Interface Edge Crack in a Bimaterial Elastic Half-Plane

The plane strain elastic half-plane problem of an edge crack lying along the interface of two perfectly bonded dissimilar quarter-planes is considered. Moreover, on the boundaries of the two quarter-planes concentrated forces are acting. For the correct formulation of the crack problem at hand, we consider the existence of a small slippage zone near the crack tip where closing stresses act. The mixed boundary value problem is subsequently reduced to a system of two functional equations of the Wiener–Hopf type which are effectively solved. The exact analytical solution of the problem is presented in series form. Numerical results, as well as asymptotic solutions for the most important physical quantities, are also presented. It is shown that there exist certain modes of surface loading of the homogeneous half-space, that result to the formation of two distinct zones at the crack tip region, one where the crack opening occurs and another adjacent to it, where frictionless contact of crack lips takes place. Also, it is demonstrated that in the case of high contrast of Young's moduli of the two quarter-planes, two opening-contact intervals appear consecutively along the crack. This revised version was published online in August 2006 with corrections to the Cover Date.     

Non-Axisymmetric Matrix Cracking and Interface Debonding with Friction in Ceramic Composites

A three-dimensional analytical model based on the principle of minimum potential energy is developed and applied to determine the stress state in a discrete fiber/matrix composite cylinder subjected to axial tensile loading in the fiber direction and containing a non-axisymmetric transverse matrix crack and an interface debond. The friction over the debonded interface is incorporated into the analysis. The strain energy release rates associated with the matrix crack and the interface debonding under the combination of the applied load and the interface frictional force are computed. The strain energy release rate criterion has been employed to evaluate the critical applied loads for the two fracture modes and to assess the competition between propagation of a matrix crack and growth of interface debonding. A parametric study has been carried out. The computed results show that the interface friction plays an important role in the failure of brittle matrix composites.     

Approximation of Strain-Stress Curves in Front of a Crack in a Non-Linear Material

Authors suggest a method to compute the parameters in the Ramberg-Osgood constitutive relation, which when used in the Hutchinson-Rice-Rosengren stress formula in front of the crack leads to a good agreement with the stress distribution computed numerically. The method is tested by finite element calculations.     

Crack Front Waves in Dynamic Fracture*

A rapidly moving tensile crack is often idealized as a one-dimensional object moving through an ideal two-dimensional material, where the crack tip is a singular point. When a material is translationally invariant in the direction normal to the crack's propagation direction, this idealization is justified. A real tensile crack, however, is a planar object whose leading edge forms a propagating one-dimensional singular front (a `crack front'). We consider the interaction of a crack front with localized material inhomogeneities (asperities), in otherwise ideal brittle amorphous materials. We review experiments in these materials which indicate that this interaction excites a new type of elastic wave, a front wave, which propagates along the crack front. We will show that front waves (FW) are highly localized nonlinear entities that propagate along the front at approximately the Rayleigh wave speed, relative to the material. We will first review some of their characteristics. We then show that by breaking the translational invariance of the material, FW effectively act as a mechanism by which initially `massless' cracks acquire inertia.     

Crack Growth Across a Strength Mismatched Bimaterial Interface

Crack growth across an interface between materials with different strength is examined by a cohesive zone model. The two materials have identical elastic properties but different fracture process properties, or different yield stresses, which is modeled by different cohesive stresses. The fracture criteria is a critical crack opening displacement. Load is represented by a stress intensity factor defining a remote square root singular stress field. The results show that the ratio between the cohesive stresses of the two materials primarily determines the behavior of the critical stress intensity factor. When the crack approaches a material with a higher cohesive stress the crack tip is shielded, but if the crack approaches a material with smaller critical crack opening displacement the maximum level of shielding is determined by the ratio between the critical crack opening displacements. When a crack approaches a material with a lower cohesive stress it is exposed to an amplified load. This revised version was published online in August 2006 with corrections to the Cover Date.     

Plane Stress Crack Tip Field Around a Rapidly Growing Ductile/Rigid Interface Crack

Plane stress dynamic crack growth along a ductile/rigid interface is investigated. The ductile material is taken to be ideally plastic and obey the J₂ flow theory of plasticity. Under steady-state conditions, the asymptotic structure of the crack-tip stress, velocity and strain fields has been obtained. The study reveals that two types of crack-tip sectors exist, namely uniform and nonuniform plastic sectors and that the stress, strain and velocity fields are bounded (nonsingular) in all sectors. In a uniform sector, the rectangular Cartesian components of the stress, strain and velocity fields are constant, and there is no plastic strain accumulation. In a nonuniform sector, the stress, strain and velocity components at a point depend on the angular position of the point in the crack-tip polar coordinate system and are governed by a system of simultaneous ordinary differential equations. This is a sector plastic strains can accumulate. A general crack-tip sector assembly is obtained for a practical range of crack growth speeds. Several nontrivial families of admissible solutions of the crack-tip fields based on this general assembly of uniform and nonuniform crack-tip sectors are presented and discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Self-similarity of the Crack Front in Stress Corrosion Fracture

