Hygrothermal modeling of polymers and polymer matrix composites

The theory of irreversible thermodynamics is applied to derive governing eq uations for history-dependent diffusion in polymers and polymer matrix comp osites from first principles. A special form for Gibbs free energy is intro duced using stress, temperatures, and moisture concentration as independent state variables. The resulting governing equations are capable of modeling the effect of interactions between complex stress, temperature, and moistu re histories on the diffusion process within an orthotropic material. Since the mathematically complex nature of the governing equations precludes a c losed-form solution, a variational formulation is used to derive the weak f orm of the nonlinear governing equations which are then solved using the fi nite element method. For model validation, the model predictions are compar ed with published experimental data for the special case of isothermal diff usion in an unstressed Graphite-Epoxy symmetric angle-ply laminate.