Modelling damage and permanent strain in fibre-reinforced composites under in-plane fatigue loading

The vast majority of the fatigue models for fibre-reinforced composites is limited to one-dimensional loading conditions. Due to the heterogeneous and anisotropic nature of composites, the extension of these models towards multi-axial fatigue loading conditions is not straightforward. This paper presents a phenomenological residual stiffness model that predicts the stiffness degradation and (possible) permanent strain in fibre-reinforced polymers under in-plane fatigue loading. The model takes into account the actual stress state in each material point and does not make any assumptions about geometry or boundary conditions of the fatigue loaded specimen. As the presented model has been developed within a larger research programme, the emphasis in this paper lies on the theoretical modelling framework, rather than on an in-depth validation of the model which would require much more detail about the close feedback between experimental data and finite element simulations. Therefore the development of the stress–strain-damage relationships and the damage growth rate equations is explained thoroughly and a few finite element results are presented for plain woven glass/epoxy composites.