. Increased rim and web compliance increased the occurrence of rim fractures. For gears with constant-pitch radii, coarser-pitch teeth increased the occurrence of tooth fractures over rim fractures. Also, 25 degree pressure angle teeth increased the occurrence of tooth fractures over rim fractures in comparison to 20 pressure angle teeth. For gears with a constant number of teeth or for gears with constant diametral pitch, varying size had little or no effect on crack propagation paths.

Reactive Behavior of Explosive Billets in Deflagration Tube of Varied Confinements

NASA Astrophysics Data System (ADS)

Hu, Haibo; Guo, Yingwen; Li, Tao; Fu, Hua; Shang, Hailin; Wen, Shanggang; Qiu, Tian; LaboratoryShock Wave; Detonation Physics Research Team

2017-06-01

The deflagration process of small size cylinder billets of pressed HMX-based explosive JO-9159 and the deflagration tube wall deformation is recorded by combined pressure velocity-meter high-speed frame photographic and radiographic diagnostic system. The influence of confinement structure strength on deflagration evolution behavior is compared with analysis of convective flame propagation along the slot between explosive billet and confinement wall.The follow-up reaction inside the cracks on the initiation site end surface on the side surfaces and between the end surfaces of explosive billets is restored with the analysis results of post experimental explosive billet remains.

Thermo-mechanically coupled fracture analysis of shape memory alloys using the extended finite element method

NASA Astrophysics Data System (ADS)

Hatefi Ardakani, S.; Ahmadian, H.; Mohammadi, S.

2015-04-01

In this paper, the extended finite element method is used for fracture analysis of shape memory alloys for both cases of super elastic and shape memory effects. Heat generation during the forward and reverse phase transformations can lead to temperature variation in the material because of strong thermo-mechanical coupling, which significantly influences the SMA mechanical behavior. First, the stationary crack mode is studied and the effects of loading rate on material behavior in the crack tip are examined. Then, the crack propagation analysis is performed in the presence of an initial crack by adopting a weighted averaging criterion, where the direction of crack propagation is determined by weighted averaging of effective stresses at all the integration points in the vicinity of the crack tip. Finally, several numerical examples are analyzed and the obtained results are compared with the available reference results.

Initiation and growth of multiple-site damage in the riveted lap joint of a curved stiffened fuselage panel: An experimental and analytical study

NASA Astrophysics Data System (ADS)

Ahmed, Abubaker Ali

As part of the structural integrity research of the National Aging Aircraft Research Program, a comprehensive study on multiple-site damage (MSD) initiation and growth in a pristine lap-joint fuselage panel has been conducted. The curved stiffened fuselage panel was tested at the Full-Scale Aircraft Structural Test Evaluation and Research (FASTER) facility located at the Federal Aviation Administration William J. Hughes Technical Center. A strain survey test was conducted to verify proper load application. The panel was then subjected to a fatigue test with constant-amplitude cyclic loading. The applied loading spectrum included underload marker cycles so that crack growth history could be reconstructed from post-test fractographic examinations. Crack formation and growth were monitored via nondestructive and high-magnification visual inspections. Strain gage measurements recorded during the strain survey tests indicated that the inner surface of the skin along the upper rivet row of the lap joint experienced high tensile stresses due to local bending. During the fatigue loading, cracks were detected by eddy-current inspections at multiple rivet holes along the upper rivet row. Through-thickness cracks were detected visually after about 80% of the fatigue life. Once MSD cracks from two adjacent rivet holes linked up, there was a quick deterioration in the structural integrity of the lap joint. The linkup resulted in a 2.87" (72.9-mm) lead fatigue crack that rapidly propagated across 12 rivet holes and crossed over into the next skin bay, at which stage the fatigue test was terminated. A post-fatigue residual strength test was then conducted by loading the panel quasi-statically up to final failure. The panel failed catastrophically when the crack extended instantaneously across three additional bays. Post-test fractographic examinations of the fracture surfaces in the lap joint of the fuselage panel were conducted to characterize subsurface crack initiation and growth. Results showed evidence of fretting damage and crack initiation at multiple locations near the rivet holes along the faying surface of the skin. The subsurface cracks grew significantly along the faying surface before reaching the outer surface of the skin, forming elliptical crack fronts. A finite element model (FE) of the panel was constructed and geometrically-nonlinear analyses conducted to determine strain distribution under the applied loads. The FE model was validated by comparing the analysis results with the strain gage measurements recorded during the strain survey test. The validated FE model was then used to determine stress-intensity factors at the crack tips. Stress-intensity factor results indicated that crack growth in the lap joint was under mixed-mode; however, the opening-mode stress intensity factor was dominant. The stress-intensity factors computed from the FE analysis were used to conduct cycle-by-cycle integration of fatigue crack growth. In the cycle-by-cycle integration, the NASGRO crack growth model was used with its parameters selected to account for the effects of plasticity-induced crack closure and the test environment on crack growth rate. Fatigue crack growth predictions from cycle-by-cycle computation were in good agreement with the experimental measured crack growth data. The results of the study provide key insights into the natural development and growth of MSD cracks in the pristine lap joint. The data provided by the study represent a valuable source for the evaluation and validation of analytical methodologies used for predicting MSD crack initiation and growth.

