FATIGUE PROPERTIES OF WOOD IN TENSION, COMPRESSION AND SHEAR

The fatigue properties of wood laminates have been investigated in tension, compression and shear. Fatigue lives in compression are significantly less than in tension, and S-N data at five R ratios has yielded a set of constant-life lines, the form of which is related to the failure mode of the wood observed by scanning electron microscopy. A point of inflection in the constant-life lines at the transition between all compressive and partially tensile fatigue loading is a new observation. S-N curves for wood laminates have been produced for shear across the radial-longitudinal and tangential-longitudinal planes, and the latter plane is observed to be more resistant to fatigue loads. Samples with four times the cross-sectional area of standard-sized samples have been fatigued at R = -1 and no significant difference in fatigue life is apparent. It appears that the absence of a size effect in tension-compression results from the orthotropic structure of wood which is insensitive to variations in the density of surface flaws.