Biofiber-plastic composites are known to exhibit nonlinear load-displacement behavior when loaded at small-scale standardized test rates (1% strain/min.). The observed nonlinear behavior in tension, compression, and shear is unique to each mode of loading. Flexural loading incorporates all three aforementioned loading modes; therefore, in accordance with conservation of energy, each unique behavior must combine to yield the observed flexural behavior. As part of a broader study to determine the large-scale behavior of wood-plastic composites (WPC), the small-scale mechanical properties of six diverse formulations of extruded WPCs were established using small coupons cut from full-size structural components. To determine the nonlinear stress-strain relationships for the six different formulations, testing was conducted in tension (ASTM D638), compression (method developed by the authors), and flexure (ASTM D790 to obtain the shear modulus as described in ASTM D198 Appendix IV). A function of strain with both linear and inverse-hyperbolic sine terms accurately represented the observed stress-strain behavior. The resulting equations were included in a user-defined material model within a finite element analysis to verify the functional representation of the different loading modes with the conservation of energy. The load-displacement output from the finite element analysis of full-size members in flexure accurately depicted the load-displacement measured for the corresponding specimens. Having verified the stress-strain relationships for small-scale, they will be utilized as a basis for large-scale model development.
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