Experimental Study on the Growth, Coalescence and Wrapping Behaviors of 3D Cross-Embedded Flaws Under Uniaxial Compression

NASA Astrophysics Data System (ADS)

Zhou, Xiao-Ping; Zhang, Jian-Zhi; Wong, Louis Ngai Yuen

2018-05-01

The crack initiation, growth, wrapping and coalescence of two 3D pre-existing cross-embedded flaws in PMMA specimens under uniaxial compression are investigated. The stress-strain curves of PMMA specimens with 3D cross-embedded flaws are obtained. The tested PMMA specimens exhibit dominant elastic deformation and eventual brittle failure. The experimental results show that four modes of crack initiation and five modes of crack coalescence are observed. The initiations of oblique secondary crack and anti-wing crack in 3D cracking behaviors are first reported as well as the coalescence of anti-wing cracks. Moreover, two types of crack wrapping are found. Substantial wrapping of petal cracks, which includes open and closed modes of wrapping, appears to be the major difference between 2D and 3D cracking behaviors of pre-existing flaws, which are also first reported. Petal crack wraps symmetrically from either the propagated wing cracks or the coalesced wing cracks. Besides, only limited growth of petal cracks is observed, and ultimate failure of specimens is induced by the further growth of the propagated wing crack. The fracture mechanism of the tested PMMA specimens is finally revealed. In addition, the initiation stress and the peak stress versus the geometry of two 3D pre-existing cross-embedded flaws are also investigated in detail.

Fatigue of Self-Healing Nanofiber-based Composites: Static Test and Subcritical Crack Propagation.

PubMed

Lee, Min Wook; Sett, Soumyadip; Yoon, Sam S; Yarin, Alexander L

2016-07-20

Here, we studied the self-healing of composite materials filled with epoxy-containing nanofibers. An initial incision in the middle of a composite sample stretched in a static fatigue test can result in either crack propagation or healing. In this study, crack evolution was observed in real time. A binary epoxy, which acted as a self-healing agent, was encapsulated in two separate types of interwoven nano/microfibers formed by dual-solution blowing, with the core containing either epoxy or hardener and the shell being formed from poly(vinylidene fluoride)/ poly(ethylene oxide) mixture. The core-shell fibers were encased in a poly(dimethylsiloxane) matrix. When the fibers were damaged by a growing crack in this fiber-reinforced composite material because of static stretching in the fatigue test, they broke and released the healing agent into the crack area. The epoxy used in this study was cured and solidified for approximately an hour at room temperature, which then conglutinated and healed the damaged location. The observations were made for at least several hours and in some cases up to several days. It was revealed that the presence of the healing agent (the epoxy) in the fibers successfully prevented the propagation of cracks in stretched samples subjected to the fatigue test. A theoretical analysis of subcritical cracks was performed, and it revealed a jumplike growth of subcritical cracks, which was in qualitative agreement with the experimental results.

Redistribution of Welding Residual Stresses of Crack Tip Opening Displacement Specimen by Local Compression.

PubMed

Kim, Young-Gon; Song, Kuk-Hyun; Lee, Dong-Hoon; Joo, Sung-Min

2018-03-01

The demand of crack tip opening displacement (CTOD) test which evaluates fracture toughness of a cracked material is very important to ensure the stability of structure under severe service environment. The validity of the CTOD test result is judged using several criterions of the specification standards. One of them is the artificially generated fatigue pre-crack length inside the specimen. For acceptable CTOD test results, fatigue pre-crack must have a reasonable sharp crack front. The propagation of fatigue crack started from the tip of the machined notch, which might have propagated irregularly due to residual stress field. To overcome this problem, test codes suggest local compression method, reversed bending method and stepwise high-R ratio method to reduce the disparity of residual stress distribution inside the specimen. In this paper, the relation between the degree of local compression and distribution of welding residual stress has been analyzed by finite element analyses in order to determine the amount of effective local compression of the test piece. Analysis results show that initial welding residual stress is dramatically varied three-dimensionally while cutting, notch machining and local compressing due to the change of internal restraint force. From the simulation result, the authors find that there is an optimum amount of local compression to modify regularly for generating fatigue pre-crack propagation. In the case of 0.5% compressions of the model width is the most effective for uniforming residual stress distribution.

A proposal for unification of fatigue crack growth law

NASA Astrophysics Data System (ADS)

Kobelev, V.

2017-05-01

In the present paper, the new fractional-differential dependences of cycles to failure for a given initial crack length upon the stress amplitude in the linear fracture approach are proposed. The anticipated unified propagation function describes the infinitesimal crack length growths per increasing number of load cycles, supposing that the load ratio remains constant over the load history. Two unification fractional-differential functions with different number of fitting parameters are proposed. An alternative, threshold formulations for the fractional-differential propagation functions are suggested. The mean stress dependence is the immediate consequence from the considered laws. The corresponding formulas for crack length over the number of cycles are derived in closed form.

Reliability analysis of structures under periodic proof tests in service

NASA Technical Reports Server (NTRS)

Yang, J.-N.

1976-01-01

A reliability analysis of structures subjected to random service loads and periodic proof tests treats gust loads and maneuver loads as random processes. Crack initiation, crack propagation, and strength degradation are treated as the fatigue process. The time to fatigue crack initiation and ultimate strength are random variables. Residual strength decreases during crack propagation, so that failure rate increases with time. When a structure fails under periodic proof testing, a new structure is built and proof-tested. The probability of structural failure in service is derived from treatment of all the random variables, strength degradations, service loads, proof tests, and the renewal of failed structures. Some numerical examples are worked out.

Study of Near-Threshold Fatigue Crack Propagation in Pipeline Steels in High Pressure Environments

NASA Technical Reports Server (NTRS)

Mitchell, M.

