On the reduction of cure-induced stresses in thermoset polymer composites

This work concerns the study of the effect of cure cycles on the development of cure induced stresses in thermoset polymer composites. The study includes experimental testing as well as theoretical modeling. The experimental part included modifying a newly developed test method to monitor the development of cure induced stresses in thermoset polymer composites. Test results were shown to be highly reproducible and gave a clear understanding of the mechanisms of stresses development as well as cancellation. Good qualitative agreement between test results and the independently measured polymer volume change data was seen. The test method was further modified with a closed loop feedback control system to modify the cure cycles to reduce cure induced stresses. Several composite material systems were studied. Test results show that the modified cure cycles reduce residual stresses in composite laminates compared to the cure cycles recommended by materials manufacturers while maintaining the same glass transition temperature. It was shown that the modified cure cycles may also enhance the matrix-dominated mechanical properties, maintain better dimensional stability, and reduce cure time. The theoretical part of the study included the development of a mechanical optimization model and a computer program, OPTICURE, to study the effect of changing the cure cycle on the development of residual stresses in composite laminates. The model and the program were used to generate cure cycles that reduce residual stresses in thermoset polymer composites and hence enhance the dimensional stability and mechanical properties. Findings of the theoretical modeling were found to agree well with those of the experimental study.