Thermo-Oxidative Stability of High Performance Composites under Thermal Cycling Conditions

Thermo-oxidative stability of high performance composites was investigated under thermal cycling conditions between room and 177°C curing temperature. For the analysis of thermal cycling experimental results, an equivalent cycle time (ECT) was developed by applying degradation-reaction kinetic theories to thermal-cycling conditions. Applying this methodology to the weight loss measurements of composite specimens, thermal cycling was found to exhibit a slower weight-loss degradation rate than isothermal conditions. This observation seemed to result from a retarded diffusion rate of oxygen through the viscoelastic relaxation of the epoxy matrix exposed to the thermal cycling conditions. In the later stage of thermal cycling, the through-thickness microcracks were induced at the exposed surfaces and then propagated into the core of the composite laminates. Subsequently, the degradation rate measured by the composite weight loss was accelerated with the thermal cycles seemingly because themicrocracks increased the effective degradation-reaction area and enhanced the diffusion rate of oxygen into the unreacted core.