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.
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
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.
3D numerical analysis of crack propagation of heterogeneous notched rock under uniaxial tension
NASA Astrophysics Data System (ADS)
Wang, S. Y.; Sloan, S. W.; Sheng, D. C.; Tang, C. A.
2016-05-01
Macroscopic notches play an important role in evaluating the fracture process zone (FPZ) and the strengths of a heterogeneous rock mass. Crack initiation, propagation and coalescence for unnotched, single-notched and double-notched rock specimens are numerically simulated in a 3-D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. For the unnotched case, special attention is given to the complete stress-strain curve and the corresponding AE events for the failure process of rock specimen. By comparing with published experimental results, the simulation results from RFPA3D are found to be satisfactory. For the single-notched case, the effect of the length and the depth of the single notch and the thickness of the specimen on the failure mode and peak stress are evaluated. The 3D FPZ is very different from that in two dimensions. For the double-notched case, the effects of the separation distance and overlap distance of the double notches, as well as influence of the homogeneity index (m) are also investigated. As the overlap distance increases, the direction of the principal tensile stress at each notch-end changes from a perpendicular direction (tensile stress field) to a nearly parallel direction (compressive stress field), which affects the evolution of the cracks from the two notches.
Ishihara, T.; Kim, J.K.; Kobayashi, Y.
1995-10-01
In this study, martensite-ferrite dual phase steel composed of martensite in hard phase and ferrite in soft phase is made 3 dimensions fabric composite for energy transport and the difference in fatigue crack propagation behavior resulting from the structural size is investigated by fracture mechanics and microstructural method. The main results obtained are as follows: (1) the fatigue crack propagation rate is influenced by the ferrite grain size, in other words, in the low {Delta}K region the fatigue crack propagation rate is decreased with decreasing of the grain size but the difference of propagation rate resulted from the structural size is decreased as {Delta}K is increased; (2) the above results are explained by the degree of crack arrest effect of the martensite phase for the fatigue crack propagation depending on the ratio of reversed plastic zone size to the ferrite grain size.
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
Crack interaction with 3-D dislocation loops
NASA Astrophysics Data System (ADS)
Gao, Huajian
CRACKS in a solid often interact with other crystal defects such as dislocation loops. The interaction effects are of 3-D character yet their analytical treatment has been mostly limited to the 2-D regime due to mathematical complications. This paper shows that distribution of the stress intensity factors along a crack front due to arbitrary dislocation loops may be expressed as simple line integrals along the loop contours. The method of analysis is based on the 3-D Bueckner-Rice weight function theory for elastic crack analysis. Our results have significantly simplified the calculations for 3-D dislocation loops produced in the plastic processes at the crack front due to highly concentrated crack tip stress fields. Examples for crack-tip 3-D loops and 2-D straight dislocations emerging from the crack tip are given to demonstrate applications of the derived formulae. The results are consistent with some previous analytical solutions existing in the literature. As further applications we also analyse straight dislocations that are parallel or perpendicular to the crack plane but are not parallel to the crack front.
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.
3D Elastic Seismic Wave Propagation Code
Energy Science and Technology Software Center (ESTSC)
1998-09-23
E3D is capable of simulating seismic wave propagation in a 3D heterogeneous earth. Seismic waves are initiated by earthquake, explosive, and/or other sources. These waves propagate through a 3D geologic model, and are simulated as synthetic seismograms or other graphical output.
NASA Astrophysics Data System (ADS)
Budarapu, P. R.; Javvaji, B.; Sutrakar, V. K.; Roy Mahapatra, D.; Zi, G.; Rabczuk, T.
2015-08-01
The crack initiation and growth mechanisms in an 2D graphene lattice structure are studied based on molecular dynamics simulations. Crack growth in an initial edge crack model in the arm-chair and the zig-zag lattice configurations of graphene are considered. Influence of the time steps on the post yielding behaviour of graphene is studied. Based on the results, a time step of 0.1 fs is recommended for consistent and accurate simulation of crack propagation. Effect of temperature on the crack propagation in graphene is also studied, considering adiabatic and isothermal conditions. Total energy and stress fields are analyzed. A systematic study of the bond stretching and bond reorientation phenomena is performed, which shows that the crack propagates after significant bond elongation and rotation in graphene. Variation of the crack speed with the change in crack length is estimated.
Automatic crack propagation tracking
NASA Technical Reports Server (NTRS)
Shephard, M. S.; Weidner, T. J.; Yehia, N. A. B.; Burd, G. S.
1985-01-01
A finite element based approach to fully automatic crack propagation tracking is presented. The procedure presented combines fully automatic mesh generation with linear fracture mechanics techniques in a geometrically based finite element code capable of automatically tracking cracks in two-dimensional domains. The automatic mesh generator employs the modified-quadtree technique. Crack propagation increment and direction are predicted using a modified maximum dilatational strain energy density criterion employing the numerical results obtained by meshes of quadratic displacement and singular crack tip finite elements. Example problems are included to demonstrate the procedure.
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.
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.
Analysis of fatigue crack propagation
NASA Technical Reports Server (NTRS)
Liu, H. W.
1972-01-01
The correlation between fatigue crack propagation and stress intensity factor is analyzed. When determining fatigue crack propagation rate, a crack increment, delta a, and its corresponding increment in load cycles, delta N, are measured. Fatigue crack propagation must be caused by a shear and/or a normal separation mode. Both of these two processes are discrete if one looks at the atomic level. If the average deformation and fracture properties over the crack increments, delta a, can be considered as homogeneous, if the characteristic discrete lengths of sigma a, if the plastic zone size is small, and if a plate is thick enough to insure a plane strain case, da/dN is proportional to delta K squared. Any deviation of empirical data from this relation must be caused by the fact that one or more of these conditions are not satisfied. The effects of plate thickness and material inhomogeneity are discussed in detail. A shear separation mode of fatigue crack propagation is described and is used to illustrate the effects of material inhomogeneity.
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.
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.
3D characterization of trans- and inter-lamellar fatigue crack in (α + β) Ti alloy
Babout, Laurent; Jopek, Łukasz; Preuss, Michael
2014-12-15
This paper presents a three dimensional image processing strategy that has been developed to quantitatively analyze and correlate the path of a fatigue crack with the lamellar microstructure found in Ti-6246. The analysis is carried out on X-ray microtomography images acquired in situ during uniaxial fatigue testing. The crack, the primary β-grain boundaries and the α lamellae have been segmented separately and merged for the first time to allow a better characterization and understanding of their mutual interaction. This has particularly emphasized the role of translamellar crack growth at a very high propagation angle with regard to the lamellar orientation, supporting the central role of colonies favorably oriented for basal 〈a〉 slip to guide the crack in the fully lamellar microstructure of Ti alloy. - Highlights: • 3D tomography images reveal strong short fatigue crack interaction with α lamellae. • Proposed 3D image processing methodology makes their segmentation possible. • Crack-lamellae orientation maps show prevalence of translamellar cracking. • Angle study comforts the influence of basal/prismatic slip on crack path.
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.
Pavement cracking measurements using 3D laser-scan images
NASA Astrophysics Data System (ADS)
Ouyang, W.; Xu, B.
2013-10-01
Pavement condition surveying is vital for pavement maintenance programs that ensure ride quality and traffic safety. This paper first introduces an automated pavement inspection system which uses a three-dimensional (3D) camera and a structured laser light to acquire dense transverse profiles of a pavement lane surface when it carries a moving vehicle. After the calibration, the 3D system can yield a depth resolution of 0.5 mm and a transverse resolution of 1.56 mm pixel-1 at 1.4 m camera height from the ground. The scanning rate of the camera can be set to its maximum at 5000 lines s-1, allowing the density of scanned profiles to vary with the vehicle's speed. The paper then illustrates the algorithms that utilize 3D information to detect pavement distress, such as transverse, longitudinal and alligator cracking, and presents the field tests on the system's repeatability when scanning a sample pavement in multiple runs at the same vehicle speed, at different vehicle speeds and under different weather conditions. The results show that this dedicated 3D system can capture accurate pavement images that detail surface distress, and obtain consistent crack measurements in repeated tests and under different driving and lighting conditions.
3D crack tip fields for FCC single crystals
Cuitino, A.M.; Ortiz, M.
1995-12-31
Cracks in single crystals are of concern in a number of structural and non-structural applications, ranging form single-crystal turbine blades and rotors to metal interconnect lines in microcircuits. In this paper we present 3D numerical simulations of the crack-tip fields of a Cu single crystal, including stress, strain and slip activity patterns. The orientation of the crack tip is along the crystallographic orientation (101), while the crack plane is (010). A material model based on dislocation mechanics is used in these simulations. This model correctly predicts the observed behavior of Cu, including the basic hardening characteristics of single crystals, orientation dependence and stage I-II-III structure of the stress-strain curves, the observed levels of latent hardening and their variation with orientation and deformation in the primary system and slip activities and dislocation densities. We use the FEM within the context of finite deformation plasticity. In the figure below, we show the finite element mesh composed by 12-noded tetrahedrons with 6-noded triangular faces. The model simulates half of a beam, which is subjected to a concentrated load at 1/8 of total length from the support. Detailed results of the stress, deformation and slip activity are presented at different radii from crack tip and at different depths from the surface. In general, the results show a strong difference in the slip activity pattern form the interior to the exterior, while smaller differences are encountered in the stress and strain fields.
Further progress on the wavy-crack model of dynamic crack propagation
Gao, H.; Pawlikowski, K.
1995-12-31
The state-of-the-art theory of dynamic crack propagation has not been able to provide an unequivocal explanation for a number of experimental findings. An important observation is that the crack surfaces, as the trace of fracture path, tend to exhibit a rough surface morphology during rapid crack propagation. In a wavy-crack model proposed recently by the author, the crack surface roughening is attributed to an inherent instability which causes the tip of the crack to propagate along an oscillatory fracture path. It appears that the wavy-crack model is capable of explaining important discrepancies currently existing between theory and experiments. In particular, experimentally observed terminal fracture speeds are significantly lower than the theoretically predicted value, i.e. the Rayleigh wave speed CR. This may be attributed to the oscillatory fracture path which makes the measured crack velocity appear lower than the actual crack speed. Also, the wavy-crack model explains how the local crack tip motion can exhibit high inertia behaviors while the measurable crack motion remains in the low inertia domain. As a result of different inertia effects associated with local and apparent crack motion, the high inertia field near the crack tip tends to induce nucleation of microcrack branches while the low inertia apparent crack field tends to suppress the microbranching. This view of dynamic fracture is not inconsistent with relevant experimental observations (e.g. see and references therein) and recent numerical simulation of fast crack motion. A planar wavy motion of a 3D crack front has been analyzed by Pice et al.. The wavy-crack model has also been applied to dynamic crack propagation along a weak interface having lower fracture resistance than the adjacent material. Further analytical and numerical developments of this model will be discussed in this presentation.
Instability and Wave Propagation in Structured 3D Composites
NASA Astrophysics Data System (ADS)
Kaynia, Narges; Fang, Nicholas X.; Boyce, Mary C.
2014-03-01
Many structured composites found in nature possess undulating and wrinkled interfacial layers that regulate mechanical, chemical, acoustic, adhesive, thermal, electrical and optical functions of the material. This research focused on the complex instability and wrinkling pattern arising in 3D structured composites and the effect of the buckling pattern on the overall structural response. The 3D structured composites consisted of stiffer plates supported by soft matrix on both sides. Compression beyond the critical strain led to complex buckling patterns in the initially straight plates. The motivation of our work is to elaborate the formation of a system of prescribed periodic scatterers (metamaterials) due to buckling, and their effect to interfere wave propagation through the metamaterial structures. Such metamaterials made from elastomers enable large reversible deformation and, as a result, significant changes of the wave propagation properties. We developed analytical and finite element models to capture various aspects of the instability mechanism. Mechanical experiments were designed to further explore the modeling results. The ability to actively alter the 3D composite structure can enable on-demand tunability of many different functions, such as active control of wave propagation to create band-gaps and waveguides.
Corrosion fatigue crack propagation in metals
Gangloff, R.P.
1990-06-01
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.
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.
Discrete Method of Images for 3D Radio Propagation Modeling
NASA Astrophysics Data System (ADS)
Novak, Roman
2016-09-01
Discretization by rasterization is introduced into the method of images (MI) in the context of 3D deterministic radio propagation modeling as a way to exploit spatial coherence of electromagnetic propagation for fine-grained parallelism. Traditional algebraic treatment of bounding regions and surfaces is replaced by computer graphics rendering of 3D reflections and double refractions while building the image tree. The visibility of reception points and surfaces is also resolved by shader programs. The proposed rasterization is shown to be of comparable run time to that of the fundamentally parallel shooting and bouncing rays. The rasterization does not affect the signal evaluation backtracking step, thus preserving its advantage over the brute force ray-tracing methods in terms of accuracy. Moreover, the rendering resolution may be scaled back for a given level of scenario detail with only marginal impact on the image tree size. This allows selection of scene optimized execution parameters for faster execution, giving the method a competitive edge. The proposed variant of MI can be run on any GPU that supports real-time 3D graphics.
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. PMID:25048684
3D Multispectral Light Propagation Model For Subcutaneous Veins Imaging
Paquit, Vincent C; Price, Jeffery R; Meriaudeau, Fabrice; Tobin Jr, Kenneth William
2008-01-01
In this paper, we describe a new 3D light propagation model aimed at understanding the effects of various physiological properties on subcutaneous vein imaging. In particular, we build upon the well known MCML (Monte Carlo Multi Layer) code and present a tissue model that improves upon the current state-of-the-art by: incorporating physiological variation, such as melanin concentration, fat content, and layer thickness; including veins of varying depth and diameter; using curved surfaces from real arm shapes; and modeling the vessel wall interface. We describe our model, present results from the Monte Carlo modeling, and compare these results with those obtained with other Monte Carlo methods.
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
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.
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
Simulation of 3D Global Wave Propagation Through Geodynamic Models
NASA Astrophysics Data System (ADS)
Schuberth, B.; Piazzoni, A.; Bunge, H.; Igel, H.; Steinle-Neumann, G.
2005-12-01
This project aims at a better understanding of the forward problem of global 3D wave propagation. We use the spectral element program "SPECFEM3D" (Komatitsch and Tromp, 2002a,b) with varying input models of seismic velocities derived from mantle convection simulations (Bunge et al., 2002). The purpose of this approach is to obtain seismic velocity models independently from seismological studies. In this way one can test the effects of varying parameters of the mantle convection models on the seismic wave field. In order to obtain the seismic velocities from the temperature field of the geodynamical simulations we follow a mineral physics approach. Assuming a certain mantle composition (e.g. pyrolite with CMASF composition) we compute the stable phases for each depth (i.e. pressure) and temperature by system Gibbs free energy minimization. Elastic moduli and density are calculated from the equations of state of the stable mineral phases. For this we use a mineral physics database derived from calorimetric experiments (enthalphy and entropy of formation, heat capacity) and EOS parameters.
Depth propagation and surface construction in 3-D vision.
Georgeson, Mark A; Yates, Tim A; Schofield, Andrew J
2009-01-01
In stereo vision, regions with ambiguous or unspecified disparity can acquire perceived depth from unambiguous regions. This has been called stereo capture, depth interpolation or surface completion. We studied some striking induced depth effects suggesting that depth interpolation and surface completion are distinct stages of visual processing. An inducing texture (2-D Gaussian noise) had sinusoidal modulation of disparity, creating a smooth horizontal corrugation. The central region of this surface was replaced by various test patterns whose perceived corrugation was measured. When the test image was horizontal 1-D noise, shown to one eye or to both eyes without disparity, it appeared corrugated in much the same way as the disparity-modulated (DM) flanking regions. But when the test image was 2-D noise, or vertical 1-D noise, little or no depth was induced. This suggests that horizontal orientation was a key factor. For a horizontal sine-wave luminance grating, strong depth was induced, but for a square-wave grating, depth was induced only when its edges were aligned with the peaks and troughs of the DM flanking surface. These and related results suggest that disparity (or local depth) propagates along horizontal 1-D features, and then a 3-D surface is constructed from the depth samples acquired. The shape of the constructed surface can be different from the inducer, and so surface construction appears to operate on the results of a more local depth propagation process. PMID:18977239
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.
3D millimeter wave imaging of vertical cracks and its application for the inspection of HDPE pipes
NASA Astrophysics Data System (ADS)
Ghasr, Mohammad Tayeb; Ying, Kuang; Zoughi, Reza
2014-02-01
Robust detection of vertical cracks in high-density polyethylene (HDPE) pipes is a challenging task for the majority of nondestructive testing (NDT) techniques. Vertical cracks are specifically referred to those whose largest planar view is parallel to the signal direction of propagation, leaving very little signal to be scattered for detection. In such pipes this commonly occurs between two pipes sections when thermally or adhesively joined. This work presents the utility and efficacy of three-dimensional (3D) millimeter wave holographical imaging based on synthetic aperture radar (SAR) algorithm for imaging such cracks. Such a 3D millimeter wave image can readily represent the type, size, and location of various flaws within a structure. Two-dimensional (2D) slices of the 3D image, at different orientations, can also be readily produced showing the cross-sectional views of the structure and flaws, further aiding in identifying, and sizing a flaw or vertical crack. Imaging results for planner and curved (pipe section) specimen with machined flaws are presented. These images are produced using a novel field-portable, small, and low-cost wideband millimeter-wave reflectometer capable of rapid 3D image production.
Burrowing mechanics: burrow extension by crack propagation.
Dorgan, Kelly M; Jumars, Peter A; Johnson, Bruce; Boudreau, B P; Landis, Eric
2005-02-01
Until now, the analysis of burrowing mechanics has neglected the mechanical properties of impeding, muddy, cohesive sediments, which behave like elastic solids. Here we show that burrowers can progress through such sediments by using a mechanically efficient, previously unsuspected mechanism--crack propagation--in which an alternating 'anchor' system of burrowing serves as a wedge to extend the crack-shaped burrow. The force required to propagate cracks through sediment in this way is relatively small: we find that the force exerted by the annelid worm Nereis virens in making and moving into such a burrow amounts to less than one-tenth of the force it needs to use against rigid aquarium walls. PMID:15690029
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.
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.
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. PMID:23897295
Crack propagation and arrest in pressurized containers
NASA Technical Reports Server (NTRS)
Erdogan, F.; Delale, F.; Owczarek, J. A.
1977-01-01
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. The problem is formulated by making two major assumptions, namely, that the shell problem is quasi-static and the gas dynamics problem is one-dimensional. 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 main results are demonstrated by considering two examples, an aluminum cylinder which may behave in a quasi-brittle manner, and a steel cylinder which undergoes ductile fracture. 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.
Dynamic delamination crack propagation in a graphite/epoxy laminate
NASA Technical Reports Server (NTRS)
Sun, C. T.; Grady, J. E.
1986-01-01
The dynamic delamination crack propagation behavior during ballistic tests of (90/0)5s T-300/934 graphite/epoxy laminates with embedded interfacial cracks was investigated using high speed photography. The impact on the beam-like specimen was produced with a silicon rubber ball, and the crack propagation speeds and the threshold impact velocities required to initiate dynamic crack propagation were determined for several crack positions. The results suggest that the mode of crack propagation depends on the specimen geometry as well as the loading condition. A simplified finite element analysis of the experimental data obtained from one of the midplane-cracked specimens was used to estimate the critical strain energy release rate, which may determine the onset of unstable crack propagation.
Irregular lattice model for quasistatic crack propagation
NASA Astrophysics Data System (ADS)
Bolander, J. E.; Sukumar, N.
2005-03-01
An irregular lattice model is proposed for simulating quasistatic fracture in softening materials. Lattice elements are defined on the edges of a Delaunay tessellation of the medium. The dual (Voronoi) tessellation is used to scale the elemental stiffness terms in a manner that renders the lattice elastically homogeneous. This property enables the accurate modeling of heterogeneity, as demonstrated through the elastic stress analyses of fiber composites. A cohesive description of fracture is used to model crack initiation and propagation. Numerical simulations, which demonstrate energy-conserving and grid-insensitive descriptions of cracking, are presented. The model provides a framework for the failure analysis of quasibrittle materials and fiber-reinforced brittle-matrix composites.
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.
Directional crack propagation of granular water systems.
Mizuguchi, Tsuyoshi; Nishimoto, Akihiro; Kitsunezaki, So; Yamazaki, Yoshihiro; Aoki, Ichio
2005-05-01
Pattern dynamics of directional crack propagation phenomena observed in drying process of starch-water mixture is investigated. To visualize the three-dimensional structure of the drying-fracture process two kinds of experiments are performed, i.e., resin solidification planing method and real-time measurement of water content distribution with MR instruments. A cross section with polygonal structure is visualized in both experiments. The depth dependency of cell size is measured. The phenomenological model for water transportation is also discussed. PMID:16089617
NASA Astrophysics Data System (ADS)
Bi, J.; Zhou, X. P.; Qian, Q. H.
