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, T-sf. 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 T-sf 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, alpha(2). From curvature-temperature profiles, it is found that alpha(2) is almost constant during the cycle, below T-g. 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. (C) 2002 Elsevier Science Ltd. All rights reserved.