Fracture characteristics of wood under mode I, mode II, and mode III loading

Wood is a highly optimized cellular orthotropic material. Its structural properties have major influence on the fracture mechanics behaviour of the material. The aim of the presented study is the evaluation and analysis of differences in the fracture process of wood subjected to fundamental fracture modes. The results of fracture mechanics experiments for mode I, mode II and mode III in two crack propagation systems were evaluated. The fracture process is discussed by means of the load-displacement diagrams recorded under stable crack propagation and the calculation of the specific fracture energy. It was found that the typical specific fracture energy is much higher for the mode II and mode III cases than for mode I loading. The differences for wood are explained by different energy-dissipating processes such as the development of larger damage zones for mode II and mode III loading than for mode I loading. The influence of the chosen orientation and its relation to structural parameters of the material are also discussed. The findings are supported by microscopic images of typical fracture surfaces.