Mechanical and time-dependent behavior of wood-plastic composites subjected to bending

The most popular use of wood-plastic composite (WPC) members in the United States has been as outdoor decking material in residential construction. If the use of these products expands into more structural applications, such as beams and joists, it is imperative that the material's mechanical behavior be understood. Since most of the potential structural uses of this material are as flexural members, it is particularly important that the response to this mode of loading is well characterized. Like many filled polymers, WPCs are anisotropic and bimodal, and thus their shear and two axial moduli (tension and compression) must be determined separately. This study determined the shear and axial moduli of six WPC formulations (mainly polypropylene, high-density polyethylene, and low-density polyethylene) by testing prismatic members in bending at multiple span-to-depth ratios. The initial moduli were determined from constant strain rate tests, and their time dependencies were found using creep tests. The resulting axial-to-shear moduli ratios were shown to be greater than 25 for all formulations. The ratios were relatively constant over time at low stress levels, while decreasing over time at high stress levels.