Dynamic crack propagation in a viscoelastic strip
NASA Astrophysics Data System (ADS)
Popelar, C. H.; Atkinson, C.
1980-04-01
THE DYNAMIC PROPAGATION of a semi-infinite crack in a finite linear viscoelastic strip subjected to Mode I loading is investigated. Through the use of integral transforms the problem is reduced to solving a Wiener-Hopf equation. The asymptotic properties of the transforms are exploited to establish the stress intensity factor. Plane-stress and plane-strain stress intensity factors as a function of crack speed for both fully-clamped and shear-free lateral boundaries are presented for the standard linear viscoelastic solid. Comparisons are made with previously obtained asymptotic stress intensity factors and with stress intensity factors for the equivalent elastic strips.
Dynamic crack propagation through nanoporous media
NASA Astrophysics Data System (ADS)
Nguyen, Thao; Wilkerson, Justin
2015-06-01
The deformation and failure of nanoporous metals may be considerably different than that of more traditional bulk porous metals. The length scales in traditional bulk porous metals are typically large enough for classic plasticity and buckling to be operative. However, the extremely small length scales associated with nanoporous metals may inhibit classic plasticity mechanisms. Here, we motivate an alternative nanovoid growth mechanism mediated by dislocation emission. Following an approach similar to Lubarda and co-workers, we make use of stability arguments applied to the analytic solutions of the elastic interactions of dislocations and voids to derive a simple stress-based criterion for emission activation. We then propose a dynamic nanovoid growth law that is motivated by the kinetics of dislocation emission. The resulting failure model is implemented into a commercial finite element software to simulate dynamic crack growth. The simulations reveal that crack propagation through a nanoporous media proceeds at somewhat faster velocities than through the more traditional bulk porous metal.
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.
Molecular dynamics studies of interfacial crack propagation in heterogeneous media
Corbett, J.M. |; Selinger, R.L.B.
1999-08-01
The authors use molecular dynamics simulation to investigate the evolution of a crack front in interfacial fracture in three dimensions. They find that when a crack passes through a localized region of heterogeneous toughness, crack front waves are initiated and propagate laterally. They also investigate the development of roughness of the crack front when the crack propagates in a region of heterogeneous toughness. They find that in steady state the mean square width W of the front scales with system size L as W {approximately} L{sup 0.35}, in agreement with recent theoretical predictions.
Dynamic delamination crack propagation in a graphite/epoxy laminate
NASA Technical Reports Server (NTRS)
Grady, J. E.; Sun, C. T.
1991-01-01
Dynamic delamination crack propagation in a (90/0) 5s Graphite/Epoxy laminate with an embedded interfacial crack was investigated experimentally using high speed photography. The dynamic motion was produced by impacting the beamlike laminate specimen with a silicon rubber ball. The threshold impact velocities required to initiate dynamic crack propagation in laminates with varying initial crack positions were determined. The crack propagation speeds were estimated from the photographs. Results show that the through the thickness position of the embedded crack can significantly affect the dominant mechanism and the threshold impact velocity for the onset of crack movement. If the initial delamination is placed near the top of bottom surface of the laminate, local buckling of the delaminated plies may cause instability of the crack. If the initial delamination lies on the midplane, local buckling does not occur and the initiation of crack propagation appears to be dominated by Mode II fracture. The crack propagation and arrest observed was seen to be affected by wave motion within the delamination region.
Dynamic initiation and propagation of cracks in unidirectional composite plates
NASA Astrophysics Data System (ADS)
Coker, Demirkan
Dynamic crack growth along weak planes is a significant mode of failure in composites and other layered/sandwiched structures and is also the principal mechanism of shallow crustal earthquakes. In order to shed light on this phenomenon dynamic crack initiation and propagation characteristics of a model fiber-reinforced unidirectional graphite/epoxy composite plate was investigated experimentally. Dynamic fracture experiments were conducted by subjecting the composite plates to in-plane, symmetric and asymmetric, impact loading. The lateral shearing interferometric technique of coherent gradient sensing (CGS) in conjunction with high-speed photography was used to visualize the failure process in real time. It was found that mode-I cracks propagated subsonically with crack speeds increasing to the neighborhood of the Rayleigh wave speed of the composite. Also in mode-I, the dependence of the dynamic initiation fracture toughness on the loading rate was determined and was found to be constant for low loading rates and to increase rapidly above K˙dI>10 5 . The dynamic crack propagation toughness, KID, was observed to decrease with crack tip speed up to the Rayleigh wave speed of the composite. For asymmetric, mode-II, types of loading the results revealed highly unstable and intersonic shear-dominated crack growth along the fibers. These cracks propagated with unprecedented speeds reaching 7400 m/s which is the dilatational wave speed of the composite along the fibers. For intersonic crack growth, the interferograms, featured a shock wave structure typical of disturbances traveling with speeds higher than one of the characteristic wave speeds in the solid. In addition high speed thermographic measurements are conducted that show concentrated hot spots behind the crack tip indicating non-uniform crack face frictional contact. In addition, shear dominated dynamic crack growth is investigated along composite/Homalite interfaces subjected to impact loading. The crack
Crack propagation through adhesive interface in glass driven by dynamic loading
NASA Astrophysics Data System (ADS)
Park, Hwun
Dynamic crack behaviors at glass interfaces were investigated to understand dynamic failure mechanisms of glass. To produce highly intensive and rapidly increasing loading, glass specimens jointed with epoxy adhesive in edge-to-edge configurations were impacted on their notched edges with plastic projectiles. Cracks developed from the notch and propagated into the interfaces between glass plates at the maximum speed. The patterns of crack propagation through the interfaces were observed to depend on the interface's conditions. Crack propagation stops at the interface where no adhesive was applied. The crack penetrates through the interface where two glass plates were bonded directly without any space. If the interface has finite thickness of an adhesive layer, a crack passing through the interface branches into multiple cracks immediately when it extends to the second glass plate. Both of the slow crack speed in the epoxy adhesive and resistance for crack initiation in the second glass account for the delay in crack propagation at the interface. The surface conditions of glass at the interface affect the resistance for crack initiation. Mirror-like polished surfaces have better resistance than rough surfaces trimmed by a water jet. If the polished surface is etched with hydrofluoric acid to remove surface flaws, the glass surfaces have higher strength and resistance for damage. This etched glass even ceases crack propagation completely with a sufficiently thick adhesive layer. Crack branching has been an open topic. Exact explanation has not been given yet. As the consistent shape of crack branching are created with the proposed method, diagnostics experiments were conducted to reveal the nature of crack branching. To investigate interaction between stress propagation and crack branching, stress histories synchronizing with high speed images were measured. Two types of specimen were used to vary stress distribution during crack propagation. The apex angle of
Molecular dynamics simulation of effect of hydrogen atoms on crack propagation behavior of α-Fe
NASA Astrophysics Data System (ADS)
Song, H. Y.; Zhang, L.; Xiao, M. X.
2016-12-01
The effect of the hydrogen concentration and hydrogen distribution on the mechanical properties of α-Fe with a pre-existing unilateral crack under tensile loading is investigated by molecular dynamics simulation. The results reveal that the models present good ductility when the front region of crack tip has high local hydrogen concentration. The peak stress of α-Fe decreases with increasing hydrogen concentration. The studies also indicate that for the samples with hydrogen atoms, the crack propagation behavior is independent of the model size and boundaries. In addition, the crack propagation behavior is significantly influenced by the distribution of hydrogen atoms.
NASA Astrophysics Data System (ADS)
Suzuki, Shinichi; Homma, Hiroomi; Kusaka, Riichiro
A METHOD OF pulsed holographic microscopy is applied to take instantaneous microscopic photographs of the neighborhoods of crack tips propagating through PMMA or through AISI 4340 steel specimens at a speed of several hundred meters per second. The cracks are in the opening mode. A fast propagating crack is recorded as a hologram at an instant during its propagation. A microscopic photograph of the crack is taken with a conventional microscope to magnify the reconstructed image from the hologram. From the microscopic photograph, crack opening displacement (COD) is measured along the crack in the vicinity of the crack tip. The COD is of the order often to one hundred microns, and in proportion to the square root of the distance from the crack tip. The dynamic fracture toughness KID is obtained using the formula for COD in the singular stress field of a fast propagating crack. Simultaneous KID measurement both through pulsed holographic microscopy and through the caustic method is furthermore carried out with PMMA specimens. The values of KID obtained through pulsed holographic microscopy are in agreement with those through the caustic method. Microcracks accompanied by a main crack are also photographed with the method of pulsed holographic microscopy.
Gear Crack Propagation Investigation
NASA Technical Reports Server (NTRS)
1995-01-01
Reduced weight is a major design goal in aircraft power transmissions. Some gear designs incorporate thin rims to help meet this goal. Thin rims, however, may lead to bending fatigue cracks. These cracks may propagate through a gear tooth or into the gear rim. A crack that propagates through a tooth would probably not be catastrophic, and ample warning of a failure could be possible. On the other hand, a crack that propagates through the rim would be catastrophic. Such cracks could lead to disengagement of a rotor or propeller from an engine, loss of an aircraft, and fatalities. To help create and validate tools for the gear designer, the NASA Lewis Research Center performed in-house analytical and experimental studies to investigate the effect of rim thickness on gear-tooth crack propagation. Our goal was to determine whether cracks grew through gear teeth (benign failure mode) or through gear rims (catastrophic failure mode) for various rim thicknesses. In addition, we investigated the effect of rim thickness on crack propagation life. A finite-element-based computer program simulated gear-tooth crack propagation. The analysis used principles of linear elastic fracture mechanics, and quarter-point, triangular elements were used at the crack tip to represent the stress singularity. The program had an automated crack propagation option in which cracks were grown numerically via an automated remeshing scheme. Crack-tip stress-intensity factors were estimated to determine crack-propagation direction. Also, various fatigue crack growth models were used to estimate crack-propagation life. Experiments were performed in Lewis' Spur Gear Fatigue Rig to validate predicted crack propagation results. Gears with various backup ratios were tested to validate crack-path predictions. Also, test gears were installed with special crack-propagation gages in the tooth fillet region to measure bending-fatigue crack growth. From both predictions and tests, gears with backup ratios
Computer simulation of crack propagation in ductile materials under biaxial dynamic loads
Chen, Y.M.
1980-07-29
The finite-difference computer program HEMP is used to simulate the crack-propagation phenomenon in two-dimensional ductile materials under truly dynamic biaxial loads. A comulative strain-damage criterion for the initiation of ductile fracture is used. To simulate crack propagation numerically, the method of equivalent free-surface boundary conditions and the method of artifical velocity are used in the computation. Centrally cracked rectangular aluminum bars subjected to constant-velocity biaxial loads at the edges are considered. Tensile and compressive loads in the direction of crack length are found, respectively, to increase and decrease directional instability in crack propagation, where the directional instability is characterized by branching or bifurcation.
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
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.
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.
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.
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.
3D dynamic simulation of crack propagation in extracorporeal shock wave lithotripsy
NASA Astrophysics Data System (ADS)
Wijerathne, M. L. L.; Hori, Muneo; Sakaguchi, Hide; Oguni, Kenji
2010-06-01
Some experimental observations of Shock Wave Lithotripsy(SWL), which include 3D dynamic crack propagation, are simulated with the aim of reproducing fragmentation of kidney stones with SWL. Extracorporeal shock wave lithotripsy (ESWL) is the fragmentation of kidney stones by focusing an ultrasonic pressure pulse onto the stones. 3D models with fine discretization are used to accurately capture the high amplitude shear shock waves. For solving the resulting large scale dynamic crack propagation problem, PDS-FEM is used; it provides numerically efficient failure treatments. With a distributed memory parallel code of PDS-FEM, experimentally observed 3D photoelastic images of transient stress waves and crack patterns in cylindrical samples are successfully reproduced. The numerical crack patterns are in good agreement with the experimental ones, quantitatively. The results shows that the high amplitude shear waves induced in solid, by the lithotriptor generated shock wave, play a dominant role in stone fragmentation.
Tracking and Motion Analysis of Crack Propagations in Crystals for Molecular Dynamics
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 are presented in terms of both feature tracking and velocity analysis.
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.
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.
Elevated temperature crack propagation
Orange, T.W.
1994-02-01
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.
NASA Astrophysics Data System (ADS)
Shilko, Evgeny V.; Psakhie, Sergey G.; Popov, Valentin L.
2016-11-01
The paper is devoted to the study of the influence of crack-normal stress on the shear strength of the brittle material with initial crack and the geometrical condition of acceleration of dynamically growing crack towards the longitudinal wave speed. We considered elastic-brittle permeable materials with nanoscale pore size. We have shown that pore fluid in nanoporous brittle materials influences mainly the condition of shear crack propagation transition from conventional sub-Rayleigh regime to supershear one. The results of the study make it possible to assess the ability of initial cracks in brittle materials to develop in supershear regime under the condition of confined longitudinal shear.
NASA Astrophysics Data System (ADS)
Kim, J. S.; Koh, H. J.; Lee, W. D.; Shin, N.; Kim, J. G.; Lee, K.-H.; Sohn, K.-S.
2008-04-01
The propagation of a macro-scale crack was visualized in yittria-tettragonal zirconia polycrystal (Y-TZP) ceramics using a mechano-luminescence (ML) of SrAl2O4:Eu, Dy. The transformation zone around the crack was also clearly detected in both the crack front and side areas of the Y-TZP in a realistic time frame. The ML made it possible to precisely detect a relatively fast crack propagating in the speed range from 20 m/s to 140 m/s, thereby realizing so-called quasi-dynamic R-curve. Effective toughening then commenced and the applied stress intensity factor increased to 20 MPa √ m . The ho values obtained from the ML observation deviated slightly from those predicted by the Evans-McMeeking model, and support Marshall's simple power law model of quasi-static crack propagation.
NASA Astrophysics Data System (ADS)
Zhang, Yanqiu; Jiang, Shuyong; Zhu, Xiaoming; Zhao, Yanan
2017-03-01
Tensile deformation of nanoscale bicrystal nickel film with twist grain boundary, which includes various twist angles, is investigated via molecular dynamics simulation to obtain the influence of twist angle on crack propagation. The twist angle has a significant influence on crack propagation. At the tensile strain of 0.667, as for the twist angles of 0°, 3.54° and 7.05°, the bicrystal nickel films are subjected to complete fracture, while as for the twist angles of 16.1° and 33.96°, no complete fracture occurs in the bicrystal nickel films. When the twist angles are 16.1° and 33.96°, the dislocations emitted from the crack tip are almost unable to go across the grain boundary and enter into the other grain along the slip planes {111}. There should appear a critical twist angle above which the crack propagation is suppressed at the grain boundary. The higher energy in the grain boundary with larger twist angle contributes to facilitating the movement of the glissile dislocation along the grain boundary rather than across the grain boundary, which leads to the propagation of the crack along the grain boundary.
NASA Astrophysics Data System (ADS)
Hedan, S.; Pop, O.; Valle, V.; Cottron, M.
2006-08-01
We propose in this paper, to analyse, the evolution of out-of-plane displacement fields for a crack propagation in brittle materials. As the crack propagation is a complex process that involves the deformation mechanisms, the out-of-plane displacement measurement gives pertinent information about the 3D effects. For investigation, we use the interferometric method. The optical device includes a laser source, a Michelson interferometer and an ultra high-speed CCD camera. To take into account the crack velocity, we dispose of a maximum frame rate of 1Mfps. The experimental tests have been carried out for a SEN (Single Edge Notch) specimen of PMMA material. The crack propagation is initiated by adding a dynamic energy given by the impact of a cutter on the initial crack. The obtained interferograms are analysed with a new phase extraction method entitled MPC [6]. This analysis, which has been developed specially for dynamic studies, gives the out-of-plane displacement with an accuracy of about 10 nm.
Omeltchenko, A.; Yu, J.; Kalia, R.K.; Vashishta, P.
1997-03-01
Crack propagation in a graphite sheet is investigated with million atom molecular-dynamics simulations based on Brenner{close_quote}s reactive empirical bond-order potential. For certain crystalline orientations, multiple crack branches with nearly equal spacing sprout as the crack tip reaches a critical speed of 0.6V{sub R}, where V{sub R} is the Rayleigh wave speed. This results in a fracture surface with secondary branches and overhangs. Within the same branch the crack-front profile is characterized by a roughness exponent, {alpha}=0.41{plus_minus}0.05. However, for interbranch fracture surface profiles the return probability yields {alpha}=0.71{plus_minus}0.10. Fracture toughness is estimated from Griffith analysis and local-stress distributions. {copyright} {ital 1997} {ital The American Physical Society}
NASA Astrophysics Data System (ADS)
Omeltchenko, Andrey; Yu, Jin; Kalia, Rajiv K.; Vashishta, Priya
1997-03-01
Crack propagation in a graphite sheet is investigated with million atom molecular-dynamics simulations based on Brenner's reactive empirical bond-order potential. For certain crystalline orientations, multiple crack branches with nearly equal spacing sprout as the crack tip reaches a critical speed of 0.6VR, where VR is the Rayleigh wave speed. This results in a fracture surface with secondary branches and overhangs. Within the same branch the crack-front profile is characterized by a roughness exponent, α = 0.41+/-0.05. However, for interbranch fracture surface profiles the return probability yields α = 0.71+/-0.10. Fracture toughness is estimated from Griffith analysis and local-stress distributions.
NASA Astrophysics Data System (ADS)
Lengliné, Olivier; Schmittbuhl, Jean; Elkhoury, Jean; Toussaint, Renaud; Daniel, Guillaume; Maloy, Knut Jurgen
2010-05-01
Observations of aseismic transients in several tectonic context suggest that they might be linked to seismicity. However a clear observation and description of these phenomena and their interaction is lacking. This owes to the difficulty of characterizing with a sufficient resolution processes taking place at depth. Here we aim to study these interactions between aseismic and seismic slip taking advantage of an unique experimental setup. We conducted a series of mode I crack propagation experiments on transparent materials (PMMA). The crack advance is trapped in a weakness plane which is the interface between two previously sandblasted and annealed plexiglass plates. A fast video camera taking up to 500 frames per second ensures the tracking of the front rupture. The acoustic system is composed of a maximum of 44 channels continuously recording at 5 MHz for a few tens of seconds. Piezo-electric sensors are composed of a 32 elements linear array and individual sensors surrounding the crack front. An automatic detection and localization procedure allows us to obtain the position of acoustic emission (A.E.) that occurred during the crack advance. Crack front image processing reveals an intermittent opening which might be linked to the time and space clustering of the AE. An analogy between the mode I (opening) and the mode III (antiplane slip) allows us to interpret our results in term of slip on faults. Our experiment thus helps to reveal the interplay between seismic and aseismic slip on faults.
Mode II fatigue crack propagation.
NASA Technical Reports Server (NTRS)
Roberts, R.; Kibler, J. J.
1971-01-01
Fatigue crack propagation rates were obtained for 2024-T3 bare aluminum plates subjected to in-plane, mode I, extensional loads and transverse, mode II, bending loads. These results were compared to the results of Iida and Kobayashi for in-plane mode I-mode II extensional loads. The engineering significance of mode I-mode II fatigue crack growth is considered in view of the present results. A fatigue crack growth equation for handling mode I-mode II fatigue crack growth rates from existing mode I data is also discussed.
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.
Three-Dimensional Gear Crack Propagation Studies
NASA Technical Reports Server (NTRS)
Lewicki, David G.; Sane, Ashok D.; Drago, Raymond J.; Wawrzynek, Paul A.
1998-01-01
Three-dimensional crack growth simulation was performed on a split-tooth gear design using boundary element modeling and linear elastic fracture mechanics. Initial cracks in the fillet of the teeth produced stress intensity factors of greater magnitude (and thus, greater crack growth rates) than those in the root or groove areas of the teeth. Crack growth simulation was performed on a case study to evaluate crack propagation paths. Tooth fracture was predicted from the crack growth simulation for an initial crack in the tooth fillet region. Tooth loads on the uncracked mesh of the split-tooth design were up to five times greater than those on the cracked mesh if equal deflections of the cracked and uncracked teeth were considered. Predicted crack shapes as well as crack propagation life are presented based on calculated stress intensity factors, mixed-mode crack propagation trajectory theories, and fatigue crack growth theories.
Confining crack propagation in defective graphene.
López-Polín, Guillermo; Gómez-Herrero, Julio; Gómez-Navarro, Cristina
2015-03-11
Crack propagation in graphene is essential to understand mechanical failure in 2D materials. We report a systematic study of crack propagation in graphene as a function of defect content. Nanoindentations and subsequent images of graphene membranes with controlled induced defects show that while tears in pristine graphene span microns length, crack propagation is strongly reduced in the presence of defects. Accordingly, graphene oxide exhibits minor crack propagation. Our work suggests controlled defect creation as an approach to avoid catastrophic failure in graphene.
Crack propagation driven by crystal growth
A. Royne; Paul Meaking; A. Malthe-Sorenssen; B. Jamtveit; D. K. Dysthe
2011-10-01
Crystals that grow in confinement may exert a force on their surroundings and thereby drive crack propagation in rocks and other materials. We describe a model of crystal growth in an idealized crack geometry in which the crystal growth and crack propagation are coupled through the stress in the surrounding bulk solid. Subcritical crack propagation takes place during a transient period, which may be very long, during which the crack velocity is limited by the kinetics of crack propagation. When the crack is sufficiently large, the crack velocity becomes limited by the kinetics of crystal growth. The duration of the subcritical regime is determined by two non-dimensional parameters, which relate the kinetics of crack propagation and crystal growth to the supersaturation of the fluid and the elastic properties of the surrounding material.
Crack propagation, arrest and statistics in heterogeneous materials.
Kierfeld, J.; Vinokur, V.; Materials Science Division; Dortmund Univ. of Technology
2008-04-01
We investigate theoretically statistics and thermally activated dynamics of crack nucleation and propagation in a two-dimensional heterogeneous material containing quenched randomly distributed defects. We consider a crack tip dynamics accounting for dissipation, thermal noise and the random forces arising from the elastic interactions of the crack opening with the defects. The equation of motion is based on the generalized Griffith criterion and the dynamic energy release rate and gives rise to Langevin-type stochastic dynamics in a quenched disordered potential. For different types of quenched random forces, which are characterized (a) by the range of elastic interactions with the crack tip and (b) the range of correlations between defects, we derive a number of static and dynamic quantities characterizing crack propagation in heterogeneous materials both at zero temperature and in the presence of thermal activation. In the absence of thermal fluctuations we obtain the nucleation and propagation probabilities, typical arrest lengths, the distribution of crack lengths and of critical forces. For thermally activated crack propagation we calculate the mean time to fracture. Depending on the range of elastic interactions between crack tip and frozen defects, heterogeneous material exhibits brittle or ductile fracture. We find that aggregations of defects generating long-range interaction forces (e.g. clouds of dislocations) lead to anomalously slow creep of the crack tip or even to its complete arrest. We demonstrate that heterogeneous materials with frozen defects contain a large number of arrested microcracks and that their fracture toughness is enhanced to the experimentally accessible timescales.
Double noding technique for mixed mode crack propagation studies
NASA Technical Reports Server (NTRS)
Liaw, B. M.; Kobayashi, A. S.; Emery, A. F.
1984-01-01
A simple dynamic finite element algorithm for analyzing a propagating mixed mode crack tip is presented. A double noding technique, which can be easily incorporated into existing dynamic finite element codes, is used together with a corrected J integral to extract modes I and II dynamic stress intensity factors of a propagating crack. The utility of the procedure is demonstrated by analyzing test problems involving a mode I central crack propagating in a plate subjected to uniaxial tension, a mixed mode I and II stationary, slanted central crack in a plate subjected to uniaxial impact loading, and a mixed mode I and II extending, slanted single edge crack in a plate subjected to uniaxial tension. Previously announced in STAR as N83-13491
Double noding technique for mixed mode crack propagation studies
NASA Technical Reports Server (NTRS)
Liaw, B. M.; Kobayashi, A. S.; Emergy, A. F.
1982-01-01
A simple dynamic finite element algorithm for analyzing a propagating mixed mode crack tip is presented. A double noding technique, which can be easily incorporated into existing dynamic finite element codes, is used together with a corrected J integral to extract modes I and II dynamic stress intensity factors of a propagating crack. The utility of the procedure is demonstrated by analyzing test problems involving a mode I central crack propagating in a plate subjected to uniaxial tension, a mixed mode I and II stationary, slanted central crack in a plate subjected to uniaxial impact loading, and a mixed mode I and II extending, slanted single edge crack in a plate subjected to uniaxial tension.
Roughening of a propagating planar crack front
NASA Astrophysics Data System (ADS)
Åström, J. A.; Alava, M. J.; Timonen, J.
2000-08-01
A numerical model of the front of a planar crack propagating between two connected elastic plates is investigated. The plates are modeled as square lattices of elastic beams. The plates are connected by similar but breakable beams with a randomly varying stiffness. The crack is driven by pulling both plates at one end in Mode I at a constant rate. We find ζ=1/3, z=4/3, and β=1/4 for the roughness, dynamical, and growth exponents, respectively, that describe the front behavior. This is similar to continuum limit analyses based on a perturbative stress-intensity treatment of the front [H. Gao and J. R. Rice, J. Appl. Mech. 56, 828 (1989)]. We discuss the differences to recent experiments.
NASA Astrophysics Data System (ADS)
Simonov, M. Yu.; Georgiev, M. N.; Shaimanov, G. S.; Simonov, Yu. N.; Zaporozhan, R. S.
2016-05-01
Comparative analysis of zones of plastic strain, dynamic crack resistance, structure, and micromechanisms of crack propagation in structural steels 09G2S, 25 and 40 in high-toughness condition is performed. The structure, the micromechanisms of crack growth, and the dynamic crack resistance of steels 09G2S, 25 and 40 are studied. Complete zones of plastic stain (CPSZ) under fracture surfaces are plotted after quenching and high tempering at 650°C. The levels of microhardness in the CPSZ are mapped for specially-designed specimens with additional 1-mm-deep side notches and relative crack length of 0.4 - 0.5. The sizes of the zones of plastic strain in the starting region are determined. Special features of the distribution of microhardness in local volumes of the CPSZ are determined. The structure under fracture surfaces of steels 09G2S, 25 and 40 is studied over the whole of the path of propagation of a dynamic crack.
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.
Crack propagation in Hastelloy X
Weerasooriya, T.; Strizak, J.P.
1980-05-01
The fatigue and creep crack growth rates of Hastelloy X were examined both in air and impure helium. Creep crack growth rate is higher in air and impure helium at 650/sup 0/C. Initial creep crack growth from the original sharp fatigue crack is by an intergranular mode of fracture. As the cracking accelerates at higher stress intensities, growth is by a mixed mode of both intergranular and transgranular fracture. Fatigue crack growth rate increases with increasing temperature and decreasing frequency for the range of stress intensities reported in the literature and is lower in impure helium than in air.
Quantity effect of radial cracks on the cracking propagation behavior and the crack morphology.
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.
MECHANICS OF CRACK BRIDGING UNDER DYNAMIC LOADS
N. SRIDHAR; ET AL
2001-02-01
A bridging law for fiber reinforced composites under dynamic crack propagation conditions has been derived. Inertial effects in the mechanism of fiber pullout during dynamic propagation of a bridged crack are critically examined for the first time. By reposing simple shear lag models of pullout as problems of dynamic wave propagation, the effect of the frictional coupling between the fibers and the matrix is accounted for in a fairly straightforward way. The solutions yield the time-dependent relationship between the crack opening displacement and the bridging traction. Engineering criteria and the role of material and geometrical parameters for significant inertial effects are identified.
Crack Propagation in Bamboo's Hierarchical Cellular Structure
Habibi, Meisam K.; Lu, Yang
2014-01-01
Bamboo, as a natural hierarchical cellular material, exhibits remarkable mechanical properties including excellent flexibility and fracture toughness. As far as bamboo as a functionally graded bio-composite is concerned, the interactions of different constituents (bamboo fibers; parenchyma cells; and vessels.) alongside their corresponding interfacial areas with a developed crack should be of high significance. Here, by using multi-scale mechanical characterizations coupled with advanced environmental electron microscopy (ESEM), we unambiguously show that fibers' interfacial areas along with parenchyma cells' boundaries were preferred routes for crack growth in both radial and longitudinal directions. Irrespective of the honeycomb structure of fibers along with cellular configuration of parenchyma ground, the hollow vessels within bamboo culm affected the crack propagation too, by crack deflection or crack-tip energy dissipation. It is expected that the tortuous crack propagation mode exhibited in the present study could be applicable to other cellular natural materials as well. PMID:24998298
Crack propagation in bamboo's hierarchical cellular structure.
Habibi, Meisam K; Lu, Yang
2014-07-07
Bamboo, as a natural hierarchical cellular material, exhibits remarkable mechanical properties including excellent flexibility and fracture toughness. As far as bamboo as a functionally graded bio-composite is concerned, the interactions of different constituents (bamboo fibers; parenchyma cells; and vessels.) alongside their corresponding interfacial areas with a developed crack should be of high significance. Here, by using multi-scale mechanical characterizations coupled with advanced environmental electron microscopy (ESEM), we unambiguously show that fibers' interfacial areas along with parenchyma cells' boundaries were preferred routes for crack growth in both radial and longitudinal directions. Irrespective of the honeycomb structure of fibers along with cellular configuration of parenchyma ground, the hollow vessels within bamboo culm affected the crack propagation too, by crack deflection or crack-tip energy dissipation. It is expected that the tortuous crack propagation mode exhibited in the present study could be applicable to other cellular natural materials as well.
Three-Dimensional Gear Crack Propagation Studied
NASA Technical Reports Server (NTRS)
Lewicki, David G.
1999-01-01
Gears used in current helicopters and turboprops are designed for light weight, high margins of safety, and high reliability. However, unexpected gear failures may occur even with adequate tooth design. To design an extremely safe system, the designer must ask and address the question, "What happens when a failure occurs?" With gear-tooth bending fatigue, tooth or rim fractures may occur. A crack that propagates through a rim will be catastrophic, leading to disengagement of the rotor or propeller, loss of an aircraft, and possible fatalities. This failure mode should be avoided. A crack that propagates through a tooth may or may not be catastrophic, depending on the design and operating conditions. Also, early warning of this failure mode may be possible because of advances in modern diagnostic systems. One concept proposed to address bending fatigue fracture from a safety aspect is a splittooth gear design. The prime objective of this design would be to control crack propagation in a desired direction such that at least half of the tooth would remain operational should a bending failure occur. A study at the NASA Lewis Research Center analytically validated the crack-propagation failsafe characteristics of a split-tooth gear. It used a specially developed three-dimensional crack analysis program that was based on boundary element modeling and principles of linear elastic fracture mechanics. Crack shapes as well as the crack-propagation life were predicted on the basis of the calculated stress intensity factors, mixed-mode crack-propagation trajectory theories, and fatigue crack-growth theories. The preceding figures show the effect of the location of initial cracks on crack propagation. Initial cracks in the fillet of the teeth produced stress intensity factors of greater magnitude (and thus, greater crack growth rates) than those in the root or groove areas of the teeth. Crack growth was simulated in a case study to evaluate crack-propagation paths. Tooth
Seismic wave propagation in cracked porous media
NASA Astrophysics Data System (ADS)
Pointer, Tim; Liu, Enru; Hudson, John A.
2000-07-01
The movement of interstitial fluids within a cracked solid can have a significant effect on the properties of seismic waves of long wavelength propagating through the solid. We consider three distinct mechanisms of wave-induced fluid flow: flow through connections between cracks in an otherwise non-porous material, fluid movement within partially saturated cracks, and diffusion from the cracks into a porous matrix material. In each case the cracks may be aligned or randomly oriented, leading, respectively, to anisotropic or isotropic wave speeds and attenuation factors. In general, seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid-filled cracks if fluid flow is significant. In the range of frequencies for which considerable fluid flow occurs there is high attenuation and dispersion of seismic waves. Fluid flow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented, resulting in completely different effects on seismic wave propagation. A numerical analysis shows that all models can have an effect over the exploration seismic frequency range.
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.
Slow crack propagation in composite restorative materials.
Montes-G, G M; Draughn, R A
1987-05-01
The double-torsion test technique was used to study slow crack propagation in a set of dental composite resins including two glass-filled and two microfilled materials. The microstructure within each pair was the same but one of the resins was selfcured and the other photocured. The fracture behavior was dependent on the filler concentration and the presence of absorbed water. Wet materials fractured by slow crack growth in the range of crack velocity studied (10(-7) to 10(-3) m/s), and the microfilled composites, which contain a lower concentration of inorganic filler, had lower stress intensity factors (K1c) than the glass-filled composites tested. Dry specimens of the microfilled materials and the selfcured, glass-filled composite also showed unstable, stick-slip fracture behavior indicative of a crack blunting mechanism which leads to an elevation of the stress intensity factor for crack initiation over K1c for stable crack growth. The plasticizing effect of water increased the viscoelastic response of the materials measured by the slope of curves of slow crack growth. Analysis of fracture surfaces showed that cracks propagated at low velocities (10(-7) to 10(-5) m/s) by the apparent failure of the filler/matrix interfacial bond, and absorbed water affected the strength or fracture resistance of the interface. At high crack velocities the properties of the composite depend on the properties of the polymeric matrix, the filler, and the filler volume fraction, but at low velocities the interface is the controlling factor in the durability of these composites exposed to an aqueous environment.
Crack propagation modeling using Peridynamic theory
NASA Astrophysics Data System (ADS)
Hafezi, M. H.; Alebrahim, R.; Kundu, T.
2016-04-01
Crack propagation and branching are modeled using nonlocal peridynamic theory. One major advantage of this nonlocal theory based analysis tool is the unifying approach towards material behavior modeling - irrespective of whether the crack is formed in the material or not. No separate damage law is needed for crack initiation and propagation. This theory overcomes the weaknesses of existing continuum mechanics based numerical tools (e.g. FEM, XFEM etc.) for identifying fracture modes and does not require any simplifying assumptions. Cracks grow autonomously and not necessarily along a prescribed path. However, in some special situations such as in case of ductile fracture, the damage evolution and failure depend on parameters characterizing the local stress state instead of peridynamic damage modeling technique developed for brittle fracture. For brittle fracture modeling the bond is simply broken when the failure criterion is satisfied. This simulation helps us to design more reliable modeling tool for crack propagation and branching in both brittle and ductile materials. Peridynamic analysis has been found to be very demanding computationally, particularly for real-world structures (e.g. vehicles, aircrafts, etc.). It also requires a very expensive visualization process. The goal of this paper is to bring awareness to researchers the impact of this cutting-edge simulation tool for a better understanding of the cracked material response. A computer code has been developed to implement the peridynamic theory based modeling tool for two-dimensional analysis. A good agreement between our predictions and previously published results is observed. Some interesting new results that have not been reported earlier by others are also obtained and presented in this paper. The final objective of this investigation is to increase the mechanics knowledge of self-similar and self-affine cracks.
Liao, B.; Nan, Y.; Hu, Y.; Kang, D.T.
1998-02-01
The influence of hydrogen on the deformation ahead of the crack tip and the crack propagation were observed and studied in situ under transmission electron microscopy with dynamic tensile deformation for steel. The results show that hydrogen can promote local plastic deformation ahead of the crack tip and change the mode of crack propagation so that the crack will propagate in a zigzag path.
Fatigue crack propagation analysis of plaque rupture.
Pei, Xuan; Wu, Baijian; Li, Zhi-Yong
2013-10-01
Rupture of atheromatous plaque is the major cause of stroke or heart attack. Considering that the cardiovascular system is a classic fatigue environment, plaque rupture was treated as a chronic fatigue crack growth process in this study. Fracture mechanics theory was introduced to describe the stress status at the crack tip and Paris' law was used to calculate the crack growth rate. The effect of anatomical variation of an idealized plaque cross-section model was investigated. The crack initiation was considered to be either at the maximum circumferential stress location or at any other possible locations around the lumen. Although the crack automatically initialized at the maximum circumferential stress location usually propagated faster than others, it was not necessarily the most critical location where the fatigue life reached its minimum. We found that the fatigue life was minimum for cracks initialized in the following three regions: the midcap zone, the shoulder zone, and the backside zone. The anatomical variation has a significant influence on the fatigue life. Either a decrease in cap thickness or an increase in lipid pool size resulted in a significant decrease in fatigue life. Comparing to the previously used stress analysis, this fatigue model provides some possible explanations of plaque rupture at a low stress level in a pulsatile cardiovascular environment, and the method proposed here may be useful for further investigation of the mechanism of plaque rupture based on in vivo patient data.
Crack Propagation in Double-Base Propellants
1976-01-01
propagation tests were conducted on a composite modified double- base ( CMDB ) propellant with the use of center-cracked strip biaxial specimens...double-base ( CMDB ) propellant. He performed a stress analysis of small, precracked, subscale STV motors formulated in terms of stress intensity factors...assumed for Solithane 113. The present program was aimed at evaluating the Schapery theory when it was applied to a CMDB propellant under similar loading
Engineering Aspects of Fatigue Crack Propagation
1962-01-01
Estimating Notch-Size Effect in Fatigue Tests on Steel. NACA TN 2805, 1952. - 37 - 19. Landers, Charles B., and Hardrath, Herbert F.: Results of Axial- Load... Charles B., and Howell, F. M.: Axial-Load Fatigue Properties of 24S-T and 75S-T Aluminum Alloy as Determined in Several Laboratories. NACA TR 1190, 1954...Hardrath, Herbert F., Leybold, Herbert A., Landers, Charles B., and Hauschild, Louis W.: Fatigue-Crack Propagation in Aluminum- Alloy Box Beams. NACA
Fatigue crack propagation behavior of ultrahigh molecular weight polyethylene.
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.
Multiscale modeling of crack initiation and propagation at the nanoscale
NASA Astrophysics Data System (ADS)
Shiari, Behrouz; Miller, Ronald E.
2016-03-01
Fracture occurs on multiple interacting length scales; atoms separate on the atomic scale while plasticity develops on the microscale. A dynamic multiscale approach (CADD: coupled atomistics and discrete dislocations) is employed to investigate an edge-cracked specimen of single-crystal nickel, Ni, (brittle failure) and aluminum, Al, (ductile failure) subjected to mode-I loading. The dynamic model couples continuum finite elements to a fully atomistic region, with key advantages such as the ability to accommodate discrete dislocations in the continuum region and an algorithm for automatically detecting dislocations as they move from the atomistic region to the continuum region and then correctly "converting" the atomistic dislocations into discrete dislocations, or vice-versa. An ad hoc computational technique is also applied to dissipate localized waves formed during crack advance in the atomistic zone, whereby an embedded damping zone at the atomistic/continuum interface effectively eliminates the spurious reflection of high-frequency phonons, while allowing low-frequency phonons to pass into the continuum region. The simulations accurately capture the essential physics of the crack propagation in a Ni specimen at different temperatures, including the formation of nano-voids and the sudden acceleration of the crack tip to a velocity close to the material Rayleigh wave speed. The nanoscale brittle fracture happens through the crack growth in the form of nano-void nucleation, growth and coalescence ahead of the crack tip, and as such resembles fracture at the microscale. When the crack tip behaves in a ductile manner, the crack does not advance rapidly after the pre-opening process but is blunted by dislocation generation from its tip. The effect of temperature on crack speed is found to be perceptible in both ductile and brittle specimens.
A Parametric Study of Crack Propagation During Sonic IR Inspection
NASA Astrophysics Data System (ADS)
Chen, J. C.; Kephart, J.; Riddell, W. T.
2006-03-01
We have developed an experiment to study the propagation of synthetic cracks under various controlled conditions during sonic IR inspection. The experiment provides for good repeatability in testing. The parameters of interest include the initial crack length, load history (stress intensity and load ratio) during crack generation, geometry of the crack, material, and also the various conditions involving the ultrasonic source. In general, we find that under typical sonic IR inspection conditions, the initial crack will propagate when subjected to sonic IR testing. The crack growth after each inspection event varies and exhibits a distribution in length of propagation. The results show that the average crack propagation decreases with increasing initial crack length and increasing stress intensity.
NASA Astrophysics Data System (ADS)
Singh Raman, R. K.; Ibrahim, R. N.; Wu, F.; Rihan, R.
2008-12-01
Corrosion-assisted propagation of an existing crack is profoundly influenced by the stress intensity at the crack tip. This article presents the first results of thermomechanical conditioning (TMC) for local manipulation of material at and ahead of the crack tip, in an attempt to retard/stop crack propagation. Prenotched round tensile specimens of mild steel were subjected to rotating bending to generate a fatigue precrack, and then to apply localized thermomechanical conditioning. The threshold stress intensity factor ( K ISCC ) for stress corrosion cracking (SCC) of precracked specimens with and without TMC was determined in a caustic environment. Results suggest that TMC can increase K ISCC . Finite element analysis of the specimens suggests development of compressive stresses at and around the crack tip, which is expected to improve the resistance to stress corrosion crack propagation (since stress corrosion cracks can propagate only under tensile loading).
Liquid metal embrittlement. [crack propagation in metals with liquid metal in crack space
NASA Technical Reports Server (NTRS)
Tiller, W. A.
1973-01-01
Crack propagation is discussed for metals with liquid metal in the crack space. The change in electrochemical potential of an electron in a metal due to changes in stress level along the crack surface was investigated along with the change in local chemistry, and interfacial energy due to atomic redistribution in the liquid. Coupled elastic-elastrostatic equations, stress effects on electron energy states, and crack propagation via surface roughening are discussed.
Semiconductor laser asymmetry cutting glass with laser induced thermal-crack propagation
NASA Astrophysics Data System (ADS)
Zhao, Chunyang; Zhang, Hongzhi; Wang, Yang
2014-12-01
Laser induced thermal-crack propagation (LITP) makes the material to produce an uneven temperature field, maximum temperature can't soften or melt the material, induces the thermal stress, then the crack separates along the cutting path. One of the problems in laser asymmetry cutting glass with LITP is the cutting deviation along scanning trajectory. This study lays great emphasis on considering the dynamic extension of crack to explain the reason of the cutting deviation in laser asymmetry cutting glass, includes asymmetric linear cutting and a quarter of a circular curve cutting. This paper indicates the experiments of semiconductor laser asymmetry cutting glass with LITP. Optical microscope photographs of the glass sheet are obtained to examine the cutting deviation. The extended finite element method (XFEM) is used to simulate the dynamic propagation of crack; the crack path does not have to be specified a priori. The cutting deviation mechanism and the crack propagation process are studied by the stress fields using finite element software ABAQUS. This work provides a theoretical basis to investigate the cutting deviation in laser asymmetry cutting glass. In semiconductor laser asymmetry cutting glass, the tensile stress is the basis of crack propagation, then the compressive stress not only makes the crack to extend stably, but also controls the direction of crack propagation.
Effects of gear crack propagation paths on vibration responses of the perforated gear system
NASA Astrophysics Data System (ADS)
Ma, Hui; Pang, Xu; Zeng, Jin; Wang, Qibin; Wen, Bangchun
2015-10-01
This paper investigates the dynamic behaviors of a perforated gear system considering effects of the gear crack propagation paths and this study focuses on the effects of a crack propagating through the rim on the time-varying mesh stiffness (TVMS) and vibration responses. Considering the effects of the extended tooth contact, a finite element (FE) model of a gear pair is established based on ANSYS software. TVMS of the perforated gear with crack propagating through tooth and rim are calculated by using the FE model. Furthermore, a lumped mass model is adopted to investigate the vibration responses of the perforated gear system. The results show that there exist three periods related to slots of the gear body in a rotating period of the perforated gear. Gear cracks propagating through tooth and rim both reduce the gear body stiffness and lead to reduction of TVMS besides the crack tooth contact moment, and the TVMS weakening for the former is less than that for the latter. Moreover, the results also show that the gear crack propagating through the rim (CPR) has a greater effect on vibration responses than the gear crack propagating through the tooth (CPT) under the same crack level. Vibration level increases with the increasing crack depth, especially for the gear with CPR.
Effect of Rim Thickness on Gear Crack Propagation Path.
1996-08-01
Analytical and experimental studies were performed to investigate the effect of rim thickness on gear tooth crack propagation. The goal was to...ANalysis Code) simulated gear tooth crack propagation. The analysis used principles of linear elastic fracture mechanics. Quarter-point, triangular
Propagation of stress corrosion cracks in alpha-brasses
Beggs, Dennis Vinton
1981-01-01
Transgranular and intergranular stress corrosion cracks were investigated in alpha-brasses in a tarnishing ammoniacal solution. Surface observation indicated that the transgranular cracks propagated discontinuously by the sudden appearance of a fine crack extending several microns ahead of the previous crack tip, often associated with the detection of a discrete acoustic emission (AE). By periodically increasing the deflection, crack front markings were produced on the resulting fracture surfaces, showing that the discontinuous propagation of the crack trace was representative of the subsurface cracking. The intergranular crack trace appeared to propagate continuously at a relatively blunt crack tip and was not associated with discrete AE. Under load pulsing tests with a time between pulses, ..delta..t greater than or equal to 3 s, the transgranular fracture surfaces always exhibited crack front markings which corresponded with the applied pulses. The spacing between crack front markings, ..delta..x, decreased linearly with ..delta..t. With ..delta..t less than or equal to 1.5 s, the crack front markings were in a one-to-one correspondence with applied pulses only at relatively long crack lengths. In this case, ..delta..x = ..delta..x* which approached a limiting value of 1 ..mu..m. No crack front markings were observed on intergranular fracture surfaces produced during these tests. It is concluded that transgranular cracking occurs by discontinuous mechanical fracture of an embrittled region around the crack tip, while intergranular cracking results from a different mechanism with cracking occurring via the film-rupture mechanism.
A study of crack propagation in metals in the presence of defects
NASA Astrophysics Data System (ADS)
Petucci, Justin M.
Molecular dynamics (MD) and molecular statics (MS) simulations of crack propagation in the presence of defects in crystalline FCC metals under mode I loading are carried out on the (001)[100] crack system using the embedded atom method (EAM) interatomic potential. Substitutional impurity point defects are introduced into a 3D thin-strip slab of 160000 atoms at various distances from the crack tip. The critical load required for the initiation of crack propagation is obtained, along with the atomic level stress distribution near the crack tip. The results indicate that the critical load is dependent on the defect species, geometry, and position. When located directly at the crack tip, the defects reduce the peak internal stress, increasing the critical load relative to the defect-free system. As the defects are moved away from the crack tip the critical load goes through a minimum and approaches the value of the pure material asymptotically.
Blunt-crack band propagation in finite-element analysis for concrete structures. [LMFBR
Pfeiffer, P.A.; Bazant, Z.P.; Marchertas, A.H.
1983-01-01
The knowledge of concrete fracture is needed in nuclear reactor safety. The question of safety arises from the potential of concrete to crack under thermal loading. It has been postulated that structural concrete could be exposed to very high temperature, which may result from hot reactor coolant or even core debris coming in direct contact with the concrete. The utilization of the blunt crack approach for simulating concrete cracking in a general-purpose code is explored. The difficulties encountered in establishing the proper direction of crack propagation in an arbitrary discretization are described. Crack propagation is considered within the context of two types of solution techniques: (1) implicit solution of the static crack advance, and (2) explicit time integration using a dynamic relaxation technique to simulate the static crack advance. Also, in both solution techniques an elastic model is used to characterize the concrete.
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.
A review of crack propagation under unsteady loading
NASA Astrophysics Data System (ADS)
Bryan, H. H.; Ahuja, K. K.
The theories and research current available on crack propagation under unsteady loadings, especially those of acoustic origin, are reviewed. Since the original theories on fatigue failure did not account for random loading conditions, modified theories which provide statistical methods for evaluating the random loading have emerged. The impact of acoustic fatigue in the aerospace industry, basic principles such as fatigue crack initiation and propagation and load interactions, and testing procedures are discussed. Attention is also given to metal and metal alloy structures, fiber-reinforced composites and nonmetallic structures, short crack growth, and the effects of temperature, moisture, and corrosion on structures. Suggestions for future research in this field are presented, namely, studies on the effect of 'snap-through' response and associated crack growth patterns, studies in microcrack and 'small crack'; propagation under unsteady loading conditions, and the development of an accurate analytical model to predict acceleration and retardation effects in fatigue crack growth under random loading conditions.
A review of crack propagation under unsteady loading
NASA Technical Reports Server (NTRS)
Bryan, H. H.; Ahuja, K. K.
1992-01-01
The theories and research current available on crack propagation under unsteady loadings, especially those of acoustic origin, are reviewed. Since the original theories on fatigue failure did not account for random loading conditions, modified theories which provide statistical methods for evaluating the random loading have emerged. The impact of acoustic fatigue in the aerospace industry, basic principles such as fatigue crack initiation and propagation and load interactions, and testing procedures are discussed. Attention is also given to metal and metal alloy structures, fiber-reinforced composites and nonmetallic structures, short crack growth, and the effects of temperature, moisture, and corrosion on structures. Suggestions for future research in this field are presented, namely, studies on the effect of 'snap-through' response and associated crack growth patterns, studies in microcrack and 'small crack'; propagation under unsteady loading conditions, and the development of an accurate analytical model to predict acceleration and retardation effects in fatigue crack growth under random loading conditions.
1984-08-10
Plenum Press , New York, pp 227-247, 1982. (3) Kanninen, M. F., "Dynamic Fracture Mechanics and its Application to Material Behavior under High Stress and...Mechanics: Sixteenth Conference, ASTM STP, in press , 1983. (9) Kanninen, M. F., Popelar, C. H., and Broek, D., "A Critical Survey on the Application of...Mechanics, D.R.J. Owen and A. R. Luxmoore, editors, Pinerldge Press , Swansea, United Kingdom, 1980. 2. Kannlnen, M. F., Gehlen, P. C., Barnes, C. R
Dynamic Crack Branching - A Photoelastic Evaluation,
1982-05-01
24. Freund, L. B., "Dynamic Crack Propagation", Mechanics of Fracture, Vol. 19, edited by F. Erdogan, ASME, 1976 , pp. 105-134. 25. Streit, R., and... spangle Dtie"Itty *I CowoffdO Reserch Laboeuotee for eM ousrme so bftgesrif UakIOŘ b.alSeg"Cl of 211135* asews t of MetasŘlosifts184 bgieaue Sellese
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.
Experimental study of thermodynamics propagation fatigue crack in metals
NASA Astrophysics Data System (ADS)
Vshivkov, A.; Iziumova, A.; Plekhov, O.
2015-10-01
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 the crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip.
Experimental study of thermodynamics propagation fatigue crack in metals
Vshivkov, A. Iziumova, A. Plekhov, O.
2015-10-27
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 the crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip.
Competition between fatigue crack propagation and wear
Fan, H.; Keer, L.M.; Cheng, W.; Cheng, H.S. )
1993-01-01
Based on a semi-empirical derivation of the Paris fatigue law, the fatigue crack length a is related to the yield limit or flow stress, which ultimately is related to the hardness of the material. The analysis considers together the cyclic loading, which tends to increase the surface crack length, and the wear, which tends to decrease the crack length at the surface, and shows that under certain conditions a stable crack length may be developed. Experiments conducted on two test groups (Rc = 58.5 and Rc = 62.7) tend to support the present analysis. 10 refs.
Fatigue crack propagation at polymer adhesive interfaces
Ritter, J.E.
1996-12-31
Delamination of polymer adhesive interfaces often occurs due to slow crack growth under either monotonic or cyclic loading. The author`s previous research showed that moisture-assisted crack growth at epoxy/glass and epoxy acrylate/glass interfaces under monotonic loading was directly related to the applied energy release rate and relative humidity and that cyclic loading could enhance crack growth. The purpose of the present research is to compare crack growth along epoxy acrylate/glass and epoxy/PMMA interfaces under monotonic and cyclic loading.
Stress analysis of fracture of atherosclerotic plaques: crack propagation modeling.
Rezvani-Sharif, Alireza; Tafazzoli-Shadpour, Mohammad; Kazemi-Saleh, Davood; Sotoudeh-Anvari, Maryam
2016-12-09
Traditionally, the degree of luminal obstruction has been used to assess the vulnerability of atherosclerotic plaques. However, recent studies have revealed that other factors such as plaque morphology, material properties of lesion components and blood pressure may contribute to the fracture of atherosclerotic plaques. The aim of this study was to investigate the mechanism of fracture of atherosclerotic plaques based on the mechanical stress distribution and fatigue analysis by means of numerical simulation. Realistic models of type V plaques were reconstructed based on histological images. Finite element method was used to determine mechanical stress distribution within the plaque. Assuming that crack propagation initiated at the sites of stress concentration, crack propagation due to pulsatile blood pressure was modeled. Results showed that crack propagation considerably changed the stress field within the plaque and in some cases led to initiation of secondary cracks. The lipid pool stiffness affected the location of crack formation and the rate and direction of crack propagation. Moreover, increasing the mean or pulse pressure decreased the number of cycles to rupture. It is suggested that crack propagation analysis can lead to a better recognition of factors involved in plaque rupture and more accurate determination of vulnerable plaques.
Rock Failure and Crack Propagation Beneath Disc Cutters
NASA Astrophysics Data System (ADS)
Entacher, Martin; Schuller, E.; Galler, R.
2015-07-01
Analyses of rock failure mechanisms beneath disc cutters are presented. Full-scale cutting tests are conducted to assess the global energy input in comparison with rock chips and excavated volume. Small-scale cutting tests are subsequently used for macro- and microscopic analyses of rupture modes and crack propagation. A high spatial resolution allows to obtain pictures of crack networks in different rock types. It is shown that all specimens develop lateral cracks in sufficiently confined areas whereas median cracks typically develop in boundary regions. Regarding cutting forces, a hypothesis is proposed that associates sudden force drops accompanied by sudden sound emission with grain crushing in the proximity of the cutter tip.
Propagation and interactions of cracks in Si induced by H supply into He-filled cracks
NASA Astrophysics Data System (ADS)
Reboh, S.; Barbot, J. F.; Beaufort, M. F.; Fitchner, P. F. P.
2011-02-01
The phenomena of interaction and propagation of cracks under the contribution of hydrogen were studied in (001) silicon substrate in which an array of scattered over-pressurized He-plates was previously introduced at a given depth. Their propagation under subcritical regime was activated through diffusional supply of H atoms introduced by implantation/annealing. Interactions between the tips of non coplanar cracks take place in a nanometric scale; they can be of plastic-type leading to the formation of extended defects or of elastic-type resulting in deviations of crack-tip propagation. While the planar interactions facilitate the propagation of cracks, those of non coplanar-type stop them. The observations were carried out by transmission electron microscopy and the results were discussed and modelled by using concepts of elasticity and fracture mechanics.
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.
NASA Technical Reports Server (NTRS)
Mahishi, J. M.; Adams, D. F.
