Dynamic response of an elastic medium containing crack

A fundamental solution for an infinite elastic medium containing a penny-shaped crack subjected to dynamic torsional surface tractions is attempted. A double Laplace–Hankel integral transform with respect to time and space is applied both to motion equation and boundary conditions yielding dual integral equations. The solution of the derived dual integral equations is based on an analytic procedure using theorems of Bessel functions and ordinary differential equations. The dynamic displacements’ field is obtained by inversion of the corresponding Laplace–Hankel transformed variable. Results of a representative example for a crack subjected to pulse surface tractions are obtained and discussed.     

Penny-shaped crack at the interface between elastic half-spaces under the action of a shear wave

The three-dimensional problem of elasticity for a bimaterial body with a penny-shaped crack at the interface under the action of a normal harmonic shear wave is solved by the boundary-element method. The distribution of displacements of crack faces and tractions and displacements at the interface is analyzed     

The computation of stress intensity factors in dissimilar materials

A reciprocal work contour integral method for calculating stress intensity factors is extended to treat the problem of two bonded dissimilar materials containing a crack along the bond. The method is based on Betti's Reciprocal work theorem from which the singular stress intensities at the crack tip may be evaluated in terms of an integral involving tractions and displacements on a contour remote from the crack tip.     

Analysis method of planar cracks of arbitrary shape in the isotropic plane of a three-dimensional transversely isotropic magnetoelectroelastic medium

The hyper-singular boundary integral equation method of crack analysis in three-dimensional transversely isotropic magnetoelectroelastic media is proposed. Based on the fundamental solutions or Green’s functions of three-dimensional transversely isotropic magnetoelectroelastic media and the corresponding Somigliana identity, the boundary integral equations for a planar crack of arbitrary shape in the plane of isotropy are obtained in terms of the extended displacement discontinuities across crack faces. The extended displacement discontinuities include the displacement discontinuities, the electric potential discontinuity and the magnetic potential discontinuity, and correspondingly the extended tractions on crack face represent the conventional tractions, the electric displacement and the magnetic induction boundary values. The near crack tip fields and the intensity factors in terms of the extended displacement discontinuities are derived by boundary integral equation approach. A solution method is proposed by use of the analogy between the boundary integral equations of the magnetoelectroelastic media and the purely elastic materials. The influence of different electric and magnetic boundary conditions, i.e., electrically and magnetically impermeable and permeable conditions, electrically impermeable and magnetically permeable condition, and electrically permeable and magnetically impermeable condition, on the solutions is studied. The crack opening model is proposed to consider the real crack opening and the electric and magnetic fields in the crack cavity under combined mechanical-electric-magnetic loadings. An iteration approach is presented for the solution of the non-linear model. The exact solution is obtained for the case of uniformly applied loadings on the crack faces. Numerical results for a square crack under different electric and magnetic boundary conditions are displayed to demonstrate the proposed method.     

Analysis of bi-material interface cracks with complex weighting functions and non-standard quadrature

A boundary element formulation is developed to determine the complex stress intensity factors associated with cracks on the interface between dissimilar materials. This represents an extension of the methodology developed previously by the authors for determination of free-edge generalized stress intensity factors on bi-material interfaces, which employs displacements and weighted tractions as primary variables. However, in the present work, the characteristic oscillating stress singularity is addressed through the introduction of complex weighting functions for both displacements and tractions, along with corresponding non-standard numerical quadrature formulas. As a result, this boundary-only approach provides extremely accurate mesh-insensitive solutions for a range of two-dimensional interface crack problems. A number of computational examples are considered to assess the performance of the method in comparison with analytical solutions and previous work on the subject. As a final application, the method is applied to study the scaling behavior of epoxy–metal butt joints.     

The fracture mechanics of slit-like cracks in anisotropic elastic media

U method of continuously distributed dislocations, the problem of a slit-like crack in an arbitrarily-anisotropic linear elastic medium stressed uniformly at infinity is formulated and solved. The crack faces may be either freely-slipping or loaded by arbitrary equal and opposite tractions. If there is no net dislocation content in the crack, then the tractions and stress concentrations on the plane of the crack are independent of the elastic constants and the anisotropy; the same is true of the elastic stress intensity factors. The crack extension force depends on anisotropy only through the inverse matrix elements K _mg⁻¹, where [K] is the pre-logarithmic energy factor matrix for a single dislocation parallel to the crack front. Numerical results for crack extension forces are presented for three media of cubic symmetry.     

