A STRAIN-GRADIENT MODEL OF CLEAVAGE FRACTURE IN PLASTICALLY DEFORMINGMATERIALS

The current analysis presents a model for cleavage fracture in the pre sence of dislocation plasticity. We build on a framework developed in an earlier paper [Lipkin et al., Acta mater. 44, 1287 (1996)], wherein an elastic core is embedded about the crack tip in a plastically defo rming medium. This model provides a mechanism by which cleavage-type c rack growth could proceed concomitantly with significant plastic dissi pation. The present model is amended to account for the large strain g radients in the immediate vicinity of the crack tip. Such gradients ar e thought to lead to extensive local hardening. A simple, continuum-ba sed model is used to identify a characteristic Length scale ahead of t he crack tip, within which the material cannot plastically deform subj ect to the crack-tip stress field. An expression is derived for the cr ack-tip shielding afforded by the plasticity prior to initiation of Fr acture. The strong dependence of toughness on the ideal work of fractu re indicates a possible mechanism for such varied phenomena as segrega tion-induced embrittlement, ductile-to-brittle transition, stress corr osion cracking and constrained fracture in metal-ceramic composites. C opyright (C) 1996 Acta Metallurgica Inc.