The initiation of the interface crack propagation is analyzed by the finite element method in which a cohesive model is embedded along the line extending ahead of the crack-tip. The constitutive equation of the cohesive model formulated by Ma and Kishimoto is employed. The critical stress intensity factors at the crack initiation are evaluated for a wide range of bimaterial constant and the fracture boundary curves are obtained. It is shown that if the characteristic length existing in the definition of the interface stress intensity factors is chosen suitably the fracture boundary curves converges to one unique boundary curve, which is considered to be the intrinsic fracture boundary curve. The effect of plastic deformation is also examined. The results show that the fracture boundary curves are influenced by the plastic deformation especially when the shear stress is dominant.
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