Mechanical and time-dependent behavior of wood–plastic composites subjected to tension and compression

Abstract

The thermoplastics within wood–plastic composites (WPCs) are known to experience significant time-dependent deformation or creep. In some formulations, creep deformation can be twice as much as the initial quasi-static strain in as little as 4 days. While extensive work has been done on the creep behavior of pure polymers, little information is available on the mechanical effects of mixing polymers with large amounts of wood-based or other bio-based fillers. As producers seek to develop structural WPC products that may be subjected to sustained loads, it is imperative that this creep behavior be understood. We characterized the quasi-static and time-dependent deformations of seven WPC formulations (primarily polypropylene, and polyethylene) in tension and compression. The quasi-static, mode-dependent response of the material to a linearly increasing strain was found to be well described by an exponential function coupled with a linear term. For most formulations, significant differences between the tension and the compression behaviors were not exhibited below 50% of the tensile capacity. The long-term creep response of the material was found to conform well to a time-dependent power-law (Findley, Shapery, etc.) at various stress levels for both loading modes.