Fractographic study of epoxy under mode I and mixed mode I/III loading

Fiber-reinforced polymeric matrix composites are widely used in structural components such as wind turbine blades, which are typically subject to complicated loading conditions. Thus, the response of these materials under mixed-mode loading is of great relevance to the reliability of these structures. In this study, a novel fracture testing methodology, the spiral notch torsion test (SNTT), was applied to study the fracture behavior of an epoxy resin. SNTT samples were tested using either monotonic loading or cyclic loading, while both mode I and mixed mode I/III loading conditions were used. Fractographic examination indicated the epoxy samples prevalently failed in mode I even when the test specimens were subject to mixed mode loading. Different fatigue precracks were observed on mode I and mixed mode samples, i.e. precracks appeared as a uniform band under mode I loading, and a semi-ellipse under mixed mode loading. Fracture toughness was also estimated using quantitative fractography.