On a unique fracture micromechanism for highly cross-linked epoxy resins

Micromechanics-based fracture criteria for highly cross-linked thermosets are heavily needed in the context of multiscale composite failure analysis. Fracture in an epoxy resin under quasi-static loadings was previously addressed using a two-parameter model, formulated in terms of the attainment of a critical tensile stress at the tip of internal defects. This model requires the identification of the aspect ratio of the defect and of the critical stress. Here, we unravel the physics underlying this fracture criterion and extend its validity to high temperature test conditions, to fatigue and to fracture toughness in an application to the RTM6 epoxy resin system. Accurate prediction of the fracture strain at high temperatures supports the stress-based nature of the criterion. Fractographic analyses justify the relevance of the attainment of a critical stress in fatigue as well. Indeed, the similitude between fracture surfaces under quasi-static and fatigue loading conditions indicates an identical failure scenario which is found to consist of crack nucleation near a defect, a crack arrest step and a re-initiation process including crack tip blunting. Finally, a combination of finite element analyses and experiments allows linking the fracture in pre-cracked specimens to the identified fracture criterion. The success of this approach encompassing such a wide range of conditions was unexpected and leads to a much simpler treatment of fracture in this class of thermoset materials compared to other existing approaches. It opens new avenues for improving failure analysis of composites based on highly cross-linked epoxy resins for conditions where matrix cracking is dominant.