1981-01-01

Near threshold fatigue crack propagation in pipeline steels in high pressure environments was studied. The objective was to determine the level of threshold stress intensity for fatigue crack growth rate behavior in a high strength low alloy X60 pipeline-type steel. Complete results have been generated for gaseous hydrogen at ambient pressure, laboratory air at ambient pressure and approximately 60% relative humidity as well as vacuum of 0.000067 Pa ( 0.0000005 torr) at R-ratios = K(min)/K(max) of 0.1, 0.5, and 0.8. Fatigue crack growth rate behavior in gaseous hydrogen, methane, and methane plus 10 percent hydrogen at 6.89 MPa (100 psi) was determined.

Simulation of Low Velocity Impact Induced Inter- and Intra-Laminar Damage of Composite Beams Based on XFEM

NASA Astrophysics Data System (ADS)

Sun, Wei; Guan, Zhidong; Li, Zengshan

2017-12-01

In this paper, the Inter-Fiber Fracture (IFF) criterion of Puck failure theory based on the eXtended Finite Element Method (XFEM) was implemented in ABAQUS code to predict the intra-laminar crack initiation of unidirectional (UD) composite laminate. The transverse crack path in the matrix can be simulated accurately by the presented method. After the crack initiation, the propagation of the crack is simulated by Cohesive Zoom Model (CZM), in which the displacement discontinuities and stress concentration caused by matrix crack is introduced into the finite element (FE) model. Combined with the usage of the enriched element interface, which can be used to simulate the inter-laminar delamination crack, the Low Velocity Impact (LVI) induced damage of UD composite laminate beam with a typical stacking of composite laminates [05/903]S is studied. A complete crack initiation and propagation process was simulated and the numerical results obtained by the XFEM are consistent with the experimental results.

Detection of crack in thin cylindrical pipes using piezo-actuated Lamb waves

NASA Astrophysics Data System (ADS)

Tua, P. S.; Quek, S. T.; Wang, Q.

2005-05-01

The detection of cracks in beams and plates using piezo-actuated Lamb waves has been presented in the last SPIE Symposium. This paper is an extension of the technique to pipes. It has been shown that for a thin-walled pipe, the assumption of Lamb wave propagation is valid. Such waves can be efficiently excited using piezoceramic transducers (PZT) with good control on the pulse characteristics to assess the health of structural components, such as the presence of cracks. In this paper, a systematic methodology to detect and locate cracks in homogenous cylinder/pipe based on the time-of-flight and strength analysis of propagating Lamb wave is proposed. By observing the attenuation in strength of the direct wave incidence at the sensor, the presence of a crack along the propagation path can be determined. At least four actuation positions, two on each end of the pipe segment of interest, are needed to exhaustively interrogate for the presence of cracks. The detailed procedure for locating and tracing the geometry of the crack(s) is described. It is shown experimentally that the detection using circular PZT actuator and sensor, with dimensions of 5.0 mm diameter and 0.5 mm thick, is possible for an aluminum pipe segment of up to at least 4.0 m in length. The proposed methodology is also explored for the aluminum pipe under more practical situations, such as burying it in sand with only the actuator and sensor positions exposed. Experimental results obtained showed the feasibility of detecting the 'concealed' crack on the pipe buried in sand.

Fatigue threshold studies in Fe, Fe-Si, and HSLA steel: Part II. thermally activated behavior of the effective stress intensity at threshold

NASA Astrophysics Data System (ADS)

Yu, W.; Esaklul, K.; Gerberich, W. W.

1984-05-01

It is shown that closure mechanisms alone cannot fully explain increasing fatigue thresholds with decreasing test temperature for a sequence of Fe-Si binary alloys and an HSLA steel. Implications are that fatigue crack propagation near threshold is a thermally activated process. The effective threshold stress intensity, which was obtained by subtracting the closure portion from the fatigue threshold, was examined. This effective stress intensity was found to correlate very well to the thermal component of the flow stress. A detailed fractographic study of the fatigue surface was performed. Water vapor in the room air was found to promote the formation of oxide and intergranular crack growth. At lower temperature, a brittle-type cyclic cleavage fatigue surface was observed but the ductile process persisted even at 123 K. Arrest marks were found on all three modes of fatigue crack growth. The regular spacings between these lines and dislocation modeling suggested that fatigue crack growth was controlled by the subcell structure near threshold. A model based on the slip-off of dislocations was examined. From this, it is shown that the effective fatigue threshold may be related to the square root of (one plus the strain rate sensitivity).

An Experimental Investigation of the Effects of Vacuum Environment on the Fatigue Life, Fatigue-Crack-Growth Behavior, and Fracture Toughness of 7075-T6 Aluminum Alloy. Ph.D. Thesis - North Carolina State Univ.

NASA Technical Reports Server (NTRS)

Hudson, C. M.

1972-01-01

Axial load fatigue life, fatigue-crack propagation, and fracture toughness tests were conducted on 0.090-inch thick specimens made of 7075-T6 aluminum alloy. The fatigue life and fatigue-crack propagation experiments were conducted at a stress ratio of 0.02. Maximum stresses ranged from 33 to 60 ksi in the fatigue life experiments, and from 10 to 40 ksi in the fatigue-crack propagation experiments, and fatigue life experiments were conducted at gas pressures of 760, 0.5, 0.05, and 0.00000005 torr. Fatigue-crack-growth and fracture toughness experiments were conducted at gas pressures of 760 and 5 x 10 to the minus 8th power torr. Residual stress measurements were made on selected fatigue life specimens to determine the effect of such stresses on fatigue life. Analysis of the results from the fatigue life experiments indicated that fatigue life progressively increased as the gas pressure decreased. 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 in air as in vacuum. Fracture toughness data showed there was essentially no difference in the fracture toughness of 7075-T6 in vacuum and in air.

