Elastic-plastic fracture mechanics of strength-mismatched interface cracks

Recent progress in rationalizing and predicting the diverse ductile and brittle cracking behaviors seen in a given structural alloy under monotonic loading (e.g., shallow vs, deep cracks; tension vs. bending; ductile void growth vs, cleavage: etc.) has rested critically on improved characterizations of elastic-plastic crack-front stress and deformation fields, For a parameter such as J or CTOD which scales the intensity of crack-tip deformation, as well as a parameter such as the T-stress or the Q parameter which accounts for the triaxiality of the crack-tip stress fields. When a crack lies along or near a planar material interface across which there is a significant gradient in plastic deformation resistance, the local stress and deformation fields exhibit features which differ both in magnitude and in kind from the 'homogeneous' fields noted above, Such conditions are encountered in strength-mismatched weldment. in which the flow strength of the weld metal differes from that of the baseplate. Major features of-such strength-mismatched welds are that (i) for a given macroscopic measure of crack-tip deformation, plastic strain preferentially concentrates in the softer material, and (ii) stress triaxiality in the softer material exceeds that attainable in even the most severe of the homogeneous crack-tip fields noted above. We review basic features of strength-mismatched interface crack fields for a range of strength mismatches and strain hardening behaviors. The crack-tip can be well-described by families of slip-line fields which depend on the level of mismatch. ill, as well as on overall deformation (J) and triaxial constraint (Q, T). Implications of these findings for the toughness of strength-mismatched interface cracks are explored.