Fracture toughness and crack propagation analysis in CF/PEKK composites with hole defect: A comparative study of MBT and CBBM methods

This study experimentally investigated the effect of an open-hole (OH, W/[Formula: see text]) configuration on the Mode I fracture behavior of CF/PEKK thermoplastic composites. Double cantilever beam (DCB) tests were performed to analyze the influence of hole-induced discontinuities on crack initiation and propagation. The load–displacement responses were evaluated using the modified beam theory (MBT) and the compliance-based beam method (CBBM) to assess their applicability to defect-containing geometries. The Mode I fracture toughness values obtained from both methods exhibited good consistency within a [Formula: see text]% deviation, confirming the reliability of the data-based evaluation. The introduction of the hole decreased the critical fracture toughness at crack initiation by 16.6% (MBT) and 14% (CBBM), primarily due to stress concentration and local deformation irregularities near the defect. However, the reduction during crack propagation was much smaller — only 6.5% (MBT) and 3.7% (CBBM) — as the crack front became realigned and stabilized after passing through the hole. Microscopic observations revealed that fiber bridging governed the fracture resistance in both configurations. Even under defect conditions, no fiber breakage occurred; instead, interfacial debonding and matrix drawing were observed, maintaining the bridging effect throughout propagation. These results demonstrate that CBBM-based evaluation can effectively characterize Mode I fracture behavior in thermoplastic composites containing geometric discontinuities and provide a foundation for the defect-tolerant structural design of CF/PEKK composite components. This study particularly emphasizes that understanding the rapid initiation and propagation of cracks induced by geometric discontinuities is crucial for the effective design and application of CF/PEKK composites in structural components with holes or other local defects.