2016-05-01
General particle dynamics (GPD), which is a novel meshless numerical method, is proposed to simulate the initiation, propagation and coalescence of 3D pre-existing penetrating and embedded flaws under biaxial compression. The failure process for rock-like materials subjected to biaxial compressive loads is investigated using the numerical code GPD3D. Moreover, internal crack evolution processes are successfully simulated using GPD3D. With increasing lateral stress, the secondary cracks keep growing in the samples, while the growth of the wing cracks is restrained. The samples are mainly split into fragments in a shear failure mode under biaxial compression, which is different from the splitting failure of the samples subjected to uniaxial compression. For specimens with macroscopic pre-existing flaws, the simulated types of cracks, the simulated coalescence types and the simulated failure modes are in good agreement with the experimental results.
Simulation of 3D Seismic Wave Propagation with Volcano Topography
NASA Astrophysics Data System (ADS)
Ripperger, J.; Igel, H.; Wassermann, J.
2001-12-01
We investigate the possibilities of using three-dimensional finite difference (FD) methods for numerical simulation of the seismic wave field at active volcanoes. We put special emphasis on the implementation of the boundary conditions for free surface topography. We compare two different approaches to solve the free surface boundary conditions. The algorithms are implemented on parallel hardware and have been tested for correctness and stability. We apply them to smooth artificial topographies and to the real topography of Mount Merapi, Indonesia. We conclude, that grid stretching type methods (e.g. Hestholm & Ruud, 1994) are not well suited for realistic volcano topography as they tend to become unstable for large topographic gradients. The representation of topography through staircase shaped grids (Ohminato & Chouet, 1997) results in stable calculations, while demanding very fine gridding. The simulations show the effects of a three-dimensional surface topography on elastic wave propagation. Ground motion at the surface is severely affected by topography. If neglected, this may jeopardize attempts to determine source location by analyzing particle motion. Numerical studies like this can help to understand wave propagation phenomena observed on field recordings in volcano seismology. Future studies will aim at separating the wave effects of internal scattering, topography and sources (tremors, tectonic events, pyroclastic flows).
NASA Technical Reports Server (NTRS)
Raju, I. S.; Newman, J. C., Jr.
1993-01-01
A computer program, surf3d, that uses the 3D finite-element method to calculate the stress-intensity factors for surface, corner, and embedded cracks in finite-thickness plates with and without circular holes, was developed. The cracks are assumed to be either elliptic or part eliptic in shape. The computer program uses eight-noded hexahedral elements to model the solid. The program uses a skyline storage and solver. The stress-intensity factors are evaluated using the force method, the crack-opening displacement method, and the 3-D virtual crack closure methods. In the manual the input to and the output of the surf3d program are described. This manual also demonstrates the use of the program and describes the calculation of the stress-intensity factors. Several examples with sample data files are included with the manual. To facilitate modeling of the user's crack configuration and loading, a companion program (a preprocessor program) that generates the data for the surf3d called gensurf was also developed. The gensurf program is a three dimensional mesh generator program that requires minimal input and that builds a complete data file for surf3d. The program surf3d is operational on Unix machines such as CRAY Y-MP, CRAY-2, and Convex C-220.
Heat pulse propagation is 3-D chaotic magnetic fields
NASA Astrophysics Data System (ADS)
Del-Castillo-Negrete, D.; Blazevski, D.
2013-10-01
Perturbative transport studies provide valuable time dependent information to construct and test transport models in magnetically confined plasmas. In these studies, the transient response of the plasma to externally applied small perturbations is followed in time. Here we present a numerical study of the radial propagation of edge heat pulse perturbations in the presence of 3-dimensional chaotic magnetic fields in cylindrical geometry. Based on the strong transport anisotropy encountered in magnetized plasmas (χ∥ /χ⊥ ~1010 in fusion plasmas, where χ∥ and χ⊥ are the parallel and perpendicular conductivities) we limit attention to the extreme anisotropic, purely parallel, χ⊥ = 0 , case. Using the Lagrangian-Green's function method we study the dependence of the pulse speed and radial penetration on the level of stochasticity of the magnetic field in regular, and reversed magnetic shear configurations. Of particular interest is the slowing down of the heat pulse due to weak chaos, islands, and shearless cantori. Work supported by the USA Department of Energy.
Axial crack propagation and arrest in pressurized fuselage
NASA Technical Reports Server (NTRS)
Kosai, M.; Shimamoto, A.; Yu, C.-T.; Walker, S. I.; Kobayashi, A. S.; Tan, P.
1994-01-01
The crack arrest capability of a tear strap in a pressurized precracked fuselage was studied through instrumented axial rupture tests of small scale models of an idealized fuselage. Upon pressurization, rapid crack propagation initiated at an axial through crack along the stringer and immediately kinked due to the mixed modes 1 and 2 state caused by the one-sided opening of the crack flap. The diagonally running crack further turned at the tear straps. Dynamic finite element analysis of the rupturing cylinder showed that the crack kinked and also ran straight in the presence of a mixed mode state according to a modified two-parameter crack kinking criterion.
Temperature evolution and heat dissipation during crack propagation
NASA Astrophysics Data System (ADS)
Lengliné, Olivier; Santucci, Stéphane; Jørgen Måløy, Knut; Vincent-Dospital, Tom; Toussaint, Renaud
2013-04-01
During a crack propagation, energy is dissipated in mainly three ways: creation of new fracture surface (possibly at microscopic scale in a process zone), emission of elastic waves that get dissipated in the far field, and local Joule heating during friction in a process zone. There is in addition some reversible elastic energy change associated to the crack advance.Since the temperature variations can have an important impact on the physics of the crack propagation, establishing properly this balance in different crack propagation scenarios is of great importance. Notably, the physics of fracture propagation has been shown to be strongly affected by thermally activated rupture, even when the heterogeneity of material properties determines strongly the microscopic fracture geometry and the intermittency in the fracture propagation. A natural question, in such kinetic crack propagation, is the temperature field during the cracks propagation. This question is also central in earth science, where a lot of attention has been set recently on thermal effects, with the possibility of thermo-pressurization of faults due to thermal expansion of fluids present in faults. Independently of thermo pressurization, the rise of temperature locally, at the zone enduring damage, could significantly affect the creep in this zone, as understood by statistical physics and Arrhenius law, and thus the crack propagation. We are interested in quantifying directly these different effects in an experimental situation. We present results based on infrared and optical imaging of the propagation of a crack in a sheet of paper. The temperature field shows local increases of the temperature of several degrees during the crack propagation. Optical images acquired with a fast video camera are correlated in order to extract the deformation field at each time step. We show how the temperature in our paper sample varies with the deformation rate at the tip of the crack. We also present some numerical
Anomalous crack propagation in reinforced natural rubber
NASA Astrophysics Data System (ADS)
Sotta, Paul; Gabrielle, Brice; Long, Didier; Vanel, Loic; Albouy, Pierre-Antoine; Peditto, Francesca
2009-03-01
In reinforced natural rubber, crack propagation in mode I exhibits rotation of the tear in a direction perpendicular to the usual one. Our objective is, first, to understand the impact of this phenomenon on fracture toughness of the material, and, secondly, to understand how this phenomenon is related to the specific properties of reinforced natural rubber. To this aim, we combine measurements of ultimate properties, measurements of the number and length of tear rotations as a function of loading velocity and temperature, and investigation of material heterogeneities at sub-micrometric scales, originating both from fillers and strain-induced crystallites (strain-induced crystallinity is measured up to failure by X ray diffraction), in natural rubber samples reinforced by nanometric aggregates. Observations suggest that tear rotation is related both to the mechanical anisotropy induced by strain-induced crystallinity and to the dissipative properties of the material at high strain.
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.
Propagation of stress corrosion cracks in Zr-1% Nb claddings
NASA Astrophysics Data System (ADS)
Bibilashvily, Yu. K.; Dolgov, Yu. N.; Nesterov, B. I.; Novikov, V. V.
1995-09-01
Experimental results on iodine induced stress corrosion cracking (SCC) are analyzed. The studies were performed at 350°C using Zr-1% Nb tubular specimens. Fatigue crack at internal surface served as an initial defect. The relationship was derived between crack propagation rate and stress intensity factor; the threshold stress intensity factor of 4.8 MPa m{1}/{2} was determined.
Laser-Based Instrument Measures Propagation Of Cracks
NASA Technical Reports Server (NTRS)
Lee, Rupert U.; Cox, Robert B.; Youngquist, Robert C.; Sentz, John T.; Rose, Kenneth A.
1995-01-01
Report describes use of commerical laser displacement meter to measure propagation of cracks in stainless-steel specimens in stress tests in corrosive (salt-spray) environment. Measurements directed toward determining time from beginning of each test until onset of propagation of crack.
Morphogenesis and propagation of complex cracks induced by thermal shocks.
Bourdin, Blaise; Marigo, Jean-Jacques; Maurini, Corrado; Sicsic, Paul
2014-01-10
We study the genesis and the selective propagation of complex crack networks induced by thermal shock or drying of brittle materials. We use a quasistatic gradient damage model to perform large-scale numerical simulations showing that the propagation of fully developed cracks follows Griffith criterion and depends only on the fracture toughness, while crack morphogenesis is driven by the material's internal length. Our numerical simulations feature networks of parallel cracks and selective arrest in two dimensions and hexagonal columnar joints in three dimensions, without any hypotheses on cracks geometry, and are in good agreement with available experimental results. PMID:24483901
Multiple Cracks Propagate Simultaneously in Polymer Liquids in Tension.
Huang, Qian; Alvarez, Nicolas J; Shabbir, Aamir; Hassager, Ole
2016-08-19
Understanding the mechanism of fracture is essential for material and process design. While the initiation of fracture in brittle solids is generally associated with the preexistence of material imperfections, the mechanism for initiation of fracture in viscoelastic fluids, e.g., polymer melts and solutions, remains an open question. We use high speed imaging to visualize crack propagation in entangled polymer liquid filaments under tension. The images reveal the simultaneous propagation of multiple cracks. The critical stress and strain for the onset of crack propagation are found to be highly reproducible functions of the stretch rate, while the position of initiation is completely random. The reproducibility of conditions for fracture points to a mechanism for crack initiation that depends on the dynamic state of the material alone, while the crack profiles reveal the mechanism of energy dissipation during crack propagation. PMID:27588883
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.
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.
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.
Characterizing the propagation of gravity waves in 3D nonlinear simulations of solar-like stars
NASA Astrophysics Data System (ADS)
Alvan, L.; Strugarek, A.; Brun, A. S.; Mathis, S.; Garcia, R. A.
2015-09-01
Context. The revolution of helio- and asteroseismology provides access to the detailed properties of stellar interiors by studying the star's oscillation modes. Among them, gravity (g) modes are formed by constructive interferences between progressive internal gravity waves (IGWs), propagating in stellar radiative zones. Our new 3D nonlinear simulations of the interior of a solar-like star allows us to study the excitation, propagation, and dissipation of these waves. Aims: The aim of this article is to clarify our understanding of the behavior of IGWs in a 3D radiative zone and to provide a clear overview of their properties. Methods: We use a method of frequency filtering that reveals the path of individual gravity waves of different frequencies in the radiative zone. Results: We are able to identify the region of propagation of different waves in 2D and 3D, to compare them to the linear raytracing theory and to distinguish between propagative and standing waves (g-modes). We also show that the energy carried by waves is distributed in different planes in the sphere, depending on their azimuthal wave number. Conclusions: We are able to isolate individual IGWs from a complex spectrum and to study their propagation in space and time. In particular, we highlight in this paper the necessity of studying the propagation of waves in 3D spherical geometry, since the distribution of their energy is not equipartitioned in the sphere.
Fatigue crack propagation in aluminum-lithium alloys
NASA Technical Reports Server (NTRS)
Rao, K. T. V.; Ritchie, R. O.; Piascik, R. S.; Gangloff, R. P.
1989-01-01
The principal mechanisms which govern the fatigue crack propagation resistance of aluminum-lithium alloys are investigated, with emphasis on their behavior in controlled gaseous and aqueous environments. Extensive data describe the growth kinetics of fatigue cracks in ingot metallurgy Al-Li alloys 2090, 2091, 8090, and 8091 and in powder metallurgy alloys exposed to moist air. Results are compared with data for traditional aluminum alloys 2024, 2124, 2618, 7075, and 7150. Crack growth is found to be dominated by shielding from tortuous crack paths and resultant asperity wedging. Beneficial shielding is minimized for small cracks, for high stress ratios, and for certain loading spectra. While water vapor and aqueous chloride environments enhance crack propagation, Al-Li-Cu alloys behave similarly to 2000-series aluminum alloys. Cracking in water vapor is controlled by hydrogen embrittlement, with surface films having little influence on cyclic plasticity.
NASA Technical Reports Server (NTRS)
Zhao, W.; Newman, J. C., Jr.; Sutton, M. A.; Wu, X. R.; Shivakumar, K. N.
1995-01-01
Stress intensity factors for quarter-elliptical corner cracks emanating from a circular hole are determined using a 3-D weight function method combined with a 3-D finite element method. The 3-D finite element method is used to analyze uncracked configuration and provide stress distribution in the region where crack is to occur. Using this stress distribution as input, the 3-D weight function method is used to determine stress intensity factors. Three different loading conditions, i.e. remote tension, remote bending and wedge loading, are considered for a wide range in geometrical parameters. The significance in using 3-D uncracked stress distribution and the difference between single and double corner cracks are studied. Typical crack opening displacements are also provided. Comparisons are made with solutions available in the literature.
Fatigue crack propagation behavior in dual-phase steel
Sarwar, M.; Priestner, R.
1999-04-01
The fatigue crack propagation in dual-phase steel was studied with the objective of developing ferritic-martensitic microstructures via intercritical annealing and thermomechanical processing. It was found that the changes in fatigue crack propagation rates and in the threshold stress intensity range, {Delta}K{sub th}, resulting from microstructural variations, were directly related to tensile strength in the same manner that was observed in other types of structural steels. it was also observed that the relationship between tensile strength and fatigue crack propagation in intercritically annealed and thermomechanically processed dual-phase steel was much the same as for conventional steels of similar strength level.
The three thresholds for fatigue crack propagation
Miller, K.J.
1997-12-01
The three governing threshold conditions in metal fatigue are considered, one relating to crack growth in single crystals, one concerned with crack growth in polycrystalline materials, and one based on linear elastic fracture mechanics (LEFM). All three conditions are examined in relation to the two physical processes of cracking, i.e., Stage I (shear) and Stage II (tensile) crack growth. The LEFM threshold is seen as a lower bound condition for fatigue crack growth rate, and the single crystal threshold is viewed in relation to the fundamental threshold pertaining to the fatigue resistance of polycrystalline metals.
On Modeling Hydrogen-Induced Crack Propagation Under Sustained Load
NASA Astrophysics Data System (ADS)
Dadfarnia, Mohsen; Somerday, Brian p.; Schembri, Philip E.; Sofronis, Petros; Foulk, James W.; Nibur, Kevin A.; Balch, Dorian K.
2014-08-01
The failure of hydrogen containment components is generally associated with subcritical cracking. Understanding subcritical crack growth behavior and its dependence on material and environmental variables can lead to methods for designing structural components in a hydrogen environment and will be beneficial in developing materials resistant to hydrogen embrittlement. In order to identify the issues underlying crack propagation and arrest, we present a model for hydrogen-induced stress-controlled crack propagation under sustained loading. The model is based on the assumptions that (I) hydrogen reduces the material fracture strength and (II) crack propagation takes place when the opening stress over the characteristic distance ahead of a crack tip is greater than the local fracture strength. The model is used in a finite-element simulation of crack propagation coupled with simultaneous hydrogen diffusion in a model material through nodal release. The numerical simulations show that the same physics, i.e., diffusion-controlled crack propagation, can explain the existence of both stages I and II in the velocity versus stress intensity factor ( V- K) curve.
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.
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.
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.
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
Scaling of crack propagation in rubber sheets
NASA Astrophysics Data System (ADS)
Chen, C. H.; Zhang, H. P.; Niemczura, J.; Ravi-Chandar, K.; Marder, M.
2011-11-01
We have conducted experiments and numerical simulations to investigate supersonic cracks. The experiments are performed at 85 °C to suppress strain-induced crystallites that complicate experiments at lower temperature. Calibration experiments were performed to obtain the parameters needed to compare with a theory including viscous dissipation. We find that both experiments and numerical simulations support supersonic cracks, and we discover a transition from subsonic to supersonic as we plot experimental crack speed curves vs. extension ratio for different sized samples. Both experiments and simulations show two different scaling regimes: the speed of subsonic cracks scales with the elastic energy density while the speed of supersonic cracks scales with the extension ratio. Crack openings have qualitatively different shapes in the two scaling regimes.
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.
NASA Astrophysics Data System (ADS)
Iqbal, AKM Asif; Arai, Yoshio
2016-02-01
The fatigue crack propagation behaviour of a cast hybrid metal matrix composite (MMC) was investigated and compared with the crack propagation behaviour of MMC with Al2O3 and Al alloy in this article. Three dimensional (3D) surface analysis is carried out to analyze the crack propagation mechanism. All three materials clearly show near threshold and stable crack growth regions, but the rapid crack growth region is not clearly understood. The crack propagation resistance is found higher in hybrid MMC than that of MMC with Al2O3 whisker and the Al alloy in the low ΔK region. The crack propagation in the hybrid MMC in the near-threshold region is directed by the debonding of reinforcement-matrix followed by void nucleation in the Al alloy matrix. Besides, the crack propagation in the stable- or midcrack-growth region is controlled by the debonding of particle-matrix and whisker-matrix interface caused by the cycle-by-cycle crack growth along the interface. The transgranular fracture of the reinforcement and void formation are also observed. Due to presence of large volume of inclusions and the microstructural inhomogeneity, the area of striation formation is reduced in the hybrid MMC, caused the unstable fracture.
Comparison of fatigue crack propagation in Modes I and III
Ritchie, R.O.
1985-06-01
The propagation behavior of fatigue cracks in Mode III (anti-plane shear), measured under cyclic torsion, is described and compared with more commonly encountered behavior under Mode I (tensile opening) loads. It is shown that a unique, global characterization of Mode III growth rates, akin to the Paris ''law'' in Mode I, is only possible if characterizating parameters appropriate to large-scale yielding are employed and allowance is made for crack tip shielding from sliding crack surface interference (i.e., friction and abrasion) between mating fracture surfaces. Based on the crack tip stress and deformation fields for Mode III stationary cracks, the cyclic crack tip displacement, (..delta..CTD/sub III/, and plastic strain intensity range ..delta..GAMMA/sub III/, have been proposed and are found to provide an adequate description of behavior in a range of steels, provided crack surface interference is minimized. The magnitude of this interference, which is somewhat analogous to crack closure in Mode I, is further examined in the light of the complex fractography of torsional fatigue failures and the question of a ''fatigue threshold'' for Mode III crack growth. Finally, micro-mechanical models for cyclic crack extension in anti-plane shear are briefly described, and the contrasting behavior between Mode III and Mode I cracks subjected to simple variable amplitude spectra is examined in terms of the differing role of crack tip blunting and closure in influencing shear, as opposed to tensile opening, modes of crack growth.
NASA Astrophysics Data System (ADS)
Bhavanam, Sharada
The aim of this thesis is to numerically evaluate the mixed-mode Stress Intensity Factors (SIFs) of complex 3D structural geometries with arbitrary 3D cracks using the Symmetric Galerkin Boundary Element Method-Finite Element Method (SGBEM-FEM) Alternating Method. Various structural geometries with different loading scenarios and crack configurations were examined in this thesis to understand the behavior and trends of the mixed-mode SIFs as well as the fatigue life for these complex structural geometries. Although some 3D structures have empirical and numerical solutions that are readily available in the open literature, some do not; therefore this thesis presents the results of fracture and fatigue analyses of these 3D complex structures using the SGBEM-FEM Alternating Method to serve as reference for future studies. Furthermore, there are advantages of using the SGBEM-FEM Alternating Method compared to traditional FEM methods. For example, the fatigue-crack-growth and fatigue life can be better estimated for a structure because different fatigue models (i.e. Walker, Paris, and NASGRO) can be used within the same framework of the SGBEM-FEM Alternating Method. The FEM (un-cracked structure)/BEM(crack model) meshes are modeled independently, which speeds up the computation process and reduces the cost of human labor. A simple coarse mesh can be used for all fracture and fatigue analyses of complex structures. In this thesis, simple coarse meshes were used for 3D complex structures, which were below 5000 elements as compared to traditional FEM, which require meshes where the elements range on the order of ˜250,000 to ˜106 and sometimes even more than that.
Fatigue crack propagation in carburized X-2M steel
NASA Astrophysics Data System (ADS)
Averbach, B. L.; Lou, Bingzhe; Pearson, P. K.; Fairchild, R. E.; Bamberger, E. N.