1982-01-01
An elastoplastic, axisymmetric finite element model has been used to predict the initiation and propagation of a crack in a composite model consisting of a single broken boron fiber embedded in an annular sheath of aluminum matrix. The accuracy of the axisymmetric finite element model for crack problems has been established by solving the classical problem of a penny-shaped crack in a thick cylindrical rod under axial tension. Also, the stress intensity factors predicted by the present numerical model are compared with continuum results. A constant displacement boundary condition applied during an increment of crack growth permits a substantial amount of stable crack growth in the matrix material. The concept of Crack Growth Resistance Curves (KR-curves) has been used to determine the point of crack instability
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.
Role of the pore fluid in crack propagation in glass
NASA Astrophysics Data System (ADS)
Mallet, Céline; Fortin, Jérôme; Guéguen, Yves; Bouyer, Fréric
2015-05-01
We investigate pore fluid effects due to surface energy variation or due to chemical corrosion in cracked glass. Both effects have been documented through experimental tests on cracked borosilicate glass samples. Creep tests have been performed to investigate the slow crack propagation behavior. We compared the dry case (saturated with argon gas), the nonreactive water saturated case (commercial mineralized water), and the distilled and deionized water saturated case (pure water). Chemical corrosion effects have been observed and evidenced from pH and water composition evolution of the pure water. Then, the comparison of the dry case, the mineral water saturated case, and the corrosion case allow to (i) evidence the mechanical effect of the presence of a pore fluid and (ii) show also the chemical effect of a glass dissolution. Both effects enhance subcritical crack propagation.
Modeling Crack Propagation in Polycrystalline Microstructure Using Variational Multiscale Method
Sun, S.; Sundararaghavan, V.
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. 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
Slow crack propagation in glass and creep prediction
NASA Astrophysics Data System (ADS)
Mallet, Celine; Fortin, Jerome; Gueguen, Yves
2013-04-01
The context of our study is the observation of the time-dependent deformation of cracked glass. The aim of our study is to observe the slow crack propagation, to quantify it and to predict finally the creep behavior. We performed creep experiments in compaction conditions in a triaxial cell, on cracked boro-silicate glass samples. The chemical composition of the investigated glass is very close to the composition of waste vitrified packages. The matrix of the original glass (OG) is perfectly amorphous, without porosity. A few isolated air bubbles are trapped during the glass flow. Cracks are introduced in the OG through thermal shocks. Strain and acoustic emission (AE) are recorded. Several experiments are performed at different confining pressures (15 or 25 MPa), different pore fluid conditions (with argon gas, considered as the dry case, with tap water saturated porosity, or with distilled water) and different temperatures (ambiant temperature, 50oC or 80oC). Linear increase of the volumetric strain is first observed. A dilatancy increase is recorded. Note that dilatancy does not appear in constant strain rate tests. Constant stress tests show that dilatancy develops during a time interval that depends on the stress level. In addition AE rate are recorded. A non zero AE rate is an evidence of crack propagation. We use a micro-mechanical model that gives the stress intensity factor at the crack tips. This factor depends on stress and geometrical parameters (all known). An exponential law describe the rate of crack propagation, as a function of temperature, environment and applied stresses. This model allows us to predict the creep rate in glass. Assuming a constant crack aspect ratio, crack length and volumetric strain are related. The volumetric strain rate is calculated from model and compared to the data.
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.
Mode-3 spontaneous crack propagation along functionally graded bimaterial interfaces
NASA Astrophysics Data System (ADS)
Kubair, D. V.; Bhanu-Chandar, B.
2007-06-01
The effects of combining functionally graded materials (FGMs) of different inhomogeneous property gradients on the mode-3 propagation characteristics of an interfacial crack are numerically investigated. Spontaneous interfacial crack propagation simulations were performed using the newly developed spectral scheme. The numerical scheme derived and implemented in the present work can efficiently simulate planar crack propagation along functionally graded bimaterial interfaces. The material property inhomogeneity was assumed to be in the direction normal to the interface. Various bimaterial combinations were simulated by varying the material property inhomogeneity length scale. Our parametric study showed that the inclusion of a softening type FGM in the bimaterial system leads to a reduction in the fracture resistance indicated by the increase in crack propagation velocity and power absorbed. An opposite trend of increased fracture resistance was predicted when a hardening material was included in the bimaterial system. The cohesive tractions and crack opening displacements were altered due to the material property inhomogeneity, but the stresses ahead of the cohesive zone remained unaffected.
Propagation of Crack in Glasses under Creep Conditions
NASA Astrophysics Data System (ADS)
Mallet, C.; Fortin, J.; Guéguen, Y.; Schubnel, A.
2012-04-01
The context of our study is the observation of the mechanical behaviour of glass used for the storage of radioactive wastes. This implies to measure the crack propagation characteristics in glass. Results on the investigation of the micromechanics of creep under triaxial loading conditions are presented in the framework of this study. We performed the experiments in a triaxial cell, with pore fluid pressure, on boro-silicate glass. The chemical composition of the investigated glass is very close to the composition of waste vitrified packages. The matrix of the original glass (OG) is perfectly amorphous, without porosity. A few isolated air bubbles are trapped during the glass flow. Cracks are introduced in the OG through thermal shocks. The evolution of deformation (axial and radial strain) is measured using strain gages. The elastic P and S wave velocities and the acoustic emissions (AE) are also recorded. An experiment in dry conditions was performed (the pore fluid was argon gas) with a confining pressure fixed at 15 MPa. Stress step tests were performed in order to get creep data. A similar experiment was performed in water saturated conditions. Crack-closure is first observed at very low strains. Then elastic deformation is observed up to a stress level where elastic anisotropy develops. This can be clearly detected from ɛ Thomsen parameter increase. At last, at a deviatoric stress of 175 MPa (in dry conditions), we observe dilatancy. This behaviour has never been observed in original glass. Indeed, the OG behaviour is perfectly elastic and brittle. In addition, the constant stress tests show that dilatancy develops during a time constant that depends on the stress level. It can be inferred that crack propagation takes place during the constant stress steps. This behaviour is under investigation. We are also quantifying the velocity of the crack propagation by modelling this phenomenon. Indeed, the crack density can be expressed as a volumic strain, ɛv =
Simulation of Ductile Crack Propagation for Pipe Structures Using X-FEM
NASA Astrophysics Data System (ADS)
Miura, Naoki; Nagashima, Toshio
Conventional finite element method is continually used for the flaw evaluation of pipe structures to investigate the fitness-for-service for power plant components, however, it is generally time consuming to make a model of specific crack configuration. The consideration of a propagating surface crack is further accentuated since the crack propagation behavior along the crack front is implicitly affected by the distribution of the crack driving force along the crack front. The authors developed a system to conduct crack propagation analysis by use of the three-dimensional elastic-plastic extended finite element method. It was applied to simulate ductile crack propagation of circumferentially surface cracks in pipe structures and could realize the simultaneous calculation of the J-integral and the consequent ductile crack propagation. Both the crack extension and the possible change of crack shape were evaluated by the developed system.
TF41 Engine Fan Disk Seeded Fault Crack Propagation Test
NASA Technical Reports Server (NTRS)
Lewicki, David G.
2003-01-01
Uncontained engine failures, although rare in occurrence, can have a catastrophic effect on aircraft performance and safety. Engine disk cracks can eventually lead to these type of failures. A number of techniques to detect engine disk cracks have been developed in recent years. However, these technologies have only been validated by disk spin pit tests, not actual engine tests. Due to this, a project was established to perform seeded fault engine tests on a TF41 engine disk fan. A defect was machined in the first stage fan disk of a TF41 engine. The disk was run in a spin pit to initiate a crack. Once initiated, the disk was run in an actual engine test facility. The engine was cycled by a number of start and stops with the goal of propagating the crack to disk burst through low cycle fatigue. Various crack detection techniques were installed on the engine and run real-time during the test to validate their abilities to detect disk cracks. These techniques were based on methods such as change in mass imbalance using vibration or shaft displacement, change in blade position, acoustic emission, and torsional resonance. At the completion of 4474 test cycles, the crack in the TF41 disk was determined to have grown approximately 0.025 inches. This was far less the predicted crack growth based on a fracture mechanics analysis and finite element stress analysis.
Energy absorption mechanisms during crack propagation in metal matrix composites
NASA Technical Reports Server (NTRS)
Murphy, D. P.; Adams, D. F.
1979-01-01
The stress distributions around individual fibers in a unidirectional boron/aluminum composite material subjected to axial and transverse loadings are being studied utilizing a generalized plane strain finite element analysis. This micromechanics analysis was modified to permit the analysis of longitudinal sections, and also to incorporate crack initiation and propagation. The analysis fully models the elastoplastic response of the aluminum matrix, as well as temperature dependent material properties and thermal stress effects. The micromechanics analysis modifications are described, and numerical results are given for both longitudinal and transverse models loaded into the inelastic range, to first failure. Included are initially cracked fiber models.
Crack propagation and fracture in engineered stress profile glass
NASA Astrophysics Data System (ADS)
Abrams, Matthew B.
Ion exchange procedures have been developed for soda lime silicate and soda alumina silicate glasses that produce a maximum compressive stress below the surface of the material. These glasses can form stable surface cracks under applied tensile stress, resulting in rising apparent R-curve behavior and reduced strength variability as a function of flaw size in the material. Glass exhibiting this behavior has been termed engineered stress profile (ESP) glass. In this work, eight ion exchange procedures and three surface preparation methods were used to produce a range of ESP glasses. An experimental stress measurement method utilizing iterated optical retardation and progressive etching was developed to determine the stress profile in the glass surfaces. Based on the measured stress profiles, a weight function approach was used to predict stress intensity factors as a function of crack geometry, and thus determine crack propagation paths as a function of initial flaw size, residual and applied stresses, and material fracture toughness. These calculations were used to predict fracture strength distribution, crack stability, and the potential for multiple surface cracking. Predicted values were compared to experimental observations of crack growth and fracture behavior, and with measured fracture strength distributions.
Fatigue Crack Propagation in Freshwater Ice
1993-07-01
Dempsey, J.P., and Wei, Y. (1989). Fracture toughness K,and fractography of SI type freshwater ice. Advances in Fracture Research, (K. Salama , K. Ravi...188-200. Perez , J., Mai, C., Tatibouet, J. and Vassoille, R. (1980). Dynamic Behaviour of Dislocations in HF-Doped Ice Ih. Journal of Glaciology, vol...Denmark, pp. 351-362. Tatibouet, J., Perez , J. and Vassoille, R. (1987). Study of Grain Boundaries in Ice by Internal Friction. J. Phys. Colloq. C1
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
Fatigue crack propagation behavior of a single crystalline superalloy
NASA Technical Reports Server (NTRS)
Lerch, B. A.; Antolovich, Stephen D.
1990-01-01
Crack propagation mechanisms occurring at various temperatures in a single crystalline Ni-base alloy, Rene N4, were investigated. The rates of crack growth at 21, 704, 927, 1038, and 1093 C were measured in specimens with 001-line and 110-line directions parallel to the load axis and the machined notch, respectively, using a pulsed dc potential drop apparatus, and the fracture surfaces at each temperature were examined using SEM. Crack growth rates (CGRs) for specimens tested at or below 927 C were similar, while at two higher temperatures, the CGRs were about an order of magnitude higher than at the lower temperatures. Results of SEM observations showed that surface morphologies depended on temperature.
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.
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.
Crack tip blunting and cleavage under dynamic conditions
NASA Astrophysics Data System (ADS)
Rajan, V. P.; Curtin, W. A.
2016-05-01
In structural materials with both brittle and ductile phases, cracks often initiate within the brittle phase and propagate dynamically towards the ductile phase. The macroscale, quasistatic toughness of the material thus depends on the outcome of this microscale, dynamic process. Indeed, dynamics has been hypothesized to suppress dislocation emission, which may explain the occurrence of brittle transgranular fracture in mild steels at low temperatures (Lin et al., 1987). Here, crack tip blunting and cleavage under dynamic conditions are explored using continuum mechanics and molecular dynamics simulations. The focus is on two questions: (1) whether dynamics can affect the energy barriers for dislocation emission and cleavage, and (2) what happens in the dynamic "overloaded" situation, in which both processes are energetically possible. In either case, dynamics may shift the balance between brittle cleavage and ductile blunting, thereby affecting the intrinsic ductility of the material. To explore these effects in simulation, a novel interatomic potential is used for which the intrinsic ductility is tunable, and a novel simulation technique is employed, termed as a "dynamic cleavage test", in which cracks can be run dynamically at a prescribed energy release rate into a material. Both theory and simulation reveal, however, that the intrinsic ductility of a material is unaffected by dynamics. The energy barrier to dislocation emission appears to be identical in quasi-static and dynamic conditions, and, in the overloaded situation, ductile crack tip behavior ultimately prevails since a single emission event can blunt and arrest the crack, preventing further cleavage. Thus, dynamics cannot embrittle a ductile material, and the origin of brittle failure in certain alloys (e.g., mild steels) appears unrelated to dynamic effects at the crack tip.
Microtubule dynamic instability: the role of cracks between protofilaments.
Li, Chunlei; Li, Jun; Goodson, Holly V; Alber, Mark S
2014-03-28
Microtubules (MTs) are cytoplasmic protein polymers that are essential for fundamental cellular processes including the maintenance of cell shape, organelle transport and formation of the mitotic spindle. Microtubule dynamic instability is critical for these processes, but it remains poorly understood, in part because the relationship between the structure of the MT tip and the growth/depolymerization transitions is enigmatic. In previous work, we used computational models of dynamic instability to provide evidence that cracks (laterally unbonded regions) between protofilaments play a key role in the regulation of dynamic instability. Here we use computational models to investigate the connection between cracks and dynamic instability in more detail. Our work indicates that while cracks contribute to dynamic instability in a fundamental way, it is not the depth of the cracks per se that governs MT dynamic instability. Instead, what matters more is whether the cracks terminate in GTP-rich or GDP-rich regions of the MT. Based on these observations, we suggest that a functional "GTP cap" (i.e., one capable of promoting MT growth) is one where the cracks terminate in pairs of GTP-bound subunits, and that the likelihood of catastrophe rises significantly with the fraction of crack-terminating subunits that contain GDP. In addition to helping clarify the mechanism of dynamic instability, this idea could also explain how MT stabilizers work: proteins that introduce lateral cross-links between protofilaments would produce islands of GDP-bound tubulin that mimic GTP-rich regions in having strong lateral bonds, thus reducing crack propagation, suppressing catastrophe and promoting rescue.
A local crack-tracking strategy to model three-dimensional crack propagation with embedded methods
Annavarapu, Chandrasekhar; Settgast, Randolph R.; Vitali, Efrem; Morris, Joseph P.
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 results 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.
A local crack-tracking strategy to model three-dimensional crack propagation with embedded methods
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
Incubation time for sub-critical crack propagation in SiC-SiC composites
El-Azab, A.; Ghoniem, N.M.
1995-04-01
The objective of this work is to investigate the time for sub-critical crack propagation is SiC-SiC composites at high temperatures. The effects of fiber thermal creep on the relaxation of crack bridging tractions in SiC-SiC ceramic matrix composites (CMCs) is considered in the present work, with the objective of studying the time-to propagation of sub-critical matrix cracks in this material at high temperatures. Under the condition of fiber stress relaxation in the bridiging zone, it is found that the crack opening and the stress intensity factor increase with time for sub-critical matrix cracks. The time elapsed before the stress intensity reaches the critical value for crack propagation is calculated as a function of the initial crack length, applied stress and temperature. Stability domains for matrix cracks are defined, which provide guidelines for conducting high-temperature crack propagation experiments.
The effect of adhesive layer on crack propagation in laminates
NASA Technical Reports Server (NTRS)
Gecit, M. R.; Erdogan, F.
1976-01-01
The effect of the adhesive layer on crack propagation in composite materials is investigated. The composite medium consists of parallel load carrying laminates and buffer strips arranged periodically and bonded with thin adhesive layers. The strips, assumed to be isotropic and linearly elastic, contain symmetric cracks of arbitrary lengths located normal to the interfaces. Two problems are considered: (1) thin adhesive layers are approximated by uncoupled tension and shear springs distributed along the interfaces of the strips for which only the case of internal cracks can be treated rigorously; (2) broken laminates and the true singular behavior in the presence of the adhesive layer are studied. The adhesive is then treated as an isotropic, linearly elastic continuum. General expressions for field quantities are obtained in terms of infinite Fourier integrals. These expressions give a system of singular integral equations in terms of the crack surface displacement derivatives. By using appropriate quadrature formulas, the integral equations reduce to a system of linear algebraic equations which are solved numerically.
NASA Astrophysics Data System (ADS)
Ota, Akihiko; Suzuki, Naoyuki; Maeda, Yoshio; Mawari, Toshio; Matsuoka, Saburo; Nishijima, Satoshi
1993-01-01
Marine structures are often constructed by welding, and they are subject to repeated loading such as waves and mechanical vibrations which can create fatigue cracks and consequently break the structures. Fatigue crack propagation properties of welded joints are studied under random loading in the air, synthetic sea water, and compressive cycling. It was found that the most crucial factor that controls fatigue crack propagation was high tensile residual stress fields of welded joints. This stress constantly kept the cracks open, simplifying fatigue crack propagation, and therefore, the rate of crack propagation could be assessed with high accuracy. In the transverse matching welded joints with cracks in the center, crack closure did not occur due to the tensile residual stress constantly induced at the crack ends in the center of the test samples. Fatigue crack propagation was accelerated both in artificial sea water and in compressive cycling compared to that in the air, and the fatigue lowest values were about half. Serious crack closures occurred in compressive cycling in which dry hours exceeded 45 minutes, and the fatigue crack propagation rate deteriorated remarkably. Mean fatigue crack propagation rate under the random loading is estimated precisely using equivalent stress intensity factor limit.
1983-11-01
KEY WORDS (Continue on reveree side It neceesear and Identify by block number) " Ignition, Deflagration-to- Detonation Transitfon, Crack Propagation...pressurization situation is typical of igni- tion transients in rocket motors and the onset of deflagration-to- detonation transition (DD’r) in confined...rocket motor and in the extreme could lead to de- flagration-to- detonation transition (DDT) or transition to detonation due to some unresolved
Avalanches and clusters in planar crack front propagation.
Laurson, Lasse; Santucci, Stephane; Zapperi, Stefano
2010-04-01
We study avalanches in a model for a planar crack propagating in a disordered medium. Due to long-range interactions, avalanches are formed by a set of spatially disconnected local clusters, the sizes of which are distributed according to a power law with an exponent tau{a}=1.5. We derive a scaling relation tau{a}=2tau-1 between the local cluster exponent tau{a} and the global avalanche exponent tau . For length scales longer than a crossover length proportional to the Larkin length, the aspect ratio of the local clusters scales with the roughness exponent of the line model. Our analysis provides an explanation for experimental results on planar crack avalanches in Plexiglas plates, but the results are applicable also to other systems with long-range interactions.
Assessment of damage localization based on spatial filters using numerical crack propagation models
NASA Astrophysics Data System (ADS)
Deraemaeker, Arnaud
2011-07-01
This paper is concerned with vibration based structural health monitoring with a focus on non-model based damage localization. The type of damage investigated is cracking of concrete structures due to the loss of prestress. In previous works, an automated method based on spatial filtering techniques applied to large dynamic strain sensor networks has been proposed and tested using data from numerical simulations. In the simulations, simplified representations of cracks (such as a reduced Young's modulus) have been used. While this gives the general trend for global properties such as eigen frequencies, the change of more local features, such as strains, is not adequately represented. Instead, crack propagation models should be used. In this study, a first attempt is made in this direction for concrete structures (quasi brittle material with softening laws) using crack-band models implemented in the commercial software DIANA. The strategy consists in performing a non-linear computation which leads to cracking of the concrete, followed by a dynamic analysis. The dynamic response is then used as the input to the previously designed damage localization system in order to assess its performances. The approach is illustrated on a simply supported beam modeled with 2D plane stress elements.
Low-pH SCC: Mechanical effects on crack propagation
Beavers, J.A.; Hagerdorn, E.L.
1996-09-06
A better definition of the role of mechanical factors on low-pH stress corrosion crack propagation is needed to aid in the prediction of crack growth rates on operating pipelines and to develop strategies to mitigate this form of cracking. The overall objective of the project was to determine the roles and synergistic effects of pressure, pressure fluctuations, and hydrotesting on low-pH stress corrosion crack growth. All testing was performed in a low-pH electrolyte (NS4 solution) under cyclic load conditions on pre-cracked specimens of one X-65 line pipe steel. The cyclic load conditions in the testing were related to field conditions using the J-integral parameter. This project consisted of the following three tasks, Task 1 - Development of Test Protocol, Task 2 - Mechanical Effects, and Task 3 - Effects of Hydrotesting. The purposes of Task 1 were to prepare the test specimens and experimental apparatus and to establish a standard test protocol for conducting the cyclic load tests and analyzing the test data. The specimen preparation procedures and environmental conditions were similar to those used in a previous project for TransCanada PipeLines (TCPL). The most significant difference between the tests performed in this project and the previous research was in the mode of loading. The previous work was performed under constant extension rate loading while this project was performed under cyclic load conditions. It is difficult to relate test conditions under constant extension rate loading with field conditions. However, the cyclic load conditions in the laboratory test can be directly related to field test conditions using the J-integral parameter. Modifications also were necessary in the data analysis procedure to account for the change in loading mode.
Quenched versus thermal disorder in crack propagation: size (and scales) matter.
NASA Astrophysics Data System (ADS)
Lengliné, Olivier; Schmittbuhl, Jean; Cochard, Alain; Jørgen Måløy, Knut; Toussaint, Renaud
2013-04-01
The slow propagation of crack in heterogeneous material is of fundamental importance for the failure of engineering structure and of natural system, such as seismic faults. Owing to the many interacting processes at play, it however still remains a challenge to describe the precise mechanical formulation that governs the dynamics of such systems. Previous studies dedicated to this issue have mostly been restricted to the zero temperature limit, giving rise to extremal dynamics, or to systems with short range interactions. Here we incorporate in a numerical model of slow crack growth the effect of temperature and long range elastic interactions. This approach provides a more realistic model of crack propagation in heterogeneous media under natural conditions. We adopt the configuration of an interfacial crack system, similar to a designed experimental setup. We recover both at the macroscopic and at the microscopic scales all the reported experimental observations. Namely we are able to observe a similar macroscopic crack evolution, a similar morphology of the crack front line and a similar distribution of local speeds: a self affine morphology with roughness exponent around 0.5 at small scale, and a lower effective roughness at larger scale for the front morphology [1], and a non Gaussian power law velocity distribution, with a fat tail P(v) v-2.6 at large speeds [2,3]. We also evidenced the competition between temperature and disorders, influencing the crack dynamics and modifying the crack pattern. We present analytical derivations that independently recover our numerical and experimental findings of two regimes dominated at small [4] and large scales [5] by quenched and annealed disorders respectively. We demonstrate that the cross-over length between these two regimes varies with the inverse of the temperature. We also show that the distribution of local speeds in our system is controlled by a parameter which depend both on temperature and disorder fluctuations
An Atomistic Simulation of Crack Propagation in a Nickel Single Crystal
NASA Technical Reports Server (NTRS)
Karimi, Majid
2002-01-01
The main objective of this paper is to determine mechanisms of crack propagation in a nickel single crystal. Motivation for selecting nickel as a case study is because we believe that its physical properties are very close to that of nickel-base super alloy. We are directed in identifying some generic trends that would lead a single crystalline material to failure. We believe that the results obtained here would be of interest to the experimentalists in guiding them to a more optimized experimental strategy. The dynamic crack propagation experiments are very difficult to do. We are partially motivated to fill the gap by generating the simulation results in lieu of the experimental ones for the cases where experiment can not be done or when the data is not available.
Crack propagation in functionally graded strip under thermal shock
NASA Astrophysics Data System (ADS)
Ivanov, I. V.; Sadowski, T.; Pietras, D.
2013-09-01
The thermal shock problem in a strip made of functionally graded composite with an interpenetrating network micro-structure of Al2O3 and Al is analysed numerically. The material considered here could be used in brake disks or cylinder liners. In both applications it is subjected to thermal shock. The description of the position-dependent properties of the considered functionally graded material are based on experimental data. Continuous functions were constructed for the Young's modulus, thermal expansion coefficient, thermal conductivity and thermal diffusivity and implemented as user-defined material properties in user-defined subroutines of the commercial finite element software ABAQUS™. The thermal stress and the residual stress of the manufacturing process distributions inside the strip are considered. The solution of the transient heat conduction problem for thermal shock is used for crack propagation simulation using the XFEM method. The crack length developed during the thermal shock is the criterion for crack resistance of the different graduation profiles as a step towards optimization of the composition gradient with respect to thermal shock sensitivity.
Factors influencing fatigue crack propagation behavior of austenitic steels
NASA Astrophysics Data System (ADS)
Kim, Sangshik; Kwon, Jaeki; Kim, Youngju; Jang, Wookil; Lee, Soongi; Choi, Jongkyo
2013-07-01
In the present study, the fatigue crack propagation (FCP) behaviors of austenitic single phase steels, including STS304, Fe18Mn and Fe22Mn with different grain sizes ranging from 12 μm to 98 μm were investigated. The FCP tests were conducted in air at an R ratio of 0.1 using compact tension specimens and the crack paths and fracture surfaces were documented by using an SEM. The highest ΔKth value of 9.9MPa·m1/2 was observed for the Fe18Mn specimen, followed by 5.2MPa·m1/2 for the Fe22Mn specimen and 4.6MPa·m1/2 for the STS304 specimen, showing a substantial difference in the near-threshold FCP resistance for each microstructure. The crack path and fractographic analyses suggested that the near-threshold FCP behavior of these austenitic steels was largely influenced by the degree of slip planarity, as determined by stacking fault energy and grain size, rather than the tensile properties. In the Paris' regime, the slip planarity still played an important role while the tensile properties began to affect the FCP. The FCP behavior of austenitic steels with different microstructural features are discussed based on detailed fractographic and micrographic observations.
1984-01-31
GR. Fatigue crack propagation, nickel-based superalloys, powder metallurgy, HIP , grain size influence t3ABSRACT (Continue on everse if neceaaary and...AD-A13L9290 ’A STUDY OF FATIGUE CRACK PROPAGATION IN POWDER METALLURGY HOT SOTATCAL.U) LEHIGHUNIVBETHEHEM PA DEPT 0F METALLURGY AND MATERIALS ENG...ClaaaficatioA Study of Fatigue Crack Propagation in Powder Metallurgy Hot Iso tatically Pressed Nickel-Base Alloy 12. PERSONAL AUTHOR(S) (Unclassified
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
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.
Cyclic-fatigue crack initiation and propagation in smooth alumina specimens
Kishimoto, Hidehiro; Ueno, Akira; Matsunaga, Atsushi; Kondo, Takuya
1998-01-01
Crack-initiation sites and crack-propagation rates of small cracks in smooth specimens of alumina with two grain sizes have been studied. The principal results that have been obtained are as follows: (1) in most cases, the crack-initiation life comprises a large portion of the fatigue life, (2) small cracks that are initiated in smooth specimens propagate with stress intensity factors that are much lower than the apparent threshold stress intensity factors (K{sub Imax}) of artificial cracks that are 200 {micro}m in length, and (3) the critical crack lengths beyond which the crack-propagation rate is described uniquely by K{sub Imax} are 120 {micro}m for fine-grained alumina and 250 {micro}m for coarse-grained alumina.
Low-pH stress corrosion crack propagation in API X-65 line pipe steel
Harle, B.A.; Beavers, J.A. )
1993-10-01
Preliminary results of ongoing crack growth studies being performed on an API X-65 line pipe steel in a low-pH cracking environment were reported. Objectives were to reproduce low-pH crack propagation in the laboratory, to identify a crack driving force parameter, and to evaluate the influence of environmental and mechanical parameters on crack growth. A J-integral test technique was used in the study. Significant crack growth was observed. The parameter J appeared to be a good driving force parameter to describe crack growth.
Salinity Effects on Cracking Morphology and Dynamics in Desiccating Clays
NASA Astrophysics Data System (ADS)
DeCarlo, K.; Shokri, N.
2013-12-01
Saline conditions induce not only chemical but physical changes in swelling clays, and have a significant influence on the crack dynamics in desiccating clays. In this study, we used X-ray computerized tomography (CT) to experimentally investigate the effects of sodium chloride on the morphology and dynamics of desiccation cracks in three-dimensional mixtures of sand-bentonite slurry under varying rheological conditions. Rectangular glass containers (40.5x40.5x56 mm^3) were packed with sand-bentonite slurries of different salt concentrations, with the top boundary exposed to air for evaporation. The growth and propagation of the cracking network that subsequently formed was visualized in 3D at multiple intervals. 3D characterization of cracking dynamics shows a high extent of localized superficial crack networks at low salinity, with a transition to less extensive but deeper, more centralized crack networks with increased salinity. The observed behavior was described in the context of rheological and colloidal properties of the clay, which suggest the transition from a voluminous and poorly-sorted stacked clay structure to a more compact and highly cohesive entangled clay structure as salt concentration increases in the evaporating samples. This is further corroborated by vertical profiles of sample water distribution, which shows localized uniform drying at the higher salt concentrations. Our results provide new insights regarding the formation, patterns, dynamics and characteristics of desiccation cracks formed during evaporation from 3D saline clay structures, which will be useful in various hydrological applications including water management, land surface evaporation, and subsurface contaminant transport.
Dynamical Realism and Uncertainty Propagation
NASA Astrophysics Data System (ADS)
Park, Inkwan
In recent years, Space Situational Awareness (SSA) has become increasingly important as the number of tracked Resident Space Objects (RSOs) continues their growth. One of the most significant technical discussions in SSA is how to propagate state uncertainty in a consistent way with the highly nonlinear dynamical environment. In order to keep pace with this situation, various methods have been proposed to propagate uncertainty accurately by capturing the nonlinearity of the dynamical system. We notice that all of the methods commonly focus on a way to describe the dynamical system as precisely as possible based on a mathematical perspective. This study proposes a new perspective based on understanding dynamics of the evolution of uncertainty itself. We expect that profound insights of the dynamical system could present the possibility to develop a new method for accurate uncertainty propagation. These approaches are naturally concluded in goals of the study. At first, we investigate the most dominant factors in the evolution of uncertainty to realize the dynamical system more rigorously. Second, we aim at developing the new method based on the first investigation enabling orbit uncertainty propagation efficiently while maintaining accuracy. We eliminate the short-period variations from the dynamical system, called a simplified dynamical system (SDS), to investigate the most dominant factors. In order to achieve this goal, the Lie transformation method is introduced since this transformation can define the solutions for each variation separately. From the first investigation, we conclude that the secular variations, including the long-period variations, are dominant for the propagation of uncertainty, i.e., short-period variations are negligible. Then, we develop the new method by combining the SDS and the higher-order nonlinear expansion method, called state transition tensors (STTs). The new method retains advantages of the SDS and the STTs and propagates
Fatigue-crack propagation in gamma-based titanium aluminide alloys at large and small crack sizes
Kruzic, J.J.; Campbell, J.P.; Ritchie, R.O.
1999-07-01
Most evaluations of the fracture and fatigue-crack propagation properties of {gamma}+{alpha}{sub 2} titanium aluminide alloys to date have been performed using standard large-crack samples, e.g., compact-tension specimens containing crack sizes which are on the order of tens of millimeters, i.e., large compared to microstructural dimensions. However, these alloys have been targeted for applications, such as blades in gas-turbine engines, where relevant crack sizes are much smaller ({lt}500 {micro}m) and where the small-crack fatigue threshold may be the most relevant design parameter. In this study, the authors compare and contrast the cyclic crack-growth behavior of both large (a {gt} 5 mm) and (c {approximately} 25--300 {micro}m) cracks in a {gamma}-TiAl based alloy, of composition Ti-47Al-2Nb-2Cr-0.2B (at.%), specifically for duplex (average grain size {approximately}17 {micro}m) and refined lamellar (average colony size {approximately}150 {micro}m) microstructures. It is found that, whereas the lamellar microstructure displays far superior fracture toughness and fatigue-crack growth resistance in the presence of large cracks, in small-crack testing the duplex microstructure exhibits a better combination of properties. The reasons for such contrasting behavior are examined in terms of the intrinsic and extrinsic (i.e., crack bridging) contributions to cyclic crack advance.
Okafor, A. Chukwujekwu; Singh, Navdeep; Singh, Navrag
2007-03-21
An aircraft is subjected to severe structural and aerodynamic loads during its service life. These loads can cause damage or weakening of the structure especially for aging military and civilian aircraft, thereby affecting its load carrying capabilities. Hence composite patch repairs are increasingly used to repair damaged aircraft metallic structures to restore its structural efficiency. This paper presents the results of Acoustic Emission (AE) monitoring of crack propagation in 2024-T3 Clad aluminum panels repaired with adhesively bonded octagonal, single sided boron/epoxy composite patch under tension-tension fatigue loading. Crack propagation gages were used to monitor crack initiation. The identified AE sensor features were used to train neural networks for predicting crack length. The results show that AE events are correlated with crack propagation. AE system was able to detect crack propagation even at high noise condition of 10 Hz loading; that crack propagation signals can be differentiated from matrix cracking signals that take place due to fiber breakage in the composite patch. Three back-propagation cascade feed forward networks were trained to predict crack length based on the number of fatigue cycles, AE event number, and both the Fatigue Cycles and AE events, as inputs respectively. Network using both fatigue cycles and AE event number as inputs to predict crack length gave the best results, followed by Network with fatigue cycles as input, while network with just AE events as input had a greater error.
NASA Astrophysics Data System (ADS)
Okafor, A. Chukwujekwu; Singh, Navdeep; Singh, Navrag
2007-03-01
An aircraft is subjected to severe structural and aerodynamic loads during its service life. These loads can cause damage or weakening of the structure especially for aging military and civilian aircraft, thereby affecting its load carrying capabilities. Hence composite patch repairs are increasingly used to repair damaged aircraft metallic structures to restore its structural efficiency. This paper presents the results of Acoustic Emission (AE) monitoring of crack propagation in 2024-T3 Clad aluminum panels repaired with adhesively bonded octagonal, single sided boron/epoxy composite patch under tension-tension fatigue loading. Crack propagation gages were used to monitor crack initiation. The identified AE sensor features were used to train neural networks for predicting crack length. The results show that AE events are correlated with crack propagation. AE system was able to detect crack propagation even at high noise condition of 10 Hz loading; that crack propagation signals can be differentiated from matrix cracking signals that take place due to fiber breakage in the composite patch. Three back-propagation cascade feed forward networks were trained to predict crack length based on the number of fatigue cycles, AE event number, and both the Fatigue Cycles and AE events, as inputs respectively. Network using both fatigue cycles and AE event number as inputs to predict crack length gave the best results, followed by Network with fatigue cycles as input, while network with just AE events as input had a greater error.
Salinity effects on the dynamics and patterns of desiccation cracks
NASA Astrophysics Data System (ADS)
Shokri, N.; Zhou, P.
2012-12-01
Cracking arising from desiccation is a ubiquitous phenomenon encountered in various industrial and geo-environmental applications including drying of clayey soil, cement, ceramics, gels, and many more colloidal suspensions. Presence of cracks in muddy sediments modifies the characteristics of the medium such as pore structure, porosity, and permeability which in turn influence various flow and transport processes. Thus it remains a topic of great interest in many disciplines to describe the dynamics of desiccation cracking under various boundary conditions. To this end, we conducted a comprehensive study to investigate effects of NaCl concentrations on cracking dynamics and patterns during desiccation of Bentonite. Mixtures of Bentonite and NaCl solutions were prepared with NaCl concentration varying from 2 to 10 percent in 0.5 percent increment (totally 17 configurations). The slurry was placed in a Petri dish mounted on a digital balance to record the evaporation dynamics. The atmospheric conditions were kept constant using an environmental chamber. An automatic camera was used to record the dynamics of macro-cracks (mm scale) at the surface of desiccating clay each minute. The obtained results illustrate the significant effects of salt concentration on the initiation, propagation, morphology and general dynamics of macro-cracks. We found that higher salt concentrations results in larger macro cracks' lengths attributed to the effects of NaCl on compressing the electric double layer of particles at increasing electrolyte concentrations which reduce considerably the repulsive forces among the particles and causing instability of the slurry and flocculation of the colloidal particles. Rheological measurements by means of a stress controlled rheometer revealed that the yield stress of the slurry decreases as NaCl concentration increases which may indicate aggregation of larger units in the slurry as a result of flocculation causing larger cracks' lengths due to
NASA Astrophysics Data System (ADS)
Choi, Jung-Hun; Kang, Min-Sung; Koo, Jae-Mean; Seok, Chang-Sung; Kim, Hyung-Ick
The fatigue crack propagation of CFRP (carbon fiber reinforced composite material) laminates is of current interest, particularly with regard to their durability under fatigue loading. Recently, carbon fiber reinforced composite materials (Woven fabric) are widely used in various fields of engineering because of its advanced properties. Then, many researchers have studied woven fabric CFRP materials but fatigue crack propagation behaviors for composites have not been still standardized . It shows the different crack propagation behavior according to load and fiber direction. Therefore, there is a need to consider fatigue crack propagation behavior in conformity with fiber arraying direction to load direction at designing structure using woven CFRP materials. In this study, therefore, the fatigue crack propagation for plain woven CFRP composite materials was investigated under two different fiber array direction (fiber arraying direction to load : 0°, 45°). Fatigue crack propagation tests of the woven CFRP composite were conducted under sinusoidal wave-form with stress ratios of 0.3 at a frequency of 10Hz, respectively. As a result of test, fatigue crack propagation rates (da/dN) were plotted against the stress-intensity factor amplitude (ΔK) and other factor. Also we compared ΔK with other factor that considering in-plain anisotropy. All of tests of fatigue crack propagation were carried out under mode I opening loading by using compact tension specimens.
1996-05-01
Analytical and experimental studies were performed to investigate the effect of rim thickness on gear tooth crack propagation. The goal was to... tooth crack propagation was simulated using a finite element based computer program. Principles of linear elastic fracture mechanics were used. Quarter...test gears to measure gear tooth crack growth rate. From both predictions and tests, gears with backup ratios (rim thickness divided by tooth height
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.
Low-Cycle Fatigue Life and Fatigue Crack Propagation of Sintered Ag Nanoparticles
NASA Astrophysics Data System (ADS)
Shioda, Ryutaro; Kariya, Yoshiharu; Mizumura, Noritsuka; Sasaki, Koji
2017-02-01
The low-cycle fatigue life and fatigue crack propagation behavior of sintered silver nanoparticles were investigated using miniature specimens sintered at two different temperatures. The fatigue crack initiation life and fatigue crack propagation rate of sintered Ag nanoparticles were extremely sensitive to changes in the range of inelastic energy density and the cyclic J integral, exhibiting brittle characteristics, in contrast to tin-based lead-free solder alloys. With increasing sintering temperature, the fatigue crack propagation rate decreased. On the other hand, the effect of sintering temperature on the fatigue crack initiation life differed depending on the use of either a smooth specimen (low-cycle fatigue test) or notched specimen (fatigue crack propagation test). For the notched specimens, the probability of grain boundaries around the notch decreased due to increased sintering temperature. Therefore, the fatigue crack initiation life was increased with an increase in sintering temperature in the fatigue crack propagation test. In the smooth specimen, however, the fatigue life decreased with an increase in sintering temperature, as the elastic modulus of the specimen increased with increasing sintering temperature. In the low-cycle fatigue test, the specimen sintered with high internal stress started to develop crack initiation early, causing a decrease in the crack initiation life.
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
NASA Astrophysics Data System (ADS)
Zhu, M.; Ou, G. F.; Jin, H. Z.; Du, C. W.; Li, X. G.; Liu, Z. Y.
2015-06-01
The crack propagation behavior of pipeline steels with or without AC application was studied in high pH solution using the crack propagation experiment (cyclic load). The results show that there is a significant difference in the crack propagation behavior of steels with or without AC interference. The crack growth rate (CGR) of steel under superimposed AC is considerably greater than that without AC. AC could cause an obvious effect on the crack propagation behavior, and enhance the CGR. The crack propagation behavior of steel under AC application in high pH solution is analogous to that in near-neutral pH solution.
Fatigue and Creep Crack Propagation behaviour of Alloy 617 in the Annealed and Aged Conditions
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 the 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
Near-neutral pH SCC in pipelines: Effects of pressure fluctuations on crack propagation
Beavers, J.A.; Jaake, C.E.
1998-12-31
Currently, there is a poor understanding of the effects of pressure related parameters (operating pressure, pressure fluctuations, and hydrostatic testings) on external stress corrosion crack propagation in pipelines in near-neutral-pH environments. A better definition of the role of these parameters on crack propagation is needed to aid in the prediction of crack growth rates on operating pipelines and to develop strategies to mitigate this form of cracking. The objective of the research described in this paper was to determine the roles and synergistic effects of operating pressure, pressure fluctuations, and hydrostatic testing on crack growth in line pipe steels in a near-neutral-pH SCC environment. All testing was performed on one X-65 line pipe steel in a near-neutral-pH cracking environment, designated NS4. Fatigue precracked compact-type specimens of the line pipe steel were cyclically loaded while immersed in the cracking environment. The desired loading regime was applied using a servo-hydraulic tensile testing machine. Crack growth was monitored using the electric potential drop technique. The loading conditions applied to the specimen were related to field conditions using the J-integral parameter. It was found that the prior load history applied to the specimens had a significant effect on crack growth behavior. Overloading inhibited crack growth while unloading stimulated crack growth. Hydrostatic testing, which combines overloading and unloading, caused some crack extension but reduced the crack velocity.
Crack propagation and the material removal mechanism of glass-ceramics by the scratch test.
Qiu, Zhongjun; Liu, Congcong; Wang, Haorong; Yang, Xue; Fang, Fengzhou; Tang, Junjie
2016-12-01
To eliminate the negative effects of surface flaws and subsurface damage of glass-ceramics on clinical effectiveness, crack propagation and the material removal mechanism of glass-ceramics were studied by single and double scratch experiments conducted using an ultra-precision machine. A self-manufactured pyramid shaped single-grit tool with a small tip radius was used as the scratch tool. The surface and subsurface crack propagations and interactions, surface morphology and material removal mechanism were investigated. The experimental results showed that the propagation of lateral cracks to the surface and the interaction between the lateral cracks and radial cracks are the two main types of material peeling, and the increase of the scratch depth increases the propagation angle of the radial cracks and the interaction between the cracks. In the case of a double scratch, the propagation of lateral cracks and radial cracks between paired scratches results in material peeling. The interaction between adjacent scratches depends on the scratch depth and separation distance. There is a critical separation distance where the normalized material removal volume reaches its peak. These findings can help reduce surface flaws and subsurface damage induced by the grinding process and improve the clinical effectiveness of glass-ceramics used as biological substitute and repair materials.
Crack propagation criteria in the framework of X-FEM-based structural analyses
NASA Astrophysics Data System (ADS)
Dumstorff, Peter; Meschke, Günther
2007-02-01
The extended finite element method (X-FEM) has proven to be capable of simulating cracking and crack propagation in quasi-brittle materials, such as cement paste or concrete, without the need for re-meshing. In the framework of the X-FEM cracks are represented as surfaces of discontinuous displacements continuously propagating through finite elements. Since crack path continuity is required in X-FEM-based analyses, the reliability of numerical analyses of cracked structures crucially depends on the correct prediction of the crack path and, consequently, on the criterion used for the determination of the crack propagation direction. In this paper four different crack propagation criteria proposed in the literature are investigated including two local and two global criteria. The two local criteria include an averaged stress criterion and the maximum circumferential stress criterion based on the linear elastic fracture mechanics. The two global criteria include a global tracking criterion proposed by Oliver and Huespe (Online Proceedings of the Fifth World Congress on Computational Mechanics, 2002) and an energy based X-FEM formulation recently proposed in (Computational Plasticity 2005. CIMNE: Barcelona, 2005; 565-568; Comput. Methods Appl. Mech. Eng. 2006, in press). Representative numerical benchmark examples, characterized by mode-I dominant fracture as well as by mixed-mode fracture, are used to study the performance and the robustness of the different crack propagation criteria. Copyright
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.
Fatigue-crack propagation in advanced aerospace materials: Aluminum-lithium alloys
Venkateswara Rao, K.T.; Ritchie, R.O.
1988-10-01
Characteristics of fatigue-crack propagation behavior are reviewed for recently developed commercial aluminum-lithium alloys, with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage-tolerant design. Specifically, crack-growth kinetics in Alcoa 2090-T8E41, Alcan 8090 and 8091, and Pechiney 2091 alloys, and in certain powder-metallurgy alloys, are examined as a function of microstructure, plate orientation, temperature, crack size, load ratio and loading sequence. In general, it is found that growth rates for long (> 10 mm) cracks are nearly 2--3 orders of magnitude slower than in traditional 2000 and 7000 series alloys at comparable stress-intensity levels. In additions, Al-Li alloys shown enhanced crack-growth retardations following the application of tensile overloads and retain superior fatigue properties even after prolonged exposure at overaging temperatures; however, they are less impressive in the presence of compression overloads and further show accelerated crack-growth behavior for microstructurally-small (2--1000 {mu}m) cracks (some three orders of magnitude faster than long cracks). These contrasting observations are attributed to a very prominent role of crack-tip shielding during fatigue-crack growth in Al-Li alloys, promoted largely by the tortuous and zig-zag nature of the crack-path morphologies. Such crack paths result in locally reduced crack-tip stress intensities, due to crack deflection and consequent crack wedging from fracture-surface asperities (roughness-induced crack closure); however, such mechanisms are far less potent in the presence of compressive loads, which act to crush the asperities, and for small cracks, where the limited crack wake severely restricts the shielding effect. 50 refs., 21 figs.
GOES dynamic propagation of attitude
NASA Technical Reports Server (NTRS)
Markley, F. Landis; Seidewitz, ED; Chu, Don; Rowe, John N.
1988-01-01
The spacecraft in the next series of Geostationary Operational Environmental Satellites (GOES-Next) are Earth pointing and have 5-year mission lifetimes. Because gyros can be depended on only for a few years of continuous use, they will be turned off during routine operations. This means attitude must, at times, be determined without benefit of gyros and, often, using only Earth sensor data. To minimize the interruption caused by dumping angular momentum, these spacecraft have been designed to reduce the environmental torque acting on them and incorporate an adjustable solar trim tab for fine adjustment. A new support requirement for GOES-Next is that of setting the solar trim tab. Optimizing its setting requires an estimate of the unbalanced torque on the spacecraft. These two requirements, determining attitude without gyros and estimating the external torque, are addressed by replacing or supplementing the gyro propagation with a dynamic one, that is, one that integrates the rigid body equations of motion. By processing quarter-orbit or longer batches, this approach takes advantage of roll-yaw coupling to observe attitude completely without Sun sensor data. Telemetered momentum wheel speeds are used as observations of the unbalanced external torques. GOES-Next provides a unique opportunity to study dynamic attitude propagation. The geosynchronous altitude and adjustable trim tab minimize the external torque and its uncertainty, making long-term dynamic propagation feasible. This paper presents the equations for dynamic propagation, an analysis of the environmental torques, and an estimate of the accuracies obtainable with the proposed method.
Threshold and Plastic Work of Fatigue Crack Propagation in HY80 and HY130 Steels.
1982-10-01
crack propagation rate near threshold versus LK of the standard heat treatment in HY80 steel . 43 HY 80 STEEL DUAL PHASE 1AA STEEL tO...650oC I HR .8- x S.T. Kmox .4 x .2 x x xj °xx x .2 I I I Ito 0 2 4 6 8 t0 12 AK (MPoa/m) Figure 29. The crack closure behavior of HY80 steel . (a) Crack ...4Figure 30. The crack closure behavior of HY80 steel . (a) Crack closure stress intensity Kcl versus A Ke ff (b) Kcl/Kmax versus
The influence of creep properties on crack propagation in thermal barrier coatings
NASA Astrophysics Data System (ADS)
Bäker, Martin
2010-07-01
Thermal barrier coatings are used to protect turbine blades from the high temperature of the process gas inside a turbine. They consist of a metallic bond coat and of a ceramic top coat with low thermal conductivity. During service, an additional oxide layer forms between bond coat and top coat that eventually causes failure. Finite element simulations show that the roughness of the interface between top and bond coat is crucial for determining the stress state. Lifetime models have been inferred that assume that cracks form in the peak positions at small oxide thickness and propagate when the oxide layer grows and the stress field shifts. A two-dimensional finite element model of crack propagation in the TBC layer is presented. Since the cracks propagate near a material interface and since plasticity may occur in the bond coat, standard tools of fracture mechanics for predicting the crack propagation direction are difficult to apply. This problem is circumvented in a very simple way by propagating short "test cracks" in different directions and optimising to find the crack direction with the maximum energy release rate. It is shown that the energy release rate and the crack propagation direction are sensitive to the details of the stress state and especially to the creep properties of the materials. Implications for failure models are discussed.
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).
High-cycle fatigue crack initiation and propagation in laser melting deposited TC18 titanium alloy
NASA Astrophysics Data System (ADS)
Wang, Yang; Zhang, Shu-quan; Tian, Xiang-jun; Wang, Hua-ming
2013-07-01
This article examines fatigue crack nucleation and propagation in laser deposited TC18 titanium alloy. The Widmanstätten structure was obtained by double-annealing treatment. High-cycle fatigue (HCF) tests were conducted at room temperature with the stress ratio of 0.1 and the notch concentration factor K t = 1. Fatigue cracks initiated preferentially at micropores, which had great effect on the HCF properties. The effect decreased with the decrease of pore size and the increase of distance from the pore location to the specimen surface. The crack initiation region was characterized by the cleavage facets of α lamella and the tearing of β matrix. The soft α precipitated-free zone formed along grain boundaries accelerated the crack propagation. Subsurface observation indicated that the crack preferred to propagate along the grain boundary α or border of α lamella or vertical to α lamella.
NASA Astrophysics Data System (ADS)
Dutta, V. B.; Suresh, S.; Ritchie, R. O.
1984-06-01
microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~10-8 to 10-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range ΔK0 values reported to date, and certainly the highest combination of strength and ΔK0 for steels ( i.e., ΔK0 values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations.
Ma, Longzhou
2012-11-30
The nickel-based superalloy INCONEL 617 is a candidate material for heat exchanger applications in the next-generation nuclear plant (NGNP) system. This project will study the crack propagation process of alloy 617 at temperatures of 650°C-950°C in air under static/cyclic loading conditions. The goal is to identify the environmental and mechanical damage components and to understand in-depth the failure mechanism. Researchers will measure the fatigue crack propagation (FCP) rate (da/dn) under cyclic and hold-time fatigue conditions, and sustained crack growth rates (da/dt) at elevated temperatures. The independent FCP process will be identified and the rate-controlled sustained loading crack process will be correlated with the thermal activation equation to estimate the oxygen thermal activation energy. The FCP-dependent model indicates that if the sustained loading crack growth rate, da/dt, can be correlated with the FCP rate, da/dn, at the full time dependent stage, researchers can confirm stress-accelerated grain-boundary oxygen embrittlement (SAGBOE) as a predominate effect. Following the crack propagation tests, the research team will examine the fracture surface of materials in various cracking stages using a scanning electron microscope (SEM) and an optical microscope. In particular, the microstructure of the crack tip region will be analyzed in depth using high resolution transmission electron microscopy (TEM) and electron energy loss spectrum (EELS) mapping techniques to identify oxygen penetration along the grain boundary and to examine the diffused oxygen distribution profile around the crack tip. The cracked sample will be prepared by focused ion beam nanofabrication technology, allowing researchers to accurately fabricate the TEM samples from the crack tip while minimizing artifacts. Researchers will use these microscopic and spectroscopic results to interpret the crack propagation process, as well as distinguish and understand the environment or
NASA Technical Reports Server (NTRS)
Everett, R. A., Jr.; Elber, W.
2000-01-01
In this paper the significance of the "small" crack effect as defined in fracture mechanics will be discussed as it relates to life managing rotorcraft dynamic components using the conventional safe-life, the flaw tolerant safe-life, and the damage tolerance design philosophies. These topics will be introduced starting with an explanation of the small-crack theory, then showing how small-crack theory has been used to predict the total fatigue life of fatigue laboratory test coupons with and without flaws, and concluding with how small cracks can affect the crack-growth damage tolerance design philosophy. As stated in this paper the "small" crack effect is defined in fracture mechanics where it has been observed that cracks on the order of 300 microns or less in length will propagate at higher growth rates than long cracks and also will grow at AK values below the long crack AK threshold. The small-crack effect is illustrated herein as resulting from a lack of crack closure and is explained based on continuum mechanics principles using crack-closure concepts in fracture mechanics.
Neutron Irradiation Effects on Fatigue Crack Propagation in Type 316 Stainless Steels at 649 C.
1980-08-01
maintaining the maximum tensile load constant for selected time periods duriig each cycle. Induction heating was employed to achieve a test temperature of...7 AD-A OB 052 NAVAL RESEAR ICH LAS WASHINGTON DC F/6 11/BNEUTRON IRRADIATION EFFECTS ON FATIGUE CRACK PROPAGATION IN TYP-EC(UlU UNL AUG 80 0 .J...and Identify by block number) Radiation Microstruture B Irradiation Fatigue Stainless steels Crack propagation Radiation effects High temperature V
Solvent control of crack dynamics in a reversible hydrogel
NASA Astrophysics Data System (ADS)
Baumberger, Tristan; Caroli, Christiane; Martina, David
2006-07-01
The resistance to fracture of reversible biopolymer hydrogels is an important control factor of the textural characteristics of food gels (such as gummy candies and aspic preparations). It is also critical for their use in tissue engineering, for which mechanical protection of encapsulated components is needed. Its dependence on loading rate and, recently, on the density and strength of crosslinks has been investigated. But, so far, no attention has been paid to solvent or to environment effects. Here we report a systematic study of crack dynamics in gels of gelatin in water/glycerol mixtures. We show in this model system that increasing solvent viscosity slows down cracks; moreover soaking with solvent markedly increases gel fragility; finally tuning the viscosity by adding a miscible liquid affects crack propagation through diffusive invasion of the crack tip vicinity. The results highlight the fact that fracture occurs by viscoplastic chain pull-out. This mechanism, as well as the related phenomenology, should be common to all reversibly crosslinked (physical) gels.
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.
Fracture mechanics of propagating 3-D fatigue cracks with parametric dislocations
NASA Astrophysics Data System (ADS)
Takahashi, Akiyuki; Ghoniem, Nasr M.
