The effect of partial delignification on the stress-strain relationship in transverse compression

Due to very significant improvements in the mechanical performance of solid wood, densification after partial or full delignification has received renewed attention. While studies predominantly focus on isolated cases, e.g., one wood species densified in one anatomical direction, systematic investigations of the effect of wood structure on the densification process are seldom performed. The present study compares the deformation patterns of one representative softwood (spruce) and hardwood (poplar), respectively, in transverse compression. In terms of variables, the native and partially delignified state, wet and dry conditions, and three different orientations of wood grain are considered. It shows that the structural inhomogeneity of spruce wood, with low-density earlywood and high-density latewood, governs its densification pattern and hinders defect-free densification. Contrarily, diffuse-porous poplar is structurally more homogeneous, which together with more efficient softening by delignification leads to favorable densification behavior. Overall, delignification led to a significant softening of both wood species in transverse compression, which greatly enhanced the softening effect of high wood moisture content.