D. Leguillon er al /Journal of the Mechanics and Physics of Solids 48(2000)2137-2161 2145 (Leguillon et al., 1999)2na=np=a leading together with Eq.(12)to the so-called LS (Leguillon and Sanchez-Palencia, 1992)criterion Kd go Once the above equality assumptions have been made, criteria Eqs. (13)and(14) are independent of any length, otherwise they are not(see He et al., 1994, for HH and Eq(12) for LS). Although they look similar, these two criteria are slightly different. HH assume the penetration and deflection geometries and study the local fields at the tip of the new extensions. It is thus consistent to carry out the analysis at the same distance of the primary crack tip. On the contrary, in the present Ls approach, the question is to determine the energy balance which allows creation of crack extensions In this context, it is consistent to examine equal crack extensions It makes an important difference in case of symmetrical double deflection along the interface. In the HH case the total interface debonding length is 2a whereas it must be a in the ls one An attempt to introduce different crack increments is proposed by Ahn et al. (1998) which is shown to fit experimental data. However their approach is not an asymptotic one, based on a structural computation; it depends on the applied loads, on the geometry of the specimen and on the actual length of the increments, not only on their ratio as above Eq.(12)(i.e. even if the increments are taken as equal, their results depend on the common value a) t A comparison between He and Hutchinson's results and the present criterion LS (14)is shown in Fig. 6 for different values of the first Dundurs parameter a 2.2 Kd/Kp Gd/Gp He Hutchinson 0.6 0,4 -1-08-06-0,40,2 020,40,60,81 dundurs Fig. 6. Comparison between HH Eq (13)and LS Eq(14) criteriaD. Leguillon et al. / Journal of the Mechanics and Physics of Solids 48 (2000) 2137–2161 2145 (Leguillon et al., 1999) 2hd=hp=a leading together with Eq. (12) to the so-called LS (Leguillon and Sanchez-Palencia, 1992) criterion Kd Kp $ G(i) c G(2) c . (14) Once the above equality assumptions have been made, criteria Eqs. (13) and (14) are independent of any length, otherwise they are not (see He et al., 1994, for HH and Eq. (12) for LS). Although they look similar, these two criteria are slightly different. HH assume the penetration and deflection geometries and study the local fields at the tip of the new extensions. It is thus consistent to carry out the analysis at the same distance of the primary crack tip. On the contrary, in the present LS approach, the question is to determine the energy balance which allows creation of crack extensions. In this context, it is consistent to examine equal crack extensions. It makes an important difference in case of symmetrical double deflection along the interface. In the HH case the total interface debonding length is 2a whereas it must be a in the LS one. An attempt to introduce different crack increments is proposed by Ahn et al. (1998) which is shown to fit experimental data. However their approach is not an asymptotic one, based on a structural computation; it depends on the applied loads, on the geometry of the specimen and on the actual length of the increments, not only on their ratio as above Eq. (12) (i.e. even if the increments are taken as equal, their results depend on the common value a). A comparison between He and Hutchinson’s results and the present criterion LS Eq. (14) is shown in Fig. 6 for different values of the first Dundurs parameter a Fig. 6. Comparison between HH Eq. (13) and LS Eq. (14) criteria