In this letter, a new method was proposed to visualize the crack front in stress corrosion fracture of high strength steel. The fractal characteristic of the crack front was analyzed by measuring its fractal dimension and roughness exponent at the same time. The experimental results show that the irregularity of the crack front was partly self-affine.     

Plane Problem of a Crack on the Interface of Orthotropic Plates with Friction of Crack Lips

We consider a plane problem of a crack on the interface of different orthotropic arbitrarily oriented semiinfinite spaces subjected to the action of uniform tensile and shear forces applied at infinity. In the vicinity of the crack tip, the crack lips smoothly close up to form a contact zone of a priori unknown length in the presence of constant friction between the lips. A transcendental equation for the length of the contact zone is deduced and analyzed. As a result, we obtain the dependences of the relative length of the contact zone and the stress intensity factors on the orientation of the principal directions of orthotropy for different values of the external load and boundary friction.     

Modelling Crack Closure for an Interface Crack Under Harmonic Loading

The paper is devoted to a linear crack located between two dissimilar elastic half-spaces under normally incident harmonic tension-compression loading. The system of boundary integral equations for displacements and tractions is derived from the dynamic Somigliana identity. The dynamic stress intensity factors (the opening and the transverse shear modes) are computed as functions of the loading frequency taking the contact interaction of the opposite crack faces into account. The results are compared with those obtained neglecting the crack closure.     

Fatigue Debonding Analysis of Repaired Aluminium Panels by Composite Patch using Interface Elements

Repaired panels with composite patches subjected to fatigue loading may fail due to the progressive debonding between the composite patch and aluminium panel. The objective of this paper is to study the initiation and propagation of a possible fatigue debonding in the adhesive layer while the crack also growths in the panel for single-side repaired aluminium panels. For this purpose three dimensional finite elements method with a thin layer solid like interface element is employed. Fracture mechanics approach is used for the analysis of crack growth in aluminium panel and the interface elements with fatigue constitutive law for mixed mode debonding growth in the adhesive layer. A user element routine and a damage model material routine were developed to include the interface element and to simulate the initiation and propagation of damage in adhesive layer under cyclic loading. It is shown that, the debonding propagation and crack growth rate of the repaired panels depend on the composite patch material and interface bonding properties significantly. It is also shown that using of patch material with higher elastic module leads to the faster damage or debonding growth in the adhesive layer during the fatigue loading.     

Analytical-Numerical Solution of Elliptical Interface Crack Problem

The problem of elliptical interface crack, located between two bonded dissimilar elastic half spaces, is considered. To obtain a solution of the problem, the traction boundary pseudodifferential equations are used. An analytical-numerical method for solving these equations is proposed. Strain energy release rates along the crack contours are calculated for some examples. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modeling of Bonding at an Interface Crack

A mechanical and mathematical model is suggested for an interface crack with bonding in its end zones. Normal and shear bond tractions occurring under the action of the external loads are searched for by solving a system of two singular integrodifferential equations. The stress intensity factors at the crack tip are calculated taking the compensating action of the bonds into account. Energetic characteristics of the interface crack (the deformation energy release rate and the rate of the energy absorption by the bonds) are analyzed. A sensitivity analysis is performed of the force and energetic characteristics of the interface crack to the end zone size, bond compliance and limit stretching. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stress Intensity Factors for an Interface Kinked Crack in a Bi-Material Plate Loaded Normal to the Interface

Stress intensity factors for a kinked crack originating at interface of two bonded dissimilar materials subjected to normal tension are found by the finite element method.     

Axisymmetric Contact Problem for an Interface Crack

We study stresses formed in two rigidly connected elastic half spaces made of different materials in the case where their interface contains a circular crack. The contact of the crack surfaces along the crack contour is taken into account and the solution of the integral equation of the problem is obtained by the Wiener–Hopf method in the closed form. The size of the zone of contact of the crack surfaces and the distributions of stresses in the contact region and on the interface of the half spaces outside the crack are found explicitly.     