Prediction of Fatigue Crack Growth in Rail Steels.

DOT National Transportation Integrated Search

1981-10-01

Measures to prevent derailments due to fatigue failures of rails require adequate knowledge of the rate of propagation of fatigue cracks under service loading. The report presents a computational model for the prediction of crack growth in rails. The...

Modeling and monitoring of tooth fillet crack growth in dynamic simulation of spur gear set

NASA Astrophysics Data System (ADS)

Guilbault, Raynald; Lalonde, Sébastien; Thomas, Marc

2015-05-01

This study integrates a linear elastic fracture mechanics analysis of the tooth fillet crack propagation into a nonlinear dynamic model of spur gear sets. An original formulation establishes the rigidity of sound and damaged teeth. The formula incorporates the contribution of the flexible gear body and real crack trajectories in the fillet zone. The work also develops a KI prediction formula. A validation of the equation estimates shows that the predicted KI are in close agreement with published numerical and experimental values. The representation also relies on the Paris-Erdogan equation completed with crack closure effects. The analysis considers that during dN fatigue cycles, a harmonic mean of ΔK assures optimal evaluations. The paper evaluates the influence of the mesh frequency distance from the resonances of the system. The obtained results indicate that while the dependence may demonstrate obvious nonlinearities, the crack progression rate increases with a mesh frequency augmentation. The study develops a tooth fillet crack propagation detection procedure based on residual signals (RS) prepared in the frequency domain. The proposed approach accepts any gear conditions as reference signature. The standard deviation and mean values of the RS are evaluated as gear condition descriptors. A trend tracking of their responses obtained from a moving linear regression completes the analysis. Globally, the results show that, regardless of the reference signal, both descriptors are sensitive to the tooth fillet crack and sharply react to tooth breakage. On average, the mean value detected the crack propagation after a size increase of 3.69 percent as compared to the reference condition, whereas the standard deviation required crack progressions of 12.24 percent. Moreover, the mean descriptor shows evolutions closer to the crack size progression.

Crack propagation modelling for high strength steel welded structural details

NASA Astrophysics Data System (ADS)

Mecséri, B. J.; Kövesdi, B.

2017-05-01

Nowadays the barrier of applying HSS (High Strength Steel) material in bridge structures is their low fatigue strength related to yield strength. This paper focuses on the fatigue behaviour of a structural details (a gusset plate connection) made from NSS and HSS material, which is frequently used in bridges in Hungary. An experimental research program is carried out at the Budapest University of Technology and Economics to investigate the fatigue lifetime of this structural detail type through the same test specimens made from S235 and S420 steel grades. The main aim of the experimental research program is to study the differences in the crack propagation and the fatigue lifetime between normal and high strength steel structures. Based on the observed fatigue crack pattern the main direction and velocity of the crack propagation is determined. In parallel to the tests finite element model (FEM) are also developed, which model can handle the crack propagation. Using the measured strain data in the tests and the calculated values from the FE model, the approximation of the material parameters of the Paris law are calculated step-by-step, and their calculated values are evaluated. The same material properties are determined for NSS and also for HSS specimens as well, and the differences are discussed. In the current paper, the results of the experiments, the calculation method of the material parameters and the calculated values are introduced.

Failure analysis of aluminum alloy components

NASA Technical Reports Server (NTRS)

Johari, O.; Corvin, I.; Staschke, J.

1973-01-01

Analysis of six service failures in aluminum alloy components which failed in aerospace applications is reported. Identification of fracture surface features from fatigue and overload modes was straightforward, though the specimens were not always in a clean, smear-free condition most suitable for failure analysis. The presence of corrosion products and of chemically attacked or mechanically rubbed areas here hindered precise determination of the cause of crack initiation, which was then indirectly inferred from the scanning electron fractography results. In five failures the crack propagation was by fatigue, though in each case the fatigue crack initiated from a different cause. Some of these causes could be eliminated in future components by better process control. In one failure, the cause was determined to be impact during a crash; the features of impact fracture were distinguished from overload fractures by direct comparisons of the received specimens with laboratory-generated failures.

Gear Crack Propagation Path Studies: Guidelines for Ultra-Safe Design

NASA Technical Reports Server (NTRS)

Lewicki, David G.

2001-01-01

Design guidelines have been established to prevent catastrophic rim fracture failure modes when considering gear tooth bending fatigue. Analysis was performed using the finite element method with principles of linear elastic fracture mechanics. Crack propagation paths were predicted for a variety of gear tooth and rim configurations. The effects of rim and web thicknesses, initial crack locations, and gear tooth geometry factors such as diametral pitch, number of teeth, pitch radius, and tooth pressure angle were considered. Design maps of tooth/rim fracture modes including effects of gear geometry, applied load, crack size, and material properties were developed. The occurrence of rim fractures significantly increased as the backup ratio (rim thickness divided by tooth height) decreased. The occurrence of rim fractures also increased as the initial crack location was moved down the root of the tooth. Increased rim and web compliance increased the occurrence of rim fractures. For gears with constant pitch radii, coarser-pitch teeth increased the occurrence of tooth fractures over rim fractures. Also, 25 deg pressure angle teeth had an increased occurrence of tooth fractures over rim fractures when compared to 20 deg pressure angle teeth. For gears with constant number of teeth or gears with constant diametral pitch, varying size had little or no effect on crack propagation paths.

