Issue 35

T. Haiyan, Frattura ed Integrità Strutturale, 35 (2016) 472-480; DOI: 10.3221/IGF-ESIS.35.53 473 wood, research on crack of bamboo is fewer. Only Amada et al. [8] has studied transverse bending crack of bamboo and pointed that could be used as structural material because of its well-matched toughness and strength of cross grain crack. To select out bamboo and wood suitable for garden landscaping and explore damage crack behavior and toughening mechanism of biomaterials comprehensively, this study performed theoretical derivation in combination with experiment to study damage crack mode of different kinds of woods and bamboos. L INEAR ELASTIC FRACTURE MECHANICAL CHARACTERISTICS OF WOOD rack and defect are inevitable in engineering materials. They produce either in production process, processing process or using process. For example, fatigue crack, compressive injury, ring shake and radial shake will generate under alternating force [9]. To conveniently study strength of cracked body, we classify cracks into three categories, opening mode (mode I) produced under external normal stress, sliding mode (mode II) produced under shear stress parallel to crack direction and tearing mode (mode III) produced under stress that can stagger crack surface, according to stress and characteristics of crack (Fig.1). Mode I Mode II Mode III Figure 1 : Illustrations for mode I, mode II and mode III crack. Particularity of application of linear elastic fracture mechanics in wood Wood is an anisotropic and heterogeneous material. Stress-strain curve of wood in different loading form is linear, consistent with linear elastic behavior [10]. Its three elastic symmetry planes are vertical to length wise direction (L), radial direction (R) and tangential direction (T). Thus, if we use the first symbol to express the normal direction and the second symbol as expansion direction of crack, then there are six kinds of crack growth forms, i.e., TL, RL, LT, LR, TR and RT, as shown in Tab. 1. Orthotropic materials usually have more complicated crack than isotropic material [11]. We derive equations for stress and displacement field of crack tip of orthogonal anisotropic material using complex variables functions [12]. 1 2 3 1 2 3 Re ( , , , , , ) 2 2 Re ( , , , , , ) ij ij ij ij ij ij K f u u u r K r v f u u u G                       (1) where a ij is elastic constant of material; u l , u 2 and u 3 are compound parameters of materials which are determined by degree of anisotropy and angle α between crack and long grain fiber (Fig. 2) and Re is real part of complex function f ij . C