Boundary Value Problems of Plane Elasticity Involving Orientations of Displacements and Tractions

This paper presents unconventional formulations of boundary problems of plane elasticity formulated in terms of orientations of tractions and displacements on a closed contour separating internal and external domains as the boundary conditions. These are combined with the conditions of continuity of tractions or displacements across the boundary. Therefore the magnitudes of neither tractions nor displacements are assumed on the contour. Four boundary value problems for both external and internal domains are investigated by analyzing the solvability of the corresponding singular integral equations. It is shown that all considered problems can have non-unique solutions expressed as linear combinations of particular solutions and, hence, contain free arbitrary parameters, the number of which is finite and determined by the contour orientations of tractions and/or displacements     

Reflection and transmission of elastic waves at five types of possible interfaces between two dipolar gradient elastic half-spaces

Reflection and transmission of an incident plane wave at five types of possible interfaces between two dipo-lar gradient elastic solids are studied in this paper. First, the explicit expressions of monopolar tractions and dipolar trac-tions are derived from the postulated function of strain energy density. Then, the displacements, the normal derivative of displacements, monopolar tractions, and dipolar tractions are used to create the nontraditional interface conditions. There are five types of possible interfaces based on all possible combinations of the displacements and the normal derivative of displacements. These interfacial conditions with consid-eration of microstructure effects are used to determine the amplitude ratio of the reflection and transmission waves with respect to the incident wave. Further, the energy ratios of the reflection and transmission waves with respect to the incident wave are calculated. Some numerical results of the reflection and transmission coefficients are given in terms of energy flux ratio for five types of possible interfaces. The influences of the five types of possible interfaces on the energy parti-tion between the refection waves and the transmission waves are discussed, and the concept of double channels of energy transfer is first proposed to explain the different influences of five types of interfaces.     

Intersonic crack growth under time-dependent loading

The problem investigated in this paper is a mode II crack extending at a uniform intersonic speed in an otherwise unbounded elastic solid subjected to time dependent crack-face tractions. The fundamental solution for this problem is obtained analytically, which is then used to construct the general solution for an intersonic crack subjected to arbitrary time-dependent loading. For time-independent loading, this solution reduces to Huang and Gao’s [J. Appl. Mech 68 (2001) 169] fundamental solution. We have also studied two crack-face loadings that are of interest for engineering applications.     

Dynamic stress intensity factors around a cylindrical crack in an infinite elastic medium subject to impact load

This paper investigates transient stresses around a cylindrical crack in an infinite elastic medium subject to impact loads. Incoming stress waves resulting from the impact load impinge on the crack in a direction perpendicular to the crack axis. In the Laplace transform domain, by means of the Fourier transform technique, the mixed boundary value equations with respect to stresses and displacements were reduced to two sets of dual integral equations. To solve the equations, the differences in the crack surface displacements were expanded in a series of functions that are zero outside the crack. The boundary conditions for the crack were satisfied by means of the Schmidt method. Stress intensity factors were defined in the Laplace transform domain and were numerically inverted to physical space. Numerical calculations were carried out for the dynamic stress intensity factors corresponding to some typical shapes assumed for the cylindrical crack.     

三维裂纹分析的主值型面力边界积分方程法

给出了一组只包含Ｃａｕｃｈｙ主值积分、不含有强奇异积分的三维静动力边界积分方程及其应用于裂纹问题的具体列式，并给出了几何轴对称问题的相应半解析边界元求解方法，将三维问题降阶为一维数值问题．文中分析了无限、半无限介质中圆裂纹、平行圆裂纹系、球面裂纹等在静载及应力波作用下的静力或瞬态动力响应问题，求得了相应的应力强度因子．     

圆弧形裂纹问题中的应力对数奇异性

研究了无限大板上的一条圆孤形裂纹, 又在裂纹表面作用有反对称载荷. 换言之, 裂纹两侧表面的载荷是大小相等方向相同的. 上述问题可用复变函数方法来解决. 应力和位移分量通过两个复位函数来表示. 经过一系列推导, 此问题可归结为复变函数的黎曼-希尔巴德(Riemann-Hilbert) 问题, 并且可用闭合形式得出解答. 裂纹端的应力强度因子用通常方法定出. 在裂纹端邻域, 得到的复位函数中有对数函数部分. 由这个对数函数部分, 可以定义和得出裂纹端的对数奇异性, 此对数奇异性系数用闭合型式得出.     

Computing bounds to mixed-mode stress intensity factors in elasticity

A bounding procedure combined with an effective error bound method for linear functionals of the displacements and a simple two points displacement extrapolation method is presented to compute the lower and upper bounds to the stress intensity factors in elastic fracture problems. First, the displacements of two nodes (or node pairs) on the crack edges are used to construct the linear extrapolation to obtain the stress intensity factors at the crack tip, so that stress intensity factors are explicitly expressed as linear functionals of the displacements. Then, a posteriori bounding method is utilized to compute the bounds to the stress intensity factors. Finally, the bounding procedure is verified by a mixed-mode homogenous elastic fracture problem and a bimaterial interface crack problem.     