United States Air Force Summer Faculty Research Program for 1990. Program Management Report

DTIC Science & Technology

1991-06-05

propagation characteristics were extensively studied using pencil lead breaks in a center notch. For the fatigue studies center crack samples of 2024 - T351 ... aluminum specimens during fatigue cycling. The experimental procedure involved excitation of Rayleigh waves on the surface of each specimen and...Research Program (SFRP) provides opportunities for research in the physical sciences, engineering, and life sciences. The program has been effective

Packaging Reliability Effect of ENIG and ENEPIG Surface Finishes in Board Level Thermal Test under Long-Term Aging and Cycling

PubMed Central

Shen, Chaobo; Hai, Zhou; Zhao, Cong; Zhang, Jiawei; Evans, John L.; Bozack, Michael J.; Suhling, Jeffrey C.

2017-01-01

This study illustrates test results and comparative literature data on the influence of isothermal aging and thermal cycling associated with Sn-1.0Ag-0.5Cu (SAC105) and Sn-3.0Ag-0.5Cu (SAC305) ball grid array (BGA) solder joints finished with ENIG and ENEPIG on the board side and ENIG on the package side compared with ImAg plating on both sides. The resulting degradation data suggests that the main concern for 0.4 mm pitch 10 mm package size BGA is package side surface finish, not board side. That is, ENIG performs better than immersion Ag for applications involving long-term isothermal aging. SAC305, with a higher relative fraction of Ag3Sn IMC within the solder, performs better than SAC105. SEM and polarized light microscope analysis show cracks propagated from the corners to the center or even to solder bulk, which eventually causes fatigue failure. Three factors are discussed: IMC, grain structure, and Ag3Sn particle. The continuous growth of Cu-Sn intermetallic compounds (IMC) and grains increase the risk of failure, while Ag3Sn particles seem helpful in blocking the crack propagation. PMID:28772811

Spiraling Cracks in Thin Sheets

NASA Astrophysics Data System (ADS)

Romero, Victor; Roman, Benoit; Cerda, Enrique

2008-03-01

A wide kind of everyday-life industrial products come in a thin package that needs to be torn open by the user, and the opening is not always easy. We built a simple setup to study crack propagation in thin sheets coupled with large out-of-plane displacement : A cylindrical tool is inserted in a straight incision in a thin sheet, and is pushed against the sheet perpendicularly to that incision, eventually propagating a crack. When the blunt tool is continually pushed against the lip, we found that the crack follows a very robust spiraling path. Experiments may be interpreted in terms of ``Spira Mirabilis'' (logarithmic spiral). Starting with crack theory argument, we will show that the early behavior of the cut path follows a portion of a logathmic spiral, and that the path tends to another spiral with a different pitch as the crack adds more turns. Our crack experiment illustrates the fact that thin sheets mechanics is deeply connected to geometry, and finally spirals characteristics allow us to measure material crack properties of the thin layer used.

A physical model study of scattering of waves by aligned cracks: Comparison between experiment and theory

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

Ass'ad, J.M.; McDonald, J.A.; Kusky, T.M.

1993-04-01

An approximation to plane-wave propagation through a composite material is examined using a physical model with oriented but randomly distributed penny-shaped rubber inclusions within an isotropic epoxy resin matrix. A pulse transmission method is used to determine velocities of shear and compressional waves as a function of angle of incidence and crack density. The experimental and theoretical results of Hudson were compared and limitations within the crack parameters used in this study have been determined. Results from both polarized shear waves (S1, S2) compare favorably with the theory for a composite with up to 7% crack density, but theory andmore » experiment diverge at higher crack densities. On the other hand, compressional-wave velocities at low crack densities (1% and 3%) compare favorably with the theory. It is also shown that the velocity ratio V[sub p]/V[sub s] for two extreme cases, i.e. propagation normal and parallel to the cracks, as a function of crack density and porosity, has a strong directional dependence.« less

High-Frequency, High-Temperature Fretting Experiments

NASA Technical Reports Server (NTRS)

Matlik, J. F.; Farris, T. N.; Haake, F. K.; Swanson, G. R.; Duke, G. C.

2005-01-01

Fretting is a structural damage mechanism observed when two nominally clamped surfaces are subjected to an oscillatory loading. A critical location for fretting induced damage has been identified at the blade/disk and blade/damper interfaces of gas turbine engine turbomachinery and space propulsion components. The high-temperature, high-frequency loading environment seen by these components lead to severe stress gradients at the edge-of-contact. These contact stresses drive crack nucleation and propagation in fretting and are very sensitive to the geometry of the contacting bodies, the contact loads, materials, temperature, and contact surface tribology (friction). To diagnose the threat that small and relatively undetectable fretting cracks pose to damage tolerance and structural integrity of in-service components, the objective of this work is to develop a well-characterized experimental fretting rig capable of investigating fretting behavior of advanced aerospace alloys subjected to load and temperature conditions representative of such turbomachinery components.

Micro-fracture Behavior of Organic Matters in Kerogen-Rich Shale (KRS) Composites from Micro- to Milli-meter Scales

NASA Astrophysics Data System (ADS)

Haque, M. H.; Han, Y.; Hull, K. L.; Abousleiman, Y. N.

