ABSTRACT

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${}^{\circ }$.

摘要

白桦树是欧洲常见的硬木树种，具有坚硬和坚固的材料特性。然而，桦木木材在压缩载荷下的机械性能并未得到很好的定义，因为生长特性的影响在实验研究中已被大大忽略。为了解决这个问题，本研究调查了在平行和垂直于纹理方向的压缩载荷下，生长特性对桦木锯材力学性能的影响。结果表明，在两个加载方向上，结的大小是主要的参数，而在没有结的样本中，密度，到髓的距离和生长环的宽度对于平行于晶粒的加载是主要的，密度和生长环的取向在垂直于晶粒的方向上是重要的加载中。在选定的标本上，通过使用新型对比模式的数字图像相关技术，进一步研究了局部变形和破坏行为。平行于晶粒加载的试样上的应变场显示出在结内和结顶上方附近裂纹处的局部应变峰值，在此出现晶粒取向的突然偏差。垂直于晶粒加载的试样在髓附近和在生长环切线角约45度处显示局部应变峰${}^{\circ }$。