Specific conditions at the tip of a crack and discontinuities in a material are the challenges in analyzing the growth of cracks using conventional methods. In recent years, a method has been developed based on the non-local mechanics, called peridynamic theory, which has improved the analysis process of such structures. In this theory, the points of matter whose displacement or displacement derivatives are discontinuous are not distinguished from other material points. In this paper, we employed the bond-based peridynamic theory to investigate the rate of crack propagation and the path of crack growth in a beam with an initial crack due to low velocity impact. Two beams made of polymethyl-methacrylate (PMMA) and steel alloy with different projectile shapes were considered. The effects of changes in the impact velocity and the fracture toughness were studied and the obtained results were validated with other conducted studies. The crack path was predicted successfully and the branching of the crack was captured. The results confirm the ability of the peridynamic theory to model the crack growth in impact problems.

In this study, cast tungsten carbide particle/steel matrix surface composites were fabricated using a vacuum evaporative pattern casting (V-EPC) infiltration process. Through thermal shock tests at 500 °C, the initiation and propagation of cracks at the interface of the composites were investigated. Owing to the mismatch in the coefficients of thermal expansion (CTE), cracks tended to appear at the interface reaction zone (IRZ) between the particles and the matrix. Because there was also a difference in the CTE between the composite and the substrate, the cracks propagated rapidly along the transition layer (TL) between the composite and the substrate, and finally connected to form macro-cracks. Based on the stress analysis and calculation, the maximum thermal stress at the TL was 63.4 MPa, while the maximum thermal stress at the IRZ was 38 MPa. It could thus be inferred that the TL is the weak link under thermal fatigue. In addition, the experimental results were verified and found to be in good agreement with the calculations.

GaN films have been grown on SiC substrates with an AlN nucleation layer by using a metal organic chemical vapor deposition technique. Micro-cracking of the GaN films has been observed in some of the grown samples. In order to investigate the micro-cracking and microstructure, the samples have been studied using various characterization techniques such as optical microscopy, atomic force microscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy (TEM). The surface morphology of the AlN nucleation layer is related to the stress evolution in subsequent overgrown GaN epilayers. It is determined via TEM evidence that, if the AlN nucleation layer has a rough surface morphology, this leads to tensile stresses in the GaN films, which finally results in cracking. Raman spectroscopy results also suggest this, by showing the existence of considerable tensile residual stress in the AlN nucleation layer. Based on these various observations and results, conclusions or propositions relating to the microstructure are presented.

Fracture analysis is performed on a cylindrical composite consisting of an outer elastic layer, an inner rigid cylinder and an intermediate sliding interface. Interaction between the sliding interface and a parallel crack under in-plane shear is explored. An interesting phenomenon of oscillatory normal stress occurs on the local interfacial region near to the crack. It leads to local sliding-prevention and promotion effects, which constitute the mechanisms for the variations of stress intensity factors versus interfacial parameters. In addition, another interesting conclusion is that a crack near and parallel to a sliding interface never has the conventional anti-symmetry, even under pure in-plane shear loading.

In this paper, a method is proposed for extracting fracture parameters in anisotropic thermoelasticity cracking via interaction integral method within the framework of extended finite element method (XFEM). The proposed method is applied to linear thermoelastic crack problems. The numerical results of the stress intensity factors (SIFs) are presented and compared with those reported in related references. The good agreement of the results obtained by the developed method with those obtained by other numerical solutions proves the applicability of the proposed approach and confirms its capability of efficiently extracting thermoelasticity fracture parameters in anisotropic materials.

A new approach in meshless methods has been introduced for stress assessment around a crack in two-dimensional elastic solids. This method with the name VDLSM (Voronoi Based Discrete Least Squares Meshless) is a pure meshless method which does not implement nodal mesh for trial and test functions. Rather, it uses a collection of the scattered nodal points and implements the discrete least squares approach to discretize the strong form of the governing differential equations on the domain of interest. This can reduce considerably the pre-processing cost of the analysis. Meshless methods generally are faced with some difficulty to accommodate the stress analysis in the vicinity of sharply concave surfaces such as cracks. Some techniques have been used to fix that problem such as visibility, transparency and diffraction, but these methods require some additional back corrective analyses for those parts of the domain located near the crack, which lengthen the time consumed for the solution and, moreover, do not provide the desired accuracy for the unknowns in these regions. VDLSM as a new, straightforward and easy applicable method has been suggested here for overcoming such deficiency using the algorithm of the Voronoi tessellation for constructing the Moving Least Squares (MLS) shape functions.

