A new method to reduce cure-induced stresses in thermoset polymer composites, part II: Closed loop feedback control system

A closed loop feedback control system has been developed to obtain cure cyc les which reduce the cure-induced stresses in single fiber/matrix model com posites. In this feedback system, thermal expansion and stress relaxation a re used to counteract the stresses resulting from the chemical shrinkage. T he feedback system is applied to two different fibers (carbon and glass) in four different resins [3501-6, 934, and 977-3 epoxies and 5250-4 bismaleim ide (BMI)]. The completeness of the cure is verified by comparing glass tra nsition temperature (T-g) for polymers cured using the standard cure cycles (cycles recommended by the prepreg manufactures) and the feedback cure cyc les. The durations of the feedback cure cycles are within that of the stand ard cycles. Test results indicate that feedback cure cycles reduce the curv ature of unsymmetrical laminates as a measure of residual stresses.