Size Effect Based Fracture Toughness Characterization of Epoxy Resin to Resolve Small-Scale Yielding Ambiguity

This work presents an experimental investigation into the fracture toughness of lab-cured epoxy resin via the size effect approach, which helps resolve the ambiguity in specimen size selection introduced by unknown ductility levels. Particular emphasis is placed on demonstrating the cure-state dependent ductility and its implications on specimen size selection. However, the findings extend to other sources affecting the epoxy ductility, such as temperature, aging, and strain rate. Mode I fracture experiments are presented on a lab-cured epoxy resin's geometrically scaled SENB specimens in two different cure states: cure state 1 (CS1, 40 h after cure initiation) and cure state 2 (CS2, 168 h after cure initiation). Basic mechanical characterization under uniaxial tension and compression is also carried out. The fracture tests are analyzed using Bazant's type II size effect law (SEL), which reveals that the CS2 exhibits predominantly brittle behavior and conforms to the small-scale yielding (SSY) assumption, implicit in ASTM standardized tests. On the other hand, CS1 displays much higher ductility, and violates the SSY assumption, despite the curing time well exceeding the manufacturer recommended duration. These findings demonstrate that epoxy continues to cure over long periods, creating difficulty in selecting specimen dimensions that consistently satisfy SSY-based validity requirements. The size-effect-based approach is shown to eliminate this ambiguity by enabling the determination of size-independent fracture toughness using the same set of specimen sizes, regardless of curing degree. Although epoxy resins are often treated as intrinsically brittle, the results demonstrate the importance and relevance of size effect based characterization for unambiguous toughness measurements. The presented experimental data also serve as a useful benchmark for calibration and validation of computational models for epoxy fracture.