2013-07-01
Propagation of 3-D fatigue cracks is analyzed using a discrete dislocation representation of the crack opening displacement. Three dimensional cracks are represented with Volterra dislocation loops in equilibrium with the applied external load. The stress intensity factor (SIF) is calculated using the Peach-Koehler (PK) force acting on the crack tip dislocation loop. Loading mode decomposition of the SIF is achieved by selection of Burgers vector components to correspond to each fracture mode in the PK force calculations. The interaction between 3-D cracks and free surfaces is taken into account through application of the superposition principle. A boundary integral solution of an elasticity problem in a finite domain is superposed onto the elastic field solution of the discrete dislocation method in an infinite medium. The numerical accuracy of the SIF is ascertained by comparison with known analytical solution of a 3-D crack problem in pure mode I, and for mixed-mode loading. Finally, fatigue crack growth simulations are performed with the Paris law, showing that 3-D cracks do not propagate in a self-similar shape, but they re-configure as a result of their interaction with external boundaries. A specific numerical example of fatigue crack growth is presented to demonstrate the utility of the developed method for studies of 3-D crack growth during fatigue.
Haddad, R.E.; Dorado, A.O.
1994-12-31
This paper describes the tests conducted to determine the conditions leading to cracking of a specified grain of metal, during the iodine stress corrosion cracking (SCC) of zirconium alloys, focusing on the crystallographic orientation of crack paths, the critical stress conditions, and the significance of the fractographic features encountered. In order to perform crystalline orientation of fracture surfaces, a specially heat-treated Zircaloy-4 having very large grains, grown up to the wall thickness, was used. Careful orientation work has proved that intracrystalline pseudo-cleavage occurs only along basal planes. the effects of anisotropy, plasticity, triaxiality, and residual stresses originated in thermal contraction have to be considered to account for the influence of the stress state. A grain-by-grain calculation led to the conclusion that transgranular cracking always takes place on those bearing the maximum resolved tensile stress perpendicular to basal planes. Propagation along twin boundaries has been identified among the different fracture modes encountered.
3D characterization of crack propagation in building stones
NASA Astrophysics Data System (ADS)
Fusi, N.; Martinez-Martinez, J.; Crosta, G. B.
2012-04-01
Opening of fractures can strongly modify mechanical characteristics of natural stones and thus significantly decrease stability of historical and modern buildings. It is commonly thought that fractures origin from pre-existing structures of the rocks, such as pores, veins, stylolythes (Meng and Pan, 2007; Yang et al., 2008). The aim of this study is to define relationships between crack formation and textural characteristics in massive carbonate lithologies and to follow the evolution of fractures with loading. Four well known Spanish building limestones and dolostones have been analysed: Amarillo Triana (AT): a yellow dolomitic marble, with fissures filled up by calcite and Fe oxides or hydroxides; Blanco Tranco (BT): a homogeneous white calcitic marble with pore clusters orientated parallel to metamorphic foliation; Crema Valencia (CV): a pinkish limestone (mudstone), characterized by abundant stilolythes, filled mainly by quartz (80%) and kaolin (11%); Rojo Cehegin (RC): a red fossiliferous limestone (packstone) with white veins, made up exclusively by calcite in crystals up to 300 micron. All lithotypes are characterized by homogeneous mineralogical composition (calcitic or dolomitic) and low porosity (<10%). Three cores 20 mm in diameter have been obtained for each lithotype. Uniaxial compressive tests have been carried out in order to induce sample fracturing by a series of successive steps with application of a progressive normal stress. Crack propagation has been checked after each stress level application by microCT-RX following Hg impregnation of the sample (in a Hg porosimeter). Combination of both tests (microCT-RX and Hg porosimeter) guarantees a better characterization of small defects and their progressive propagation inside low-porous rocks than by employing solely microCT-RX (Fusi et al., 2009). Due to the reduced dimensions of sample holder (dilatometers) in porosimeter, cores have been cut with a non standard h/d = 1.5. Several cycles of: a) Hg
Peak Stress Intensity Factor Governs Crack Propagation Velocity In Crosslinked UHMWPE
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
Sirimamilla, Abhiram; Furmanski, Jevan; Rimnac, Clare
2013-04-01
Ultrahigh-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. 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 quasistatic manner. This 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.
Crack propagation in disordered materials: how to decipher fracture surfaces
NASA Astrophysics Data System (ADS)
Ponson, L.
For a half-century, engineers know how to describe and predict the propagation of a crack in a model elastic homogeneous medium. The case of real materials is much more complex. Indeed, we do not know how to relate their lifetime or their resistance to their microstructure. To achieve such a prediction, understanding the role of the microstructural disorder on the behavior of a crack is determinant. Fracture surfaces represent a promising field of investigation to address this question. From the study of various disordered materials, we propose a statistical description of their roughness and determine to which extent their properties are dependent of the material. We show that fracture surfaces display an anisotropic scale invariant geometry characterized by two universal exponents. Glass ceramics is then studied because its microstructure can be tuned in a controlled manner. Their fracture surfaces display the same general anisotropic properties but with surprisingly low exponents independent of the detail of the ceramics microstructure. This suggests the existence of a second universality class in failure problems. Using finally theoretical tools from out-of-equilibrium statistical physics and fracture mechanics, we relate the statistical properties of fracture surfaces with the mechanisms occurring at the microscopic scale during the failure of a material. In particular, we show that the first class of fracture surfaces results from a failure involving damage processes while the second one results from a perfectly brittle failure. Propagation de fissures dans les matériaux désordonnés : comment déchiffrer les surfaces de rupture. Depuis près d'un demi-siècle, les ingénieurs savent décrire et prévoir la propagation d'une fissure dans un milieu élastique homogène modèle. Le cas des matériaux réels est beaucoup plus complexe. En effet, on ne sait pas relier leur durée de vie ou leur résistance à leur microstructure. Passage obligé avant de telles
Uncommon deformation mechanisms during fatigue-crack propagation in nanocrystalline alloys.
Cheng, Sheng; Lee, Soo Yeol; Li, Li; Lei, Changhui; Almer, Jon; Wang, Xun-Li; Ungar, Tamas; Wang, Yinmin; Liaw, Peter K
2013-03-29
The irreversible damage at cracks during the fatigue of crystalline solids is well known. Here we report on in situ high-energy x-ray evidence of reversible fatigue behavior in a nanocrystalline NiFe alloy both in the plastic zone and around the crack tip. In the plastic zone, the deformation is fully recoverable as the crack propagates, and the plastic deformation invokes reversible interactions of dislocation and twinning in the nanograins. But around the crack tip lies a regime with reversible grain lattice reorientation promoted by a change of local stress state. These observations suggest unprecedented fatigue deformation mechanisms in nanostructured systems that are not addressed theoretically.
Effect of Microstructural Parameters on Fatigue Crack Propagation in an API X65 Pipeline Steel
NASA Astrophysics Data System (ADS)
Mohtadi-Bonab, M. A.; Eskandari, M.; Ghaednia, H.; Das, S.
2016-11-01
In the current research, we investigate fatigue crack growth in an API X65 pipeline steel by using an Instron fatigue testing machine. To this, first the microstructure of steel was accurately investigated using scanning electron microscope. Since nonmetallic inclusions play a key role during crack propagation, the type and distribution of such inclusions were studied through the thickness of as-received X65 steel using energy-dispersive spectroscopy technique. It was found that the accumulation of such defects at the center of thickness of the pipe body was higher than in other regions. Our results showed that there were very fine oxide inclusions (1-2 µm in length) appeared throughout the cross section of X65 steel. Such inclusions were observed not at the fatigue crack path nor on both sides of the fatigue crack. However, we found that large manganese sulfide inclusions (around 20 µm in length) were associated with fatigue crack propagation. Fatigue experiments on CT specimens showed that the crack nucleated when the number of fatigue cycles was higher than 340 × 103. On fracture surfaces, crack propagation also occurred by joining the microcracks at tip of the main crack.
Effects of Water Intrusion on Mechanical Properties of and Crack Propagation in Coal
NASA Astrophysics Data System (ADS)
Yao, Qiangling; Chen, Tian; Ju, Minghe; Liang, Shun; Liu, Yapeng; Li, Xuehua
2016-12-01
Studying the mechanical properties of and crack propagation in coal after water intrusion is necessary to tackle a number of geological engineering problems such as those associated with underground water storage in collieries and support for underground roadways in coal mines. To study the mechanical properties and crack development, 12 coal samples with moisture contents of 0, 2.37, 3.78 and 5.29 % were prepared for acoustic emission tests under uniaxial compression. Over about 6 days, the coal samples absorbed moisture from a humidifier in three different phases. In this period, uniaxial tests show that the peak stress, elastic modulus, strain softening modulus and post-peak modulus decreased with rising moisture content in the samples while the peak strain increased. It was further found that, by analysing the relationship between the stiffness and stress and the accumulated acoustic emission counts, all the phases of crack development can be evaluated. This is useful for studying the effect of water intrusion on crack propagation and for calculating the mechanical properties of the coal such as the elastic modulus. This investigation also quantifies the percentage of the stress thresholds for crack closure, crack initiation, and crack damage that constitutes the peak stress. These stress thresholds do not change with moisture content. Our results are of great significance for water storage in coal mines, for determination of pillar dimensions in coal mines, and for expanding the knowledge base of the mechanical properties of coal and the characteristics of crack propagation.
Effect of relative humidity on crack propagation in barrier films for flexible electronics
NASA Astrophysics Data System (ADS)
Vellinga, W. P.; De Hosson, J. Th. M.; Bouten, P. C. P.
2012-10-01
A set of propagating cracks in a SiN barrier film on poly ethylene naphthalate (PEN) were subjected to differing levels of relative humidity. It was observed that the propagation speed of the cracks increased for increasing levels of relative humidity. This was shown using two independent, simultaneous techniques. One of the techniques (a resistance measurement) gives a qualitative measure of the averaged crack tip speed and the other (a microscopic technique) a quantitative measure. An attempt is made to quantify the resistance measurements in terms of crack tip speed. The effects that humidity may have on the crack driving force through differences in hygroscopic expansion are discussed, using independent determination of the diffusion constant of water into PEN. It is concluded that hygroscopic expansion alone cannot account for the observations.
Fracture behaviors of pre-cracked monolayer molybdenum disulfide: A molecular dynamics study
Li, Zhen-huan; Tian, Xiao-geng
2016-01-01
Summary The fracture strength and crack propagation of monolayer molybdenum disulfide (MoS2) sheets with various pre-existing cracks are investigated using molecular dynamics simulation (MDS). The uniaxial tensions of pre-cracked monolayer MoS2 sheets with different crack tips, different locations of crack, different crack lengths and angled cracks are simulated and studied. The results show that the configuration of crack tip can influence significantly the fracture behaviors of monolayer MoS2 sheets while the location of crack does not influence the fracture strength. With the increase of crack length, the fracture strength of monolayer MoS2 sheets reduces almost linearly, and the fracture of monolayer MoS2 sheets is transformed from almost brittle to ductile. By making comparison between the MDS results and the predictions of continuum fracture mechanics theories, including Inglis' model, Griffith's model with and without finite size effect, it is found that MDS results agree well with the predictions of Griffith's model with finite size effect, differ from the predictions of Inglis' model and Griffith's model without finite size effect. Finally, the MDS results of monolayer MoS2 sheets with different angled crack are also analyzed based on the continuum fracture mechanics model. PMID:27826515
Dynamic response and time-frequency analysis for gear tooth crack detection
NASA Astrophysics Data System (ADS)
Mohammed, Omar D.; Rantatalo, Matti
2016-01-01
Vibration health monitoring is a non-destructive technique which can be applied to detect cracks propagating in gear teeth. This paper studies gear tooth crack detection by investigating the natural frequencies and by performing time-frequency analysis of a 6 DOF dynamic gear model. The gear mesh stiffness used in the model was calculated analytically for different cases of crack sizes. The frequency response functions (FRFs) of the model were derived for healthy and faulty cases and dynamic simulation was performed to obtain the time signal responses. A new approach involving a short-time Fourier transform (STFT) was applied where a fast Fourier transform (FFT) was calculated for successive blocks with different sizes corresponding to the time segments of the varying gear mesh stiffness. The relationship between the different crack sizes and the mesh-stiffness-dependent eigenfrequencies was studied in order to detect the tooth crack and to estimate its size.
Rao Venkateswara, K.T.; Ritchie, R.O. California Univ., Berkeley, CA . Dept. of Materials Science and Mineral Engineering); Bucci, R.J. . Alcoa Labs.)
1989-12-01
Micromechanisms influencing the propagation of long (>10 mm) fatigue cracks in aluminum-lithium alloys are examined by specifically comparing crack-growth kinetics in a peak-aged Al-Li-Cu-Zr alloy 2090, processed as 1.6-mm thin (T83) sheet and 12.7-mm thick (T81) plate. It is found that in general crack-growth rates are significantly faster in the sheet material at equivalent stress-intensity levels, due to differences in the role of crack-tip shielding, resulting from crack deflection and consequent crack closure from wedging of fracture-surface asperities. Microstructurally, such differences are related to variations in the degree of recrystallization, grain structure and deformation texture in the two wrought-product forms. 14 refs., 4 figs.
Fracture tolerance of reaction wood (yew and spruce wood in the TR crack propagation system).
Stanzl-Tschegg, Stefanie E; Keunecke, Daniel; Tschegg, Elmar K
2011-07-01
The fracture properties of spruce and yew were studied by in-situ loading in an environmental scanning microscope (ESEM). Loading was performed with a micro-wedge splitting device in the TR-crack propagation direction. The emphasis was laid on investigating the main mechanisms responsible for a fracture tolerant behavior with a focus on the reaction wood. The fracture mechanical results were correlated with the features of the surface structure observed by the ESEM technique, which allows loading and observation in a humid environment. Some important differences between the reaction wood and normal wood were found for both investigated wood species (spruce and yew), including the formation of cracks before loading (ascribed to residual stresses) and the change of fracture mode during crack propagation in the reaction wood. The higher crack propagation resistance was attributed mainly to the different cell (i.e. fiber) geometries (shape, cell wall thickness) and fiber angle to the load axis of the reaction wood, as basic structural features are responsible for more pronounced crack deflection and branching, thus leading to crack growth retardation. Fiber bridging was recognized as another crack growth retarding mechanism, which is effective in both wood species and especially pronounced in yew wood.
Study of Sub-interfacial Quasi-static Crack Propagation Using Shearing Interferometry
NASA Astrophysics Data System (ADS)
Lee, Hansuk; Krishnaswamy, Sridhar
Cracks on the interface between two materials have been extensively studied in view of their applications to failure processes in composite materials [1-3]. In this work, we look at the case of cracks that are off but close to an interface. Some early studies have indicated that under certain circumstances such sub-interfacial cracks tend to grow along a path that is parallel to the interface at a characteristic distance from the interface depending on the loading and material properties of the two materials. In this study, we optically map crack-tip stress fields for cracks that start off the interface, and track them as they subsequently propagate off the interface. The optical technique that was developed in our laboratory and which is used in this study will be explained. This shearing interferometer is used in conjunction with a 1000 frame/sec video camera. The resulting fringe patterns are evaluated to obtain information about the stress-state during initiation and propagation. The conditions for crack propagation parallel to the interface are explained. The experimental results are compared with crack trajectories predicted by finite element simulations.
Visualization of non-propagating Lamb wave modes for fatigue crack evaluation
NASA Astrophysics Data System (ADS)
An, Yun-Kyu; Sohn, Hoon
2015-03-01
This article develops a non-propagating Lamb wave mode (NPL) imaging technique for fatigue crack visualization. NPL has a great potential for crack evaluation in that it significantly contributes local mode amplitudes in the vicinity of a crack without spatial propagation. Such unique physical phenomenon is theoretically proven and experimentally measured through laser scanning. Although its measurement is a quite challenging work due to the fact that it is quite localized and coexists with complex propagating Lamb wave modes, a NPL filter proposed in this article overcomes the technical challenge by eliminating all propagating Lamb modes from laser scanned full Lamb wavefields. Through the NPL filtering process, only fatigue crack-induced NPLs can be measured and retained. To verify such physical observation and the corresponding NPL filter, a real micro fatigue crack is created by applying repeated tensile loading, and its detectability is tested using a surface-mounted piezoelectric transducer for generating Lamb waves and a laser Doppler vibrometer for measuring the corresponding responses. The experimental results confirm that even an invisible fatigue crack can be instantaneously visualized and effectively evaluated through the proposed NPL measurement and filtering processes.
NASA Astrophysics Data System (ADS)
Gaume, Johan; van Herwijnen, Alec; Chambon, Guillaume; Wever, Nander; Schweizer, Jürg
2017-01-01
The failure of a weak snow layer buried below cohesive slab layers is a necessary, but insufficient, condition for the release of a dry-snow slab avalanche. The size of the crack in the weak layer must also exceed a critical length to propagate across a slope. In contrast to pioneering shear-based approaches, recent developments account for weak layer collapse and allow for better explaining typical observations of remote triggering from low-angle terrain. However, these new models predict a critical length for crack propagation that is almost independent of slope angle, a rather surprising and counterintuitive result. Based on discrete element simulations we propose a new analytical expression for the critical crack length. This new model reconciles past approaches by considering for the first time the complex interplay between slab elasticity and the mechanical behavior of the weak layer including its structural collapse. The crack begins to propagate when the stress induced by slab loading and deformation at the crack tip exceeds the limit given by the failure envelope of the weak layer. The model can reproduce crack propagation on low-angle terrain and the decrease in critical length with increasing slope angle as modeled in numerical experiments. The good agreement of our new model with extensive field data and the ease of implementation in the snow cover model SNOWPACK opens a promising prospect for improving avalanche forecasting.
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.
Laser cutting silicon-glass double layer wafer with laser induced thermal-crack propagation
NASA Astrophysics Data System (ADS)
Cai, Yecheng; Yang, Lijun; Zhang, Hongzhi; Wang, Yang
2016-07-01
This study was aimed at introducing the laser induced thermal-crack propagation (LITP) technology to solve the silicon-glass double layer wafer dicing problems in the packaging procedure of silicon-glass device packaged by WLCSP technology, investigating the feasibility of this idea, and studying the crack propagation process of LITP cutting double layer wafer. In this paper, the physical process of the 1064 nm laser beam interact with the double layer wafer during the cutting process was studied theoretically. A mathematical model consists the volumetric heating source and the surface heating source has been established. The temperature and stress distribution was simulated by using finite element method (FEM) analysis software ABAQUS. The extended finite element method (XFEM) was added to the simulation as the supplementary features to simulate the crack propagation process and the crack propagation profile. The silicon-glass double layer wafer cutting verification experiment under typical parameters was conducted by using the 1064 nm semiconductor laser. The crack propagation profile on the fracture surface was examined by optical microscope and explained from the stress distribution and XFEM status. It was concluded that the quality of the finished fracture surface has been greatly improved, and the experiment results were well supported by the numerical simulation results.
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.
Atomistic Simulation of Environment-Assisted Crack Propagation Behavior of SiO2
NASA Astrophysics Data System (ADS)
Yasukawa, Akio
A modified extended Tersoff interatomic potential function is proposed to simulate environment-assisted crack propagation behavior. First, the physical properties of Si, O2, H2, SiO2, and H2O were calculated by this modified function. It was confirmed that the calculated values agreed with the measured values very well. Next, the potential surface of the H2O molecular transporting process to the crack tip of SiO2 material was calculated by the same function. The relationship between the velocity of crack propagation "υ" and the stress intensity factor "K" was calculated based on this surface. The results agreed with the experimental results well. This simulation clarified that the crack velocity is controlled by the H2O transporting process in both regions I and II of the "υ-K curve". In region I, H2O molecules have physically limited access to the crack tip due to the small opening in the crack. This works as an energy barrier in transporting H2O molecules. Due to the relatively large crack opening in region II, H2O molecules have free access to the crack tip without any energy barrier. This difference makes a bend in the "υ-K curve" between regions I and II.
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 conventional 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.
Two-scale extended finite element method for studying crack propagation of porous bioceramic
NASA Astrophysics Data System (ADS)
Chen, Jinlong; Wang, Mingguo; Zhan, Nan; Ji, Xinhua
2008-11-01
Extended finite element method (X-FEM) is a new method to solve the discontinuous problems, the basic theory of XFEM is presented in this paper, then the X-FEM is used to simulate the crack growth process of the hydroxyapatite material by three points bending test, and its deformation and stress field distribution is analyzed. The numerical results show the effectiveness of the method, the mesh in extended finite element method is independent of the internal geometry and physical interfaces, such that the trouble of high density meshing and re-meshing in the discontinuous field can be avoided. This greatly simplifies the analysis of the crack propagation process, showing the unique advantages of the extended finite element method in fracture expansion analysis of bioceramic. We also propose a two-scale strategy for crack propagation which enables one to use a refined mesh only in the crack's vicinity where it is required.
NASA Astrophysics Data System (ADS)
Park, Sang Yoon; Kim, Jun Hwan; Lee, Myoung Ho; Jeong, Yong Hwan
2008-01-01
Iodine-induced stress-corrosion cracking (ISCC) properties and the associated ISCC process of Zircaloy-4 fuel cladding were evaluated. Cladding was heat-treated to have either stress-relieved (SR) or recrystallized (RX) microstructures, and then an internal pressurization with a smooth and pre-cracked specimen was performed at 350 °C, in an iodine environment. The results showed that the threshold stress-intensity factor ( KISCC) of the SR and RX Zircaloy-4 claddings were 3.3 and 4.8 MPa m 0.5, respectively. The crack propagation rate of the RX Zircaloy-4 was 10 times lower than that of the SR one. Crack initiation and propagation mechanisms of Zircaloy-4 claddings, which had different microstructures, were proposed by a grain-boundary pitting model and a pitting-assisted slip cleavage model; they showed reasonable results.
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.
Extreme stress gradient effects on microstructural fatigue crack propagation rates in Ni microbeams
Sadeghi-Tohidi, F.; Pierron, O. N.
2015-05-18
The fatigue crack propagation behavior of microstructurally small cracks growing under extreme stress gradients was investigated in Ni microbeams under fully reversed cyclic loading. A technique to calculate the crack growth rates in microbeams with two different normalized stress gradients (17% and 50% μm{sup −1}) is developed and validated. Decreasing crack propagation rates are observed over the first 2 μm, and the rates are more than 1 order of magnitude slower for the devices with 50% μm{sup −1} stress gradients. This fundamental knowledge is critical to predict the fatigue reliability of advanced metallic microcomponents under bending such as in microelectromechanical systems or flexible/stretchable electronics.
Lewicki, D.G.
1996-05-01
Analytical and experimental studies were performed to investigate the effect of rim thickness on gear tooth crack propagation. The goal was to determine whether cracks grew through gear teeth (benign failure mode) or through gear rims (catastrophic failure mode) for various rim thicknesses. Gear tooth crack propagation was simulated using a finite element based computer program. Principles of linear elastic fracture mechanics were used. Quarter-point, triangular elements were used at the crack tip to represent the stress singularity. Crack tip stress intensity factors were estimated and used to determine crack propagation direction and fatigue crack growth rate. The computer program used had an automated crack propagation option in which cracks were grown numerically using an automated re-meshing scheme. In addition, experimental studies were performed in the NASA Lewis Spur Gear Fatigue Rig. Gears with various backup ratios were tested to validate crack path predictions. Also, specialized crack propagation gages were installed on the test gears to measure gear tooth crack growth rate. From both predictions and tests, gears with backup ratios (film thickness divided by tooth height) of 3.3 and 1.0 produced tooth fractures while a backup ratio of 0.3 produced rim fractures. For a backup ratio of 0.5, the experiments produced rim fractures and the predictions produced both rim and tooth fractures, depending on the initial crack conditions. Good correlation between the predicted number of crack propagation cycles and measured number of cycles was achieved using both the Paris fatigue crack growth method and the Collipfiest crack growth equation when fatigue crack closure was considered.
Salinity effects on cracking morphology and dynamics in 3-D desiccating clays
NASA Astrophysics Data System (ADS)
DeCarlo, Keita F.; Shokri, Nima
2014-04-01
Saline conditions induce not only chemical but physical changes in swelling clays, and have a significant influence on the crack dynamics and morphology of desiccating clays. In this study, we used X-ray microtomography to experimentally investigate the effects of sodium chloride on the morphology and dynamics of desiccation cracks in three-dimensional mixtures of sand-bentonite slurry under varying rheological conditions. Rectangular glass containers were packed with slurries of different salt concentrations, with the top boundary exposed to air for evaporation. The growth and propagation of the cracking network that subsequently formed was visualized in 3-D at multiple intervals. The characterization of cracking and branching behavior shows a high extent of localized surficial crack networks at low salinity, with a transition to less extensive but more centralized crack networks with increased salinity. The observed behavior was described in the context of the physicochemical properties of the montmorillonite clay, where shifts from an "entangled" (large platelet spacing, small pore structure) to a "stacked" (small platelet spacing, open pore structure) network influence fluid distribution and thus extent of cracking and branching behavior. This is further corroborated by vertical profiles of water distribution, which shows localized desiccation fronts that shift to uniform desaturation with increasing salt concentration. Our results provide new insights regarding the formation, dynamics, and patterns of desiccation cracks formed during evaporation from 3-D saline clay structures, which will be useful in hydrological applications including water management, land surface evaporation, and subsurface contaminant transport.
NASA Astrophysics Data System (ADS)
Hassanifard, S.; Bonab, M. A. Mohtadi; Jabbari, Gh.
2013-01-01
In this paper, fatigue crack propagation life of resistance spot welds in tensile-shear specimens is investigated based on the calculation of stress intensity factors and J-integral using three-dimensional finite element method. For comparison, experimental works on 5083-O aluminum alloy spot-welded joints have been carried out to verify the numerical predictions of fatigue crack propagation of welded joints. A lot of analyses have been performed to obtain stress intensity factors and J-integral in tensile-shear specimens of spot-welded joints by using commercial software ANSYS. These gathered data have been formulated by using statistical software SPSS. The results of fatigue propagation life and predicted fatigue crack path revealed very good agreement with the experimental fatigue test data and photograph of cross-section of the fatigued spot-weld specimens.
Acoustic emission during fatigue crack propagation in SiC particle reinforced Al matrix composites
Niklas, A.; Froyen, L.; Wevers, M.; Delaey, L.
1995-12-01
The acoustic emission (AE) behavior during fatigue propagation in aluminum 6061 and aluminum 6061 matrix composites containing 5, 10, and 20 wt pct SiC particle reinforcement was investigated under tension-tension fatigue loading. The purpose of this investigation was to monitor fatigue crack propagation by the AE technique and to identify the source(s) of AE. Most of the AEs detected were observed at the top of the load cycles. The cumulative number of AE events was found to correspond closely to the fatigue crack growth and to increase with increasing SiC content. Fractographic studies revealed an increasing number of fractured particles and to a lesser extent decohered particles on the fatigue fracture surface as the crack propagation rate (e.g., {Delta}K) or the SiC content was increased.
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.
NASA Astrophysics Data System (ADS)
Hemery, Samuel; Berdin, Clotilde; Auger, Thierry; Bourhi, Mariem
2016-12-01
Liquid metal embrittlement (LME) of T91 steel is numerically modeled by the finite element method to analyse experimental results in an axisymmetric notched geometry. The behavior of the material is identified from tensile tests then a crack with a constant crack velocity is introduced using the node release technique in order to simulate the brittle crack induced by LME. A good agreement between the simulated and the experimental macroscopic behavior is found: this suggests that the assumption of a constant crack velocity is correct. Mechanical fields during the embrittlement process are then extracted from the results of the finite element model. An analysis of the crack initiation and propagation stages: the ductile fracture probably breaks off the LME induced brittle fracture.
Experimental detection of cracks at rivets using structural wave propagation
NASA Astrophysics Data System (ADS)
Fromme, Paul; Sayir, Mahir B.
2001-04-01
Guided bending waves are used to detect defects in aluminum plates. The scattered field of a notched hole is studied as a model for a crack at a rivet hole in an airplane fuselage. The first anti-symmetric Lamb wave mode is excited using piezoelectric transducers. The scattered field is measured with a heterodyne laser-interferometer. Good agreement between measurements and an analytical model was found. A small notch changes the scattered field and can be detected.
Fatigue of Self-Healing Nanofiber-based Composites: Static Test and Subcritical Crack Propagation.
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.
In-situ acousto-ultrasonic monitoring of crack propagation in Al2024 alloy
NASA Astrophysics Data System (ADS)
Vanniamparambil, Prashanth A.; Bartoli, Ivan; Hazeli, Kavan; Cuadra, Jefferson; Schwartz, Eric; Saralaya, Raghavendra; Kontsos, Antonios
2012-04-01
A data fusion technique implementing the principles of acoustic emission (AE), ultrasonic testing (UT) and digital image correlation (DIC) was employed to in situ monitor crack propagation in an Al 2024 alloy compact tension (CT) specimen. The specimen was designed according to ASTM E647-08 and was pre-cracked under fatigue loading to ensure stable crack growth. Tensile (Mode I) loads were applied according to ASTM E1290-08 while simultaneously recording AE activity, transmitting ultrasonic pulses and measuring full-field surface strains. Realtime 2D source location AE algorithms and visualization provided by the DIC system allowed the full quantification of the crack growth and the cross-validation of the recorded non-destructive testing data. In post mortem, waveform features sensitive to crack propagation were extracted and visible trends as a function of computed crack length were observed. In addition, following a data fusion approach, features from the three independent monitoring systems were combined to define damage sensitive correlations. Furthermore a novelty detector based on the Mahalanobis outlier analysis was implemented to quantify the extent of crack growth and to define a more robust sensing basis for the proposed system.
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
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.
Dynamic fracture mechanics analysis for an edge delamination crack
NASA Technical Reports Server (NTRS)
Rizzi, Stephen A.; Doyle, James F.
1994-01-01
A global/local analysis is applied to the problem of a panel with an edge delamination crack subject to an impulse loading to ascertain the dynamic J integral. The approach uses the spectral element method to obtain the global dynamic response and local resultants to obtain the J integral. The variation of J integral along the crack front is shown. The crack behavior is mixed mode (Mode 2 and Mode 3), but is dominated by the Mode 2 behavior.
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.
Research on a Lamb Wave and Particle Filter-Based On-Line Crack Propagation Prognosis Method
Chen, Jian; Yuan, Shenfang; Qiu, Lei; Cai, Jian; Yang, Weibo
2016-01-01
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. PMID:26950130
Research on a Lamb Wave and Particle Filter-Based On-Line Crack Propagation Prognosis Method.
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.
Paul, S.C.; Pirskawetz, S.; Zijl, G.P.A.G. van; Schmidt, W.
2015-03-15
This paper presents the analysis of crack propagation in strain-hardening cement-based composite (SHCC) under tensile and flexural load by using acoustic emission (AE). AE is a non-destructive technique to monitor the development of structural damage due to external forces. The main objective of this research was to characterise the cracking behaviour in SHCC in direct tensile and flexural tests by using AE. A better understanding of the development of microcracks in SHCC will lead to a better understanding of pseudo strain-hardening behaviour of SHCC and its general performance. ARAMIS optical deformation analysis was also used in direct tensile tests to observe crack propagation in SHCC materials. For the direct tensile tests, SHCC specimens were prepared with polyvinyl alcohol (PVA) fibre with three different volume percentages (1%, 1.85% and 2.5%). For the flexural test beam specimens, only a fibre dosage of 1.85% was applied. It was found that the application of AE in SHCC can be a good option to analyse the crack growth in the specimens under increasing load, the location of the cracks and most importantly the identification of matrix cracking and fibre rupture or slippage.
Small fatigue crack propagation in Y2O3 strengthened steels
NASA Astrophysics Data System (ADS)
Hutař, P.; Kuběna, I.; Ševčík, M.; Šmíd, M.; Kruml, T.; Náhlík, L.
2014-09-01
This paper is focused on two type of Y2O3 strengthened steels (Fe-14Cr ODS and ODS-EUROFER). Small fatigue crack propagation was experimentally measured using special small cylindrical specimens (diameter 2 and 2.6 mm) with shallow notch grinded in the gauge length. In the middle of this notch, a pre-crack of length of 50 μm was fabricated using a focused ion beam technique. Fatigue crack growth rate was measured for different applied total strain amplitudes and described using plastic part of the J-integral. Obtained results were compared with published data of EUROFER 97. The effect of the oxide dispersion on small fatigue crack propagation was found rather insignificant. Ferritic Fe-14Cr ODS steel shows more brittle behaviour, i.e. for the same cyclic plasticity, characterised by the plastic part of the J-integral, the small cracks grow faster. A new methodology for residual lifetime prediction of structures containing physically small cracks, based on plastic part of the J-integral, is presented.
NASA Astrophysics Data System (ADS)
Hao, Wenfeng; Ma, Liting; Chen, Xinwen; Yuan, Yanan; Ma, Yinji
2016-02-01
The fatigue crack propagation behavior of two different forms of PMMA was studied using two-stage zone model. First, the fatigue crack length and fatigue crack propagation velocities of different specimens were obtained experimentally. Then the effect of material forms and specimen types on the fatigue crack propagation velocities was analyzed. Finally, the data scatter of da/ dN-Δ K curves in different forms and different types of specimens was analyzed. The results show that the expressions of fatigue crack propagation velocities of middle crack tension (MT) specimens and compact tension (CT) specimens in the same form PMMA are similar. And the scatter of MT specimens is larger than CT specimens in two forms of PMMA.
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.
Modeling of Propagation of Interacting Cracks Under Hydraulic Pressure Gradient
Huang, Hai; Mattson, Earl Douglas; Podgorney, Robert Karl
2015-04-01
A robust and reliable numerical model for fracture initiation and propagation, which includes the interactions among propagating fractures and the coupling between deformation, fracturing and fluid flow in fracture apertures and in the permeable rock matrix, would be an important tool for developing a better understanding of fracturing behaviors of crystalline brittle rocks driven by thermal and (or) hydraulic pressure gradients. In this paper, we present a physics-based hydraulic fracturing simulator based on coupling a quasi-static discrete element model (DEM) for deformation and fracturing with conjugate lattice network flow model for fluid flow in both fractures and porous matrix. Fracturing is represented explicitly by removing broken bonds from the network to represent microcracks. Initiation of new microfractures and growth and coalescence of the microcracks leads to the formation of macroscopic fractures when external and/or internal loads are applied. The coupled DEM-network flow model reproduces realistic growth pattern of hydraulic fractures. In particular, simulation results of perforated horizontal wellbore clearly demonstrate that elastic interactions among multiple propagating fractures, fluid viscosity, strong coupling between fluid pressure fluctuations within fractures and fracturing, and lower length scale heterogeneities, collectively lead to complicated fracturing patterns.
Fatigue crack propagation path across the dentinoenamel junction complex in human teeth.
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.
Investigation of Crack Propagation in Rock using Discrete Sphero-Polyhedral Element Method
NASA Astrophysics Data System (ADS)
Behraftar, S.; Galindo-torres, S. A.; Scheuermann, A.; Li, L.; Williams, D.
2014-12-01
In this study a micro-mechanical model is developed to study the fracture propagation process in rocks. The model is represented by an array of bonded particles simulated by the Discrete Sphero-Polyhedral Element Model (DSEM), which was introduced by the authors previously and has been shown to be a suitable technique to model rock [1]. It allows the modelling of particles of general shape, with no internal porosity. The motivation behind using this technique is the desire to microscopically investigate the fracture propagation process and study the relationship between the microscopic and macroscopic behaviour of rock. The DSEM method is used to model the Crack Chevron Notch Brazilian Disc (CCNBD) test suggested by the International Society of Rock Mechanics (ISRM) for determining the fracture toughness of rock specimens. CCNBD samples with different crack inclination angles, are modelled to investigate their fracture mode. The Crack Mouth Opening Displacement (CMOD) is simulated and the results are validated using experimental results obtained from a previous study [2]. Fig. 1 shows the simulated and experimental results of crack propagation for different inclination angles of CCNBD specimens. The DSEM method can be used to predict crack trajectory and quantify crack propagation during loading. References: 1. Galindo-Torres, S. A., et al. "Breaking processes in three-dimensional bonded granular materials with general shapes." Computer Physics Communications 183.2 (2012): 266-277. 2. Erarslan, N., and D. J. Williams. "Mixed-mode fracturing of rocks under static and cyclic loading." Rock mechanics and rock engineering 46.5 (2013): 1035-1052.
Grain boundary oxidation and oxidation accelerated fatigue crack nucleation and propagation
NASA Technical Reports Server (NTRS)
Liu, H. W.; Oshida, Y.
1986-01-01
Fatigue life at elevated temperatures is often shortened by oxidation. Grain boundary oxidation penetrates deeper than the surface oxidation. Therefore, grain boundary oxide penetration could be the primary cause of accelerated fatigue crack nucleation and propagation, and the shortened fatigue life at elevated temperatures. Grain boundary oxidation kinetics was studied and its statistical scatter was analyzed by the Weibull's distribution function. The effects of grain boundary oxidation on shortened fatigue life was analyzed and discussed. A model of intermittent microruptures of the grain boundary oxide was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate in the low frequency region with hold time at K sub max is inversely proportional to cyclic frequency and that crack growth is intergranular.
The Evolution of Stress Intensity Factors and the Propagation of Cracks in Elastic Media
NASA Astrophysics Data System (ADS)
Friedman, Avner; Hu, Bei; Velazquez, Juan J. L.
When a crack Γs propagates in an elastic medium the stress intensity factors evolve with the tip x(s) of Γs. In this paper we derive formulae which describe the evolution of these stress intensity factors for a homogeneous isotropic elastic medium under plane strain conditions. Denoting by ψ=ψ(x,s) the stress potential (ψ is biharmonic and has zero traction along the crack Γs) and by κ(s) the curvature of the crack at the tip x(s), we prove that the stress intensity factors A1(s), A2(s), as functions of s, satisfy:
Nonlinear analysis of flexural wave propagation through 1D waveguides with a breathing crack
NASA Astrophysics Data System (ADS)
Joglekar, D. M.; Mitra, M.
2015-05-01
An analytical-numerical approach is presented to investigate the flexural wave propagation through a slender semi-infinite beam with a breathing edge-crack. A Fourier transform based spectral finite element method is employed in an iterative manner to analyze the nonlinear response of the cracked beam subjected to a transverse tone burst excitation. Results obtained using the spectral finite element method are corroborated using 1D finite element analysis that involves the formulation and solution of a linear complementarity problem at every time step. In both the methods, an equivalent rotational spring is used to model the local flexibility caused by an open crack and the respective damaged beam element is formulated. The effect of crack-breathing is accounted for by an intermittent contact force acting at the nodes of the damaged beam element. A parallel study involving the open crack model is performed in the same setting to facilitate a comparison between the open and the breathing crack model. An illustrative case study reveals clearly the existence of higher order harmonics originating from the crack-breathing phenomenon which are absent if the crack is assumed to remain open throughout. A thorough investigation of the wrap-around effect associated with spectral finite element method reveals that the relative strengths of the higher order harmonics are not influenced by the wrap-around effect. A brief parametric study involving the variation of crack depth is presented at the end which suggests that the magnitudes of the higher harmonic peaks increase with increasing levels of crack severity. The present study can be potentially useful in the efforts geared toward the development of damage detection/localization strategies based on the nonlinear wave-damage interaction.
The accuracy of dynamic attitude propagation
NASA Technical Reports Server (NTRS)
Harvie, E.; Chu, D.; Woodard, M.
1990-01-01
Propagating attitude by integrating Euler's equation for rigid body motion has long been suggested for the Earth Radiation Budget Satellite (ERBS) but until now has not been implemented. Because of limited Sun visibility, propagation is necessary for yaw determination. With the deterioration of the gyros, dynamic propagation has become more attractive. Angular rates are derived from integrating Euler's equation with a stepsize of 1 second, using torques computed from telemetered control system data. The environmental torque model was quite basic. It included gravity gradient and unshadowed aerodynamic torques. Knowledge of control torques is critical to the accuracy of dynamic modeling. Due to their coarseness and sparsity, control actuator telemetry were smoothed before integration. The dynamic model was incorporated into existing ERBS attitude determination software. Modeled rates were then used for attitude propagation in the standard ERBS fine-attitude algorithm. In spite of the simplicity of the approach, the dynamically propagated attitude matched the attitude propagated with good gyros well for roll and yaw but diverged up to 3 degrees for pitch because of the very low resolution in pitch momentum wheel telemetry. When control anomalies significantly perturb the nominal attitude, the effect of telemetry granularity is reduced and the dynamically propagated attitudes are accurate on all three axes.
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.
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.
Effect of crack meandering on dynamic, ductile fracture
NASA Astrophysics Data System (ADS)
Tvergaard, V.; Needleman, A.
DYNAMIC crack growth is analyzed numerically for a plane strain edge cracked specimen subject to impulsive tensile loading at one end. An elastic—viscoplastic constitutive relation for a porous plastic solid is used to model ductile fracture by the nucleation and subsequent growth of voids to coalescence. Two populations of second-phase particles are represented: large inclusions with low strength, which result in large voids near the crack tip at an early stage, and small second-phase particles, which require large strains before cavities nucleate. Adiabatic heating due to plastic dissipation and the resulting thermal softening are accounted for in the analyses. Various two-dimensional distributions of the larger inclusions in front of the crack tip are considered, while the small second-phase particles are taken to be uniformly distributed. It is found that in most cases cracks grow in a zig-zag manner, dependent on the distribution of larger inclusions. Predictions for the dynamic crack growth behavior and for the time variation of crack tip characterizing parameters are obtained for each case analyzed. The computed crack growth paths and speeds are entirely based on the ductile failure predictions of the material model, so that the present study is free from ad hoc assumptions regarding appropriate dynamic crack growth criteria.
NASA Astrophysics Data System (ADS)
Popelyukh, A. I.; Popelyukh, P. A.; Bataev, A. A.; Nikulina, A. A.; Smirnov, A. I.
2016-03-01
The processes of the fracture of 40Kh and U8 steels under cyclic dynamic compression are studied. It has been found that the main cause for the fracture of the cyclically compressed specimens is the propagation of cracks due to the effect of residual tensile stresses, which arise near the tips of the cracks at the stage of the unloading of the specimens. The growth rate of a crack has the maximum value at the initial stage of its propagation in the vicinity of the stress concentrator. As the crack propagates deep into the specimen, its growth rate decreases and depends only slightly on the real cross section of the specimen. The model of the process of the fatigue fracture of the steels under dynamic loading by a cyclically varied compressive force is proposed. It has been found that the high fatigue endurance is provided by tempering at 200°C for the 40Kh steel and at 300°C for the U8 steel.
Modeling time-dependent corrosion fatigue crack propagation in 7000 series aluminum alloys
NASA Technical Reports Server (NTRS)
Mason, Mark E.; Gangloff, Richard P.
1994-01-01
Stress corrosion cracking and corrosion fatigue experiments were conducted with the susceptible S-L orientation of AA7075-T651, immersed in acidified and inhibited NaCl solution, to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as NASA FLAGRO. This environment enhances da/dN by five to ten-fold compared to fatigue in moist air. Time-based crack growth rates from quasi-static load experiments are an order of magnitude too small for accurate linear superposition prediction of da/dN for loading frequencies above 0.001 Hz. Alternate methods of establishing da/dt, based on rising-load or ripple-load-enhanced crack tip strain rate, do not increase da/dt and do not improve linear superposition. Corrosion fatigue is characterized by two regimes of frequency dependence; da/dN is proportional to f(exp -1) below 0.001 Hz and to F(exp 0) to F(exp -0.1) for higher frequencies. Da/dN increases mildly both with increasing hold-time at K(sub max) and with increasing rise-time for a range of loading waveforms. The mild time-dependence is due to cycle-time-dependent corrosion fatigue growth. This behavior is identical for S-L nd L-T crack orientations. The frequency response of environmental fatigue in several 7000 series alloys is variable and depends on undefined compositional or microstructural variables. Speculative explanations are based on the effect of Mg on occluded crack chemistry and embritting hydrogen uptake, or on variable hydrogen diffusion in the crack tip process zone. Cracking in the 7075/NaCl system is adequately described for life prediction by linear superposition for prolonged load-cycle periods, and by a time-dependent upper bound relationship between da/dN and delta K for moderate loading times.
Crack propagation analysis of surface enhanced titanium alloys with fretting induced damage
NASA Astrophysics Data System (ADS)
Garcia, Daniel Benjamin
2005-11-01
The objectives of this research project were to analyze, characterize, and predict the influences that surface treatments have on crack propagation in the presence of fretting fatigue damage. The titanium alloys, Ti-6Al-4V and Ti-17, were implemented for this research, and the surface enhancement methods consisted of shot peening and laser shock peening. The approach was to incorporate methods of contact mechanics, fractography, and fracture mechanics so that the influence of surface enhancements on fretting fatigue could be better understood. The specimens were obtained from prior fretting fatigue experiments that consisted of dog-bones and contact pads with both surface enhanced and bare conditions. The dog-bone specimens had fretting fatigue damage, which is a combination of a cyclic bulk load and the fretting induced damage. These specimens were incorporated in life prediction analyses in which a procedure for calculating fretting fatigue life by correlating nucleation and propagation through a non-arbitrary crack initiation criterion was introduced. The life prediction results show that the fretting fatigue life can be determined with knowledge of the fretting stress field and nature of the fretting cracks. The results also show that surface enhancements do not stop fretting fatigue cracks from forming, do slow the propagation and increase the fretting fatigue life. The contact pads had what is known as 'pure fretting' damage, which consists of the damage from the contact stresses but no cyclic bulk load. The contact pads are the basis for the development of the C-specimen experiment. The contact pads were machined into C-specimens that help measure the threshold stress intensity factor. The objective of the C-specimen experiment is to increase the cyclically applied load of the specimen through step testing until a fatigue crack propagates from the existing fretting induced crack. The testing technique provides for the threshold stress intensity factor to be
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.
NASA Technical Reports Server (NTRS)
Gupta, Vipul; Hochhalter, Jacob; Yamakov, Vesselin; Scott, Willard; Spear, Ashley; Smith, Stephen; Glaessgen, Edward
2013-01-01
A systematic study of crack tip interaction with grain boundaries is critical for improvement of multiscale modeling of microstructurally-sensitive fatigue crack propagation and for the computationally-assisted design of more durable materials. In this study, single, bi- and large-grain multi-crystal specimens of an aluminum-copper alloy are fabricated, characterized using electron backscattered diffraction (EBSD), and deformed under tensile loading and nano-indentation. 2D image correlation (IC) in an environmental scanning electron microscope (ESEM) is used to measure displacements near crack tips, grain boundaries and within grain interiors. The role of grain boundaries on slip transfer is examined using nano-indentation in combination with high-resolution EBSD. The use of detailed IC and EBSD-based experiments are discussed as they relate to crystal-plasticity finite element (CPFE) model calibration and validation.
1993-10-01
Volume ,: Introduction NJ 08405 Fracture Mechanics Fatigue Crack Propagation Research and Special Programs Administration John A. Volpe National...Load-displacement plot [Adapted from John M. Barson/Stanley T. Rolfe, Fracture and Fatigue Control in Structures. Applications of Fracture Mechanics...Methods ASTM STP 527, American Society for Testing and Materials, Philadelphia, PA, 1973. 2-19. Ratwani, M.M. and Wilhem , DP. Develonment and EvaluAtion of
NASA Astrophysics Data System (ADS)
Myftiu, Eglind; Roche, Matthieu; Kim, Pilnam; Stone, Howard A.
2010-11-01
Many highly concentrated particle suspensions are shear thickening; the viscosity increases with shear rate. The physics underlying shear thickening is still under discussion. In recent years, it was pointed out that shear thickening may be connected with a liquid-to-solid phase transition of the suspension. We provide direct evidence of this transition by studying the behavior of aqueous cornstarch suspensions of various concentration and layer thicknesses after impact of a free-falling cylindrical rod, which induces high strain rates and stresses. We observe patterns of regularly distributed radial cracks growing outwards from the impact region. Just after impact, a wave propagates on the surface of the layer and in the neighborhood of the impact a cavity expands. During this expansion, the cavity boundary is torn, and cracks start to grow. These cracks have rough boundaries, as is seen in solids. Once the cracks have reached their maximal extension, the suspension relaxes. The solvent slowly fills the cracks, until the layer returns to its initial shape. We discuss the influence of the layer thickness, starch concentration and impact energy on the dynamics of these cracks. We also discuss some properties of the solid phase of these suspensions as well as their relaxation dynamics.
Dynamic behaviour of a rotating cracked beam
NASA Astrophysics Data System (ADS)
Yashar, Ahmed; Ghandchi-Tehrani, Maryam; Ferguson, Neil
2016-09-01
This paper presents a new approach to investigate and analyse the vibrational behaviour of cracked rotating cantilever beams, which can for example represent helicopter or wind turbine blades. The analytical Hamiltonian method is used in modelling the rotating beam and two numerical methods, the Rayleigh-Ritz and FEM, are used to study the natural frequencies and the mode shapes of the intact rotating beams. Subsequently, a crack is introduced into the FE model and simulations are performed to identify the modal characteristics for an open cracked rotating beam. The effect of various parameters such as non-dimensional rotating speed, hub ratio and slenderness ratio are investigated for both the intact and the cracked rotating beam, and in both directions of chordwise and flapwise motion. The veering phenomena in the natural frequencies as a function of the rotational speed and the buckling speed are considered with respect to the slenderness ratio. In addition, the mode shapes obtained for the flapwise vibration are compared using the modal assurance criterion (MAC). Finally, a new three dimensional design chart is produced, showing the effect of crack location and depth on the natural frequencies of the rotating beam. This chart will be subsequently important in identifying crack defects in rotating blades.
NASA Astrophysics Data System (ADS)
Takada, Akira
1990-06-01
The three-dimensional shape and velocity of propagating cracks in the hydrostatic stress condition were studied by using gelatin, the physical properties of which were controlled to be constant. Various liquids (with various densities, viscosities, and volumes as the governed parameters) were injected in gelatin to form liquid-filled cracks. The directions of the crack growth and the propagation of an isolated crack are governed by the density difference between injected liquid and gelatin (Δρ), that is, a buoyancy. The propagation of a crack has two critical values: the first is the transition value to brittle fracture; the second is the value where segmentation begins to occur. The condition of a stable isolated crack formation is discussed. The crack shape of an isolated crack in the direction perpendicular to the crack plane is different from that of a growing crack with a fat tear drop form: the former has an elliptical top and a nearly flat bottom. The upper termination of an isolated crack in the vertical cross section has an elliptical shape, and the lower termination has a cusped shape. The lower part of the crack occupies the preexiting fracture which has formed by fracturing at the crack top. The crack thickness (w)/crack height (h) ratio is proportional to Δρ A, if the elastic moduli are constant. The crack length l/h ratio increase with h in the primary fracture, while the l/h ratio decreases with h in the preexisting fracture except for air-filled cracks. The ascending velocity of an isolated crack is proportional to Δρ3 h4, that is, Δρ w2, if the other physical properties are constant. The height and length of a growing penny-shaped crack are approximately proportional to A 3d1/3t4/9, so that the growth rate of height is in proportion to A3d3t-5/9 (A3d is constant injection rale). Some comparisons with the two-dimensional crack theory and applications for magma-filled cracks are discussed on the basis of these results.
NASA Astrophysics Data System (ADS)
Park, Sang Yoon; Kim, Jun Hwan; Choi, Byung Kwon; Jeong, Yong Hwan
2007-04-01
Tests of iodine-induced stress corrosion cracking (ISCC) were carried out to elucidate the initiation and propagation of cracks in the claddings of zirconium alloys. Zircaloy-4 cladding and Nb-contained zirconium cladding were pressurized with and without a pre-cracked state at 350°C in an iodine environment. The results show that pitting nucleation and growth play an important role in initiating ISCC. Pits preferentially grow and agglomerate around the grain boundary, where the number of pits increases with the iodine concentration and the hoop stress of the claddings. A model of grain-boundary pitting coalescence and a model of pitting-assisted slip cleavage, which were proposed to clearly elucidate the crack initiation and propagation process under ISCC, produce reasonable results. The Nb-contained zirconium cladding exhibits higher ISCC resistance than Zircaloy-4 from the standpoint of a higher threshold stress-intensity factor and a lower crack propagation rate.
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.
Analysis of crack propagation in nuclear graphite using three-point bending of sandwiched specimens
NASA Astrophysics Data System (ADS)
Shi, Li; Li, Haiyan; Zou, Zhenmin; Fok, Alex S. L.; Marsden, Barry J.; Hodgkins, Andrew; Mummery, Paul M.; Marrow, James
2008-01-01
The aim of this paper was to assess the suitability of the sandwiched beam in three-point bending as a technique for determining fracture toughness and R-curve behaviour of nuclear graphite using small beam specimens. Surface displacements of the cracked beam specimen were measured using Electronic Speckle Pattern Interferometry (ESPI) and Image Correlation in order to accurately monitor crack propagation and frictional contact between the test specimen and the sandwiching beams. The results confirmed that solutions based on the simple beam theory could overestimate the fracture toughness of graphite. Finite element analysis using a Continuum Damage Mechanics failure model indicated that both friction and shape of the notch played an important part in providing resistance to crack growth. Inclusion of these factors and the use of more accurate load vs. crack length curves derived from the FE model would provide a satisfactory measure of fracture toughness in small beam specimens under such a loading configuration. The particular graphite tested, IG-110, showed a decrease in fracture toughness with increasing crack length.
Simulation of crack propagation in fiber-reinforced concrete by fracture mechanics
Zhang Jun; Li, Victor C
2004-02-01
Mode I crack propagation in fiber-reinforced concrete (FRC) is simulated by a fracture mechanics approach. A superposition method is applied to calculate the crack tip stress intensity factor. The model relies on the fracture toughness of hardened cement paste (K{sub IC}) and the crack bridging law, so-called stress-crack width ({sigma}-{delta}) relationship of the material, as the fundamental material parameters for model input. As two examples, experimental data from steel FRC beams under three-point bending load are analyzed with the present fracture mechanics model. A good agreement has been found between model predictions and experimental results in terms of flexural stress-crack mouth opening displacement (CMOD) diagrams. These analyses and comparisons confirm that the structural performance of concrete and FRC elements, such as beams in bending, can be predicted by the simple fracture mechanics model as long as the related material properties, K{sub IC} and ({sigma}-{delta}) relationship, are known.
