An analytical–numerical framework for the study of ageing in fibre reinforced polymer composites

In the last few years there has been available a growing amount of information on composites ageing that considers effects such as temperature, oxidation, UV radiation, permanent loading, etc. In this paper, an analytical–numerical framework adequate for the compilation, interpretation and application of experimental data to actual engineering analysis and design is discussed. The formulation proposed includes elastic anisotropic relations; ageing viscoelastic anisotropic constitutive equations in terms of state variables; age-adjusted failure and degradation criteria; all in a setting of large displacements with small strains. The essential differences between hardening and softening ageing processes are described together with the constitutive relations adequate for each case. These equations are written in a form suitable for numerical analysis in a finite elements context, using an incremental-iterative solution procedure that accounts for post-critical effects. Several examples, including elastic, viscoelastic and failure behaviour in a large displacement context are included to check the algorithms.