Cure cycle effect on composite structures manufactured by resin transfer molding

An experimental investigation was conducted of the effect of curing cycle on the development of material properties, residual strains and stresses in composite parts during the resin transfer molding (RTM) process. The material investigated consisted of AS4 carbon fibers as the preform and a three-part epoxy system. Unidirectional and crossply carbon/epoxy laminates were prepared by the RTM process in aluminum molds. Several different curing cycles were designed by changing the peak cure temperature and heat up rate based on a cure kinetic model. Strains were measured in the composite laminates during curing using embedded fiber optic strain sensors and electrical resistance strain gages. It was found that significant strain was developed by interference between composite and mold during cure, resulting in constraint-induced strain. The magnitude of residual stresses was also assessed by measuring the warpage curvature of asymmetric cross ply laminates. The data showed that the residual stress was significantly dependent on the cure cycle and the interference.