The defense mechanism that nacre (mother-of-pearl) uses to protect its living organism against high-speed predatory attack can provide lessons for engineered armor design. However, the underlying physics responsible for nacre's dynamic energy dissipation has hitherto remained a mystery to be uncovered. Here we demonstrate a new energy dissipation mechanism hidden in nacre and activated only upon dynamic loading, where the crack terminates its propagation along nacre's biopolymer interlayers but straightly impinges the aragonite platelets (95 vol%) in a transgranular manner. This intergranular–transgranular transition promotes the fracture energy dissipation, far exceeding that of the currently-used engineered ceramics. The mechanistic origin accounting for the enhancement of fracture energy dissipation is attributed to the unique nanoparticle architectured aragonite platelets. The dynamic manifestation in nacre can inspire a new route to design stronger-and-tougher engineered ceramic armors.

This study applies the numerical inversion of Laplace transform methods to study the piezoelectric dynamic fracture problem, recalculating Chen and Karihaloo's [1] analysis on the transient response of a impermeable crack subjected to anti-plane mechanical and in-plane electric impacts. Three numerical methods were adopted for calculating the dynamic stress intensity factor: Durbin method, Zhao method 1, and Zhao method 2. The results obtained were more accurate than the results in Chen and Karihaloo's [1] study. Through the calculation, this study presents a better range of parameters for the above three methods, and compares the advantages and disadvantages of each method in detail.

Fiber metal laminates (FMLs) are widely used in aerospace industries nowadays. Repairingof the cracks in these advanced materials was first done by some aeronautical laboratoriesin early 1970s. In this study, experimental investigations were done on the effect ofrepairing the center-cracked aluminum plates using the FML patches. The repairingprocesses were conducted to characterize the response of the repaired structures totensile tests. The composite patches were made of one aluminum layer and two wovenglass-epoxy composite layers. Three different crack lengths in three crack angles anddifferent patch lay-ups were examined. It was observed that no matter what the cracklength was, the more the crack angle is larger, the more ultimate tensile strength of thestructure became. It was also indicated that the patch lay-up had an important effect onthe tensile response of the repaired specimens. When the aluminum layer of the patches wasfarther from the repair zone, the ultimate tensile strength reached to its maximum value.

The purpose of this paper is to provide a priori error estimates on the approximation of contact conditions in the framework of the eXtended Finite-Element Method (XFEM) for two dimensional elastic bodies. This method allows to perform finite-element computations on cracked domains by using meshes of the non-cracked domain. We consider a stabilized Lagrange multiplier method whose particularity is that no discrete inf-sup condition is needed in the convergence analysis. The contact condition is prescribed on the crack with a discrete multiplier which is the trace on the crack of a finite-element method on the non-cracked domain, avoiding the definition of a specific mesh of the crack. Additionally, we present numerical experiments which confirm the efficiency of the proposed method.

In this study, numerical models based on finite-element method are developed for severalapplications of fracture mechanics. The use of cracked specimens formed by differentmaterial properties has allowed us to analyse the effect of the crack position to theinterface and to have a well understanding of mismatch effect for the case of mixed modeof fracture. In other hand, we have noticed that it is very interesting to see the effectof the interface position and loading for the case of symmetrically cracked ring specimenunder compression loading.

Welded structures are vulnerable to fracture due to cracks, especially at the welds. To investigate the safety of T-Shape welded structures used in some construction sites, a method is proposed in this paper to evaluate the crack occurrence probabilities of the structures. Three major factors that affect the crack occurrence are taken into consideration. They are residual stress, diffusible hydrogen content and chemical composition of the weld metal. In the analysis, finite element analysis is performed to find the residual stress distribution of the structures. The uncertainties of diffusible hydrogen content and chemical composition are treated as random variables. The critical cooling time is found and utilized for evaluating the crack occurrence probability of the welded structure. Numerical results indicate that T-shape welded joints lead to higher residual stresses and higher crack occurrence probabilities in comparison with the traditional butt joints. Therefore, more attention should be paid to this kind of welded joints when they are used.

In this study, we present a fully automated method to investigate and reconstruct the three-dimensional crack structure beneath an indent in a highly insulating material. This work concentrates on issues arising from a long automatic acquisition process, the insulating nature of the specimen, and the introduction of minimal damage to the original cracks resulting from indentation.

Notre approche vise à tester les modèles classiques de piègage de fissure pour des motifsde ténacité contrôlée et des géométries d’éprouvette réalistes, c’est-à-dire de taillesfinies. Nous modélisons notamment l’effet de l’épaisseur du matériau. Les prédictions dumodèle théorique développé dans [L. Legrand, S. Patinet, J.-B. Leblond, J. Frelat, V.Lazarus, D. Vandembroucq, Coplanar perturbation of a crack lying on the mid-plane of aplate, Int. J. Frac.] sont validées par des calculs de type éléments-finis. Ces derniersconvergent en fonction de la fréquence de perturbation du front de fissure vers deuxrégimes asymptotiques : milieu semi-infini et plaque mince. Dans le cas d’une fissureinteragissant avec un défaut unique, nous confrontons nos calculs à une configurationexpérimentale de fissuration. Nous montrons une remarquable amélioration des prédictionsdes modèles de ligne élastique par la prise en compte de l’épaisseur de l’éprouvette.

