Prediction of the non-linear viscoelastic response of unidirectional fiber composites

A methodology for predicting the nonlinear viscoelastic behaviour of unidirectional fibre composites is proposed. The prediction, which is based on a modified Schapery formulation, is easy to apply by using a combination of analytical formulations and numerical procedures. In addition a generic function is developed for the description of the stress dependence of the creep-recovery response over the whole stress range examined. All the parameters included in the proposed generic function used for the prediction of the nonlinear viscoelastic behaviour of unidirectional fibre composites, such as the stress threshold of nonlinearity and the ultimate tensile strength of the material, have a clear physical meaning. The accuracy of both the generic function and the numerical technique is checked by creep-recovery tests on 90° unidirectional carbon-fibre/epoxy-matrix composites in which predicted response is compared with measured response. Good-to-reasonable agreement between experimental and analytical results is observed. The method is presented in a generalised manner and could be applicable to a large class of UD fibre composite systems.