Mechanical behaviour of sawn timber of silver birch under compression loading

Silver birch is a common European hardwood species possessing stiff and strong material properties. The mechanical properties of birch timber under compression loads are, however, not well defined as the influence of growth characteristics have largely been disregarded in experimental investigations. To address this, the present study investigated the effect of growth characteristics on the mechanical behaviour of birch sawn timber under compression loading, parallel and perpendicular to the grain direction. The results indicate knot size as the dominant parameter for both loading directions, whereas in specimen without knots, density, distance to pith and growth-ring width are dominant for parallel to grain loading and density and growth-ring orientation are dominant under perpendicular to grain loading. On selected specimens, local deformation and failure behaviour is further investigated by means of digital image correlation technique using a novel contrast pattern. Strain fields on specimens loaded parallel to grain reveal localized strain peaks at the cracks within knots and around the topside of knots where abrupt deviations in the grain orientation occur. Specimens loaded perpendicular to the grain display localized strain peaks near the pith and at growth-ring tangential angles around 45∘.