1985-07-01
The growth rates of fatigue cracks propagating through the case and into the core have been studied for carburized X-2M steel (0.14 C, 4.91 Cr, 1.31 Mo, 1.34 W, 0.42 V). Fatigue cracks were propagated at constant stress intensities, ΔK, and also at a constant cyclic peak load, and the crack growth rates were observed to pass through a minimum value as the crack traversed the carburized case. The reduction in the crack propagation rates is ascribed to the compressive stresses which were developed in the case, and a pinched clothespin model is used to make an approximate calculation of the effects of internal stress on the crack propagation rates. We define an effective stress intensity, Ke = Ka + Ki, where Ka is the applied stress intensity, Ki = σid{i/1/2}, σi is the internal stress, and di is a characteristic distance associated with the depth of the internal stress field. In our work, a value of di = 11 mm (0.43 inch) fits the data quite well. A good combination of resistance to fatigue crack propagation in the case and fracture toughness in the core can be achieved in carburized X-2M steel, suggesting that this material will be useful in heavy duty gears and in aircraft gas turbine mainshaft bearings operating under high hoop stresses.
Finite Element Code For 3D-Hydraulic Fracture Propagation Equations (3-layer).
Energy Science and Technology Software Center (ESTSC)
1992-03-24
HYFRACP3D is a finite element program for simulation of a pseudo three-dimensional fracture geometries with a two-dimensional planar solution. The model predicts the height, width and winglength over time for a hydraulic fracture propagating in a three-layered system of rocks with variable rock mechanics properties.
Effect of fluid salinity on subcritical crack propagation in calcite
NASA Astrophysics Data System (ADS)
Rostom, Fatma; Røyne, Anja; Dysthe, Dag Kristian; Renard, François
2013-01-01
The slow propagation of cracks, also called subcritical crack growth, is a mechanism of fracturing responsible for a ductile deformation of rocks under crustal conditions. In the present study, the double-torsion technique was used to measure the effect of fluid chemistry on the slow propagation of cracks in calcite single crystals at room temperature. Time-lapse images and measurements of force and load-point displacement allowed accurate characterization of crack velocities in a range of 10- 8 to 10- 4 m/s. Velocity curves as a function of energy-release rates were obtained for different fluid compositions, varying NH4Cl and NaCl concentrations. Our results show the presence of a threshold in fluid composition, separating two regimes: weakening conditions where the crack propagation is favored, and strengthening conditions where crack propagation slows down. We suggest that electrostatic surface forces that modify the repulsion forces between the two surfaces of the crack may be responsible for this behavior.
Metal crack propagation monitoring by photoluminescence enhancement of quantum dots.
Zhao, Ziming; Luan, Weiling; Yin, Shaofeng; Brandner, Juergen J
2015-07-20
A visualization method for monitoring minor metal crack propagation is presented in this paper. Through CdS@ZnS core-shell quantum dots (QDs) enhanced emission of photoluminescence (PL), this crack detection method provides a visualization signal in real time and through a noncontact fashion. The crack of the CdS@ZnS core-shell QDs-epoxy resin kept a synchronous propagation with the metal crack. Detection of the tip growth in the film layers demonstrated that the actual crack propagation on the metal surface could be deduced from the tips in the film layers. The fluorescence peak tended to increase along the crack from the initial opening to the tip. Crack width as small as 10 μm can be detected with a precision of 0.1 μm and the minimum crack tip width of the QDs-epoxy resin was measured as 0.72 μm. PMID:26367834
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.
Fatigue crack propagation in aerospace aluminum alloys
NASA Technical Reports Server (NTRS)
Gangloff, R. P.; Piascik, R. S.; Dicus, D. L.; Newman, J. C., Jr.
1990-01-01
This paper reviews fracture mechanics based, damage tolerant characterizations and predictions of fatigue crack growth in aerospace aluminum alloys. The results of laboratory experimentation and modeling are summarized in the areas of: (1) fatigue crack closure, (2) the wide range crack growth rate response of conventional aluminum alloys, (3) the fatigue behavior of advanced monolithic aluminum alloys and metal matrix composites, (4) the short crack problem, (5) environmental fatigue, and (6) variable amplitude loading. Remaining uncertainties and necessary research are identified. This work provides a foundation for the development of fatigue resistant alloys and composites, next generation life prediction codes for new structural designs and extreme environments, and to counter the problem of aging components.
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.
NASA Astrophysics Data System (ADS)
Yang, Zhuofei; Kang, Jidong; Wilkinson, David S.
2015-08-01
AM60 high pressure die castings have been used in automobile applications to reduce the weight of vehicles. However, the pore defects that are inherent in die casting may negatively affect mechanical properties, especially the fatigue properties. Here we have studied damage ( e.g., pore defects, fatigue cracks) during strained-controlled fatigue using 3-dimensional X-ray computed tomography (XCT). The fatigue test was interrupted every 2000 cycles and the specimen was removed to be scanned using a desktop micro-CT system. XCT reveals pore defects, cracks, and fracture surfaces. The results show that pores can be accurately measured and modeled in 3D. Defect bands are found to be made of pores under 50 µm (based on volume-equivalent sphere diameter). Larger pores are randomly distributed in the region between the defect bands. Observation of fatigue cracks by XCT is performed in three ways such that the 3D model gives the best illustration of crack-porosity interaction while the other two methods, with the cracks being viewed on transverse or longitudinal cross sections, have better detectability on crack initiation and crack tip observation. XCT is also of value in failure analysis on fracture surfaces. By assessing XCT data during fatigue testing and observing fracture surfaces on a 3D model, a better understanding on the crack initiation, crack-porosity interaction, and the morphology of fracture surface is achieved.
User's manual for FRAC3D: Supplement to report on stress analysis for structures with surface cracks
NASA Technical Reports Server (NTRS)
Bell, J. C.; Hopper, A. T.; Hayes, P. A.
1978-01-01
The FRAC3D computer program, designed for use in analyzing stresses in structures (including plates, bars, or blocks) which may contain part-circular surface cracks or embedded circular cracks is described. Instructions are provided for preparing input, including that for the supporting programs LATTICE and MATSOL as well as for FRAC3D. The course of a substantial illustrative calculation is shown with both input and output. The formulas underlying the calculations are summarized and related to the subroutines in which they are used. Many issues of strategy in using this program for analysing stresses around surface cracks are elucidated.
NASA Technical Reports Server (NTRS)
Zhao, W.; Newman, J. C., Jr.; Sutton, M. A.; Shivakumar, K. N.; Wu, X. R.
1995-01-01
Parallel with the work in Part-1, stress intensity factors for semi-elliptical surface cracks emanating from a circular hole are determined. The 3-D weight function method with the 3D finite element solutions for the uncracked stress distribution as in Part-1 is used for the analysis. Two different loading conditions, i.e. remote tension and wedge loading, are considered for a wide range in geometrical parameters. Both single and double surface cracks are studied and compared with other solutions available in the literature. Typical crack opening displacements are also provided.
ZIP3D: An elastic and elastic-plastic finite-element analysis program for cracked bodies
NASA Technical Reports Server (NTRS)
Shivakumar, K. N.; Newman, J. C., Jr.
1990-01-01
ZIP3D is an elastic and an elastic-plastic finite element program to analyze cracks in three dimensional solids. The program may also be used to analyze uncracked bodies or multi-body problems involving contacting surfaces. For crack problems, the program has several unique features including the calculation of mixed-mode strain energy release rates using the three dimensional virtual crack closure technique, the calculation of the J integral using the equivalent domain integral method, the capability to extend the crack front under monotonic or cyclic loading, and the capability to close or open the crack surfaces during cyclic loading. The theories behind the various aspects of the program are explained briefly. Line-by-line data preparation is presented. Input data and results for an elastic analysis of a surface crack in a plate and for an elastic-plastic analysis of a single-edge-crack-tension specimen are also presented.
Crack propagation in aluminum sheets reinforced with boron-epoxy
NASA Technical Reports Server (NTRS)
Roderick, G. L.
1979-01-01
An analysis was developed to predict both the crack growth and debond growth in a reinforced system. The analysis was based on the use of complex variable Green's functions for cracked, isotropic sheets and uncracked, orthotropic sheets to calculate inplane and interlaminar stresses, stress intensities, and strain-energy-release rates. An iterative solution was developed that used the stress intensities and strain-energy-release rates to predict crack and debond growths, respectively, on a cycle-by-cycle basis. A parametric study was made of the effects of boron-epoxy composite reinforcement on crack propagation in aluminum sheets. Results show that the size of the debond area has a significant effect on the crack propagation in the aluminum. For small debond areas, the crack propagation rate is reduced significantly, but these small debonds have a strong tendency to enlarge. Debond growth is most likely to occur in reinforced systems that have a cracked metal sheet reinforced with a relatively thin composite sheet.
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.
Improving light propagation Monte Carlo simulations with accurate 3D modeling of skin tissue
Paquit, Vincent C; Price, Jeffery R; Meriaudeau, Fabrice; Tobin Jr, Kenneth William
2008-01-01
In this paper, we present a 3D light propagation model to simulate multispectral reflectance images of large skin surface areas. In particular, we aim to simulate more accurately the effects of various physiological properties of the skin in the case of subcutaneous vein imaging compared to existing models. Our method combines a Monte Carlo light propagation model, a realistic three-dimensional model of the skin using parametric surfaces and a vision system for data acquisition. We describe our model in detail, present results from the Monte Carlo modeling and compare our results with those obtained with a well established Monte Carlo model and with real skin reflectance images.
Fatigue-Crack Propagation in Aluminum-Alloy Tension Panels
NASA Technical Reports Server (NTRS)
Whaley, Richard E.; Kurzhals, Peter R.
1960-01-01
Results are presented of a series of fatigue tests to study crack propagation and the resulting stress distributions in tension panels. The panels were all of the same general design, and configurations varied mainly in the relative amount of cross-sectional area in the skin, stiffeners, and flanges. The panels were constructed of 2024-T3 and 7075-T6 aluminum alloys. It was found that the average rate of crack growth was slower in panels made of 2024-T3 aluminum alloy than in panels made of 7075-T6 aluminum alloy. All cracks initiated in the skin, and the slowest crack growth was measured in configurations where the highest percentage of cross-sectional area was in the stiffeners. Strain-gage surveys were made to determine the redistribution of stress as the crack grew across the panels. As a crack approached a given point in the skin, the stress at that point increased rapidly. The stress in the stiffeners also increased as the crack approached the stiffeners. During the propagation of the crack the stress was not distributed uniformly in the remaining area.
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
ATHENA 3D: A finite element code for ultrasonic wave propagation
NASA Astrophysics Data System (ADS)
Rose, C.; Rupin, F.; Fouquet, T.; Chassignole, B.
2014-04-01
The understanding of wave propagation phenomena requires use of robust numerical models. 3D finite element (FE) models are generally prohibitively time consuming. However, advances in computing processor speed and memory allow them to be more and more competitive. In this context, EDF R&D developed the 3D version of the well-validated FE code ATHENA2D. The code is dedicated to the simulation of wave propagation in all kinds of elastic media and in particular, heterogeneous and anisotropic materials like welds. It is based on solving elastodynamic equations in the calculation zone expressed in terms of stress and particle velocities. The particularity of the code relies on the fact that the discretization of the calculation domain uses a Cartesian regular 3D mesh while the defect of complex geometry can be described using a separate (2D) mesh using the fictitious domains method. This allows combining the rapidity of regular meshes computation with the capability of modelling arbitrary shaped defects. Furthermore, the calculation domain is discretized with a quasi-explicit time evolution scheme. Thereby only local linear systems of small size have to be solved. The final step to reduce the computation time relies on the fact that ATHENA3D has been parallelized and adapted to the use of HPC resources. In this paper, the validation of the 3D FE model is discussed. A cross-validation of ATHENA 3D and CIVA is proposed for several inspection configurations. The performances in terms of calculation time are also presented in the cases of both local computer and computation cluster use.
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
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.
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.
Tracking of cracks in bridges using GPR: a 3D approach
NASA Astrophysics Data System (ADS)
Benedetto, A.
2012-04-01
Corrosion associated with reinforcing bars is the most significant contributor to bridge deficiencies. The corrosion is usually caused by moisture and chloride ion exposure. In particular, corrosion products FeO, Fe2O3, Fe3O4 and other oxides along reinforcement bars. The reinforcing bars are attacked by corrosion and yield expansive corrosion products. These oxidation products occupy a larger volume than the original intact steel and internal expansive stresses lead to cracking and debonding. There are some conventional inspection methods for detection of reinforcing bar corrosion but they can be invasive and destructive, often laborious, lane closures is required and it is difficult or unreliable any quantification of corrosion. For these reasons, bridge engineers are always more preferring to use the Ground Penetrating Radar (GPR) technique. In this work a novel numerical approach for three dimensional tracking and mapping of cracks in the bridge is proposed. The work starts from some interesting results based on the use of the 3D imaging technique in order to improve the potentiality of GPR to detect voids, cracks or buried object. The numerical approach has been tested on data acquired on some bridges using a pulse GPR system specifically designed for bridge deck and pavement inspection that is called RIS Hi Bright. The equipment integrates two arrays of Ultra Wide Band ground coupled antennas, having a main working frequency of 2 GHz. The two arrays within the RIS Hi Bright are using antennas arranged with different polarization. One array includes sensors with parallel polarization with respect to the scanning direction (VV array), the other has sensors in orthogonal polarization (HH array). Overall the system collects 16 profiles within a single scan (8 HH + 8 VV). The cracks, associated often to moisture increasing and higher values of the dielectric constant, produce a not negligible increasing of the signal amplitude. Following this, the algorithm
Simulating Seismic Wave Propagation in 3-D Structure: A Case Study For Istanbul City
NASA Astrophysics Data System (ADS)
Yelkenci, Seda; Aktar, Mustafa
2013-04-01
Investigation of the wave propagation around the Marmara Sea, in particular for the city of Istanbul is critical because this target area is identified as one of the megacities with the highest seismic risk in the world. This study makes an attempt for creating an integrated 3D seismic/geologic model and precise understanding of 3-D wave propagation in the city of Istanbul. The approach is based on generating synthetic seismograms using realistic velocity structures as well as accurate location, focal mechanism and source parameters of reference earthquakes. The modarate size reference earthquakes occured in the Marmara Sea and were recorded by the National Seismic Network of Turkey as well as the network of Istanbul Early Warning and Rapid Response System. The seismograms are simulated by means of a 3-D finite difference method operated on parallel processing environment. In the content of creating a robust velocity model; 1D velocity models which are derived fom previous crustal studies of Marmara region such as refraction seismic and receiver functions have been conducted firstly for depths greater than 1km. Velocity structure in shallower part of the study region is then derived from recent geophysical and geotechnical surveys. To construct 3-D model from the obtained 1-D model data, a variety of interpolation methods are considered. According to the observations on amplitude and arrival time based on comparison of simulated seismograms, the considered velocity model is refined the way that S delay times are compensated. Another important task of this work is an application of the finite difference method to estimate three-dimensional seismic responses for a specified basin structure including soft sediments with low shear velocities in respect of the surrounded area in the Asian part of Istanbul. The analysis performed both in the time and frequency domain, helps in understanding of the comprehensive wave propagation characteristics and the distribution of
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.
Fatigue crack propagation behavior of stainless steel welds
NASA Astrophysics Data System (ADS)
Kusko, Chad S.
The fatigue crack propagation behavior of austenitic and duplex stainless steel base and weld metals has been investigated using various fatigue crack growth test procedures, ferrite measurement techniques, light optical microscopy, stereomicroscopy, scanning electron microscopy, and optical profilometry. The compliance offset method has been incorporated to measure crack closure during testing in order to determine a stress ratio at which such closure is overcome. Based on this method, an empirically determined stress ratio of 0.60 has been shown to be very successful in overcoming crack closure for all da/dN for gas metal arc and laser welds. This empirically-determined stress ratio of 0.60 has been applied to testing of stainless steel base metal and weld metal to understand the influence of microstructure. Regarding the base metal investigation, for 316L and AL6XN base metals, grain size and grain plus twin size have been shown to influence resulting crack growth behavior. The cyclic plastic zone size model has been applied to accurately model crack growth behavior for austenitic stainless steels when the average grain plus twin size is considered. Additionally, the effect of the tortuous crack paths observed for the larger grain size base metals can be explained by a literature model for crack deflection. Constant Delta K testing has been used to characterize the crack growth behavior across various regions of the gas metal arc and laser welds at the empirically determined stress ratio of 0.60. Despite an extensive range of stainless steel weld metal FN and delta-ferrite morphologies, neither delta-ferrite morphology significantly influence the room temperature crack growth behavior. However, variations in weld metal da/dN can be explained by local surface roughness resulting from large columnar grains and tortuous crack paths in the weld metal.
3D simulation of seismic wave propagation around a tunnel using the spectral element method
NASA Astrophysics Data System (ADS)
Lambrecht, L.; Friederich, W.
2010-05-01
We model seismic wave propagation in the environment of a tunnel for later application to reconnaissance. Elastic wave propagation can be simulated by different numerical techniques such as finite differences and pseudospectral methods. Their disadvantage is the lack of accuracy on free surfaces, numerical dispersion and inflexibility of the mesh. Here we use the software package SPECFEM3D_SESAME in an svn development version, which is based on the spectral element method (SEM) and can handle complex mesh geometries. A weak form of the elastic wave equation leads to a linear system of equations with a diagonal mass matrix, where the free surface boundary of the tunnel can be treated under realistic conditions and can be effectively implemented in parallel. We have designed a 3D external mesh including a tunnel and realistic features such as layers and holes to simulate elastic wave propagation in the zone around the tunnel. The source is acting at the tunnel surface so that we excite Rayleigh waves which propagate to the front face of the tunnel. A conversion takes place and a high amplitude S-wave is radiated in the direction of the tunnel axis. Reflections from perturbations in front of the tunnel can be measured by receivers implemented on the tunnel face. For a shallow tunnel the land surface has high influence on the wave propagation. By implementing additional receivers at this surface we intent to improve the prediction. It shows that the SEM is very capable to handle the complex geometry of the model and especially incorporates the free surfaces of the model.
Modeling of the general astigmatic Gaussian beam and its propagation through 3D optical systems.
Kochkina, Evgenia; Wanner, Gudrun; Schmelzer, Dennis; Tröbs, Michael; Heinzel, Gerhard
2013-08-20
The paper introduces the complete model of the general astigmatic Gaussian beam as the most general case of the Gaussian beam in the fundamental mode. This includes the laws of propagation, reflection, and refraction as well as the equations for extracting from the complex-valued beam description its real-valued parameters, such as the beam spot radii and the radii of curvature of the wavefront. The suggested model is applicable to the case of an oblique incidence of the beam at any 3D surface that can be approximated by the second-order equation at the point of incidence. Thus it can be used in simulations of a large variety of 3D optical systems. The provided experimental validation of the model shows good agreement with simulations. PMID:24085008
3D Modeling of Laser Propagation in Ionizing Gas and Plasma
NASA Astrophysics Data System (ADS)
Cooley, J.; Antonsen, T., Jr.; Huang, C.; Mori, W.
2003-10-01
The interaction of a high intensity laser with ionizing gas and plasmas is of current interest for both Laser Wakefield Accelerators and x-ray generation. We have developed a 3D fluid simulation code based on the same quasistatic approximation used in the 2D code WAKE [1]. The object oriented structure of the code also allows it to couple to the quickPIC particle code [2]. We will present 3D studies of the ionization scattering instability [3], which occurs when a laser pulse propagates in an ionizing gas. [1] P. Mora and T. Antonsen, Jr., Phys. Plasmas 4(1), January 1997 [2] J. Cooley, T. Antonsen, Jr., C. Huang, etal., Proceedings, Advanced Accelerator Concepts, 2002 [3] Z. Bian and T. Antonsen, Jr., Phys. Plasmas 8(7), July 2001 * work supported by NSF and DOE
On the steady propagation of a semi-infinite crack
Paukshto, M.V.; Sulimov, M.G.
1994-12-25
We consider the rectilinear propagation of a semi-infinite crack with constant velocity in a crystal structure. We obtain the solutions of homogeneous boundary-value problems for the corresponding difference-differential operators in spaces of one and two dimensions. We give a justification of the computational aspect of the problem.
Fatigue crack propagation resistance of highly crosslinked polyethylene.
Bradford, Letitia; Baker, David; Ries, Michael D; Pruitt, Lisa A
2004-12-01
A higher degree of cross-linking has been shown to improve wear properties of ultra-high molecular weight polyethylene in laboratory studies. However, cross-linking can also affect the mechanical properties of ultra-high molecular weight polyethylene. Fatigue crack propagation resistance was determined for electron beam cross-linked ultra-high molecular weight polyethylene and compared with gamma irradiation cross-linked and noncross-linked polyethylene fatigue specimens. Crosslinking was done with different dosages of irradiation followed by melting. For one irradiation dose (50 kGy) extrusion and molding processes were compared. A fracture mechanics approach was used to determine how the degree of cross-linking affects resistance to crack propagation in ultra-high molecular weight polyethylene. Fatigue crack propagation resistance was reduced in proportion to the irradiation dose. The type of irradiation (gamma or electron beam) or manufacturing method (extrusion or molding) did not affect fatigue crack propagation resistance. The reduced fatigue strength of highly cross-linked ultra-high molecular weight polyethylene could lead to mechanical failure in conditions that are associated with cyclic local tensile stresses. PMID:15577468
Upscaling small heterogeneities for seismic wave propagation in 3D complex media
NASA Astrophysics Data System (ADS)
Cupillard, P.; Capdeville, Y.
