Investigation of open-hole and temperature effects on mode I fracture behavior of unidirectional CF/PEKK composites

This study investigated the combined influence of open-hole geometry and temperature on the Mode I fracture behavior of unidirectional CF/PEKK thermoplastic composites. Double cantilever beam specimens with varying ligament (L = 5-15 mm), and width-to-diameter ratios (W/D = 3.3 and 6.6) were tested at −60, 25, and 110 °C. For no-hole specimens, the initiation apparent fracture toughness ( ) increased from 767 J/m 2 at −60 °C to 1167 J/m 2 at 110 °C, as the PEKK matrix transitioned from a brittle regime to a viscoelastic–plastic response, promoting shear deformation, enlargement of the fracture process zone, and delayed crack initiation. In open-hole specimens, short ligaments at W/D = 3.3 (L = 5 mm) reduced by 27–45% compared with the baseline, whereas increasing ligament length (L ≥ 9 mm) or W/D to 6.6 limited the loss to ∼20% and recovered up to ∼90% of the no-hole toughness. Temperature–ligament maps of normalized show that, within the examined range, short ligaments at W/D = 3.3 and −60 °C reduce the normalized toughness below ∼0.8, whereas increasing ligament length (L ≥ 9 mm) or W/D to 6.6 at 25–110 °C keeps it at or above ∼0.9, thereby defining quantitative boundaries for Mode I delamination resistance in CF/PEKK laminates. The findings demonstrate that Mode I fracture behavior in CF/PEKK laminates is jointly governed by temperature and geometric discontinuities, and provide design insight for open-hole and mechanically fastened structures that must retain Mode I delamination resistance over a wide temperature range.