2017-12-01

Understanding the failure behavior of kerogen-rich shale (KRS) at multiscale is critical to efficient hydraulic fracture stimulations in unconventional source shale reservoirs. As a composite material consisting of compacted clay particles, silt-sized grains, and organic matter (OM), KRS is highly complex both structurally and mechanically. The OM, which is intertwined within the shale matrix, presents a particular challenge as it can be much more compliant than its surrounding minerals while at the same time have a significantly higher tensile strength. The mode-I fracture toughness and tensile failure behavior of KRS has been studied at the core scale by traditional rock mechanics methods i.e., Brazilian tests and more recently with non-traditional approaches at the micro-scale using nanoindentation techniques. However, core scale testing fails in precisely capturing the effects of OM due to its coarse resolution, while nanoindention may capture the behavior of isolated component but in some cases miss the collective properties of the composite system. To bridge this gap, while still complying with ASTM/ISRM standards in principle, we investigate fracture initiation and propagation in KRS using the single-edge notched beam (SENB) miniature samples with span length in the millimeter scale. The size scale attempts to isolate the contributions from individual components, especially the OM, to the emergent and systematic fracturing behavior of KRS. Crack propagation along and across the bedding planes have left noticeable signatures on fractured OM while travelling through and around an OM body depending upon its size and spatial position along the crack path illustrating what looks like crack arrest and/or crack bridging in a composite porous matrix. The fractured surface of OM, even being polymeric in nature, exhibits smooth and even surface profile when ripped apart but not in all observed surfaces. Unique microscale features such as- ridges, twists, and inclusions have also been observed for the OM indicating a mix of complex modes of failures. This study helps further the understanding of fracture morphologies in source rock reservoirs.

Compilation of fatigue, fatigue-crack propagation, and fracture data for 2024 and 7075 aluminum, Ti-6Al-4V titanium, and 300M steel. Volume 1: Description of data and data storage on magnetic tape. Volume 2: Data tape (7-track magnetic tape)

NASA Technical Reports Server (NTRS)

Rice, R. C.; Reynolds, J. L.

1976-01-01

Fatigue, fatigue-crack-propagation, and fracture data compiled and stored on magnetic tape are documented. Data for 202 and 7075 aluminum alloys, Ti-6Al-4V titanium alloy, and 300M steel are included in the compilation. Approximately 4,500 fatigue, 6,500 fatigue-crack-propagation, and 1,500 fracture data points are stored on magnetic tape. Descriptions of the data, an index to the data on the magnetic tape, information on data storage format on the tape, a listing of all data source references, and abstracts of other pertinent test information from each data source reference are included.

Gamma irradiation alters fatigue-crack behavior and fracture toughness in 1900H and GUR 1050 UHMWPE.

PubMed

Cole, Jantzen C; Lemons, Jack E; Eberhardt, Alan W

2002-01-01

Pitting and delamination remain causative factors of polyethylene failure in total knee replacement. Gamma irradiation induces cross linking in ultra-high-molecular-weight polyethylene, which has been shown to improve wear resistance. Irradiation may reduce fracture toughness and fatigue strength, however, and the effects of irradiation are dependent upon the resin, processing technique, and radiation dose. The effects of varying levels of gamma irradiation (0, 33, 66, and 100 kGy) on the fracture toughness and fatigue-crack resistance of UHMWPE, isostatically molded from 1900H and GUR 1050 resins, were examined. Paris law regressions were performed to quantify fatigue-crack propagation rates as functions of change in stress intensity, and J-integral methods were used to quantify the elastic-plastic fracture toughness. The results indicated that gamma irradiation reduced the resistance of both materials to fatigue-crack growth, and that the reductions were radiation dosage and resin dependent. Irradiation at any level was detrimental to the fracture toughness of the 1900H specimens. Irradiation at 33 kGy increased fracture toughness for the GUR 1050 specimens, and substantial reductions were observed only at the highest irradiation level. Scanning electron microscopy of the fracture surface revealed diamond-like fracture patterns of the nonirradiated specimens indicative of ductile, multilevel fracture. Pronounced striations were apparent on these fracture surfaces, oriented perpendicular to the direction of crack growth. The striations appeared as folds in surface layers of the GUR 1050 specimens. At the highest irradiation levels, the striations were nearly eliminated on the fracture surfaces of the 1900H specimens, and were markedly less severe for the GUR 1050. These results demonstrated that at higher irradiation levels the materials became more brittle in fatigue, with less ductile folding and tearing of the fracture surfaces. Copyright 2002 Wiley Periodicals, Inc. J Biomed Mater Res (Appl Biomater) 63: 559-566, 2002

Field evidence for control of quarrying by rock bridges in jointed bedrock

NASA Astrophysics Data System (ADS)

Hooyer, T. S.; Cohen, D. O.; Iverson, N. R.

2011-12-01

Quarrying is generally thought to be the most important mechanism by which glaciers erode bedrock. In quarrying models it is assumed that slow, subcritical, growth of pre-existing cracks rate-limits the process and occurs where there are large stress differences in the bed, such as near rock bumps where ice separates from the bed to form water-filled cavities. Owing to the direction of principal stresses in rocks associated with sliding and resultant cavity formation, models predict that quarrying will occur along cracks oriented perpendicular to the ice flow direction or parallel to zones of ice-bed contact. Preglacial cracks in rocks will tend to propagate mainly downward, and in sedimentary or some metamorphic rocks will merge with bedding planes, thereby helping to isolate rock blocks for dislodgement. In contrast to these model assumptions, new measurements of quarried surface orientations in the deglaciated forefield of nine glaciers in Switzerland and Canada indicate a strong correlation between orientations of pre-existing joints and quarried bedrock surfaces, independent of ice flow direction or ice-water contact lines. The strong correlation persists across all rock types, and rocks devoid of major joints lack quarried surfaces. We propose a new conceptual model of quarrying that idealizes the bedrock as a series of blocks separated by discontinuous preglacial joints containing intact rock bridges. Bridges concentrate stress differences caused by normal and shear forces acting at the rock surface. Failure of bridges is caused by slow subcritical crack growth enhanced by water pressure fluctuations. To lend credibility to this new model, we show field evidence of failed rock bridges in quarried surfaces and of rib marks on plumose structures that we interpret as arrest fracture fronts due to transient subglacial water-pressure fluctuations.

