Issue 35

T. Haiyan, Frattura ed Integrità Strutturale, 35 (2016) 472-480; DOI: 10.3221/IGF-ESIS.35.53 474 R T L R T L Table 1 : Classification extension of wood basic crack. Figure 2 : Illustration for direction of crack and fiber. It can be found from the above equations that, crack of orthogonal anisotropic material is much more complicated than isotropic material. The differences are as follows. First, crack of orthogonal anisotropic material expands in the direction of fiber rather than the initial direction. But linear elastic fracture mechanics presupposes that, crack always expands in the initial direction [13]. Secondly, even if under a simple-form loading, crack tip of orthogonal anisotropic material can also displace in a compound way, which is different from linear elastic fracture mechanics principle based on distinction means of mode I, II and III. Stress field of crack tip of orthogonal anisotropic material is a function with regard to composite parameters of materials, and meanwhile these parameters are a function with regard to material property and angle α between crack and fiber direction. It is also different from linear elastic fracture mechanics principle that develops from the statement that stress distribution of crack tip is in no correlation to property and direction of material texture. Based on the above facts, linear elastic fracture mechanics theory is not applicable to orthogonal anisotropic material. That is because we cannot define crack toughness of orthogonal anisotropic material with three material constants K IC ,