Nonlinear material behavior and fatigue-accumulated damage of wood plastic composites

Derek A. Brosious

Wood-plastic composites (WPC) are quickly growing as useful materials in the development of structural elements by combining some of the advantages of both wood and plastic. However, their structural use has been somewhat limited due to a lack of knowledge concerning the mechanical behavior of WPC, most notably the stress-strain nonlinearity and the lack of design predictability due to damage accumulation. This paper attempts to address and mitigate these two limitations. The stress-strain behaviors of several formulations of WPC were evaluated in quasi-static tension and flexure under various strain rates, where highly nonlinear performance was observed. A nonlinear hyperbolic tangent constitutive relation was used for stress-strain and force-displacement analysis in an attempt to model the behavior of coupled wood-polypropylene in axial members and in flexural members. It was found that the hyperbolic tangent function is an excellent tool for describing nonlinear stressstrain behavior in tension and flexure, and deflections of structural members in both loading modes can be predicted quite successfully. Through this process, it was also discovered that the strain rate, or rate of applied load, had a notable effect on material properties. Increases in stiffness and ultimate strength were observed for increased load rates, and the nonlinearity of stress-strain relation reduced as the load rate increased. An analytical model using energy method was proposed to predict the nonlinear loaddeformation history of structural components. A stress-cycle to failure (S-N) curve was developed for a coupled woodpolypropylene by fatiguing flexural samples to failure. The fatigue data indicated that the formulation behaved extremely well at lower stress ratios, although the data was somewhat erratically dispersed. Based on these findings, coupons were conditioned to varying fractions of ultimate failure cycles and then tested in quasi-static flexure to measure reduced stiffness and in dynamic mechanical analysis (DMA) to measure reduced storage modulus. Two cumulative damage models were proposed using random distributions and two-parameter Weibull distributions of the measured fatigue data. Damage parameters were proposed based on cumulative probability density functions, and they were applied in the context of Continuum Damage Mechanics (CDM) to predict reduced stiffness and reduced storage modulus. The Weibull distribution provided good comparisons with test data, but further investigations are recommended. The present study sheds light on nonlinear material behavior of WPCs under quasi-static loading and accumulated damage under fatigue and provides related prediction models for such phenomena.

Nonlinear material behavior and fatigue-accumulated damage of wood plastic composites

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