Experimental simulation of frost wedging-induced crack propagation in alpine rockwall
NASA Astrophysics Data System (ADS)
Jia, Hailiang; Leith, Kerry; Krautblatter, Michael
2016-04-01
Frost wedging is widely presumed to be the principal mechanism responsible for shattering jointed low-porosity rocks in high alpine rockwalls. The interaction of ice and rock physics regulates the efficacy of frost wedging. In order to better understand temporal aspects of this interaction, we present results of a series of laboratory experiments monitoring crack widening as a result of ice formation in an artificial crack (4mm wide, 80mm deep) cut 20 mm from the end of a rectangular granite block. Our results indicate that i) freezing direction plays a key role in determining the magnitude of crack widening; in short-term (1 day) experiments, maximum crack widening during top-down freezing (associated with 'autumn' conditions) was around 0.11mm, while inside-out freezing (resulting from 'spring' conditions) produced only 0.02 mm of deformation; ii) neither ice, nor water pressure (direct tension and hydraulic fracturing respectively) caused measurable irreversible crack widening during short-term tests, as the calculated maximum stress intensity at the crack tip was less than the fracture toughness of our granite sample; iii) development of ice pressure is closely related to the mechanical properties of the fracture in which it forms, and as such, the interaction of ice and rock is intrinsically dynamic; iv) irreversible crack widening (about 0.03mm) was only observed following a long-term (53 day) experiment representing a simplified transition from autumn to winter conditions. We suggest this is the result of stress corrosion aided by strong opening during freezing, and to a lesser degree by ice segregation up to one week after the initial freezing period, and downward migration of liquid water during the remainder of the test. Our results suggest the fundamental assumption of frost wedging, that rapid freezing from open ends of cracks can seal water inside the crack and thus cause damage through excessive stresses induced by volumetric expansion seems
NASA Astrophysics Data System (ADS)
Qin, Ming; Li, Jianfeng; Chen, Songying; Qu, Yanpeng
FV520B steel is a kind of precipitation hardening Martensitic stainless steel, it has high-strength, good plasticity and good corrosion resistance. Stress corrosion cracking (SCC) is one of the main corrosion failure mode for FV520B in industrial transportation of natural gas operation. For a better understanding the effect on SCC of FV520B, the improved wedge opening loading (WOL) specimens and constant displacement loading methods were employed in experimental research in carbon dioxide and hydrogen sulfide solution. The test results showed that the crack propagation rate is 1.941 × 10-7-5.748 × 10-7 mm/s, the stress intensity factor KISCC is not more than 36.83 MPa √{ m } . The rate increases with the increasing of the crack opening displacement. Under the condition of different initial loading, KISCC generally shows a decreasing tendency with the increase in H2S concentration, and the crack propagation rate showed an increasing trend substantially. For the enrichment of sulfur ion in the crack tip induced the generation of pitting corrosion, promoting the surrounding metal formed the corrosion micro batteries, the pit defects gradually extended and connected with the adjacent pit to form a small crack, leading to further propagation till cracking happened. Fracture microscopic morphology displayed typical brittle fracture phenomena, accompanying with trans-granular cracking, river shape and sector, many second cracks on the fracture surface.
NASA Astrophysics Data System (ADS)
Xiao, Si; Wang, He-Ling; Liu, Bin; Hwang, Keh-Chih
2015-11-01
The J-integral based criterion is widely used in elastic-plastic fracture mechanics. However, it is not rigorously applicable when plastic unloading appears during crack propagation. One difficulty is that the energy density with plastic unloading in the J-integral cannot be defined unambiguously. In this paper, we alternatively start from the analysis on the power balance, and propose a surface-forming energy release rate (ERR), which represents the energy available for separating the crack surfaces during the crack propagation and excludes the loading-mode-dependent plastic dissipation. Therefore the surface-forming ERR based fracture criterion has wider applicability, including elastic-plastic crack propagation problems. Several formulae are derived for calculating the surface-forming ERR. From the most concise formula, it is interesting to note that the surface-forming ERR can be computed using only the stress and deformation of the current moment, and the definition of the energy density or work density is avoided. When an infinitesimal contour is chosen, the expression can be further simplified. For any fracture behaviors, the surface-forming ERR is proven to be path-independent, and the path-independence of its constituent term, so-called Js-integral, is also investigated. The physical meanings and applicability of the proposed surface-forming ERR, traditional ERR, Js-integral and J-integral are compared and discussed. Besides, we give an interpretation of Rice paradox by comparing the cohesive fracture model and the surface-forming ERR based fracture criterion.
Mitigation of sub-surface crack propagation in railroad rails by laser surface modification
DiMelfi, R.J.; Sanders, P.G.; Hunter, B.; Eastman, J.A.; Leong, K.; Kramer, J.M.; Sawley, K.J.
1997-10-01
The authors address the mitigation of sub-surface crack propagation in railroad rails via laser surface modification. The goal is to reduce the shear forces from rail-wheel friction, which contribute significantly to the nucleation and propagation of cracks in the sub-surface region at rail gage corners. Microhardness scans and tensile tests were performed on samples from cross-sections of unused and heavily used rail heads. The results of these tests indicate that the severe cyclic plastic deformation that occurs at the gage corners, during service, significantly hardens the sub-surface region there, which leads to cracking. Laser glazing, the rapid melting and rapid solidification of a thin surface layer, was used to reduce the friction coefficient of rail steel. The advantages of this process are that specific regions of the rail surface can be targeted; the treatment does not wash away as the currently used liquid lubricants do; it is more environmentally sound than liquid lubricants; and it can be applied in service, during re-work or during rail fabrication. A number of laser treatments were conducted on AISI 1080 steel plates, similar to rail steel, from which friction samples were extracted. Static block-on-ring friction experiments performed on a variety of laser treated surfaces showed reductions in the friction coefficient by about 25% relative to untreated surfaces at loads corresponding to prototypic rail service loads. The authors laser-glazed two areas on the top surface of a 6-ft length of rail with multiple pass treatments, one with adjacent passes overlapping, and one with adjacent passes separated by 1 mm. Friction measurements were made after they were subjected to 20,000 run-in cycles. The laser treatments remained intact after these cycles. Reductions of friction coefficient of ca. 40%, relative to untreated surfaces, were observed, corresponding to a reduction in the calculated mixed mode crack propagation rate by ca. 79%.
Jeong, Dae-Ho; Choi, Myung-Je; Goto, Masahiro; Lee, Hong-Chul; Kim, Sangshik
2014-09-15
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 mechanism 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.
NASA Astrophysics Data System (ADS)
Gasem, Zuhair Mattoug
The need to predict the fatigue performance of aging aerospace structures has focused interest on environmentally assisted cracking in thick-section damage-tolerant aluminum alloys (AA). The objective of this research is to characterize and understand the time-dependent processes that govern environmental fatigue crack propagation (EFCP) in 7XXX series aluminum alloys exposed to an aggressive environment. Results are utilized to identify the rate-controlling step in growth enhancement in order to develop a mechanistic model describing the time dependency of EFCP. Aluminum alloy 7075, tested in the sensitive (SL) orientation and exposed to aqueous chloride solution, is studied. Da/dNcrit for different D K levels depends on 1/√fcrit, as predicted by process zone hydrogen-diffusion-limited crack growth modeling. A model based on hydrogen diffusion controlled growth is modified to include a stress-dependent critical hydrogen concentration normalized with the crack tip hydrogen concentration (Ccrit/CS). It is proposed that da/dNcrit for a given D K and R corresponds to the distance ahead of the crack tip where the local tensile stress associated with Kmax is maximum. The reversed plasticity estimate of this location equals da/dNcrit for two aging conditions of 7075 (SL)/NaCl at R = 0.1. The EFCP dependencies on alloy microstructure (T6 vs. T7), crack orientation (SL vs. LT), and stress ratio are measured and interpreted based on their effect on da/dN crit and fcrit as well as environmental closure. Chromate addition to the chloride solution eliminates the environmental acceleration of crack growth and reduces corrosion-product induced closure. In chromate-inhibited solution, the frequency dependence of EFCP in 7075 (SL) is unique. Da/dN is reduced at moderate and low frequencies to a value similar to crack growth rate in moist air, probably due to formation of a passive film which inhibits hydrogen uptake. Inhibition is mitigated by increasing frequency or increasing
The effect of thickness on fatigue crack propagation in 7475-T731 aluminum alloy sheet
NASA Technical Reports Server (NTRS)
Daiuto, R. A.; Hillberry, B. M.
1984-01-01
Tests were conducted on three thicknesses of 7475-T731 aluminum alloy sheet to investigate the effect of thickness on fatigue crack propagation under constant amplitude loading conditions and on retardation following a single peak overload. Constant amplitude loading tests were performed at stress ratios of 0.05 and 0.75 to obtain data for conditions with crack closure and without crack closure, respectively. At both stress ratios a thickness effect was clearly evident, with thicker specimens exhibiting higher growth rates in the transition from plane strain to plane stress region. The effect of thickness for a stress ratio of 0.05 corresponded well with the fracturing mode transitions observed on the specimens. A model based on the strain energy release rate which accounted for the fracture mode transition was found to correlate the thickness effects well. The specimens tested at the stress ratio of 0.75 did not make the transition from tensile mode to shear mode, indicating that another mechanism besides crack closure or fracture mode transition was active.
Effect of tangential traction and roughness on crack initiation/propagation during rolling contact
NASA Technical Reports Server (NTRS)
Soda, N.; Yamamoto, T.
1980-01-01
Rolling fatigue tests of 0.45 percent carbon steel rollers were carried out using a four roller type rolling contact fatigue tester. Tangential traction and surface roughness of the harder mating rollers were varied and their effect was studied. The results indicate that the fatigue life decreases when fraction is applied in the same direction as that of rolling. When the direction of fraction is reversed, the life increases over that obtained with zero traction. The roughness of harder mating roller also has a marked influence on life. The smoother the mating roller, the longer the life. Microscopic observation of specimens revealed that the initiation of cracks during the early stages of life is more strongly influenced by the surface roughness, while the propagation of these cracks in the latter stages is affected mainly by the tangential traction.
NASA Astrophysics Data System (ADS)
Uribe, David; Steeb, Holger
2016-04-01
The use of imaged based methods to determine properties of geological materials is becoming an alternative to laboratory experiments. Furthermore, the combination of laboratory experiments and image based methods using micro computer tomography have advanced the understanding of geophysical and geochemical processes. Within the scope of the "Shynergie" project, two special topics have been studied using such combination: a) the generation and propagation of cracks in rocks (specially wing cracks) and b) the time dependence of transport properties of rocks due to chemical weathering. In this publication, we describe the design considerations of our micro CT scanner to manipulate rock samples that have been subjected to the experiments to determine the above mentioned phenomena. Additionally, we discuss the preliminary experimental results and the initial interpretations we have gathered from the observations of the digitized rock samples.
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.
NASA Astrophysics Data System (ADS)
Gheldane, Farid; Souya, Lotfi Ain; Bouras, Seddik
2011-12-01
We studied resistance to the propagation of cracks on composites mullite zirconia and mullite alumina zirconia using the flexure tests SENB. The second nuance presents an R-curve effect interesting compared to mullite zirconia where the effect hardly appears. For understanding the mechanisms toughening, we used the SEM observations which showed that resistance to the propagation is mainly connected to the cracks bridging. The crack lengths are often calculated on the basis of compliance evolution during the R-curve tests. We show that the cracks lengths calculated starting from compliance underestimate in an important way the crack true values. The not fissured ligaments, responsible of the bridging mechanisms, are indeed also the cause of the error induced on compliance.
Mohammadi, H; Klassen, R J; Wan, W-K
2008-10-01
Pyrolytic carbon mechanical heart valves (MHVs) are widely used to replace dysfunctional and failed heart valves. As the human heart beats around 40 million times per year, fatigue is the prime mechanism of mechanical failure. In this study, a finite element approach is implemented to develop a model for fatigue analysis of MHVs due to the impact force between the leaflet and the stent and cavitation in the aortic position. A two-step method to predict crack propagation in the leaflets of MHVs has been developed. Stress intensity factors (SIFs) are computed at a small initiated crack located on the leaflet edge (the worst case) using the boundary element method (BEM). Static analysis of the crack is performed to analyse the stress distribution around the front crack zone when the crack is opened; this is followed by a dynamic crack analysis to consider crack propagation using the finite element approach. Two factors are taken into account in the calculation of the SIFs: first, the effect of microjet formation due to cavitation in the vicinity of leaflets, resulting in water hammer pressure; second, the effect of the impact force between the leaflet and the stent of the MHVs, both in the closing phase. The critical initial crack length, the SIFs, the water hammer pressure, and the maximum jet velocity due to cavitation have been calculated. With an initial crack length of 35 microm, the fatigue life of the heart valve is greater than 60 years (i.e. about 2.2 x 10(9) cycles) and, with an initial crack length of 170 microm, the fatigue life of the heart valve would be around 2.5 years (i.e. about 9.1 x 10(7) cycles). For an initial crack length greater than 170 microm, there is catastrophic failure and fatigue cracking no longer occurs. A finite element model of fatigue analysis using Patran command language (PCL custom code) in MSC software can be used to evaluate the useful lifespan of MHVs. Similar methodologies can be extended to other medical devices under cyclic
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 growth of mixed-mode shear cracks
Andrews, D.J.
1994-01-01
A pure mode II (in-plane) shear crack cannot propagate spontaneously at a speed between the Rayleigh and S-wave speeds, but a three-dimensional (3D) or two-dimensional (2D) mixed-mode shear crack can propagate in this range, being driven by the mode III (antiplane) component. Two different analytic solutions have been proposed for the mode II component in this case. The first is the solution valid for crack speed less than the Rayleigh speed. When applied above the Rayleigh speed, it predicts a negative stress intensity factor, which implies that energy is generated at the crack tip. Burridge proposed a second solution, which is continuous at the crack tip, but has a singularity in slip velocity at the Rayleigh wave. Spontaneous propagation of a mixed-mode rupture has been calculated with a slip-weakening friction law, in which the slip velocity vector is colinear with the total traction vector. Spontaneous trans-Rayleigh rupture speed has been found. The solution depends on the absolute stress level. The solution for the in-plane component appears to be a superposition of smeared-out versions of the two analytic solutions. The proportion of the first solution increases with increasing absolute stress. The amplitude of the negative in-plane traction pulse is less than the absolute final sliding traction, so that total in-plane traction does not reverse. The azimuth of the slip velocity vector varies rapidly between the onset of slip and the arrival of the Rayleigh wave. The variation is larger at smaller absolute stress.
Bramwell, I.L.; Tice, D.R.; Worswick, D.; Heys, G.B.
1995-12-31
The growth of sub-critical cracks in pressure boundary materials in light water reactors is assessed using codified procedures, but the presence of the overlay-welded stainless steel cladding on the pressure vessel is not normally taken into consideration because of the difficulty in demonstrating clad integrity for the lifetime of the plant. In order to investigate any possible effect of the cladding layer on crack propagation, tests have been performed using two types of specimen. The first was sputter ion plated with a thin layer of austenitic stainless steel to simulate the electrochemical and oxide effects due to the cladding, whilst the second used an overlay clad specimen to investigate the behavior of a crack propagating from the austenitic into the ferritic material. Testing was carried out under cyclic loading conditions in well controlled simulated PWR primary water. At 288 C, the presence of stainless steel in contact with the low alloy steel did not enhance crack propagation in PWR primary coolant compared to unclad or unplated specimens. There was limited evidence that at 288 C under certain loading conditions, in both air and PWR water, there may be an effect of the cladding which reduces crack growth rates, at least for a short distance of crack propagation into the low alloy steel. Crack growth rates in the ferritic steel at 130 C were higher for both the plated and clad specimens than found in previous tests under similar conditions on the unclad material. However, the crack growth rates were bounded by current ASME 11 Appendix A recommendations for defects exposed to water and at low R ratio. There was no evidence of environmental enhancement of crack propagation in the stainless steel in clad specimens. The results indicate that the current approach of ignoring the cladding for assessment purposes is conservative at plant operating temperature.
Oscillatory instability in slow crack propagation in rubber under large deformation
NASA Astrophysics Data System (ADS)
Endo, Daiki; Sato, Katsuhiko; Hayakawa, Yoshinori
2012-07-01
We performed experiments to investigate slow fracture in thin rubber films under uniaxial tension using high-viscosity oils. In this system we observed an oscillating instability in slowly propagating cracks for small applied strains. The transition between oscillatory and straight patterns occurred near the characteristic strain at which rubber exhibits a nonlinear stress-strain relation. This suggests that nonlinear elasticity plays an important role in the formation of the observed pattern. This was confirmed by numerical simulation for neo-Hookean and linear elasticity models.
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.
Inhibition of environmental fatigue crack propagation in age-hardenable aluminum alloys
NASA Astrophysics Data System (ADS)
Warner, Jenifer S.
Age-hardenable aluminum alloys, such as C47A-T86 (Al-Cu-Li) and 7075-T651 (Al-Zn-Mg-Cu), used in aerospace structures are susceptible to environment assisted fatigue crack propagation (EFCP) by hydrogen environment embrittlement. This research demonstrates effective inhibition of EFCP in C47A-T86 and 7075-T651 under both full immersion in aqueous chloride solution and atmospheric exposure which more accurately describes aircraft service conditions. Inhibition is attributed to the presence of a crack tip passive film reducing H production and uptake, as explained by the film rupture-hydrogen embrittlement mechanism, and can be accomplished through both addition of a passivating ion (ion-assisted inhibition) and localized-alloy corrosion creating passivating conditions (self inhibition). Addition of molybdate to both bulk chloride solution and surface chloride droplets eliminates the effect of environment on fatigue crack propagation in C47A-T86 and 7075-1651 at sufficiently low loading frequencies and high stress ratio by yielding crack growth rates equivalent to those for fatigue in ultra high vacuum. The preeminent corrosion inhibitor, chromate, has not been reported to produce such complete inhibition. Inhibition is promoted by reduced loading frequency, increased crack tip molybdate concentration, and potential at or anodic to free corrosion; each of which favors passivity. The inhibiting effect of molybdate parallels chromate, establishing molybdate as a viable chromate replacement inhibitor. The ability of molybdate to inhibit EFCP is enhanced by atmospheric exposures producing surface electrolyte droplets; crack growth rates are reduced by an order of magnitude under loading frequencies as high as 30 Hz, a frequency at which inhibition was not possible under full immersion. Al-Cu-Mg/Li alloys, including 2024-T351, are capable of self inhibition of EFCP. This behavior is attributed to localized corrosion through dealloying of anodic Al2CuMg or Al2Cu
NASA Astrophysics Data System (ADS)
Li, Lifei; Zhang, Zheng; Shen, Gongtian
2015-07-01
The effect of grain size on fatigue crack propagation and the corresponding acoustic emission (AE) characteristics of commercial pure titanium (CP-Ti) were investigated at room temperature. After a four-point bending fatigue testing, the fatigue features and AE source mechanisms were discussed, combined with microstructural and fractographic observations. The results showed that the increased grain size had little effect on the stable propagation rate of fatigue crack; however, a significant increase in the AE counts rate was observed. During crack stable propagation, the relationship between the AE counts rate and the fatigue stress intensity factor range was generally in accordance with the Pairs law, with the exception of some local fluctuations due to regional twin paling. While lenticular twins appeared dispersively along the crack, twin palings were observed occasionally at the edge of the crack. Twin paling occurrence was more frequent in the specimens with larger grains than in those with smaller grains. This suggests that twin discontinuously played a role in the fatigue process in this CP-Ti, and that the AE technique is sensitive to crack propagation and twinning events during fatigue.
Dynamical observations on the crack tip zone and stress corrosion of two-dimensional MoS2
NASA Astrophysics Data System (ADS)
Ly, Thuc Hue; Zhao, Jiong; Cichocka, Magdalena Ola; Li, Lain-Jong; Lee, Young Hee
2017-01-01
Whether and how fracture mechanics needs to be modified for small length scales and in systems of reduced dimensionality remains an open debate. Here, employing in situ transmission electron microscopy, atomic structures and dislocation dynamics in the crack tip zone of a propagating crack in two-dimensional (2D) monolayer MoS2 membrane are observed, and atom-to-atom displacement mapping is obtained. The electron beam is used to initiate the crack; during in situ observation of crack propagation the electron beam effect is minimized. The observed high-frequency emission of dislocations is beyond previous understanding of the fracture of brittle MoS2. Strain analysis reveals dislocation emission to be closely associated with the crack propagation path in nanoscale. The critical crack tip plastic zone size of nearly perfect 2D MoS2 is between 2 and 5 nm, although it can grow to 10 nm under corrosive conditions such as ultraviolet light exposure, showing enhanced dislocation activity via defect generation.
Dynamical observations on the crack tip zone and stress corrosion of two-dimensional MoS2
Ly, Thuc Hue; Zhao, Jiong; Cichocka, Magdalena Ola; Li, Lain-Jong; Lee, Young Hee
2017-01-01
Whether and how fracture mechanics needs to be modified for small length scales and in systems of reduced dimensionality remains an open debate. Here, employing in situ transmission electron microscopy, atomic structures and dislocation dynamics in the crack tip zone of a propagating crack in two-dimensional (2D) monolayer MoS2 membrane are observed, and atom-to-atom displacement mapping is obtained. The electron beam is used to initiate the crack; during in situ observation of crack propagation the electron beam effect is minimized. The observed high-frequency emission of dislocations is beyond previous understanding of the fracture of brittle MoS2. Strain analysis reveals dislocation emission to be closely associated with the crack propagation path in nanoscale. The critical crack tip plastic zone size of nearly perfect 2D MoS2 is between 2 and 5 nm, although it can grow to 10 nm under corrosive conditions such as ultraviolet light exposure, showing enhanced dislocation activity via defect generation. PMID:28098140
Bannikov, Mikhail E-mail: oborin@icmm.ru Oborin, Vladimir E-mail: oborin@icmm.ru Naimark, Oleg E-mail: oborin@icmm.ru
2014-11-14
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 and 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.
Mechanisms of decrease in fatigue crack propagation resistance in irradiated and melted UHMWPE#
Oral, Ebru; Malhi, Arnaz S.; Muratoglu, Orhun K.
2005-01-01
Adhesive/abrasive wear in ultra-high molecular weight polyethylene (UHMWPE) has been minimized by radiation cross-linking. Irradiation is typically followed by melting to eliminate residual free radicals that cause oxidative embrittlement. Irradiation and subsequent melting reduce the strength and fatigue resistance of the polymer. We determined the radiation dose dependence and decoupled the effects of post-irradiation melting on the crystallinity, mechanical properties and fatigue crack propagation resistance of room temperature irradiated UHMWPE from those of irradiation alone. Stiffness and yield strength, were largely not affected by increasing radiation dose but were affected by changes in crystallinity, whereas plastic properties, ultimate tensile strength and elongation at break, were dominated at different radiation dose ranges by changes in radiation dose or crystallinity. Fatigue crack propagation resistance was shown to decrease with increase in radiation dose and with decrease in crystalline content. Morphology of fracture surfaces revealed loss of ductility with increase in radiation dose and more detrimental effects on ductility at lower radiation doses after post-irradiation melting. PMID:16105682
Dynamic crack initiation toughness : experiments and peridynamic modeling.
Foster, John T.
2009-10-01
This is a dissertation on research conducted studying the dynamic crack initiation toughness of a 4340 steel. Researchers have been conducting experimental testing of dynamic crack initiation toughness, K{sub Ic}, for many years, using many experimental techniques with vastly different trends in the results when reporting K{sub Ic} as a function of loading rate. The dissertation describes a novel experimental technique for measuring K{sub Ic} in metals using the Kolsky bar. The method borrows from improvements made in recent years in traditional Kolsky bar testing by using pulse shaping techniques to ensure a constant loading rate applied to the sample before crack initiation. Dynamic crack initiation measurements were reported on a 4340 steel at two different loading rates. The steel was shown to exhibit a rate dependence, with the recorded values of K{sub Ic} being much higher at the higher loading rate. Using the knowledge of this rate dependence as a motivation in attempting to model the fracture events, a viscoplastic constitutive model was implemented into a peridynamic computational mechanics code. Peridynamics is a newly developed theory in solid mechanics that replaces the classical partial differential equations of motion with integral-differential equations which do not require the existence of spatial derivatives in the displacement field. This allows for the straightforward modeling of unguided crack initiation and growth. To date, peridynamic implementations have used severely restricted constitutive models. This research represents the first implementation of a complex material model and its validation. After showing results comparing deformations to experimental Taylor anvil impact for the viscoplastic material model, a novel failure criterion is introduced to model the dynamic crack initiation toughness experiments. The failure model is based on an energy criterion and uses the K{sub Ic} values recorded experimentally as an input. The failure model
NASA Astrophysics Data System (ADS)
Schweizer, Jürg; Reuter, Benjamin; van Herwijnen, Alec; Richter, Bettina; Gaume, Johan
2016-11-01
If a weak snow layer below a cohesive slab is present in the snow cover, unstable snow conditions can prevail for days or even weeks. We monitored the temporal evolution of a weak layer of faceted crystals as well as the overlaying slab layers at the location of an automatic weather station in the Steintälli field site above Davos (Eastern Swiss Alps). We focussed on the crack propagation propensity and performed propagation saw tests (PSTs) on 7 sampling days during a 2-month period from early January to early March 2015. Based on video images taken during the tests we determined the mechanical properties of the slab and the weak layer and compared them to the results derived from concurrently performed measurements of penetration resistance using the snow micro-penetrometer (SMP). The critical cut length, observed in PSTs, increased overall during the measurement period. The increase was not steady and the lowest values of critical cut length were observed around the middle of the measurement period. The relevant mechanical properties, the slab effective elastic modulus and the weak layer specific fracture, overall increased as well. However, the changes with time differed, suggesting that the critical cut length cannot be assessed by simply monitoring a single mechanical property such as slab load, slab modulus or weak layer specific fracture energy. Instead, crack propagation propensity is the result of a complex interplay between the mechanical properties of the slab and the weak layer. We then compared our field observations to newly developed metrics of snow instability related to either failure initiation or crack propagation propensity. The metrics were either derived from the SMP signal or calculated from simulated snow stratigraphy (SNOWPACK). They partially reproduced the observed temporal evolution of critical cut length and instability test scores. Whereas our unique dataset of quantitative measures of snow instability provides new insights into the
Dynamic antiplane rupture propagation crossing a material interface: modelling with BIEM
NASA Astrophysics Data System (ADS)
Hirano, Shiro; Yamashita, Teruo
2015-02-01
Because the Earth's crust is quite heterogeneous and consists of various types of rocks, many faults are located near material interfaces. Recent observational studies have revealed that some faults cross the interfaces, but theoretical treatment of their dynamic behaviour is still undeveloped. In this paper, we develop a boundary integral equation method (BIEM) to analyse the dynamic behaviour of antiplane rupture propagation crossing a material interface. First, we obtain an exact solution of the stress response due to a rectangular slip rate function on one discretized crack element embedded in one of the elastic half-spaces welded along a planar interface. This solution is helpful not only in BIEM simulations of rupture propagation but also in benchmark testing or extension of other numerical schemes. Next, we simulate the dynamic propagation of an antiplane rupture crossing the material interface by using the exact solution for a stress response in a BIEM framework. We find that a shear wave reflected from the material interface generates a significant change in the slip rate on the crack depending on the angle between the interface and the crack, contrast in the elastic properties, and rupture velocity. Moreover, we infer from our numerical results and previous related works that a backward propagating healing front emerges under rate-weakening friction owing to interaction between the reflected wave and friction.
NASA Astrophysics Data System (ADS)
Terasaki, Hidenori; Miyahara, Yu; Ohata, Mitsuru; Moriguchi, Koji; Tomio, Yusaku; Hayashi, Kotaro
2015-12-01
Cleavage-crack propagation behavior was investigated in the simulated coarse-grained heat-affected zone (CGHAZ) of bainitic steel using electron backscattering diffraction (EBSD) pattern analysis when a low heat input welding was simulated. From viewpoint of crystallographic analysis, it was the condition in which the Bain zone was smaller than the close-packed plane (CP) group. It was clarified that the Bain zone and CP group boundaries provided crack-propagation resistance. The results revealed that when the Bain zone was smaller than the CP group, crack length was about one quarter the size of that measured when the CP group was smaller than the Bain zone because of the increasing Bain-zone boundaries. Furthermore, it was clarified that the plastic work associated with crack opening and resistance at the Bain and CP boundaries could be visualized by the kernel average misorientation maps.
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.
NASA Astrophysics Data System (ADS)
Li, Huichao; Koyama, Motomichi; Sawaguchi, Takahiro; Tsuzaki, Kaneaki; Noguchi, Hiroshi
2015-06-01
We investigated the roles of deformation-induced ε-martensitic transformation on strain-controlled low-cycle fatigue (LCF) through crack-propagation analysis involving a notching technique that used a focused ion beam (FIB) setup on Fe-30Mn-4Si-2Al austenitic steel. Using the FIB notch, we separated the microstructure evolution into macroscopic cyclic deformation-induced and crack-propagation-induced microstructures. Following this, we clarified the fatigue crack-propagation-induced ε-martensitic transformation to decelerate crack propagation at a total strain range of 2%, obtaining an extraordinary LCF life of 1.1 × 104 cycles.
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.
NASA Astrophysics Data System (ADS)
Lu, Mingyu; Qu, Yongwei; Lu, Ye; Ye, Lin; Zhou, Limin; Su, Zhongqing
2011-11-01
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.
Crack propagation in SiC f/SiC ceramic matrix composite under static and cyclic loading conditions
NASA Astrophysics Data System (ADS)
Raghuraman, S.; Stubbins, J. F.; Ferber, M. K.; Wereszczak, A. A.
1994-09-01
{SiC f}/{SiC} ceramic matrix composite material is of high interest for potential application as a structural and barrier material in fusion systems. It possesses reasonable fracture toughness over a range of temperatures and, due to the low atomic number of its constituents, is appealing for low activation reasons. This study examines the mechanical durability of a Nicalon fiber-SiC composite which has been tested at temperatures up to 1400°C to determine its resistance to crack propagation under static and cyclic loading conditions. The crack growth characteristics are governed by the fiber and interface failure modes. These, in turn are affected by loading parameters, temperature and environmental effects. The material shows R-curve behavior, due to fiber bridging of the crack wake. The material also shows time dependent crack growth at elevated temperature, but not at room temperature. However, cyclic loading does induce crack extension at room temperature.
NASA Astrophysics Data System (ADS)
Riesch, J.; Höschen, T.; Linsmeier, Ch; Wurster, S.; You, J.-H.
2014-04-01
Tungsten is a promising candidate for the plasma-facing components of a future fusion reactor, but its use is strongly restricted by its inherent brittleness. An innovative concept to overcome this problem is tungsten fibre-reinforced tungsten composite. In this paper we present the first mechanical test of such a composite material using a sample containing multiple fibres. The in situ fracture experiment was performed in a scanning electron microscope for close observation of the propagating crack. Stable crack propagation accompanied with rising load bearing capacity is observed. The fracture toughness is estimated using the test results and the surface observation.
Nucleation and propagation of fatigue cracks in {beta}-titanium alloys
Peters, J.O.; Sauer, C.; Luetjering, G.
1999-07-01
The influence of microstructure on nucleation and propagation of fatigue cracks in two {beta} titanium alloys, {beta}-CEZ (developed by CEZUS, France) and VT 22 (Russia), was investigated. For the {beta}-CEZ alloy a comparison between lamellar ({beta} processed) and bi-modal microstructures ({alpha}+{beta} processed) at a yield stress level of 1200 MPa was performed. Bi-modal microstructures showed higher ductility, higher LCF and HCF strength level and a higher resistance against microcrack propagation whereas lamellar microstructures showed a higher resistance against macrocrack propagation and fracture toughness. These findings could be explained on the basis of the {beta} grain size and of the {alpha} plate dimensions. In the second part of this work a comparison between the bi-modal condition of the {beta}-CEZ alloy and the VT 22 alloy (at the same yield stress level of 1,200 MPa) was made. The differences in the mechanical properties will be discussed in terms of differences in {alpha} plate dimensions and uniformity of the {beta} grain structure.
NASA Astrophysics Data System (ADS)
Liu, J.; Zhu, W. D.; Charalambides, P. G.; Shao, Y. M.; Xu, Y. F.; Fang, X. M.
2016-11-01
As one of major failure modes of mechanical structures subjected to periodic loads, embedded cracks due to fatigue can cause catastrophic failure of machineries. Understanding the dynamic characteristics of a structure with an embedded crack is helpful for early crack detection and diagnosis. In this work, a new three-segment beam model with local flexibilities at crack tips is developed to investigate the vibration of a cantilever beam with a closed, fully embedded horizontal crack, which is assumed to be not located at its clamped or free end or distributed near its top or bottom side. The three-segment beam model is assumed to be a linear elastic system, and it does not account for the nonlinear crack closure effect; the top and bottom segments always stay in contact at their interface during the beam vibration. It can model the effects of local deformations in the vicinity of the crack tips, which cannot be captured by previous methods in the literature. The middle segment of the beam containing the crack is modeled by a mechanically consistent, reduced bending moment. Each beam segment is assumed to be an Euler-Bernoulli beam, and the compliances at the crack tips are analytically determined using a J-integral approach and verified using commercial finite element software. Using compatibility conditions at the crack tips and the transfer matrix method, the nature frequencies and mode shapes of the cracked cantilever beam are obtained. The three-segment beam model is used to investigate the effects of local flexibilities at crack tips on the first three natural frequencies and mode shapes of the cracked cantilever beam. A stationary wavelet transform (SWT) method is used to process the mode shapes of the cracked cantilever beam; jumps in single-level SWT decomposition detail coefficients can be used to identify the length and location of an embedded horizontal crack.
Cascaded image analysis for dynamic crack detection in material testing
NASA Astrophysics Data System (ADS)
Hampel, U.; Maas, H.-G.
Concrete probes in civil engineering material testing often show fissures or hairline-cracks. These cracks develop dynamically. Starting at a width of a few microns, they usually cannot be detected visually or in an image of a camera imaging the whole probe. Conventional image analysis techniques will detect fissures only if they show a width in the order of one pixel. To be able to detect and measure fissures with a width of a fraction of a pixel at an early stage of their development, a cascaded image analysis approach has been developed, implemented and tested. The basic idea of the approach is to detect discontinuities in dense surface deformation vector fields. These deformation vector fields between consecutive stereo image pairs, which are generated by cross correlation or least squares matching, show a precision in the order of 1/50 pixel. Hairline-cracks can be detected and measured by applying edge detection techniques such as a Sobel operator to the results of the image matching process. Cracks will show up as linear discontinuities in the deformation vector field and can be vectorized by edge chaining. In practical tests of the method, cracks with a width of 1/20 pixel could be detected, and their width could be determined at a precision of 1/50 pixel.
Shiota, Tadashi Sato, Yoshitaka; Yasuda, Kouichi
2014-03-10
Simultaneous time-resolved measurements of photon emission (PE) and fast crack propagation upon bending fracture were conducted in silica glass and soda lime glass. Observation of fracture surfaces revealed that macroscopic crack propagation behavior was similar between the silica glass and soda lime glass when fracture loads for these specimens were comparable and cracks propagated without branching. However, a large difference in the PE characteristics was found between the two glasses. In silica glass, PE (645–655 nm) was observed during the entire crack propagation process, whereas intense PE (430–490 nm and 500–600 nm) was observed during the initial stages of propagation. In contrast, only weak PE was detected in soda lime glass. These results show that there is a large difference in the atomic processes involved in fast crack propagation between these glasses, and that PE can be used to study brittle fracture on the atomic scale.
NASA Technical Reports Server (NTRS)
Bass, B. R.; Pugh, C. E.; Swindeman, R. W.
1988-01-01
Applications of nonlinear techniques to the first series of six HSST wide-plate crack-arrest tests that were performed are described. The experiments include crack initiations at low temperatures and relatively long (20 cm) cleavage propagation phases which are terminated by arrest in high temperature regions. Crack arrest are then followed by ductile tearing events. Consequently, the crack front regions are exposed to wide ranges of strain rates and temperatures.
Effects of friction and high torque on fatigue crack propagation in mode III. [AISI 4140 and 4340
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 (R /SUB B/ 88, 590 MN/m/sup 2/ 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) /SUB III/ can be related to the alternating stress intensity factor ..delta..K /SUB III/ for conditions of small-scale yielding. However, to describe crack propagation behavior over an extended range of crack growth rates (about 10/sup -6/ to 10/sup -2/ mm per cycle), where crack growth proceeds under elastic-plastic and full plastic conditions, no correlation between (dc/dN) /SUB III/ and ..delta..K /SUB III/ is possible. Accordingly, a new parameter for torsional crack growth, termed the plastic strain intensity GAMMA /SUB 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 A micro-mechanical model for the main radial Mode III growth 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 (..delta..GAMMA /SUB III/) 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 4140 steel from 10/sup -6/ to 10/sup -2/ mm per cycle.
Microstructural effects on the creep and crack propagation behaviors of {gamma}-Ti aluminide alloy
Lupinc, V.; Onofrio, G.; Nazmy, M.; Staubli, M.
1999-07-01
Gamma titanium aluminides class of materials possess several unique physical and mechanical properties. These characteristics can be attractive for specific industrial applications. By applying different heat treatment schedules one can change the microstructural features of this class of materials. In the present investigation, two heat treatment schedules were used to produce two different microstructures, duplex (D) and nearly lamellar (NL) in the cast and HIP'ed Ti-47Al-2W-0.5Si alloy. The tensile strength and creep behavior, in the 700--850 C temperature range, of this alloy have been determined and correlated to the corresponding microstructures. In addition, the fatigue crack propagation behavior in this alloy has been studied at different temperatures. The results on the creep behavior showed that the alloy with nearly lamellar microstructure has a strongly improved creep strength as compared with that of the duplex microstructure.
Zhou, Xiaoxue; Halladin, David K; Rojas, Enrique R; Koslover, Elena F; Lee, Timothy K; Huang, Kerwyn Casey; Theriot, Julie A
2015-05-01
When Staphylococcus aureus undergoes cytokinesis, it builds a septum, generating two hemispherical daughters whose cell walls are only connected via a narrow peripheral ring. We found that resolution of this ring occurred within milliseconds ("popping"), without detectable changes in cell volume. The likelihood of popping depended on cell-wall stress, and the separating cells split open asymmetrically, leaving the daughters connected by a hinge. An elastostatic model of the wall indicated high circumferential stress in the peripheral ring before popping. Last, we observed small perforations in the peripheral ring that are likely initial points of mechanical failure. Thus, the ultrafast daughter cell separation in S. aureus appears to be driven by accumulation of stress in the peripheral ring and exhibits hallmarks of mechanical crack propagation.
NASA Technical Reports Server (NTRS)
Richey, Edward, III
1995-01-01
This research aims to develop the methods and understanding needed to incorporate time and loading variable dependent environmental effects on fatigue crack propagation (FCP) into computerized fatigue life prediction codes such as NASA FLAGRO (NASGRO). In particular, the effect of loading frequency on FCP rates in alpha + beta titanium alloys exposed to an aqueous chloride solution is investigated. The approach couples empirical modeling of environmental FCP with corrosion fatigue experiments. Three different computer models have been developed and incorporated in the DOS executable program. UVAFAS. A multiple power law model is available, and can fit a set of fatigue data to a multiple power law equation. A model has also been developed which implements the Wei and Landes linear superposition model, as well as an interpolative model which can be utilized to interpolate trends in fatigue behavior based on changes in loading characteristics (stress ratio, frequency, and hold times).
Mode I and mixed I/III crack initiation and propagation behavior of V-4Cr-4Ti alloy at 25{degrees}C
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 plane 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.
Kirkwood, B.L.
1982-01-01
The high temperature fatigue behavior of a nickel base superalloy was studied to determine the effect of grain boundary cavitation on the crack propagation rate. It was found that the introduction of cavities into a specimen prior to the fatigue test increases the crack propagation rate significantly over specimens which did not have cavities introduced into them. Companion fatigue specimens were cycled under similar conditions until they attained different levels of cyclic stress intensity in order to observe the changes that occur in the cavity spacing within the plastic zone as the stress intensity increases. The cavity spacing was observed with shadowed two stage TEM replicas taken from the plastic zone near the crack tip. It was found that the cavities nucleate continuously throughout the test, with the cavity spacing becoming progressively smaller as the cyclic stress intensity increases. It also was found that the cavity spacing decreases as one goes through the plastic zone toward the crack tip. A computer analysis of the diffusional growth rate of a void in the plastic zone was done to determine a theoretical value of the minimum cavity spacing which would give the observed crack propagation.
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Baker, D A; Bellare, A; Pruitt, L
2003-07-01
Crosslinked ultrahigh molecular weight polyethylene (UHMWPE) has been recently approved by the Food and Drug Administration for use in orthopedic implants. The majority of commercially available UHMWPE orthopedic components are crosslinked using e-beam or gamma radiation. The level of crosslinking is controlled with radiation dose and free radicals are eliminated through heat treatments to prevent long-term degradation associated with chain scission or oxidation mechanisms. Laboratory studies have demonstrated a substantial improvement in the wear resistance of crosslinked UHMWPE. However, a concern about the resistance to fatigue damage remains in the clinical community, especially for tibial components that sustain high cyclic contact stresses. The objective of this study was to investigate both the initiation and propagation aspects of fatigue cracks in radiation crosslinked medical-grade UHMWPE. This work evaluated three levels of radiation, which induced three crosslink densities, on the fatigue crack propagation and total fatigue life behavior. Both as-received UHMWPE, as well as those that underwent an identical thermal history as the crosslinked UHMWPE were used as controls. Fractured crack propagation specimens were examined using scanning electron microscopy to elucidate fatigue fracture mechanisms. The results of this work indicated that a low crosslink density may optimize the fatigue resistance from both a crack initiation and propagation standpoint.
Cohesive Laws for Analyzing Through-Crack Propagation in Cross Ply Laminates
NASA Technical Reports Server (NTRS)
Bergan, Andrew C.; Davila, Carlos G.
2015-01-01
The laminate cohesive approach (LCA) is a methodology for the experimental characterization of cohesive through-the-thickness damage propagation in fiber-reinforced polymer matrix composites. LCA has several advantages over other existing approaches for cohesive law characterization, including: visual measurements of crack length are not required, structural effects are accounted for, and LCA can be applied when the specimen is too small to achieve steady-state fracture. In this work, the applicability of this method is investigated for two material systems: IM7/8552, a conventional prepreg, and AS4/VRM34, a non-crimp fabric cured using an out-of-autoclave process. The compact tension specimen configuration is used to propagate stable Mode I damage. Trilinear cohesive laws are characterized using the fracture toughness and the notch tip opening displacement. Test results are compared for the IM7/8552 specimens with notches machined by waterjet and by wire slurry saw. It is shown that the test results are nearly identical for both notch tip preparations methods, indicating that significant specimen preparation time and cost savings can be realized by using the waterjet to notch the specimen instead of the wire slurry saw. The accuracy of the cohesive laws characterized herein are assessed by reproducing the structural response of the test specimens using computational methods. The applicability of the characterization procedure for inferring lamina fracture toughness is also discussed.
Williford, R.E.
1989-09-01
Transverse cracking of polymeric matrix materials is an important fatigue damage mechanism in continuous-fiber composite laminates. The propagation of an array of these cracks is a stochastic problem usually treated by Monte Carlo methods. However, this exploratory work proposes an alternative approach wherein the Monte Carlo method is replaced by a more closed-form recursion relation based on fractional Brownian motion.'' A fractal scaling equation is also proposed as a substitute for the more empirical Paris equation describing individual crack growth in this approach. Preliminary calculations indicate that the new recursion relation is capable of reproducing the primary features of transverse matrix fatigue cracking behavior. Although not yet fully tested or verified, this cursion relation may eventually be useful for real-time applications such as monitoring damage in aircraft structures.
1976-08-01
indicators to check the pH at the tip of a frozen crack as the solution at the crack tip thawed, reported that for cracks growing in 7075 aluminum and...which time the crack growth was arrested), the specimen was incrementally loaded and the crack started to grow once again at KI = 77 ksi /I-nch...to 50 ksi inch and the crack growth in region II increasedIPOC3xl0- 5 in/min. Again in Fig. 10, the results are seen for a crack allowed to grow in
New Developments in the Embedded Statistical Coupling Method: Atomistic/Continuum Crack Propagation
NASA Technical Reports Server (NTRS)
Saether, E.; Yamakov, V.; Glaessgen, E.
2008-01-01
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain has been enhanced. The concurrent MD-FEM coupling methodology uses statistical averaging of the deformation of the atomistic MD domain to provide interface displacement boundary conditions to the surrounding continuum FEM region, which, in turn, generates interface reaction forces that are applied as piecewise constant traction boundary conditions to the MD domain. The enhancement is based on the addition of molecular dynamics-based cohesive zone model (CZM) elements near the MD-FEM interface. The CZM elements are a continuum interpretation of the traction-displacement relationships taken from MD simulations using Cohesive Zone Volume Elements (CZVE). The addition of CZM elements to the concurrent MD-FEM analysis provides a consistent set of atomistically-based cohesive properties within the finite element region near the growing crack. Another set of CZVEs are then used to extract revised CZM relationships from the enhanced embedded statistical coupling method (ESCM) simulation of an edge crack under uniaxial loading.
Problems of dynamic fracture mechanics without contact of the crack faces
NASA Astrophysics Data System (ADS)
Guz', A. N.; Zozulya, V. V.
1994-10-01
In this, the first part of the survey we have discussed only certain aspects of dynamic fracture mechanics. The surveyed material has been selected with a preference for the most highly developed parts of the theory, specifically those elements which have direct bearing on the second part of the survey. We have also included information on the dynamic fracture mechanics of initially stressed materials, in the development of which one of the authors has been a major contributor. Since many problems of dynamic fracture mechanics have been overlooked in the survey, we have added supplementary references to the literature. Various aspects of the strength and fracture of materials under dynamic loading are set forth in [11, 12, 40, 57, 60, 73, 80, 83]. Criteria of the initiation, motion, branching, and arrest of cracks are discussed in [7, 9, 60, 102, 111, 113, 124]. Among the most interesting elements of dynamic fracture mechanics are the problems of crack propagation. Certain analytical results pertinent to this topic have been obtained in [43-45, 47, 67-72, 78, 87, 92, 96, 97].
NASA Astrophysics Data System (ADS)
Wahalathantri, Buddhi L.; Thambiratnam, David P.; Chan, Tommy H. T.; Fawzia, Sabrina
2015-05-01
Structural Health Monitoring (SHM) schemes are useful for proper management of the performance of structures and for preventing their catastrophic failures. Vibration based SHM schemes has gained popularity during the past two decades resulting in significant research. It is hence evitable that future SHM schemes will include robust and automated vibration based damage assessment techniques (VBDAT) to detect, localize and quantify damage. In this context, the Damage Index (DI) method which is classified as non-model or output based VBDAT, has the ability to automate the damage assessment process without using a computer or numerical model along with actual measurements. Although damage assessment using DI methods have been able to achieve reasonable success for structures made of homogeneous materials such as steel, the same success level has not been reported with respect to Reinforced Concrete (RC) structures. The complexity of flexural cracks is claimed to be the main reason to hinder the applicability of existing DI methods in RC structures. Past research also indicates that use of a constant baseline throughout the damage assessment process undermines the potential of the Modal Strain Energy based Damage Index (MSEDI). To address this situation, this paper presents a novel method that has been developed as part of a comprehensive research project carried out at Queensland University of Technology, Brisbane, Australia. This novel process, referred to as the baseline updating method, continuously updates the baseline and systematically tracks both crack formation and propagation with the ability to automate the damage assessment process using output only data. The proposed method is illustrated through examples and the results demonstrate the capability of the method to achieve the desired outcomes.
NASA Astrophysics Data System (ADS)
Piascik, Robert S.; Gangloff, Richard P.
1991-10-01
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 ΔK and K maxI loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water’ vapor, moist air, and NaCl with cathodic polarization. While da/dN depend on ΔK4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate “plateau.” Environmental effects are strongest at low ΔK. Crack tip damage is ascribed to hydrogen embrittlement because of accelerated da/dN due to parts-per-million (ppm) levels of H2O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence of da/dN as affected by mean crack opening, the lack of an effect of film-forming O2, the likelihood for crack tip hydrogen production in NaCl, and the environmental and ΔK-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. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.
Effects of Dynamic and Static Loading on Eddy Current Nde of Fatigue Cracks
NASA Astrophysics Data System (ADS)
Lo, C. C. H.; Nakagawa, N.
2009-03-01
This paper reports on a study of the effects of dynamic and static loading on eddy current (EC) crack signals in aerospace materials. In situ EC measurements were performed on a series of fatigue crack samples grown in Al 6061 bars under dynamic loading. The EC signals detected at a fixed location on the fatigue cracks were found to vary periodically with cyclic loading. Under dynamic tension, the amplitude of the vertical component of the crack signal, which is perpendicular to the lift off direction, increases with the load amplitude and the effect is stronger for longer cracks. Such changes in EC signals are attributable to crack morphology changes (i.e. crack opening) under tension. In contrast, the amplitude of crack signal remains relatively unchanged under compression, possibly due to the insulation effect of the oxidation layer on the crack face. The loading effects on crack signals in the Al samples were confirmed by obtaining c-scan images of the fatigue cracks under static loads. C-scans conducted on a series of Ti 64 fatigue crack samples under static loads revealed significantly stronger stress-induced changes of crack signal for Ti 64 than for the Al samples. The results are interpreted in terms of load-induced crack morphology changes, taking into account of likely differences between the Al and Ti alloy samples.
NASA Astrophysics Data System (ADS)
Stewart, R. D.; Abou Najm, M. R.; Rupp, D. E.; Selker, J. S.
2010-12-01
Shrinking/swelling soils are characterized by transient crack networks which function as dominant controls on the partitioning of surface and subsurface flow, the rate and depth of percolation, and evaporation rates. For such soils, understanding the dynamics of cracks is critical to accurately quantify their influence on groundwater recharge, stream-flow generation, and solute transport, among other component of a site’s hydrology. We propose a low-cost method for measuring transient crack-volume using a sealed plastic bag connected by a hose to a PVC standpipe. The empty bag is placed into the crack, and then water is added via the standpipe, until the bag has expanded to the boundaries of the crack and some water remains in the standpipe. As the crack shrinks or swells, its volume changes, causing water displacement within the bag, which is measured as a corresponding change in water level in the standpipe. An automated level logger within the standpipe is used to record changes in water level, which are converted to volumetric changes from the known internal cross-sectional area of the standpipe. The volume of water filling the bag is accurately measured at the start and completion of the experiment (to check for leakage). Adding the startup volume to the cumulative temporal volumetric change in the standpipe provides a simple and accurate method for monitoring transient crack volume. Currently, the design is undergoing preliminary testing in a field site in Ninhue, Chile, and field and laboratory testing in Corvallis, Oregon. Initial results from the Chilean field site suggest that the crack-o-meters are responding to the closing of cracks, but further effort is needed to calibrate and validate the results. We hope that these low-cost “crack-o-meters” will become useful and simple tools for researchers to quantify temporal changes in crack volume with the objective of incorporating these results into hydrological modeling efforts.
A Constitutive Relationship between Crack Propagation and Specific Damping Capacity in Steel
1990-10-01
crack results from the accumulation of damping effects (i.e., from the fact that under a variable stress higher than the fatigue limit, constantly...flow occurs at the crack ends which are stress concentration sites, so that the crack growth is temporarily stayed. Stresses subsequently increase...theory was used to calculate the displacement in the vicinity of the crack tips due to this shear stress . It was then shown that, under certain
Asymmetric crack propagation near waterfall cliff and its influence on the waterfall lip shape
NASA Astrophysics Data System (ADS)
Vastola, G.
2011-11-01
By means of Finite Element Method (FEM) calculations and fatigue fracture mechanics analysis, we show that crack propagation in bedrocks close to the waterfall cliff is preferential towards the cliff face rather than upstream the river. Based on this effect, we derive the corresponding expression for the velocity of recession vr of the waterfall lip, and find that vr has a quadratic dependence on the hydrostatic pressure. Quantitatively, this erosion mechanism generates recession rates of the order of ~cm-dm/y, consistent with the recession rates of well-known waterfalls. We enclose our expression for vr into a growth model to investigate the time evolution of a waterfall lip subject to this erosional mechanism. Because of the dependence on hydrostatic pressure, the shape of the waterfall is influenced by the transverse profile of the river that generates the waterfall. If the river has a transverse concavity, the waterfall evolves a curved shape. Evolution for the case of meanders with asymmetric transverse profile is also given.
The dynamic behaviour of slender structures with cross-sectional cracks
NASA Astrophysics Data System (ADS)
Gudmundson, P.
A DYNAMIC model for beams with cross-sectional cracks is discussed. It is shown that a crack can be represented by a consistent, static flexibility matrix. Two different methods for the determination of the flexibility matrix are discussed. If the static stress intensity factors are known, the flexibility matrix can be determined from an integration of these stress intensity factors. Alternatively, static finite element calculations can be used for the determination of the flexibility matrix. Both methods are demonstrated in the present paper. The mathematical model was applied to an edge-cracked cantilevered beam and the eigenfrequencies were determined for different crack lengths and crack positions. These results were compared to experimentally obtained eigenfrequencies. In the experiments, the cracks were modelled by sawing cuts. The theoretical results were, for all crack lengths, in excellent agreement with the experimental data. The dynamic stress intensity factor for a longitudinally vibrating, centrally cracked bar was determined as well. The results compared very well with dynamic finite element calculations. The crack closure effect was experimentally investigated for an edge-cracked beam with a fatigue crack. It was found that the eigenfrequencies decreased, as functions of crack length, at a much slower rate than in the case of an open crack.
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.
Wang, Julia; Kaplan, Jonah A; Colson, Yolonda L; Grinstaff, Mark W
2016-02-18
The concept of using crack propagation in polymeric materials to control drug release and its first demonstration are reported. The composite drug delivery system consists of highly-textured superhydrophobic electrosprayed microparticle coatings, composed of biodegradable and biocompatible polymers poly(caprolactone) and poly(glycerol monostearate carbonate-co-caprolactone), and a cellulose/polyester core. The release of entrapped agents is controlled by the magnitude of applied strain, resulting in a graded response from water infiltration through the propagating patterned cracks in the coating. Strain-dependent delivery of the anticancer agents cisplatin and 7-ethyl-10-hydroxycamptothecin to esophageal cancer cells (OE33) in vitro is observed. Finally the device is integrated with an esophageal stent to demonstrate delivery of fluorescein diacetate, using applied tension, to an ex vivo esophagus.
1977-10-01
tangent modulus curves for 15 - 5PH in H1025 and H1150 conditions were constructed. Fatigue crack propagation data were obtained for 7075-T7351, 7475...and G .... 82 42 Working Curve Showing Effect of Temperature on Compressive Yield Strength (Fcy) of 15 - 5PH (H1025) Stainless Steel Bar . 92 43...Compressive Stress-Strain Curves for 15 - 5PH Bar H1025 ........ 96 40 Determination of Room Temperature of Ramberg-Osgood Parameters for Long Transverse
Local Crack Branching as a Mechanism for Instability in Dynamic Fracture
NASA Astrophysics Data System (ADS)
Sharon, Eran; Gross, Steven P.; Fineberg, Jay
1995-06-01
The motion of a crack in dynamic fracture has been shown to be governed by a dynamical instability causing oscillations in its velocity and structure on the fracture surface. We present experimental evidence indicating that the mechanism for instability is attempted local crack branching. At the instability onset, a crack will locally change its topology and sprout small, microscopic side branches. The trajectories of these local branches are independent of the crack velocity and exhibit scaling behavior. A connection between microscopic and macroscopic crack branching is established.
