Effect of Strain Rate on Flexural Properties of Wood Plastic Composite Sheet Pile

Wood plastic composite (WPC) materials are being developed for load-bearing structural applications; therefore, the strain rate-dependent mechanical properties of WPC materials need to be characterized. Extruded WPC Z-section sheet piles composed of 46 percent wood flour, 41 percent polypropylene, and additives were investigated. ASTM D6109 was adopted for assessing flexural properties of plastic lumber in a four-point loading configuration. Coupons were cut from the flanges of the sheet pile section and conditioned for 2 weeks at 21 degrees C and 65 percent relative humidity. The flexural tests were conducted at three different strain rates: 0.55, 1.0, and 5.5 percent per minute. The mode of failure was in tension in the middle third of the bending span. It was found that the mean apparent modulus of elasticity (MOE) increased with the strain rate; e.g., the MOE increased 9.5 percent when the strain rate was increased from 1.0 to 5.5 percent per minute. The variations in mean strain at failure with the strain rate were not statistically significant based on analysis of variance testing. The variation in flexural MOE with the strain rate was compared with the published tensile and compressive MOE values for the same material. The effect of strain rate on the flexural MOE of the polypropylene WPC material was also correlated with the published results for WPC materials with other polymer matrices (high-density polyethylene and polyvinyl chloride). The strain rate effects in the MOE of the WPC material was predicted based on a viscoelastic standard solid model calibrated with the coupon level data.