Adapted diffusion processes for effective forging dies

NASA Astrophysics Data System (ADS)

Paschke, H.; Nienhaus, A.; Brunotte, K.; Petersen, T.; Siegmund, M.; Lippold, L.; Weber, M.; Mejauschek, M.; Landgraf, P.; Braeuer, G.; Behrens, B.-A.; Lampke, T.

2018-05-01

Hot forging is an effective production method producing safety relevant parts with excellent mechanical properties. The economic efficiency directly depends on the occurring wear of the tools, which limits service lifetime. Several approaches of the presenting research group aim at minimizing the wear caused by interacting mechanical and thermal loads by using enhanced nitriding technology. Thus, by modifying the surface zone layer it is possible to create a resistance against thermal softening provoking plastic deformation and pronounced abrasive wear. As a disadvantage, intensely nitrided surfaces may possibly include the risk of increased crack sensitivity and therefore feature the chipping of material at the treated surface. Recent projects (evaluated in several industrial applications) show the high technological potential of adapted treatments: A first approach evaluated localized treatments by preventing areas from nitrogen diffusion with applied pastes or other coverages. Now, further ideas are to use this principle to structure the surface with differently designed patterns generating smaller ductile zones beneath nitrided ones. The selection of suitable designs is subject to certain geo-metrical requirements though. The intention of this approach is to prevent the formation and propagation of cracks under thermal shock conditions. Analytical characterization methods for crack sensitivity of surface zone layers and an accurate system of testing rigs for thermal shock conditions verified the treatment concepts. Additionally, serial forging tests using adapted testing geometries and finally, tests in the industrial production field were performed. Besides stabilizing the service lifetime and decreasing specific wear mechanisms caused by thermal influences, the crack behavior was influenced positively. This leads to a higher efficiency of the industrial production process and enables higher output in forging campaigns of industrial partners.

Statistical distribution of time to crack initiation and initial crack size using service data

NASA Technical Reports Server (NTRS)

Heller, R. A.; Yang, J. N.

1977-01-01

Crack growth inspection data gathered during the service life of the C-130 Hercules airplane were used in conjunction with a crack propagation rule to estimate the distribution of crack initiation times and of initial crack sizes. A Bayesian statistical approach was used to calculate the fraction of undetected initiation times as a function of the inspection time and the reliability of the inspection procedure used.

A total life prediction model for stress concentration sites

NASA Technical Reports Server (NTRS)

Hartman, G. A.; Dawicke, D. S.

1983-01-01

Fatigue crack growth tests were performed on center crack panels and radial crack hole samples. The data were reduced and correlated with the elastic parameter K taking into account finite width and corner crack corrections. The anomalous behavior normally associated with short cracks was not observed. Total life estimates for notches were made by coupling an initiation life estimate with a propagation life estimate.

A total life prediction model for stress concentration sites

NASA Technical Reports Server (NTRS)

Hartman, G. A.; Dawicke, D. S.

1983-01-01

Fatigue crack growth tests were performed on center crack panels and radial crack hole samples. The data were reduced and correlated with the elastic parameter-K taking into account finite width and corner crack corrections. The anomalous behavior normally associated with short cracks was not observed. Total life estimates for notches were made by coupling an initiation life estimate with a propagation life estimate.

Effect of thermal tempering on strength and crack propagation behavior of feldspathic porcelains.

PubMed

Anusavice, K J; Hojjatie, B

1991-06-01

The objective of this study was to test the hypothesis that tempering stress can retard the growth of surface cracks in layered porcelain discs with variable levels of contraction mismatch. Porcelain discs, 16 mm in diameter and 2 mm thick, were prepared with a 0.5-mm-thick layer of opaque porcelain (O) and a 1.5-mm-thick layer of body porcelain (B). The materials were selected to produce contraction coefficient differences, alpha O-alpha B, of +3.2, +0.7, -0.9, and -1.5 ppm/degrees C. Body porcelain discs with a thickness of 2 mm were used as the thermally compatible control specimens (delta alpha = 0). The discs were fired to the maturing temperature of body porcelain (982 degrees C) and were then subjected to three cooling procedures: slow cooling (SC) in a furnace, fast cooling (FC) in air, and tempering (T) by blasting the surface of the body porcelain with compressed and dried air for 90 s. The dimensions of cracks induced by a Vickers microhardness indenter under a load of 4.9 N were measured at baseline and six months after indentation at 80 points along diametral lines within the surface of body porcelain. In addition, biaxial flexure tests were performed to determine the influence of mismatch and tempering on flexure strength. The results of ANOVA indicate that crack dimensions were influenced significantly by the interaction of cooling rate and contraction mismatch (p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Synchrotron X-ray measurement of residual strain within the nose of a worn manganese steel railway crossing

NASA Astrophysics Data System (ADS)

Dhar, S.; Zhang, Y.; Xu, R.; Danielsen, HK; Jensen, D. Juul

2017-07-01

Switches and crossings are an integral part of any railway network. Plastic deformation associated with wear and rolling contact fatigue due to repeated passage of trains cause severe damage leading to the formation of surface and sub-surface cracks which ultimately may result in rail failure. Knowledge of the internal stress distribution adds to the understanding of crack propagation and may thus help to prevent catastrophic rail failures. In this work, the residual strains inside the bulk of a damaged nose of a manganese railway crossing that was in service for five years has been investigated by using differential aperture synchrotron X-ray diffraction. The main purpose of this paper is to describe how this method allows non-destructive measurement of residual strains in selected local volumes in the bulk of the rail. Measurements were conducted on the transverse surface at a position about 6.5 mm from the rail running surface of a crossing nose. The results revealed the presence of significant compressive residual strains along the running direction of the rail.

