Morphology of crack surfaces: A way to understand the fracture behaviour in quasi-brittle materials

Wood can be considered as a quasi-brittle material. During the damage process preceding the fracture. the microcracks which occur ahead of the crack front have an important influence on the local deviations of the macrocrack and hence on the morphology of the fracture surfaces. It is naturally expected to correlate the fracture surface morphology to mechanical properties such as fracture energy or toughness. From a scaling law describing the roughening of fracture surfaces, we discuss implications of the crack morphology for fracture mechanics. On the basis of the Griffith criterion, we show that the roughening of fracture surfaces leads to a material dependent R-curve behaviour. For the post R-curve regime, corresponding to the saturation state of the roughness, the value of the resistance to crack growth G(R) does not follows the R-curve but remains constant and corresponds to the critical resistance G(RC) The link between morphology of fracture surface and fracture properties also provides a size effect on this critical resistance GRC Predicted results are confirmed by comparison with fracture experiments made on two wood species (Norway spruce and Maritime pine).