AN INTERFACE CRACK UNDER MIXED-MODE LOADING

An elastic-plastic finite-element calculation based on finite-strain theory is carried out to estimate the fracture toughness of a ductile bimaterial with an interface crack under mixed-mode loading. A semi-infinite crack lying on an interface between an elastic-plastic material and a rigid substratum is considered, and the small-scale yielding condition and the plane-strain condition are assumed. The crack is modelled as a sharp notch. The modified Gurson constitutive equation for porous plastic materials is used to take into account the nucleation and growth of microvoids near a crack tip, and the element-vanish technique introduced by Tvergaard et al is employed to simulate the initiation of ductile fracture. It is found from the computational results that ductile fracture in a bimaterial initiates always at the crossing point of the blunted crack tip and the interface for any mixed-mode ratio, while for a crack in a homogeneous material ductile fracture initiates at a different point on the blunted crack tip depending on the mixed-mode ratio. It is also found that the values of the fracture toughness (i.e., the critical values of the J-integral) of a bimaterial are not only much lower than those of a homogeneous material, but also depend strongly on the mixed-mode ratio and take the minimum value at a certain critical ratio.