Global and local approaches to fracture normal to interfaces

The problem of a crack perpendicularly approaching a bimaterial interface i s examined using both global and local approaches to fracture. The global a pproach is based on the J-integral with a second parameter, Q, 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 Trace y model). In the first case, the Weibull stress over the plastic zone is ca lculated. In the second case, the void growth rate is calculated at the tip of the crack over a representative volume (generally associated with a cha racteristic length of the material). After a brief summary of each approach , the results for a crack near an elastically homogeneous, plastically mism atched interface are presented. The behaviour of the bimaterial is expresse d in relation to the behavior of the homogeneous material. It is shown that there is an effect 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 effect is examined as a function of change in yield stren gth ratio and hardening exponent, n. For the case of brittle fracture, the effect of changing the Weibull modulus, m, is also examined. The models bas ed on the local approach show that both stress- and strain-controlled fract ure mechanisms must be accounted for. This implies the necessity of using t he two parameters J and Q in the global approach. This is due to the fact t hat the stress-strain fields ahead of the crack tip are affected by the nat ure of the second material. (C) 1998 Elsevier Science Ltd. All rights reser ved.