Fracture Toughness Characterization of CF/Epoxy Composites Under Varying Loading Rates Using Single Edge Notch Bend Testing

This study investigates the Mode-I fracture toughness of hand lay-up CFRP laminates using the Single Edge Notch Bend (SENB) configuration in accordance with ASTM D5045 at four quasistatic crosshead speeds (1, 3, 5, and 7 mm/min). The conditional fracture toughness (KQ) was determined from load-displacement data obtained using a 100 kN universal testing machine, with three valid specimens tested at each speed (n = 3). Because the tensile yield strength and the specimen-level crack length were not measured, the values are reported as conditional fracture toughness, KQ, rather than strict plane-strain KIC. The group-average KQ values were 8.22, 8.66, 8.33, and 8.37 MPa√m at 1, 3, 5, and 7 mm/min, respectively, corresponding to less than 5% variation with no monotonic rate-dependent trend. A formal uncertainty analysis following the Guide to the Expression of Uncertainty in Measurement (GUM) yielded Type A uncertainty of 3–11% and Type B uncertainty of 9.4%, resulting in a combined standard uncertainty of 9.9–14.7% for the group average (KQ). The combined uncertainty exceeded the differences between the speed groups, indicating that the measured fracture toughness values were not significantly distinguishable at the present sample size. The average peak fracture load also showed limited variation, ranging from 828 to 851 N. Overall, the results indicate that specimen-level variability and measurement/geometry uncertainty have a greater influence on the measured fracture toughness than crosshead speed within the investigated quasi-static range. These findings provide experimental data for CFRP fracture-toughness assessment and highlight the need for direct crack-length measurement and testing at higher loading rates, including dynamic and impact regimes, to further clarify rate-dependent fracture behavior.