Engineering Mechanics Institute Conference 2013

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Prediction of creep behavior of biobased composites

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Sabbie Miller
Stanford University
United States

Michael Lepech
Stanford University
United States

Sarah Billington
Stanford University
United States

Biosynthesized polymers, such as polyhydroxyalkanoates (PHAs) and their co-polymers have demonstrated suitability for acting as a matrix in wood-plastic composites (WPCs) . WPCs have found application as replacement for wood and engineered wood materials in the construction industry. Unlike current WPCs in industry, the combination of a biosynthesized PHA matrix with natural fibers can potentially provide composites that are not dependent on petrochemical carbon feedstocks. However, for structural application of such biobased WPCs, assessment of creep properties and prediction of composite deformation properties as a function of time is necessary. Currently, creep model development for natural fiber/polymer composites is not well investigated and commonly modeling efforts for WPCs are limited to one-dimensional spring and dashpot models based on empirical results .

This presentation will discuss modeling methods to predict macroscopic creep behavior of WPC composites and considerations necessary for natural fiber composites. Using Mori-Tanaka’s theory of materials with misfitting inclusions , a formulation was assessed to be included in a finite element implementation. To account for effects of incompatibility of hydrophilic natural fibers and the hydrophobic polymer matrix and the use of compatabilizing agents, an interfacial region between the inclusion and the matrix as well as the influence of imperfect adhesion between fiber and matrix were incorporated. The parameters for the model were based predominantly on properties obtained from quasi-static tensile testing and were used to predict composite creep behavior.

The creep predictions determined from the model developed were then incorporated into life cycle assessment models to offer an expected material service life. By including these service life predictions, a more complete environmental impact assessment was made than assuming equivalent time dependent behavior (the current practice in life cycle modeling of novel materials). Based on coupled environmental impact of materials and their service performance, the most sustainable composite constituents for a deflection-based application were determined.


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