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 material with a homogeneous or a compositionally graded interface. The effects of plasticity mismatch between the layers (which have no elastic mismatch) on the shielding or amplification of the crack tip driving force 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 crack-tip J integral is smaller than the remotely imposed J integral; if the crack is situated in the plastically weaker material. For this situation, introducing an interlayer between two dissimilar solids provides greater crack-tip shielding than joining the two solids without an interlayer (i.e., with a sharp interface). An interlayer with a homogeneous yield;behavior (i.e., where the yield strength of the interlayer is the average of that of the two constituent layers) provides a greater shielding effect than a graded interface within which the yield strength varies linearly from one end of the interlayer to the other. When the crack approaches the interlayer from the plastically stronger material, the crack tip driving force is amplified as the plastic zone spreads across the interlayer. This amplification is the maximum for the interlayer with homogeneous properties, and essentially the same for the situations with a graded interlayer or no interlayer. The dependence of the shielding and amplification effects on the thickness of the interlayer, on the distance from the crack-tip to the interlayer, and on the remote loading are systematically examined. (C) 1997 Elsevier Science Ltd.