Quasi-static mechanics of reactively formed epoxy–thermoplastic interphases in CFRP laminates

This study compares amorphous PEI, PES, PSU, and PC films co-cured as functional interlayers in aerospace-grade CFRP laminates based on HexPly® M18/1. A unified quasi-static test matrix comprising single lap shear, interlaminar shear strength, and Mode I and Mode II fracture toughness was applied to establish a directly comparable mechanical ranking. All interlayer systems preserved structural integrity, increasing single-lap shear strength by 6.2% to 13.3% to about 16 to 18 MPa. Interlaminar shear strength remained in a range of 48 to 59 MPa, but depended on interlayer placement. Fracture testing revealed two distinct response types. PEI and PES showed the most effective toughening, with PEI reaching the highest Mode I toughness at 590 J/m² (+59.0%) and PES the highest Mode II toughness at 6180 J/m² (+518.6%). In contrast, PSU and PC showed poor opening-mode resistance. Overall, the results demonstrate that interlayer selection governs both the fracture mechanism and achievable damage tolerance.