Constitutive Modelling of Clear Spruce Wood under Biaxial Loading by means of an Orthotropic Single-Surface Model under Consideration of Hardening and Softening Mechanisms

The goal of this contribution is the development of a plane stress orthotropic plasticity material model for clear spruce wood. Such a model has to consider an initially linear elastic domain as well as hardening and softening behaviour at higher states of stress and strain, respectively. Combining the advantage of a smooth single-surface plasticity model with the identification of distinct hardening and softening laws for tension and compression loading in longitudinal and radial direction is the key to the novel mathematical formulation presented in this contribution. By associating characteristic strength values of the single-surface model with the respective characteristic failure modes, one can define a set of nonlinear evolution laws for six parameters of the criterion by TSAI & WU [10]. Such a modification redefines the meaning of the original failure envelope as a yield surface. Depending on the stress state, plastic loading causes hardening or softening. This is represented by changing size and location of the elliptic yield surface in the orthotropic stress space [8]. The verification of the developed model is given for the entire plane stress in the orthotropic stress space by means of back-calculation of the biaxial experiments by EBERHARDSTEINER [3].