Characterization of wood-plastic composites by dissipated energy

Matthew M Zawlocki

Dissipated energy is a method of characterizing non-linear materials based on the difference between the total energy and the recoverable energy in a material system. Wood-plastic composites with high wood-fiber content are an example of a material that exhibits a non-linear load-displacement behavior. High-density polyethylene, highdensity polyethylene with an ethylene-maleic anhydride polymer additive, and poly-vinyl chloride are three wood-plastic formulations chosen to be characterized using dissipated energy. Tension and biaxial (axial and shear) tests were used to create dissipated energy density functions for each of the three formulations. The dissipated energy density functions obtained were then used with beam bending theory to predict the total energy dissipated by test coupons in 3-point bending. The nonlinear behavior of wood-plastics means that there is no distinct point that marks the onset of material damage as in metals. As an alternative means to assign design strengths, not based on a distinct yield point, the maximum curvature in the dissipated energy function was used to define an allowable design strength. The maximum curvature in the dissipated energy function provides allowable design strengths in a range where damage accumulation rates are kept relatively low. Because the strength of wood is traditionally based on a statistical analysis of the 5% parametric tolerance limit (PTL), the 5% PTL for the three wood-plastic formulations was also calculated. The 5% PTL stresses and strains were higher than the stresses and strains at maximum DE curvature. The 5% PTL strengths provided no consistent rate of damage accumulation. A small change in 5% PTL strengths has a large effect on the rate of damage accumulation. Failure criteria (maximum stress, maximum strain, maximum shear stress, TsaiWu and Yeh-Stratton) were evaluated to determine their accuracy in predicting failure of the three wood-plastic composites. An envelope for each wood-plastic formulation based on 1) the maximum curvature of the dissipated energy function, 2) the 5% PTL and 3) the ultimate strength was created for each criterion. Biaxial test results on HDPE and HDPE with MAPE materials revealed shear failures which were accurately predicted by the maximum shear stress criterion.

Characterization of wood-plastic composites by dissipated energy

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