Anisotropic Plasticity and Failure Prediction in Wood Composites

Two case studies are summarized using anisotropic plasticity and the Weibull weakest link failure model to predict the behavior of wood and wood composites. The linear elastic orthotropic model normally used for wood is shown to be insufficient for the prediction of ultimate strength in wood. Non-linear, non-recoverable behavior in compression in each of the orthogonal material directions is of particular interest in cases where localized crushing governs the load-displacement behavior of wood. The anisotropic plasticity option in ANSYS was used to predict yielding and post-yield behavior for the wood composite used in this study because yield stresses in orthogonal directions were found to be in the ratio of 1:1.2:5. A user-programmable Fortran subroutine was developed for ANSYS to carry out the failure analysis (and to calculate the volume integration of the stresses) at each loadstep in post-processing. The model was verified using two cases: I) the ASTM D143 shear block test, and II) bolted connections. CASE I: A three-dimensional model of the ASTM D143 shear block was constructed. SOLID45 elements were used for the wood block and for the rigid steel surfaces. CONTAC49 elements were used on the interfaces between the steel loading platens and the wood block. Load-displacement results for three loading orientations and two block sizes agreed with experimental findings. CASE II: A three-dimensional model of a single-bolt connection was used to predict loaddisplacement behavior and ultimate strength. A three-dimensional analysis was required because experimental results have shown that non-uniform displacements occur through the thickness of the member due to bolt yielding and wood crushing near the member faces. SOLID45 elements were used for the bolt and wood with a friction contact interface of CONTAC49 elements. Bilinear kinematic hardening was assumed for bolt yielding behavior. Load-displacement curves of the connection compared well with experimental results.