Evaluation of time dependent behavior of a wood flour/high density polyethylene composite

Short-term flexural creep and stress relaxation tests were conducted on a wood plastic composite containing 30 percent high density polyethylene (HDPE), 67 percent fir wood flour, 2 percent compatibilizer (MAPE), and 1percent lubricant. In creep tests, applied stress levels ranged from 30 to 60 percent of measured flexural strength. The principle of time-stress superposition was applied to form a master curve extending for a maximum of 4 years. The horizontal shift factors conformed to an Arrhenius type equation. Stress relaxation tests were also carried out at strain levels ranging from 30 to 60 percent of the ultimate strain. The principle of time-strain superposition was applied to form a stress relaxation master curve that extended for 67 days. The horizontal shift factors also conformed to an Arrhenius type equation. The resulting master curves were compared with extrapolated creep and stress relaxation models. To determine whether time-stress superposition is valid for the studied composite material, creep shift factors were applied to stress relaxation data and vice versa. In both creep and stress relaxation tests, it was found that the application of superposition was verified. The results indicated that the studied composite material was rheologically simple, and a single horizontal shifting on time axis was adequate to predict the long term performance of the material.