Rate-independent mechanical behavior of biaxially stressed wood: Experimental observations and constitutive modeling as an orthotropic two-surface elasto-plastic material

Recent biaxial experiments on spruce wood show that consideration of an elliptic failure surface according to Tsai and Wu and an elastic model for stress states within this envelope lead to an insufficient description of the mechanical behavior. As compression perpendicular to the grain occurs, a non-linear stress path results from a proportional biaxial strain path. Investigation of characteristic samples with respect to loading-unloading-reloading cycles for states of stress below failure reveals behavior similar to what is known as hardening type plasticity. The experimentally observed mechanical behavior is described by means of a two-surface plasticity model addressing both failure and non-linear stress response below failure as separate mechanisms. Prediction of failure is achieved by means of a second-order failure envelope according to Tsai and Wu. The non-linear stress response has to be covered by a novel orthotropic hardening type plasticity model. Since available experimental data covers only plane stress in the LR-plane, both orthotropic failure and yield surfaces, respectively, are restricted to this case.