2012-04-01
Seismic waves propagating in the Earth are affected by different sizes of heterogeneities. When modelling these waves (using numerical methods such as the SEM), taking into account heterogeneities that are much smaller than the minimum wavelength is a challenge because meshing small heterogeneities often requires important efforts and leads to high numerical costs. In this work, we present a technique which allows to upscale the small heterogeneities that can lie in an elastic medium. This technique yields a smooth effective medium and effective equations. We describe its implementation in the 3D case and we show relevant examples.
Singh, S. S.; Williams, J. J.; Lin, M. F.; Xiao, X.; De Carlo, F.; Chawla, N.
2014-05-14
In situ X-ray synchrotron tomography was used to investigate the stress corrosion cracking behavior of under-aged Al–Zn–Mg–Cu alloy in moisture. The discontinuous surface cracks (crack jumps) mentioned in the literature are actually a single continuous and tortuous crack when observed in three dimension (3D). Contrary to 2D measurements made at the surface which suggest non-uniform crack growth rates, 3D measurements of the crack length led to a much more accurate measurement of crack growth rates.
Singh, S. S.; Williams, J. J.; Lin, M. F.; Xiao, X.; De Carlo, F.; Chawla, N.
2014-05-14
In situ X-ray synchrotron tomography was used to investigate the stress corrosion cracking behavior of under-aged Al–Zn–Mg–Cu alloy in moisture. The discontinuous surface cracks (crack jumps) mentioned in the literature are actually a single continuous and tortuous crack when observed in three dimension (3D). Contrary to 2D measurements made at the surface which suggest non-uniform crack growth rates, 3D measurements of the crack length led to a much more accurate measurement of crack growth rates.
Simulation of Crack Propagation in Metal Powder Compaction
NASA Astrophysics Data System (ADS)
Tahir, S. M.; Ariffin, A. K.
2006-08-01
This paper presents the fracture criterion of metal powder compact and simulation of the crack initiation and propagation during cold compaction process. Based on the fracture criterion of rock in compression, a displacement-based finite element model has been developed to analyze fracture initiation and crack growth in iron powder compact. Estimation of fracture toughness variation with relative density is established in order to provide the fracture parameter as compaction proceeds. A finite element model with adaptive remeshing technique is used to accommodate changes in geometry during the compaction and fracture process. Friction between crack faces is modelled using the six-node isoparametric interface elements. The shear stress and relative density distributions of the iron compact with predicted crack growth are presented, where the effects of different loading conditions are presented for comparison purposes.
NASA Astrophysics Data System (ADS)
Pathak, Himanshu; Singh, Akhilendra; Singh, Indra Vir
2016-06-01
In this work, finite element method (FEM) and element free Galerkin method (EFGM) are coupled for solving 3D crack domains subjected to cyclic thermal load of constant amplitude. Crack growth contours and fatigue life have been obtained for each of the considered numerical examples. Thermo-elastic problems are decoupled into thermal and elastic problems . Firstly, the unknown temperature field is obtained by solving heat conduction equation, then, it is used as the input load in the elastic problem to calculate the displacement and stress fields. The geometrical discontinuity across crack surface is modelled by extrinsically enriched EFGM and the remaining part of the domain is approximated by standard finite element method. At the crack interface, a ramp function based interpolation scheme has been implemented. This coupled approach combines the advantages of both EFGM and FEM. A linear successive crack increment approach is used to model crack growth. The growing crack surface is traced by level set function. Standard Paris law is used for life estimation of the three-dimensional crack models. Different cases of planar and non-planar crack problems have been solved and their results are compared with the results obtained using extended finite element method to check accuracy, efficiency and robustness of the coupled FE-EFG approach implemented in this study.
NASA Astrophysics Data System (ADS)
Wu, Chensheng; Nelson, William; Davis, Christopher C.
2014-10-01
Plenoptic functions are functions that preserve all the necessary light field information of optical events. Theoretical work has demonstrated that geometric based plenoptic functions can serve equally well in the traditional wave propagation equation known as the "scalar stochastic Helmholtz equation". However, in addressing problems of 3D turbulence simulation, the dominant methods using phase screen models have limitations both in explaining the choice of parameters (on the transverse plane) in real-world measurements, and finding proper correlations between neighboring phase screens (the Markov assumption breaks down). Though possible corrections to phase screen models are still promising, the equivalent geometric approach based on plenoptic functions begins to show some advantages. In fact, in these geometric approaches, a continuous wave problem is reduced to discrete trajectories of rays. This allows for convenience in parallel computing and guarantees conservation of energy. Besides the pairwise independence of simulated rays, the assigned refractive index grids can be directly tested by temperature measurements with tiny thermoprobes combined with other parameters such as humidity level and wind speed. Furthermore, without loss of generality one can break the causal chain in phase screen models by defining regional refractive centers to allow rays that are less affected to propagate through directly. As a result, our work shows that the 3D geometric approach serves as an efficient and accurate method in assessing relevant turbulence problems with inputs of several environmental measurements and reasonable guesses (such as Cn 2 levels). This approach will facilitate analysis and possible corrections in lateral wave propagation problems, such as image de-blurring, prediction of laser propagation over long ranges, and improvement of free space optic communication systems. In this paper, the plenoptic function model and relevant parallel algorithm computing
Mode III fatigue crack propagation in low alloy steel
NASA Astrophysics Data System (ADS)
Ritchie, R. O.; McClintock, F. A.; Nayeb-Hashemi, H.; Ritter, M. A.
1982-01-01
To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects, showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 104 cycles at nominal yield to 106 cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to be slower than in Mode I, although still power-law related to the alternating stress intensity (△K III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements (△CTD III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTDIII per cycle.
Crack propagation induced heating in crystalline energetic materials
NASA Astrophysics Data System (ADS)
Holmes, W.; Francis, R. S.; Fayer, M. D.
1999-02-01
A model is presented for time and spatial dependences of the heating of molecular vibrations and the possible initiation of chemical reaction from heat dissipated in the vicinity of a propagating crack in a molecular crystal. In the model, energy from a moving crack tip is released as phonons in proximity to the crack. Initially the phonons and the molecular vibrations are not in thermal equilibrium. Subsequently, there is a competition between excitation of molecular vibrations by multiphonon up-pumping and diffusion of phonons from the crack region. If the coupling between the locally hot phonon bath and the molecular vibrations is sufficiently large, a transitory high vibrational temperature will be achieved prior to eventual thermal equilibration with the bulk of the crystal. It is found that the peak vibrational temperature can be sufficiently high for a significant time period for chemical reactions to occur. The model calculates the local time-dependent vibrational temperature using reasonable values of the physical input parameters. For a crack tip moving near the speed of sound, the calculations show that vibrational temperatures can reach ˜800 K in 55 ps and exceed 550 K for ˜1 ns after the initial heating. This temperature change is sufficient to produce chemical reaction in a secondary explosive such as HMX, but given the duration and size of the heated region, a single crack should not result in self-sustaining chemical reaction. The role that cracks may play in shock sensitivity is discussed.
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.
Numerical homogenization for seismic wave propagation in 3D geological media
NASA Astrophysics Data System (ADS)
Cupillard, P.; Capdeville, Y.; Botella, A.
2014-12-01
Despite the important increase of the computational power in the last decades, simulating the seismic wave propagation through realistic geological models is still a challenge. By realistic models we here mean 3D media in which a broad variety (in terms of amplitude and extent) of heterogeneities lies, including discontinuities with complex geometry such as faulted and folded horizons, intrusive geological contacts and fault systems. To perform accurate numerical simulations, these discontinuities require complicated meshes which usually contain extremely small elements, yielding large, sometimes prohibitive, computation costs. Fortunately, the recent development of the non-periodic homogenization technique now enables to overcome this problem by computing smooth equivalent models for which a coarse mesh is sufficient to get an accurate wavefield. In this work, we present an efficient implementation of the technique which now allows for the homogenization of large 3D geological models. This implementation relies on a tetrahedral finite-element solution of the elasto-static equation behind the homogenization problem. Because this equation is time-independent, solving it is numerically cheaper than solving the wave equation, but it nevertheless requires some care because of the large size of the stiffness matrix arising from the fine mesh of realistic geological structures. A domain decomposition is therefore adopted. In our strategy, the obtained sub-domains overlap but they are independent so the solution within each of them can be computed either in series or in parallel. In addition, well-balanced loads, efficient search algorithms and multithreading are implemented to speed up the computation. The resulting code enables the homogenization of 3D elastic media in a time that is neglectable with respect to the simulation time of the wave propagation within. This is illustrated through a sub-surface model of the Furfooz karstic region, Belgium.
Spatial parallelism of a 3D finite difference, velocity-stress elastic wave propagation code
Minkoff, S.E.
1999-12-01
Finite difference methods for solving the wave equation more accurately capture the physics of waves propagating through the earth than asymptotic solution methods. Unfortunately, finite difference simulations for 3D elastic wave propagation are expensive. The authors model waves in a 3D isotropic elastic earth. The wave equation solution consists of three velocity components and six stresses. The partial derivatives are discretized using 2nd-order in time and 4th-order in space staggered finite difference operators. Staggered schemes allow one to obtain additional accuracy (via centered finite differences) without requiring additional storage. The serial code is most unique in its ability to model a number of different types of seismic sources. The parallel implementation uses the MPI library, thus allowing for portability between platforms. Spatial parallelism provides a highly efficient strategy for parallelizing finite difference simulations. In this implementation, one can decompose the global problem domain into one-, two-, and three-dimensional processor decompositions with 3D decompositions generally producing the best parallel speedup. Because I/O is handled largely outside of the time-step loop (the most expensive part of the simulation) the authors have opted for straight-forward broadcast and reduce operations to handle I/O. The majority of the communication in the code consists of passing subdomain face information to neighboring processors for use as ghost cells. When this communication is balanced against computation by allocating subdomains of reasonable size, they observe excellent scaled speedup. Allocating subdomains of size 25 x 25 x 25 on each node, they achieve efficiencies of 94% on 128 processors. Numerical examples for both a layered earth model and a homogeneous medium with a high-velocity blocky inclusion illustrate the accuracy of the parallel code.
Spatial Parallelism of a 3D Finite Difference, Velocity-Stress Elastic Wave Propagation Code
MINKOFF,SUSAN E.
1999-12-09
Finite difference methods for solving the wave equation more accurately capture the physics of waves propagating through the earth than asymptotic solution methods. Unfortunately. finite difference simulations for 3D elastic wave propagation are expensive. We model waves in a 3D isotropic elastic earth. The wave equation solution consists of three velocity components and six stresses. The partial derivatives are discretized using 2nd-order in time and 4th-order in space staggered finite difference operators. Staggered schemes allow one to obtain additional accuracy (via centered finite differences) without requiring additional storage. The serial code is most unique in its ability to model a number of different types of seismic sources. The parallel implementation uses the MP1 library, thus allowing for portability between platforms. Spatial parallelism provides a highly efficient strategy for parallelizing finite difference simulations. In this implementation, one can decompose the global problem domain into one-, two-, and three-dimensional processor decompositions with 3D decompositions generally producing the best parallel speed up. Because i/o is handled largely outside of the time-step loop (the most expensive part of the simulation) we have opted for straight-forward broadcast and reduce operations to handle i/o. The majority of the communication in the code consists of passing subdomain face information to neighboring processors for use as ''ghost cells''. When this communication is balanced against computation by allocating subdomains of reasonable size, we observe excellent scaled speed up. Allocating subdomains of size 25 x 25 x 25 on each node, we achieve efficiencies of 94% on 128 processors. Numerical examples for both a layered earth model and a homogeneous medium with a high-velocity blocky inclusion illustrate the accuracy of the parallel code.
3-D Measurement and Visualization of Electrical Propagation on Heart Surface
NASA Astrophysics Data System (ADS)
Lin, Shien-Fong; Wikswo, John P.
1997-11-01
Optical recording of the cardiac transmembrane potential (Vm) has recently become a powerful tool to reveal patterns of electrical wave front dynamics on the heart surface. The optical mapping techniques have been previously applied to observe a portion of the heart due to its 3-D geometry. We extended our 2-D optical mapping technique to include one front view and two back mirror views for measuring and visualizing the transmembrane potential distribution simultaneously over entire surface of an isolated rabbit heart. The heart was illuminated with an argon laser delivered through an optical fiber bundle consisting of seven 1-mm fibers. These fibers were positioned around the heart to induce a near-uniform fluorescence intensity distribution on the heart surface. A single high-speed CCD camera with a long depth of field recorded the laser-stimulated epifluorescence from all three views in a single frame. Sequences of 100 to 600 frames of 12-bit/pixel digital images were recorded during regular pacing or induced ventricular fibrillation at 335 frames/second. Image processing then yielded the Vm distribution at a resolution of 128x64 pixels/frame. The propagating wave front images were obtained by subtracting two subsequent Vm images. The geometry of the heart was obtained by profilometry. The wave front information obtained from image processing could be texture-mapped to the heart geometry for visualization. Our 3-D imaging technique provides simultaneous, dynamic information of wave front activation and propagation over entire heart surface, and thereby can offer a more complete knowledge of wave front dynamics in a whole heart model. Future work involves automatic procedure for digitizing the heart shape and measuring the wave front dimensions using the 3-D geometry.
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.
Cohen, Laurent D; Deschamps, Thomas
2007-08-01
We present a new fast approach for segmentation of thin branching structures, like vascular trees, based on Fast-Marching (FM) and Level Set (LS) methods. FM allows segmentation of tubular structures by inflating a "long balloon" from a user given single point. However, when the tubular shape is rather long, the front propagation may blow up through the boundary of the desired shape close to the starting point. Our contribution is focused on a method to propagate only the useful part of the front while freezing the rest of it. We demonstrate its ability to segment quickly and accurately tubular and tree-like structures. We also develop a useful stopping criterion for the causal front propagation. We finally derive an efficient algorithm for extracting an underlying 1D skeleton of the branching objects, with minimal path techniques. Each branch being represented by its centerline, we automatically detect the bifurcations, leading to the "Minimal Tree" representation. This so-called "Minimal Tree" is very useful for visualization and quantification of the pathologies in our anatomical data sets. We illustrate our algorithms by applying it to several arteries datasets. PMID:17671862
Crack propagation and fracture in silicon wafers under thermal stress
Danilewsky, Andreas; Wittge, Jochen; Kiefl, Konstantin; Allen, David; McNally, Patrick; Garagorri, Jorge; Elizalde, M. Reyes; Baumbach, Tilo; Tanner, Brian K.
2013-01-01
The behaviour of microcracks in silicon during thermal annealing has been studied using in situ X-ray diffraction imaging. Initial cracks are produced with an indenter at the edge of a conventional Si wafer, which was heated under temperature gradients to produce thermal stress. At temperatures where Si is still in the brittle regime, the strain may accumulate if a microcrack is pinned. If a critical value is exceeded either a new or a longer crack will be formed, which results with high probability in wafer breakage. The strain reduces most efficiently by forming (hhl) or (hkl) crack planes of high energy instead of the expected low-energy cleavage planes like {111}. Dangerous cracks, which become active during heat treatment and may shatter the whole wafer, can be identified from diffraction images simply by measuring the geometrical dimensions of the strain-related contrast around the crack tip. Once the plastic regime at higher temperature is reached, strain is reduced by generating dislocation loops and slip bands and no wafer breakage occurs. There is only a small temperature window within which crack propagation is possible during rapid annealing. PMID:24046487
3D frequency-domain finite-difference modeling of acoustic wave propagation
NASA Astrophysics Data System (ADS)
Operto, S.; Virieux, J.
2006-12-01
We present a 3D frequency-domain finite-difference method for acoustic wave propagation modeling. This method is developed as a tool to perform 3D frequency-domain full-waveform inversion of wide-angle seismic data. For wide-angle data, frequency-domain full-waveform inversion can be applied only to few discrete frequencies to develop reliable velocity model. Frequency-domain finite-difference (FD) modeling of wave propagation requires resolution of a huge sparse system of linear equations. If this system can be solved with a direct method, solutions for multiple sources can be computed efficiently once the underlying matrix has been factorized. The drawback of the direct method is the memory requirement resulting from the fill-in of the matrix during factorization. We assess in this study whether representative problems can be addressed in 3D geometry with such approach. We start from the velocity-stress formulation of the 3D acoustic wave equation. The spatial derivatives are discretized with second-order accurate staggered-grid stencil on different coordinate systems such that the axis span over as many directions as possible. Once the discrete equations were developed on each coordinate system, the particle velocity fields are eliminated from the first-order hyperbolic system (following the so-called parsimonious staggered-grid method) leading to second-order elliptic wave equations in pressure. The second-order wave equations discretized on each coordinate system are combined linearly to mitigate the numerical anisotropy. Secondly, grid dispersion is minimized by replacing the mass term at the collocation point by its weighted averaging over all the grid points of the stencil. Use of second-order accurate staggered- grid stencil allows to reduce the bandwidth of the matrix to be factorized. The final stencil incorporates 27 points. Absorbing conditions are PML. The system is solved using the parallel direct solver MUMPS developed for distributed
Fatigue crack propagation in carburized high alloy bearing steels
NASA Astrophysics Data System (ADS)
Averbach, B. L.; Lou, Bingzhe; Pearson, P. K.; Fairchild, R. E.; Bamberger, E. N.
1985-07-01
Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates, da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, and da/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, Ki where Ki, = σid{i/1/2} (σi = internal stress, di = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes Ke = Ka + Ki where Ka is the applied stress intensity. Values of Ki were calculated as a function of distance from the surface using experimental measurements of σi and a value of di = 11 mm (0.43 inch). The resultant values of Ke were taken to be equivalent to effective ΔK values, and da/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the
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.
Formation and propagation of cracks on the flame surface
NASA Astrophysics Data System (ADS)
Kuznetsov, E. A.; Minaev, S. S.
1996-02-01
In the framework of the simplified Sivashinsky equation it is shown that the process of formation, propagation and interactions of cracks on flame fronts can be considered as the interference between nonlinear effects and instabilities of plane flame fronts. It is demonstrated that in a model with a constant growth rate cracks can form webs with triple vertices. At late times, the crack web consists of straight line segments. In agreement with recent observations [D. Breadly and C.M. Hamer, Comb. Flames 99 (1994) 562], the fourth order (and higher) vertices are rare and can appear as a result of scattering of triple vertices with each other. Within this model we predict the merging of small cells into neighboring larger cells.
Dynamic crack propagation in single-crystalline silicon
Cramer, T.; Gumbsch, P.; Wanner, A.
1999-08-01
Tensile tests on notched plates of single-crystalline silicon were carried out at high overloads. Cracks were forced to propagate on {l_brace}110{r_brace} planes in a {l_angle}1{bar 1}0{r_angle} direction. The dynamics of the fracture process was measured using the potential drop technique and correlated with the fracture surface morphology. Crack propagation velocity did not exceed a terminal velocity of v{sub c} = 3,800 m/s, which corresponds to 83% of the Rayleigh wave velocity v{sub R}. Specimens fractured at low stresses exhibited crystallographic cleavage whereas a transition from mirror-like smooth regions to rougher hackle zones was observed in case of the specimens fractured at high stresses. Inspection of the mirror zone at high magnification revealed a deviation of the {l_brace}110{r_brace} plane onto {l_brace}111{r_brace}crystallographic facets.
Fatigue crack propagation rate model based on a dislocation mechanism
NASA Technical Reports Server (NTRS)
Mazumdar, P. K.; Jeelani, S.
1986-01-01
It has been noted that the crack propagation exponent p for most metals usually varies between values of 2 and 4, and that the motion of dislocations plays an important part in determining the exponent p. Attention is presently given to the significance of the exponent p in terms of the motion of dislocations, in view of the theory of thermally activated plastic flow and the cumulative plastic strain concept for a failure criterion.
Mode III fatigue crack propagation in low alloy steel
Ritchie, R.O.; McClintock, F.A.; Nayeb-Hashemi, H.; Ritter, M.A.
1982-01-01
To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI 4340 steel. Results are compared to analogous behavior in Mode I. The approach investigated the feasibility of using continuum fracture mechanics and preliminary mechanistic modeling to serve as a basis for defect-tolerant life estimation procedures. 38 refs.
Automatic 3D segmentation of spinal cord MRI using propagated deformable models
NASA Astrophysics Data System (ADS)
De Leener, B.; Cohen-Adad, J.; Kadoury, S.
