Three-dimensional crack-face weight functions for the semi-infinite interface crack - I: Variation of the stress intensity factors due to some small perturbation of the crack front

The main goal of this paper and its companion is to provide the expressions of Bueckner's fundamental three-dimensional crack-face weight functions for a semi-infinite interface crack in an infinite body. The method of solution avoids the calculation of the full mechanical fields in the elasticity problems implied but concentrates instead on the sole feature of interest, namely the distribution of the stress intensity factors along the crack front. As a part of the method, one must derive the expression for the variation of the stress intensity factors along the crack front arising from an infinitesimal coplanar perturbation of that front. The present Part I is devoted to this question. As an application, we examine the problem of the stability of the fundamental, straight configuration of the propagating crack front versus small in-plane perturbations.