Prediction of the residual structural integrity of a polymer matrix composite construction element

Polymers are time-dependent materials. Polymer matrix composites behave as viscoelastic-viscoplastic anisotropic continua. Their thermomechanical characteristics are not only time-dependent, but are strongly influenced by the variations of the environmental conditions (temperature, moisture diffusion, radiation; etc.). This concerns not only the stiffness but also the strength characteristics, related to the damage development. Due to the composite nature, the number of potential damage sources is high and the damage development has to take into account all the interactions between simple mechanical loading modes,,with or without, changing environmental conditions. Following a non-linear viscoelastic-viscoplastic analysis, based on the Schapery equations and a Zapas-Crissman functional, the stiffness degradation is obtained based on short-term creep and creep-recovery measurements at different stress levels and different temperatures, A damage component is added to the viscoelastic-viscoplastic model in order to account for damage development and associated strength degradation. Results obtained in the transverse direction of an unidirectional graphite-epoxy composite are presented. The extension to complete laminates with different stacking sequences is discussed. An assumption. on the interlaminar behaviour must be introduced in order to take into account the important damage source not considered in classical laminate theories.