2014-03-01
Spinal cord diseases or injuries can cause dysfunction of the sensory and locomotor systems. Segmentation of the spinal cord provides measures of atrophy and allows group analysis of multi-parametric MRI via inter-subject registration to a template. All these measures were shown to improve diagnostic and surgical intervention. We developed a framework to automatically segment the spinal cord on T2-weighted MR images, based on the propagation of a deformable model. The algorithm is divided into three parts: first, an initialization step detects the spinal cord position and orientation by using the elliptical Hough transform on multiple adjacent axial slices to produce an initial tubular mesh. Second, a low-resolution deformable model is iteratively propagated along the spinal cord. To deal with highly variable contrast levels between the spinal cord and the cerebrospinal fluid, the deformation is coupled with a contrast adaptation at each iteration. Third, a refinement process and a global deformation are applied on the low-resolution mesh to provide an accurate segmentation of the spinal cord. Our method was evaluated against a semi-automatic edge-based snake method implemented in ITK-SNAP (with heavy manual adjustment) by computing the 3D Dice coefficient, mean and maximum distance errors. Accuracy and robustness were assessed from 8 healthy subjects. Each subject had two volumes: one at the cervical and one at the thoracolumbar region. Results show a precision of 0.30 +/- 0.05 mm (mean absolute distance error) in the cervical region and 0.27 +/- 0.06 mm in the thoracolumbar region. The 3D Dice coefficient was of 0.93 for both regions.
NASA Astrophysics Data System (ADS)
Anderson, T. S.; Miller, R.; Greenfield, R.; Fisk, D.
2002-12-01
The propagation of seismic waves through regions of complex topography is not thoroughly understood. Surface waves, are of particular interest, as they are large in amplitude and can characterize the source depth, magnitude, and frequency content. The amplitude and frequency content of seismic waves that propagate in regions with large topographical variations are affected by both the scattering and blockage of the wave energy. The ability to predict the 3-d scattering due to topography will improve the understanding of both regional scale surface wave magnitudes, and refine surface wave discriminants as well as at the local scale (<2 km ) where it will aid in the development of rule of thumb guide lines for array sensor placement for real time sensing technologies. Ideally, when validating the numerical accuracy of a propagation model against field data, the input geologic parameters would be known and thus eliminates geology as a source of error in the calculation. In March of 2001, Kansas Geological Survey (KGS) performed a detailed seismic site characterization at the Smart Weapons Test Range, Yuma Proving Ground, Arizona. The result of the KGS characterization study is a high-resolution 3-d model that is used in our seismic simulations. The velocities Vs, Vp are calculated by tomography and refraction, attenuation coefficients estimated from the surface wave and from p-waves and are provided in a model with attributes resolved in 3-d to 0.5 meters. In the present work, we present comparisons of synthetic data with seismic data collected at the Smart Weapons Test Range to benchmark the accuracy achieved in simulating 3-d wave propagation in the vicinity of a topographical anomaly (trench). Synthetic seismograms are generated using a 3-d 8th order staggered grid visco-elastic finite difference code that accounts for topography. The geologic model is based on the Yuma site characterization. The size of these calculations required use of the DoD High Performance
Error propagation in the computation of volumes in 3D city models with the Monte Carlo method
NASA Astrophysics Data System (ADS)
Biljecki, F.; Ledoux, H.; Stoter, J.
2014-11-01
This paper describes the analysis of the propagation of positional uncertainty in 3D city models to the uncertainty in the computation of their volumes. Current work related to error propagation in GIS is limited to 2D data and 2D GIS operations, especially of rasters. In this research we have (1) developed two engines, one that generates random 3D buildings in CityGML in multiple LODs, and one that simulates acquisition errors to the geometry; (2) performed an error propagation analysis on volume computation based on the Monte Carlo method; and (3) worked towards establishing a framework for investigating error propagation in 3D GIS. The results of the experiments show that a comparatively small error in the geometry of a 3D city model may cause significant discrepancies in the computation of its volume. This has consequences for several applications, such as in estimation of energy demand and property taxes. The contribution of this work is twofold: this is the first error propagation analysis in 3D city modelling, and the novel approach and the engines that we have created can be used for analysing most of 3D GIS operations, supporting related research efforts in the future.
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.
NASA Astrophysics Data System (ADS)
Chen, Wei-Qiu
2015-10-01
Significant progress has been made in mixed boundary-value problems associated with three-dimensional (3D) crack and contact analyses of advanced materials featuring more complexities compared to the conventional isotropic elastic materials. These include material anisotropy and multifield coupling, two typical characteristics of most current multifunctional materials. In this paper we try to present a state-of-the-art description of 3D exact/analytical solutions derived for crack and contact problems of elastic solids with both transverse isotropy and multifield coupling in the latest decade by the potential theory method in the spirit of V. I. Fabrikant, whose ingenious breakthrough brings new vigor and vitality to the old research subject of classical potential theory. We are particularly interested in crack and contact problems with certain nonlinear features. Emphasis is also placed on the coupling between the temperature field (or the like) and other physical fields (e.g., elastic, electric, and magnetic fields). We further highlight the practical significance of 3D contact solutions, in particular in applications related to modern scanning probe microscopes.
Modeling of slow crack propagation in heterogeneous rocks
NASA Astrophysics Data System (ADS)
Lengliné, Olivier; Stormo, Arne; Hansen, Alex; Schmittbuhl, Jean
2015-04-01
Crack propagation in heterogeneous media is a rich problem which involves the interplay of various physical processes. The problem has been intensively investigated theoretically, numerically, and experimentally, but a unifying model capturing all the experimental features has not been entirely achieved despite its broad range of implications in Earth sciences problems. The slow propagation of a crack front where long range elastic interactions are dominant, is of crucial importance to fill the gap between experiments and models. Several theoretical and numerical works have been devoted to quasi-static models. Such models give rise to an intermittent local activity characterized by a depinning transition and can be viewed as a critical phenomenon. However these models fail to reproduce all experimental conditions, notably the front morphology does not display any cross-over length with two different roughness exponents above and below the cross-over as observed experimentally. Here, we compare experimental observations of a slow interfacial crack propagation along an heterogeneous interface to numerical simulations from a cantilever fiber bundle model. The model consists of a planar set of brittle fibers between an elastic half-space and a rigid square root shaped plate which loads the system in a cantilever configuration. The latter is shown to provide an improved opening and stress field in the process zone around the crack tip. The model shares a similar scale invariant roughening of the crack front both at small and large scales and a similar power law distribution of the local velocity of the crack front to experiments. Implications for induced seismicity at the brittle-creep transition are discussed. We show that a creep route for induced seismicity is possible when heterogeneities exist along the fault. Indeed, seismic event occurrences in time and space are in strong relation with the development of the aseismic motion recorded during the experiment and the
NASA Astrophysics Data System (ADS)
Daeubler, M. A.; Thompson, A. W.; Bernstein, I. M.
1988-02-01
The fatigue behavior of the iron-base superalloy A-286 was studied at room temperature in air for three aging conditions: underaged, peak aged, and overaged. A fatigue strength at 107 cycles of about 200 MPa, independent of aging condition, was measured for an applied load ratio of R =0.1. Surface crack initiation and propagation were measured using hourglass specimens. Surface cracks were invariably initiated in slip bands orientated between 45 and 55 deg to the load axis, and an average ratio of crack depth to crack length of about 0.45 for these semi-elliptical cracks was measured. These earliest observable short surface cracks grew at an accelerated propagation rate in the near-threshold regime but were retarded in a transition stage, resulting in a minimum in crack growth rate. This behavior was correlated to the interaction of the crack with specific microstructure features. Following this minimum, the crack growth accelerated again with increasing Δ K and appeared to converge with the crack growth behavior expected for long through cracks. The crack propagation rate at fixed Δ K was lowest in underaged, compared to peak aged and overaged microstructures. The minimum and trends in crack growth rate appeared to depend on the development of roughness-induced closure.
Fatigue crack detection in metallic structures with Lamb waves and 3D laser vibrometry
NASA Astrophysics Data System (ADS)
Staszewski, W. J.; Lee, B. C.; Traynor, R.
2007-03-01
The paper presents the application of ultrasonic guided waves for fatigue crack detection in metallic structures. The study involves a simple fatigue test performed to introduce a crack into an aluminium plate. Lamb waves generated by a low-profile, surface-bonded piezoceramic transducer are sensed using a tri-axis, multi-position scanning laser vibrometer. The results demonstrate the potential of laser vibrometry for simple, rapid and robust detection of fatigue cracks in metallic structures. The method could be used in quality inspection and in-service maintenance of metallic structures in aerospace, civil and mechanical engineering industries.
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.
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.
Quantitative analysis of accuracy of seismic wave-propagation codes in 3D random scattering media
NASA Astrophysics Data System (ADS)
Galis, Martin; Imperatori, Walter; Mai, P. Martin
2013-04-01
Several recent verification studies (e.g. Day et al., 2001; Bielak et al., 2010, Chaljub et al., 2010) have demonstrated the importance of assessing the accuracy of available numerical tools at low frequency in presence of large-scale features (basins, topography, etc.). The fast progress in high-performance computing, including efficient optimization of numerical codes on petascale supercomputers, has permitted the simulation of 3D seismic wave propagation at frequencies of engineering interest (up to 10Hz) in highly heterogeneous media (e.g. Hartzell et al, 2010; Imperatori and Mai, 2013). However, high frequency numerical simulations involving random scattering media, characterized by small-scale heterogeneities, are much more challenging for most numerical methods, and their verification may therefore be even more crucial than in the low-frequency case. Our goal is to quantitatively compare the accuracy and the behavior of three different numerical codes for seismic wave propagation in 3D random scattering media at high frequency. We deploy a point source with omega-squared spectrum, and focus on the near-source region, being of great interest in strong motion seismology. We use two codes based on finite-difference method (FD1 and FD2) and one code based on support-operator method (SO). Both FD1 and FD2 are 4-th order staggered-grid finite-difference codes (for FD1 see Olsen et al., 2009; for FD2 see Moczo et al., 2007). The FD1 and FD2 codes are characterized by slightly different medium representations, since FD1 uses point values of material parameters in each FD-cell, while FD2 uses the effective material parameters at each grid-point (Moczo et al., 2002). SO is 2-nd order support-operator method (Ely et al., 2008). We considered models with random velocity perturbations described by van Karman correlation function with different correlation lengths and different standard deviations. Our results show significant variability in both phase and amplitude as
NASA Astrophysics Data System (ADS)
Bayona, Victor; Kindelan, Manuel
2013-10-01
Laminar flame propagation is an important problem in combustion modelling for which great advances have been achieved both in its theoretical understanding and in the numerical solution of the governing equations in 2D and 3D. Most of these numerical simulations use finite difference techniques on simple geometries (channels, ducts, ...) with equispaced nodes. The objective of this work is to explore the applicability of the radial basis function generated finite difference (RBF-FD) method to laminar flame propagation modelling. This method is specially well suited for the solution of problems with complex geometries and irregular boundaries. Another important advantage is that the method is independent of the dimension of the problem and, therefore, it is very easy to apply in 3D problems with complex geometries. In this work we use the RBF-FD method to compute 2D and 3D numerical results that simulate premixed laminar flames with different Lewis numbers propagating in open ducts.
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.
Propagation of Electromagnetic Waves in 3D Opal-based Magnetophotonic Crystals
NASA Astrophysics Data System (ADS)
Pardavi-Horvath, Martha; Makeeva, Galina S.; Golovanov, Oleg A.; Rinkevich, Anatolii B.
2013-03-01
Opals, a class of self-organized 3D nanostructures, are typical representatives of photonic bandgap structures. The voids inside of the opal structure of close packed SiO2 spheres can be infiltrated by a magnetic material, creating magnetically tunable magnetophotonic crystals with interesting and potentially useful properties at GHz and THz frequencies. The propagation of electromagnetic waves at microwave frequencies was investigated numerically in SiO2 opal based magnetic nanostructures, using rigorous mathematical models to solve Maxwell's equations complemented by the Landau-Lifshitz equation with electrodynamic boundary conditions. The numerical approach is based on Galerkin's projection method using the decomposition algorithm on autonomous blocks with Floquet channels. The opal structure consists of SiO2 nanospheres, with inter-sphere voids infiltrated with nanoparticles of Ni-Zn ferrites. Both the opal matrix and the ferrite are assumed to be lossy. A model, taking into account the real structure of the ferrite particles in the opal's voids was developed to simulate the measured FMR lineshape of the ferrite infiltrated opal. The numerical technique shows an excellent agreement when applied to model recent experimental data on similar ferrite opals.
NASA Technical Reports Server (NTRS)
Raju, I. S.
1992-01-01
A computer program that generates three-dimensional (3D) finite element models for cracked 3D solids was written. This computer program, gensurf, uses minimal input data to generate 3D finite element models for isotropic solids with elliptic or part-elliptic cracks. These models can be used with a 3D finite element program called surf3d. This report documents this mesh generator. In this manual the capabilities, limitations, and organization of gensurf are described. The procedures used to develop 3D finite element models and the input for and the output of gensurf are explained. Several examples are included to illustrate the use of this program. Several input data files are included with this manual so that the users can edit these files to conform to their crack configuration and use them with gensurf.
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.
3D parallel computations of turbofan noise propagation using a spectral element method
NASA Astrophysics Data System (ADS)
Taghaddosi, Farzad
2006-12-01
A three-dimensional code has been developed for the simulation of tone noise generated by turbofan engine inlets using computational aeroacoustics. The governing equations are the linearized Euler equations, which are further simplified to a set of equations in terms of acoustic potential, using the irrotational flow assumption, and subsequently solved in the frequency domain. Due to the special nature of acoustic wave propagation, the spatial discretization is performed using a spectral element method, where a tensor product of the nth-degree polynomials based on Chebyshev orthogonal functions is used to approximate variations within hexahedral elements. Non-reflecting boundary conditions are imposed at the far-field using a damping layer concept. This is done by augmenting the continuity equation with an additional term without modifying the governing equations as in PML methods. Solution of the linear system of equations for the acoustic problem is based on the Schur complement method, which is a nonoverlapping domain decomposition technique. The Schur matrix is first solved using a matrix-free iterative method, whose convergence is accelerated with a novel local preconditioner. The solution in the entire domain is then obtained by finding solutions in smaller subdomains. The 3D code also contains a mean flow solver based on the full potential equation in order to take into account the effects of flow variations around the nacelle on the scattering of the radiated sound field. All aspects of numerical simulations, including building and assembling the coefficient matrices, implementation of the Schur complement method, and solution of the system of equations for both the acoustic and mean flow problems are performed on multiprocessors in parallel using the resources of the CLUMEQ Supercomputer Center. A large number of test cases are presented, ranging in size from 100 000-2 000 000 unknowns for which, depending on the size of the problem, between 8-48 CPU's are
Burrowing in marine muds by crack propagation: kinematics and forces.
Dorgan, Kelly M; Arwade, Sanjay R; Jumars, Peter A
2007-12-01
The polychaete Nereis virens burrows through muddy sediments by exerting dorsoventral forces against the walls of its tongue-depressor-shaped burrow to extend an oblate hemispheroidal crack. Stress is concentrated at the crack tip, which extends when the stress intensity factor (KI) exceeds the critical stress intensity factor (KIc). Relevant forces were measured in gelatin, an analog for elastic muds, by photoelastic stress analysis, and were 0.015+/-0.001 N (mean +/- s.d.; N=5). Measured elastic moduli (E) for gelatin and sediment were used in finite element models to convert the forces in gelatin to those required in muds to maintain the same body shapes observed in gelatin. The force increases directly with increasing sediment stiffness, and is 0.16 N for measured sediment stiffness of E=2.7 x 10(4) Pa. This measurement of forces exerted by burrowers is the first that explicitly considers the mechanical behavior of the sediment. Calculated stress intensity factors fall within the range of critical values for gelatin and exceed those for sediment, showing that crack propagation is a mechanically feasible mechanism of burrowing. The pharynx extends anteriorly as it everts, extending the crack tip only as far as the anterior of the worm, consistent with wedge-driven fracture and drawing obvious parallels between soft-bodied burrowers and more rigid, wedge-shaped burrowers (i.e. clams). Our results raise questions about the reputed high energetic cost of burrowing and emphasize the need for better understanding of sediment mechanics to quantify external energy expenditure during burrowing. PMID:18025018
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. PMID:27347531
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
Preferred propagation patterns of axial surface cracks in thick-walled cylinders
Perez, E.H.; Kendall, D.P.
1996-12-01
Semi-elliptical axial surface cracks, growing due to cyclic pressure loading in thick-walled cylinders undergo significant shape change during the propagation process. These growing cracks change their shapes such that they approach and follow preferred propagation patterns (PPPs). These PPPs depend on the diameter ratio of the cylinder and on the fatigue crack propagation constant, ``m`` in the Paris equation. The objective of this paper is to show the crack shape variation during fatigue crack growth using linear elastic fracture mechanics. It is shown that a crack whose initial shape does not agree with this preferred propagation pattern will grow such that its shape converges to the preferred pattern. The results of this study also show the effect of autofrettage on the PPPs and the final shape of the cracks at breakthrough.
NASA Astrophysics Data System (ADS)
Huang, Peiyan; Liu, Guangwan; Guo, Xinyan; Huang, Man
2008-11-01
The experimental research on fatigue crack propagation rate of reinforced concrete (RC) beams strengthened with carbon fiber laminate (CFL) is carried out by MTS system in this paper. The experimental results show that, the main crack propagation on strengthened beam can be summarized into three phases: 1) fast propagation phase; 2) steady propagation and rest phase; 3) unsteady propagation phase. The phase 2-i.e. steady propagation and rest stage makes up about 95% of fatigue life of the strengthened beam. The propagation rate of the main crack, da/dN, in phase 2 can be described by Paris formula, and the constant C and m can be confirmed by the fatigue crack propagation experiments of the RC beams strengthened with CFL under three-point bending loads.
Fatigue crack propagation in ceria-partially-stabilized zirconia (Ce-TZP)-alumina composites
Tsai, J.F.; Yu, C.S.; Shetty, D.K.
1990-10-10
Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO{sub 2}-12 mol% CeO{sub 2}-10 wt% Al{sub 2}O{sub 3} ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs. The fatigue crack growth behavior was strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates.
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.
A 3-D Propagation Model for Emerging Land Mobile Radio Cellular Environments
Ahmed, Abrar; Nawaz, Syed Junaid; Gulfam, Sardar Muhammad
2015-01-01
A tunable stochastic geometry based Three-Dimensional (3-D) scattering model for emerging land mobile radio cellular systems is proposed. Uniformly distributed scattering objects are assumed around the Mobile Station (MS) bounded within an ellipsoidal shaped Scattering Region (SR) hollowed with an elliptically-cylindric scattering free region in immediate vicinity of MS. To ensure the degree of expected accuracy, the proposed model is designed to be tunable (as required) with nine degrees of freedom, unlike its counterparts in the existing literature. The outer and inner boundaries of SR are designed as independently scalable along all the axes and rotatable in horizontal plane around their origin centered at MS. The elevated Base Station (BS) is considered outside the SR at a certain adjustable distance and height w.r.t. position of MS. Closed-form analytical expressions for joint and marginal Probability Density Functions (PDFs) of Angle-of-Arrival (AoA) and Time-of-Arrival (ToA) are derived for both up- and down-links. The obtained analytical results for angular and temporal statistics of the channel are presented along with a thorough analysis. The impact of various physical model parameters on angular and temporal characteristics of the channel is presented, which reveals the comprehensive insight on the proposed results. To evaluate the robustness of the proposed analytical model, a comparison with experimental datasets and simulation results is also presented. The obtained analytical results for PDF of AoA observed at BS are seen to fit a vast range of empirical datasets in the literature taken for various outdoor propagation environments. In order to establish the validity of the obtained analytical results for spatial and temporal characteristics of the channel, a comparison of the proposed analytical results with the simulation results is shown, which illustrates a good fit for 107 scattering points. Moreover, the proposed model is shown to degenerate to
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.
Yuan, Shen-fang; Jin, Xin; Qiu, Lei; Huang, Hong-mei
2015-03-01
In order to improve the security of aircraft repaired structures, a method of crack propagation monitoring in repaired structures is put forward basing on characteristics of Fiber Bragg Grating (FBG) reflecting spectra in this article. With the cyclic loading effecting on repaired structure, cracks propagate, while non-uniform strain field appears nearby the tip of crack which leads to the FBG sensors' reflecting spectra deformations. The crack propagating can be monitored by extracting the characteristics of FBG sensors' reflecting spectral deformations. A finite element model (FEM) of the specimen is established. Meanwhile, the distributions of strains which are under the action of cracks of different angles and lengths are obtained. The characteristics, such as main peak wavelength shift, area of reflecting spectra, second and third peak value and so on, are extracted from the FBGs' reflecting spectral which are calculated by transfer matrix algorithm. An artificial neural network is built to act as the model between the characteristics of the reflecting spectral and the propagation of crack. As a result, the crack propagation of repaired structures is monitored accurately and the error of crack length is less than 0.5 mm, the error of crack angle is less than 5 degree. The accurately monitoring problem of crack propagation of repaired structures is solved by taking use of this method. It has important significance in aircrafts safety improvement and maintenance cost reducing. PMID:26117887
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
Alcala, J.; Anglada, M.
