The fracture behaviour of wood and the relations between toughness and morphology

Wood is a complex composite material that possesses an excellent combination of mechanical properties. In particular the toughness, or work of fracture, is very high, reaching a value in excess of 1.0 $\times $ 10$^{4}$ J/m$^{2}$, which, mass for mass, is roughly equal to the toughness of steel and higher than that of artificial composites. The high work of fracture of wood is shown to be due to the arrangement of the cellulose microfibrils in the secondary cell wall (S$_{2}$). The helically wound pattern of these fibrillae is such as to induce a novel form of buckling failure in tension, which produces an elastic behaviour analogous to the yield point of ductile metals. When glass-fibre composites were constructed with a similar morphology, a large increase in work of fracture was observed.