Issue 39

M.A. Tashkinov, Frattura ed Integrità Strutturale, 39 (2017) 248-262; DOI: 10.3221/IGF-ESIS.39.23 250   I I E X u Z w 1 2      (1) where I X and I Z are shear and opening forces at the node I , u  and w  are displacements, corresponding to shear and opening at the node L (Fig. 1). Figure 1 : VCCT scheme. The energy release rate can be calculated then as: ΔE G ΔA  (2) where ∆ ܣ is formed crack surface. This hypothesis can be used if the crack profile does not change significantly during its growth, and when the value of the distance, for which the rate of strain energy release is being calculated, is relatively small compared with the overall dimensions of the crack. In the real materials, the crack growth during delamination occurs simultaneously in all three modes of deformation (opening, in-plane shear and out-of-plane shear). In order to take account of this fact, the energy release rate is calculated for each mode ( I II III G G G , ,  ) , then they are summed up: T I II III G G G G    (3) and are compared with the critical value C G . The opening of the two nodes and the crack growth occurs with satisfaction of condition: T c G G 1  (4) The critical value C G depends on all three modes of deformation and is determined by the mixed criterion. The three dimensional Benzeggah and Kenane (B-K) criterion is often used [21]:   II III C Ic IIC IC T G G G G G G G            (5) where values Ic G , IIc G , and exponential parameter  are determined experimentally. Such experiments are in details described in [22, 23]. Besides, guidelines for experiments determining the critical strain energy release rate are presented in the ASTM standards [24].