Time-temperature dependence of fracture toughness for bisphenol A epoxy resin

The relationship between the curing conditions and the time-temperature dependence of fracture toughness was investigated for bisphenol A epoxy resin. The glass transition temperature and Angell's fragility parameter, which are obtained from thermoviscoelasticity measurements, were used to characterize epoxy resins cured under various conditions. Examination of the fracture toughness at various temperatures and displacement rates showed that it depends on both temperature and time, and that it follows the time-temperature equivalence principle. The time-temperature dependence of the fracture toughness was greatly affected by the fragility parameter. The fracture toughness of the resin with a smaller fragility parameter increased from lower temperatures to the brittle-ductile transition temperature than that of the resin with a larger fragility parameter when their glass transition temperatures were approximately 400 K. It was also found that the brittle-ductile transition temperature did not depend on the fragility parameter. This means that epoxy resin with a smaller fragility parameter has better fracture characteristics than epoxy resin with a larger fragility parameter if their glass transition temperatures are approximately 400 K.