Longitudinal Shear Properties of European Larch Wood Related to Cell-Wall Structure

Using a new method to determine the longitudinal shear modulus (G) and shear strength (τ) of solid wood in a single test, the observed shear properties of normal (NW) and compression wood (CW) of larch samples were related to their microstructure, i.e., density, microfibril angle (MFA), and lignin content. To estimate the effective G of the solid cell wall, a semi-empirical model, which calculates G on the basis of porosity by extrapolation from experimental data, was used. For comparison, the effective G was derived from an analytical model, which considers the cell wall as a unidirectional laminate consisting of fiber and matrix material. The analytical model proved that the effect of increased MFA and higher lignin content on G in CW balance each other to a large degree. The effective G of the cell wall calculated by the analytical unidirectional laminate model was close to the estimate of the effective cell wall G performed by extrapolation from experimental data. Both models and mechanical test results demonstrated that effects of variability in cell-wall ultrastructure on G are minor, compared to effects of porosity and density, respectively. A multivariate regression model combining G and density showed that a good estimate of τ can be achieved using these input data