Elastodynamic Crack Problems

Begins with both a non-hypersingular time-domain traction boundary integral equation formulation for transient elastodynamic crack analysis and a time-stepping scheme for solving the boundary integral equations. The scheme is applied to analyze three-dimensional rectangular and penny-shaped cracks, and to investigate pulse shape effects on the dynamic stress intensity factor. The corresponding frequency-domain boundary integral equation is given, and time- harmonic wave propagation in randomly cracked solids is treated. The second half of the book deals with the elastodynamic analysis of a periodic array of cracks in plane strain and of anti-plane interface cracks between two different materials, and the effect of the material anistrophy on the near-tip quantities, the scattered far-field, and wave attenuation and dispersion. No index. Annotation copyrighted by Book News, Inc., Portland, OR

The purpose of this study is to develop a range of elastodynamic solutions that will be applicable to the investigation of crack growth and to non-destructive evaluation. A number of problems of dynamic crack propagation in brittle solids are solved by applying the Smirnov and Sobolev method of self-similar potentials and extending the method in several ways. The method of self-similar potentials in conjunction with function-theoretic approach leads to a direct approach to the solutions of two-dimensional problems of mode-I, mode-II, and mode-III cracks in homogeneous solids as well as to the solution of problem of the mode-III interface crack. The technique of self-similar with a new application of rotational superposition was used to solve dynamic problems of an expanding penny-shaped crack under torsional loading. The problem of sudden arrest of a crack is treated by the method of self-similar potentials with the aid of appropriate time delays and origin shifts and by using a scheme of weighted superposition of fundamental problems. The solutions of fundamental problems require a full of the function-theoretic approach. In all the problems considered, it is assumed that the crack-tip speed is less than that of the Rayleigh-wave speed for inplane problems and less than that of the shear-wave speed for antiplane problems. For the sub-Rayleigh cases, the singularities of velocities and stresses occurring at crack tips are always of inverse square root type. The dynamic stress intensity factor is used to describe the square root singularities at the crack tips. The asymptotic solutions at wave fronts are also obtained for some problems.

It is weH known that the traditional failure criteria cannot adequately explain failures which occur at a nominal stress level considerably lower than the ultimate strength of the material. The current procedure for predicting the safe loads or safe useful life of a structural member has been evolved around the discipline oflinear fracture mechanics. This approach introduces the concept of a crack extension force which can be used to rank materials in some order of fracture resistance. The idea is to determine the largest crack that a material will tolerate without failure. Laboratory methods for characterizing the fracture toughness of many engineering materials are now available. While these test data are useful for providing some rough guidance in the choice of materials, it is not clear how they could be used in the design of a structure. The understanding of the relationship between laboratory tests and fracture design of structures is, to say the least, deficient. Fracture mechanics is presently at astandstill until the basic problems of scaling from laboratory models to fuH size structures and mixed mode crack propagation are resolved. The answers to these questions require some basic understanding ofthe theory and will not be found by testing more specimens. The current theory of fracture is inadequate for many reasons. First of aH it can only treat idealized problems where the applied load must be directed normal to the crack plane.

This book is concerned with the numerical solution of crack problems. The techniques to be developed are particularly appropriate when cracks are relatively short, and are growing in the neighbourhood of some stress raising feature, causing a relatively steep stress gradient. It is therefore practicable to represent the geometry in an idealised way, so that a precise solution may be obtained. This contrasts with, say, the finite element method in which the geometry is modelled exactly, but the subsequent solution is approximate, and computationally more taxing. The family of techniques presented in this book, based loosely on the pioneering work of Eshelby in the late 1950's, and developed by Erdogan, Keer, Mura and many others cited in the text, present an attractive alternative. The basic idea is to use the superposition of the stress field present in the unfiawed body, together with an unknown distribution of 'strain nuclei' (in this book, the strain nucleus employed is the dislocation), chosen so that the crack faces become traction-free. The solution used for the stress field for the nucleus is chosen so that other boundary conditions are satisfied. The technique is therefore efficient, and may be used to model the evolution of a developing crack in two or three dimensions. Solution techniques are described in some detail, and the book should be readily accessible to most engineers, whilst preserving the rigour demanded by the researcher who wishes to develop the method itself.