Global and local approaches to fracture normal to interfaces

The problem of a crack perpendicularly approaching a bimaterial interface is examined using both global and local approaches to fracture[ The global approach is based on the J!integral with a second parameterQ\ which scales the stress triaxiality ahead of the crack[ The local approach is based on either brittle fracture "Beremin model# or ductile fracture "Rice and Tracey model#[ In the _rst case\ the Weibull stress over the plastic zone is calculated[ In the second case\ the void growth rate is calculated at the tip of the crack over a representative volume "generally associated with a characteristic length of the material#[ After a brief summary of each approach\ the results for a crack near an elastically homogeneous\ plastically mismatched interface are presented[ The behaviour of the bimaterial is expressed in relation to the behavior of the homogeneous material[ It is shown that there is an e}ect on the crack behavior which depends on the direction of crack propagation\ i[e[ from the harder material to the softer material or vice versa[ This e}ect is examined as a function of change in yield strength ratio and hardening exponent\ n[ For the case of brittle fracture\ the e}ect of changing the Weibull modulus\ m\ is also examined[ The models based on the local approach show that both stress! and strain!controlled fracture mechanisms must be accounted for[ This implies the necessity of using the two parameters J and Q in the global approach[ This is due to the fact that the stress strain _elds ahead of the crack tip are a}ected by the nature of the second material