Crack propagation in various woods has been examined by scanning electron microscopy, and the observations related to measurements of fracture toughness. It is found that the toughness is related in a simple way to the density of the wood, which is explained by a straightforward model. The apparent fracture toughness of wood for cracks that lie normal to the grain is larger, by a factor of about 10, than that for cracks which propagate parallel to the grain. This difference can be explained in terms of the fracture mechanics of very anisotropic solids.
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Ashby et al. (1985) studied this question.
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