Orthotropy of wood mechanosorption in Norway spruce

Mechanosorption refers to the increase in creep deformation under load and simultaneous moisture changes. It is the dominant deformation mechanism in wood, yet the least understood. Due to difficulties in isolating it from the total deformation, there is a lack of consistent experimental data regarding anisotropy and species. This led to oversimplified rheological models of wood. To overcome this, the present study assesses a full set of orthotropic mechanosorptive parameters for a single Norway spruce stem. To account for nonuniform stress and moisture fields, a coupled experimental-numerical approach is implemented. It considers moisture-dependent elasticity, plasticity, viscoelasticity, and hygroexpansion, obtained in a previous work for the same tree. The present study identifies the most relevant orthotropic differences, including compression-tension and shear asymmetries. Similarities in directional behavior are clustered into non-trivial scaling groups with respect to elastic and viscoelastic behavior. The findings of this study are integrated into a popular rheological model for Norway spruce, implemented as a publicly available Abaqus User Material. The obtained data have a degree of completeness to serve as a reference dataset for future rheological models of wood.