The application of non-linear fracture mechanics to wood is a relatively new topic in the area of wood science, however, linear elastic fracture mechanics (LEFM) was first applied to wood in the nineteen sixties. The fracture propagation in wood is mainly governed by two aspects, namely the direction of the principal stresses, and the microstructure. There are six principal crack propagation systems in wood, which are illustrated in Fig 2, see eg. Smith
] or Reiterer and Tschegg [
]. The usual presumption is that wood is perfectly brittle-elastic (LEFM). When a crack propagates perpendicular to grain, the observed fracture behaviour is quasi-brittle, i.e. a tensile softening branch exists [
]. This indicates that a fracture process zone of some length exists. The LEFM approach is not an adequate approximation if the specimen size is of the same order of magnitude as the length of fracture process zone.