1997-11-01
The influence of precracking techniques in the crack growth behavior of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) is investigated. Load-bridge and cyclic-compression precracking enhance subsequent tensile crack growth rates, in comparison to results that are found with precracks that are extended under four-point bending prior to testing. The actual influence of these precracking techniques in the near-threshold crack growth regime is remarkably different. Although load-bridge precracking produces a pattern of crack growth fluctuations for stress intensity factors, K, lower than the effective crack-growth threshold of the material, compression-fatigue precracks start to propagate under far-field tensile loads at very fast growth rates and for K values that are slightly higher than the effective threshold. Crack-tip shielding by tetragonal-to-monoclinic transformation develops gradually, influencing the crack growth behavior in Y-TZP. Proposed fatigue crack growth micromechanisms involve damage accumulation at the crack-tip region. For K{sub max} > 3 MPa{center_dot}m{sup 1/2}, fatigue crack growth rates are strongly affected by environmental interactions at the crack tip, and postulated fatigue micromechanisms include the cyclic degradation of crack-tip shielding.
Modeling of crack propagation in weak snowpack layers using the discrete element method
NASA Astrophysics Data System (ADS)
Gaume, J.; van Herwijnen, A.; Chambon, G.; Schweizer, J.; Birkeland, K. W.
2015-01-01
Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e. to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. For instance, it is not uncommon to perform field measurements with widespread crack propagation on one day, while a few days later, with very little changes to the snowpack, crack propagation does not occur anymore. Thus far, there is no clear theoretical framework to interpret such observations, and it is not clear how and which snowpack properties affect dynamic crack propagation. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, the relation between mechanical parameters of the snowpack was taken into account so as to compare numerical and experimental results, which were in good agreement, suggesting that the simulations can reproduce crack propagation in PSTs. Finally, an in-depth analysis of the mechanical
Pressure-Induced Crack Propagation Behavior in a Particle-Reinforced Composite
NASA Astrophysics Data System (ADS)
Ha, Jae-Seok; Kim, Jae-Hoon
An experimental investigation was conducted to study pressure-induced crack propagation behavior of a particle-reinforced composite (PRC) under various pressurization rate conditions. A pre-cracked specimen of a metallic particle-reinforced rubbery composite was fixed in a holder which is installed in a windowed test chamber, and then high compressed nitrogen gas rapidly pressurized the chamber and the specimen. Chamber pressures were measured during the test, and detailed sequences of crack initiation and propagation were recorded by a high-speed digital video camera. Pressure vs. time traces were obtained from test results, and pressurization rates were defined from them. The crack propagation contours and lengths under various pressurization rates were observed through a stereoscopic microscope. Also, a progression of the crack initiation and propagation was observed by the sequences of the crack recorded by the high-speed digital video camera.
Micromechanisms of fatigue crack propagation in particulate-reinforced metal-matrix composites
Shang, Jianku.
1989-01-01
Consequences of the interaction of cracks with SiC particles are examined with emphasis on micromechanisms influencing fatigue crack propagation in high strength aluminum alloy matrix composites. Fatigue crack propagation is found to show three distinct regimes; each accompanied by growth mechanisms reflecting different roles of SiC particles. At near-threshold levels, SiC particles impeded fatigue crack growth by deflecting the crack to promote roughness-induced crack closure and by acting as crack traps along the crack front. A two-dimensional crack trapping analysis based on the interaction of a finite crack with a SiC particle indicates that a limiting criterion for fatigue crack growth in SiC{sub p}/Al composites can be established, which requires that the maximum plastic-zone size exceed the effective mean particle size or that the tensile stress in the matrix beyond the particle on the crack front exceed the yield strength of the material. Implications of crack closure and crack trapping to near-threshold crack growth, including load-ration and particle-size dependence of fatigue thresholds, are discussed in terms of contributions from each mechanism. At higher stress intensities, limited fracture of SiC particles ahead of the crack tip leads to the development of uncracked ligaments along the crack length, resulting in a reduced crack-tip stress intensity from ligament bridging. Micromechanical models are developed for such bridges induced by both overlapping cracks and co-planar ligaments, based on the notion of a limiting crack opening displacement or limiting strain in the ligament. The predicted reduction in crack tip stress intensity is shown to be consistent with experimental observations.
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.
Time-dependent corrosion fatique crack propagation in 7000 series aluminum alloys. M.S. Thesis
Mason, M.E.
1995-10-01
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.
NASA Astrophysics Data System (ADS)
Miyashita, Yukio; Mogi, Masashi; Hasegawa, Hirotaka; Sujatanod, Supamard; Mutoh, Yoshiharu
Laser cutting is one of methods for breaking a brittle material by using local thermal stress due to laser irradiation without melting and vaporization of the material. In this study, a method for controlling crack nucleation and propagation behavior was studied experimentally as well as numerically. In case of a specimen with a starter notch, crack propagated by following a laser spot. However, crack did not follow the laser spot trace when the laser scanning direction changed. It was found from the result of FEM analysis that crack propagation behavior was controlled by a stress intensity factor for the maximum tangential stress, Kθmax ahead of crack tip. Twin beam is considered as an effective method to control crack propagation direction for the laser cutting. Crack nucleation behavior was studied with a glass specimen without a starter notch. A crack could nucleate from an edge for staring of laser irradiation in case of the specimen with defects induced by polishing with abrasive papers. However, crack nucleation and propagation behavior was unstable in case of the specimen with mirror-like smooth surface. Effect of laser spot radius on crack nucleation behavior was also studied by FEM analysis.
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.
NASA Astrophysics Data System (ADS)
Woodbury, D.; Kubota, S.; Johnson, I.
2014-10-01
Computer simulations of electromagnetic wave propagation in magnetized plasmas are an important tool for both plasma heating and diagnostics. For active millimeter-wave and microwave diagnostics, accurately modeling the evolution of the beam parameters for launched, reflected or scattered waves in a toroidal plasma requires that calculations be done using the full 3-D geometry. Previously, we reported on the application of GPGPU (General-Purpose computing on Graphics Processing Units) to a 3-D vacuum Maxwell code using the FDTD (Finite-Difference Time-Domain) method. Tests were done for Gaussian beam propagation with a hard source antenna, utilizing the parallel processing capabilities of the NVIDIA K20M. In the current study, we have modified the 3-D code to include a soft source antenna and an induced current density based on the cold plasma approximation. Results from Gaussian beam propagation in an inhomogeneous anisotropic plasma, along with comparisons to ray- and beam-tracing calculations will be presented. Additional enhancements, such as advanced coding techniques for improved speedup, will also be investigated. Supported by U.S. DoE Grant DE-FG02-99-ER54527 and in part by the U.S. DoE, Office of Science, WDTS under the Science Undergraduate Laboratory Internship program.
Fatigue crack propagation thresholds for long and short cracks in Rene 95 nickel-base super alloy
McCarver, J.F.; Ritchie, R.O.
1981-10-01
A study has been made of the near-threshold fatigue crack propagation behavior of a wroght Ni-base superalloy, Rene 95, with reference to the effect of crack size on the threshold stress intensity ..delta..K/sub 0/ for no detectable crack growth. Measured threshold ..delta..K/sub 0/ values at low load ratios (R = 0.1) for physically short cracks (0.01 to 0.20 mm) were found to be 60% smaller than the corresponding ..delta..K/sub 0/ values for long cracks (approx. 25 mm). However, short crack threshold values at R = 0.1 were found to be similar to long crack thresholds at R = 0.8. Such behavior is rationalized in terms of fatigue crack closure, specifically involving the role of fracture surface roughness from crystallographic crack growth in Ni-base alloys. The large difference observed in threshold values for long and physically-short cracks serves to illustrate the potential problems in applying conventional (long crack) fatigue data to defect-tolerant lifetime predictions for structural components containing small flaws.
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
NASA Astrophysics Data System (ADS)
Deng, Y.; Ebert-Uphoff, I.; Chen, J.
2015-12-01
Causal discovery seeks to discover potential cause-effect relationships from observational data. Here we adopt the idea of interpreting large-scale atmospheric dynamical processes, particularly those tied to propagation of large-scale waves, as information flow around the globe, which can then be calculated using causal discovery methods. We apply a well-established causal discovery algorithm - based on constraint-based structure learning of probabilistic graphical models - toward 51 years of 6-hourly, atmospheric isobaric-level geopotential height data to construct the first-ever graphs of 3D information flow in the atmosphere. These graphs are created globally for different seasons and their connection to phase/energy propagation of atmospheric waves are investigated. Specifically, we examine the information flows 1) in the topical region that represent horizontal and vertical propagations of Kelvin and Rossby-gravity waves whose associated momentum transfer are known to play a key role in the Quasi-Biennial Oscillation (QBO), and 2) in the northern extratropics that represent propagations of planetary-scale waves whose heat/momentum fluxes are responsible for vacillations in the polar stratospheric vortex and occurrences of extreme events such as the stratospheric sudden warming. The sensitivity of the constructed graphs of 3D information flow to data resolution and pre-processing methods (e.g., spatial and temporal filtering) will be discussed.
Kallinderis, Yannis; Vitsas, Panagiotis A.; Menounou, Penelope
2012-07-15
A low-order flow/acoustics interaction method for the prediction of sound propagation and diffraction in unsteady subsonic compressible flow using adaptive 3-D hybrid grids is investigated. The total field is decomposed into the flow field described by the Euler equations, and the acoustics part described by the Nonlinear Perturbation Equations. The method is shown capable of predicting monopole sound propagation, while employment of acoustics-guided adapted grid refinement improves the accuracy of capturing the acoustic field. Interaction of sound with solid boundaries is also examined in terms of reflection, and diffraction. Sound propagation through an unsteady flow field is examined using static and dynamic flow/acoustics coupling demonstrating the importance of the latter.
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. PMID:27479896
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.
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.
Capturing atmospheric effects on 3D millimeter wave radar propagation patterns
NASA Astrophysics Data System (ADS)
Cook, Richard D.; Fiorino, Steven T.; Keefer, Kevin J.; Stringer, Jeremy
2016-05-01
Traditional radar propagation modeling is done using a path transmittance with little to no input for weather and atmospheric conditions. As radar advances into the millimeter wave (MMW) regime, atmospheric effects such as attenuation and refraction become more pronounced than at traditional radar wavelengths. The DoD High Energy Laser Joint Technology Offices High Energy Laser End-to-End Operational Simulation (HELEEOS) in combination with the Laser Environmental Effects Definition and Reference (LEEDR) code have shown great promise simulating atmospheric effects on laser propagation. Indeed, the LEEDR radiative transfer code has been validated in the UV through RF. Our research attempts to apply these models to characterize the far field radar pattern in three dimensions as a signal propagates from an antenna towards a point in space. Furthermore, we do so using realistic three dimensional atmospheric profiles. The results from these simulations are compared to those from traditional radar propagation software packages. In summary, a fast running method has been investigated which can be incorporated into computational models to enhance understanding and prediction of MMW propagation through various atmospheric and weather conditions.
Effect of the coating properties on crack propagation in fiber-reinforced materials
Kumar, S.; Singh, R.N.
1996-12-31
A finite element technique is used to study the effects of the coating properties on the crack propagation in fiber-reinforced materials. Crack opening stresses and energy release rates for the crack penetration and deflection have been studied in SCS6 (fiber) -C/BN (coating) - Zircon (matrix) composites. Effect of the coating thickness on crack propagation has been also studied. In general, no significant changes are found in stress ratio (ratio of hoop stresses along the crack and at the interface) and energy release rate ratio (ratio of energy release rates for the crack penetration and crack deflection), but the magnitudes of the stresses and energy release rates change substantially with the change in the coating thickness.
NASA Technical Reports Server (NTRS)
Williams, J. H., Jr.; Lee, S. S.; Kousiounelos, P. N.
1981-01-01
An orthotropic double cantilever beam (DCB) model is used to study dynamic crack propagation and arrest in 90 deg unidirectional Hercules AS/3501-6 graphite fiber epoxy composites. The dynamic fracture toughness of the composite is determined from tests performed on the long-strip specimen and DCB crack arrest experiments are conducted. By using the dynamic fracture toughness in a finite-difference solution of the DCB governing partial differential equations, a numerical solution of the crack propagation and arrest events is computed. Excellent agreement between the experimental and numerical crack arrest results are obtained.
Diffraction-based study of fatigue crack initiation and propagation in aerospace aluminum alloys
NASA Astrophysics Data System (ADS)
Gupta, Vipul K.
The crack initiation sites and microstructure-sensitive growth of small fatigue cracks are experimentally characterized in two precipitation-hardened aluminum alloys, 7075-T651 and 7050-T7451, stressed in ambient temperature moist-air (warm-humid) and -50°C dry N2 (cold-dry) environmental conditions. Backscattered electron imaging (BSE) and energy dispersive spectroscopy (EDS) of the fracture surfaces showed that Fe-Cu rich constituent particle clusters are the most common initiation sites within both alloys stressed in either environment. The crack growth within each alloy, on average, was observed to be slowed in the cold-dry environment than in the warm-humid environment, but only at longer crack lengths. Although no overwhelming effects of grain boundaries and grain orientations on small-crack growth were observed, crack growth data showed local fluctuations within individual grains. These observations are understood as crack propagation through the underlying substructure at the crack surface and frequent interaction with low/high-angle grain and subgrain boundaries, during cyclic loading, and, are further attributed to periodic changes in crack propagation path and multiple occurrences of crack-branching observed in the current study. SEM-based stereology in combination with electron backscattered diffraction (EBSD) established fatigue crack surface crystallography within the region from ˜1 to 50 mum of crack initiating particle clusters. Fatigue crack facets were parallel to a wide variety of crystallographic planes, with pole orientations distributed broadly across the irreducible stereographic triangle between the {001} and {101}-poles within both warm-humid and cold-dry environments. The results indicate environmentally affected fatigue cracking in both cases, given the similarity between the observed morphology and crystallography with that of a variety of aerospace aluminum alloys cracked in the presence of moist-air. There was no evidence of
NASA Astrophysics Data System (ADS)
Yoon, Han Ki; Kim, Sa Woong; Lee, Sang Pill; Katoh, Yutai; Kohyama, Akira
Recently, reduced activation ferritic/martensitic steel, vanadium alloy and SiC/SiC composite are embossed for nuclear fusion reactor in accordance with the coolant. Especially, reduced activation ferritic/martensitic steel is very suitable material for nuclear fusion reactor, because it has low coefficient of thermal expansion and excellent heat conductivity. The objective of this study is to investigate fatigue crack propagation behavior in the Reduced Activation Ferritic Steel (JLF-1). The fatigue crack propagation behavior of the JLF-1 steel was investigated by the constant-amplitude loading test for the stress ratios R = 0.1, 0.3 and 0.5 respectively. The fatigue crack growth tests carried out at room temperature and 400°C for base metal and weld metal. The effects of stress ratio, test temperature, specimen size and TIG welding on the fatigue crack propagation behaviors for JLF-1 steel were discussed within the Paris law. Particularly, the fatigue crack propagation rate of a weld metal was similar to that of base metal at the stress ratio of 0.3. Also, the fatigue crack propagation rate of a half size specimen was similar to that of a full size specimen at the stress ratios of 0.1, 0.3 and 0.5 respectively. From this result, we can recognize that the fatigue crack propagation behavior of this material can be evaluated by using the half size specimens.
Luquet, David; Marchiano, Régis; Coulouvrat, François
2015-10-28
Many situations involve the propagation of acoustical shock waves through flows. Natural sources such as lightning, volcano explosions, or meteoroid atmospheric entries, emit loud, low frequency, and impulsive sound that is influenced by atmospheric wind and turbulence. The sonic boom produced by a supersonic aircraft and explosion noises are examples of intense anthropogenic sources in the atmosphere. The Buzz-Saw-Noise produced by turbo-engine fan blades rotating at supersonic speed also propagates in a fast flow within the engine nacelle. Simulating these situations is challenging, given the 3D nature of the problem, the long range propagation distances relative to the central wavelength, the strongly nonlinear behavior of shocks associated to a wide-band spectrum, and finally the key role of the flow motion. With this in view, the so-called FLHOWARD (acronym for FLow and Heterogeneous One-Way Approximation for Resolution of Diffraction) method is presented with three-dimensional applications. A scalar nonlinear wave equation is established in the framework of atmospheric applications, assuming weak heterogeneities and a slow wind. It takes into account diffraction, absorption and relaxation properties of the atmosphere, quadratic nonlinearities including weak shock waves, heterogeneities of the medium in sound speed and density, and presence of a flow (assuming a mean stratified wind and 3D turbulent ? flow fluctuations of smaller amplitude). This equation is solved in the framework of the one-way method. A split-step technique allows the splitting of the non-linear wave equation into simpler equations, each corresponding to a physical effect. Each sub-equation is solved using an analytical method if possible, and finite-differences otherwise. Nonlinear effects are solved in the time domain, and others in the frequency domain. Homogeneous diffraction is handled by means of the angular spectrum method. Ground is assumed perfectly flat and rigid. Due to the 3D
NASA Astrophysics Data System (ADS)
Luquet, David; Marchiano, Régis; Coulouvrat, François
2015-10-01
Many situations involve the propagation of acoustical shock waves through flows. Natural sources such as lightning, volcano explosions, or meteoroid atmospheric entries, emit loud, low frequency, and impulsive sound that is influenced by atmospheric wind and turbulence. The sonic boom produced by a supersonic aircraft and explosion noises are examples of intense anthropogenic sources in the atmosphere. The Buzz-Saw-Noise produced by turbo-engine fan blades rotating at supersonic speed also propagates in a fast flow within the engine nacelle. Simulating these situations is challenging, given the 3D nature of the problem, the long range propagation distances relative to the central wavelength, the strongly nonlinear behavior of shocks associated to a wide-band spectrum, and finally the key role of the flow motion. With this in view, the so-called FLHOWARD (acronym for FLow and Heterogeneous One-Way Approximation for Resolution of Diffraction) method is presented with three-dimensional applications. A scalar nonlinear wave equation is established in the framework of atmospheric applications, assuming weak heterogeneities and a slow wind. It takes into account diffraction, absorption and relaxation properties of the atmosphere, quadratic nonlinearities including weak shock waves, heterogeneities of the medium in sound speed and density, and presence of a flow (assuming a mean stratified wind and 3D turbulent ? flow fluctuations of smaller amplitude). This equation is solved in the framework of the one-way method. A split-step technique allows the splitting of the non-linear wave equation into simpler equations, each corresponding to a physical effect. Each sub-equation is solved using an analytical method if possible, and finite-differences otherwise. Nonlinear effects are solved in the time domain, and others in the frequency domain. Homogeneous diffraction is handled by means of the angular spectrum method. Ground is assumed perfectly flat and rigid. Due to the 3D
Monitoring of solidification crack propagation mechanism in pulsed laser welding of 6082 aluminum
NASA Astrophysics Data System (ADS)
von Witzendorff, P.; Kaierle, S.; Suttmann, O.; Overmeyer, L.
2016-03-01
Pulsed laser sources with pulse durations in the millisecond regime can be used for spot welding and seam welding of aluminum. Seam welds are generally produced with several overlapping spot welds. Hot cracking has its origin in the solidification process of individual spot welds which determines the cracking morphology along the seam welding. This study used a monitoring unit to capture the crack geometry within individual spot welds during seam welding to investigate the conditions for initiation, propagation and healing (re-melting) of solidification cracking within overlapping pulsed laser welds. The results suggest that small crack radii and high crack angles with respect to welding direction are favorable conditions for crack healing which leads to crack-free seam welds. Optimized pulse shapes were used to produce butt welds of 0.5 mm thick 6082 aluminum alloys. Tensile tests were performed to investigate the mechanical strength in the as-welded condition.
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.
Study on subsurface-inclined crack propagation during machining of brittle crystal materials
NASA Astrophysics Data System (ADS)
Guo, Jiawen; Chen, Jianbin; Li, Jia; Fang, Qihong; Liu, Youwen
2016-05-01
There is an immense need to obtain high-quality surface and subsurface on brittle material owing to the advantage of its improved performance. Thus, in this paper, we proposed a mechanical and numerical study of fracture mechanics from the perspective of external loading and indentation geometry in brittle machining. Stress intensity factors are computed to analyze various impacts of external loading and indentation configuration on subsurface crack propagation. Results indicate that the main fracture mode for inclined crack is shear rather than opening and the apex angle of the indentation plays an important role in fracture behavior. As a certain external loading is exerted to the surface of the silicon, a large apex angle of indentation may lead to strong shielding effect on mode II crack propagation. A relationship between critical value of external loading to the crack propagation and the apex angle of the indentation is given in this paper that shows quantitative indication for suppression of crack growth.
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
The effect of temperature upon the fatigue crack propagation behavior of Alloy 625
James, L.A.
1990-12-31
Fatigue crack propagation of annealed Alloy 625 was studied in air at 24--649 C. Crack growth rates tend to increase with temperature. Two heats were studied; differences in behavior between them suggest a heat-to-heat variability. Characterization of stress ratio (R=K{sub min}/K{sub max}) effects was also done at 538 C.
