A general weight function for inclined kinked edge cracks in a semi-plane

A general method for evaluating the Stress Intensity Factors (SIFs) of an inclined kinked edge crack in a semi-plane is presented. An analytical Weight Function (WF) with a matrix structure was derived by extending a method developed for an inclined edge crack. The effects of the principal geometrical parameters governing the problem were studied through a parametric Finite Element (FE) analysis, carried out for different reference loading conditions. The WF can be used to produce efficient and accurate evaluations of the SIFs for cracks with initial inclination angle in the range −60° to +60° and kinked angle in the range from −90° to +90°. The agreement between the results with those obtained by accurate FE solutions suggests that the proposed WF can be used as a general tool for evaluating the fracture mechanics parameters of an inclined kinked crack.     

Distributed cracks and kinked cracks in bonded dissimilar half planes with an interface crack

This paper is concerned with the interactions between an interface crack and other arbitrarily distributed cracks in two bonded dissimilar half planes. Special emphasis is placed on the cracks kinked at a tip of the interface crack, which remain unsolved as far as the authors are concerned. For the present, we pay attention to the stress intensity factors at the tips of the kinks or the distributed cracks, and not to those at the tips of the interface crack. The analysis is based on continuous distributions of the body forces along the cracks, and their densities are determined with a new procedure in order to get highly accurate results. The present analysis for distributed line cracks applies to kinked cracks, branched cracks and those piercing the interface just by joining some of the line cracks. Numerical calculations are performed for various important problems, and the effects of geometric and mechanical parameters on the stress intensity factors are examined.     

Stresses in an orthotropic semi-infinite strip due to edge loads

Summary The stresses in an orthotropic elastic semi-infinite strip subject to plane strain are investigated. Symmetrical distributions of surface tractions are prescribed on the sides of the strip, while along the end the boundary conditions are arbitrary. By using an integral transform method the problem is reduced to a singular integral equation. The dependence of the stress singularity and the stress-intensity factors on the orthotropic properties of the strip is investigated. Stress distributions over the strip end are evaluated numerically.With 5 Figures     

A model for interface cracks in layered orthotropic solids: Convergence of modal decomposition using the interaction integral method

The mode‐partitioning problem for bimaterial interfaces is still not resolved by the classical fracture mechanics approach in a satisfactory manner. Stress oscillations and overlapping crack faces are a direct consequence of the rigorous solution of the elastic boundary value problem, if the constitutive law changes discontinuously across the interface. Conversely, continuously varying material properties, also referred to as functionally graded materials (FGM), avoid these physically not admissible drawbacks. In this case the crack tip fields are of the same nature as those known from homogeneous materials. Therefore, the well‐established stress intensity factor concept can be used without any changes. Following this motivation an FGM‐interface model for delaminated composite beam structures was developed and its characteristics with respect to the modal decomposition of the crack tip fields were investigated. The considered beam structures consisted of two orthotropic layers, each of a different material. The spatial variation of the material properties in the interface region was modeled by a tanh ‐function introducing one transition parameter that controlled the FGM‐gradient. Four load cases were analyzed for each structural configuration: either a unit normal force or a unit bending moment was imposed on each end of the split beam. Thus, any load case can be simply reconstructed from the presented results by means of superposition. The stress intensity factors for modes I and II were then evaluated using an interaction integral method along with the finite element method. The corresponding results are given depending on the mesh density of the interface region, the integration domain and the transition parameter. In this manner, the influence of the transition parameter on the mode ratio and on the convergence behavior of the modal decomposition scheme with respect to its integration domain was identified. Finally, the ability of the FGM‐interface model to represent bimaterial interfaces while still maintaining the advantages of crack analysis in homogeneous materials was highlighted. Copyright © 2008 John Wiley & Sons, Ltd.     

