A thermodynamic approach to the influence of plastic deformation on crack instability

Abstract

It is argued that the role of plastic deformation in crack growth has been widely misinterpreted. In particular the notion that the work of plastic deformation contributes to the energy balance in such a way as to give rise to an increased effective surface energy is shown to be physically incorrect. A fundamental reappraisal of the role of plastic deformation on the instability of cracks is undertaken and a thermodynamic formulation of the problem of the energetics of crack extension is developed. This is capable of treating both moving cracks and the dislocations producing plastic deformation in a unified manner and provides a powerful tool for the study of combined crack extension and plastic deformation. A rigorous definition of plastic work is also given and it is shown that the crack extension force is the only physically meaningful parameter which can be used in the formulation of a crack instability criterion. The work of plastic deformation is not a part of the crack extension force and should not be included in the criterion for crack instability. It is found that one of the important effects of plastic deformation on crack instability is the ‘shielding’ effect or the reduction it causes in the magnitude of the stress at the crack tip resulting in a smaller crack extension force. Other possible effects of plastic deformation on crack propagation are briefly discussed.