Fatigue damage and hysteresis in wood-epoxy laminates

Wood-epoxy laminates were subjected to constant amplitude fatigue tests in tension-tension (R = 0.1), compression-compression (R = 10) and reverse loa ding (R = -1) in order to follow property changes and fatigue damage accumu lation. Hysteresis loops were captured during these tests and the form of s tress versus number of cycles to failure (S-N) curves was established. Reve rsed loading is the most damaging mode of cyclic stress application. In ter ms of static strengths, the wood laminate is weaker in compression than in tension. However at low levels of stress, following many fatigue cycles, th e fatigue life is greater in compression-compression than in tension-tensio n. The shape of captured hysteresis loops is strongly influenced by loading mode. As subcritical damage develops, loop area increases and dynamic modu lus falls. In reversed loading, loop bending and distortion is observed dep ending on whether the damage is tension- or compression-dominated or both. Maximum and minimum fatigue strains, the dynamic modulus and loop area have been plotted as a function of the number of fatigue cycles. The majority o f damage occurs towards the end of the sample life but property changes can be detected throughout fatigue tests. Normalisation of fatigue data demons trates that the fatigue behaviour of wood-epoxy laminates is consistent. (C ) 2001 Kluwer Academic Publishers.