Comparison of critical strain energy release rate and fracture energy for timber mode II failure

This study compares two approaches for evaluating mode II fracture properties in Japanese larch: the critical strain energy release rate ( G c ) based on linear elastic fracture mechanics and the fracture energy ( G f ) from non-linear fracture mechanics. While these properties are theoretically equivalent for brittle materials, experimental confirmation of this relationship in mode II failure of wood has been limited. The study employed a modified ASTM D7905 methodology and the compliance-based beam method to measure G c using end-notched flexure specimens. For G f measurements, a single-lap shear test incorporating displacement control and high system stiffness was developed to ensure stable crack propagation. Among 30 specimens tested for G f , 13 exhibited stable failure behavior, yielding an average mode II fracture energy of 1.53 N/mm, which was reduced to 1.42 N/mm after correcting for the dissipated energy in the bulk of the specimen. The G c measurements from 24 specimens (13 stable fracture specimens) using CBBM averaged 1.37 N/mm. Although the ASTM D7905-based method yielded 2.06 N/mm, this result appeared unreliable due to the limited available crack propagation length and was, therefore, excluded from the comparison. Statistical analysis using t tests and Kolmogorov–Smirnov tests showed no significant difference between G c and both original and corrected G f values ( p > 0.05). The difference between G c and G f decreased from 11.5% to only 3.7% after correction, confirming the theoretical equivalence when fracture process zone effects and energy dissipation mechanisms are properly accounted for. These findings contribute to the fundamental understanding of wood fracture mechanics and have practical implications for enabling flexible experimental design choices.