Issue 35

S. El Kabir et alii, Frattura ed Integrità Strutturale, 35 (2016) 64-73; DOI: 10.3221/IGF-ESIS.35.08 67 C 1 and C 2 are the reduced elastic compliances and K 1 and K 2 and are the stress intensity factors in opening mode and shear mode, respectively. In order to decouple each fracture mode and obtain the reel stress intensity factors u I K and u II K , we perform two computations of particular values of virtual stress intensity factors v I K and v II K [1] as follows     1 2 1, 0 0, 1 8 and 8 v v v v I II I II u u I II M K K M K K K K C C         (6) Finally, we can compute the energy release rate for each mode by combining expressions (4) and (5) according to real intensity factors:     2 2 1 2 8 8 u u I II I II K K G G G C C       (7) MMCG SPECIMEN Design and geometry s described in introduction, the Mixed Mode Crack Growth (MMCG) specimen is a combination between modified DCB and CTS [1]. In this part, we recall the wood specimen dimensions of MMCG device. Fig. 3 presents the dimensions in millimeters of the initial wood specimen proposed in [1]. In this wood specimen, four holes are machined in order to fixe the Arcan device. This allows a various mixed ratio with seven loading fixations with angle θ = (0° ; 15° ; 30° ; 45° ; 60° ; 75° ; 90°). The geometry of the MMCG specimen has been optimized by using a finite element computation. The main goal of this specimen is to obtain a stable crack growth rate during propagation. Figure 3 : MMCG specimen. The finite element computation is realized in plane stress state for an elastic orthotropic behavior. Wood material used is Douglas fire and has the following elastic characteristics: longitudinal modulus Ex =14100 MPa , transversal modulus Ey=2040 MPa , shear modulus Gxy=925 MPa , Poisson ratio NUxy=0.4 . Crack growth stability MMCG specimen was designed in order to obtain the crack growth stability. The decrease of the energy release rate translates stability of the crack growth. In mixed mode loading and according to the Griffith approach, Moutou Pitti et al. have introduced f threshold function [1] such as: 1 2 1 2 s s G G f k G G   (8) A