Perturbation analysis of Mode III interfacial cracks advancing in a dilute heterogeneous material

The paper addresses the problem of a Mode III interfacial crack advancing quasi-statically in a heterogeneous composite material, that is a two-phase material containing elastic inclusions, both soft and stiff, and defects, such as microcracks, rigid line inclusions and voids. It is assumed that the bonding between dissimilar elastic materials is weak so that the interface is a preferential path for the crack. The perturbation analysis is made possible by means of the fundamental solutions (symmetric and skew-symmetric weight functions) derived in Piccolroaz et al. (2009). We derive the dipole matrices of the defects in question and use the corresponding dipole fields to evaluate “effective” tractions along the crack faces and interface to describe the interaction between the main interfacial crack and the defects. For a stable propagation of the crack, the perturbation of the stress intensity factor induced by the defects is then balanced by the elongation of the crack along the interface, thus giving an explicit asymptotic formula for the calculation of the crack advance. The method is general and applicable to interfacial cracks with general distributed loading on the crack faces, taking into account possible asymmetry in the boundary conditions.The analytical results are used to analyse the shielding and amplification effects of various types of defects in different configurations. Numerical computations based on the explicit analytical formulae allows for the analysis of crack propagation and arrest.     

An elastic wave scattering problem relating to the rapid movement of a fracture in a plate

We consider the problem of a semiinfinite through crack in an elastic plate, which starts suddenly under the application of a step load (mode-I). As the crack propagates, microfractures grow rapidly at a small distance ahead of the tip, releasing pressure pulses. This process of microfracture nucleation, pulse emission and subsequent coalescence with the main fracture front continues to occur during crack motion. We crudely simulate this process by employing a single point dilatational source that is situated ahead of the crack tip and moves in unison with it, emitting pulses periodically. The total wavefield is then due to the effect of this point source, as well as the scattering of the pulses by the crack front. We model this elastodynamic problem under the plane stress assumption. A closed form solution is developed for the in-plane displacements of the crack faces due to the scattered field. The surface wave contribution can be pulled out separately and is expected to be significant. In particular, the results are cast into a form that is readily amenable to numerical analysis. We will be presenting the numerical results in Part II of this two part paper.     

Exact solution for mixed boundary value problems at anisotropic piezoelectric bimaterial interface and unification of various interface defects

The special mixed boundary value problem in which a debonded conducting rigid line inclusion is embedded at the interface of two piezoelectric half planes is solved analytically by employing the 8-D Stroh formalism. Different from existing interface insulating crack model and interface conducting rigid line inclusion model, the presently analyzed model is based on the assumption that all of the physical quantities, i.e., tractions, displacements, normal component of electric displacements and electric potential, are discontinuous across the interface defect. Explicit solutions for stress singularities at the tips of debonded conducting rigid line inclusion are obtained. Closed form solutions for the distribution of tractions on the interface, surface opening displacements and jump in electric potential on the debonded inclusion are also obtained, in addition real form solutions for these physical quantities are derived. Various forms of interface defect problems encountered in practice are solved within a unified framework and the stress singularities induced by those interface defects are discussed in detail. Particularly, we find that the analysis of interface cracks between the embedded electrode layer and piezoelectric ceramics can also be carried out within the unified framework.     

Elastic displacements along the flanks of internal cracks in rubber

Biaxial tensile experiments with thin rubber sheets, containing an internal crack, reveal the possibility to simulate and readily check the exact linear-elastic crack-flank displacements. The resulting deformed shape and the final position during loading of an internal inclined crack in an infinite, biaxially loaded elastic plate, was defined by measuring the crack-flank displacements, and the deformation features of the internal crack in rubber sheets. The results were compared with the linear-elastic displacements and the respective features, which have been obtained from an infinitesimal elasticity theory.The calculation of these displacement and deformation properties for a given crack presupposes the determination of two parameters, which characterize the loading conditions of the boundaries of the elastic cracked plate. These parameters have been determined as Lagrangian or Eulerian ones from the homogeneous strains at the boundaries of the elastic sheet, assuming either a Hookean, or a neo-Hookean or a Mooney material behavior for the elastic sheet.It has been shown that, except for the vicinities of the crack tips and for the regions of the imposed boundary strains in the experiments, the observed crack-flank displacements agree satisfactorily with the respective displacements obtained from the infinitesimal theory, if the material behavior is assumed as a neo-Hookean one, and the boundary-loading parameters are calculated as Eulerian ones.     

反平面剪切裂纹和刚性线问题和带裂纹圆轴扭转问题中的新型积分方程

如果把通常裂纹问题中奇异积分方程中的右端项由应力改为合力,此时积分方程的核也要由奇异核改为对数型奇异核。文中对于反乎面剪切裂纹和刚性线问题和带裂纹圆轴扭转问题,推导出了这种带对数核的积分方程。     

Stochastic behavior of a crack with random length and tractions

A linear elastic crack with random length and tractions is analyzed by application of the collocation method for solving the governing singular integral equation. Obtained are the stochastic behavior of the crack opening and stress intensity factor. Numerical examples are provided to illustrate the method and approximation of the solution.