FRACTURE NORMAL TO A BIMATERIAL INTERFACE - EFFECTS OF PLASTICITY ON CRACK-TIP SHIELDING AND AMPLIFICATION

The problem of a crack approaching a bimaterial interface is considered in this paper. Attention is focused on an interface between two elastoplastic solids whose elastic properties are identical and whose plastic properties are different. For the case of a crack approaching a bimaterial interface perpendicularly, it is shown by recourse to detailed finite element analyses that the near-tip ''driving force'' for fracture is strongly influenced by whether the crack approaches the interfaces from the lower strength or the higher strength materials. Specifically, it is demonstrated that the crack-tip is ''shielded'' from the remote loads when it approaches the interface from the weaker material, and that the effective J-integral at the crack tip is greater than the remote J when it approaches the interface from the stronger material. This plasticity effect determines whether a crack approaching the bimaterial interface continues to advance through the interface or is arrested before penetrating the interface. These theoretical findings are substantiated using controlled experiments of fatigue crack growth perpendicular to a ferrite-austenite bimaterial interface. The effect of the non-singular T-stress, acting parallel to the crack plane, on shielding and amplification of the stress fields is also discussed.