NASA Astrophysics Data System (ADS)
Qiu, Lei; Yuan, Shenfang; Bao, Qiao; Mei, Hanfei; Ren, Yuanqiang
2016-05-01
For aerospace application of structural health monitoring (SHM) technology, the problem of reliable damage monitoring under time-varying conditions must be addressed and the SHM technology has to be fully validated on real aircraft structures under realistic load conditions on ground before it can reach the status of flight test. In this paper, the guided wave (GW) based SHM method is applied to a full-scale aircraft fatigue test which is one of the most similar test status to the flight test. To deal with the time-varying problem, a GW-Gaussian mixture model (GW-GMM) is proposed. The probability characteristic of GW features, which is introduced by time-varying conditions is modeled by GW-GMM. The weak cumulative variation trend of the crack propagation, which is mixed in time-varying influence can be tracked by the GW-GMM migration during on-line damage monitoring process. A best match based Kullback-Leibler divergence is proposed to measure the GW-GMM migration degree to reveal the crack propagation. The method is validated in the full-scale aircraft fatigue test. The validation results indicate that the reliable crack propagation monitoring of the left landing gear spar and the right wing panel under realistic load conditions are achieved.
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 of 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.
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.
Cryogenic S-N Fatigue and Fatigue Crack Propagation Behaviors of High Manganese Austenitic Steels
NASA Astrophysics Data System (ADS)
Jeong, Dae-Ho; Lee, Soon-Gi; Jang, Woo-Kil; Choi, Jong-Kyo; Kim, Young-Ju; Kim, Sangshik
2013-10-01
In the current study, the S-N fatigue and the fatigue crack propagation (FCP) behaviors of high manganese austenitic steels, including Fe24Mn and Fe22Mn, were studied, and the results were compared with STS304 (Fe-1Si-2Mn-20Cr-10Ni). The S-N fatigue tests were conducted at 298 K and 110 K (25 °C and -163 °C), respectively, and at an R ratio of 0.1 under a uniaxial loading condition. The FCP tests were conducted at 298 K and 110 K (25 °C and -163°C), respectively, and at R ratios of 0.1 and 0.5, respectively, using compact tension specimens. The resistance to S-N fatigue of each specimen increased greatly with decreasing temperature from 298 K to 110 K (25 °C to -163 °C) and showed a strong dependency on the flow stress. The FCP behaviors of the austenitic steels currently studied substantially varied depending on testing temperature, applied Δ K (stress intensity factor range), and R ratio. The enhanced FCP resistance was observed for the Fe24Mn and the Fe22Mn specimens particularly in the near-threshold Δ K regime, while the enhancement was significant over the entire Δ K regimes for the STS304 specimen, with decreasing temperature from 298 K to 110 K (25 °C to -163 °C). The S-N fatigue and the FCP behaviors of high manganese austenitic steels are compared with STS304 and discussed based on the fractographic and the micrographic observations.
Propagating confined states in phase dynamics
NASA Technical Reports Server (NTRS)
Brand, Helmut R.; Deissler, Robert J.
1992-01-01
Theoretical treatment is given to the possibility of the existence of propagating confined states in the nonlinear phase equation by generalizing stationary confined states. The nonlinear phase equation is set forth for the case of propagating patterns with long wavelengths and low-frequency modulation. A large range of parameter values is shown to exist for propagating confined states which have spatially localized regions which travel on a background with unique wavelengths. The theoretical phenomena are shown to correspond to such physical systems as spirals in Taylor instabilities, traveling waves in convective systems, and slot-convection phenomena for binary fluid mixtures.
NASA Astrophysics Data System (ADS)
Xu, Tengfei; Castel, Arnaud
2016-04-01
In this paper, a model, initially developed to calculate the stiffness of cracked reinforced concrete beams under static loading, is used to assess the dynamic stiffness. The model allows calculating the average inertia of cracked beams by taking into account the effect of bending cracks (primary cracks) and steel-concrete bond damage (i.e. interfacial microcracks). Free and forced vibration experiments are used to assess the performance of the model. The respective influence of bending cracks and steel-concrete bond damage on both static and dynamic responses is analyzed. The comparison between experimental and simulated deflections confirms that the effects of both bending cracks and steel-concrete bond loss should be taken into account to assess reinforced concrete stiffness under service static loading. On the contrary, comparison of experimental and calculated dynamic responses reveals that localized steel-concrete bond damages do not influence significantly the dynamic stiffness and the fundamental frequency.
Sudden bending of cracked laminates
NASA Technical Reports Server (NTRS)
Sih, G. C.; Chen, E. P.
1980-01-01
A dynamic approximate laminated plate theory is developed with emphasis placed on obtaining effective solution for the crack configuration where the 1/square root of r stress singularity and the condition of plane strain are preserved. The radial distance r is measured from the crack edge. The results obtained show that the crack moment intensity tends to decrease as the crack length to laminate plate thickness is increased. Hence, a laminated plate has the desirable feature of stabilizing a through crack as it increases its length at constant load. Also, the level of the average load intensity transmitted to a through crack can be reduced by making the inner layers to be stiffer than the outer layers. The present theory, although approximate, is useful for analyzing laminate failure to crack propagation under dynamic load conditions.
Chong, Alexander C M; Miller, Forrest; Buxton, McKee; Friis, Elizabeth A
2007-08-01
Third-generation mechanical analogue bone models and synthetic analogue cortical bone materials manufactured by Pacific Research Laboratories, Inc. (PRL) are popular tools for use in mechanical testing of various orthopedic implants and biomaterials. A major issue with these models is that the current third-generation epoxy-short fiberglass based composite used as the cortical bone substitute is prone to crack formation and failure in fatigue or repeated quasistatic loading of the model. The purpose of the present study was to compare the tensile and fracture mechanics properties of the current baseline (established PRL "third-generation" E-glass-fiber-epoxy) composite analogue for cortical bone to a new composite material formulation proposed for use as an enhanced fourth-generation cortical bone analogue material. Standard tensile, plane strain fracture toughness, and fatigue crack propagation rate tests were performed on both the third- and fourth-generation composite material formulations using standard ASTM test techniques. Injection molding techniques were used to create random fiber orientation in all test specimens. Standard dog-bone style tensile specimens were tested to obtain ultimate tensile strength and stiffness. Compact tension fracture toughness specimens were utilized to determine plane strain fracture toughness values. Reduced thickness compact tension specimens were also used to determine fatigue crack propagation rate behavior for the two material groups. Literature values for the same parameters for human cortical bone were compared to results from the third- and fourth-generation cortical analogue bone materials. Tensile properties of the fourth-generation material were closer to that of average human cortical bone than the third-generation material. Fracture toughness was significantly increased by 48% in the fourth-generation composite as compared to the third-generation analogue bone. The threshold stress intensity to propagate the crack
Effect of cold rolling on fatigue crack propagation of TiNi/Al6061 shape memory composite
NASA Astrophysics Data System (ADS)
Park, Young Chul; Kang, Jung Ho; Lee, Jin Kyung; Lee, Gyu Chang; Furuya, Yasybumi
2007-08-01
A TiNi alloy fiber was used to recover the original shape of materials using its shape memory effect. The shape memory alloy plays an important role within the metal matrix composite. The shape memory alloy can control the crack propagation in the metal matrix composite, and improve the tensile strength of the composite. In this study, TiNi/Al6061 shape memory alloy (SMA) composite was fabricated by the hot press method, and pressed by a roller for its strength improvement. The four kinds of specimens were fabricated with 0%, 3.2%, 5.2% and 7% volume fraction of TiNi alloy fiber, respectively. A fatigue test has been performed to evaluate the crack initiation and propagation for the TiNi/Al6061 SMA composite fabricated by this method. In order to study the shape memory effect of the TiNi alloy fiber, the test has also been done under both room temperature and high temperature conditions. The relationship between the crack growth rate and the stress intensity factor was clarified for the composite, and the cold rolling effect was also studied.
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
NASA Astrophysics Data System (ADS)
Gubeljak, N.; Predan, J.; Senčič, B.; Chapetti, M. D.
2016-03-01
An integrated fracture mechanics approach is proposed to account for the estimation of the fatigue resistance of component. Applications, estimations and results showed very good agreements with experimental results. The model is simple to apply, accounts for the main geometrical, mechanical and material parameters that define the fatigue resistance, and allows accurate predictions. It offers a change in design philosophy: It could be used for design, while simultaneously dealing with crack propagation thresholds. Furthermore, it allows quantification of the material defect sensitivity. In the case of the set of fatigue tests carried out by rotational bending of specimens without residual stresses, the estimated results showed good agreement and that an initial crack length of 0.5 mm can conservatively explain experimental data. In the case of fatigue tests carried out on the springs at their final condition with bending at R = 0.1 our data shows the influence of compressive residual stresses on fatigue strength. Results also showed that the procedures allow us to analyze the different combinations of initial crack length and residual stress levels, and how much the fatigue resistance can change by changing that configuration. For this set of tests, the fatigue resistance estimated for an initial crack length equal to 0.35 mm, can explain all testing data observed for the springs.
NASA Astrophysics Data System (ADS)
Ferdous, Md. Shafiul; Setyabudi, Sofyan Arief; Makabe, Chobin; Fujikawa, Masaki
2013-05-01
The fatigue and fracture behavior of C/C composites fabricated using fine-woven carbon fiber laminates with α = 0/90° direction were investigated. Also, the phenomenon of crack growth behavior and the shear damage in the fiber bundle was discussed. Slits of several sizes were cut on both sides of a test section and different sizes of slit length were chosen. The effect of the slit configuration on crack initiation and growth behavior was observed. Specimens with blunt-notches and center-holes were also used to compare the fatigue strength and crack growth behavior. Non-propagating cracks were observed and fatigue limit was defined as the maximum stress at which specimen did not break for N = 107 cycles stress application. The longest fatigue life was obtained in the case of specimens with shorter slits. The relationships between fatigue strengths and specimen shapes were analyzed by stress concentration, Kt, and stress intensity factor, KI. The effect of slit configuration on fatigue strength was then discussed regarding both the experimental and calculated consequences.
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 of 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.
The use of XFEM to assess the influence of intra-cortical porosity on crack propagation.
Rodriguez-Florez, Naiara; Carriero, Alessandra; Shefelbine, Sandra J
2017-03-01
This study aimed at using eXtended finite element method (XFEM) to characterize crack growth through bone's intra-cortical pores. Two techniques were compared using Abaqus: (1) void material properties were assigned to pores; (2) multiple enrichment regions with independent crack-growth possibilities were employed. Both were applied to 2D models of transverse images of mouse bone with differing porous structures. Results revealed that assigning multiple enrichment regions allows for multiple cracks to be initiated progressively, which cannot be captured when the voids are filled. Therefore, filling pores with one enrichment region in the model will not create realistic fracture patterns in Abaqus-XFEM.
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.
May, Robert A.; Smith, R. Scott; Kay, Bruce D.
2013-03-14
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 distribution 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.
The use of electrical anisotropy measurements to monitor soil crack dynamics - laboratory evaluation
NASA Astrophysics Data System (ADS)
Sahraei, Amirhossein; Huisman, Johan Alexander; Zimmermann, Egon; Vereecken, Harry
2016-04-01
Swelling and shrinking of soil cracks is a key factor determining water fluxes in many irrigated soils. Most previous studies have used time-intensive and destructive methods for crack characterization, such as depth and volume determination from simplified geometrical measurements or liquid latex filling. Because of their destructive and time-consuming nature, these methods have only provided instantaneous estimates of the geometry and/or volume of cracks. The aim of this study is to evaluate the use of anisotropy in electrical resistivity measured with a square electrode array to determine crack depth dynamics. In a first step, the performance of the method was analyzed using a laboratory experiment where an artificial soil crack was emulated using a plastic plate in a water bath. Since cracking depth was precisely known, this experiment allowed to develop a method to estimate soil crack depth from measurements of the electrical anisotropy. In a second step, electrical anisotropy was measured during soil crack development within a soil monolith consisting of a mix of sand and bentonite. The cracking depth estimated from electrical measurement compared well with reference ruler measurements. These laboratory measurements inspired confidence in the use of electrical anisotropy for soil crack investigations, and consequently the developed methods will be applied to investigate soil crack dynamics in the field in a next step.
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.
Path dependence of J in three numerical examples. [J integral in three crack propagation problems
NASA Technical Reports Server (NTRS)
Karabin, M. E., Jr.; Swedlow, J. L.
1979-01-01
Three cracked geometries are studied with the aid of a new finite element model. The procedure employs a variable singularity at the crack tip that tracks changes in the material response during the loading process. Two of the problems are tension-loaded center-crack panels and the other is a three-point bend specimen. Results usually agree with other numerical and analytical analyses, except the finding that J is path dependent as a substantial plastic zone develops. Credible J values are obtained near the crack tip and J shows a significant increase as the radius of J path increases over two orders of magnitude. Incremental and deformation theories are identical provided the stresses exhibit proportionality found in the far field stresses but not near the tip.
Wave propagation analysis of edge cracked circular beams under impact force.
Akbaş, Şeref Doğuşcan
2014-01-01
This paper presents responses of an edge circular cantilever beam under the effect of an impact force. The beam is excited by a transverse triangular force impulse modulated by a harmonic motion. The Kelvin-Voigt model for the material of the beam is used. The cracked beam is modelled as an assembly of two sub-beams connected through a massless elastic rotational spring. The considered problem is investigated within the Bernoulli-Euler beam theory by using energy based finite element method. The system of equations of motion is derived by using Lagrange's equations. The obtained system of linear differential equations is reduced to a linear algebraic equation system and solved in the time domain by using Newmark average acceleration method. In the study, the effects of the location of crack, the depth of the crack, on the characteristics of the reflected waves are investigated in detail. Also, the positions of the cracks are calculated by using reflected waves.
Wave Propagation Analysis of Edge Cracked Circular Beams under Impact Force
Akbaş, Şeref Doğuşcan
2014-01-01
This paper presents responses of an edge circular cantilever beam under the effect of an impact force. The beam is excited by a transverse triangular force impulse modulated by a harmonic motion. The Kelvin–Voigt model for the material of the beam is used. The cracked beam is modelled as an assembly of two sub-beams connected through a massless elastic rotational spring. The considered problem is investigated within the Bernoulli-Euler beam theory by using energy based finite element method. The system of equations of motion is derived by using Lagrange's equations. The obtained system of linear differential equations is reduced to a linear algebraic equation system and solved in the time domain by using Newmark average acceleration method. In the study, the effects of the location of crack, the depth of the crack, on the characteristics of the reflected waves are investigated in detail. Also, the positions of the cracks are calculated by using reflected waves. PMID:24972050
Dynamics of crack penetration vs. branching at a weak interface: An experimental study
NASA Astrophysics Data System (ADS)
Sundaram, Balamurugan M.; Tippur, Hareesh V.
2016-11-01
In this paper, the dynamic crack-interface interactions and the related mechanics of crack penetration vs. branching at a weak interface are studied experimentally. The interface is oriented perpendicular to the incoming mode-I crack in an otherwise homogeneous bilayer. The focus of this investigation is on the effect of interface location and the associated crack-tip parameters within the bilayer on the mechanics of the ensuing fracture behavior based on the optical methodologies laid down in Ref. Sundaram and Tippur (2016). Time-resolved optical measurement of crack-tip deformations, velocity and stress intensity factor histories in different bilayer configurations is performed using Digital Gradient Sensing (DGS) technique in conjunction with high-speed photography. The results show that the crack path selection at the interface and subsequently the second layer are greatly affected by the location of the interface within the geometry. Using optically measured fracture parameters, the mechanics of crack penetration and branching are explained. Counter to the intuition, a dynamically growing mode-I approaching a weak interface at a lower velocity and stress intensity factor penetrates the interface whereas a higher velocity and stress intensity factor counterpart gets trapped by the interface producing branched daughter cracks until they kink out into the next layer. An interesting empirical observation based on measured crack-tip parameters for crack penetration and branching is also made.
Dynamic analysis of a geared rotor system considering a slant crack on the shaft
NASA Astrophysics Data System (ADS)
Han, Qinkai; Zhao, Jingshan; Chu, Fulei
2012-12-01
The vibration problems associated with geared systems have been the focus of research in recent years. As the torque is mainly transmitted by the geared system, a slant crack is more likely to appear on the gear shaft. Due to the slant crack and its breathing mechanism, the dynamic behavior of cracked geared system would differ distinctly with that of uncracked system. Relatively less work is reported on slant crack in the geared rotor system during the past research. Thus, the dynamic analysis of a geared rotor-bearing system with a breathing slant crack is performed in the paper. The finite element model of a geared rotor with slant crack is presented. Based on fracture mechanics, the flexibility matrix for the slant crack is derived that accounts for the additional stress intensity factors. Three methods for whirling analysis, parametric instability analysis and steady-state response analysis are introduced. Then, by taking a widely used one-stage geared rotor-bearing system as an example, the whirling frequencies of the equivalent time-invariant system, two types of instability regions and steady-state response under the excitations of unbalance forces and tooth transmission errors, are computed numerically. The effects of crack depth, position and type (transverse or slant) on the system dynamic behaviors are considered in the discussion. The comparative study with slant cracked geared rotor is carried out to explore distinctive features in their modal, parametric instability and frequency response behaviors.
NASA Astrophysics Data System (ADS)
Ozevin, Didem; Fazel, Hossein; Cox, Justin; Hardman, William; Kessler, Seth S.; Timmons, Alan
2014-04-01
Gearbox components of aerospace structures are typically made of brittle materials with high fracture toughness, but susceptible to fatigue failure due to continuous cyclic loading. Structural Health Monitoring (SHM) methods are used to monitor the crack growth in gearbox components. Damage detection methodologies developed in laboratory-scale experiments may not represent the actual gearbox structural configuration, and are usually not applicable to real application as the vibration and wave properties depend on the material, structural layers and thicknesses. Also, the sensor types and locations are key factors for frequency content of ultrasonic waves, which are essential features for pattern recognition algorithm development in noisy environments. Therefore, a deterministic damage detection methodology that considers all the variables influencing the waveform signature should be considered in the preliminary computation before any experimental test matrix. In order to achieve this goal, we developed two dimensional finite element models of a gearbox cross section from front view and shaft section. The cross section model consists of steel revolving teeth, a thin layer of oil, and retention plate. An ultrasonic wave up to 1 MHz frequency is generated, and waveform histories along the gearbox are recorded. The received waveforms under pristine and cracked conditions are compared in order to analyze the crack influence on the wave propagation in gearbox, which can be utilized by both active and passive SHM methods.
NASA Astrophysics Data System (ADS)
Han, Xianhong; Yang, Kun; Chen, Sisi; Chen, Jun
2015-10-01
Mechanical trimming process for ultra-high strength boron steel after hot stamping was carried out in this study. Shear and tensile tests were designed to analyze the influences of stress state on the fracture mode; trimmed fracture surface and profile were observed and compared to other commonly used steels such as DP980 and Q235 etc.; the crack propagation during trimming process was studied through step-by-step tests. The observation and analysis reveal that the fracture mode of hot-stamped boron steel is highly related to the stress state, it belongs to cleavage fracture on low stress triaxiality but dimple fracture on high stress triaxiality. Such phenomenon is reflected in the trimming process, during which the stress state changes from shear-dominated state to tensile-dominated state. In addition, the burnish zone of trimmed boron steel is much smaller than other high strength steels, and the profile of cutting surface shows an `S'-like shape which is destructive to the trimming tool. Moreover, during the trimming process, most martensite laths near the cutting edge are stretched and rotated markedly to the direction of the shear band, and the main crack expands along those grain boundaries, which may penetrate through a few martensite laths and form small crack branches.
NASA Astrophysics Data System (ADS)
Simonov, M. Yu.; Simonov, Yu. N.; Khanov, A. M.; Shaimanov, G. S.
2013-03-01
The structure of steels 09G2S, 25 and 40 after quenching and tempering at from 200 to 650°C is studied. An own method of the authors is used to estimate the impact toughness and the dynamic crack resistance of structural steels in various structural states. The micromechanisms of crack growth are studied in steels with various structures.
A numerical study of crack initiation in a bcc iron system based on dynamic bifurcation theory
Li, Xiantao
2014-10-28
Crack initiation under dynamic loading conditions is studied under the framework of dynamic bifurcation theory. An atomistic model for BCC iron is considered to explicitly take into account the detailed molecular interactions. To understand the strain-rate dependence of the crack initiation process, we first obtain the bifurcation diagram from a computational procedure using continuation methods. The stability transition associated with a crack initiation, as well as the connection to the bifurcation diagram, is studied by comparing direct numerical results to the dynamic bifurcation theory [R. Haberman, SIAM J. Appl. Math. 37, 69–106 (1979)].
A Dynamic Discrete Dislocation Plasticity study of elastodynamic shielding of stationary cracks
NASA Astrophysics Data System (ADS)
Gurrutxaga-Lerma, B.; Balint, D. S.; Dini, D.; Sutton, A. P.
2017-01-01
Employing Dynamic Discrete Dislocation Plasticity (D3P), an elastodynamic analysis of the shielding of a stationary crack tip by dislocations is studied. Dislocations are generated via Frank-Read sources, and make a negligible contribution to the shielding of the crack tip, whereas dislocations generated at the crack tip via homogeneous nucleation dominate the shielding. Their effect is found to be highly localised around the crack, leading to a magnification of the shielding when compared to time-independent, elastostatic predictions. The resulting attenuation of KI(t) is computed, and is found to be directly proportional to the applied load and to √{ t }.
Crack fusion dynamics: A model for large earthquakes
Newman, W.I.; Knopoff, L.
1982-07-01
The physical processes of the fusion of small cracks into larger ones are nonlinear in character. A study of the nonlinear properties of fusion may lead to an understanding of the instabilities that give rise to clustering of large earthquakes. We have investigated the properties of simple versions of fusion processes to see if instabilities culminating in repetitive massive earthquakes are possible. We have taken into account such diverse phenomena as the production of aftershocks, the rapid extension of large cracks to overwhelm and absorb smaller cracks, the influence of anelastic creep-induced time delays, healing, the genesis of ''juvenile'' cracks due to plate motions, and others. A preliminary conclusion is that the time delays introduced by anelastic creep may be responsible for producing catastrophic instabilities characteristic of large earthquakes as well as aftershock sequences. However, it seems that nonlocal influences, i.e., the spatial diffusion of cracks, may play a dominant role in producing episodes of seismicity and clustering.
Effect of Microstructure on the Fatigue Crack Propagation Behavior of Ni-Base Superalloys.
1987-12-01
N= cycles to initiate crack in process zone A Coffin - Manson exponent Sp = plastic strain range ef = fatigue ductility CO = constant From this the...initiation at an average plastic strain range Aep in the process zone. ANi can be calculated from the Coffln- Manson equation: where (-NI)PAP = C 0 f .(8
Crack Propagation Analysis Using Acoustic Emission Sensors for Structural Health Monitoring Systems
Kral, Zachary; Horn, Walter; Steck, James
2013-01-01
Aerospace systems are expected to remain in service well beyond their designed life. Consequently, maintenance is an important issue. A novel method of implementing artificial neural networks and acoustic emission sensors to form a structural health monitoring (SHM) system for aerospace inspection routines was the focus of this research. Simple structural elements, consisting of flat aluminum plates of AL 2024-T3, were subjected to increasing static tensile loading. As the loading increased, designed cracks extended in length, releasing strain waves in the process. Strain wave signals, measured by acoustic emission sensors, were further analyzed in post-processing by artificial neural networks (ANN).more » Several experiments were performed to determine the severity and location of the crack extensions in the structure. ANNs were trained on a portion of the data acquired by the sensors and the ANNs were then validated with the remaining data. The combination of a system of acoustic emission sensors, and an ANN could determine crack extension accurately. The difference between predicted and actual crack extensions was determined to be between 0.004 in. and 0.015 in. with 95% confidence. These ANNs, coupled with acoustic emission sensors, showed promise for the creation of an SHM system for aerospace systems.« less
Crack propagation analysis using acoustic emission sensors for structural health monitoring systems.
Kral, Zachary; Horn, Walter; Steck, James
2013-01-01
Aerospace systems are expected to remain in service well beyond their designed life. Consequently, maintenance is an important issue. A novel method of implementing artificial neural networks and acoustic emission sensors to form a structural health monitoring (SHM) system for aerospace inspection routines was the focus of this research. Simple structural elements, consisting of flat aluminum plates of AL 2024-T3, were subjected to increasing static tensile loading. As the loading increased, designed cracks extended in length, releasing strain waves in the process. Strain wave signals, measured by acoustic emission sensors, were further analyzed in post-processing by artificial neural networks (ANN). Several experiments were performed to determine the severity and location of the crack extensions in the structure. ANNs were trained on a portion of the data acquired by the sensors and the ANNs were then validated with the remaining data. The combination of a system of acoustic emission sensors, and an ANN could determine crack extension accurately. The difference between predicted and actual crack extensions was determined to be between 0.004 in. and 0.015 in. with 95% confidence. These ANNs, coupled with acoustic emission sensors, showed promise for the creation of an SHM system for aerospace systems.
Crack detection in a wheel end spindle using wave propagation via modal impacts and piezo actuation
NASA Astrophysics Data System (ADS)
Ackers, Spencer; Evans, Ronald; Johnson, Timothy; Kess, Harold; White, Jonathan; Adams, Douglas E.; Brown, Pam
2006-03-01
This research demonstrates two methodologies for detecting cracks in a metal spindle housed deep within a vehicle wheel end assembly. First, modal impacts are imposed on the hub of the wheel in the longitudinal direction to produce broadband elastic wave excitation spectra out to 7000 Hz. The response data on the flange is collected using 3000 Hz bandwidth accelerometers. It is shown using frequency response analysis that the crack produces a filter, which amplifies the elastic response of the surrounding components of the wheel assembly. Experiments on wheel assemblies mounted on the vehicle with the vehicle lifted off the ground are performed to demonstrate that the modal impact method can be used to nondestructively evaluate cracks of varying depths despite sources of variability such as the half shaft angular position relative to the non-rotating spindle. Second, an automatic piezo-stack actuator is utilized to excite the wheel hub with a swept sine signal extending from 20 kHz. Accelerometers are then utilized to measure the response on the flange. It is demonstrated using frequency response analysis that the crack filters waves traveling from the hub to the flange. A simple finite element model is used to interpret the experimental results. Challenges discussed include variability from assembly to assembly, the variability in each assembly, and the high amount of damping present in each assembly due to the transmission gearing, lubricant, and other components in the wheel end. A two-channel measurement system with a graphical user interface for detecting cracks was also developed and a procedure was created to ensure that operators properly perform the test.
FRANC2D: A two-dimensional crack propagation simulator. Version 2.7: User's guide
NASA Technical Reports Server (NTRS)
Wawrzynek, Paul; Ingraffea, Anthony
1994-01-01
FRANC 2D (FRacture ANalysis Code, 2 Dimensions) is a menu driven, interactive finite element computer code that performs fracture mechanics analyses of 2-D structures. The code has an automatic mesh generator for triangular and quadrilateral elements. FRANC2D calculates the stress intensity factor using linear elastic fracture mechanics and evaluates crack extension using several methods that may be selected by the user. The code features a mesh refinement and adaptive mesh generation capability that is automatically developed according to the predicted crack extension direction and length. The code also has unique features that permit the analysis of layered structure with load transfer through simulated mechanical fasteners or bonded joints. The code was written for UNIX workstations with X-windows graphics and may be executed on the following computers: DEC DecStation 3000 and 5000 series, IBM RS/6000 series, Hewlitt-Packard 9000/700 series, SUN Sparc stations, and most Silicon Graphics models.
Crack propagation and arrest in CFRP materials with strain softening regions
NASA Astrophysics Data System (ADS)
Dilligan, Matthew Anthony
Understanding the growth and arrest of cracks in composite materials is critical for their effective utilization in fatigue-sensitive and damage susceptible applications such as primary aircraft structures. Local tailoring of the laminate stack to provide crack arrest capacity intermediate to major structural components has been investigated and demonstrated since some of the earliest efforts in composite aerostructural design, but to date no rigorous model of the crack arrest mechanism has been developed to allow effective sizing of these features. To address this shortcoming, the previous work in the field is reviewed, with particular attention to the analysis methodologies proposed for similar arrest features. The damage and arrest processes active in such features are investigated, and various models of these processes are discussed and evaluated. Governing equations are derived based on a proposed mechanistic model of the crack arrest process. The derived governing equations are implemented in a numerical model, and a series of simulations are performed to ascertain the general characteristics of the proposed model and allow qualitative comparison to existing experimental results. The sensitivity of the model and the arrest process to various parameters is investigated, and preliminary conclusions regarding the optimal feature configuration are developed. To address deficiencies in the available material and experimental data, a series of coupon tests are developed and conducted covering a range of arrest zone configurations. Test results are discussed and analyzed, with a particular focus on identification of the proposed failure and arrest mechanisms. Utilizing the experimentally derived material properties, the tests are reproduced with both the developed numerical tool as well as a FEA-based implementation of the arrest model. Correlation between the simulated and experimental results is analyzed, and future avenues of investigation are identified
1986-01-01
pre ization rates of 30 GPa/s or higher, multiple branching in various dirm--ions is observed exclusively (branched mode), and at intermediate, 3p/at...Vrginia Contract No. N00014-79-C-0762 Acceso nFrNTIS CRA&I DTIC TABPrepared by Unannounced 0 Kenneth K. Kuo Justificatio -............... John A. Moreci...88 X LIST OF TABLES Page Table 1 Film Interpretation of Multiple Crack Propc.gation and Branching (Test Firing No. DNICP-28
Application of the cyclic J-integral to fatigue crack propagation
NASA Astrophysics Data System (ADS)
Volpert, Yehuda; Banks-Sills, Leslie
1992-02-01
A cyclic J-integral method is investigated considering correlation of fatigue crack growth rate under elastic and small scale yielding conditions. An experimental/numerical study is carried out in order to explain more clearly the meaning of the cyclic J-integral delta J, for fatigue crack growth studies. Compact tension specimens fabricated from Al 2024-T351 were subjected to constant amplitude loading with two R-ratios (R approximately equal to 0.05 and 0.5). A simulation of conducted tests is carried out numerically, employing the finite element method with the material modeled to be elasto-plastic. From the numerical results, values of delta J are calculated from both a path independent integral and load versus load-line displacement data. Comparison between these values, as well as with those determined from experiments are seen to be reasonable. The delta J parameter is seen to correlate properly with the crack growth rate under elastic and small-scale yielding conditions.
Unveiling the propagation dynamics of self-accelerating vector beams
NASA Astrophysics Data System (ADS)
Bar-David, Jonathan; Voloch-Bloch, Noa; Mazurski, Noa; Levy, Uriel
2016-09-01
We study theoretically and experimentally the varying polarization states and intensity patterns of self-accelerating vector beams. It is shown that as these beams propagate, the main intensity lobe and the polarization singularity gradually drift apart. Furthermore, the propagation dynamics can be manipulated by controlling the beams’ acceleration coefficients. We also demonstrate the self-healing dynamics of these accelerating vector beams for which sections of the vector beam are being blocked by an opaque or polarizing obstacle. Our results indicate that the self-healing process is almost insensitive for the obstacles’ polarization direction. Moreover, the spatial polarization structure also shows self- healing properties, and it is reconstructed as the beam propagates further beyond the perturbation plane. These results open various possibilities for generating, shaping and manipulating the intensity patterns and space variant polarization states of accelerating vector beams.
Unveiling the propagation dynamics of self-accelerating vector beams
Bar-David, Jonathan; Voloch-Bloch, Noa; Mazurski, Noa; Levy, Uriel
2016-01-01
We study theoretically and experimentally the varying polarization states and intensity patterns of self-accelerating vector beams. It is shown that as these beams propagate, the main intensity lobe and the polarization singularity gradually drift apart. Furthermore, the propagation dynamics can be manipulated by controlling the beams’ acceleration coefficients. We also demonstrate the self-healing dynamics of these accelerating vector beams for which sections of the vector beam are being blocked by an opaque or polarizing obstacle. Our results indicate that the self-healing process is almost insensitive for the obstacles’ polarization direction. Moreover, the spatial polarization structure also shows self- healing properties, and it is reconstructed as the beam propagates further beyond the perturbation plane. These results open various possibilities for generating, shaping and manipulating the intensity patterns and space variant polarization states of accelerating vector beams. PMID:27671745
Analysis of the dynamic characteristics of a slant-cracked cantilever beam
NASA Astrophysics Data System (ADS)
Ma, Hui; Zeng, Jin; Lang, Ziqiang; Zhang, Long; Guo, Yuzhu; Wen, Bangchun
2016-06-01
In this study, the dynamic characteristics of a slant-cracked cantilever beam are studied based on a new finite element (FE) model where both plane and beam elements are used to reduce the computational costs. Simulation studies show that the proposed model has the same system natural frequencies and vibration responses as those in the pure plane element model but is computationally more efficient. Based on the new model, the effects of loads such as gravity Fg, excitation force amplitude F0 and direction angles of excitation force φ, and crack parameters including slant crack angle θ, dimensionless crack depth s and dimensionless crack location p, on system dynamics have been analyzed. The results indicate that (1) the gravity has a more significant effect on the sub-harmonic resonance responses than on the super-harmonic resonance and resonance responses; (2) The amplitudes of the system responses at both excitation force frequencies fe and its harmonics such as 2fe and 3fe increase almost linearly with the increase of the excitation force amplitude F0; (3) Under the constant excitation force in the flexural direction, the tensile and compressive forces along the longitudinal direction can lead to opposite breathing behaviors of the crack within the super-harmonic and sub-harmonic resonance frequency regions; (4) Vibration is most severe under the straight crack angle (θ=90°) and near the straight crack angle such as θ=100° and 110°, and the vibration responses under smaller or larger crack angles such as θ=30° and θ=150° become weaker; (5) The resonance at 2fe is sensitive to the faint crack signals when s is small and p is large. In addition, the significant vibration responses at the multiple frequency of 3fe and the fractional frequency of 0.5fe can be regarded as a distinguishable feature of the serious crack with large s and small p.
The effects of solidification on sill propagation dynamics and geometry
NASA Astrophysics Data System (ADS)
Lola, Chanceaux; Thierry, Menand
2015-04-01
The effects of solidification on sill propagation dynamics and geometry are studied by means of analogue laboratory experiments. Hot fluid vegetable oil (a magma analogue), that solidifies during its propagation, is injected as a sill in a colder layered gelatine solid (an elastic host rock analogue). The injection flux and temperature are maintained constant during an experiment. In order to vary the importance of solidification and quantify its effect on sill propagation, the injection flux and temperature are systematically varied between each experiment. Depending on the importance of solidification effects, two extreme behaviours for sill propagation dynamics and geometry are observed. When solidification effects are small (high injection temperatures and fluxes), the propagation is continuous and the sill has a regular and smooth surface. Inversely, when solidification effects are important (low injection temperatures and fluxes), sill propagation is discontinuous and occurs by steps. After each propagation step, the sill stalls, thickens progressively by storing hot fluid vegetable oil beneath the partially solidified intrusion, without growing neither in length nor in breadth, and after a pause, the propagation initiates again, soon followed by a new episode of momentary arrest. The geometry of these sills displays folds, ropy structures on their surface, and lobes with imprints of the leading fronts that correspond to each step of surface creation. These experiments show that for a given, constant injected volume, as solidification effects increase, the surface of the sills decreases, their thickness increases, and the number of propagation steps increases. In the same way lower solidification effects promote larger sill surfaces, lower thicknesses, and a lower number of propagation steps. These results have various geological and geophysical implications. Regarding the geometry of sills, 3D seismic studies in sedimentary basins reveal sills with lobate
Effect of superplastic forming exposure on fatigue crack propagation behavior of Ti-6Al-4V alloy
NASA Astrophysics Data System (ADS)
Jeong, Daeho; Kwon, Yongnam; Goto, Masahiro; Kim, Sangshik
2016-09-01
The effect of superplastic forming (SPF) exposure on the ɛ (strain)-N (number of cycles to failure) fatigue and fatigue crack propagation (FCP) behaviors of Ti-6Al-4V (Ti64) alloy was examined at 298 and 473 K. To simulate the thermal exposure during superplastic forming process, the mill-annealed Ti64 alloy sheet was heated in the vacuum chamber with the pre-determined temperature profile. Notable microstructural change during the SPF exposure included the shape of transformed β phase from fine and round particles in the as-received specimen to coarse angular particles in the as-exposed specimen. The effective grain size tended to increase with the exposure, enhancing the slip reversibility and the resistance to FCP. However, the crack hindering effect by fine, particle-like β phase became weak with the exposure, offseting the beneficial effect associated with the increment of effective grain size. The effect of SPF exposure on ɛ-N fatigue and FCP behavior of mill-annealed Ti64 alloy was therefore marginal, excluding the effect of α-case (the oxygen-enriched phase) on the surface.
Why is nacre strong? II. Remaining mechanical weakness for cracks propagating along the sheets.
Okumura, K
2002-04-01
In our previous paper (Eur. Phys. J. E 4, 121 (2001)) we proposed a coarse-grained elastic energy for nacre, or stratified structure of hard and soft layers found in certain seashells . We then analyzed a crack running perpendicular to the layers and suggested one possible reason for the enhanced toughness of this substance. In the present paper, we consider a crack running parallel to the layers. We propose a new term added to the previous elastic energy, which is associated with the bending of layers. We show that there are two regimes for the parallel-fracture solution of this elastic energy; near the fracture tip the deformation field is governed by a parabolic differential equation while the field away from the tip follows the usual elliptic equation. Analytical results show that the fracture tip is lenticular, as suggested in a paper on a smectic liquid crystal (P.G. de Gennes, Europhys. Lett. 13, 709 (1990)). On the contrary, away from the tip, the stress and deformation distribution recover the usual singular behaviors ( and 1/, respectively, where x is the distance from the tip). This indicates there is no enhancement in toughness in the case of parallel fracture.
Why is nacre strong? II. Remaining mechanical weakness for cracks propagating along the sheets
NASA Astrophysics Data System (ADS)
Okumura, K.
2002-04-01
In our previous paper (Eur. Phys. J. E 4, 121 (2001)) we proposed a coarse-grained elastic energy for nacre, or stratified structure of hard and soft layers found in certain seashells . We then analyzed a crack running perpendicular to the layers and suggested one possible reason for the enhanced toughness of this substance. In the present paper, we consider a crack running parallel to the layers. We propose a new term added to the previous elastic energy, which is associated with the bending of layers. We show that there are two regimes for the parallel-fracture solution of this elastic energy; near the fracture tip the deformation field is governed by a parabolic differential equation while the field away from the tip follows the usual elliptic equation. Analytical results show that the fracture tip is lenticular, as suggested in a paper on a smectic liquid crystal (P.G. de Gennes, Europhys. Lett. 13, 709 (1990)). On the contrary, away from the tip, the stress and deformation distribution recover the usual singular behaviors (sqrt{x} and 1/sqrt{x}, respectively, where x is the distance from the tip). This indicates there is no enhancement in toughness in the case of parallel fracture.
Dynamic dependence to domain wall propagation through artificial spin ice
NASA Astrophysics Data System (ADS)
Burn, D. M.; Chadha, M.; Branford, W. R.
2017-03-01
Domain wall propagation dynamics has been studied in nanostructured artificial kagome spin-ice structures. A stripline circuit has been used to provide localized pulsed magnetic fields within the artificial spin-ice (ASI) structure. This provides control of the system through electrically assisted domain wall nucleation events. Synchronization of the pulsed fields with additional global magnetic fields and the use of a focused magneto-optical Kerr effect magnetometer allows our experiments to probe the domain wall transit through an extended ASI structure. We find that the propagation distance depends on the driving field revealing field-driven properties of domain walls below their intrinsic nucleation field.
Microscale spatiotemporal dynamics during neocortical propagation of human focal seizures.
Wagner, Fabien B; Eskandar, Emad N; Cosgrove, G Rees; Madsen, Joseph R; Blum, Andrew S; Potter, N Stevenson; Hochberg, Leigh R; Cash, Sydney S; Truccolo, Wilson
2015-11-15
Some of the most clinically consequential aspects of focal epilepsy, e.g. loss of consciousness, arise from the generalization or propagation of seizures through local and large-scale neocortical networks. Yet, the dynamics of such neocortical propagation remain poorly understood. Here, we studied the microdynamics of focal seizure propagation in neocortical patches (4×4 mm) recorded via high-density microelectrode arrays (MEAs) implanted in people with pharmacologically resistant epilepsy. Our main findings are threefold: (1) a newly developed stage segmentation method, applied to local field potentials (LFPs) and multiunit activity (MUA), revealed a succession of discrete seizure stages, each lasting several seconds. These different stages showed characteristic evolutions in overall activity and spatial patterns, which were relatively consistent across seizures within each of the 5 patients studied. Interestingly, segmented seizure stages based on LFPs or MUA showed a dissociation of their spatiotemporal dynamics, likely reflecting different contributions of non-local synaptic inputs and local network activity. (2) As previously reported, some of the seizures showed a peak in MUA that happened several seconds after local seizure onset and slowly propagated across the MEA. However, other seizures had a more complex structure characterized by, for example, several MUA peaks, more consistent with the succession of discrete stages than the slow propagation of a simple wavefront of increased MUA. In both cases, nevertheless, seizures characterized by spike-wave discharges (SWDs, ~2-3 Hz) eventually evolved into patterns of phase-locked MUA and LFPs. (3) Individual SWDs or gamma oscillation cycles (25-60 Hz), characteristic of two different types of recorded seizures, tended to propagate with varying degrees of directionality, directions of propagation and speeds, depending on the identified seizure stage. However, no clear relationship was observed between the MUA
Effects of disorder and internal dynamics on vortex wall propagation.
Min, Hongki; McMichael, Robert D; Donahue, Michael J; Miltat, Jacques; Stiles, M D
2010-05-28
Experimental measurements of domain wall propagation are typically interpreted by comparison to reduced models that ignore both the effects of disorder and the internal dynamics of the domain wall structure. Using micromagnetic simulations, we study vortex wall propagation in magnetic nanowires induced by fields or currents in the presence of disorder. We show that the disorder leads to increases and decreases in the domain wall velocity depending on the conditions. These results can be understood in terms of an effective damping that increases as disorder increases. As a domain wall moves through disorder, internal degrees of freedom get excited, increasing the energy dissipation rate.
Microscale Spatiotemporal Dynamics during Neocortical Propagation of Human Focal Seizures
Wagner, Fabien B.; Eskandar, Emad N.; Cosgrove, G. Rees; Madsen, Joseph R.; Blum, Andrew S.; Potter, N. Stevenson; Hochberg, Leigh R.; Cash, Sydney S.; Truccolo, Wilson
2015-01-01
Some of the most clinically consequential aspects of focal epilepsy, e.g. loss of consciousness, arise from the generalization or propagation of seizures through local and large-scale neocortical networks. Yet, the dynamics of such neocortical propagation remain poorly understood. Here, we studied the microdynamics of focal seizure propagation in neocortical patches (4 × 4 mm) recorded via high-density microelectrode arrays (MEAs) implanted in people with pharmacologically resistant epilepsy. Our main findings are threefold: (1) A newly developed stage segmentation method, applied to local field potentials (LFPs) and multi-unit activity (MUA), revealed a succession of discrete seizure stages, each lasting several seconds. These different stages showed characteristic evolutions in overall activity and spatial patterns, which were relatively consistent across seizures within each of the 5 patients studied. Interestingly, segmented seizure stages based on LFPs or MUA showed a dissociation of their spatiotemporal dynamics, likely reflecting different contributions of non-local synaptic inputs and local network activity. (2) As previously reported, some of the seizures showed a peak in MUA that happened several seconds after local seizure onset and slowly propagated across the MEA. However, other seizures had a more complex structure characterized by, for example, several MUA peaks, more consistent with the succession of discrete stages than the slow propagation of a simple wavefront of increased MUA. In both cases, nevertheless, seizures characterized by spike-wave discharges (SWDs, ~ 2–3Hz) eventually evolved into patterns of phase-locked MUA and LFPs. (3) Individual SWDs or gamma oscillation cycles (25–60 Hz), characteristic of two different types of recorded seizures, tended to propagate with varying degrees of directionality, directions of propagation and speeds, depending on the identified seizure stage. However, no clear relationship was observed between the
Kanninen, M.F.; Dexter, R.J.; Polch, E.Z.; Popelar, C.H.
1986-12-31
The extensive safe-operating experience that has been accumulated in LNG storage tanks is strongly indicative of a high degree of structural integrity. Nevertheless, concern has arisen with regard to the presence of weld-induced fabrication flaws in these tanks. When such flaws exist, it may not be possible to preclude the initiation of unstable crack growth during operation. However, if it can be shown that the material has the ability to arrest a running crack, an adequate margin of safety can still be demonstrated. A comprehensive program of experimentation was undertaken in companion programs to develop the crack-arrest properties that are needed for such a demonstration. But, because the 9% nickel steel used for LNG tank materials is very ductile and tough, its fracture properties are not well characterized by the quasi-static linear elastic-fracture-mechanics techniques available to these researchers. The research was therefore aimed at assisting the experimental work by providing more-realistic dynamic and nonlinear fracture-mechanics analyses. These analyses were intended to enhance the experimental program and to help insure that the results of the research are appropriate for the assessment of LNG storage-tank integrity. Specifically, the application of these advanced analysis techniques has demonstrated the mildly anticonservative nature of conventional fracture-mechanics treatments for the materials and structural geometry of interest. Nevertheless, provided a defected region in a storage tank is not overly large, the tank materials examined in this research possess high-enough-crack arrest toughness values to preclude complete tank rupture.
An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth.
Mi, Bao; Michaels, Jennifer E; Michaels, Thomas E
2006-01-01
Attached ultrasonic sensors can detect changes caused by crack initiation and growth if the wave path is directed through the area of critical crack formation. Dynamics of cracks opening and closing under load cause nonlinear modulation of received ultrasonic signals, enabling small cracks to be detected by stationary sensors. A methodology is presented based upon the behavior of ultrasonic signals versus applied load to detect and monitor formation and growth of cracks originating from fastener holes. Shear wave angle beam transducers operating in through transmission mode are mounted on either side of the hole such that the transmitted wave travels through the area of expected cracking. Time shift is linear with respect to load, and is well explained by path changes due to strain combined with wave speed changes due to acoustoelasticity. During subsequent in situ monitoring with unknown loads, the measured time of flight is used to estimate the load, and behavior of the received energy as a function of load is the basis for crack detection. Results are presented from low cycle fatigue tests of several aluminum specimens and illustrate the efficacy of the method in both determining the applied load and monitoring crack initiation and growth.
Dynamical chiral symmetry with an infrared finite gluon propagator
NASA Astrophysics Data System (ADS)
Cardona, J. C.; Aguilar, A. C.
2016-04-01
In this work we study dynamical quark mass generation using an infrared finite gluon propagator obtained from quenched lattice simulations. The quark gap equation is solved using a purely non-Abelian Ansatz for the quark-gluon vertex, which displays a dependence on the ghost dressing function and the scalar component of quark-ghost scattering kernel. For the former quantity we use quenched lattice results, while for the latter we derive its own integral equation at the one-loop-dressed approximation. This latter quantity is then coupled to the system of equations governing the two Dirac structures of the quark propagator. It turns out that when a current quark mass of 5 MeV is introduced, the constituent quark mass generated from the gap equation is of the order of 310 MeV. In addition, the pion decay constant computed from the resulting quark propagator is in good agreement with the physical value.
Repassivation and crack propagation of alloy 600 in 288 C water
Angeliu, T.M.; Andresen, P.L.; Pollick, M.L.
1996-10-01
The polarization and repassivation behavior of Alloy 600 was evaluated at 288 C in 0.1 M boric acid titrated with NaOH (pH{sub 25 C} 7.9) as a function of dissolved hydrogen (0 to 48 scc/Kg) and zinc (O and 60 wppb). Potentiodynamic scans measured the polarization behavior, while a combination of drop-weight straining and cathodic reduction/potential pulse techniques measured the repassivation behavior. Potentiodynamic scans revealed larger current densities, especially over the range of {minus}800 to {minus}550 mV{sub SHE}, with the addition of H{sub 2}. At a H{sub 2} content of 0 scc/Kg, dissolved zinc at 60 wppb reduced the current density at {approximately}{minus}650mV{sub SHE}. However, 60 wppb zinc did not effect the repassivation kinetics at 0 and 48 scc/Kg of H{sub 2}. Repassivation kinetics experiments conducted slightly above the open circuit potential, E{sub OC}, revealed a monotonic decrease in the oxidation current transient with increasing H{sub 2} at short times. Reduced current transients at higher levels of H{sub 2} agree with the Pourbaix diagram that shows Ni metal becoming more stable at potentials associated with the addition of H{sub 2}. The residual oxidation transients exhibited with 18 and 48 scc/Kg of H{sub 2} are most likely associated with H{sub 2} oxidation on the oxide surface, which is unaffected by time. The repassivation results are consistent with recent observations of a decrease in the crack growth rate of Alloy 600 in water at 288 C as the H{sub 2} content increased from 0 to 18 scc/Kg, indicating that slip oxidation is the mechanism for crack advance under the conditions investigated in this study.
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.
Molecular dynamics simulation of the burning front propagation in PETN
NASA Astrophysics Data System (ADS)
Yanilkin, A. V.; Sergeev, O. V.
2014-05-01
One of the models of detonation development in condensed explosives under shock loading is the concept of "hot spots." According to this model, the reaction initially starts at various defects and inhomogeneities, where energy is localized during shock wave propagation. In such a region the reaction may start and the heat flux sufficient for the ignition of the adjacent layers of matter may be formed. If the reaction propagates fast enough, the merging of the burning fronts from several hot spots may lead to detonation. So there is an interest in determining the burning propagation rate from the hot spot in various conditions. In this work we investigate the propagation of plane burning front from initially heated layer in PETN single crystal using molecular dynamics method with the reactive force field (ReaxFF). The burning rate depends on the direction in crystal. The kinetics of chemical transformations is considered. The dependence of the burning front propagation rate along [100] direction on the external pressure in the pressure range from normal to 30 GPa is calculated, it is shown that it grows linearly in the considered range from 50 m/s to 320 m/s. The results are compared with the data from experiments and quantum chemical calculations.
Molecular dynamics simulation of the burning front propagation in PETN
NASA Astrophysics Data System (ADS)
Yanilkin, Alexey; Sergeev, Oleg; Computational materials science Team
2013-06-01
One of the models of detonation development in condensed explosives under shock loading is the concept of ``hot spots.'' According to this model, the reaction initially starts at various defects and inhomogeneities, where energy is localized during shock wave propagation. In such a region the exothermic reaction may start with heat yield sufficient for the ignition of the adjacent layers of matter. If the reaction propagates fast enough, the merging of the burning fronts from several hot spots may lead to detonation. So there is an interest in determining the burning propagation rate from the hot spot in various conditions. In this work we investigate the propagation of plane burning front from initially heated layer in PETN single crystal using molecular dynamics method with reaction force field (ReaxFF). It is shown that the burning rate depends on the direction in crystal. The kinetics of chemical transformations is considered, main reaction paths are determined. The dependence of the burning front propagation rate on the external pressure in the pressure range of normal to 30 GPa is calculated, it is shown that it grows linearly in the considered range from 50 m/s to 320 m/s. The results are compared with the data from experiments and quantum chemical calculations.
Dynamics in Layer Models of Solid Flame Propagation
NASA Technical Reports Server (NTRS)
Aldushin, A. P.; Bayliss, A.; Matkowsky, B. J.; Gokoglu, S. (Technical Monitor)
2000-01-01
Self-propagating high-temperature synthesis (SHS) is a process in which combustion waves, e.g., "solid flames", which are considered here, are employed to synthesize desired materials. Like many other systems, SHS is a pattern forming system. The problem of describing experimentally observed patterns and of predicting new, as yet unobserved, patterns continues to attract the attention of scientists and mathematicians due to the fundamental significance of the phenomena in combustion in particular, and in nonlinear science in general. Here, we analyze the dynamics of solid flame propagation in a 2D region by considering the region to be composed of parallel, identical layers aligned along the direction of propagation and having thermal contact. Each layer is then described by wave propagation in 1D, with the transverse Laplacian replaced by a term describing heat exchange between neighboring layers. This configuration is the simplest model of a 2D system because it accounts, in a simple way, for the principal feature of the problem, i.e., heat exchange between neighbors in the transverse direction. For simplicity, we describe the situation for two layers. Because the layers are identical, uniformly propagating waves in each layer must be identical, independent of the heat exchange rate alpha. When the Zeldovich number Z exceeds a critical value Z(sub c), which depends on alpha, uniformly propagating waves become unstable. The stability diagram for the two coupled layers reproduces that for the full 2D problem after appropriate identification of parameters in the two problems. Depending on parameter values, we determine three different steady-state dynamical behaviors (though additional behaviors are also expected to occur). The three behaviors are: (i) waves in each layer which pulsate in phase as they propagate, so that together they form a single pulsating propagating wave; (ii) the waves in each layer are no longer identical, and antiphase pulsations occur, with
NASA Technical Reports Server (NTRS)
Hardrath, H. F.
1974-01-01
Fracture mechanics is a rapidly emerging discipline for assessing the residual strength of structures containing flaws due to fatigue, corrosion or accidental damage and for anticipating the rate of which such flaws will propagate if not repaired. The discipline is also applicable in the design of structures with improved resistance to such flaws. The present state of the design art is reviewed using this technology to choose materials, to configure safe and efficient structures, to specify inspection procedures, to predict lives of flawed structures and to develop reliability of current and future airframes.
Dynamics and Predictability of Deep Propagating Atmospheric Gravity Waves
NASA Astrophysics Data System (ADS)
Doyle, J.; Fritts, D. C.; Smith, R.; Eckermann, S. D.