The stress state of a magnetized elastic half-plane with a uniformly pressurized crack parallel to the free surface subjected to a uniform magnetic induction Bo is considered. The linear theory for a soft ferromagnetic elastic solid with muti-domain structure, which has been developed by Pao and Yeh [1] is adopted to investigate this problem. A numerical method is developed to determine the magnetoelastic stress intensity factor. The effect of the magnetic field and the boundary conditions on the magnetoelasitc stress intensity factor are shown graphically and numerically.

Associé à des singularités mathématiques, le phénomène optique des caustiques représente l'ensemble des valeurs singulières de la projection d'une sous-variété lagrangienne de l'espace de phases (i.e. système de rayon) sur l'espace de configurations défini par le champ des déplacements. Dans ce cas, en supposant que la transition entre le domaine élastique et le domaine plastique qui entoure la pointe de fissure se fait par un changement de la configuration du champ de déplacements, nous nous proposons d'analyser les modifications que cela entraîne dans la formation des caustiques.

Le défaut de fatigue de contact des rails dénommé "head-check" comporte après amorçage une phase de propagation en mode II débouchant soit sur une bifurcation fatale, soit sur un blocage de la fissure. Pour prévoir au cas par cas les conditions qui peuvent conduire à l'une de ces issues, une modélisation de la fissuration en mode II est nécessaire. Dans cette étude, nous montrons que la méthode des dislocations est idéale pour établir les ΔKII qui permettent de décrire le comportement de la fissure en utilisant les paramètres de la loi de Paris. Nous analysons ainsi notamment l'effet de proximité dans le cas de paires de fissures, le rôle du frottement entre les faces de celles-ci et sa synergie avec les contraintes résiduelles.

L'objectif de ce travail est l'étude du comportement à la fatigue cyclique de disques sollicités en compression diamétrale en biocéramiques orthopédiques d'alumine et de zircone (Al2O3, ZrO2) utilisées pour la conception des prothèses ostéo-articulaires. Nous étudions, grâce aux calculs numériques, l'état de contraintes sur un cotyle d'une prothèse totale de hanche en se basant sur les travaux de Pauwels [Pauwels, Biomécanique de la hanche saine et pathologique, principe, technique et résultats d'une thérapeutique causale, Springer-Verlag, Berlin, Heidelberg, New York, 1977]. La zone la plus sollicitée en contrainte de traction potentiellement à l'origine d'une rupture par fatigue est déterminée. Nous rassemblons des résultats sur la propagation et l'amorçage des fissures de fatigue sous chargement cyclique en compression sur des éprouvettes de simulation de type disque brésilien chargées en mode I et en mode mixte I+II.

It is commonly accepted that Black and White Americans hold divergent views about the criminal justice system. Furthermore, many accept the view that U.S. public opinion is unflinchingly punitive where issues of criminal justice policy are concerned, with this punitiveness among White Americans deriving to a significant degree from anti-Black prejudice. Using a series of survey-based experiments and large, nationally representative samples of White and African American respondents, we subject the questions of Black-White polarization, unyielding punitiveness, and the influence of racial prejudice to close scrutiny. Our results, first, confirm large Black-White differences in opinion with Blacks consistently less punitive than Whites. These differences are substantially a result of beliefs about the extent of racial bias in the criminal justice system. Second, the framing experiments suggest that responses to the death penalty are very different than responses to drug-related crimes like crack or powder cocaine use, with the former exhibiting far less malleability than the latter. Third, racial prejudice is a consistently large influence on White public opinion and a weaker, but sometimes important influence among Blacks as well. Implications for discourse on race and crime are also discussed.

Studies of Alzheimer's disease patients show that individuals with larger premorbid brains have a later onset of disease, or a lessened severity of cognitive impairment, or both. This may be due to a “functional reserve” associated with the greater number of neurons and synapses available in larger brains. We used magnetic resonance imaging and the MicroCog Assessment of Cognitive Functioning to examine the association between intracranial volume (premorbid brain size) and neuropsychological function in abstinent crack-cocaine and crack-cocaine–alcohol dependent individuals. There were no significant differences between the crack-only and the crack–alcohol dependent participants in neuropsychological performance or in intracranial volume. The abstinent cocaine-dependent individuals (both crack-only and crack–alcohol) were significantly impaired in many neuropsychological domains. Intracranial volume accounted for a significant proportion of the variance in neuropsychological performance. This result is consistent with the finding in the Alzheimer's literature that larger brains can maintain function to a greater degree, or for a longer period of time, in the face of cerebral disease or insult. Functional reserve may be a heretofore little recognized protective mechanism of the brain that has consequences for the severity of expression of cerebral disease or insult throughout life. (JINS, 1998, 4, 559–565.)