Hygro-thermal residual stresses in unsymmetrical multi-stable composite laminates

In this study, an approach to predict and analyse the effects of moisture ingress on residual stresses in multi-stable composite laminates is developed. Residual stresses are a common consequence of the manufacturing process of composite laminates (e.g. formed thermally, following cool-down from manufacture). Imbalance in these stresses about the mid-plane can lead to warping, and so composite laminates are usually restricted to symmetrical lay-ups. In certain cases, unbalanced residual stresses can be used advantageously, such as in novel morphing structures by use of multi-stable parts. These are parts which feature two or more stable shape configurations, which are obtainable through a force application. With energy only being required to alternate between shapes, multi-stable laminates have been proposed as morphing aerodynamic surfaces for aerospace and wind-energy applications. In these cases (and others in which the laminates are sensitive to residual stresses such as thin plates, ply drop off and bonded repairs) a thorough understanding of the residual stresses (both as-manufactured and in-service) is required. The residual stresses in fibre-resin composites are known to be sensitive to environmental effects, which can be encountered under in-service conditions. One such effect is moisture absorption, which alters the residual stress state of a laminate through matrix swelling and plasticisation. These changes may lead to a change in the laminate’s shape, and in the case of multi-stable laminates, a change in the multi-stable behaviour. Applications based upon these unsymmetrical laminates therefore require consideration to moisture effects at a design stage. In this work, a combined numerical/experimental approach is presented whereby the macro-scale through-thickness residual stresses of dry and saturated unsymmetrical composite laminates can be predicted and analysed. A range of unsymmetrical laminates were manufactured from carbon-fibre reinforced plastic (unidirectional continuous fibres pre-impregnated in a polymer-resin matrix), featuring both square cross-ply and tailored (i.e. featuring local variations in lay-up and/or thickness – representative of laminates that would be used in complex applications) laminate configurations. Following manufacture, the dry laminate shapes were measured, with, in the case of the tailored laminates, laser scanning – a full-field, non-contact surface measuring technique. Three-dimensional continuum based finite element models were created (using the software Abaqus) to simulate the thermal deformation of the laminates. The models were benchmarked using analytical approaches, and subsequently calibrated to match the experimentally measured laminate shapes by means of equivalent orthotropic thermal expansion coefficients, negating the need to account for individual residual stress contributors. Subsequently, laminates were immersed in water until saturation, and the change in shape due to matrix swelling was measured. The numerical models were then adapted to take into account moisture induced matrix swelling by use of the analogy between thermal expansion and moisture induced swelling. Subsequently, the variation in shape and residual stress distribution in the laminates following moisture saturation could be analysed. Using laser scanning to measure the tailored laminate shapes allowed for a detailed analysis of the full-field variation between numerically-predicted and experimentally-measured laminate shapes. Using this analysis technique, macro-scale through-thickness residual stress profiles were extracted for each of the cross-ply and tailored laminate configurations. It was found that peak residual stresses can drop by over 70% following moisture saturation resulting in a significant loss of curvature. Likewise, laminate potential energy can drop by over 90%, impacting upon the laminates multi-stable behaviour.