2012-12-01
An overview will be provided of the first field campaign that attempts to follow deeply propagating gravity waves (GWs) from their tropospheric sources to their mesospheric breakdown. The DEEP propagating gravity WAVE experiment over New Zealand (DEEPWAVE-NZ) is a comprehensive, airborne and ground-based measurement and modeling program focused on providing a new understanding of GW dynamics and impacts from the troposphere through the mesosphere and lower thermosphere (MLT). This program will employ the new NSF/NCAR GV (NGV) research aircraft from a base in New Zealand in a 6-week field measurement campaign in June-July 2014. The NGV will be equipped with new lidar and airglow instruments for the DEEPWAVE measurement program, providing temperatures and vertical winds spanning altitudes from immediately above the NGV flight altitude (~13 km) to ~100 km. The region near New Zealand is chosen since all the relevant GW sources occur strongly here, and upper-level winds in austral winter permit GWs to propagate to very high altitudes. Given large-amplitude GWs that propagate routinely into the MLT, the New Zealand region offers an ideal natural laboratory for studying these important GW dynamics and effects impacting weather and climate over a much deeper atmospheric layer than previous campaigns have attempted (0-100 km altitude). The logistics of making measurements in the vicinity of New Zealand are potentially easier than from the Andes and Drake Passage region. A suite of GW-focused modeling and predictability tools will be used to guide NGV flight planning to GW events of greatest scientific significance. These models will also drive scientific interpretation of the GW measurements, together providing answers to the key science questions posed by DEEPWAVE about GW dynamics, morphology, predictability and impacts from 0-100 km. Preliminary results will be presented from high-resolution and adjoint models applied over areas featuring deep wave propagation. The high
Cherepanov, Genady P
2015-03-28
By way of introduction, the general invariant integral (GI) based on the energy conservation law is presented, with mention of cosmic, gravitational, mass, elastic, thermal and electromagnetic energy of matter application to demonstrate the approach, including Coulomb's Law generalized for moving electric charges, Newton's Law generalized for coupled gravitational/cosmic field, the new Archimedes' Law accounting for gravitational and surface energy, and others. Then using this approach the temperature track behind a moving crack is found, and the coupling of elastic and thermal energies is set up in fracturing. For porous materials saturated with a fluid or gas, the notion of binary continuum is used to introduce the corresponding GIs. As applied to the horizontal drilling and fracturing of boreholes, the field of pressure and flow rate as well as the fluid output from both a horizontal borehole and a fracture are derived in the fluid extraction regime. The theory of fracking in shale gas reservoirs is suggested for three basic regimes of the drill mud permeation, with calculating the shape and volume of the local region of the multiply fractured rock in terms of the pressures of rock, drill mud and shale gas.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Gyekenyesi, John P.
2002-01-01
The life prediction analysis based on an exponential crack velocity formulation was examined using a variety of experimental data on glass and advanced structural ceramics in constant stress-rate ("dynamic fatigue") and preload testing at ambient and elevated temperatures. The data fit to the strength versus In (stress rate) relation was found to be very reasonable for most of the materials. It was also found that preloading technique was equally applicable for the case of slow crack growth (SCG) parameter n > 30. The major limitation in the exponential crack velocity formulation, however, was that an inert strength of a material must be known priori to evaluate the important SCG parameter n, a significant drawback as compared to the conventional power-law crack velocity formulation.
Dislocation emission from a three-dimensional crack -- A large-scale molecular dynamics study
Zhou, S.J.; Beazley, D.M.; Lomdahl, P.S.; Voter, A.F.; Holian, B.L.
1996-12-31
A series of massively parallel molecular dynamics simulations with up to 35 million atoms is performed to investigate dislocation emission from a three-dimensional crack. The authors observe dislocation loops emitted from the crack front--the first time this has been seen in computer simulations. The sequence of dislocation emission in the process of crack blunting process strongly depends on the crystallographic orientation of the crack front and differs strikingly from anything previously conjectured. This finding is essential to establish a precise dislocation emission criterion (i.e., intrinsic ductility criterion). They also find that boundary conditions and interatomic force laws have a significant effect on jogging or blunting dislocation emission modes.
Dynamical states, possibilities and propagation of stress signal
Malik, Md. Zubbair; Ali, Shahnawaz; Singh, Soibam Shyamchand; Ishrat, Romana; Singh, R. K. Brojen
2017-01-01
The stress driven dynamics of Notch-Wnt-p53 cross-talk is subjected to a few possible dynamical states governed by simple fractal rules, and allowed to decide its own fate by choosing one of these states which are contributed from long range correlation with varied fluctuations due to active molecular interaction. The topological properties of the networks corresponding to these dynamical states have hierarchical features with assortive structure. The stress signal driven by nutlin and modulated by mediator GSK3 acts as anti-apoptotic signal in this system, whereas, the stress signal driven by Axin and modulated by GSK3 behaves as anti-apoptotic for a certain range of Axin and GSK3 interaction, and beyond which the signal acts as favor-apoptotic signal. However, this stress system prefers to stay in an active dynamical state whose counterpart complex network is closest to hierarchical topology with exhibited roles of few interacting hubs. During the propagation of stress signal, the system allows the propagator pathway to inherit all possible properties of the state to the receiver pathway/pathways with slight modifications, indicating efficient information processing and democratic sharing of responsibilities in the system via cross-talk. The increase in the number of cross-talk pathways in the system favors to establish self-organization. PMID:28106087
NASA Astrophysics Data System (ADS)
Riahi, Mohammad; Gholami, Pouya
2016-04-01
Ultrasonic guided waves have rapidly become an effective device in the field of NDT in recent years. Main reason for this is the ability of transmission from one point on the pipe to travel a long distance along it length. These waves are typically used in relatively low frequencies, and as a result, die out in longer periods of time. In this study, by designing and building a system to generate the needed signal for the stimulation of guided waves through using a piezoelectric crystal, these waves were generated and transmitted along a pipe. After propagation, waves were relieved by an ultrasonic probe and were saved by a digital oscilloscope. The received waves were then processed and filtered to eliminate noise and compared with each other. In order to compare the results and study the effective parameters of inspecting ability by these waves, the receiving probe was moved along the length of the pipe and through clanging the number of entering sinusoidal pulses along with altering the frequency signal; the data was recorded in the highest amplitude frequency. By adjusting the frequency within 30-40 KHz range, it would be possible to receive signals at any point in the experiment. Although the received signals weaken by further distance, however; through increase in the number of pulses of inlet signals, the guided waves better stimulate and become stronger at the outlet signal.
NASA Technical Reports Server (NTRS)
Kalluri, Sreeramesh; Telesman, Jack
1988-01-01
Electrical potential methods have been used in the past primarily to monitor crack length in long crack specimens subjected to fatigue loading. An attempt was made to develop test procedures for monitoring the fatigue crack initiation and the growth of short fatigue cracks in a turbine disk alloy with the electrical potential drop technique (EPDT). In addition, the EPDT was also applied to monitor the fatigue crack growth in long crack specimens of the same alloy. The resolution of the EPDT for different specimen geometries was determined. Factors influencing the EPDT are identified and the applicability of EPDT in implementing damage tolerant design concepts for turbine disk materials is discussed. The experimental procedure adopted and the results obtained is discussed. No substantial differences were observed between the fatigue crack growth data of short and long crack specimens.
Molecular dynamics study of sound propagation in a gas
NASA Astrophysics Data System (ADS)
Yano, Takeru
2012-09-01
Large scale molecular dynamics simulations have been performed to clarify the nonlinear and non-equilibrium processes of large-amplitude and high-frequency progressive sound waves in a gas. The statistical errors in calculating macroscopic variables are reduced by putting more than 0.3 million molecules in a simulation box with the length of several micrometers in the direction of wave propagation. The one-dimensional sound wave is generated by a harmonic oscillation of sound source with the Lennard-Jones intermolecular potential, which is the same as that of gas molecules. As a result, we find that the large-amplitude and high-frequency sound propagates with strong attenuation, in some cases, exhibiting a stream-like profile accompanied with mass, momentum, and energy transports. This leads to a completely different picture and a different dispersion relation from a classical theory of high-frequency sound based on the linear standing wave analysis.
Chordwise propagation of dynamic stall cells on an oscillating airfoil
NASA Technical Reports Server (NTRS)
Carta, F. O.
1975-01-01
The dynamic stall phenomenon was examined in detail by analyzing a set of unsteady pressure data obtained on an airfoil oscillating in pitch. These data were for sinusoidal oscillations which penetrated the stall region in varying degrees, and here the effort was concentrated on the chordwise propagation of pressure waves associated with the dynamic stall. It was found that this phenomenon could be quantified in terms of a pressure wave velocity which is consistently much less than free-stream velocity, and which varies directly with frequency. It was also found that even when the stall region has been deeply penetrated and a substantial dynamic stall occurs during the downstroke, stall recovery near minimum incidence will occur, followed by a potential flow behavior up to stall inception.
The effects of solidification on sill propagation dynamics and morphology
NASA Astrophysics Data System (ADS)
Chanceaux, L.; Menand, T.
2016-05-01
Sills are an integral part of the formation and development of larger plutons and magma reservoirs. Thus sills are essential for both the transport and the storage of magma in the Earth's crust. However, although cooling and solidification are central to magmatism, their effects on sills have been so far poorly studied. Here, the effects of solidification on sill propagation dynamics and morphology are studied by means of analogue laboratory experiments. Hot fluid vegetable oil (magma analogue), that solidifies during its propagation, is injected as a sill in a colder layered gelatine solid (elastic host rock analogue). The injection flux and temperature are maintained constant during an experiment and systematically varied between each experiment, in order to vary and quantify the amount of solidification between each experiments. The oil is injected directly at the interface between the two gelatine layers. When solidification effects are small (high injection temperatures and fluxes), the propagation is continuous and the sill has a regular and smooth surface. Inversely, when solidification effects are important (low injection temperatures and fluxes), sill propagation is discontinuous and occurs by steps of surface-area creation interspersed with periods of momentary arrest. The morphology of these sills displays folds, ropy structures on their surface, and lobes with imprints of the leading fronts that correspond to each step of area creation. These experiments show that for a given, constant injected volume, as solidification effects increase, the area of the sills decreases, their thickness increases, and the number of propagation steps increases. These results have various geological and geophysical implications. The morphology of sills, such as lobate structures (interpretation of 3D seismic studies in sedimentary basin) and ropy flow structures (field observations) can be related to solidification during emplacement. Moreover, a non-continuous morphology
Dynamical propagation of nanosecond pulses in Naphthalocyanines and Phthalocyanines
NASA Astrophysics Data System (ADS)
Miao, Quan; Liang, Min; Liu, Qixin; Wang, Jing-Jing; Sun, Erping; Xu, Yan
2016-11-01
Dynamical propagation and optical limiting of nanosecond pulses in peripherally substituted Naphthalocyanines (Npcs) and Phthalocyanines (Pcs) with central metals gallium and indium were theoretically studied using paraxial field and rate equations. The results demonstrated that both Npcs and Pcs have good optical limiting performances, and Npc with heavier central mental indium shows better optical limiting properities due to the stronger reverse saturable absorption, which is mainly strengthened by the larger one-photo absorption cross section of excited state and the faster intersystem crossing rate.
Radial propagation in population dynamics with density-dependent diffusion
NASA Astrophysics Data System (ADS)
Ngamsaad, Waipot
2014-01-01
Population dynamics that evolve in a radial symmetric geometry are investigated. The nonlinear reaction-diffusion model, which depends on population density, is employed as the governing equation for this system. The approximate analytical solution to this equation is found. It shows that the population density evolves from the initial state and propagates in a traveling-wave-like manner for a long-time scale. If the distance is insufficiently long, the curvature has an ineluctable influence on the density profile and front speed. In comparison, the analytical solution is in agreement with the numerical solution.
Tsuruta, T.; Okamoto, S.; Kitera, T.; Takamatsu, H.; Matsunaga, T.
1995-12-31
Two major research objectives, IGA/SCC crack propagation rate measurement using predefected Alloy 600 tubes and the evaluation of crevice environments, were tested in side stream model boilers at Ohi Unit 1. The results of IGA/SCC propagation rate measurements using several types of predefected tubes were previously reported at the 6th symposium. The results of crevice environment evaluation by analyzing the directly sampled concentrated crevice solutions were also previously reported at the 5th symposium. These measurements have been continued using side stream model boilers at Ohi unit 1, and interim results are presented in this report.
Dynamic propagation channel characterization and modeling for human body communication.
Nie, Zedong; Ma, Jingjing; Li, Zhicheng; Chen, Hong; Wang, Lei
2012-12-18
This paper presents the first characterization and modeling of dynamic propagation channels for human body communication (HBC). In-situ experiments were performed using customized transceivers in an anechoic chamber. Three HBC propagation channels, i.e., from right leg to left leg, from right hand to left hand and from right hand to left leg, were investigated under thirty-three motion scenarios. Snapshots of data (2,800,000) were acquired from five volunteers. Various path gains caused by different locations and movements were quantified and the statistical distributions were estimated. In general, for a given reference threshold è = -10 dB, the maximum average level crossing rate of the HBC was approximately 1.99 Hz, the maximum average fade time was 59.4 ms, and the percentage of bad channel duration time was less than 4.16%. The HBC exhibited a fade depth of -4 dB at 90% complementary cumulative probability. The statistical parameters were observed to be centered for each propagation channel. Subsequently a Fritchman model was implemented to estimate the burst characteristics of the on-body fading. It was concluded that the HBC is motion-insensitive, which is sufficient for reliable communication link during motions, and therefore it has great potential for body sensor/area networks.
Investigation of Dynamic Crack Coalescence Using a Gypsum-Like 3D Printing Material
NASA Astrophysics Data System (ADS)
Jiang, Chao; Zhao, Gao-Feng; Zhu, Jianbo; Zhao, Yi-Xin; Shen, Luming
2016-10-01
Dynamic crack coalescence attracts great attention in rock mechanics. However, specimen preparation in experimental study is a time-consuming and difficult procedure. In this work, a gypsum-like material by powder bed and inkjet 3D printing technique was applied to produce specimens with preset cracks for split Hopkinson pressure bar (SHPB) test. From micro X-ray CT test, it was found that the 3D printing technique could successfully prepare specimens that contain preset cracks with width of 0.2 mm. Basic mechanical properties of the 3D printing material, i.e., the elastic modulus, the Poisson's ratio, the density, the compressive strength, the indirect tensile strength, and the fracture toughness, were obtained and reported. Unlike 3D printed specimens using polylactic acid, these gypsum-like specimens can produce failure patterns much closer to those observed in classical rock mechanical tests. Finally, the dynamic crack coalescence of the 3D printed specimens with preset cracks were captured using a high-speed camera during SHPB tests. Failure patterns of these 3D printed specimens are similar to the specimens made by Portland cement concrete. Our results indicate that sample preparation by 3D printing is highly competitive due to its quickness in prototyping, precision and flexibility on the geometry, and high material homogeneity.
Dynamic response of cracked rotor-bearing system under time-dependent base movements
NASA Astrophysics Data System (ADS)
Han, Qinkai; Chu, Fulei
2013-12-01
Dynamic response of cracked rotor-bearing system under time-dependent base movements is studied in this paper. Three base angular motions, including the rolling, pitching and yawing motions, are assumed to be sinusoidal perturbations superimposed upon constant terms. Both the open and breathing transverse cracks are considered in the analysis. The finite element model is established for the base excited rotor-bearing system with open or breathing cracks. Considering the time-varying base movements and transverse cracks, the second-order differential equations of the system will not only have time-periodic gyroscopic and stiffness coefficients, but also the multi-frequency external excitations. An improved harmonic balance method is introduced to obtain the steady-state response of the system under both base and unbalance excitations. The response spectra, orbits of shaft center and frequency response characteristics, are analyzed accordingly. The effects of various base angular motions, frequency and amplitude of base excitations, and crack depths on the system dynamic behaviors are considered in the discussions.
Challenges and limitations in studying the shrink-swell and crack dynamics of vertisol soils
Technology Transfer Automated Retrieval System (TEKTRAN)
The need to study the shrink-swell and crack properties of vertic soils has long been recognized given their dynamics in time and space, which modifies the physical properties that impact water and air movement in the soil, flow of water into the subsoil and ground water, and generally alter the hyd...
Dynamic behavior of anchors in cracked and uncracked concrete: A progress report
Rodriguez, M.; Yong-gang Zhang; Lotze, D.
1995-04-01
In early 1993, the US Nuclear Regulatory Commission began a research program at The University of Texas at Austin, dealing with the dynamic behavior of anchors in cracked and uncracked concrete. In this paper, the progress of that research program is reviewed. The test program is summarized, the work performed to date is reviewed, with emphasis on the dynamic and static behavior of single tensile anchors in uncracked concrete. General conclusions from that work are discussed, and future plans are presented.
Fatigue-Crack Propagation and Residual Static Strength of PH 15-7 Mo (TH 1050) Stainless Steel
1965-12-01
section prior to the application of load.) A method of quantitatively predicting fatigue-crack growth rates in aluminum alloys was developed in...section stress. A method of calculating stress-concentration factors based on Neuber’s analysis of stresses around sharp notches (ref. 2) was...developed in the crack-growth analysis. A simple engineering method for predicting the strength of cracked aluminum parts under static loading was
S-N Fatigue and Fatigue Crack Propagation Behaviors of X80 Steel at Room and Low Temperatures
NASA Astrophysics Data System (ADS)
Jung, Dae-Ho; Kwon, Jae-Ki; Woo, Nam-Sub; Kim, Young-Ju; Goto, Masahiro; Kim, Sangshik
2014-02-01
In the present study, the S-N fatigue and the fatigue crack propagation (FCP) behaviors of American Petroleum Institute X80 steel were examined in the different locations of the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) at 298 K, 223 K, and 193 K (25 °C, -50 °C, and -80 °C). The resistance to S-N fatigue of X80 BM specimen increased greatly with decreasing temperature from 298 K to 193 K (25 °C to -80 °C) and showed a strong dependency on the flow strength (½(yield strength + tensile strength)). The FCP rates of X80 BM specimen were substantially reduced with decreasing temperature from 298 K to 223 K (25 °C to -50 °C) over the entire ∆ K regime, while further reduction in FCP rates was not significant with temperature from 223 K to 193 K (-50 °C to -80 °C). The FCP rates of the X80 BM and the WM specimens were comparable with each other, while the HAZ specimen showed slightly better FCP resistance than the BM and the WM specimens over the entire ∆K regime at 298 K (25 °C). Despite the varying microstructural characteristics of each weld location, the residual stress appeared to be a controlling factor to determine the FCP behavior. The FCP behaviors of high strength X80 steel were discussed based on the microstructural and the fractographic observations.
NASA Astrophysics Data System (ADS)
Morishita, Yoshihiro; Tsunoda, Katsuhiko; Urayama, Kenji
2016-04-01
The crack growth dynamics of the carbon-black (CB) filled elastomers is studied experimentally and analyzed while focusing on both kinetics and crack tip profiles. The CB amounts are varied to change the mechanical properties of the elastomers. Static crack growth measurements simultaneously reveal the discontinuous-like transition of the crack growth rate v between the "slow mode" (v ≈10-5-10-3 m/s) and "fast mode" (v ≈10-1-102 m/s) in a narrow range of the input tearing energy Γ and the accompanying changes in the crack tip profiles from blunt to sharp shapes. The crack tip profiles are characterized by two specific parameters, i.e., the deviation δ from the parabolic profile and the opening displacement a in the loading direction. The analysis based on the linear and weakly nonlinear elasticity theories of fracture dynamics demonstrates that the Γ dependence of δ and a is simply classified into three groups depending on the mode (slow or fast) and the magnitudes of δ , independent of CB volume fractions. The theories well explain the results in the slow and fast modes with small magnitudes of δ , while they fail to describe the data in the fast mode with large magnitudes of δ , where the contributions of the strong nonlinearity and/or energy dissipation become significant. The correlation between a power-law relationship Γ ˜vα observed in the fast mode and the linear viscoelasticity spectrum is also discussed. The correlation in elastomers with low CB volume fractions is quantitatively explained by the theory of Persson and Brener [Phys. Rev. E 71, 036123 (2005), 10.1103/PhysRevE.71.036123], whereas the deviation from the theory becomes appreciable for elastomers with higher CB volume fractions which exhibit strong nonlinear viscoelasticity.
Investigation of Helicopter Longeron Cracks
NASA Technical Reports Server (NTRS)
Newman, John A.; Baughman, James; Wallace, Terryl A.
2009-01-01
Four cracked longerons, containing a total of eight cracks, were provided for study. Cracked regions were cut from the longerons. Load was applied to open the cracks, enabling crack surface examination. Examination revealed that crack propagation was driven by fatigue loading in all eight cases. Fatigue crack initiation appears to have occurred on the top edge of the longerons near geometric changes that affect component bending stiffness. Additionally, metallurgical analysis has revealed a local depletion in alloying elements in the crack initiation regions that may be a contributing factor. Fatigue crack propagation appeared to be initially driven by opening-mode loading, but at a crack length of approximately 0.5 inches (12.7 mm), there is evidence of mixed-mode crack loading. For the longest cracks studied, shear-mode displacements destroyed crack-surface features of interest over significant portions of the crack surfaces.
Low frequency piezoresonance defined dynamic control of terahertz wave propagation
Dutta, Moumita; Betal, Soutik; Peralta, Xomalin G.; Bhalla, Amar S.; Guo, Ruyan
2016-01-01
Phase modulators are one of the key components of many applications in electromagnetic and opto-electric wave propagations. Phase-shifters play an integral role in communications, imaging and in coherent material excitations. In order to realize the terahertz (THz) electromagnetic spectrum as a fully-functional bandwidth, the development of a family of efficient THz phase modulators is needed. Although there have been quite a few attempts to implement THz phase modulators based on quantum-well structures, liquid crystals, or meta-materials, significantly improved sensitivity and dynamic control for phase modulation, as we believe can be enabled by piezoelectric-resonance devices, is yet to be investigated. In this article we provide an experimental demonstration of phase modulation of THz beam by operating a ferroelectric single crystal LiNbO3 film device at the piezo-resonance. The piezo-resonance, excited by an external a.c. electric field, develops a coupling between electromagnetic and lattice-wave and this coupling governs the wave propagation of the incident THz beam by modulating its phase transfer function. We report the understanding developed in this work can facilitate the design and fabrication of a family of resonance-defined highly sensitive and extremely low energy sub-millimeter wave sensors and modulators. PMID:27901070
Low frequency piezoresonance defined dynamic control of terahertz wave propagation
NASA Astrophysics Data System (ADS)
Dutta, Moumita; Betal, Soutik; Peralta, Xomalin G.; Bhalla, Amar S.; Guo, Ruyan
2016-11-01
Phase modulators are one of the key components of many applications in electromagnetic and opto-electric wave propagations. Phase-shifters play an integral role in communications, imaging and in coherent material excitations. In order to realize the terahertz (THz) electromagnetic spectrum as a fully-functional bandwidth, the development of a family of efficient THz phase modulators is needed. Although there have been quite a few attempts to implement THz phase modulators based on quantum-well structures, liquid crystals, or meta-materials, significantly improved sensitivity and dynamic control for phase modulation, as we believe can be enabled by piezoelectric-resonance devices, is yet to be investigated. In this article we provide an experimental demonstration of phase modulation of THz beam by operating a ferroelectric single crystal LiNbO3 film device at the piezo-resonance. The piezo-resonance, excited by an external a.c. electric field, develops a coupling between electromagnetic and lattice-wave and this coupling governs the wave propagation of the incident THz beam by modulating its phase transfer function. We report the understanding developed in this work can facilitate the design and fabrication of a family of resonance-defined highly sensitive and extremely low energy sub-millimeter wave sensors and modulators.
Comparison of localised spalling and crack damage from dynamic modelling of spur gear vibrations
NASA Astrophysics Data System (ADS)
Jia, Shengxiang; Howard, Ian
2006-02-01
This paper presents a 26 degree of freedom gear dynamic model of three shafts and two pairs of spur gears in mesh for comparison of localised tooth spalling and damage. This paper details how tooth spalling and cracks can be included in the model by using the combined torsional mesh stiffness of the gears. The FEA models developed for calculation of the torsional stiffness and tooth load sharing ratio of the gears in mesh with the spalling and crack damage are also described. The dynamic simulation results of vibration from the gearbox were obtained by using Matlab and Simulink models, which were developed from the equations of motion. The simulation results were found to be consistent with results from previously published mathematical analysis and experimental investigations. The difference and comparison between the vibration signals with the tooth crack and spalling damage are discussed by investigating some of the common diagnostic functions and changes to the frequency spectra results. The result of this paper indicates that the amplitude and phase modulation of the coherent time synchronous vibration signal average can be effective in indicating the difference between localised tooth spalling and crack damage.
Application of a novel finite difference method to dynamic crack problems
NASA Technical Reports Server (NTRS)
Chen, Y. M.; Wilkins, M. L.
1976-01-01
A versatile finite difference method (HEMP and HEMP 3D computer programs) was developed originally for solving dynamic problems in continuum mechanics. It was extended to analyze the stress field around cracks in a solid with finite geometry subjected to dynamic loads and to simulate numerically the dynamic fracture phenomena with success. This method is an explicit finite difference method applied to the Lagrangian formulation of the equations of continuum mechanics in two and three space dimensions and time. The calculational grid moves with the material and in this way it gives a more detailed description of the physics of the problem than the Eulerian formulation.
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.
Modeling Impact of Cross Drainage Works on Flood Propagation Dynamics
NASA Astrophysics Data System (ADS)
Haldar, R.; Khosa, R.; Gupta, S.
2013-12-01
River bed and flood plain geometries are formed as a response to centuries of natural erosional and depositional processes. Any human intervention made in the course of a river has the potential to create a disturbance in its flow pattern. The present study considers the possible consequences of changes made in the flood plain of a river and is an attempt to show how investigation and modeling prior to execution of water resources projects can be largely beneficial in desisting from unintended disasters. The Ghaggar River is a non- perennial stream that has its origin in the Shivalik Hills of Himachal Pradesh, India. It passes through the two states of Punjab and Haryana into Rajasthan. A flood investigation and modeling study was done for the Ghaggar River where it was attempted to simulate the change in the pattern of the flow in the main channel, and the propagation of excess waters in the flood plains, as a result of impediment created by the embankments of the Hansi-Butana Link canal, constructed recently during 2007-09. The study used daily rainfall data for the 2009 and 2010 monsoon seasons which was obtained from the India Meteorological Department. The modeling was done with the help of the MIKE SHE hydrologic model coupled with the MIKE 11 hydrodynamic model in order to estimate the peak river stage, the time to peak, and the recession time that was needed for the flood plains to get back to their normal dry state. It was found that the maximum impact of the canal embankment was felt on the flood recession time. The importance of the study was felt, when in the 2010 monsoons, the canal embankment that was acting as an obstruction to the speeding flood wave, cracked at places and fragments of the canal body were washed away by the flood water. Large areas of cultivated and inhabited land became inundated and stayed under water for weeks, when the volume of water captured by the canal embankments gradually drained through the various outlets made in the
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Salem, Jonathan A.; Nemeth, Noel; Gyekenyesi, John P.
1994-01-01
The slow crack growth parameters of a hot-pressed silicon nitride were determined at 1200 and 1300 C in air by statically, dynamically and cyclicly loading bend specimens. The fatigue parameters were estimated using the recently developed CARES/Life computer code. Good agreement exists between the flexural results. However, fatigue susceptibility under static uniaxial tensile loading, reported elsewhere, was greater than in flexure. Cyclic flexural loading resulted in the lowest apparent flexural fatigue susceptibility.
NASA Astrophysics Data System (ADS)
Joglekar, D. M.; Mitra, M.
2016-08-01
An analytical-numerical method, based on the use of wavelet spectral finite elements (WSFE), is presented for studying the nonlinear interaction of flexural waves with a breathing crack present in a slender beam. The cracked beam is discretized using wavelet spectral finite elements which use compactly supported Daubechies scaling functions for approximating the temporal dependence of the transverse displacement. Rotational spring is used to model the open crack condition, and behavior of the beam in closed-crack condition is assumed to be similar to that of an intact beam. An intermittent switching between the open- and closed-crack conditions simulates crack-breathing, leading to a set of nonlinear equations which is solved using an iterative method. Results of the proposed method are compared with those obtained using the Fourier spectral finite element (FSFE) and 1D finite element (FE) methods, which show a close agreement. Existence of the higher-order harmonic components, indicative of the crack-induced bilinearity, is confirmed in the frequency domain response. Moreover, the time domain analysis reveals separation of harmonics resulting from the dispersive nature of the waveguide, which is further used for localizing the damage. A parametric study is presented to bring out the influence of crack-severity and -location on the extent of harmonic separation and on the relative strength of higher order harmonic. In addition to elaborating the use of WSFE in addressing the nonlinear wave-damage interaction, results of the present investigation can be potentially useful in devising strategies for an inverse analysis.
1984-10-01
Baseline Effective T (0) 79 5.3.5 Accounting for Bolt Eoad Transfer 80 5.3.5.1 Superposition Model for 80 Estimating 1-ma x at Edge of Hole 5.3.5.2...Intensity Factor for Loaded 97 Bolt Holes vi ’ . . ." , .- ," " " " " " " q "". r " " " " " " . NADC-83126-60-VOl. III TABLE OF CONTENTS (CONTD...Notch Stress Due 87 to Maximum Linear Stress at Edge of Hole Based on Stress-Strain Relationship 18 Essential Elements of the CF Crack 89 Propagation
Molecular dynamics approach to dissipative relativistic hydrodynamics: Propagation of fluctuations
NASA Astrophysics Data System (ADS)
Shahsavar, Leila; Ghodrat, Malihe; Montakhab, Afshin
2016-12-01
Relativistic generalization of hydrodynamic theory has attracted much attention from a theoretical point of view. However, it has many important practical applications in high energy as well as astrophysical contexts. Despite various attempts to formulate relativistic hydrodynamics, no definitive consensus has been achieved. In this work, we propose to test the predictions of four types of first-order hydrodynamic theories for nonperfect fluids in the light of numerically exact molecular dynamics simulations of a fully relativistic particle system in the low density regime. In this regard, we study the propagation of density, velocity, and heat fluctuations in a wide range of temperatures using extensive simulations and compare them to the corresponding analytic expressions we obtain for each of the proposed theories. As expected, in the low temperature classical regime all theories give the same results, consistent with the numerics. In the high temperature extremely relativistic regime, not all considered theories are distinguishable from one another. However, in the intermediate regime, a meaningful distinction exists in the predictions of various theories considered here. We find that the predictions of the recent formulation due to Tsumura, Kunihiro, and Ohnishi are more consistent with our numerical results than the traditional theories: the Meixner, modified Eckart, and modified Marle-Stewart theories.
John, R.; Lackey, A.F.; Ashbaugh, N.E.
1996-09-15
The primary weakness of a component made of unidirectionally reinforced SCS-6/Timetal{reg_sign}21S is its susceptibility to transverse loads. The strength of the component in the transverse direction is significantly lower than that in the longitudinal direction under monotonic, sustained and fatigue loading conditions. The previous investigations did not investigate the effect of applied stress ratio on the crack growth behavior of the composite. This paper describes extensive characterization of the fatigue crack growth parallel to fibers in unidirectional SCS-6/Timetal{reg_sign}21S using two geometries, three applied stress ratios and closure measurements.
NASA Astrophysics Data System (ADS)
Shiozawa, Kazuaki; Morii, Yuuichi; Nishino, Seiichi
In order to study the subsurface crack initiation and propagation mechanism of high strength steel under a very high cycle fatigue regime, computational simulation with fracture surface topographic analysis (FRASTA) was carried out for subsurface fatigue crack initiated specimens of high speed tool steel (JIS SKH51) obtained from the rotating bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye region on the fracture surface, which is a feature on the fracture surface in super long fatigue. This so-called GBF (granular-bright-facet) was observed in detail by a scanning probe microscope and a three-dimensional SEM. The GBF area, in which a rich carbide distribution was detected by EPMA, revealed a very rough and granular morphology in comparison with the area inside the fish-eye. It was clearly simulated by FRASTA that multiple microcracks were initiated and dispersed by the decohesion of a spherical carbide from the matrix around a nonmetallic inclusion, and converged into the GBF area during the fatigue process. After the formation of the GBF area, interior cracks grew radially and a fish-eye pattern formed on the fracture surface.
Thermographic characterization of stress during crack growth
NASA Technical Reports Server (NTRS)
Cramer, K. E.; Dawicke, David S.; Welch, Christopher S.
1992-01-01
A full-field-thermographic technique for imaging stress patterns in dynamically loaded structures using general purpose IR imaging and image processing hardware is described. The inspection technique is based on the thermoelastic effect. A simple geometry is examined, and the experimentally determined values for the stress invariant are shown to be consistent with theoretical and numerical calculations. The application of full-field-thermographic measurement would ensure that the observed stress field has a common sampling period, thus allowing the observation of rapidly occurring stress anomalies such as the propagation of a fatigue crack. Fatigue crack propagation in two consecutive thermoelastic stress images from an aluminum sample is shown.
Fracture dynamics in implanted silicon
Massy, D.; Tardif, S.; Penot, J. D.; Ragani, J.; Rieutord, F.; Mazen, F.; Madeira, F.; Landru, D.; Kononchuk, O.
2015-08-31
Crack propagation in implanted silicon for thin layer transfer is experimentally studied. The crack propagation velocity as a function of split temperature is measured using a designed optical setup. Interferometric measurement of the gap opening is performed dynamically and shows an oscillatory crack “wake” with a typical wavelength in the centimetre range. The dynamics of this motion is modelled using beam elasticity and thermodynamics. The modelling demonstrates the key role of external atmospheric pressure during crack propagation. A quantification of the amount of gas trapped inside pre-existing microcracks and released during the fracture is made possible, with results consistent with previous studies.
Faunal Drivers of Soil Flux Dynamics via Alterations in Crack Structure
NASA Astrophysics Data System (ADS)
DeCarlo, Keita; Caylor, Kelly
2016-04-01
Organismal activity, in addition to its role in ecological feedbacks, has the potential to serve as instigators or enhancers of atmospheric and hydrologic processes via alterations in soil structural regimes. We investigated the biomechanical effect of faunal activity on soil carbon dynamics via changes in soil crack structure, focusing on three dryland soil systems: bioturbated, biocompacted and undisturbed soils. Carbon fluxes were characterized using a closed-system respiration chamber, with CO2 concentration differences measured using an infrared gas analyzer (IRGA). Results show that faunal influences play a divergent biomechanics role in bulk soil cracking: bioturbation induced by belowground fauna creates "surficial" (shallow, large, well-connected) networks relative to the "systematic" (deep, moderate, poorly connected) networks created by aboveground fauna. The latter also shows a "memory" of past wetting/drying events in the consolidated soil through a crack layering effect. These morphologies further drive differences in soil carbon flux: under dry conditions, bioturbated and control soils show a persistently high and low mean carbon flux, respectively, while biocompacted soils show a large diurnal trend, with daytime lows and nighttime highs comparable to the control and bioturbated soils, respectively. Overall fluxes under wet conditions are considerably higher, but also more variable, though higher mean fluxes are observed in the biocompacted and bioturbated soils. Our results suggest that the increased surface area in the bioturbated soils create enhanced but constant diffusive processes, whereas the increased thermal gradient in the biocompacted soils create novel convective processes that create high fluxes that are diurnal in nature.
The Role of Structural Dynamics and Testing in the Shuttle Flowliner Crack Investigation
NASA Technical Reports Server (NTRS)
Frady, Gregory P.
2005-01-01
During a normal inspection of the main propulsion system at Kennedy Space Center, small cracks were noticed near a slotted region of a gimbal joint flowliner located just upstream from one of the Space Shuttle Main Engines (SSME). These small cracks sparked an investigation of the entire Space Shuttle fleet main propulsion feedlines. The investigation was initiated to determine the cause of the small cracks and a repair method that would be needed to return the Shuttle fleet back to operation safely. The cracks were found to be initiated by structural resonance caused by flow fluctuations from the SSME low pressure fuel turbopump interacting with the flowliner. The pump induced backward traveling wakes that excited the liner and duct acoustics which also caused the liner to vibrate in complex mode shapes. The investigation involved an extensive effort by a team of engineers from the NASA civil servant and contractor workforce with the goal to characterize the root cause of the cracking behavior of the fuel side gimbal joint flowliners. In addition to working to identify the root cause, a parallel path was taken to characterize the material properties and fatigue capabilities of the liner material such that the life of the liners could be ascertained. As the characterization of the material and the most probable cause matured, the combination of the two with pump speed restrictions provided a means to return the Shuttle to flight in a safe manner. This paper traces the flowliner investigation results with respect to the structural dynamics analysis, component level testing and hot-fire flow testing on a static testbed. The paper will address the unique aspects of a very complex problem involving backflow from a high performance pump that has never been characterized nor understood to such detail. In addition, the paper will briefly address the flow phenomena that excited the liners, the unique structural dynamic modal characteristics and the variability of SSME
(Computational analysis of dynamic crack run-arrest phenomena in reactor pressure vessel steels)
Bass, B.R.
1987-04-28
The traveler visited Kernforschungszentrum Karlsruhe (KFK), Federal Republic of Germany (FRG) to confer with Dr. H.K. Stamm and other staff members to accomplish several objectives that relate to the computational analysis of dynamic crack run-arrest phenomena in reactor implementation which is the product of approximately three years of KFK work in the area of unified constitutive theories for rate-dependent materials. In exchange, the traveler would discuss with KFK a computer implementation of five elastic and inelastic candidate fracture parameters utilized in the ORNL ADINA computer program for elasto-dynamic and viscoplastic-dynamic fracture analysis. An essential feature of this exchange would be in-depth discussions with the KFK staff concerning implementation, application, and future development of these computational techniques.
The Dynamic Modelling of a Spur Gear in Mesh Including Friction and a Crack
NASA Astrophysics Data System (ADS)
Howard, Ian; Jia, Shengxiang; Wang, Jiande
2001-09-01
To improve the current generation of diagnostic techniques, many researchers are actively developing advanced dynamic models of gear case vibration to ascertain the effect of different types of gear train damage. This paper details a simplified gear dynamic model aimed at exploring the effect of friction on the resultant gear case vibration. The model incorporates the effect of variations in gear tooth torsional mesh stiffness, developed using finite element analysis, as the gears mesh together. The method of introducing the frictional force between teeth into the dynamic equations is given. The comparison between the results with friction and without friction was investigated using Matlab and Simulink models developed from the differential equations. The effects the single tooth crack has on the frequency spectrum and on the common diagnostic functions of the resulting gearbox component vibrations are also shown.
A study of crack in functionally graded material under dynamic loading
Nakagaki, Michihiko; Sasaki, Hiroyuki; Hagihara, Seiya
1995-11-01
The paper addresses a numerical treatment of a fracture occurring in the functionally graded materials (FGM) under a dynamic load. The FGM is composed of a titanium alloy as an inclusion and zirconia as the matrix, where a generation of microcracks is considered to occur in the ceramic phase of the high stressed area. A spherical grain model is used to describe thee elastic constitutive law for the FGM composite, in which the nonlinear effects due to the microcracking are accounted for. The most appropriate fracture parameter, T*, is used to assess the crack-tip severity in the highly inhomogeneous materials such as the present.
Elastic Relaxation of Fluid-Driven Cracks and the Resulting Backflow
NASA Astrophysics Data System (ADS)
Lai, Ching-Yao; Zheng, Zhong; Dressaire, Emilie; Ramon, Guy Z.; Huppert, Herbert E.; Stone, Howard A.
2016-12-01
Cracks filled with fluid propagation when the pressurized fluid is injected into the crack. Subsequently, when the fluid inlet is exposed to a lower pressure, the fluid flows backwards (backflow) and the crack closes due to the elastic relaxation of the solid. Here we study the dynamics of the crack closure during the backflow. We find that the crack radius remains constant and the fluid volume in the crack decreases with time in a power-law manner at late times. The balance between the viscous stresses in the fluid and elastic stresses in the fluid and the elastic stresses in the solid yields a scaling law that agrees with the experimental results for different fluid viscosities, Young's moduli of the solid, and initial radii of the cracks. Furthermore, we visualize the time-dependent crack shapes, and the convergence to a universal dimensionless shape demonstrates the self-similarity of the crack shapes during the backflow process.
NASA Astrophysics Data System (ADS)
Hu, Yu
Slow crack growth in PVC pipe was studied in order to develop a methodology for predicting long-term creep fracture from short-term tension-tension fatigue tests. In all cases, the crack propagated continuously through a crack-tip craze. In fatigue, the density of drawn craze fibrils gradually increased with decreased frequency and increased temperature. At the lowest frequency, 0.01 Hz, the fibril density in fatigue approached that in creep. The kinetics of fatigue and creep crack growth followed the conventional Paris law formulations with the same power 2.7, da/dt = AfDK2.7I , da/dt = BK2.7I , respectively. The effects of frequency, temperature and R-ratio (the ratio of minimum to maximum stress intensity factor in the fatigue loading cycle) on the Paris law prefactors were characterized. A linear correlation allowed extrapolation of the creep prefactor ( B) from fatigue data. The extrapolated values were systematically higher than the values measured directly from creep and only converged at Tg. The difference was attributed to damage of the craze fibrils during crack closure upon unloading in the fatigue cycle. Blending poly(ethylene glycol) (PEG) with poly(lactide) (PLA) decreases the Tg and improves the mechanical properties. It was demonstrated that a PLA of lower stereoregularity was miscible with up to 30 wt% PEG. Aging was due to slow crystallization of PEG from the homogeneous amorphous blend. Crystallization of PEG depleted the amorphous phase of PEG and gradually increased the Tg until aging essentially ceased when Tg of the amorphous phase reached the aging temperature. Different aging mechanism was found with a crystallizable PLA of higher stereoregularity. The effect of cooling rate on crystallization and subsequent aging of high stereoregular PLA blended PEG was studied by thermal analysis and by direct observation of the solid state structure with atomic force microscopy (AFM). The partially crystallized blend obtained with a cooling rate of 30
NASA Astrophysics Data System (ADS)
Kwon, Kibum
A dynamic analysis of the interaction between a crack and an auxetic (negative Poisson ratio)/non-auxetic inclusion is presented. The two most important fracture parameters, namely the stress intensity factors and the T-stress are analyzed by using the symmetric Galerkin boundary element method in the Laplace domain for three different models of crack-inclusion interaction. To investigate the effects of auxetic inclusions on the fracture behavior of composites reinforced by this new type of material, comparisons of the dynamic stress intensity factors and the dynamic T-stress are made between the use of auxetic inclusions as opposed to the use of traditional inclusions. Furthermore, the technique presented in this research can be employed to analyze for the interaction between a crack and a cluster of auxetic/non-auxetic inclusions. Results from the latter models can be employed in crack growth analysis in auxetic-fiber-reinforced composites.
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.
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).
Dynamic Analysis of a Reaction-Diffusion Rumor Propagation Model
NASA Astrophysics Data System (ADS)
Zhao, Hongyong; Zhu, Linhe
2016-06-01
The rapid development of the Internet, especially the emergence of the social networks, leads rumor propagation into a new media era. Rumor propagation in social networks has brought new challenges to network security and social stability. This paper, based on partial differential equations (PDEs), proposes a new SIS rumor propagation model by considering the effect of the communication between the different rumor infected users on rumor propagation. The stabilities of a nonrumor equilibrium point and a rumor-spreading equilibrium point are discussed by linearization technique and the upper and lower solutions method, and the existence of a traveling wave solution is established by the cross-iteration scheme accompanied by the technique of upper and lower solutions and Schauder’s fixed point theorem. Furthermore, we add the time delay to rumor propagation and deduce the conditions of Hopf bifurcation and stability switches for the rumor-spreading equilibrium point by taking the time delay as the bifurcation parameter. Finally, numerical simulations are performed to illustrate the theoretical results.
Effects of dynamical FLIC fermions in the quark and gluon propagator
NASA Astrophysics Data System (ADS)
Kamleh, W.; Bowman, P. O.; Leinweber, D. B.; Williams, A. G.; Zhang, J.-B.
2006-11-01
In this work we examine the FLIC overlap quark propagator and the gluon propagator on both dynamical and quenched lattices. The tadpole improved Luscher-Weisz gauge action is used in both cases. The dynamical gauge fields use the FLIC fermion action for the sea quark contribution. We observe that the presence of sea quarks causes a suppression of the mass function, quark renormalisation function and gluon dressing function in the infrared. The ultraviolet physics is unaffected.
NASA Astrophysics Data System (ADS)
Ma, Longzhou; Roy, Shawoon K.
2013-04-01
The fatigue crack propagation (FCP) behavior of two solid-solution-strengthened Ni-based superalloys, INCONEL 617 and HAYNES 230, were studied simultaneously in laboratory air using a constant stress intensity factor (K)-controlled mode with different load ratios (R-ratio) at 700 °C. The FCP tests were performed in both cycle and time-dependent FCP domains to examine the effect of R-ratio on the FCP rate, da/dn. For cycle-dependent FCP test, a 1-s sinusoidal fatigue was applied for a compact tension (CT) specimen of INCONEL 617 and HAYNES 230 to measure their FCP rates. For time-dependent FCP test, a 3-s sinusoidal fatigue with a hold time of 300 s at maximum load was applied. Both cycle/time-dependent FCP behaviors were characterized and analyzed. The results showed that increasing R-ratio would introduce the fatigue incubation and decrease the FCP rates at cycle-dependent FCP tests. On the contrary, fatigue incubation was not observed at time-dependent FCP tests for both INCONEL 617 and HAYNES 230 at each tested R-ratio, suggesting that association of maximum load (Kmax) with crack tip open displacement (CTOD) and environmental factor governed the FCP process. Also, for time-dependent FCP, HAYNES 230 showed lower FCP rates than INCONEL 617 regardless of R-ratio. However, for cycle-dependent FCP, HAYNES 230 showed the lower FCP rates only at high R-ratios. Fracture surface of specimens were examined using SEM to investigate the cracking mechanism under cycle/time-dependent FCP condition with various R-ratios.
Dynamic 8-state ICSAR rumor propagation model considering official rumor refutation
NASA Astrophysics Data System (ADS)
Zhang, Nan; Huang, Hong; Su, Boni; Zhao, Jinlong; Zhang, Bo
2014-12-01
With the rapid development of information networks, negative impacts of rumor propagation become more serious. Nowadays, knowing the mechanisms of rumor propagation and having an efficient official rumor refutation plan play very important roles in reducing losses and ensuring social safety. In this paper we first develop the dynamic 8-state ICSAR (Ignorance, Information Carrier, Information Spreader, Information Advocate, Removal) rumor propagation model to study the mechanism of rumor propagation. Eight influencing factors including information attraction, objective identification of rumors, subjective identification of people, the degree of trust of information media, spread probability, reinforcement coefficient, block value and expert effects which are related to rumor propagation were analyzed. Next, considering these factors and mechanisms of rumor propagation and refutation, the dynamic 8-state ICSAR rumor propagation model is verified by the SIR epidemic model, computer simulation and actual data. Thirdly, through quantitative sensitivity analysis, the detailed function of each influencing factor was studied and shown in the figure directly. According to these mechanisms, we could understand how to block a rumor in a very efficient way and which methods should be chosen in different situations. The ICSAR model can divide people into 8 states and analyze rumor and anti-rumor dissemination in an accurate way. Furthermore, official rumor refutation is considered in rumor propagation. The models and the results are essential for improving the efficiency of rumor refutation and making emergency plans, which help to reduce the possibility of losses in disasters and rumor propagation.
Dai, Xiaoxu; Hu, Minghua; Tian, Wen; Xie, Daoyi; Hu, Bin
2016-01-01
This paper presents a propagation dynamics model for congestion propagation in complex networks of airspace. It investigates the application of an epidemiology model to complex networks by comparing the similarities and differences between congestion propagation and epidemic transmission. The model developed satisfies the constraints of actual motion in airspace, based on the epidemiology model. Exploiting the constraint that the evolution of congestion cluster in the airspace is always dynamic and heterogeneous, the SIR epidemiology model (one of the classical models in epidemic spreading) with logistic increase is applied to congestion propagation and shown to be more accurate in predicting the evolution of congestion peak than the model based on probability, which is common to predict the congestion propagation. Results from sample data show that the model not only predicts accurately the value and time of congestion peak, but also describes accurately the characteristics of congestion propagation. Then, a numerical study is performed in which it is demonstrated that the structure of the networks have different effects on congestion propagation in airspace. It is shown that in regions with severe congestion, the adjustment of dissipation rate is more significant than propagation rate in controlling the propagation of congestion.
1983-03-01
Time-To-Crack-Initiation TYFI Tim-To-Falure ** To be determined FAST I service life in 2 days SUPW 1 service life In 16 days 1 I 171 ,I .*1i, " e -D...JOINTS VOLUME I - PHASE I DOCUMENTATION - Y. H. Kim S. M. Speaker D. E . Gordon S. D. Manning STRUCTURES AND DESIGN DEPARTMENT GENERAL DYNAMICS FORT...PERFoRING OTG. REPORT NUMDER ically Fastened Joints’Vol.I- Phase I ,Dpg9tat~nI. CONTRACT OR GRANT NUMMER(s) Y. H. Kim, R. P. Wei, D. E . Gordon, S. M
Dynamic fracture toughnesses of reaction-bonded silicon nitride
NASA Technical Reports Server (NTRS)
Kobayashi, A. S.; Emery, A. F.; Liaw, B. M.
1983-01-01
The room-temperature dynamic fracture response of reaction-bonded silicon nitride is investigated using a hybrid experimental-numerical procedure. In this procedure, experimentally determined crack velocities are utilized to drive a dynamic finite-element code or dynamic finite-difference code in its generation mode in order to extract numerically the dynamic stress intensity factor of the fracturing specimen. Results show that the dynamic fracture toughness vs crack velocity relations of the two reaction-bonded silicon nitrides do not follow the general trend in those relations of brittle polymers and steel. A definite slow crack velocity during the initial phase of dynamic crack propagation is observed in reaction-bonded silicon nitride, which results in a nonunique dynamic fracture toughness vs crack velocity relation. In addition, it is found that a propagating crack will continue to propagate under a static stress intensity factor substantially lower than K(IC).
Investigation of Cracks Found in Helicopter Longerons
NASA Technical Reports Server (NTRS)
Newman, John A.; Baughman, James M.; Wallace, Terryl A.
2009-01-01
Four cracked longerons, containing a total of eight cracks, were provided for study. Cracked regions were cut from the longerons. Load was applied to open the cracks, enabling crack surface examination. Examination revealed that crack propagation was driven by fatigue loading in all eight cases. Fatigue crack initiation appears to have occurred on the top edge of the longerons near geometric changes that affect component bending stiffness. Additionally, metallurigical analysis has revealed a local depletion in alloying elements in the crack initiation regions that may be a contributing factor. Fatigue crack propagation appeared to be initially driven by opening-mode loading, but at a crack length of approximately 0.5 inches (12.7 mm), there is evidence of mixed-mode crack loading. For the longest cracks studied, shear-mode displacements destroyed crack-surface features of interest over significant portions of the crack surfaces.
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.
Alloying propagation in nanometric Ni/Al multilayers: A molecular dynamics study
NASA Astrophysics Data System (ADS)
Turlo, V.; Politano, O.; Baras, F.
2017-02-01
In nanometric metallic multilayers such as Ni/Al, the alloying reaction proceeds in the form of a propagating wave. We studied the different phase transformations involved in the reactive wave propagation by means of molecular dynamics. The focus was on a specific regime that involves melting of reactants, intermixing of reactants, and formation of an intermetallic compound. We found that the wave consists of two stages. The first front is associated with a dissolution process and propagates at several meters per second, while the second front is due to the crystallization of the final product and is slower, leading to a specific microstructure with alternated large grains of NiAl and liquid regions in the front propagation direction. Three main exothermic processes were identified, including grain coarsening. Their respective contributions were evaluated. We developed a new texture analysis tool that allowed us to follow the evolution of the microstructure and the dynamics of the grain orientation.
Fracture Mechanics of Crack Growth During Sonic-IR Inspection
NASA Astrophysics Data System (ADS)
Chen, J. C.; Riddell, W. T.; Lick, Kyle; Wong, Chang-Hwa
2007-03-01
In past studies, we showed that cracks synthesized under carefully controlled conditions will propagate when subjected to sonic IR testing. The extent or severity of the propagation observed depended on several parameters including the stress intensity factor (which corresponds to crack growth rate) under which the crack was synthesized, the tightness of the crack closure, and the initial crack length. Furthermore, we showed that crack propagation during sonic IR testing occurs for 2024 aluminum, titanium and 304 stainless steel specimens. In this study, we extend the range of experimental conditions for synthesizing cracks to further elucidate their effect on the crack propagation, and we focus more specifically on the stress intensity factor. The stress intensity factor not only determines the rate of crack growth, but it has two profound effects on crack characteristics: the establishment of plastic zones around the crack tip and the variation of the topography of the mating crack surfaces. These two factors strongly affect crack propagation.
Ulaganathan, Jaganathan Newman, Roger C.
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 within 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.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Nemeth, Noel N.; Salem, Jonathan A.; Powers, Lynn M.; Gyekenyesi, John P.
1995-01-01
The slow crack growth of a hot-pressed silicon nitride was determined at 1300 C in air using dynamic fatigue testing under both uniaxial and biaxial stress states. Good agreement in fatigue parameter exists between the data obtained from uniaxial and biaxial loading conditions. A reasonable prediction of dynamic fatigue from one stress state to another was made using the recently developed CARES/LIFE computer code.
Visualization of stress propagation in dynamically compacted wetted particle beds
NASA Astrophysics Data System (ADS)
Marr, Bradley J.; Frost, David L.
2017-01-01
The high-strain-rate response of granular media has received considerable attention due to increasing interest in granular penetration. Introduction of a liquid phase into the particle bed alters the global deformation response of the system as the liquid is capable of supporting stresses. In the present study, we investigate the response of arrays of stacked glass rods, both dry and immersed in liquid, under varying drop weight-induced stress loadings. We examine the role of saturation on particle and bed deformation, using well-defined loading conditions and particle bed arrangements. Using high-speed photograph and the photoelastic nature of the glass rods, the propagation of the stress wave through the two-phase system can be visualized. The liquid phase was seen to contribute to the mean stress transfer within the system, resulting in reduced total driver displacements as well as increased bed strains at the time when particle fracturing was first observed.
Mobility propagation and dynamic facilitation in superionic conductors
Annamareddy, Ajay Eapen, Jacob
2015-11-21
In an earlier work [V. A. Annamareddy et al., Phys. Rev. E 89, 010301(R) (2014)], we showed the manifestation of dynamical heterogeneity (DH)—the presence of clustered mobile and immobile regions—in UO{sub 2}, a model type II superionic conductor. In the current work, we demonstrate the mechanism of dynamic facilitation (DF) in two superionic conductors (CaF{sub 2} and UO{sub 2}) using atomistic simulations. Using the mobility transfer function, DF is shown to vary non-monotonically with temperature with the intensity of DF peaking at temperatures close to the superionic transition temperature (T{sub λ}). Both the metrics quantifying DH and DF show remarkable correspondence implying that DF, in the framework of kinematically constrained models, underpins the heterogeneous dynamics in type II superionic conductors.
