144 D. Leguillon er al /Journal of the Mechanics and Physics of Solids 48(2000)2137-2167 material 2 interf crackI material 1 material 2 nterface crack material 1 Fig. 4. Penetration or deflection of a crack impinging on an interface. naterial 2 material 2 1 interface material 1 material 1 interface ck crac crack Fig. 5. Stretched inner domain with a unit length penetration or deflection To make possible the comparison between the two crack behaviours He and Hut- chinson add the assumption that the two perturbation lengths(penetration and deflection) are equal. The HH criterion reads Ga ga where Ga and Gp are the deflection and penetration energy release rates. He and Hutchinson calculate these quantities, using integral equations and Muskhelishvili method, at a same distance a from the impinging point, on the deflected and penetrat ing branches (i.e. n=np=a). This assumption is slightly different from our own2144 D. Leguillon et al. / Journal of the Mechanics and Physics of Solids 48 (2000) 2137–2161 Fig. 4. Penetration or deflection of a crack impinging on an interface. Fig. 5. Stretched inner domain with a unit length penetration or deflection. To make possible the comparison between the two crack behaviours He and Hut￾chinson add the assumption that the two perturbation lengths (penetration and deflection) are equal. The HH criterion reads Gd Gp $ G(i) c G(2) c , (13) where Gd and Gp are the deflection and penetration energy release rates. He and Hutchinson calculate these quantities, using integral equations and Muskhelishvili’s method, at a same distance a from the impinging point, on the deflected and penetrat￾ing branches (i.e. hd=hp=a). This assumption is slightly different from our own