Influence of repetitive stiffness variation on crack growth behaviour in wood

Softwoods have a repetitive variation in stiffness over their growth rings, which is due to the difference in cellular structure between the latewood and earlywood. In this paper, the influence of the repetitive stiffness var iation on radially growing cracks is studied by detailed finite element ana lyses, in which the wood material is represented by a layered orthotropic c ontinuum. The distribution of stress around the crack is found to be very d ifferent from crack tip stress fields in homogenous isotropic materials. Th e latewood layer ahead of the crack experiences a significant tensile stres s, which indicates that formation of new secondary cracks ahead of the prim ary crack front is a likely mechanism for crack propagation. This mechanism is also favoured by the fact that the primary crack is subjected to a sign ificant shielding from the stiff latewood, which tends to arrest the primar y crack in the soft earlywood layer. Analyses are performed for materials w ith various growth ring widths, and the calculated results are compared wit h reported experimental observations. (C) 2000 Kluwer Academic Publishers.