Effect of polar anisotropy of wood loaded perpendicular to grain

This paper reports on an analysis and experimental verification of the effect of cylindrical anisotropy of softwood loaded in the radial r and tangential t growth plane, i.e., perpendicular to grain. This off-axis loading mode is especially design-relevant for curved and tapered glulam beams. The very low shear stiffness in the r.t on-axis growth plane, together with a complex shear coupling effect in polar coordinates, contributes essentially to extremely variable off-axis strain and stress fields. Thus far, no explicit empiric proof exists for these effects with considerable consequences on damage-relevant stresses. The exemplary off-axis investigations were performed with a cross-sectional slab of a spruce board loaded in tension parallel to the longer edges. True on-axis stiffnesses were determined for the simulations from two on-axis specimens cut from the test volume. Throughout a good agreement between the finite-element predictions and the measured results was obtained. The experiments, for example, proved that off-axis strains parallel and normal to load axes may vary along specimen length by factors of about 4.5 and 9, respectively.