Elastoplastic behavior of two-dimensional solids with rectilinear cracks under mixed-mode loading

This paper investigates the elastoplastic behavior of a two-dimensional solid with rectilinear cracks. Plastic strip model is used to reduce plasticity problem to the equivalent linear eleasticity formulation. The effective mechanical response predictions are obtained for mode I and mixed-mode cracks. For mixed-mode cracks, the modification of Dugdale (1960) model proposed by Becker and Gross (1988) is applied. Results are compared with known elastic solutions.     

Frictional contact problems of kinked cracks modelled by a boundary integral method

A numerical method is presented for the solution of two dimensional crack problems including the effects of crack kinks and frictional contact between crack faces. The metod is based on an integral equation for the resultant forces along a crack. Coulomb friction between contacting crack surfaces is taken into account. The numerical implementation is demonstrated by considerations of surface and sub-surface piece-wise straight line cracks in a half-plane. Numerical results are presented to illustrate the efficiency and the reliability of the presented method.     

Application of the Finite Element Iterative Method to cracks and sharp notches in orthotropic media

In this paper, the asymptotic singular fields at cracks and sharp notches in isotropic and orthotropic media are numerically investigated. The Finite Element Iterative Method (FEIM) is used for evaluating the power of the singular field at the notch root under three different loading conditions: tension, shear, and a combination of tension and shear. The numerical results for the isotropic case, obtained by the FEIM, are in excellent agreement with the results from analytical solutions. In tension, the singularity at the notch root converges to the strong singularity % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4UdW2aaS% baaSqaaiaadggaaeqaaaaa!38B3!\[\lambda _a \], while under shear, convergence is to the weak singularity % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4UdW2aaS% baaSqaaiaadkgaaeqaaaaa!38B4!\[\lambda _b \]. When the loading is a combination of tension and shear, the singularity at the notch root always converges to % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4UdW2aaS% baaSqaaiaadggaaeqaaaaa!38B3!\[\lambda _a \] The stress singularity results for orthotropic materials followed the same trend. However, the stress singularities % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4UdW2aaS% baaSqaaiaadggaaeqaaaaa!38B3!\[\lambda _a \] and % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4UdW2aaS% baaSqaaiaadkgaaeqaaaaa!38B4!\[\lambda _b \] are strongly dependent on the elastic properties of the material. Under combined tension and shear loading, the number of iterations for convergence significantly increases, as the loading changes from tension-dominated to shear-dominated. This study has demonstrated the usefulness of the FEIM for the evaluation of asymptotic singular fields at sharp notches in orthotropic materials.     

Analysis of multiple interfacial cracks in an orthotropic bi-material subjected to anti-plane shear loading

The present work concerns with the elasto-static problem of double interfacial cracks located between two dissimilar orthotropic plates. The dimensions of the bi-material composite, are assumed to be finite. The crack faces are subjected to anti-plane shear traction. Finite Fourier transforms are applied to reduce the problem to a triple series equations, and then to a system of singular integral equations with Cauchy type singularity. That are solved numerically using Gauss-Chebyshev integration formulae. The stress intensity factors, are determined in a closed form expressions. The obtained results agreed with the previous analytical ones. Further, a parametric study is introduced to investigate the effects of the geometric and elastic characteristics of the composite on the values of the stress intensity factors.     

Thermoelasticity problem of an orthotropic plate with two collinear cracks

In this article a rigorous formulation of, and an exact solution to the plane thermoelasticity problem of an orthotropic plate having two collinear cracks are presented. Explicit expressions for the temperature, thermal displacements, thermal stresses and thermal stress intensity factors are obtained assuming that uniform or linear heat flow has been applied on the crack surfaces. Numerical values of thermal stress intensity factors and thermal crack sliding displacements and other quantities are worked out and presented in graphic form. A number of conclusions of practical significance are derived from the above results, including the assertion that an extremely large magnitude of stress singularity may occur in the geometry of two closely neighbouring cracks under the action of linear heat flow.

