The effect of moisture ingress on through-thickness residual stresses in unsymmetric composite laminates: A combined experimental–numerical analysis

Some unsymmetric polymer–matrix composite laminates show multi-stable behaviour, which can provide opportunities for morphing composite structures. It is known that through-thickness residual stresses are introduced in unsymmetric laminates during the manufacturing process. In this research work, a combined experimental–numerical analysis is proposed to study the effect of moisture ingress on unsymmetric composite laminates by analysing through-thickness residual stresses at dry and saturation states. A range of unsymmetric cross-ply laminates were manufactured using carbon–fibre epoxy pre-preg material. The laminates were subsequently immersed in water to full saturation. The curvatures of dry and saturated test coupons were measured and compared to quantify the effects of moisture ingress on the shapes of the unsymmetric laminates. In addition, a finite element model was developed to predict the experimentally measured curvatures of the dry laminates. By calibrating the numerical model, the through-thickness residual stresses were analysed for different unsymmetric cross-ply layup conditions. The calibrated dry model was then extended to develop a moisture-dependent numerical model in order to incorporate the effect of moisture ingress on the shapes of unsymmetric laminates. The moisture-dependent model was calibrated with the experimentally measured curvatures and then used to predict the through-thickness residual stresses in the fully saturated unsymmetric cross-ply laminates. It is shown that moisture ingress can considerably influence through-thickness residual stresses and thus the shapes of unsymmetric laminates.