Computational Analysis of Quality Reduction during Drying of Lumber due to Irrecoverable Deformation. I: Orthotropic Viscoelastic-Mechanosorptive-Plastic Material Model for the Transverse Plane of Wood

This paper presents the development of an orthotropic material model for the mechanical analysis of wood in the plane perpendicular to the growth direction. It is based on an earlier uniaxial development and experimental verification. The novel features are the biaxial extension and the description of partially irrecoverable creep deformation by enhancing a viscoelasticmechanosorptive creep model by coupling it with orthotropic plasticity. The mathematical description of both the equations of state and the evolution laws are formulated on a thermodynamical basis. A semianalytical solution algorithm is derived for the obtained nonlinear system of differential equations. The model is applicable over a wide range of temperature as well as moisture content (20-120°C; nearly 0% moisture content to fiber saturation), which is achieved through application of the time-temperature-moisture superposition principle and the introduction of a moisture-change-temperature superposition principle. A set of material parameters suitable for this range of conditions is given for Pinus silvestris.