Fracture Toughness Micromechanics by Energy Methods With a Photocure Fiber-Reinforced Composite

A fracture toughness analysis for discontinuous fiber reinforcement was evaluated as a function of fiber volume percent (V_f) using advanced flexural bend tests. Fully articulated fixtures with 40-mm spans were used to examine specimens (2 × 2 × 50 mm³) under conditions of Euler-type bending to reduce shearing effects. Testing for fracture toughness in standardized international units (kJ/m²) using fundamental mechanics-of-materials energy methods by strain energy was then applied for assessment of resilience and work of fracture (WOF). Fracture toughness was also measured as strain energy release (SER_IC) for the condition of unstable fracture between peak load and 5% maximum deflection past peak load. Energies were calculated by numerical integration using the trapezoidal rule from the area under the load-deflection curve. Fracture depths were normalized using sample dimensions from microscopy imaging for a combined correlation matrix analysis of all mechanical test data. V_f significantly correlated with resilience, WOF, and SER_IC, but negatively correlated with degree of crack depth with p < 0.0000005. All measured interrelated properties also significantly correlated with one another (p < 0.000001). Significant fracture toughness differences between particulate-filled and fiber-reinforced composites began when adding fiber reinforcement at 10.3 V_f for resilience, 5.4 V_f for WOF, and 5.4 V_f for SER_IC (p < 0.05).