Axial compression strength of Norway spruce related to structural variability and lignin content

Variability in the axial tensile strength and Young's modulus of wood is mostly due to changes in the main orientation of the cellulose microfibrils with respect to the cell axis. By contrast, the causes of variability in the axial compression strength of wood are less well understood. Therefore, the axial compression strength and density as well as microfibril angle and lignin content of Norway spruce specimens were examined. 84% of the variability of compression strength could be explained by density. After normalisation of compression strength for density, the experimental results showed that variability in the microfibril angle in the secondary cell wall is not responsible for variability in the axial compression strength of the cell wall. This finding is supported by theoretical considerations using a composite failure criterion. Deviations of the microfibrils from a strictly axial alignment in the vicinity of rays are most probably the cause for the initiation of compression failure in Norway spruce. The lignin content of the secondary cell wall showed a positive relationship at low statistical significance with the compression strength of the cell wall. A positive effect of increasing lignin content on compression strength seems therefore possible, but very weak. (C) 2002 Elsevier Science Ltd. All rights reserved.