Development of curvature during the cure of AS4/8552 [0/90] unsymmetric composite plates

The development of residual stresses in [0/90] unsymmetric flat laminates (AS4/8552 composite system) was monitored by stopping the cure cycle at pre-determined points and evaluating the related level of deformation. Cylindrical shapes of consistent curvature are obtained and the “cured� curvature can be eliminated by re-heating samples up to a certain temperature, designated as the stress free temperature, Tsf. The stress free temperature has also been measured after the interrupted cure cycles: the evolution of the stress free temperature during the cure cycle allowed the vitrification point to be estimated. Both curvature and Tsf increase with increasing cure but level off after the vitrification point, attaining a constant value. The stress free temperature of samples cured beyond vitrification is systematically higher than their cure temperature. It is evident that a certain percentage of non-thermoelastic stress is present in the structure, possibly due to resin chemical shrinkage. The “cured� curvature is mostly driven by the transverse coefficient of thermal expansion, α2. From curvature–temperature profiles, it is found that α2 is almost constant during the cycle, below Tg. Post-cure of laminates that have not been fully cured tends to increase their “residual� curvature, but the effect of the post-cure is relatively small for specimens cured beyond the vitrification point.