PLASTICITY EFFECTS ON FRACTURE NORMAL TO INTERFACES WITH HOMOGENEOUS AND GRADED COMPOSITIONS

This paper deals with the problem of a crack in a multi-layered materi al with a homogeneous or a compositionally graded interface. The effec ts of plasticity mismatch between the layers (which have no elastic mi smatch) on the shielding or amplification of the crack tip driving for ce are examined by recourse to finite element analyses for the case of a crack perpendicularly approaching the interface between the layers. When the near-lip plastic zone spreads across the interlayer, the cra ck-tip J integral is smaller than the remotely imposed J integral; if the crack is situated in the plastically weaker material. For this sit uation, introducing an interlayer between two dissimilar solids provid es greater crack-tip shielding than joining the two solids without an interlayer (i.e., with a sharp interface). An interlayer with a homoge neous yield;behavior (i.e., where the yield strength of the interlayer is the average of that of the two constituent layers) provides a grea ter shielding effect than a graded interface within which the yield st rength varies linearly from one end of the interlayer to the other. Wh en the crack approaches the interlayer from the plastically stronger m aterial, the crack tip driving force is amplified as the plastic zone spreads across the interlayer. This amplification is the maximum for t he interlayer with homogeneous properties, and essentially the same fo r the situations with a graded interlayer or no interlayer. The depend ence of the shielding and amplification effects on the thickness of th e interlayer, on the distance from the crack-tip to the interlayer, an d on the remote loading are systematically examined. (C) 1997 Elsevier Science Ltd.