The effect of temperature and moisture content on the fracture behaviour of spruce and birch

The fracture behaviour of wood is affected by, amongst other factors, its microscopic structure, moisture content and temperature, as well as the direction of crack propagation. Spruce (Picea abies [L.] Karst.) and birch (Betula pendula Roth.) compact tension specimens were subjected to pure mode I loading in both the radial-tangential (RT) and tangential-radial (TR) orientations under four climatic conditions. The effect of microscopic structure on crack propagation was observed, and the "crack mouth opening displacement vs. load" histories were recorded, from which characteristic fracture parameters were calculated. The fracture surfaces were subsequently examined by optical microscopy. In both species, distinct changes in the fracture behaviour were observed as moisture content (MC) and temperature (T) were varied. The results were specific for the wood species, but MC and T influenced fracture behaviour in a similar way. The effects were more pronounced in birch than in spruce. Elevated T and MC did not affect the failure mode, except in the case of spruce in the RT orientation.