A Baseline Nonlinear Material Characterization for Predicting the Long-term Durability of Composite Structures
Prediction of the long-term durability of continuous fiber, polymer matrix composite structures exposed to sever temperature and moisture conditions requires a good understanding of the time-dependent nonlinear properties of these materials. Both theory and accelerated testing methods need to be developed to make such a prediction. The present focus of this research is to use experimental techniques to derive ply-level stress-strain behavior and mathematically characterize in the nonlinear range the initial constant stress rate response of a rubber-toughened carbon/epoxy in the dry state, at room temperature. With these conditions, a baseline for isolating the effect of damage growth from intrinsic viscoelasticity and for determining temperature and moisture dependence may be established.
A mathematical model which accounts for both viscoelasticity and changes in microstructure has been used to characterize the material behavior. It is found for a large portion of the loading curve that the shear and transverse compliances can be described by a single function of stress, a ratio of compliances, a time or rate component, and two elastic terms. Special specimen design considerations for using off-axis and angle-ply coupons are examined. Material behavior derived from the two specimen types and from thick and thin samples are compared. Finally, the accuracy of the material characterization is checked by comparing experimental stress-strain response to a predicted response based on the mathematical model.