---

Résumé On présente une formulation rigoureuse et une solution exacte d'un problème de thermo-élasticité plane dans une tôle orthotrope comportant deux fissures colinéaires. On obtient les expressions explicites de la température, des dilatations thermiques, des contraintes thermiques et des facteurs d'intensité des contraintes thermiques en supposant appliquer un flux de chaleur uniforme ou linéaire sur les faces de la fissure. On élabore les valeurs numériques des facteurs d'intensité des contraintes thermiques, des déformations thermiques de la fissure par glissements, et d'autres grandeurs, et on les présente sous une forme graphique. On tire de ces résultats un certain nombre de conclusions pratiques, y compris la constatation que des singularités de contraintes d'extrêmmeent grande amplitude peuvent se rencontrer dans une géométrie de deux fissures très voisines sujettes à un flux de chaleur linéaire.

     

Rapid propagation of cracks in orthotropic materials

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 5, pp. 70–71, September–October, 1989.     

Contact model of cracks in orthotropic materials

We consider a plane strain problem for an orthotropic half plane loaded at infinity and containing a crack along its fixed edge. To remove a singularity near the right crack tip, we introduce an artificial contact zone. The problem under consideration is reduced to the mixed Dirichlet-Riemann boundaryvalue problem. We present the exact solution of this problem and deduce formulas for stresses in the contact zone and on the continuation of the crack and for the stress intensity factors. By using both analytic and numerical methods, we prove that the energy-release rate is quasiinvariant in the process of crack propagation relative to the size of the contact zone. On the basis of these results, we propose an algorithm for the evaluation of the paramenters of fracture of composites of finite dimensions with interface cracks. As a special case, we develop a model of interface cracks with actual contact zone and establish the dependences of the length of this zone on the external load and elasticity moduli of the material. Dnepropetrovsk State University, Dnepropetrovsk. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 5, pp. 59–66, September–October, 1999     

An interaction integral method for analysis of cracks in orthotropic functionally graded materials

This paper presents two new interaction integrals for calculating stress-intensity factors (SIFs) for a stationary crack in two-dimensional orthotropic functionally graded materials of arbitrary geometry. The method involves the finite element discretization, where the material properties are smooth functions of spatial co-ordinates and two newly developed interaction integrals for mixed-mode fracture analysis. These integrals can also be implemented in conjunction with other numerical methods, such as meshless method, boundary element method, and others. Three numerical examples including both mode-I and mixed-mode problems are presented to evaluate the accuracy of SIFs calculated by the proposed interaction integrals. Comparisons have been made between the SIFs predicted by the proposed interaction integrals and available reference solutions in the literature, generated either analytically or by finite element method using various other fracture integrals or analyses. An excellent agreement is obtained between the results of the proposed interaction integrals and the reference solutions. The authors would like to acknowledge the financial support of the U.S. National Science Foundation (NSF) under Award No. CMS-9900196. The NSF program director was Dr. Ken Chong.     

Application of an accelerated boundary-based mesh-free method to two-dimensional problems in potential theory

The boundary node method (BNM) is a boundary-only meshfree method based on boundary integral equations (BIE). One drawback of the usual BNM, however, is that it typically requires much more computer time than the usual boundary element method (BEM). The multipole method (MM) has been demonstrated, in the context of the n body problem, and the BEM, to greatly accelerate these methods while still maintaining sufficient accuracy. The present paper explores, for the first time, a coupling of the BNM with the MM (called the BNMM) in the context of 2-D potential theory. Numerical results (for selected problems) from the BNM are compared with those from the BNMM with regard to accuracy and computational efficiency. Sincere thanks are expressed to Mr. Zhongping Bao for the use of his computer for the numerical study of problem 4.     