Propagation of 3D nonlinear waves over complex bathymetry using a High-Order Spectral method
NASA Astrophysics Data System (ADS)
Gouin, Maïté; Ducrozet, Guillaume; Ferrant, Pierre
2016-04-01
Scattering of regular and irregular surface gravity waves propagating over a region of arbitrary three-dimensional varying bathymetry is considered here. The three-dimensional High-Order Spectral method (HOS) with an extension to account for a variable bathymetry is used. The efficiency of the model has been proved to be conserved even with this extension. The method is first applied to a bathymetry consisting of an elliptical lens, as used in the Vincent and Briggs (1989) experiment. Incident waves passing across the lens are transformed and a strong convergence region is observed after the elliptical mound. The wave amplification depends on the incident wave. Numerical results for regular and irregular waves are analysed and compared with other methods and experimental data demonstrating the efficiency and practical applicability of the present approach. Then the method is used to model waves propagating over a real bathymetry: the canyons of Scripps/La Jolla in California. The implementation of this complex bathymetry in the model is presented, as well as the first results achieved. They will be compared to the ones obtained with another numerical model.
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.
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.
NASA Technical Reports Server (NTRS)
Wang, John T.; Pineda, Evan J.; Ranatunga, Vipul; Smeltzer, Stanley S.
2015-01-01
A simple continuum damage mechanics (CDM) based 3D progressive damage analysis (PDA) tool for laminated composites was developed and implemented as a user defined material subroutine to link with a commercially available explicit finite element code. This PDA tool uses linear lamina properties from standard tests, predicts damage initiation with an easy-to-implement Hashin-Rotem failure criteria, and in the damage evolution phase, evaluates the degradation of material properties based on the crack band theory and traction-separation cohesive laws. It follows Matzenmiller et al.'s formulation to incorporate the degrading material properties into the damaged stiffness matrix. Since nonlinear shear and matrix stress-strain relations are not implemented, correction factors are used for slowing the reduction of the damaged shear stiffness terms to reflect the effect of these nonlinearities on the laminate strength predictions. This CDM based PDA tool is implemented as a user defined material (VUMAT) to link with the Abaqus/Explicit code. Strength predictions obtained, using this VUMAT, are correlated with test data for a set of notched specimens under tension and compression loads.
NASA Astrophysics Data System (ADS)
Luo, Cong; Friederich, Wolfgang
2016-04-01
Realistic shallow seismic wave propagation simulation is an important tool for studying induced seismicity (e.g., during geothermal energy development). However over a long time, there is a significant problem which constrains computational seismologists from performing a successful simulation conveniently: pre-processing. Conventional pre-processing has often turned out to be inefficient and unrobust because of the miscellaneous operations, considerable complexity and insufficiency of available tools. An integrated web-based platform for shallow seismic wave propagation simulation has been built. It is aiming at providing a user-friendly pre-processing solution, and cloud-based simulation abilities. The main features of the platform for the user include: revised digital elevation model (DEM) retrieving and processing mechanism; generation of multi-layered 3D shallow Earth model geometry (the computational domain) with user specified surface topography based on the DEM; visualization of the geometry before the simulation; a pipeline from geometry to fully customizable hexahedral element mesh generation; customization and running the simulation on our HPC; post-processing and retrieval of the results over cloud. Regarding the computational aspect, currently the widely accepted specfem3D is chosen as the computational package; packages using different types of elements can be integrated as well in the future. According to our trial simulation experiments, this web-based platform has produced accurate waveforms while significantly simplifying and enhancing the pre-processing and improving the simulation success rate.
Cao, Yong; Zhang, Yi; Yin, Xianzheng; Lu, Hongbin; Hu, Jianzhong; Duan, Chunyue
2016-01-01
Lumbar facet joint (LFJ) degeneration is believed to be an important cause of low back pain (LBP). Identifying the morphological changes of the LFJ in the degeneration process at a high-resolution level could be meaningful for our better understanding of the possible mechanisms underlying this process. In the present study, we determined the 3D morphology of the LFJ using propagation phase contrast micro-tomography (PPCT) in rats to assess the subtle changes that occur during the degeneration process. PPCT provides vivid 3D images of micromorphological changes in the LFJ during its degeneration process, and the changes in the subchondral bone occurred earlier than in the cartilage during the early stage of degeneration of the LFJ. The delineation of this alteration was similar to that with the histological method. Our findings demonstrated that PPCT could serve as a valuable tool for 3D visualization of the morphology of the LFJ by providing comprehensive information about the cartilage and the underlying subchondral bone and their changes during degeneration processes. It might also have great potential for providing effective diagnostic tools to track changes in the cartilage and to evaluate the effects of therapeutic interventions for LFJ degeneration in preclinical studies. PMID:26907889
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
Dynamic crack propagation in a 2D elastic body: The out-of-plane case
NASA Astrophysics Data System (ADS)
Nicaise, Serge; Sandig, Anna-Margarete
2007-05-01
Already in 1920 Griffith has formulated an energy balance criterion for quasistatic crack propagation in brittle elastic materials. Nowadays, a generalized energy balance law is used in mechanics [F. Erdogan, Crack propagation theories, in: H. Liebowitz (Ed.), Fracture, vol. 2, Academic Press, New York, 1968, pp. 498-586; L.B. Freund, Dynamic Fracture Mechanics, Cambridge Univ. Press, Cambridge, 1990; D. Gross, Bruchmechanik, Springer-Verlag, Berlin, 1996] in order to predict how a running crack will grow. We discuss this situation in a rigorous mathematical way for the out-of-plane state. This model is described by two coupled equations in the reference configuration: a two-dimensional scalar wave equation for the displacement fields in a cracked bounded domain and an ordinary differential equation for the crack position derived from the energy balance law. We handle both equations separately, assuming at first that the crack position is known. Then the weak and strong solvability of the wave equation will be studied and the crack tip singularities will be derived under the assumption that the crack is straight and moves tangentially. Using the energy balance law and the crack tip behavior of the displacement fields we finally arrive at an ordinary differential equation for the motion of the crack tip.
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.
Modeling of crack propagation in weak snowpack layers using the discrete element method
NASA Astrophysics Data System (ADS)
Gaume, J.; van Herwijnen, A.; Chambon, G.; Birkeland, K. W.; Schweizer, J.
2015-10-01
Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e., to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, in order to compare the numerical and experimental results, the slab mechanical properties (Young's modulus and strength) which are not measured in the field were derived from density. The simulations nicely reproduced the process of crack propagation observed in field PSTs. Finally, the mechanical processes at play were analyzed in depth which led to suggestions for minimum column length in field PSTs.
Solar Energetic Particle Observations and Propagation in the 3-D Heliosphere in December 2006
NASA Astrophysics Data System (ADS)
Malandraki, Olga E.; Marsden, Richard G.; Lario, David; Tranquille, Cecil; Heber, Bernd; Mewaldt, Richard A.; Cohen, Christina M. S.; Lanzerotti, Louis J.; Forsyth, Robert B.; Elliott, Heather A.
2010-05-01
Ulysses is the first spacecraft to fly over the poles of the Sun. Although the Sun was again close to its activity minimum during the recently completed third polar orbit of Ulysses, solar activity has been more prevalent during the declining phase of solar cycle 23 than was the case in the declining phase of the 22nd solar cycle, when the first polar passes occurred (1994-1995). In December 2006, an unexpected rise of solar activity occurred. Active Region 10930 produced a series of major solar flares with the strongest one (X9.0) recorded on December 5, after it rotated into view on the east limb of the Sun. In this work, we present in detail energetic particle observations obtained by various instruments onboard Ulysses, located at > 70 degrees south heliographic latitude during this period and discuss their implications for particle propagation to solar polar regions. The observed events are also compared with high latitudes measurements obtained previously by Ulysses close to solar maximum. Furthermore, comparisons with data acquired by the STEREO and ACE spacecraft near the ecliptic plane are discussed.
Matrix Cracking in 3D Orthogonal Melt-Infiltrated SiC/SiC Composites with Various Z-Fiber Types
NASA Technical Reports Server (NTRS)
Morscher, Gregory N.; Yun, Hee Mann; DiCarlo, James A.
2003-01-01
The occurrence of matrix cracks in melt-infiltrated SiC/SiC composites with a 3D orthogonal architecture was determined at room temperature for specimens tested in tension oriented in the X-direction (parallel to Z-bundle weave direction) and Y-direction (perpendicular to Z-bundle weave direction) and Y-direction (perpendicular to Z-bundle weave direction). The fiber-types were Sylramic and Sylramic-IBN in the X and Y-directions and lower modulus ZMI, T300, and rayon in the Z-direction. Acoustic emission (AE) was used to monitor the matrix cracking activity. For Y-direction composites, the AE data was used to determine the exact (+/- 0.25 mm) location where matrix cracks occurred in the 3D orthogonal architecture. This enabled the determination of the stress-dependent matrix crack distributions for small but repeatable matrix rich 'unidirectional' and the matrix poor 'cross-ply' regions within the architecture. It was found that matrix cracking initiated at very low stresses (approx. 40 MPa) in the 'unidirectional' regions for the largest z-direction fiber tow composites. Decreasing the size of the z-fiber bundle, increased the stress for matrix cracking in the 'unidirectional' regions. Matrix cracking in the 'cross-ply' regions always occurred at higher stresses than in 'unidirectional' regions, and the stress-dependent matrix crack distribution of the 'cross-ply' regions was always over a wider stress-range than the 'unidirectional' regions. For composites tested in the X-direction, a lower elastic modulus and a narrower and lower stress-range for matrix cracking were observed compared to composites tested in the Y-direction.
Effect of random microstructure on crack propagation in cortical bone tissue under dynamic loading
NASA Astrophysics Data System (ADS)
Gao, X.; Li, S.; Adel-Wahab, A.; Silberschmidt, V.
2013-07-01
A fracture process in a cortical bone tissue depends on various factors, such as bone loss, heterogeneous microstructure, variation of its material properties and accumulation of microcracks. Therefore, it is crucial to comprehend and describe the effect of microstructure and material properties of the components of cortical bone on crack propagation in a dynamic loading regime. At the microscale level, osteonal bone demonstrates a random distribution of osteons imbedded in an interstitial matrix and surrounded by a thin layer known as cement line. Such a distribution of osteons can lead to localization of deformation processes. The global mechanical behavior of bone and the crack-propagation process are affected by such localization under external loads. Hence, the random distribution of microstructural features plays a key role in the fracture process of cortical bone. The purpose of this study is two-fold: firstly, to develop two-dimensional microstructured numerical models of cortical bone tissue in order to examine the interaction between the propagating crack and bone microstructure using an extended finite-element method under both quasi-static and dynamic loading conditions; secondly, to investigate the effect of randomly distributed microstructural constituents on the crack propagation processes and crack paths. The obtained results of numerical simulations showed the influence of random microstructure on the global response of bone tissue at macroscale and on the crack-propagation process for quasi-static and dynamic loading conditions.
Theory for accelerated slow crack propagation in polyethylene fuel pipes. Annual report, 1987-1988
Moet, A.; Chudnovsky, A.; Chaoui, K.; Strebel, J.
1988-06-01
The report describes a test for assessing the resistance of polyethylene fuel gas pipe materials to brittle crack propagation. The test employs fatigue loading to a notched specimen. Pipe specimens prepared from 2306-IIC and 2306-IA exhibit an initial stage of brittle crack propagation which becomes progressively ductile as it approaches ultimate failure by tearing. The complete test duration is extremely short in comparison to others currently employed, yet it similarly ranks both materials tested. Further, crack layer analysis is employed to evaluate the specific energy of fracture, gamma, a fundamental parameter characteristic of the material's resistance to brittle-crack propagation. It is also found from microscopic examinations that brittle fatigue involves a crazing mechanism known to occur under creep condition.
Effect of Microstructure on the Fatigue Crack Propagation Behavior of TC4-DT Titanium Alloy
NASA Astrophysics Data System (ADS)
Guo, Ping; Zhao, Yongqing; Zeng, Weidong; Liu, Jianglin
2015-05-01
This paper focused on the fatigue crack growth behavior of TC4-DT titanium alloy with different microstructures. Heat treatments were performed to produce different microstructures, which varied in α lamella width and cluster size. The fatigue crack propagation route was observed for different microstructures. The deformation characteristic of the crack tip plastic zone was analyzed. The results demonstrated that, for adequate mechanical properties of the alloy, the microstructure formed after performing two treatments (first, air cooling from the β-phase field, and then annealing at 550 °C for 4 h) exhibited a better fatigue anti-crack propagation ability. This result was related to the existing higher plastic deformation field in the crack tip. Wide α lamellae and coarse α colonies were found to contribute to the improvement of the fracture toughness.
NASA Astrophysics Data System (ADS)
Todoriki, M.; Furumura, T.; Maeda, T.
2013-12-01
We have studied the effect of topography and a seawater layer on the propagation of seismic wave propagation towards the realization of a high-resolution 3D FDM simulation of strong ground motions expected from future large subduction zone earthquakes along the Nankai Trough. Although most of the former studies on seismic wave propagation simulation did not consider a seawater layer in their simulation model, some of the recent studies claimed the importance of topography and a seawater layer on the simulation of strong ground motions (e.g., Petukhin et al., 2010; Nakamura, 2012; Maeda et al., 2013). In this study, we examined the effect of these two features on seismic wave propagation by introducing the high-resolution topography with a seawater layer over a wide frequency band. The area of 3D FDM simulation is 1200 km x 1000 km for horizontal directions and 200 km in depth, which covers entirely the area of southwestern Japan centered at 136E and 34.8N. This model was discretized with small grid interval of 0.5 km in horizontal direction and 0.25 km in depth. We used 2400 nodes of the K-computer, which is about 2.9% of its total resources, with a total memory of 1TB. We used a 3D velocity model of Koketsu et al. (2008) and an original source-rupture model from a recent study on the expansion of source-rupture area of the 1707 Hoei earthquake (Furumura et al., 2011). The result of simulation shows that the effect of a seawater layer on ground motion is small in almost all parts of Japan Island with a change of the seismic wave amplitude of less than +-20%. However, around the Northern Kanto area characterized by a belt-shaped anomalous zone, the amplitude of ground motion grows twice as large as that without seawater. This was possibly brought about from amplification of the amplitudes of surface waves generated on the Philippine Sea plate in the Suruga Trough located in the eastern end of the Nankai Trough. It is quite likely that the amplitude of surface wave
Origin of moisture effects on crack propagation in composites
NASA Technical Reports Server (NTRS)
Mandell, J. F.
1978-01-01
A study has been made of the origin of unexpected moisture effects on crack extension in fiberglass laminates. Water immersion has been found to greatly reduce the rate of crack growth under constant loading, while increasing the rate under cyclic loading, the latter effect being the expected one. Observations were made of the extension of the stable damage zone at the tip of precut notches in wet and dry environments. The damage zone size is postulated as a critical element in the relaxation of high stress concentrations in composites, such as those at notch or crack tips. Under constant load, moisture is shown to greatly expand the interply delamination region in the damage zone, thus reducing the local fiber stresses and increasing crack resistance. Under cyclic loading moisture has little effect on the delamination region, which is large even for dry environments, and the only effect is weakening of the material and acceleration of cracks. Severe hygrothermal conditions can so weaken the material that the crack resistance is reduced under constant loading as well.
Tsunami Generation and Propagation by 3D deformable Landslides and Application to Scenarios
NASA Astrophysics Data System (ADS)
McFall, Brian C.; Fritz, Hermann M.
2014-05-01
Tsunamis generated by landslides and volcano flank collapse account for some of the most catastrophic natural disasters recorded and can be particularly devastative in the near field region due to locally high wave amplitudes and runup. The events of 1958 Lituya Bay, 1963 Vajont reservoir, 1980 Spirit Lake, 2002 Stromboli and 2010 Haiti demonstrate the danger of tsunamis generated by landslides or volcano flank collapses. Unfortunately critical field data from these events is lacking. Source and runup scenarios based on real world events are physically modeled using generalized Froude similarity in the three dimensional NEES tsunami wave basin at Oregon State University. A novel pneumatic landslide tsunami generator (LTG) was deployed to simulate landslides with varying geometry and kinematics. The bathymetric and topographic scenarios tested with the LTG are the basin-wide propagation and runup, fjord, curved headland fjord and a conical island setting representing a landslide off an island or a volcano flank collapse. The LTG consists of a sliding box filled with 1,350 kg of landslide material which is accelerated by means of four pneumatic pistons down a 2H:1V slope. The landslide is launched from the sliding box and continues to accelerate by gravitational forces up to velocities of 5 m/s. The landslide Froude number at impact with the water is in the range 1
Observation of crack propagation in saline ice and freshwater ice with fluid inclusion
NASA Astrophysics Data System (ADS)
Arakawa, M.; Petrenko, V. F.
2003-01-01
A key process of crack propagation in saline ice is the interaction between the crack and fluid inclusions. We observed their interaction in freshwater ice using very high-speed photography (VHSP) and found that the low-density fluids (air and inert liquid, Fluorinert, 1.78 g/cm(3)) could not impede the crack effectively, interrupting the propagation for less than 10 mus. The high-density liquid mercury, (13.8 g/cm(3)) impeded the crack more effectively, stalling the development of the crack for more than 20 mus. The crack velocity in saline ice was measured using two different methods: electrical resistance method (ERM) and VHSP. These two methods returned very different mean velocities, 15 m/s for the ERM and 250 m/s for the VHSP. We found that in ice with conductive liquid inclusions, the ERM measured the time it took to break liquid bridges stretched across a crack rather than the crack velocity. Results from the VHSP show that the maximum crack velocity in saline ice was 500 m/s, which is one-half of that found in freshwater ice. From our results using freshwater ice with inclusions, we conclude that liquid inclusions in saline ice may play a role in this retardation.
Dislocation mechanism based model for stage II fatigue crack propagation rate
NASA Technical Reports Server (NTRS)
Mazumdar, P. K.
1986-01-01
Repeated plastic deformation, which of course depends on dislocation mechanism, at or near the crack tip leads to the fatigue crack propagation. By involving the theory of thermally activated flow and the cumulative plastic strain criterion, an effort is made here to model the stage II fatigue crack propagation rate in terms of the dislocation mechanism. The model, therefore, provides capability to ascertain: (1) the dislocation mechanism (and hence the near crack tip microstructures) assisting the crack growth, (2) the relative resistance of dislocation mechanisms to the crack growth, and (3) the fracture surface characteristics and its interpretation in terms of the dislocation mechanism. The local microstructure predicted for the room temperature crack growth in copper by this model is in good agreement with the experimental results taken from the literature. With regard to the relative stability of such dislocation mechanisms as the cross-slip and the dislocation intersection, the model suggests an enhancement of crack growth rate with an ease of cross-slip which in general promotes dislocation cell formation and is common in material which has high stacking fault energy (produces wavy slips). Cross-slip apparently enhances crack growth rate by promoting slip irreversibility and fracture surface brittleness to a greater degree.
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.
NASA Technical Reports Server (NTRS)
Nikishkov, G. P.; Park, J. H.; Atluri, S. N.
2001-01-01
The highly accurate and efficient Symmetric Galerkin Boundary Element Method (SGBEM), a Finite Element Method (FEM)-based alternating method, is proposed for analyzing three-dimensional non-planar cracks and their growth. The cracks are modeled using the symmetric Galerkin boundary element method as a distribution of displacement discontinuities, simulating an infinite medium. The finite element method only analyzes the stress for the uncracked body. The solution for the cracked structural component is determined by an iteration procedure. This process alternates between an FEM solution for the uncracked body and the SGBEM solution for a crack in an infinite body. Numerical analysis, and the Java code used, evaluate stress intensity factors and model fatigue crack growth. Examples of non-planar cracks in infinite media and planar cracks in finite bodies, as well as growth under fatigue, show the accuracy of the method.
NASA Astrophysics Data System (ADS)
Trovato, Claudio; Aochi, Hideo; De Martin, Florent
2014-05-01
Understanding the source mechanism of long-period (LP) seismic signals on volcanoes is an important key point in volcanology and for the hazard forecasting. In the last decades, moment tensor inversions have led to various descriptions of the kinematic source mechanism. These inversions suppose a relatively simple structure of the medium. However, the seismic wave propagation in a realistic 3-D volcano model should be taken into account for understanding the complicated physical processes of magma and gas behaviors at depth. We are studying Etna volcano, Italy, to understand the volcanic processes during different stages of activity. We adopt a spectral element method (SEM), a code EFISPEC3D (De Martin, BSSA, 2011), which shows a good accuracy and numerical stability in the simulations of seismic wave propagation. First we construct the geometrical model. We use a digital elevation model (DEM) to generate finite element meshes with a spacing of 50 m on the ground surface. We aim to calculate the ground motions until 3 Hz for the shallowest layer with Vs = ~500 m/s. The minimal size of the hexahedral elements is required to be around 100 m, with a total number of elements n = ~2 10 ^ 6 for the whole model. We compare different velocity structure configurations. We start with a homogeneous medium and add complexities taking in account the shallow low velocity structure. We also introduce a velocity gradient towards depth. Simulations performed in the homogeneous medium turn in approximately 20 hours for calculations parallelized on 16 CPUs. Complex velocity models should take approximately the same time of computation. We then try to simulate the ground motion from the LP sources (0.1-1.5 Hz) obtained by the inversion for the Etna volcano in 2008 (De Barros, GRL, 2009 and De Barros, JGR, 2011). Some vertical and horizontal structures can be added to reproduce injected dikes or sills respectively.