Cybersim: geographic, temporal, and organizational dynamics of malware propagation
Santhi, Nandakishore; Yan, Guanhua; Eidenbenz, Stephan
2010-01-01
Cyber-infractions into a nation's strategic security envelope pose a constant and daunting challenge. We present the modular CyberSim tool which has been developed in response to the need to realistically simulate at a national level, software vulnerabilities and resulting mal ware propagation in online social networks. CyberSim suite (a) can generate realistic scale-free networks from a database of geocoordinated computers to closely model social networks arising from personal and business email contacts and online communities; (b) maintains for each,bost a list of installed software, along with the latest published vulnerabilities; (d) allows designated initial nodes where malware gets introduced; (e) simulates, using distributed discrete event-driven technology, the spread of malware exploiting a specific vulnerability, with packet delay and user online behavior models; (f) provides a graphical visualization of spread of infection, its severity, businesses affected etc to the analyst. We present sample simulations on a national level network with millions of computers.
Ray dynamics in a long-range acoustic propagation experiment
NASA Astrophysics Data System (ADS)
Beron-Vera, Francisco J.; Brown, Michael G.; Colosi, John A.; Tomsovic, Steven; Virovlyansky, Anatoly L.; Wolfson, Michael A.; Zaslavsky, George M.
2003-09-01
A ray-based wave-field description is employed in the interpretation of broadband basin-scale acoustic propagation measurements obtained during the Acoustic Thermometry of Ocean Climate program's 1994 Acoustic Engineering Test. Acoustic observables of interest are wavefront time spread, probability density function (PDF) of intensity, vertical extension of acoustic energy in the reception finale, and the transition region between temporally resolved and unresolved wavefronts. Ray-based numerical simulation results that include both mesoscale and internal-wave-induced sound-speed perturbations are shown to be consistent with measurements of all the aforementioned observables, even though the underlying ray trajectories are predominantly chaotic, that is, exponentially sensitive to initial and environmental conditions. Much of the analysis exploits results that relate to the subject of ray chaos; these results follow from the Hamiltonian structure of the ray equations. Further, it is shown that the collection of the many eigenrays that form one of the resolved arrivals is nonlocal, both spatially and as a function of launch angle, which places severe restrictions on theories that are based on a perturbation expansion about a background ray.
Ray dynamics in a long-range acoustic propagation experiment.
Beron-Vera, Francisco J; Brown, Michael G; Colosi, John A; Tomsovic, Steven; Virovlyansky, Anatoly L; Wolfson, Michael A; Zaslavsky, George M
2003-09-01
A ray-based wave-field description is employed in the interpretation of broadband basin-scale acoustic propagation measurements obtained during the Acoustic Thermometry of Ocean Climate program's 1994 Acoustic Engineering Test. Acoustic observables of interest are wavefront time spread, probability density function (PDF) of intensity, vertical extension of acoustic energy in the reception finale, and the transition region between temporally resolved and unresolved wavefronts. Ray-based numerical simulation results that include both mesoscale and internal-wave-induced sound-speed perturbations are shown to be consistent with measurements of all the aforementioned observables, even though the underlying ray trajectories are predominantly chaotic, that is, exponentially sensitive to initial and environmental conditions. Much of the analysis exploits results that relate to the subject of ray chaos; these results follow from the Hamiltonian structure of the ray equations. Further, it is shown that the collection of the many eigenrays that form one of the resolved arrivals is nonlocal, both spatially and as a function of launch angle, which places severe restrictions on theories that are based on a perturbation expansion about a background ray.
Dynamic Causal Modelling of epileptic seizure propagation pathways: a combined EEG-fMRI study.
Murta, Teresa; Leal, Alberto; Garrido, Marta I; Figueiredo, Patrícia
2012-09-01
Simultaneous EEG-fMRI offers the possibility of non-invasively studying the spatiotemporal dynamics of epileptic activity propagation from the focus towards an extended brain network, through the identification of the haemodynamic correlates of ictal electrical discharges. In epilepsy associated with hypothalamic hamartomas (HH), seizures are known to originate in the HH but different propagation pathways have been proposed. Here, Dynamic Causal Modelling (DCM) was employed to estimate the seizure propagation pathway from fMRI data recorded in a HH patient, by testing a set of clinically plausible network connectivity models of discharge propagation. The model consistent with early propagation from the HH to the temporal-occipital lobe followed by the frontal lobe was selected as the most likely model to explain the data. Our results demonstrate the applicability of DCM to investigate patient-specific effective connectivity in epileptic networks identified with EEG-fMRI. In this way, it is possible to study the propagation pathway of seizure activity, which has potentially great impact in the decision of the surgical approach for epilepsy treatment.
NASA Astrophysics Data System (ADS)
Bajpai, Shubhra; Gupta, Ankur; Pradhan, Siddhartha Kumar; Mandal, Tapendu; Balani, Kantesh
2014-09-01
Hydroxyapatite (HA) is a widely used bioceramic known for its chemical similarity with that of bone and teeth (Ca/P ratio of 1.67). But, owing to its extreme brittleness, α-Al2O3 is reinforced with HA and processed as a coating via pulsed laser deposition (PLD). Reinforcement of α-Al2O3 (50 wt.%) in HA via PLD on 316L steel substrate has shown modulus increase by 4% and hardness increase by 78%, and an improved adhesion strength of 14.2 N (improvement by 118%). Micro-scratching has shown an increase in the coefficient-of-friction from 0.05 (pure HA) to 0.17 (with 50 wt.% Al2O3) with enhancement in the crack propagation resistance (CPR) up to 4.5 times. Strong adherence of PLD HA-Al2O3 coatings (~4.5 times than that of HA coating) is attributed to efficient release of stored tensile strain energy (~17 × 10-3 J/m2) in HA-Al2O3 composites, making it a potential damage-tolerant bone-replacement surface coating.
Propagation Dynamics of a Light Beam in a Fractional Schrödinger Equation.
Zhang, Yiqi; Liu, Xing; Belić, Milivoj R; Zhong, Weiping; Zhang, Yanpeng; Xiao, Min
2015-10-30
The dynamics of wave packets in the fractional Schrödinger equation is still an open problem. The difficulty stems from the fact that the fractional Laplacian derivative is essentially a nonlocal operator. We investigate analytically and numerically the propagation of optical beams in the fractional Schrödinger equation with a harmonic potential. We find that the propagation of one- and two-dimensional input chirped Gaussian beams is not harmonic. In one dimension, the beam propagates along a zigzag trajectory in real space, which corresponds to a modulated anharmonic oscillation in momentum space. In two dimensions, the input Gaussian beam evolves into a breathing ring structure in both real and momentum spaces, which forms a filamented funnel-like aperiodic structure. The beams remain localized in propagation, but with increasing distance display an increasingly irregular behavior, unless both the linear chirp and the transverse displacement of the incident beam are zero.
Zimmerman, Paul M; Tranca, Diana C; Gomes, Joseph; Lambrecht, Daniel S; Head-Gordon, Martin; Bell, Alexis T
2012-11-28
Product selectivity of alkane cracking catalysis in the H-MFI zeolite is investigated using both static and dynamic first-principles quantum mechanics/molecular mechanics simulations. These simulations account for the electrostatic- and shape-selective interactions in the zeolite and provide enthalpic barriers that are closely comparable to experiment. Cracking transition states for n-pentane lead to a metastable intermediate (a local minimum with relatively small barriers to escape to deeper minima) where the proton is shared between two hydrocarbon fragments. The zeolite strongly stabilizes these carbocations compared to the gas phase, and the conversion of this intermediate to more stable species determines the product selectivity. Static reaction pathways on the potential energy surface starting from the metastable intermediate include a variety of possible conversions into more stable products. One-picosecond quasiclassical trajectory simulations performed at 773 K indicate that dynamic paths are substantially more diverse than the potential energy paths. Vibrational motion that is dynamically sampled after the cracking transition state causes spilling of the metastable intermediate into a variety of different products. A nearly 10-fold change in the branching ratio between C2/C3 cracking channels is found upon inclusion of post-transition-state dynamics, relative to static electronic structure calculations. Agreement with experiment is improved by the same factor. Because dynamical effects occur soon after passing through the rate-limiting transition state, it is the dynamics, and not only the potential energy barriers, that determine the catalytic selectivity. This study suggests that selectivity in zeolite catalysis is determined by high temperature pathways that differ significantly from 0 K potential surfaces.
Ultrasonic characterization of fatigue crack closure
Thompson, R.B.; Buck, O.; Rehbein, D.K.
1991-01-01
The characterization of fatigue crack closure is an important objective because of its influence on fatigue crack propagation, particularly under conditions of variable amplitude loading. This paper describes a nontraditional technique for characterizing closure, in which ultrasonic scattering measurements are used to obtain estimates of the number density and size of asperities bridging the crack faces, with subsequent estimates of the crack tip shielding being based on those geometrical parameters. The paper first reviews the experimental configuration and the basic elasto-dynamic theory underlying the technique. It then presents recent results obtained in studies of the influence of block overloads and load shedding on the growth of fatigue cracks in aluminum alloys. In both cases, the change in the closure state after the overload can be unambiguously seen even in the raw data. Moreover, data analysis suggests that it may be possible to predict when the crack will reinitiate based on more subtle changes in the ultrasonically inferred closure state. In the case of load shedding, a massive closure region is observed, whose characteristics appear consistent with the notion that threshold phenomena can be explained in terms of crack closure. 20 refs., 10 figs.
NASA Astrophysics Data System (ADS)
Zhu, Guanghu; Chen, Guanrong; Zhang, Haifeng; Fu, Xinchu
2015-01-01
Based on the fact that most human pathogens originate from animals, this paper attempts to illustrate the propagation dynamics of some zoonotic infections, which spread in two separated networks of populations (human network I and animal network II) and cross-species (vectors, or infective media). An epidemic time-evolution model is proposed via mean-field approximation and its global dynamics are investigated. It is found that the basic reproduction number in terms of epidemiological parameters and the network structure is the threshold condition determining the propagation dynamics. Further, the influences of various infection rates and contact patterns are verified. Numerical results show that the heterogeneity in connection patterns and inner infection in network I can easily trigger endemic dynamics, but when a pathogen, such as H7N9, has weak infectivity in humans, the effects of animal-animal interactions and the contacts with vectors tend to induce endemic states and enhance the prevalence in all the populations.
Momentum conserving Brownian dynamics propagator for complex soft matter fluids
Padding, J. T.; Briels, W. J.
2014-12-28
We present a Galilean invariant, momentum conserving first order Brownian dynamics scheme for coarse-grained simulations of highly frictional soft matter systems. Friction forces are taken to be with respect to moving background material. The motion of the background material is described by locally averaged velocities in the neighborhood of the dissolved coarse coordinates. The velocity variables are updated by a momentum conserving scheme. The properties of the stochastic updates are derived through the Chapman-Kolmogorov and Fokker-Planck equations for the evolution of the probability distribution of coarse-grained position and velocity variables, by requiring the equilibrium distribution to be a stationary solution. We test our new scheme on concentrated star polymer solutions and find that the transverse current and velocity time auto-correlation functions behave as expected from hydrodynamics. In particular, the velocity auto-correlation functions display a long time tail in complete agreement with hydrodynamics.
Dynamic population flow based risk analysis of infectious disease propagation in a metropolis.
Zhang, Nan; Huang, Hong; Duarte, Marlyn; Zhang, Junfeng Jim
2016-09-01
Knowledge on the characteristics of infectious disease propagation in metropolises plays a critical role in guiding public health intervention strategies to reduce death tolls, disease incidence, and possible economic losses. Based on the SIR model, we established a comprehensive spatiotemporal risk assessment model to compute infectious disease propagation within an urban setting using Beijing, China as a case study. The model was developed for a dynamic population distribution using actual data on location, density of residences and offices, and means of public transportation (e.g., subways, buses and taxis). We evaluated four influencing factors including biological, behavioral, environmental parameters and infectious sources. The model output resulted in a set of maps showing how the four influencing factors affected the trend and characteristics of airborne infectious disease propagation in Beijing. We compared the scenarios for the long-term dynamic propagation of infectious disease without governmental interventions versus scenarios with government intervention and hospital coordinated emergency responses. Lastly, the sensitivity of the average number of people at different location in spreading infections is analyzed. Based on our results, we provide valuable recommendations to governmental agencies and the public in order to minimize the disease propagation.
Dynamical chiral symmetry breaking and confinement with an infrared-vanishing gluon propagator
Roberts, C.D.; Hawes, F.T.; Williams, A.G.
1995-08-01
We have studied a model Dyson-Schwinger equation for the quark propagator, constructed using an Ansatz for the gluon propagator of the form D(q) {approximately} q{sup 2}/[(q{sup 2}){sup 2} + b{sup 4}] and two Ansatze for the quark-gluon vertex: the minimal Ball-Chiu and the modified form suggested by Curtis and Pennington. The aim was to determine whether such a form of the gluon propagator, which was suggested by a number of authors and which recent lattice simulations of QCD suggest may be plausible, can support dynamical chiral symmetry breaking and ensure quark confinement. The form of the gluon propagator at small space-like momenta is crucial to the nature of the strong interaction spectrum but is presently unknown and information gathered in such studies is invaluable in supporting or invalidating given hypotheses. It was found that there is a critical value of b = b{sub c} such that the model does not support dynamical chiral symmetry breaking for b > b{sub c}. Further, it was shown that this form of gluon propagator cannot confine quarks. As a consequence this form represents a physically unreasonable model. In addition, these results formed the basis for an invited presentation at a workshop on quantum infrared physics and will be published in the proceedings.
Huo, Pengfei; Coker, David F
2012-12-14
Powerful approximate methods for propagating the density matrix of complex systems that are conveniently described in terms of electronic subsystem states and nuclear degrees of freedom have recently been developed that involve linearizing the density matrix propagator in the difference between the forward and backward paths of the nuclear degrees of freedom while keeping the interference effects between the different forward and backward paths of the electronic subsystem described in terms of the mapping Hamiltonian formalism and semi-classical mechanics. Here we demonstrate that different approaches to developing the linearized approximation to the density matrix propagator can yield a mean-field like approximate propagator in which the nuclear variables evolve classically subject to Ehrenfest-like forces that involve an average over quantum subsystem states, and by adopting an alternative approach to linearizing we obtain an algorithm that involves classical like nuclear dynamics influenced by a quantum subsystem state dependent force reminiscent of trajectory surface hopping methods. We show how these different short time approximations can be implemented iteratively to achieve accurate, stable long time propagation and explore their implementation in different representations. The merits of the different approximate quantum dynamics methods that are thus consistently derived from the density matrix propagator starting point and different partial linearization approximations are explored in various model system studies of multi-state scattering problems and dissipative non-adiabatic relaxation in condensed phase environments that demonstrate the capabilities of these different types of approximations for treating non-adiabatic electronic relaxation, bifurcation of nuclear distributions, and the passage from nonequilibrium coherent dynamics at short times to long time thermal equilibration in the presence of a model dissipative environment.
Application of the cracked pipe element to creep crack growth prediction
Brochard, J.; Charras, T.
1997-04-01
Modifications to a computer code for ductile fracture assessment of piping systems with postulated circumferential through-wall cracks under static or dynamic loading are very briefly described. The modifications extend the capabilities of the CASTEM2000 code to the determination of fracture parameters under creep conditions. The main advantage of the approach is that thermal loads can be evaluated as secondary stresses. The code is applicable to piping systems for which crack propagation predictions differ significantly depending on whether thermal stresses are considered as primary or secondary stresses.
2000-02-01
REFERENCES Mechs., Vol. 7, No. 2, pp 235-247, 1975. 1. Palmgren , A., Ball and Roller Bearing 11. Zocher, H., (ed.), "Behavior of Short Cracks in 1...flaw (not a sharp fatigue times. The replacement times are determined using crack) pre-exists in the structure. The flaw tolerant the Palmgren /Miner...nominal stress rule [Ref. 1,2] method still uses the Palmgren /Miner rule to establish where the stress versus life curves, S/N curves, are retirement
Regulatory effects on the population dynamics and wave propagation in a cell lineage model.
Wang, Mao-Xiang; Ma, Yu-Qiang; Lai, Pik-Yin
2016-03-21
We consider the interplay of cell proliferation, cell differentiation (and de-differentiation), cell movement, and the effect of feedback regulations on the population and propagation dynamics of different cell types in a cell lineage model. Cells are assumed to secrete and respond to negative feedback molecules which act as a control on the cell lineage. The cell densities are described by coupled reaction-diffusion partial differential equations, and the propagating wave front solutions in one dimension are investigated analytically and by numerical solutions. In particular, wavefront propagation speeds are obtained analytically and verified by numerical solutions of the equations. The emphasis is on the effects of the feedback regulations on different stages in the cell lineage. It is found that when the progenitor cell is negatively regulated, the populations of the cell lineage are strongly down-regulated with the steady growth rate of the progenitor cell being driven to zero beyond a critical regulatory strength. An analytic expression for the critical regulation strength in terms of the model parameters is derived and verified by numerical solutions. On the other hand, if the inhibition is acting on the differentiated cells, the change in the population dynamics and wave propagation speed is small. In addition, it is found that only the propagating speed of the progenitor cells is affected by the regulation when the diffusion of the differentiated cells is large. In the presence of de-differentiation, the effect on down-regulating the progenitor population is weakened and there is no effect on the propagation speed due to regulation, suggesting that the effect of regulatory control is diminished by de-differentiation pathways.
NASA Astrophysics Data System (ADS)
Kame, N.; Uchida, K.
2006-12-01
We simulate dynamic rupture propagation in which two mode II cracks coalesce on a planar fault using a boundary integral equation method. Our main interests are in the rupture complexity and resultant seismic radiation due to coalescence and in the reconstruction of seismically equivalent another dynamic model that could be inferred only from the waveforms. First we analyze crack coalescence model (CCM) with homogeneous source parameters except on two pre- slipped regions. In CCM, a main crack nucleates, propagates and coalesces with a nucleating subsidiary crack. Our analysis shows that local high slip-rate pulse is generated by coalescence and a secondary Rayleigh slip pulse subsequently begins to propagate trailing the rupture front. Second we reconstruct a single crack model (SCM) with heterogeneous source parameters that can reproduce the same slip-rate history in CCM, that is, both models are seismically equivalent. In SCM we found singular increase in the stress drop and sudden decrease in the strength excess corresponding to the coalescence pulse region, which means that these two inhomogeneities appeared in SCM originally resulted from the stress interaction between approaching crack tips in CCM. Third we synthesize seismic radiation from CCM and successfully identify distinct phases associated with two pulses: the coalescence pulse phase shows seismic radiation similar to the stopping phase that has a typical ω-2 behavior at high frequency, which is also consistent with theoretically predicted radiation by the singular stress drop in SCM. Rayleigh slip-pulse phase appears dominantly in transverse component with strong forward directivity similar to rupture front phase although disappears in parallel component except very near the fault.
Restrictions on dynamically propagating surfaces of strong discontinuity in elastic-plastic solids
NASA Astrophysics Data System (ADS)
Drugan, W. J.; Shen, Yinong
F OR DYNAMIC three-dimensional deformations of elastic-plastic materials, we elicit conditions necessary for the existence of propagating surfaces of strong discontinuity (across which components of stress, strain or material velocity jump). This is accomplished within a small-displacement-gradient formulation of standard weak continuum-mechanical assumptions of momentum conservation and geometrical compatibility, and skeletal constitutive assumptions which permit very general elastic and plastic anisotropy including yield surface vertices and anisotropic hardening. In addition to deriving very explicit restrictions on propagating strong discontinuities in general deformations, we prove that for anti-plane strain and incompressible plane strain deformations, such strong discontinuities can exist only at elastic wave speeds in generally anisotropic elastic-ideally plastic materials unless a material's yield locus in stress space contains a linear segment. The results derived seem essential for correct and complete construction of solutions to dynamic elastic-plastic boundary-value problems.
Pattern transition and sluggish cracking of colloidal droplet deposition with polymer additives
NASA Astrophysics Data System (ADS)
Zhang, YongJian; Liu, ZhengTang; Zang, DuYang; Qian, YiMeng; Lin, KeJun
2013-09-01
Drying of colloidal droplets often develops versatile depositions. We study the drying deposition of both colloidal droplets containing silica nanoparticles and the silica colloidal droplets with polyethylene oxide (PEO) additives. We focus on the effect of polymer additives on the deposition formation and the cracking dynamics by using in-situ microscope observation. With PEO additives, a transition from ring-like deposition to uniform deposition is observed, and the cracking dynamics is greatly reduced. The PEO additives enhance the adsorption of particles at the air-water interface, thus forming the network structure at the interface which blocks the outward capillary flow. This contributes to the uniform deposition. Meanwhile, the multi-distribution of the aggregates size enhances the non-homogeneity of the drying film and consequently results in multi-nucleation of cracks. This reduces the stress accumulation that drives the crack propagation and may be responsible for the sluggish cracking dynamics.
Nabika, Hideki; Sato, Mami; Unoura, Kei
2013-01-07
The ability to control chemical wave propagation dynamics could stimulate the science and technology of artificial and biological spatiotemporal oscillating phenomena. In contrast to the conventional chemical approaches to control the wave front dynamics, here we report a physical approach to tune the propagation dynamics under the same chemical conditions. By using well-designed microchannels with different channel widths and depths, the propagation velocity was successfully controlled based on two independent effects: (i) a transition in the proton diffusion mode and (ii) the formation of a slanted wave front. Numerical analysis yielded a simple relationship between the propagation velocity and the microchannel configuration, which offers a simple and general way of controlling chemical wave propagation.
NASA Astrophysics Data System (ADS)
Jiang, Shan; Sewell, Thomas D.; Thompson, Donald L.
2015-06-01
We are interested in understanding the fundamental processes that occur during propagation of shock waves across the crystal-melt interface in molecular substances. We have carried out molecular dynamics simulations of shock passage from the nitromethane (100)-oriented crystal into the melt and vice versa using the fully flexible, non-reactive Sorescu, Rice, and Thompson force field. A stable interface was established for a temperature near the melting point by using a combination of isobaric-isothermal (NPT) and isochoric-isothermal (NVT) simulations. The equilibrium bulk and interfacial regions were characterized using spatial-temporal distributions of molecular number density, kinetic and potential energy, and C-N bond orientations. Those same properties were calculated as functions of time during shock propagation. As expected, the local temperatures (intermolecular, intramolecular, and total) and stress states differed significantly between the liquid and crystal regions and depending on the direction of shock propagation. Substantial differences in the spatial distribution of shock-induced defect structures in the crystalline region were observed depending on the direction of shock propagation. Research supported by the U.S. Army Research Office.
Henager, Charles H.
2007-10-02
PNNL has performed subcritical crack growth tests under constant applied load at various elevated temperatures in inert environments using subscale compact tension (CT) specimens of two types of SiC-composite materials. The use of CT specimens is preferred over the usual dingle-edge notched beam (SENB) specimens due to more uniform applied stresses over the crack growth region. This study will compare crack growth data taken between two materials as well as specimen geometry types, CT compared to SENB. Plain weave [0/90] Hi-Nicalon CT specimens were tested in argon atmospheres and compared to similar tests of 5-harness satin weave [0/90] Hi-Nicalon Type-S composites. We report here some of the preliminary fractographic examinations of the two materials and an initial assessment of the crack growth data. Additional information on this study will be presented later and also at ICFRM-13 in December.
Formation and interpretation of dilatant echelon cracks.
Pollard, D.D.; Segall, P.; Delaney, P.T.
1982-01-01
The relative displacements of the walls of many veins, joints, and dikes demonstrate that these structures are dilatant cracks. We infer that dilatant cracks propagate in a principal stress plane, normal to the maximum tensile or least compressive stress. Arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks. Breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction. Near the parent-crack tip, a rotation of the local principal stresses is induced in the same sense, but not necessarily through the same angle. Incipient echelon cracks form at the parent-crack tip normal to the local maximum tensile stress. Further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane. The walls of these twisted cracks may be idealized as helicoidal surfaces. An array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle. Thus, many echelon cracks grow from a single parent because the work done in creating the array, as measured by its surface area decreases as the number of cracks increases. -from Authors
NASA Astrophysics Data System (ADS)
Pan, Zhao; Whitehead, Jared; Truscott, Tadd
2016-11-01
Little research has been done to investigate the dynamics of error propagation from PIV-based velocity measurements to the pressure calculation. Rather than measure experimental error, we analytically investigate error propagation by examining the properties of the Poisson equation directly. Our results provide two contributions to the PIV community. First, we quantify the error bound in the pressure field by illustrating the mathematical roots of why and how PIV-based pressure calculations propagate. Second, we design the "worst case error" for a pressure Poisson solver. In other words, we provide a systematic example where the relatively small errors in the experimental data can lead to maximum error in the corresponding pressure calculations. The 2D calculation of the worst case error surprisingly leads to the classic Kirchhoff plates problem, and connects the PIV-based pressure calculation, which is a typical fluid problem, to elastic dynamics. The results can be used for optimizing experimental error minimization by avoiding worst case scenarios. More importantly, they can be used to design synthetic velocity error for future PIV-pressure challenges, which can be the hardest test case in the examinations.
NASA Astrophysics Data System (ADS)
Nadkarni, Neel; Daraio, Chiara; Kochmann, Dennis M.
2014-08-01
We investigate the nonlinear dynamics of a periodic chain of bistable elements consisting of masses connected by elastic springs whose constraint arrangement gives rise to a large-deformation snap-through instability. We show that the resulting negative-stiffness effect produces three different regimes of (linear and nonlinear) wave propagation in the periodic medium, depending on the wave amplitude. At small amplitudes, linear elastic waves experience dispersion that is controllable by the geometry and by the level of precompression. At moderate to large amplitudes, solitary waves arise in the weakly and strongly nonlinear regime. For each case, we present closed-form analytical solutions and we confirm our theoretical findings by specific numerical examples. The precompression reveals a class of wave propagation for a partially positive and negative potential. The presented results highlight opportunities in the design of mechanical metamaterials based on negative-stiffness elements, which go beyond current concepts primarily based on linear elastic wave propagation. Our findings shed light on the rich effective dynamics achievable by nonlinear small-scale instabilities in solids and structures.
Sokol, Serguei; Portais, Jean-Charles
2015-01-01
The dynamics of label propagation in a stationary metabolic network during an isotope labeling experiment can provide highly valuable information on the network topology, metabolic fluxes, and on the size of metabolite pools. However, major issues, both in the experimental set-up and in the accompanying numerical methods currently limit the application of this approach. Here, we propose a method to apply novel types of label inputs, sinusoidal or more generally periodic label inputs, to address both the practical and numerical challenges of dynamic labeling experiments. By considering a simple metabolic system, i.e. a linear, non-reversible pathway of arbitrary length, we develop mathematical descriptions of label propagation for both classical and novel label inputs. Theoretical developments and computer simulations show that the application of rectangular periodic pulses has both numerical and practical advantages over other approaches. We applied the strategy to estimate fluxes in a simulated experiment performed on a complex metabolic network (the central carbon metabolism of Escherichia coli), to further demonstrate its value in conditions which are close to those in real experiments. This study provides a theoretical basis for the rational interpretation of label propagation curves in real experiments, and will help identify the strengths, pitfalls and limitations of such experiments. The cases described here can also be used as test cases for more general numerical methods aimed at identifying network topology, analyzing metabolic fluxes or measuring concentrations of metabolites.
Whole heart modeling - Spatiotemporal dynamics of electrical wave conduction and propagation.
Hui Yang; Yun Chen; Leonelli, Fabio M
2016-08-01
Cardiac electrical activities are varying in both space and time. Human heart consists of a fractal network of muscle cells, Purkinje fibers, arteries and veins. Whole-heart modeling of electrical wave conduction and propagation involves a greater level of complexity. Our previous work developed a computer model of the anatomically realistic heart and simulated the electrical conduction with the use of cellular automata. However, simplistic assumptions and rules limit its ability to provide an accurate approximation of real-world dynamics on the complex heart surface, due to sensitive dependence of nonlinear dynamical systems on initial conditions. In this paper, we propose new reaction-diffusion methods and pattern recognition tools to simulate and model spatiotemporal dynamics of electrical wave conduction and propagation on the complex heart surface, which include (i) whole heart model; (ii) 2D isometric graphing of 3D heart geometry; (iii) reaction-diffusion modeling of electrical waves in 2D graph, and (iv) spatiotemporal pattern recognition. Experimental results show that the proposed numerical solution has strong potentials to model the space-time dynamics of electrical wave conduction in the whole heart, thereby achieving a better understanding of disease-altered cardiac mechanisms.
2012-03-01
are hard to detect with conventional ultrasonic testing methods because ultrasound can propagate through a tightly closed crack [16,17]. Research on...Michaels and T. E. Michaels, “An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth,” Journal of the Acoustical Society of...America, 119(1), pp. 74-85, 2005. 20. Y. Ohara, S. Horinouchi, M. Hashimoto, Y. Shintaku, and K. Yamanaka, “Nonlinear ultrasonic imaging method for
Crack Formation in Cement-Based Composites
NASA Astrophysics Data System (ADS)
Sprince, A.; Pakrastinsh, L.; Vatin, N.
2016-04-01
The cracking properties in cement-based composites widely influences mechanical behavior of construction structures. The challenge of present investigation is to evaluate the crack propagation near the crack tip. During experiments the tension strength and crack mouth opening displacement of several types of concrete compositions was determined. For each composition the Compact Tension (CT) specimens were prepared with dimensions 150×150×12 mm. Specimens were subjected to a tensile load. Deformations and crack mouth opening displacement were measured with extensometers. Cracks initiation and propagation were analyzed using a digital image analysis technique. The formation and propagation of the tensile cracks was traced on the surface of the specimens using a high resolution digital camera with 60 mm focal length. Images were captured during testing with a time interval of one second. The obtained experimental curve shows the stages of crack development.
Experimental study on crack coalescence mechanisms of pre-existing flaws under blast loading
NASA Astrophysics Data System (ADS)
Yue, Zhong-wen; Yang, Ren-shu; Ma, Xin-min; Guo, Dong-ming
2008-11-01
The PMMA model transmission-type experiment of dynamic caustics was carried out to simulate the fracture blasting process of material containing pre-existing flaws using the dynamic caustic-test system. The mechanism of the fracture coalescence among four prefabricated flaws with echelon geometry distribution was studied under blast loading. The experiment results show that two wing cracks respectively coalescing with the flaw F2 and flaw F3 appear at both tips of the flaw F1 closest to the blasthole. Whereas the flaw F4 doesn't produce wing cracks, the flaw F2 and flaw F3 also respectively generate two wing cracks which don't link up the flaw F4. Crack propagation is greatly affected by preexisting flaws. During the whole fracture process, the wing crack velocity oscillates with the increase of crack propagating time. The value of dynamic stress intensity factor reaches the maximum in a moment and then gradually decreases. The changes of dynamic stress intensity factor also oscillate in the whole time. Furthermore, the value of dynamic stress intensity factor KdII is smaller than that of dynamic stress intensity factor KdI. The results of the present research can provide the theoretical basis for the study on blasting of rock containing flaws.
Zhu, Bo O.; Chen, Ke; Jia, Nan; Sun, Liang; Zhao, Junming; Jiang, Tian; Feng, Yijun
2014-01-01
Transmission and reflection are two fundamental properties of the electromagnetic wave propagation through obstacles. Full control of both the magnitude and phase of the transmission and reflection independently are important issue for free manipulation of electromagnetic wave propagation. Here we employed the equivalent principle, one fundamental theorem of electromagnetics, to analyze the required surface electric and magnetic impedances of a passive metasurface to produce either arbitrary transmission magnitude and phase or arbitrary reflection magnitude and phase. Based on the analysis, a tunable metasurface is proposed. It is shown that the transmission phase can be tuned by 360° with the unity transmissivity or the transmissivity can be tuned from 0 to 1 while the transmission phase is kept around 0°. The reflection magnitude and phase can also been tuned similarly with the proposed metasurface. The proposed design may have many potential applications, such as the dynamic EM beam forming and scanning.
2D dynamical magma propagation modeling: application to the 2001 Mount Etna eruption
NASA Astrophysics Data System (ADS)
Pinel, Virginie; Carrara, Alexandre; Maccaferri, Francesco; Rivalta, Eleonora; Corbi, Fabio
2016-04-01
Numerical and analog studies of dike propagation in a stress field induced by volcanic edifice construction have shown that surface loading tends both to attract the magma and to reduce its velocity. Available numerical models can either calculate the trajectory or the velocity of the ascending dikes, but not both of them simultaneously. We developed a hybrid model of dyke propagation in two dimensions solving both for the magma trajectory and velocity as a function of the source overpressure, the magma physical properties (density and viscosity) as well as the crustal density and stress field. We first calculate a dyke trajectory in 2D and secondly run a 1D dynamical model of dyke propagation along this trajectory taken into account the influence of the stress field seen by the magma along this path. This model is used to characterize the influence of surface load on magma migration towards the surface and compared to previous results obtained by analog modeling.We find that the amplitude of dyke deflection and magma velocity variation depend on the ratio between the dyke driving pressure (source overpressure as well buoyancy) and the stress field perturbation. Our model is then applied to the July 2001 eruption of Etna, where the final dyke deflection had been previously interpreted as due to the topographic load by Bonaccorso et al. [2010]. We show that the velocity decrease observed during the last stage of the propagation can also be attributed to the local stress field. We use the dyke propagation duration to estimate the magma overpressure at the dyke bottom to be less than 4 MPa.
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 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.
Shaft vibrations in turbomachinery excited by cracks
NASA Technical Reports Server (NTRS)
Grabowski, B.
1982-01-01
During the past years the dynamic behavior of rotors with cracks has been investigated mainly theoretically. This paper deals with the comparison of analytical and experimental results of the dynamics of a rotor with an artificial crack. The experimental results verify the crack model used in the analysis. They show the general possibility to determine a crack by extended vibration control.
NASA Astrophysics Data System (ADS)
Okada, Akinori; Mizutani, Yusuke; Subagyo, Agus; Hosoi, Hirotaka; Nakamura, Motonori; Sueoka, Kazuhisa; Kawahara, Koichi; Okajima, Takaharu
2011-12-01
We investigated dynamic force propagation between focal adhesions of fibroblast cells cultured on polydimethylsiloxane micropost substrates, by atomic force microscopy. Live cells were mechanically modulated by the atomic force microscopy probe bound to cell apical surfaces at 0.01-0.5 Hz, while microposts served as a force sensor at basal surfaces. We observed that cells exhibited rheological behavior at the apical surface but had no apparent out-of-phase response at the basal surface, indicating that the dynamic force propagating through cytoskeletal filaments behaves in an elastic manner. Moreover, the direction of the propagated force was observed to be intimately associated with the prestress.
NASA Astrophysics Data System (ADS)
Uenishi, Koji
2016-04-01
This contribution outlines our experimental observations of seismicity-related fast fracture (rupture) propagation in solids utilising high-speed analog and digital photography (maximum frame rate 1,000,000 frames per second) over the last two decades. Dynamic fracture may be triggered or initiated in the monolithic or layered seismic models by detonation of micro explosives, a projectile launched by a gun, laser pulses and electric discharge impulses, etc. First, we have investigated strike-slip rupture along planes of weakness in transparent photoelastic (birefringent) materials at a laboratory scale and shown (at that time) extraordinarily fast rupture propagation in a bi-material system and its possible effect on the generation of large strong motion in the limited narrow areas in the Kobe region on the occasion of the 1995 Hyogo-ken Nanbu, Japan, earthquake (Uenishi Ph.D. thesis 1997, Uenishi et al. BSSA 1999). In this series of experiments, we have also modelled shallow dip-slip earthquakes and indicated a possible origin of the asymmetric ground motion in the hanging and foot-walls. In the photoelastic photographs, we have found the unique dynamic wave interaction and generation of specific shear and interface waves numerically predicted by Uenishi and Madariaga (Eos 2005), and considered as a case study the seismic motion associated with the 2014 Nagano-ken Hokubu (Kamishiro Fault), Japan, dip-slip earthquake (Uenishi EFA 2015). Second, we have experimentally shown that even in a monolithic material, rupture speed may exceed the local shear wave speed if we employ hyperelasically behaving materials like natural rubber (balloons) (Uenishi Eos 2006, Uenishi ICF 2009, Uenishi Trans. JSME A 2012) but fracture in typical monolithic thin fluid films (e.g. soap bubbles, which may be treated as a solid material) propagates at an ordinary subsonic (sub-Rayleigh) speed (Uenishi et al. SSJ 2006). More recent investigation handling three-dimensional rupture propagation
Dynamic analyses of a crack run-arrest experiment in a nonisothermal plate
Bass, B.R.; Pugh, C.E.; Stamm, H.K.
1985-06-01
In crack-arrest studies sponsored by the Heavy-Section Steel Technology (HSST) Program, a primary objective has been to produce fracture toughness data for reactor pressure vessel materials at temperatures approaching the Charpy upper-shelf regime. Wide-plate tests being conducted at the National Bureau of Standards for the HSST Program are providing an opportunity to obtain significant numbers of data points at affordable costs. In these tests, a single-edge crack in a wide-plate which is subjected to tensile loading initiates at low temperature and arrests in a region of increased fracture toughness. The gradient in toughness is achieved by applying a linear transverse temperature profile across the plate. The second test in this series for A 533 grade B class 1 steel involves crack initiation in cleavage followed by arrest at a temperature corresponding to Charpy upper-shelf behavior. The plate geometry, the material properties, and the instrumentation are discussed along with conditions and results for this test. Pretest static and elastodynamic analyses are described, and posttest analyses based on actual boundary conditions are shown to compare favorably with the observed run-arrest events.
NASA Astrophysics Data System (ADS)
Angelikopoulos, Panagiotis; Papadimitriou, Costas; Koumoutsakos, Petros
2012-10-01
We present a Bayesian probabilistic framework for quantifying and propagating the uncertainties in the parameters of force fields employed in molecular dynamics (MD) simulations. We propose a highly parallel implementation of the transitional Markov chain Monte Carlo for populating the posterior probability distribution of the MD force-field parameters. Efficient scheduling algorithms are proposed to handle the MD model runs and to distribute the computations in clusters with heterogeneous architectures. Furthermore, adaptive surrogate models are proposed in order to reduce the computational cost associated with the large number of MD model runs. The effectiveness and computational efficiency of the proposed Bayesian framework is demonstrated in MD simulations of liquid and gaseous argon.
Angelikopoulos, Panagiotis; Papadimitriou, Costas; Koumoutsakos, Petros
2012-10-14
We present a Bayesian probabilistic framework for quantifying and propagating the uncertainties in the parameters of force fields employed in molecular dynamics (MD) simulations. We propose a highly parallel implementation of the transitional Markov chain Monte Carlo for populating the posterior probability distribution of the MD force-field parameters. Efficient scheduling algorithms are proposed to handle the MD model runs and to distribute the computations in clusters with heterogeneous architectures. Furthermore, adaptive surrogate models are proposed in order to reduce the computational cost associated with the large number of MD model runs. The effectiveness and computational efficiency of the proposed Bayesian framework is demonstrated in MD simulations of liquid and gaseous argon.
Modeling the dynamics of several particles behind a propagating shock wave
NASA Astrophysics Data System (ADS)
Bedarev, I. A.; Fedorov, A. V.
2017-01-01
The interaction of a shock wave in a gas phase with a system of particles moving in this gas has been numerically simulated. The wave pattern of the nonstationary interaction of the propagating shock wave with these particles is described in detail. The mathematical model and computational technology employed is compared with experimental data on the dynamics of particles behind the shock wave. It is established that the approximate model of separate particles used to calculate relaxation of their velocities unsatisfactorily operates in the presence of a mutual influence of particles, whereby one particle can occur in the aerodynamic shadow of an adjacent particle.
Uncertainty in Damage Detection, Dynamic Propagation and Just-in-Time Networks
2015-08-03
AFRL-OSR-VA-TR-2015-0216 Uncertainty in Damage Detection, Dynamic Propagation and Just-in- Time Networks Harvey Banks North Carolina State University...ApprovedOMB No. 0704-0188 The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for... Time Networks 5a. CONTRACT NUMBER 5b. GRANT NUMBER FA9550-12-1-0188 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Harvey Banks 5d. PROJECT NUMBER 5e
Mean field study of a propagation-turnover lattice model for the dynamics of histone marking
NASA Astrophysics Data System (ADS)
Yao, Fan; Li, FangTing; Li, TieJun
2017-02-01
We present a mean field study of a propagation-turnover lattice model, which was proposed by Hodges and Crabtree [Proc. Nat. Acad. Sci. 109, 13296 (2012)] for understanding how posttranslational histone marks modulate gene expression in mammalian cells. The kinetics of the lattice model consists of nucleation, propagation and turnover mechanisms, and exhibits second-order phase transition for the histone marking domain. We showed rigorously that the dynamics essentially depends on a non-dimensional parameter κ = k +/ k -, the ratio between the propagation and turnover rates, which has been observed in the simulations. We then studied the lowest order mean field approximation, and observed the phase transition with an analytically obtained critical parameter. The boundary layer analysis was utilized to investigate the structure of the decay profile of the mark density. We also studied the higher order mean field approximation to achieve sharper estimate of the critical transition parameter and more detailed features. The comparison between the simulation and theoretical results shows the validity of our theory.
The Initiation and Propagation of Dynamic Abnormal Grain Growth in Molybdenum
NASA Astrophysics Data System (ADS)
Noell, Philip J.; Worthington, Daniel L.; Taleff, Eric M.
2015-12-01
Plastic straining can initiate and propagate abnormal grains at temperatures significantly lower than is possible by static annealing. This phenomenon is termed dynamic abnormal grain growth (DAGG). Experiments that produce DAGG in commercial-purity molybdenum sheet materials are used to study the initiation and propagation of abnormal grains by plastic straining at temperatures from 1673 K to 2073 K (1400° C to 1800° C). The minimum strain necessary to initiate DAGG, termed the critical strain, decreases approximately linearly with increasing temperature. The variation in critical strain values observed at a single temperature and strain rate is well described by a normal distribution. An increased fraction of grains aligned with the < 110rangle along the tensile axis, a preferred orientation for DAGG grains, appears to decrease the critical strain for DAGG initiation. DAGG grains preferentially grow into the finest-grained polycrystalline regions, which suggests that the driving force for DAGG propagation is primarily from grain-boundary curvature. No effects of local crystallographic texture variation on growth are evident in microstructures containing DAGG grains. Together, these observations support the hypothesis that plastic straining during DAGG acts primarily to increase boundary mobility, rather than to increase the driving force for boundary migration.
Computational fluid dynamics simulation of sound propagation through a blade row.
Zhao, Lei; Qiao, Weiyang; Ji, Liang
2012-10-01
The propagation of sound waves through a blade row is investigated numerically. A wave splitting method in a two-dimensional duct with arbitrary mean flow is presented, based on which pressure amplitude of different wave mode can be extracted at an axial plane. The propagation of sound wave through a flat plate blade row has been simulated by solving the unsteady Reynolds average Navier-Stokes equations (URANS). The transmission and reflection coefficients obtained by Computational Fluid Dynamics (CFD) are compared with semi-analytical results. It indicates that the low order URANS scheme will cause large errors if the sound pressure level is lower than -100 dB (with as reference pressure the product of density, main flow velocity, and speed of sound). The CFD code has sufficient precision when solving the interaction of sound wave and blade row providing the boundary reflections have no substantial influence. Finally, the effects of flow Mach number, blade thickness, and blade turning angle on sound propagation are studied.
Distributed dynamical computation in neural circuits with propagating coherent activity patterns.
Gong, Pulin; van Leeuwen, Cees
2009-12-01
Activity in neural circuits is spatiotemporally organized. Its spatial organization consists of multiple, localized coherent patterns, or patchy clusters. These patterns propagate across the circuits over time. This type of collective behavior has ubiquitously been observed, both in spontaneous activity and evoked responses; its function, however, has remained unclear. We construct a spatially extended, spiking neural circuit that generates emergent spatiotemporal activity patterns, thereby capturing some of the complexities of the patterns observed empirically. We elucidate what kind of fundamental function these patterns can serve by showing how they process information. As self-sustained objects, localized coherent patterns can signal information by propagating across the neural circuit. Computational operations occur when these emergent patterns interact, or collide with each other. The ongoing behaviors of these patterns naturally embody both distributed, parallel computation and cascaded logical operations. Such distributed computations enable the system to work in an inherently flexible and efficient way. Our work leads us to propose that propagating coherent activity patterns are the underlying primitives with which neural circuits carry out distributed dynamical computation.
NASA Astrophysics Data System (ADS)
Uenishi, K.
2011-12-01
In the previous investigations (Earthq. Eng. Struct. Dyn. 2000, Int. J. Geomech. 2010, Rock Mech. Rock Eng. 2010) we have shown the possibility of evaluating the mechanical characteristics of relevant seismic waves from earthquake-induced dynamic structural failure patterns. Slopes are one of the typical structures that may experience mechanical destabilization by seismic waves and thus can be used as "sensors" to detect specific properties of the waves, some of which, especially those related to high frequencies, might be difficult to be identified by using ordinary seismological recordings. One notable example is the fill slopes in the Midorigaoka area in Sendai City, Japan, where the dynamic loading associated with the 1978 Miyagi-ken-oki earthquake (Japan Meteorological Agency Magnitude Mj = 7.4) generated crack openings in the top surface along the upper edge (crest). Although the slopes were reinforced by steel pipe piles after this 1978 event, tensile cracking was induced again along the upper edge by the off the Pacific coast of Tohoku earthquake in March, 2011 (moment magnitude Mw = 9.0). Similar tensile cracks were found not only in California after the 1906 and 1957 San Francisco and the 1989 Loma Prieta earthquakes but also in the South Island along the cliff edge at Redcliffs, Sumner and Sumner Head on the occasion of the February 2011 Christchurch, New Zealand, earthquake (Mw = 6.1). In this study, using the moving particle semi-implicit (MPS) method, we perform two-dimensional numerical simulations for body or Rayleigh surface wave interaction with an archetypal linear elastic slope. The results suggest, at least from the stress point of view, that in order to systematically explain the generation mechanism of the tensile cracking, it may be more reasonable to assume the existence of Rayleigh waves in a high frequency range. That is, body waves and low-frequency Rayleigh waves may not be able to generate such failure patterns. Further analysis shows
Newtonian propagation methods applied to the photodissociation dynamics of I3 -
NASA Astrophysics Data System (ADS)
Ashkenazi, Guy; Kosloff, Ronnie; Ruhman, Sanford; Tal-Ezer, Hillel
1995-12-01
A uniformly convergent propagation scheme designed for non-hermitian Hamiltonian operators is presented. The method is based on a Newtonian interpolation polynomial which is created by a recursive application of the Hamiltonian operator on an initial wavefunction. The interpolation points used to construct the Newtonian polynomial are located in the complex eigenvalue space of the Hamiltonian. A new algorithm is developed to construct the interpolation points. Both time dependent and time independent quantities can be obtained using the same polynomial expansion. The method is particularly useful when negative imaginary potentials are used. The photodissociation dynamics of I3- is studied as an example of the utility of the scheme to gain insight on a dynamical encounter. The bond cleavage is followed in time simultaneously with the calculation of the Raman spectra. The study addresses the role of vibrational excitation of the reactant I3- on the nascent I2- spectral modulations and Raman spectra.
NASA Astrophysics Data System (ADS)
Tanigaki, Kenichi; Idouji, Toru; Horikawa, Keitaro; Kobayashi, Hidetoshi; Ogawa, Kinya
2015-09-01
Finite element models of closed-cell foam structures were created using the three-dimensional Voronoi tessellation method coupled with the random sequential addition algorithm. The dynamic compressive deformation behaviors of the models were numerically studied using LS-DYNA code. The deformation mode of the models changed gradually as the deformation rate increases. Also, the generation and the propagation of plastic wave was clearly observed with the rate of 100 m/s. The longitudinal elastic wave velocity showed a weak negative dependency on the deformation rate although the strain rate dependence of material properties was not considered. Furthermore, a prediction method for the dynamic stress state on the impact side based on the static stress-strain relationship was presented.
Emergence of Metastable State Dynamics in Interconnected Cortical Networks with Propagation Delays
Kutchko, Katrina M.; Fröhlich, Flavio
2013-01-01
The importance of the large number of thin-diameter and unmyelinated axons that connect different cortical areas is unknown. The pronounced propagation delays in these axons may prevent synchronization of cortical networks and therefore hinder efficient information integration and processing. Yet, such global information integration across cortical areas is vital for higher cognitive function. We hypothesized that delays in communication between cortical areas can disrupt synchronization and therefore enhance the set of activity trajectories and computations interconnected networks can perform. To evaluate this hypothesis, we studied the effect of long-range cortical projections with propagation delays in interconnected large-scale cortical networks that exhibited spontaneous rhythmic activity. Long-range connections with delays caused the emergence of metastable, spatio-temporally distinct activity states between which the networks spontaneously transitioned. Interestingly, the observed activity patterns correspond to macroscopic network dynamics such as globally synchronized activity, propagating wave fronts, and spiral waves that have been previously observed in neurophysiological recordings from humans and animal models. Transient perturbations with simulated transcranial alternating current stimulation (tACS) confirmed the multistability of the interconnected networks by switching the networks between these metastable states. Our model thus proposes that slower long-range connections enrich the landscape of activity states and represent a parsimonious mechanism for the emergence of multistability in cortical networks. These results further provide a mechanistic link between the known deficits in connectivity and cortical state dynamics in neuropsychiatric illnesses such as schizophrenia and autism, as well as suggest non-invasive brain stimulation as an effective treatment for these illnesses. PMID:24204238
NASA Astrophysics Data System (ADS)
Beatty, J. H.; Shiflet, G. J.; Jata, K. V.
1988-04-01
Dual-phase microstructures consisting of ferrite with carbides (Mo2C) surrounding equiaxed martensite packets have been developed in two alloys, Fe-O. 2C-4Mo and Fe-O. 2C-2Mo. These alloys were chosen because of the presence of two distinct carbide morphologies: (1) a needle-shaped interphase carbide structure, and (2) a fibrous carbide structure. Isothermal transformations were used to control the carbide morphology and distribution in the ferritic regions of the dual-phase microstructures. In the present research the effects of changes in carbide structure on low cycle fatigue (LCF) and fatigue crack growth (FCG) behavior were studied. Crack initiation was observed at prior austenite grain boundaries in the fibrous microstructure, and along intrusion/extrusion defects in the interphase needle microstructures for LCF tests. TEM studies revealed a carbide free region at prior austenite grain boundaries where initiation occurs for the fibrous case. The cyclic stress/strain response of the ferritic portions of the microstructure is determined by the ability of the carbides to homogenize the strain found there. This affects the stress/strain distribution in the composite ferrite-martensite microstructure by changing the hardness ratio of the two phases and subsequently alters the fatigue crack growth behavior and the macroscopic cyclic stress/strain response. Strain localization was also found to affect the roughness induced closure found for fatigue crack growth tests for low R tests ( R = 0.1).
Dynamic Granger-Geweke causality modeling with application to interictal spike propagation.
Lin, Fa-Hsuan; Hara, Keiko; Solo, Victor; Vangel, Mark; Belliveau, John W; Stufflebeam, Steven M; Hämäläinen, Matti S
2009-06-01
A persistent problem in developing plausible neurophysiological models of perception, cognition, and action is the difficulty of characterizing the interactions between different neural systems. Previous studies have approached this problem by estimating causal influences across brain areas activated during cognitive processing using structural equation modeling (SEM) and, more recently, with Granger-Geweke causality. While SEM is complicated by the need for a priori directional connectivity information, the temporal resolution of dynamic Granger-Geweke estimates is limited because the underlying autoregressive (AR) models assume stationarity over the period of analysis. We have developed a novel optimal method for obtaining data-driven directional causality estimates with high temporal resolution in both time and frequency domains. This is achieved by simultaneously optimizing the length of the analysis window and the chosen AR model order using the SURE criterion. Dynamic Granger-Geweke causality in time and frequency domains is subsequently calculated within a moving analysis window. We tested our algorithm by calculating the Granger-Geweke causality of epileptic spike propagation from the right frontal lobe to the left frontal lobe. The results quantitatively suggested that the epileptic activity at the left frontal lobe was propagated from the right frontal lobe, in agreement with the clinical diagnosis. Our novel computational tool can be used to help elucidate complex directional interactions in the human brain.
Dynamic Granger-Geweke causality modeling with application to interictal spike propagation
Lin, Fa-Hsuan; Hara, Keiko; Solo, Victor; Vangel, Mark; Belliveau, John W.; Stufflebeam, Steven M.; Hamalainen, Matti S.
2010-01-01
A persistent problem in developing plausible neurophysiological models of perception, cognition, and action is the difficulty of characterizing the interactions between different neural systems. Previous studies have approached this problem by estimating causal influences across brain areas activated during cognitive processing using Structural Equation Modeling and, more recently, with Granger-Geweke causality. While SEM is complicated by the need for a priori directional connectivity information, the temporal resolution of dynamic Granger-Geweke estimates is limited because the underlying autoregressive (AR) models assume stationarity over the period of analysis. We have developed a novel optimal method for obtaining data-driven directional causality estimates with high temporal resolution in both time and frequency domains. This is achieved by simultaneously optimizing the length of the analysis window and the chosen AR model order using the SURE criterion. Dynamic Granger-Geweke causality in time and frequency domains is subsequently calculated within a moving analysis window. We tested our algorithm by calculating the Granger-Geweke causality of epileptic spike propagation from the right frontal lobe to the left frontal lobe. The results quantitatively suggested the epileptic activity at the left frontal lobe was propagated from the right frontal lobe, in agreement with the clinical diagnosis. Our novel computational tool can be used to help elucidate complex directional interactions in the human brain. PMID:19378280
Weare, Jonathan; Dinner, Aaron R.; Roux, Benoît
2016-01-01
A multiple time-step integrator based on a dual Hamiltonian and a hybrid method combining molecular dynamics (MD) and Monte Carlo (MC) is proposed to sample systems in the canonical ensemble. The Dual Hamiltonian Multiple Time-Step (DHMTS) algorithm is based on two similar Hamiltonians: a computationally expensive one that serves as a reference and a computationally inexpensive one to which the workload is shifted. The central assumption is that the difference between the two Hamiltonians is slowly varying. Earlier work has shown that such dual Hamiltonian multiple time-step schemes effectively precondition nonlinear differential equations for dynamics by reformulating them into a recursive root finding problem that can be solved by propagating a correction term through an internal loop, analogous to RESPA. Of special interest in the present context, a hybrid MD-MC version of the DHMTS algorithm is introduced to enforce detailed balance via a Metropolis acceptance criterion and ensure consistency with the Boltzmann distribution. The Metropolis criterion suppresses the discretization errors normally associated with the propagation according to the computationally inexpensive Hamiltonian, treating the discretization error as an external work. Illustrative tests are carried out to demonstrate the effectiveness of the method. PMID:26918826
Reconstruction of laser-induced cavitation bubble dynamics based on a Fresnel propagation approach.
