Fracturing of Wood Under Torsional Loading: Fracture Mechanisms and Mechanics

Damage of wood under mechanical loading is mostly induced by some kind of mixed mode loading, like for example, superposition of a crack opening mode (mode I) plus torsional load (mode III). In addition, the high anisotropy of wood itself, with directions and paths for easy crack propagation in the neighbourhood of areas with high crack resistance, leads to a mixed-mode loading condition. In order to characterize superimposed loading, systematic studies have been performed of loading under pure mode I, and it was tried, to also realise pure mode II or mode III testing conditions. It could be shown, however, that sliding modes cannot be realized in practice without any mode I component being present, as fracture surface roughness, causes crack tip opening. Therefore, Ehart et al. [

1

] and Frühmann et al. [

2

] have performed studies on this fracture surface interaction and resulting mixed mode fracture, as well as on damage and fracture mechanisms during nominal mode III loading of wood and wood composites. In addition, a testing technique has been developed [

3

] to perform mixed mode (mode I plus II) fracture experiments, with different defined mode II and, respectively, mode I components.