NASA Astrophysics Data System (ADS)
Wang, S.; De Hoop, M. V.; Xia, J.; Li, X.
2011-12-01
We consider the modeling of elastic seismic wave propagation on a rectangular domain via the discretization and solution of the inhomogeneous coupled Helmholtz equation in 3D, by exploiting a parallel multifrontal sparse direct solver equipped with Hierarchically Semi-Separable (HSS) structure to reduce the computational complexity and storage. In particular, we are concerned with solving this equation on a large domain, for a large number of different forcing terms in the context of seismic problems in general, and modeling in particular. We resort to a parsimonious mixed grid finite differences scheme for discretizing the Helmholtz operator and Perfect Matched Layer boundaries, resulting in a non-Hermitian matrix. We make use of a nested dissection based domain decomposition, and introduce an approximate direct solver by developing a parallel HSS matrix compression, factorization, and solution approach. We cast our massive parallelization in the framework of the multifrontal method. The assembly tree is partitioned into local trees and a global tree. The local trees are eliminated independently in each processor, while the global tree is eliminated through massive communication. The solver for the inhomogeneous equation is a parallel hybrid between multifrontal and HSS structure. The computational complexity associated with the factorization is almost linear with the size of the Helmholtz matrix. Our numerical approach can be compared with the spectral element method in 3D seismic applications.
NASA Astrophysics Data System (ADS)
Roselli, Ivan; Testa, Pierluigi; Caronna, Gaetano; Barbagelata, Andrea; Ferrando, Alessandro
2005-09-01
The present paper describes some of the main acoustic issues connected with the SAFE-AIRPORT European Project for the development of an innovative acoustic system for the improvement of air traffic management. The system sensors are two rotating passive phased-array antennas with 512 microphones each. In particular, this study focused on the propagation of sound waves in the atmosphere and its influence on the system detection efficiency. The effects of air temperature and wind gradients on aircraft tracking were analyzed. Algorithms were implemented to correct output data errors on aircraft location due to acoustic ray deviation in 3D environment. Numerical simulations were performed using several temperature and wind profiles according to common and critical meteorological conditions. Aircraft location was predicted through 3D acoustic ray triangulation methods, taking into account variation in speed of sound waves along rays path toward each antenna. The system range was also assessed considering aircraft noise spectral emission. Since the speed of common airplanes is not negligible with respect to sound speed during typical airport operations such as takeoff and approach, the influence of the Doppler effect on range calculation was also considered and most critical scenarios were simulated.
NASA Astrophysics Data System (ADS)
Shen, J. L.; Zhou, L.; Rowshandel, H.; Nicholson, G. L.; Davis, C. L.
2015-11-01
Alternating current field measurement (ACFM) probes are used to detect and size cracks in a range of engineering components. Crack sizing for this, and other electromagnetic (EM) based NDT systems, relies on relating the signal obtained to the actual crack length. For cracks that do not propagate vertically, such as rolling contact fatigue cracks in rails, predicting the crack depth, which determines the rail depth to be removed by grinding, requires an assumed propagation angle into the material as no method to determine crack vertical angle from the EM signals has been reported. This paper discusses the relationship between ACFM signals and propagation angles for surface-breaking cracks using a COMSOL model. The Bx signal accurately predicts the crack pocket length when the vertical angle is 30-90° but underestimates pocket length for shallower angles, e.g. a 50% underestimate is seen for a 3.2 mm pocket length crack propagating at a vertical angle of 10°. A new measure, the Bz trough-peak ratio, is proposed to determine the crack vertical angle. These are verified by experimental measurements using a commercial ACFM pencil probe for cracks with a range of vertical angles between 10° and 90°.
Atomistic aspects of crack propagation along high angle grain boundaries
Farkas, D.
1997-12-31
The author presents atomistic simulations of the crack tip configuration near a high angle {Sigma} = 5 [001](210) symmetrical tilt grain boundary in NiAl. The simulations were carried out using molecular statics and embedded atom (EAM) potentials. The cracks are stabilized near a Griffith condition involving the cohesive energy of the grain boundary. The atomistic configurations of the tip region are different in the presence of the high angle grain boundary than in the bulk. Three different configurations of the grain boundary were studied corresponding to different local compositions. It was found that in ordered NiAl, cracks along symmetrical tilt boundaries show a more brittle behavior for Al rich boundaries than for Ni-rich boundaries. Lattice trapping effects in grain boundary fracture were found to be more significant than in the bulk.
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.
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.
NASA Astrophysics Data System (ADS)
Butt, Ali
Crack propagation in a solid rocket motor environment is difficult to measure directly. This experimental and analytical study evaluated the viability of real-time radiography for detecting bore regression and propellant crack propagation speed. The scope included the quantitative interpretation of crack tip velocity from simulated radiographic images of a burning, center-perforated grain and actual real-time radiographs taken on a rapid-prototyped model that dynamically produced the surface movements modeled in the simulation. The simplified motor simulation portrayed a bore crack that propagated radially at a speed that was 10 times the burning rate of the bore. Comparing the experimental image interpretation with the calibrated surface inputs, measurement accuracies were quantified. The average measurements of the bore radius were within 3% of the calibrated values with a maximum error of 7%. The crack tip speed could be characterized with image processing algorithms, but not with the dynamic calibration data. The laboratory data revealed that noise in the transmitted X-Ray intensity makes sensing the crack tip propagation using changes in the centerline transmitted intensity level impractical using the algorithms employed.
A methodology for the investigation of toughness and crack propagation in mouse bone.
Carriero, Alessandra; Zimmermann, Elizabeth A; Shefelbine, Sandra J; Ritchie, Robert O
2014-11-01
Bone fracture is a health concern for those with aged bone and brittle bone diseases. Mouse bone is widely used as a model of human bone, especially to investigate preclinical treatment strategies. However, little is known about the mechanisms of mouse bone fracture and its similarities and differences from fracture in human bone. In this work we present a methodology to investigate the fracture toughness during crack initiation and crack propagation for mouse bone. Mouse femora were dissected, polished on their periosteal surface, notched on the posterior surface at their mid-diaphysis, and tested in three-point bending under displacement control at a rate of 0.1mm/min using an in situ loading stage within an environmental scanning electron microscope. We obtained high-resolution real-time imaging of the crack initiation and propagation in mouse bone. From the images we can measure the crack extension at each step of the crack growth and calculate the toughness of the bone (in terms of stress intensity factor (K) and work to fracture (Wf)) as a function of stable crack length (Δa), thus generating a resistance curve for the mouse bone. The technique presented here provides insight into the evolution of microdamage and the toughening mechanisms that resist crack propagation, which are essential for preclinical development of treatments to enhance bone quality and combat fracture risk. PMID:25084121
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.
Hoffman, M.J. Sydney Univ., NSW . Dept. of Mechanical Engineering); Dauskardt, R.H.; Ritchie, R.O. ); Mai, Y.W. . Dept. of Mechanical Engineering)
1992-05-01
Damage and cyclic fatigue failure under alternating loading in transformation-toughened zirconia ceramics is reviewed and compared to corresponding behavior under quasi-static loading (static fatigue). Current understanding of the role of transformation toughening in influencing cyclic fatigue-crack propagation behavior is examined based on studies which altered the extent of the tetragonal-to-monoclinic phase transformation in MG-PSZ through subeutectoid aging. These studies suggest that near-tip computations of the crack-driving force (in terms of the local stress intensity) can be used to predict crack-growth behavior under constant amplitude and variable-amplitude (spectrum) loading, using spatially resolved Raman spectroscopy to measure the extent of the transformation zones. In addition, results are reviewed which rationalize distinctions between the crack-growth behavior of preexisting, long'' (> 2 mm), through-thickness cracks and naturally-occurring, small'' (1 to 100 [mu]m), surface cracks in terms of variations in crack-tip shielding with crack size. In the present study, the effect of grain size variations on crack-growth behavior under both monotonic (R-curve) and cyclic fatigue loading are examined. Such observations are used to speculate on the mechanisms associated with cyclic crack advance, involving such processes as alternating shear via transformation-band formation, cyclic modification of the degree of transformation toughening, and uncracked-ligament (or grain) bridging.
Hoffman, M.J. |; Dauskardt, R.H.; Ritchie, R.O.; Mai, Y.W.
1992-05-01
Damage and cyclic fatigue failure under alternating loading in transformation-toughened zirconia ceramics is reviewed and compared to corresponding behavior under quasi-static loading (static fatigue). Current understanding of the role of transformation toughening in influencing cyclic fatigue-crack propagation behavior is examined based on studies which altered the extent of the tetragonal-to-monoclinic phase transformation in MG-PSZ through subeutectoid aging. These studies suggest that near-tip computations of the crack-driving force (in terms of the local stress intensity) can be used to predict crack-growth behavior under constant amplitude and variable-amplitude (spectrum) loading, using spatially resolved Raman spectroscopy to measure the extent of the transformation zones. In addition, results are reviewed which rationalize distinctions between the crack-growth behavior of preexisting, ``long`` (> 2 mm), through-thickness cracks and naturally-occurring, ``small`` (1 to 100 {mu}m), surface cracks in terms of variations in crack-tip shielding with crack size. In the present study, the effect of grain size variations on crack-growth behavior under both monotonic (R-curve) and cyclic fatigue loading are examined. Such observations are used to speculate on the mechanisms associated with cyclic crack advance, involving such processes as alternating shear via transformation-band formation, cyclic modification of the degree of transformation toughening, and uncracked-ligament (or grain) bridging.
Gangloff, R.P.; Kim, S.
1993-09-01
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.
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.
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.
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.
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.
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.
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.
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.
Technology Transfer Automated Retrieval System (TEKTRAN)
Crack development in the field is the result of the complex interaction of multiple processes relating to the soil’s structure, moisture condition, and stress level. Visualizing and characterizing the cracking behavior of soils across the soil depth has always been a key challenge and major barrier ...
Substructuring FE-XFE approaches applied to three-dimensional crack propagation
NASA Astrophysics Data System (ADS)
Wyart, E.; Duflot, M.; Coulon, D.; Martiny, P.; Pardoen, T.; Remacle, J. F.; Lani, F.
2008-06-01
Two substructuring methods are investigated in order to allow for the use of the eXtended Finite Element Method (X-FEM) within commercial finite element (FE) codes without need for modifying their kernel. The global FE problem is decomposed into two subdomains, the safe domain and the cracked domain based on the value of the level sets representing the crack. The safe domain is treated by the host FE software while the cracked domain is treated by an independent XFE code. The first substructuring method consists of calculating the Schur matrix of a cracked super-element with the XFE code. The second technique introduces the finite element tearing and interconnecting method (FETI) which ensures the compatibility of the displacements at the interface between the cracked and safe subdomains. The stiffness matrices and nodal forces are provided by the XFE and FE codes for the cracked and safe subdomains, respectively. The solutions obtained with these two techniques are rigorously equivalent to those computed with the stand-alone XFE code. First, the computational efficiency of the two approaches is demonstrated. Second, a validation is proposed towards comparison with reference values of the stress intensity factors in simple 3D cracked geometries. Finally, this contribution presents an application of the FE-XFE-FETI method to the computation of the stress intensity factor induced by a crack inside a hydraulic cylinder under internal pressure.
Cyclic fatigue-crack propagation in sapphire in air and simulated physiological environments.
Asoo, B; McNaney, J M; Mitamura, Y; Ritchie, R O
2000-12-01
Single-crystal aluminas are being considered for use in the manufacture of prosthetic heart valves. To characterize such materials for biomedical application, subcritical crack growth by stress corrosion (static fatigue) and by cyclic fatigue has been examined in sapphire along (1100) planes in 24 degrees C humid air and 37 degrees C Ringer's solution (the latter as a simulated physiological environment). The relationships between crack-propagation rates and the linear-elastic stress intensity have been determined for the first time in sapphire for both modes of subcritical cracking. It was found that growth rates were significantly faster at a given stress intensity in the Ringer's solution compared to the humid air environment. Mechanistically, a true cyclic fatigue effect was not found in sapphire as experimentally measured cyclic fatigue-crack growth rates could be closely predicted simply by integrating the static fatigue-crack growth data over the cyclic loading cycle. PMID:11007616
SH0 Guided Wave Interaction with a Crack Aligned in the Propagation Direction in a Plate
NASA Astrophysics Data System (ADS)
Ratassepp, M.; Lowe, M. J. S.
2009-03-01
Ultrasonic guided waves are currently of interest for structural health monitoring of large structures such as storage tanks and pipelines. This study focuses on the scattering of the fundamental horizontal shear (SH0) mode at a through-thickness notch or crack in a plate whose alignment is in the direction of the wave propagation. The reflection and diffraction of the wave at a crack are examined using 2D finite element simulations. It is shown that the main reflection is generated by Rayleigh-like surface waves created on the faces of the crack, which radiate energy back into the plate. The amplitudes of the reflected and diffracted signals are verified experimentally.
Finite Element Simulations on Erosion and Crack Propagation in Thermal Barrier Coatings
NASA Astrophysics Data System (ADS)
Ma, Z. S.; Fu, L. H.; Yang, L.; Zhou, Y. C.; Lu, C.
2015-07-01
Erosion of thermal barrier coatings occurs when atmospheric or carbon particles from the combustion chamber are ingested into aviation turbine engines. To understand the influence of erosion on the service life of thermal barrier coatings, we introduce the erosion and crack propagation models, and then by using finite element simulations, determine the relationship between the penetrating depth, the maximum principle stress and impingement variables such as velocity and angle. It is shown that cracks nucleate and extend during the erosion process and the length of a crack increases with the increase of the particle velocity and impact angle.
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.
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. PMID:27332924
Prediction of fatigue crack propagation life in notched members under variable amplitude loading
NASA Astrophysics Data System (ADS)
Khan, Z.; Rauf, A.; Younas, M.
1997-06-01
One of the interesting phenomenon in the study of fatigue crack propagation under variable amplitude load cycling is the crack growth retardation that normally occurs due to the application of a periodic overload. Fatigue crack growth rate under simple variable amplitude loading sequence incorporating period overloads is studied using single edge notched specimens of AISI304 stainless steel. Load interaction effects due to single and multiple overload have been addressed. Substantial retardation of fatigue crack growth rate is observed due to the introduction of periodic tensile overloads. Estimates of fatigue life have been obtained employing Wheeler model (using Paris and modified Paris equations) and Elber’s model. Analytical predictions are compared with experimental results. Results of these analytical fatigue life predictions show good agreement with the experimental fatigue life data. Fatigue crack propagation rates also have been evaluated from the fractographic study of fatigue striations seen on the fracture surface. Good agreement was found between the experimentally observed crack growth rates and the fatigue crack growth rates determined by the fractographic studies.
The influence of edge effects on crack propagation in snow stability tests
NASA Astrophysics Data System (ADS)
Bair, E. H.; Simenhois, R.; van Herwijnen, A.; Birkeland, K.
2014-08-01
The Extended Column Test (ECT) and the Propagation Saw Test (PST) are two commonly used tests to assess the likelihood of crack propagation in a snowpack. Guidelines suggest beams with lengths of around 1 m, yet little is known about how test length affects propagation. Thus, we performed 163 ECTs and PSTs 1.0-10.0 m long. On days with full crack propagation in 1.0-1.5 m tests, we then made videos of tests 2.0-10.0 m long. We inserted markers for particle tracking to measure collapse amplitude, propagation speed, and wavelength. We also used a finite element (FE) model to simulate the strain energy release rate at fixed crack lengths. We find that (1) the proportion of tests with full propagation decreased with test length; (2) collapse was greater at the ends of the beams than in the centers; (3) collapse amplitude was independent of beam length and did not reach a constant value; (4) collapse wavelengths in the longer tests were around 3 m, two times greater than what is predicted by the anticrack model. We also confirmed the prediction that centered PSTs had double the critical length of edge PSTs. Based on our results, we conclude that cracks propagated more frequently in the shorter tests because of increased stress concentration from the far edge. The FE model suggests this edge effect occurs for PSTs of up to 2 m long or a crack to beam length ratio ≥ 0.20. Our results suggest that ECT and PST length guidelines may need to be revisited.
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
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.
The influence of edge effects on crack propagation in snow stability tests
NASA Astrophysics Data System (ADS)
Bair, E. H.; Simenhois, R.; van Herwijnen, A.; Birkeland, K.
2014-01-01
Propagation tests are used to assess the likelihood of crack propagation in a snowpack, yet little is known about how test length affects propagation. Guidelines suggest beams with lengths around 1 m for Extended Column Tests (ECTs) and Propagation Saw Tests (PSTs). To examine how test length affects propagation, we performed 163 ECTs and PSTs 1 to 10 m long. On days with full crack propagation in 1.0 to 1.5 m tests, we then made videos of tests 2 to 10 m long. We inserted markers for particle tracking to measure collapse amplitude, collapse wave speed, and wavelength. We also used a finite element model to simulate the strain energy release rate at fixed crack lengths. We find that: (1) the proportion of tests with full propagation decreased with test length; (2) collapse was greater at the ends of the beams than in the centers; (3) collapse amplitudes in the longer tests were consistent with the shorter tests and did not reach a constant value; (4) collapse wavelengths in the longer tests were around 3 m, 2 × greater than what is predicted by the anticrack model. Based on our field tests and FE models, we conclude that the shorter tests fully propagated more frequently because of increased stress concentration from the far edge. The FE model suggests this edge effect occurs for PSTs up to 2 m long or a crack to beam length ratio ≥ 0.20. Our results suggest that ECT and PST length guidelines may need to be revisited.
NASA Astrophysics Data System (ADS)
Vitousek, S.; Fletcher, C. H.; Storlazzi, C. D.
2006-12-01
Nearshore currents are driven by a number of components including tides, waves winds and even internal tides. To adequately simulate transport of sand and other constituents, the realistic behavior of the dominant current-generating phenomena should be resolved. This often requires sufficient observations and calibration/validation efforts to achieve realistic modeling results. The work explores the capabilities of modeling the currents along West Maui. The West Maui coast has a propagating tide where the observed peak tidal currents, which are directed parallel to the coast, occur very closely to the peak tidal water levels. In 2003, the USGS collected an extensive set of current observations along West Maui, Hawaii, with the goal of better understanding transport mechanisms of sediment, larvae, pollutants and other particles in coral reef settings. The observations included vessel mounted ADCP surveys and an array seafloor instruments at the 10m isobath along the coast. A simple 2DH model of West Maui using Delft3D shows good comparison of the modeled and observed currents. Nearshore currents driven by waves and winds are also considered. During the data collection period a significant erosion event occurred within the study domain at Kaanapali Beach. This event undermined several trees on the shoreline and threatened resort infrastructure. In modeling the nearshore currents of this region we hope to determine the potential for sand transport and shoreline change to hindcast this event.
Consolidation of fatigue and fatigue-crack-propagation data for design use
NASA Technical Reports Server (NTRS)
Rice, R. C.; Davies, K. B.; Jaske, C. E.; Feddersen, C. E.
1975-01-01
Analytical methods developed for consolidation of fatigue and fatigue-crack-propagation data for use in design of metallic aerospace structural components are evaluated. A comprehensive file of data on 2024 and 7075 aluminums, Ti-6Al-4V alloy, and 300M steel was established by obtaining information from both published literature and reports furnished by aerospace companies. Analyses are restricted to information obtained from constant-amplitude load or strain cycling of specimens in air at room temperature. Both fatigue and fatigue-crack-propagation data are analyzed on a statistical basis using a least-squares regression approach. For fatigue, an equivalent strain parameter is used to account for mean stress or stress ratio effects and is treated as the independent variable; cyclic fatigue life is considered to be the dependent variable. An effective stress-intensity factor is used to account for the effect of load ratio on fatigue-crack-propagation and treated as the independent variable. In this latter case, crack-growth rate is considered to be the dependent variable. A two term power function is used to relate equivalent strain to fatigue life, and an arc-hyperbolic-tangent function is used to relate effective stress intensity to crack-growth rate.
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.
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.
Fatigue crack propagation under variable amplitude loading in PMMA and bone cement.
Evans, S L
2007-09-01
Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements. PMID:17483908
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.
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
2016-01-01
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
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.
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.
NASA Astrophysics Data System (ADS)
Gao, Huajian; Rice, James R.
) and the 3-D m(φ = 0) results given here for shear dislocation loops in the crack plane. It compares well to the exact results.
NASA Astrophysics Data System (ADS)
Belashov, Vasily
We study the formation, structure, stability and dynamics of the multidimensional soliton-like beam structures forming on the low-frequency branch of oscillation in the ionospheric and magnetospheric plasma for cases when beta=4pinT/B(2) <<1 and beta>1. In first case with the conditions omega
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.