Variability of fracture toughness by the crack tip position in an annual ring of coniferous wood

Abstract

The effects of the location of the crack tip in an annual ring and the direction of crack propagation on the fracture toughness of the TR crack propagation system of coniferous wood (T, direction normal to the notch plane; R, propagation direction) were analyzed by the finite element method in regard of the changes in elastic modulus and strength within an annual ring. The critical point of the fracture was defined as the state where the stress of a square element (0.125 × 0.125 mm) in contact with the crack tip equals the tensile strength. The distribution of specific gravity was measured by soft X-ray densitometry. The elastic moduli in the T and R directions were estimated by the sound velocity. The tensile strengths in the T and R directions were measured by the tensile test using small specimens of l mm span length. Regarding the variability of fracture toughness (K _IC), the experimental and calculated results had the same tendency. Therefore, it was concluded that the variability ofK _IC is caused by the (1) heterogeneity of the elastic modulus and strength within an annual ring; and (2) changes in the degree of stress concentration at the crack tips, according to the direction of crack propagation.