A new method to reduce cure-induced stresses in thermoset polymer composites, part III: Correlating stress history to viscosity, degree of cure, and cure shrinkage

Non-thermoelastic effects such as cure shrinkage of a polymer can play a ro le in residual stresses in composite parts. Studies have shown that cure sh rinkage can place significant stresses on fibers. Therefore. the cure cycle of 3501-6 epoxy resins was modified to change its curl shrinkage character istics to minimize the stresses. New curl strategies were developed using v olumetric dilatometry. differential scanning calorimetry, dielectric cure m onitoring, and a unique single fiber stress test method. Cure cycles were m odified to balance the resin's thermal expansion with its cure shrinkage. I n some cases, a region of constant volume was achieved for a short time. Ho wever. the cure shrinkage eventually dominated over thermal expansion in al l cycles as the polymer gelled. Changing the cure cycle affected the degree of cure at the point where the fiber/matrix interface developed as well as the amount of cure shrinkage occurring afterwards. A higher degree of cure at this point leads to longer stress relaxation time. Furthermore, less cu re shrinkage at this point leads to less stress on the fibers. Also, slow h eating rates allow more time for the polymer to relax and relieve stresses caused by cure shrinkage. Finally, a cure cycle that minimizes stresses due to cure shrinkage has been demonstrated.