Issue 29

R. Dimitri et alii, Frattura ed Integrità Strutturale, 29 (2014) 266-283; DOI: 10.3221/IGF-ESIS.29.23 269 for various applications in the literature for debonding problems. The cohesive law proposed in [10], herein indicated as CZM3, is not derived from a potential and is characterized by an effective opening displacement  as coupling parameter defined as 2 2 1 N T Nu Tu g g g g                 (7) g Nu , and g Tu being the critical separations in mode I and II, after which the stresses are equal to zero, i.e. the surfaces are not locally held together. This cohesive law is defined by six characteristic parameters of the interface, i.e. the fracture energies,  N and  T , the cohesive strengths p Nmax and p Tmax , and their corresponding displacements g Nmax and g Tmax . The analytical laws in both directions can be expressed as follows   max 1 0 1 1 1 N P P Nu N N N P P Nu g for g p g p for g                     (8)   max 1 0 1 1 1 T P P Tu T T T P P Tu g for g p g p for g                     (9) where  P is the effective opening displacement at which the softening behavior of the interface begins, and is given by 2 2 2 2 max max N N T T P Nu Tu N T g g g g g g g g                             (10) The fracture energies for pure modes I and II are computed as max 0 0 1 ( ) 2 Nu g N N N N N Nu gT p g dg p g      (11) max 0 0 1 ( ) 2 Tu g T T T T T Tu gN p g dg p g      (12) whereas the uncoupled stresses in pure modes are depicted in Fig. 2. (a) (b) Figure 2 : CZMs 3 and 4: (a) Pure mode I; (b) Pure mode II. . The last analyzed model is the bilinear cohesive law proposed by Camanho et al. [11], denoted as CZM4. This model is