A fracture toughness analysis for discontinuous fiber reinforcement was evaluated as a function of fiber volume percent (Vf) 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 (SERIC) 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. V1 significantly correlated with resilience, WOF, and SERIC, 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 Vf for WOF, and 5.4 Vf for SERIC (p < 0.05). © 2007 Society of Plastics Engineers.