Development of Design Values for Wood-Plastic Composite Materials

As the use of wood-plastic composite (WPC) materials extends to include more structural applications, there is an increasing need to determine design values appropriate for designing structural WPC elements. Depending on specific formulations, WPCs can have significantly different stress-strain relationships. Common, however, to WPCs is a highly nonlinear relationship. Due to both the variability and nonlinearity of the responses, a consistent method for deriving design values for WPCs is needed. Variability exists not only in the material response but, to a greater extent, in the mix designs themselves. This creates added complexity in predicting a design value for all WPCs based upon the ultimate strength of a particular mix. A method for deriving WPC design values is developed herein by examining the energy dissipation ability of the material rather than using traditional ultimate strength or deflection criteria. The design values are based upon an equivalent yield point in the material response, up to which a linear-elastic response would represent the actual-material response within some reasonable level of accuracy. By comparing the energy dissipated as assumed by this approximate material model to the actual energy dissipated by the material (i.e., utilizing the actual stress-strain relation), it is found that the approximate energy-based model provides an accurate representation of the material behavior and can be used consistently to derive design values for WPC materials.