Thermal Analysis by Numerical Methods of Debonding Effects near the Crack Tip under Composite Repairs

Composite patch repair of metallic structures has become a rapidly grown technology in the aerospace field due to the demand for significant increases in the useful life of both military and civilian aircraft. This has led to significant advances overall in the repair technology of cracked metallic structures. Adhesively bonded composite reinforcements offer remarkable advantages such as mechanical efficiency, repair time, cost reduction, high structural integrity, repair inspectability, damage tolerance to further causes of future strains, anticorrosion and antifretting properties. However, because of the different nature and properties of the materials that form a repair (metals, composites, adhesives), side-effects may occur: debonding due to high stress concentration in the vicinity of the crack, thermal residual stresses because of different thermal expansion coefficients of the adherents, etc. In this paper a three-dimensional finite elements analysis of the area around a patch repaired crack of a typical aircraft fuselage is performed, taking into account both the properties and the geometry of the involved materials. Examined in this case are 2024-T3 aluminum alloy as base material, FM-73 as the adhesive system and F4/5521 boron/epoxy prepreg as the patch material. Through the thickness stresses near the crack tip and along the patch edges with and without temperature effects are calculated and debonding near the crack tip is examined. Finally, the calculated results are compared with existing theories.     

凝固界面前沿颗粒行为的研究进展

对凝固界面前沿颗粒被推斥或被吞没行为的热力学、动力学判据进行了综合评述,探讨了影响颗粒行为的动力学参数,认为从动力学角度而言,颗粒行为主要取决于颗柱所受相互对立的两种类型力的平衡关系,进而介绍了利用外场力控制凝固界面颗粒行为设想的可行性。     

Initiation of a Crack from the Edge of a Notch with Oblique Front in Specimens of Brittle Materials

The possibility of stable initiation and propagation of cracks from the edge of a notch with oblique front is established for specimens made of brittle tool materials. The compliance calibration of compact specimens weakened by notches with oblique front is obtained with the help of approximation of the experimental data by the method of least squares and applied to the evaluation of fracture toughness along the oblique profile in the process of wedging of a compact specimen.     

界面具有垂直裂纹的热障涂层的失效模拟

采用有限元分析软件ANSYS构造了一个在热生长氧化层（TGO）与陶瓷层界面具有一个垂直裂纹的纳米结构热障涂层的有限元模型。并计算了在热震过程中裂纹处的应力分布图,及裂纹尖端的应力场强度因子K1变化图。计算结果表明：裂纹处存在应力集中现象,且裂纹尖端的应力场强度因子K1在热障涂层热循环的冷却过程中随着时间的延长而减小,且在冷却最开始阶段,温度梯度变化最大,K1值也变化最大,裂纹在冷却的初始具有最大的扩展可能性。且涂层最有可能发生开裂失效。     

Crack Extension at an Interface: Prediction of Fracture Toughness and Simulation of Crack Path Deviation

Ductile tearing of laser welded Al sheets is studied both experimentally and numerically. The mechanical behaviour and the microstructure of the various zones of the weld are characterised. Mechanical tests on compact tension (C(T))-specimen are carried out, with the position of the initial crack in the heat affected zone. Due to the asymmetry of the configuration, crack path deviation towards the softer fusion zone is observed. The topography of the non-planar fracture surface is measured using laser equipment. This work is focussed on the prediction of the fracture resistance and the simulation of crack path deviation for the respective configuration. The numerical simulations are based on two different models for ductile damage: the micromechanical Gurson–Tvergaard– Needleman (GTN) model and the phenomenological cohesive model. In the case of the GTN-model, the crack front may follow an arbitrary path. In contrast, the crack propagation direction for the cohesive model is prescribed by the morphology of the finite element mesh. The GTN-model is used to investigate crack path deviation and to derive limits for simplifications used together with the cohesive approach. The latter allows for a cost-efficient 2D simulation. Good agreement between experimental results and numerical simulations could be achieved in all cases     

A Novel Hexahedral Interface Element for Nonlinear Crack Analysis

Existence of a crack in structures would lead to a sudden failure and damage. Establishing a precise analytical model for the cracked element would be a powerful tool to achieve the right answers in the analysis of the structure. The main aim of this article is to formulate a hexahedral interface element for use in nonlinear crack analysis. In this investigation, the kinematics of the discontinuous displacement field along with the virtual work principle, for a body with an internal discontinuity, is utilized. Based on the suggested interpolation functions for the discrete segments, and also the element displacement field, the element stiffness matrix is calculated. The proposed element can be used for modeling of the discrete cracks in three-dimensional problems, such as a concrete dam. Several numerical examples are analyzed for the accuracy test and a few of them are presented here. The results indicated that utilizing sufficient elements yields suitable answers.