Fracture Toughness to Understand Stretch-Flangeability and Edge Cracking Resistance in AHSS

NASA Astrophysics Data System (ADS)

Casellas, Daniel; Lara, Antoni; Frómeta, David; Gutiérrez, David; Molas, Sílvia; Pérez, Lluís; Rehrl, Johannes; Suppan, Clemens

2017-01-01

The edge fracture is considered as a high risk for automotive parts, especially for parts made of advanced high strength steels (AHSS). The limited ductility of AHSS makes them more sensitive to the edge damage. The traditional approaches, such as those based on ductility measurements or forming limit diagrams, are unable to predict this type of fractures. Thus, stretch-flangeability has become an important formability parameter in addition to tensile and formability properties. The damage induced in sheared edges in AHSS parts affects stretch-flangeability, because the generated microcracks propagate from the edge. Accordingly, a fracture mechanics approach may be followed to characterize the crack propagation resistance. With this aim, this work addresses the applicability of fracture toughness as a tool to understand crack-related problems, as stretch-flangeability and edge cracking, in different AHSS grades. Fracture toughness was determined by following the essential work of fracture methodology and stretch-flangeability was characterized by means of hole expansions tests. Results show a good correlation between stretch-flangeability and fracture toughness. It allows postulating fracture toughness, measured by the essential work of fracture methodology, as a key material property to rationalize crack propagation phenomena in AHSS.

Self-healing fiber-reinforced composite

NASA Astrophysics Data System (ADS)

Lee, Minwook; Yoon, Sam; Yarin, Alexander

In the present work two parts of the healing agent (commercially available epoxy resin and hardener) are encapsulated in separate polymeric nanofibers. The fibers are generated by a single-step dual coaxial solution blowing. The core-shell fibers with the diameters in the 200-2600 nm range are encased in the PDMS (polydimethyl siloxane) matrix to form a self-healing composite material. Under fatigue conditions, the core-shell fibers inside the composite material are ruptured and the healing agents released into the surrounding matrix. Various fatigue conditions including repeated bending and stretching are used to damage the composites and the degree of self-healing is quantified after that. Also, an incision resembling a crack is pre-notched and crack propagation is studied. It is found that the presence of the self-healing agents in the fibers significantly retards crack propagation due to curing by the epoxy at the ruptured site. The stiffness of the composites is also measured for the samples containing self-healing fibers inside them before and after the fatigue tests. A novel theory of crack propagation is proposed, which explains the observed jump-like growth of sub-critical cracks. This work was supported by the International Collaboration Program funded by the Agency for Defense Development.

Acoustic emission monitoring of crack propagation in additively manufactured and conventional titanium components

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

Strantza, Maria; Van Hemelrijck, Danny; Guillaume, Patrick

We report that additive manufacturing (AM) is a novel and innovative production technology that can produce complex and lightweight engineering products. In AM components, as in all engineering materials, fatigue is considered as one of the principle causes of unexpected failure. In order to detect, localise and characterise cracks in various material components and metals, acoustic emission (AE) is used as a non-destructive monitoring technique. One of the main advantages of AE is that it can be also used for dynamic damage characterisation and specifically for crack propagation monitoring. In this research, we use AE to monitor the fatigue crackmore » growth behaviour of Ti6Al4V components under four-point bending. The samples were produced by means of AM as well as conventional material. Notched and unnotched specimens were investigated with respect to the crack severity and crack detection using AE. The main AE signal parameters –such as cumulative events, hits, duration, average frequency and rise time– were evaluated and indicate sensitivity to damage propagation in order to lead to a warning against the final fracture occurrence. Finally, this is the first time that AE is applied in AM components under fatigue.« less

Acoustic emission monitoring of crack propagation in additively manufactured and conventional titanium components

DOE PAGES

Strantza, Maria; Van Hemelrijck, Danny; Guillaume, Patrick; ...

2017-05-31

We report that additive manufacturing (AM) is a novel and innovative production technology that can produce complex and lightweight engineering products. In AM components, as in all engineering materials, fatigue is considered as one of the principle causes of unexpected failure. In order to detect, localise and characterise cracks in various material components and metals, acoustic emission (AE) is used as a non-destructive monitoring technique. One of the main advantages of AE is that it can be also used for dynamic damage characterisation and specifically for crack propagation monitoring. In this research, we use AE to monitor the fatigue crackmore » growth behaviour of Ti6Al4V components under four-point bending. The samples were produced by means of AM as well as conventional material. Notched and unnotched specimens were investigated with respect to the crack severity and crack detection using AE. The main AE signal parameters –such as cumulative events, hits, duration, average frequency and rise time– were evaluated and indicate sensitivity to damage propagation in order to lead to a warning against the final fracture occurrence. Finally, this is the first time that AE is applied in AM components under fatigue.« less