THICKNESS AND MOISTURE-CONTENT EFFECT IN THE FRACTURE-TOUGHNESS OF SCOTS PINE

Fracture toughness tests were performed on specially prepared specimens taken from a single tree. These were kiln dried, machined to various thicknesses and then sets of them conditioned to various moisture contents. It was found that about half the specimens were rejected because of warping and splitting for thicknesses greater than 10 mm, but below this the rejection rate decreased. Fracture toughness values showed thickness variations in the kiln dried state, but the toughness decreased and became constant when the moisture content was changed. An explanation is proposed in terms of residual stresses induced by the drying which increases the toughness and constraint stresses which cause a decrease. A characteristic length of the annual ring spacing seems appropriate in defining the various transitions.