Wood strength is highly anisotropic, due to the inherent structural
hierarchy of the material. In the framework of a combined random-periodic
multiscale poro-micromechanics model, we here translate compositional
information throughout this hierarchy into the resulting anisotropic
strength at the softwood level, based on “universal” elastic properties of
cellulose, hemicelluloses, and lignin, and on the shear strength of the
latter elementary constituent. Therefore, derivation of the elastic energy
in a piece (representative volume element – RVE) of softwood, stemming from
homogeneous macroscopic strains prescribed in terms of displacements at the
boundary of the RVE and from pressure exerted by water filling the
nanoporous space between the hemicelluloses-lignin network within the cell
walls, with respect to the shear stiffness of lignin, yields higher order
strains in the lignin phase, approximating micro-stress peaks leading to
local lignin failure. Relating this (quasi-brittle) failure to overall
softwood failure (or strictly speaking, elastic limit of softwood) results
in a macroscopic microstructure-dependent failure criterion for softwood.
The latter satisfactorily predicts the biaxial strength of spruce at various
loading angles with respect to the grain direction. The model also predicts
the experimentally well-established fact that uniaxial tensile and
compressive strengths, as well as the shear strength of wood, depend
quasi-linearly on the cell water content, but highly nonlinearly on the
lumen porosity.