Devia-Cruz, Luis Felipe; Camacho-López, Santiago; Cortés, Víctor Ruiz; Ramos-Muñiz, Victoria; Pérez-Gutiérrez, Francisco G; Aguilar, Guillermo
2015-12-10
A single laser-induced cavitation bubble in transparent liquids has been studied through a variety of experimental techniques. High-speed video with varying frame rate up to 20×10(7) fps is the most suitable to study nonsymmetric bubbles. However, it is still expensive for most researchers and more affordable (lower) frame rates are not enough to completely reproduce bubble dynamics. This paper focuses on combining the spatial transmittance modulation (STM) technique, a single shot cavitation bubble and a very simple and inexpensive experimental technique, based on Fresnel approximation propagation theory, to reproduce a laser-induced cavitation spatial dynamics. Our results show that the proposed methodology reproduces a laser-induced cavitation event much more accurately than 75,000 fps video recording. In conclusion, we propose a novel methodology to reproduce laser-induced cavitation events that combine the STM technique with Fresnel propagation approximation theory that properly reproduces a laser-induced cavitation event including a very precise identification of the first, second, and third collapses of the cavitation bubble.
Krishnaswamy, Manjunath Mysore
2016-01-01
Introduction Success of any endodontic treatment depends on strict adherence to ‘endodontic triad’. Preparation of root canal system is recognized as being one of the most important stages in root canal treatment. At times, we inevitably end up damaging root dentin which becomes a Gateway for infections like perforation, zipping, dentinal cracks and minute intricate fractures or even vertical root fractures, thereby resulting in failure of treatment. Several factors may be responsible for the formation of dentinal cracks like high concentration of sodium hypochlorite, compaction methods and various canal shaping methods. Aim To compare and evaluate the effects of root canal preparation techniques and instrumentation length on the development of apical root cracks. Materials and Methods Seventy extracted premolars with straight roots were mounted on resin blocks with simulated periodontal ligaments, exposing 1-2 mm of the apex followed by sectioning of 1mm of root tip for better visualization under stereomicroscope. The teeth were divided into seven groups of 10 teeth each – a control group and six experimental groups. Subgroup A & B were instrumented with: Stainless Steel hand files (SS) up to Root Canal Length (RCL) & (RCL –1 mm) respectively; sub group C & D were instrumented using ProTaper Universal (PTU) up to RCL and (RCL -1mm) respectively; subgroup E & F were instrumented using ProTaper Next (PTN) up to RCL & (RCL -1 mm) respectively. Stereomicroscopic images of the instrumentation sequence were compared for each tooth. The data was analyzed statistically using descriptive analysis by ‘Phi’ and ‘Cramers’ test to find out statistical significance between the groups. The level of significance was set at p< 0.05 using SPSS software. Results Stainless steel hand file group showed most cracks followed by ProTaper Universal & ProTaper Next though statistically not significant. Samples instrumented up to 1mm short of working length (RCL-1mm) showed
NASA Astrophysics Data System (ADS)
Trifonova, Tatiana; Tulenev, Nikita; Trifonov, Dmitriy; Arakelian, Sergei
2014-05-01
stimulating a trigger mechanism for releasing of groundwater; (ii) the crackness/fracturing structure as a characteristic property for all rocks, being dissecting by totality of cracks/fissures and along which (in the case when a good development crack becomes a fault) a vertical and/or lateral movement (of both groundwater and surface water mass) occurs as a result of excessive strain; (iii) areas of formation and modification in time of groundwater transit system, and especially the modalities for it exit on surface by different factors including tectonic processes under adjustable conditions for both localization of earthquake epicenters/volcanos activity areas and occurring floods in respect of propagating of seismic waves and dislocation of border for lithospheric plates/magma objects in the river basin region; (iv) the way of distribution over surface for water flows/fronts in the further, which can be described by nonlinear hydrodynamic approach, e.g. by different classes of solutions for Korteweg-de Vries equation, associated with observable natural phenomena. 4. Monitoring in dynamics of state of hydrostatic/hydrodynamic pressures in underground aquifers (e.g. by artesian wells in comparison with two databases: before and after the events) is an important factor in assessing of acceptable risk for the events. Combining it with monitoring of seismic activity should allow to make a more detailed forecasting and zoning of potentially dangerous areas for such natural disasters.
NASA Astrophysics Data System (ADS)
Acharyya, Muktish; Halder, Ajay
2017-03-01
The dynamical responses of Blume-Capel (S=1) ferromagnet to the plane propagating (with fixed frequency and wavelength) and standing magnetic field waves are studied separately in two dimensions by extensive Monte Carlo simulation. Depending on the values of temperature, amplitude of the propagating magnetic field and the strength of anisotropy, two different dynamical phases are observed. For a fixed value of anisotropy and the amplitude of the propagating magnetic field, the system undergoes a dynamical phase transition from a driven spin wave propagating phase to a pinned or spin frozen state as the system is cooled down. The time averaged magnetisation over a full cycle of the propagating magnetic field plays the role of the dynamic order parameter. A comprehensive phase diagram is plotted in the plane formed by the amplitude of the propagating wave and the temperature of the system. It is found that the phase boundary shrinks inward as the anisotropy increases. The phase boundary, in the plane described by the strength of the anisotropy and temperature, is also drawn. This phase boundary was observed to shrink inward as the field amplitude increases.
Tomita, N; Kitakura, T; Onmori, N; Ikada, Y; Aoyama, E
1999-01-01
Flaking-type wear, so-called delamination, is often observed in polyethylene joint components. This is thought to occur partly due to crack formation and propagation at grain boundaries. This study examined the effect of vitamin E on the crack formation and/or propagation in UHMWPE by using 2-dimensional sliding fatigue testing and micro indenter testing. An in vitro sliding fatigue test was performed under two simplified articulating movements, and the cracks produced were observed by scanning acoustic tomography (SAT). Gamma-irradiated ultrahigh molecular weight polyethylene (UHMWPE) specimens demonstrated a smaller area of accumulated cracks as compared to virgin specimens, when the loading movement was reciprocated on a single linear locus. However, four out of five gamma-irradiated UHMWPE specimens exhibited severe flaking-like destruction under the complicated sliding condition, suggesting that gamma irradiation accelerated crack propagation under multidirectional loading. All the gamma-irradiated vitamin-E-containing specimens demonstrated no subsurface crack formation and no flaking-like destruction. Results using micro indenter testing showed that the dynamic hardness at grain boundary was higher than that in grain, and was increased by gamma irradiation. This hardening at grain boundary was reduced by adding vitamin E. It is possible that the presence of vitamin E prevents crack propagation partly due to reduced hardness at grain boundaries. The gamma-irradiated vitamin-E-containing UHMWPE is a promising material to prevent flaking-like destruction of polyethylene joint components.
DYNAMIC BEHAVIOR OF CONCRETE GRAVITY DAM ON JOINTED ROCK FOUNDATION DURING LARGE-SCALE EARTHQUAKE
NASA Astrophysics Data System (ADS)
Kimata, Hiroyuki; Fujita, Yutaka; Horii, Hideyuki; Yazdani, Mahmoud
Dynamic cracking analysis of concrete gravity dam has been carried out during large-scale earthquake, considering the progressive failure of jointed rock foundation. Firstly, in order to take into account the progressive failure of rock foundation, the constitutive law of jointed rock is assumed and its validity is evaluated by simulation analysis based on the past experimental model. Finally, dynamic cracking analysis of 100-m high dam model is performed, using the previously proposed approach with tangent stiffness-proportional damping to express the propagation behavior of crack and the constitutive law of jointed rock. The crack propagation behavior of dam body and the progressive failure of jointed rock foundation are investigated.
Effect of laser pulse propagation on ultrafast magnetization dynamics in a birefringent medium
NASA Astrophysics Data System (ADS)
de Jong, J. A.; Kalashnikova, A. M.; Pisarev, R. V.; Balbashov, A. M.; Kimel, A. V.; Kirilyuk, A.; Rasing, Th
2017-04-01
Light propagation effects can strongly influence the excitation and the detection of laser-induced magnetization dynamics. We investigated experimentally and analytically the effects of crystallographic linear birefringence on the excitation and detection of ultrafast magnetization dynamics in the rare-earth orthoferrites (Sm0.5Pr0.5)FeO3 and (Sm0.55Tb0.45)FeO3, which possess weak and strong linear birefringence, respectively. Our finding is that the effect of linear birefringence on the result of a magneto-optical pump-probe experiment strongly depends on the mechanism of excitation. When magnetization dynamics, probed by means of the Faraday effect, is excited via a rapid, heat-induced phase transition, the measured rotation of the probe pulse polarization is strongly suppressed due to the birefringence. This contrasts with the situation for magnetization dynamics induced by the ultrafast inverse Faraday effect, where the corresponding probe polarization rotation values were larger in the orthoferrite with strong linear birefringence. We show that this striking difference results from an interplay between the polarization transformations experienced by pump and probe pulses in the birefringent medium.
Crack velocity jumps engendered by a transformational process zone.
Boulbitch, A; Korzhenevskii, A L
2016-06-01
We study a concerted propagation of a fast crack with the process zone where a rearrangement of the solid structure takes place. The latter is treated as a second-order local phase transformation. We demonstrate that the propagation of such a zone gives rise to a nonlinear frictionlike force exerted on the crack tip, resisting its propagation. Depending on the temperature, it produces three regimes of crack motion, which differ in the behavior of the crack tip process zone: (i) always existing, (ii) only emerging at a high crack speed, and (iii) flickering. We show that the latter regime exhibits crack velocity jumps.
2014-07-01
Information Center (DTIC) (http://www.dtic.mil). AFRL-RX-WP-JA-2014-0172 HAS BEEN REVIEWED AND IS APPROVED FOR PUBLICATION IN ACCORDANCE WITH...another approach, Pompidou and Lamon [17] employed the Cook and Gordon model [13] to estimate Distribution A. Approved for public release; distribution...of Solids and Structures, vol. 41, pp. 6937-6948, 2004. [17] S. Pompidou and J. Lamon, "Analysis of crack deviation in ceramic matrix xomposites
NASA Astrophysics Data System (ADS)
Seifi, Mohsen; Dahar, Matthew; Aman, Ron; Harrysson, Ola; Beuth, Jack; Lewandowski, John J.
2015-03-01
This preliminary work documents the effects of test orientation with respect to build and beam raster directions on the fracture toughness and fatigue crack growth behavior of as-deposited EBM Ti-6Al-4V. Although ASTM/ISO standards exist for determining the orientation dependence of various mechanical properties in both cast and wrought materials, these standards are evolving for materials produced via additive manufacturing (AM) techniques. The current work was conducted as part of a larger America Makes funded project to begin to examine the effects of process variables on the microstructure and fracture and fatigue behavior of AM Ti-6Al-4V. In the fatigue crack growth tests, the fatigue threshold, Paris law slope, and overload toughness were determined at different load ratios, R, whereas fatigue precracked samples were tested to determine the fracture toughness. The as-deposited material exhibited a fine-scale basket-weave microstructure throughout the build, and although fracture surface examination revealed the presence of unmelted powders, disbonded regions, and isolated porosity, the resulting mechanical properties were in the range of those reported for cast and wrought Ti-6Al-4V. Remote access and control of testing was also developed at Case Western Reserve University to improve efficiency of fatigue crack growth testing.
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.
The interplay of crack hopping, delamination and interface failure in drying nanoparticle films
Yang, Bin; Sharp, James S.; Smith, Mike I.
2016-01-01
Films formed through the drying of nanoparticle suspensions release the build-up of strain through a variety of different mechanisms including shear banding, crack formation and delamination. Here we show that important connections exist between these different phenomena: delamination depends on the dynamics of crack hopping, which in turn is influenced by the presence of shear bands. We also show that delamination does not occur uniformly across the film. As cracks hop they locally initiate the delamination of the film which warps with a timescale much longer than that associated with the hopping of cracks. The motion of a small region of the delamination front, where the shear component of interfacial crack propagation is believed to be enhanced, results in the deposition of a complex zig-zag pattern on the supporting substrate. PMID:27558989
NASA Astrophysics Data System (ADS)
Pan, Zhao; Whitehead, Jared; Thomson, Scott; Truscott, Tadd
2016-08-01
Obtaining pressure field data from particle image velocimetry (PIV) is an attractive technique in fluid dynamics due to its noninvasive nature. The application of this technique generally involves integrating the pressure gradient or solving the pressure Poisson equation using a velocity field measured with PIV. However, very little research has been done to investigate the dynamics of error propagation from PIV-based velocity measurements to the pressure field calculation. Rather than measure the error through experiment, we investigate the dynamics of the error propagation by examining the Poisson equation directly. We analytically quantify the error bound in the pressure field, and are able to illustrate the mathematical roots of why and how the Poisson equation based pressure calculation propagates error from the PIV data. The results show that the error depends on the shape and type of boundary conditions, the dimensions of the flow domain, and the flow type.
Pan, Zhao; Whitehead, Jared; Thomson, Scott; Truscott, Tadd
2016-08-01
Obtaining pressure field data from particle image velocimetry (PIV) is an attractive technique in fluid dynamics due to its noninvasive nature. The application of this technique generally involves integrating the pressure gradient or solving the pressure Poisson equation using a velocity field measured with PIV. However, very little research has been done to investigate the dynamics of error propagation from PIV-based velocity measurements to the pressure field calculation. Rather than measure the error through experiment, we investigate the dynamics of the error propagation by examining the Poisson equation directly. We analytically quantify the error bound in the pressure field, and are able to illustrate the mathematical roots of why and how the Poisson equation based pressure calculation propagates error from the PIV data. The results show that the error depends on the shape and type of boundary conditions, the dimensions of the flow domain, and the flow type.
Implications of heterogeneity in the shock wave propagation of dynamically shocked materials
NASA Astrophysics Data System (ADS)
LaJeunesse, Jeff
The field of shock physics as a whole has only recently begun to pay particular attention to modeling heterogeneous materials under shock loading. These materials are important because of their practicality in terms of creating stronger, more shock resistant materials. To understand why they absorb shock impact energy better than homogeneous materials means that the small-scale processes that occur during the shock loading of these heterogeneous materials needs to be understood. Recent computational experiments, called mesoscale simulations, have shown that explicitly incorporating small-scale heterogeneous features into hydrocode simulations allows the bulk shock response of the heterogeneous material to be observed while not requiring the use of empirically determined constitutive equations. Including these features in simulations can offer insights into the irreversible mechanisms that dominate the propagation of shock waves in heterogeneous materials. Three cases where the mesoscale approach for modeling the dynamic shock loading of heterogeneous materials are presented. These materials fall into three categories: granular - dry sand, granular with binder - concrete, and granular contained in a metal foam with a binder - granular explosive contained in an aluminum foam. The processes in which shock waves propagate through each material are addressed and relationships between the three materials are discussed. Particle velocity profiles for dry sand and concrete was obtained from Harvard University and Eglin Air Force Base, respectively. Mesoscale simulations using CTH are conducted for each type of heterogeneous material and the results are compared to the experimental data.
Li, Hua; Thériault, Jolaine; Rousselle, Bruno; Subramanian, Balaji; Robichaud, Jacques; Djaoued, Yahia
2014-02-28
A 'dynamic hard-template' infiltration strategy for crack-free large-area synthesis of 2D WO3 inverse opal (IO) films on ITO substrates using a wide range of sizes of sacrificial PS spheres is reported. Thus prepared WO3 IO films were successfully used as an active electrode in the fabrication of an electrochromic device.
Lymperopoulos, Ilias N; Ioannou, George D
2016-10-01
We develop and validate a model of the micro-level dynamics underlying the formation of macro-level information propagation patterns in online social networks. In particular, we address the dynamics at the level of the mechanism regulating a user's participation in an online information propagation process. We demonstrate that this mechanism can be realistically described by the dynamics of noisy spiking neurons driven by endogenous and exogenous, deterministic and stochastic stimuli representing the influence modulating one's intention to be an information spreader. Depending on the dynamically changing influence characteristics, time-varying propagation patterns emerge reflecting the temporal structure, strength, and signal-to-noise ratio characteristics of the stimulation driving the online users' information sharing activity. The proposed model constitutes an overarching, novel, and flexible approach to the modeling of the micro-level mechanisms whereby information propagates in online social networks. As such, it can be used for a comprehensive understanding of the online transmission of information, a process integral to the sociocultural evolution of modern societies. The proposed model is highly adaptable and suitable for the study of the propagation patterns of behavior, opinions, and innovations among others.
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.
Khambhati, Ankit N.; Davis, Kathryn A.; Oommen, Brian S.; Chen, Stephanie H.; Lucas, Timothy H.; Litt, Brian; Bassett, Danielle S.
2015-01-01
The epileptic network is characterized by pathologic, seizure-generating ‘foci’ embedded in a web of structural and functional connections. Clinically, seizure foci are considered optimal targets for surgery. However, poor surgical outcome suggests a complex relationship between foci and the surrounding network that drives seizure dynamics. We developed a novel technique to objectively track seizure states from dynamic functional networks constructed from intracranial recordings. Each dynamical state captures unique patterns of network connections that indicate synchronized and desynchronized hubs of neural populations. Our approach suggests that seizures are generated when synchronous relationships near foci work in tandem with rapidly changing desynchronous relationships from the surrounding epileptic network. As seizures progress, topographical and geometrical changes in network connectivity strengthen and tighten synchronous connectivity near foci—a mechanism that may aid seizure termination. Collectively, our observations implicate distributed cortical structures in seizure generation, propagation and termination, and may have practical significance in determining which circuits to modulate with implantable devices. PMID:26680762
NASA Astrophysics Data System (ADS)
Lu, Hua; Franzke, Christian; Martius, Olivia; Jarvis, Martin J.; Phillips, Tony
2013-05-01
We provide statistical evidence of the effect of the solar wind dynamic pressure (Psw) on the northern winter and spring circulations. We find that the vertical structure of the Northern Annular Mode (NAM), the zonal mean circulation, and Eliassen-Palm (EP)-flux anomalies show a dynamically consistent pattern of downward propagation over a period of ~45 days in response to positive Psw anomalies. When the solar irradiance is high, the signature of Psw is marked by a positive NAM anomaly descending from the stratosphere to the surface during winter. When the solar irradiance is low, the Psw signal has the opposite sign, occurs in spring, and is confined to the stratosphere. The negative Psw signal in the NAM under low solar irradiance conditions is primarily governed by enhanced vertical EP-flux divergence and a warmer polar region. The winter Psw signal under high solar irradiance conditions is associated with positive anomalies of the horizontal EP-flux divergence at 55°N-75°N and negative anomalies at 25°N-45°N, which corresponds to the positive NAM anomaly. The EP-flux divergence anomalies occur ~15 days ahead of the mean-flow changes. A significant equatorward shift of synoptic-scale Rossby wave breaking (RWB) near the tropopause is detected during January-March, corresponding to increased anticyclonic RWB and a decrease in cyclonic RWB. We suggest that the barotropic instability associated with asymmetric ozone in the upper stratosphere and the baroclinic instability associated with the polar vortex in the middle and lower stratosphere play a critical role for the winter signal and its downward propagation.
NASA Astrophysics Data System (ADS)
Lisinetskaya, Polina G.; Röhr, Merle I. S.; Mitrić, Roland
2016-06-01
We present a theoretical approach for the simulation of the electric field and exciton propagation in ordered arrays constructed of molecular-sized noble metal clusters bound to organic polymer templates. In order to describe the electronic coupling between individual constituents of the nanostructure we use the ab initio parameterized transition charge method which is more accurate than the usual dipole-dipole coupling. The electronic population dynamics in the nanostructure under an external laser pulse excitation is simulated by numerical integration of the time-dependent Schrödinger equation employing the fully coupled Hamiltonian. The solution of the TDSE gives rise to time-dependent partial point charges for each subunit of the nanostructure, and the spatio-temporal electric field distribution is evaluated by means of classical electrodynamics methods. The time-dependent partial charges are determined based on the stationary partial and transition charges obtained in the framework of the TDDFT. In order to treat large plasmonic nanostructures constructed of many constituents, the approximate self-consistent iterative approach presented in (Lisinetskaya and Mitrić in Phys Rev B 89:035433, 2014) is modified to include the transition-charge-based interaction. The developed methods are used to study the optical response and exciton dynamics of Ag3+ and porphyrin-Ag4 dimers. Subsequently, the spatio-temporal electric field distribution in a ring constructed of ten porphyrin-Ag4 subunits under the action of circularly polarized laser pulse is simulated. The presented methodology provides a theoretical basis for the investigation of coupled light-exciton propagation in nanoarchitectures built from molecular size metal nanoclusters in which quantum confinement effects are important.
NASA Technical Reports Server (NTRS)
Parker, Robert G.; Guo, Yi; Eritenel, Tugan; Ericson, Tristan M.
2012-01-01
Vibration and noise caused by gear dynamics at the meshing teeth propagate through power transmission components to the surrounding environment. This study is devoted to developing computational tools to investigate the vibro-acoustic propagation of gear dynamics through a gearbox using different bearings. Detailed finite element/contact mechanics and boundary element models of the gear/bearing/housing system are established to compute the system vibration and noise propagation. Both vibration and acoustic models are validated by experiments including the vibration modal testing and sound field measurements. The effectiveness of each bearing type to disrupt vibration propagation is speed-dependent. Housing plays an important role in noise radiation .It, however, has limited effects on gear dynamics. Bearings are critical components in drivetrains. Accurate modeling of rolling element bearings is essential to assess vibration and noise of drivetrain systems. This study also seeks to fully describe the vibro-acoustic propagation of gear dynamics through a power-transmission system using rolling element and fluid film wave bearings. Fluid film wave bearings, which have higher damping than rolling element bearings, could offer an energy dissipation mechanism that reduces the gearbox noise. The effectiveness of each bearing type to disrupt vibration propagation in explored using multi-body computational models. These models include gears, shafts, rolling element and fluid film wave bearings, and the housing. Radiated noise is mapped from the gearbox surface to surrounding environment. The effectiveness of rolling element and fluid film wave bearings in breaking the vibro-acoustic propagation path from the gear to the housing is investigated.
NASA Astrophysics Data System (ADS)
Chuvashova, Irina; Sun, Yi-min
2016-04-01
mantle beneath the northern Songliao basin and that admixture of the common sub-lithospheric component was locally introduced into the melted region by mechanism of propagating crack. This study is based on analytical data obtained for volcanic rocks in the Chinese-Russian Wudalianchi-Baikal Research Center on recent volcanism and environment. Major oxides were determined by "wet chemistry" at the Institute of the Earth's Crust SB RAS, Irkutsk. Trace-elements were measured by ICP-MS technique using mass-spectrometer Agilent 7500ce of the Center for collective use "Microanalysis" (Limnological Institute of SB RAS, Irkutsk) and Nd, Pb, and Sr isotopes by TIMS technique using mass-spectrometer Finnigan MAT 262 of the Center for collective use "Geodynamics and geochronology" (Institute of the Earth's Crust SB RAS). The work was supported by the RFBR grant № 16-05-00774. References Chuvashova, I.S., Rasskazov, S.V., Liu, J., Meng, F., Yasnygina, T.A., Fefelov, N.N., Saranina, E.V., 2009. Isotopically-enriched components in evolution of Late Cenozoic potassic magmatism in Heilongjiang province, northeast China, Proceedings of the Irkutsk State University. Series of Earth Sciences, 2 (2), pp. 181-198. Guide book for field mission to Wudalianchi National Park, China, 2010. Prepared by Wudalianchi National Park and Nature Management Committee Heilongjiang province, 50 p. Foulger, G.R., 2010. Plates vs. plumes: a geological controversy. Wiley-Blackwell, 328 p. Rasskazov, S.V., Yasnygina, T.A., Chuvashova, I.S. Mantle sources of the Cenozoic volcanic rocks of East Asia: Derivatives of slabs, the sub-lithospheric convection, and the lithosphere. Russian Journal of Pacific Geology. 2014. V. 8 (5), 355-371. Wang, Y., Chen, H., 2005. Tectonic controls on the Pleistocene-Holocene Wudalianchi volcanic field (northeastern China), Journal of Asian Earth Sciences, 24, pp. 419-431.
NASA Astrophysics Data System (ADS)
Gelman, David; Schwartz, Steven D.
2008-07-01
The recently developed mixed quantum-classical propagation method is extended to treat tunneling effects in multidimensional systems. Formulated for systems consisting of a quantum primary part and a classical bath of heavier particles, the method employs a frozen Gaussian description for the bath degrees of freedom, while the dynamics of the quantum subsystem is governed by a corrected propagator. The corrections are defined in terms of matrix elements of zeroth-order propagators. The method is applied to a model system of a double-well potential bilinearly coupled to a harmonic oscillator. The extension of the method, which includes nondiagonal elements of the correction propagator, enables an accurate treatment of tunneling in an antisymmetric double-well potential.
Zhang, Lifu; Li, Chuxin; Zhong, Haizhe; Xu, Changwen; Lei, Dajun; Li, Ying; Fan, Dianyuan
2016-06-27
We have investigated the propagation dynamics of super-Gaussian optical beams in fractional Schrödinger equation. We have identified the difference between the propagation dynamics of super-Gaussian beams and that of Gaussian beams. We show that, the linear propagation dynamics of the super-Gaussian beams with order m > 1 undergo an initial compression phase before they split into two sub-beams. The sub-beams with saddle shape separate each other and their interval increases linearly with propagation distance. In the nonlinear regime, the super-Gaussian beams evolve to become a single soliton, breathing soliton or soliton pair depending on the order of super-Gaussian beams, nonlinearity, as well as the Lévy index. In two dimensions, the linear evolution of super-Gaussian beams is similar to that for one dimension case, but the initial compression of the input super-Gaussian beams and the diffraction of the splitting beams are much stronger than that for one dimension case. While the nonlinear propagation of the super-Gaussian beams becomes much more unstable compared with that for the case of one dimension. Our results show the nonlinear effects can be tuned by varying the Lévy index in the fractional Schrödinger equation for a fixed input power.
Normal compression wave scattering by a permeable crack in a fluid-saturated poroelastic solid
NASA Astrophysics Data System (ADS)
Song, Yongjia; Hu, Hengshan; Rudnicki, John W.
2017-03-01
A mathematical formulation is presented for the dynamic stress intensity factor (mode I) of a finite permeable crack subjected to a time-harmonic propagating longitudinal wave in an infinite poroelastic solid. In particular, the effect of the wave-induced fluid flow due to the presence of a liquid-saturated crack on the dynamic stress intensity factor is analyzed. Fourier sine and cosine integral transforms in conjunction with Helmholtz potential theory are used to formulate the mixed boundary-value problem as dual integral equations in the frequency domain. The dual integral equations are reduced to a Fredholm integral equation of the second kind. It is found that the stress intensity factor monotonically decreases with increasing frequency, decreasing the fastest when the crack width and the slow wave wavelength are of the same order. The characteristic frequency at which the stress intensity factor decays the fastest shifts to higher frequency values when the crack width decreases.
NASA Astrophysics Data System (ADS)
Gelman, David; Schwartz, Steven D.
2010-05-01
The recently developed quantum-classical method has been applied to the study of dissipative dynamics in multidimensional systems. The method is designed to treat many-body systems consisting of a low dimensional quantum part coupled to a classical bath. Assuming the approximate zeroth order evolution rule, the corrections to the quantum propagator are defined in terms of the total Hamiltonian and the zeroth order propagator. Then the corrections are taken to the classical limit by introducing the frozen Gaussian approximation for the bath degrees of freedom. The evolution of the primary part is governed by the corrected propagator yielding the exact quantum dynamics. The method has been tested on two model systems coupled to a harmonic bath: (i) an anharmonic (Morse) oscillator and (ii) a double-well potential. The simulations have been performed at zero temperature. The results have been compared to the exact quantum simulations using the surrogate Hamiltonian approach.
NASA Astrophysics Data System (ADS)
Parkhill, John A.; Markovich, Thomas; Tempel, David G.; Aspuru-Guzik, Alan
2012-12-01
In this work, we develop an approach to treat correlated many-electron dynamics, dressed by the presence of a finite-temperature harmonic bath. Our theory combines a small polaron transformation with the second-order time-convolutionless master equation and includes both electronic and system-bath correlations on equal footing. Our theory is based on the ab initio Hamiltonian, and is thus well-defined apart from any phenomenological choice of basis states or electronic system-bath coupling model. The equation-of-motion for the density matrix we derive includes non-Markovian and non-perturbative bath effects and can be used to simulate environmentally broadened electronic spectra and dissipative dynamics, which are subjects of recent interest. The theory also goes beyond the adiabatic Born-Oppenheimer approximation, but with computational cost scaling such as the Born-Oppenheimer approach. Example propagations with a developmental code are performed, demonstrating the treatment of electron-correlation in absorption spectra, vibronic structure, and decay in an open system. An untransformed version of the theory is also presented to treat more general baths and larger systems.
NASA Astrophysics Data System (ADS)
Nickisch, L. J.; Fridman, Sergey; Hausman, Mark; Kraut, Shawn; Zunich, George
2016-03-01
The ionospheric data assimilation algorithm called GPS Ionospheric Inversion (GPSII; pronounced "gypsy") has been extended and employed to model the dynamic ionosphere, including medium-scale traveling ionospheric disturbances (MS-TIDs). MS-TIDs are characterized by periods of 10-30 min. GPSII can assimilate many forms of ionospheric-related data, including ionogram data and GPS L1/L2 beacon data. For this present effort, GPSII was extended to assimilate propagation time delay, integrated Doppler shift, and angle-of-arrival (AoA) measurements of HF transmissions from known reference points (KRPs). GPSII applies a regularization technique that constrains the solver to find the smoothest 3-D ionosphere model that still reproduces the input data to within their respective errors of measurement. A companion paper documents the development of the assimilation capability for KRPs. In this paper we show test results of the model's performance in reproducing measured AoA variations in the presence of medium-scale traveling ionospheric disturbances (MS-TIDs) using near vertical incidence skywave data collected at White Sands Missile Range by the Intelligence Advanced Research Projects Activity HFGeo Program Government team. We find that using three KRPs within approximately 50 km of reference/check/nonassimilated transmitters, we can reproduce the measured AoAs of the nonassimilated transmitters to within 1.9° with 90% confidence even in the presence of highly dynamic MS-TIDs.
Parkhill, John A; Markovich, Thomas; Tempel, David G; Aspuru-Guzik, Alan
2012-12-14
In this work, we develop an approach to treat correlated many-electron dynamics, dressed by the presence of a finite-temperature harmonic bath. Our theory combines a small polaron transformation with the second-order time-convolutionless master equation and includes both electronic and system-bath correlations on equal footing. Our theory is based on the ab initio Hamiltonian, and is thus well-defined apart from any phenomenological choice of basis states or electronic system-bath coupling model. The equation-of-motion for the density matrix we derive includes non-markovian and non-perturbative bath effects and can be used to simulate environmentally broadened electronic spectra and dissipative dynamics, which are subjects of recent interest. The theory also goes beyond the adiabatic Born-Oppenheimer approximation, but with computational cost scaling such as the Born-Oppenheimer approach. Example propagations with a developmental code are performed, demonstrating the treatment of electron-correlation in absorption spectra, vibronic structure, and decay in an open system. An untransformed version of the theory is also presented to treat more general baths and larger systems.
NASA Astrophysics Data System (ADS)
Olson, T. M.; Kwon, Y. W.; Hart, D. C.; Loup, D. C.; Rasmussen, E. A.
2015-10-01
The paper investigates a carbon nanotube-based sensor to detect crack propagation in aluminum structures underneath composite patching. Initial tests are conducted to determine the correct procedure and materials to properly fabricate a carbon nanotube (CNT) based sensor, which is then placed in between a composite patch and the aluminum structure. The CNTs have been utilized as sensors in previous studies but only for sensing crack propagation within the composite itself. This study focuses on crack propagation in the base material and is not concerned with the composite. In this application, the composite is only a patch and can be replaced if damaged. The study conducts both tension and fatigue testing to determine the usefulness of the CNT sensor. The CNT sensor is shown to be effective in giving an indication of the crack propagation in the aluminum. Correlation is done between the crack propagation length and the increase in electrical resistance in the CNT sensor under tensile and cyclic loading, respectively.
NASA Astrophysics Data System (ADS)
Johri, Madhur; Dunham, Eric M.; Zoback, Mark D.; Fang, Zijun
2014-02-01
We use a two-dimensional plane strain dynamic rupture model with strongly rate-weakening friction and off-fault Drucker-Prager plasticity to model damage zones associated with buried second-order thrust faults observed in the SSC reservoir. The modeling of ruptures propagating as self-sustaining pulses is performed in the framework of continuum plasticity where the plasticity formulation includes both deviatoric and volumetric plastic strains. The material deforming inelastically due to stress perturbations generated by the propagating rupture is assumed to be the damage zone associated with the fault. Dilatant plastic strains are converted into a fracture population by assuming that the dilatant plastic strain is manifested in the form of fractures. The cumulative effect of multiple slip events is considered by superposition of the plastic strain field obtained from individual slip events. The relative number of various magnitude slip events is chosen so as to honor the Gutenberg-Richter law. Results show that the decay of fracture density (F) with distance (r) from the fault can be described by a power law F = F0r- n. The fault constant F0 represents the fracture density at unit distance from the fault. The decay rate (n) in fracture density is approximately 0.85 close to the fault and increases to ~1.4 at larger distances (>10 m). Modeled damage zones are approximately 60-100 m wide. These attributes are similar to those observed in the SSC reservoir using wellbore image logs and those reported in outcrop studies. Considering fault roughness affects local damage zone characteristics, these characteristics are similar to those modeled around planar faults at a scale (~10 m) that affects bulk fluid-flow properties.
Cracks in Sheets Draped on Curved Surfaces
NASA Astrophysics Data System (ADS)
Mitchell, Noah P.; Koning, Vinzenz; Vitelli, Vincenzo; Irvine, William T. M.
Conforming materials to surfaces with Gaussian curvature has proven a versatile tool to guide the behavior of mechanical defects such as folds, blisters, scars, and pleats. In this talk, we show how curvature can likewise be used to control material failure. In our experiments, thin elastic sheets are confined on curved geometries that stimulate or suppress the growth of cracks, and steer or arrest their propagation. By redistributing stresses in a sheet, curvature provides a geometric tool for protecting certain regions and guiding crack patterns. A simple model captures crack behavior at the onset of propagation, while a 2D phase-field model successfully captures the crack's full phenomenology.
Crack instabilities of a heated glass strip
NASA Astrophysics Data System (ADS)
Adda-Bedia, Mokhtar; Pomeau, Yves
1995-10-01
Recently, Yuse and Sano [Nature (London) 362, 329 (1993)] have observed that a crack traveling in a glass strip submitted to a nonuniform thermal diffusion field undergoes numerous instabilities. We study two cases of quasistatic crack propagation. The crack extension condition in straight propagation is determined. An asymptotic analysis of the elastic free energy is introduced and scaling laws are derived. A linear stability analysis of the straight propagation is performed, based on the assumption that the crack tip propagation deviates from the centered straight one as soon as it is submitted to a ``physical'' singular shear stress. It is shown that a straight propagation can become unstable after which a wavy instability appears. The condition for instability as well as the selected wavelength is calculated quantitatively. The results are compared with experiments and the agreement is favorable.
Scott, P.; Olson, R.; Wilkowski, O.G.; Marschall, C.; Schmidt, R.
1997-06-01
This report presents the results from Subtask 1.3 of the International Piping Integrity Research Group (IPIRG) program. The objective of Subtask 1.3 is to develop data to assess analysis methodologies for characterizing the fracture behavior of circumferentially cracked pipe in a representative piping system under combined inertial and displacement-controlled stresses. A unique experimental facility was designed and constructed. The piping system evaluated is an expansion loop with over 30 meters of 16-inch diameter Schedule 100 pipe. The experimental facility is equipped with special hardware to ensure system boundary conditions could be appropriately modeled. The test matrix involved one uncracked and five cracked dynamic pipe-system experiments. The uncracked experiment was conducted to evaluate piping system damping and natural frequency characteristics. The cracked-pipe experiments evaluated the fracture behavior, pipe system response, and stability characteristics of five different materials. All cracked-pipe experiments were conducted at PWR conditions. Material characterization efforts provided tensile and fracture toughness properties of the different pipe materials at various strain rates and temperatures. Results from all pipe-system experiments and material characterization efforts are presented. Results of fracture mechanics analyses, dynamic finite element stress analyses, and stability analyses are presented and compared with experimental results.
NASA Astrophysics Data System (ADS)
Zhang, N.; Huang, H.; Duarte, M.; Zhang, J.
2016-06-01
Social media has developed extremely fast in metropolises in recent years resulting in more and more rumors disturbing our daily lives. Knowing the characteristics of rumor propagation in metropolises can help the government make efficient rumor refutation plans. In this paper, we established a dynamic spatio-temporal comprehensive risk assessment model for rumor propagation based on an improved 8-state ICSAR model (Ignorant, Information Carrier, Information Spreader, Advocate, Removal), large personal activity trajectory data, and governmental rumor refutation (anti-rumor) scenarios. Combining these relevant data with the 'big' traffic data on the use of subways, buses, and taxis, we simulated daily oral communications among inhabitants in Beijing. In order to analyze rumor and anti-rumor competition in the actual social network, personal resistance, personal preference, conformity, rumor intensity, government rumor refutation and other influencing factors were considered. Based on the developed risk assessment model, a long-term dynamic rumor propagation simulation for a seven day period was conducted and a comprehensive rumor propagation risk distribution map was obtained. A set of the sensitivity analyses were conducted for different social media and propagation routes. We assessed different anti-rumor coverage ratios and the rumor-spreading thresholds at which the government started to launch anti-rumor actions. The results we obtained provide worthwhile references useful for governmental decision making towards control of social-disrupting rumors.
Dynamic modelling of a one-stage spur gear system and vibration-based tooth crack detection analysis
NASA Astrophysics Data System (ADS)
Mohammed, Omar D.; Rantatalo, Matti; Aidanpää, Jan-Olov
2015-03-01
For the purpose of simulation and vibration-based condition monitoring of a geared system, it is important to model the system with an appropriate number of degrees of freedom (DOF). In earlier papers several models were suggested and it is therefore of interest to evaluate their limitations. In the present study a 12 DOF gear dynamic model including a gyroscopic effect was developed and the equations of motions were derived. A one-stage reduction gear was modelled using three different dynamic models (with 6, 8 and 8 reduced to 6 DOF), as well as the developed model (with 12 DOF), which is referred as the fourth model in this paper. The time-varying mesh stiffness was calculated, and dynamic simulation was then performed for different crack sizes. Time domain scalar indicators (the RMS, kurtosis and the crest factor) were applied for fault detection analysis. The results of the first model show a clearly visible difference from those of the other studied models, which were made more realistic by including two more DOF to describe the motor and load. Both the symmetric and the asymmetric disc cases were studied using the fourth model. In the case of disc symmetry, the results of the obtained response are close to those obtained from both the second and third models. Furthermore, the second model showed a slight influence from inter-tooth friction, and therefore the third model is adequate for simulating the pinion's y-displacement in the case of the symmetric disc. In the case of the asymmetric disc, the results deviate from those obtained in the symmetric case. Therefore, for simulating the pinion's y-displacement, the fourth model can be considered for more accurate modelling in the case of the asymmetric disc.
Crack growth resistance in nuclear graphites
NASA Astrophysics Data System (ADS)
Ouagne, Pierre; Neighbour, Gareth B.; McEnaney, Brian
2002-05-01
Crack growth resistance curves for the non-linear fracture parameters KR, JR and R were measured for unirradiated PGA and IM1-24 graphites that are used as moderators in British Magnox and AGR nuclear reactors respectively. All the curves show an initial rising part, followed by a plateau region where the measured parameter is independent of crack length. JR and R decreased at large crack lengths. The initial rising curves were attributed to development of crack bridges in the wake of the crack front, while, in the plateau region, the crack bridging zone and the frontal process zone, ahead of the crack tip, reached steady state values. The decreases at large crack lengths were attributed to interaction of the frontal zone with the specimen end face. Microscopical evidence for graphite fragments acting as crack bridges showed that they were much smaller than filler particles, indicating that the graphite fragments are broken down during crack propagation. There was also evidence for friction points in the crack wake zone and shear cracking of some larger fragments. Inspection of KR curves showed that crack bridging contributed ~0.4 MPa m0.5 to the fracture toughness of the graphites. An analysis of JR and R curves showed that the development of the crack bridging zone in the rising part of the curves contributed ~20% to the total work of fracture. Energies absorbed during development of crack bridges and steady state crack propagation were greater for PGA than for IM1-24 graphite. These differences reflect the greater extent of irreversible processes occurring during cracking in the coarser microtexture of PGA graphite.
Mesoscopic approach to subcritical fatigue crack growth
NASA Astrophysics Data System (ADS)
Araújo, Maycon S.; Vieira, André P.; Andrade, José S.; Herrmann, Hans J.
2016-10-01
We investigate a model for fatigue crack growth in which damage accumulation is assumed to follow a power law of the local stress amplitude, a form that can be generically justified on the grounds of the approximately self-similar aspect of microcrack distributions. Our aim is to determine the relation between model ingredients and the Paris exponent governing subcritical crack-growth dynamics at the macroscopic scale, starting from a single small notch propagating along a fixed line. By a series of analytical and numerical calculations, we show that, in the absence of disorder, there is a critical damage-accumulation exponent γ , namely γc=2 , separating two distinct regimes of behavior for the Paris exponent m . For γ >γc , the Paris exponent is shown to assume the value m =γ , a result that proves robust against the separate introduction of various modifying ingredients. Explicitly, we deal here with (i) the requirement of a minimum stress for damage to occur, (ii) the presence of disorder in local damage thresholds, and (iii) the possibility of crack healing. On the other hand, in the regime γ <γc , the Paris exponent is seen to be sensitive to the different ingredients added to the model, with rapid healing or a high minimum stress for damage leading to m =2 for all γ <γc , in contrast with the linear dependence m =6 -2 γ observed for very long characteristic healing times in the absence of a minimum stress for damage. Upon the introduction of disorder on the local fatigue thresholds, which leads to the possible appearance of multiple cracks along the propagation line, the Paris exponent tends to m ≈4 for γ ≲2 while retaining the behavior m =γ for γ ≳4 .
Shuttle Fuel Feedliner Cracking Investigation
NASA Technical Reports Server (NTRS)
Nesman, Tomas E.; Turner, Jim (Technical Monitor)
2002-01-01
This presentation provides an overview of material covered during 'Space Shuttle Fuel Feedliner Cracking Investigation MSFC Fluids Workshop' held November 19-21, 2002. Topics covered include: cracks on fuel feed lines of Orbiter space shuttles, fluid driven cracking analysis, liner structural modes, structural motion in a fluid, fluid borne drivers, three dimensional computational fluid dynamics models, fluid borne drivers from pumps, amplification mechanisms, flow parameter mapping, and flight engine flow map.
NASA Astrophysics Data System (ADS)
Smilowitz, L.; Henson, B. F.; Romero, J. J.; Asay, B. W.; Saunders, A.; Merrill, F. E.; Morris, C. L.; Kwiatkowski, K.; Grim, G.; Mariam, F.; Schwartz, C. L.; Hogan, G.; Nedrow, P.; Murray, M. M.; Thompson, T. N.; Espinoza, C.; Lewis, D.; Bainbridge, J.; McNeil, W.; Rightley, P.; Marr-Lyon, M.
2012-05-01
We report proton transmission images obtained subsequent to the laser assisted thermal ignition of a sample of PBX 9501 (a plastic bonded formulation of the explosive nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)). We describe the laser assisted thermal ignition technique as a means to synchronize a non-linear thermal ignition event while preserving the subsequent post-ignition behavior. We have obtained dynamic proton transmission images at two spatial magnifications and viewed both the radial and transverse axis of a solid cylindrical sample encased in aluminum. Images have been obtained with 3 to 15 μs temporal resolution and approximately 100 μm spatial resolution at the higher magnification. We observe case expansion from very early in the experiment, until case fragmentation. We observe spatially anisotropic features in the transmission which we attribute to cracking in the solid explosive, in agreement with previous measurements conducted on two dimensional samples with optical viewing. Digital analysis of the images also reveals spatially isotropic features which we attribute to the evolution of the loss of density by burning subsequent to thermal ignition.
Crack branching in carbon steel. Fracture mechanisms
NASA Astrophysics Data System (ADS)
Syromyatnikova, A. S.; Alekseev, A. A.; Levin, A. I.; Lyglaev, A. V.
2010-04-01
The fracture surfaces of pressure vessels made of carbon steel that form during crack branching propagation are examined by fractography. Crack branching is found to occur at a crack velocity higher than a certain critical value V > V c . In this case, the material volume that is involved in fracture and depends on the elastoplastic properties of the material and the sample width has no time to dissipate the energy released upon crack motion via the damage mechanisms intrinsic in the material under given deformation conditions (in our case, via cracking according to intragranular cleavage).
Analysis of the interaction of two parallel surface cracks
NASA Astrophysics Data System (ADS)
Hahn, Jeeyeon
The objective of this research is to analyze and predict the interaction of surface cracks that occur in parallel planes. Multiple cracks may form in aging aircraft that forms at stress concentrations such as fastener holes and notched components by stress corrosion and fatigue cracking. The lifetime of the structures are significantly affected by the interaction between these cracks. Depending on relative positions and orientations of neighboring cracks, local stress fields and crack driving forces can be affected by the presence of adjacent cracks. Even small subcritical cracks may rapidly grow to a size that will cause failure in service due to interaction and coalescence with other cracks. The interaction behavior and crack propagation direction of two parallel surface cracks is studied using three-dimensional finite element analysis (FEA). FEA models with wide range of crack configurations in a finite plate under tension are evaluated to investigate the correlation between the crack shapes and the separation distance between two cracks. The relative distance (vertical and horizontal) between two cracks and size and shape of these cracks are varied to create different stress interaction fields. Stress intensity factors (SIF) along the crack fronts are obtained from FEA, and then, cracking behaviors of the cracks are predicted by considering the influence of the interaction on the SIF and the coalescence of two cracks. The results obtained are then compared with existing experimental and analytical data for validation. All of the data analyses are presented in tabular forms and figures.
Dynamics of the off axis intense beam propagation in a spiral inflector
NASA Astrophysics Data System (ADS)
Goswami, A.; Sing Babu, P.; Pandit, V. S.
2017-01-01
In this paper the dynamics of space charge dominated beam in a spiral inflector is discussed by developing equations of motion for centroid and beam envelope for the off axis beam propagation. Evolution of the beam centroid and beam envelope is studied as a function of the beam current for various input beam parameters. The transmission of beam through the inflector is also estimated as a function of the beam current for an on axis and off axis beam by tracking a large number of particles. Simulation studies show that shift of the centroid from the axis at the inflector entrance affects the centroid location at the exit of the inflector and causes reduction in the beam transmission. The centroid shift at the entrance in the horizontal plane (h plane) is more critical as it affects the centroid shift in the vertical plane (u plane) by a large amount near the inflector exit where the available aperture is small. The beam transmission is found to reduce with increase in the centroid shift as well as with the beam current.
NASA Astrophysics Data System (ADS)
Tago, J.; Cruz-Atienza, V. M.; Etienne, V.; Virieux, J.; Benjemaa, M.; Sanchez-Sesma, F. J.
2010-12-01
Simulating any realistic seismic scenario requires incorporating physical basis into the model. Considering both the dynamics of the rupture process and the anelastic attenuation of seismic waves is essential to this purpose and, therefore, we choose to extend the hp-adaptive Discontinuous Galerkin finite-element method to integrate these physical aspects. The 3D elastodynamic equations in an unstructured tetrahedral mesh are solved with a second-order time marching approach in a high-performance computing environment. The first extension incorporates the viscoelastic rheology so that the intrinsic attenuation of the medium is considered in terms of frequency dependent quality factors (Q). On the other hand, the extension related to dynamic rupture is integrated through explicit boundary conditions over the crack surface. For this visco-elastodynamic formulation, we introduce an original discrete scheme that preserves the optimal code performance of the elastodynamic equations. A set of relaxation mechanisms describes the behavior of a generalized Maxwell body. We approximate almost constant Q in a wide frequency range by selecting both suitable relaxation frequencies and anelastic coefficients characterizing these mechanisms. In order to do so, we solve an optimization problem which is critical to minimize the amount of relaxation mechanisms. Two strategies are explored: 1) a least squares method and 2) a genetic algorithm (GA). We found that the improvement provided by the heuristic GA method is negligible. Both optimization strategies yield Q values within the 5% of the target constant Q mechanism. Anelastic functions (i.e. memory variables) are introduced to efficiently evaluate the time convolution terms involved in the constitutive equations and thus to minimize the computational cost. The incorporation of anelastic functions implies new terms with ordinary differential equations in the mathematical formulation. We solve these equations using the same order
NASA Astrophysics Data System (ADS)
Arahata, E.; Nikuni, T.
2013-05-01
We study sound propagation in a Bose-condensed gas confined in a highly elongated harmonic trap at finite temperatures. Our analysis is based on Zaremba-Nikuni-Griffin (ZNG) formalism, which consists of Gross-Pitaevskii equation for the condensate and the kinetic equation for a thermal cloud. We extend ZNG formalism to deal with a highly-anisotropic trap potential, and use it to simulate sound propagation in the two fluid hydrodynamic regime. We use the trap parameters for the experiment that has reported second sound propagation. Our simulation results show that propagation of two sound pulses corresponding to first and second sound can be observed in an intermediate temperature.
Cottrell, Don M; Davis, Jeffrey A; Berg, Cassidy A; Freeman, Christopher Li
2014-04-01
There is great interest in Airy beams because they appear to propagate in a curved path. These beams are usually generated by inserting a cubic phase mask onto the input plane of a Fourier transform system. Here, we utilize a fast Fresnel diffraction algorithm to easily derive both the propagation dynamics and the Gouy phase shift for these beams. The trajectories of these beams can be modified by adding additional linear and quadratic phase terms onto the cubic phase mask. Finally, we have rewritten the equations regarding the propagating Airy beams completely in laboratory coordinates for use by experimentalists. Experimental results are included. We expect that these results will be of great importance in applications of Airy beams.
NASA Astrophysics Data System (ADS)
Martakos, G.; Andreasen, J. H.; Berggreen, C.; Thomsen, O. T.
2017-02-01
A novel crack arresting device has been implemented in sandwich panels and tested using a special rig to apply out-of-plane loading on the sandwich panel face-sheets. Fatigue crack propagation was induced in the face-core interface of the sandwich panels which met the crack arrester. The effect of the embedded crack arresters was evaluated in terms of the achieved enhancement of the damage tolerance of the tested sandwich panels. A finite element (FE) model of the experimental setup was used for predicting propagation rates and direction of the crack growth. The FE simulation was based on the adoption of linear fracture mechanics and a fatigue propagation law (i.e. Paris law) to predict the residual fatigue life-time and behaviour of the test specimens. Finally, a comparison between the experimental results and the numerical simulations was made to validate the numerical predictions as well as the overall performance of the crack arresters.
Dynamical chiral symmetry breaking and confinement with an infrared-vanishing gluon propagator
Hawes, F.T. ); Roberts, C.D. ); Williams, A.G. )
1994-05-01
We study a model Dyson-Schwinger equation for the quark propagator closed using an [ital Ansatz] for the gluon propagator of the form [ital D]([ital q])[similar to][ital q][sup 2]/[([ital q][sup 2])[sup 2]+[ital b][sup 4
NASA Astrophysics Data System (ADS)
Bergsaker, Anne Schad; Røyne, Anja; Ougier-Simonin, Audrey; Aubry, Jérôme; Renard, François
2016-03-01
Chemically activated processes of subcritical cracking in calcite control the time-dependent strength of this mineral, which is a major constituent of the Earth's brittle upper crust. Here experimental data on subcritical crack growth are acquired with a double torsion apparatus to characterize the influence of fluid pH (range 5-7.5) and ionic strength and species (Na2SO4, NaCl, MgSO4, and MgCl2) on the propagation of microcracks in calcite single crystals. The effect of different ions on crack healing has also been investigated by decreasing the load on the crack for durations up to 30 min and allowing it to relax and close. All solutions were saturated with CaCO3. The crack velocities reached during the experiments are in the range 10-9-10-2 m/s and cover the range of subcritical to close to dynamic rupture propagation velocities. Results show that for calcite saturated solutions, the energy necessary to fracture calcite is independent of pH. As a consequence, the effects of fluid salinity, measured through its ionic strength, or the variation of water activity have stronger effects on subcritical crack propagation in calcite than pH. Consequently, when considering the geological sequestration of CO2 into carbonate reservoirs, the decrease of pH within the range of 5-7.5 due to CO2 dissolution into water should not significantly alter the rate of fracturing of calcite. Increase in salinity caused by drying may lead to further reduction in cracking and consequently a decrease in brittle creep. The healing of cracks is found to vary with the specific ions present.
Measurement of fatigue crack closure for negative stress ratio
Romeiro, F.F.J.; Domingos, C.A.; Freitas, M.J.M. de
1999-07-01
The concept of fatigue crack closure, introduced in the 60's by Elber, has been used to explain a wide range of positive stress ratio crack propagation results (R {ge} 0). Less attention has been given to fatigue loading for negative stress ratios (R {lt} 0). In this work the results of crack propagation tests of middle-crack tension M(T) specimens of a normalized medium carbon steel DIN Ck 45 for a wide range of stress ratios from 0.7 {ge} R {ge} {minus}3 are presented. Crack closure loads were measured with the compliance technique at test frequency, using a data acquisition system. Negative crack closure loads were found for negative stress ratios R {le} {minus}1, which can explain higher crack propagation rates and accelerations in crack growth during variable-amplitude tests where compressive loads of different stress ratios are present.
NASA Astrophysics Data System (ADS)
Sen, Seema; Lake, Markus; Kroppen, Norman; Farber, Peter; Wilden, Johannes; Schaaf, Peter
2017-02-01
This study describes the self-propagating exothermic reaction in Ti/Al reactive multilayer foils by using experiments and computational fluid dynamics simulation. The Ti/Al foils with different molar ratios of 1Ti/1Al, 1Ti/2Al and 1Ti/3Al were fabricated by magnetron sputtering method. Microstructural characteristics of the unreacted and reacted foils were analyzed by using electronic and atomic force microscopes. After an electrical ignition, the influence of ignition potentials on reaction propagation has been experimentally investigated. The reaction front propagates with a velocity of minimum 0.68 ± 0.4 m/s and maximum 2.57 ± 0.6 m/s depending on the input ignition potentials and the chemical compositions. Here, the 1Ti/3Al reactive foil exhibits both steady state and unsteady wavelike reaction propagation. Moreover, the numerical computational fluid dynamics (CFD) simulation shows the time dependent temperature flow and atomic mixing in a nanoscale reaction zone. The CFD simulation also indicates the potentiality for simulating exothermic reaction in the nanoscale Ti/Al foil.
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.