Modeling the hygrothermal creep behavior of wood plastic composite (WPC) lumber made from thermally modified wood

The viscoelastic behavior of an extruded wood plastic composite (WPC) made from thermally modified wood under hygrothermal treatment was studied and modeled. Multiple three-point bending creep/recovery tests were carried out using a dynamic mechanical thermal analyzer (DMTA) equipped with a submersible clamp. WPC specimens with a 15-mm span were subjected to two initial applied stresses; 9% and 14% of the flexural strength in 30 min of creep and 30 min of creep recovery under the combined effects of temperature (25°C, 35°C, and 45°C) and water immersion (saltwater (SW) and distilled water). A dry condition WPC control was used to compare the hygrothermal effects with respect to the control conditions. The WPC material in this article exhibited a linear viscoelastic behavior under the effect of temperature, whereas a nonlinear viscoelastic behavior was observed under immersion conditions. A power law model is considered a useful model to describe the creep behavior of WPC specimens with a 15-mm span in the control and the SW conditions and at 45°C. A power law model was used to describe 180-day creep deflection of WPC lumber beams with an 853-mm span subjected to 12 MPa of the flexural strength in four-point bending at 50% relative humidity and at 21°C. The power law model predicts that the WPC lumber will reach a flexural strain in outer fiber of 1% in approximately 150 years.