Mechanical characterization and physics-based modeling of highly-crosslinked epoxy resin

Highly-crosslinked epoxy resins are used as matrix material for high-performance composites, typically developed for weight-reduction purposes in structural aerospace applications. If the clever assembling of different materials into large integrated composite panels can lead to outstanding mechanical properties with a guaranteed weight reduction, it also involves complex deformation mechanisms and failure scenarios which are not yet fully understood today. The development of more predictive modeling capabilities based on multi-scale modeling strategies therefore requires a better modeling of the epoxy matrix behavior. In this thesis, a thorough study of the mechanical and fracture behavior of the HexFlow RTM6 epoxy resin, certified for aeronautics, is performed. A wide mechanical testing campaign provides the basis for the identification of a phenomenological elastic-viscoplastic constitutive model, exhibiting most of the mechanical features of usual amorphous thermoplastic polymer systems. This model provides good prediction of the RTM6 deformation response under monotonic loading, accounting for rate, temperature and pressure dependence. The failure behavior of the resin is also accounted for through a macroscopic fracture criterion. Nevertheless, the unloading behavior and complex time-dependent mechanisms (e.g. creep, recovery and rate-reversal phenomena) are rarely well accounted for by phenomenological models, stressing a lack of physical insights in the modeling approach. A new physics-based mesoscopic modeling approach, inspired from the concept of shear transformation zones as the vehicle for elementary plastic deformation, is developed by coupling a FE framework with a time-dependent Monte-Carlo kinetic model. The proposed model, which requires only 6 parameters, captures all the observed experimental trends of RTM6. This strongly supports the theoretical foundations of this relatively simple micro-mechanical approach and opens the route to promising applications and extensions in the field of glassy polymers in general.