Reduction of cure-induced stresses in thermoset polymer composites via chemical and thermal methods

This thesis discusses three different experiments in which we examine in-cure residual stresses in carbon/epoxy thermoset polymer composites. Throughout this research the Cure Induced Stress Test (CIST), developed at UT, has been used. The CIST is a unique experimental procedure in which the effect of polymer volume change on fiber stress is determined by monitoring the change in fiber load. In the first part of the research, we addressed the role of fiber-matrix interface on the fiber-load measurement. For this purpose, the CIST experiments were conducted with as-received and Teflon-coated fibers. The Teflon coating was intended to weaken the fiber-matrix interface. The use of a Teflon coating was shown to have no effect on the fiber load measurements during cure. Thus, it is concluded that the fiber-matrix interface does not significantly influence the data obtained in the CIST experiments. Second, we use CIST to verify a new methodology to reduce the internal stresses involving a chemically engineered ring-opening reaction that is still under development. The purpose of the ring-opening reaction is to offset the polymer shrinkage. Finally, we use CIST to expand upon a previous study to determine the build-up of polymer stiffness during cure. Comparison of stiffness curves obtained from CIST and measured stiffness values of fully and partially cured polymer samples show a good agreement.