Predicting the Flexure Response of Wood-Plastic Composites from Uni-Axial and Shear Data Using a Finite-Element Model

Abstract

Wood-plastic composites (WPCs), commonly used in residential decks and railings, exhibit mechanical behavior that is bimodal, anisotropic, and nonlinear viscoelastic. They exhibit different stress-strain responses to tension and compression, both of which are nonlinear. Their mechanical properties vary with respect to extrusion direction, their deformation under sustained load is time-dependent (they experience creep), and the severity of creep is stress-dependent. Because of these complexities, it is beneficial to create a mechanics-based predictive model that will calculate the material’s response in situations that are too difficult or expensive to test experimentally. Such a model would also be valuable in designing and optimizing new structural shapes. Analysis and prediction of WPC members begins with the time-dependent characterization of the material’s axial and shear behaviors. The data must then be combined with a tool that can simulate mode-dependence, anisotropy, and nonlinear axial stress distributions that vary over the length of a member and evolve with time. Time-dependent finite-element (FE) modeling is the most practical way to satisfy all of these requirements. This paper presents an FE material model that was developed to predict the deflection of flexural members subjected to both quasi-static ramp loading and long-term creep. Predictions were made for six different WPC products, encompassing a variety of polymers and cross-sections. These predictions were compared with experimental testing and the model shows some success, particularly in the quasi-static response. Creep predictions were more accurate for solid polyethene-based materials than polypropylene-based hollow box sections.