Two singular points finite elements in the analysis of kinked cracks

A formulation for incorporating two singular points (TSP) of variable orders in a single finite element is presented. Though the element does not satisfy any of the convergence criterion, its performance is found to be good, which has been demonstrated by considering number of examples on kinked cracks. In each case only one such element is incorporated in the whole discretization. These examples illustrate the usefulness of the element to analyse kinked cracks of various sizes and shapes and subjected to different loading and boundary conditions. Computed J-integrals are compared with analytical solutions, wherever possible, and the accuracy appears quite good. Effect of size of the element on, and the path independence of, J are also examined.List of symbols , conventional natural coordinate system - ₁, ₂ another elemental natural coordinate system - L _ij equation of side ij of an element - A _ij , B _ij constants - N _i shape function associated node i - ₁, ₂ constants associated with the order of singularities - , an elemental natural coordinate system - u _i , v _i displacement components in the Cartesian directions     

A modified Kachanov method for analysis of solids with multiple cracks

Kachanov proposed an approximate method for the analysis of multiple cracks by assuming that traction in each crack can be represented as a sum of a uniform component and a non-uniform component, and the interaction among the cracks are only due to the uniform components. These assumptions simplify considerably the mathematics and allow ‘closed-form’ solutions to be obtained for some cases. However, it is noted that the assumptions may not be valid when the cracks are very close. Therefore, an improved method of elastic solids with closely spaced multiple cracks is proposed. Unlike the Kachanov method, traction in a crack is decomposed into a linearly varying component and a non-uniform component so that the sum of the two components to be equal to the traction along the crack length. It is further assumed that the interaction effect due to the non-uniform component can be neglected, and therefore, only the effect of the linearly varying component has to be considered. The accuracy of the present method is validated by comparing the results of two and three collinear open cracks obtained by the present method with those of the exact solutions and the original Kachanov method. Applications of the approach in solving non-collinear parallel crack and friction crack problems are also presented to demonstrate the versatility and accuracy of the method.     

Atomically sharp cracks in brittle solids: an electron microscopy study

The issue of bond rupture versus microplasticity as an essential mechanism of crack propagation in brittle solids is addressed. A detailed survey of existing theoretical and experimental evidence relating to this issue highlights the need for direct observations of events within the crack-tip process zone, at a level approaching 10 nm. Transmission electron microscopy is accordingly used to study arrested cracks about sharp-contact (Vickers indentation and particle impact) sites in Si, Ge, SiC and Al₂O₃. The nature of the deformation which accommodates the irreversible contact impression is first investigated, in the light of Marsh's proposal of an equivalence between indentation and crack-tip zone processes. Interfacial and tip regions of the surrounding cracks are then examined for any trace of a plasticity-controlled fracture process. Dislocation-like images are indeed evident at the crack planes, but these are shown to be totally inconsistent with any conventional slip mechanism. The close connection between the dislocation patterns and moiré fringe systems along the cracks points to lattice mismatch contrast in association with a partial closure and healing operation at the interface. Analysis of all other details in the crack patterns, e.g. the presence of a crack-front contrast band indicative of a residual strain field and the disposition of interfacial fracture steps relative to the dislocation/moiré system, reinforces this interpretation. It is concluded that the concept of an atomically sharp crack provides a sound basis for the theory of fracture of brittle solids.     

The stress intensity for an edge crack in a semi-infinite orthotropic body

The problem of an edge crack in a semi-infinite plane of linear elastic orthotropic material is studied. The correction factor which relates the stress intensity factor for this problem to that for an isolated crack in an infinite body is evaluated for a range of orthotropic material properties. Calculations are restricted to mode I problems. The method requires the numerical solution of an integral equation, the integrands in which are derived from related complex variable solutions.

---

Resumé On étudie le problème d'une fissure de bord dans un plan semi-infini en un matériau linéaire eastique et orthotrope. On évalue pour une gamme de propriétés orthotropes du matériau le facteur de correction qui permet de relier le facteur d'intensité de contraintes relatif à ce problème à celui relatif à une fissure isolée dans un corps infini. Ces calculs sont limités aux problèmes de rupture selon le mode. I. La méthode exige de trouver la solution numérique d'une intégrale dont les intégrants sont tirés de solutions associées à variables complexes.