Characterization on Mode-I/II Interlaminar Strength and Fracture Toughness of Co-Cured Fiber–Metal Laminates

This study systematically evaluates the mode-I (opening) and mode-II (shearing) interlaminar strength and fracture toughness of four co-cured fiber-metal laminates (FMLs): AL-CF (aluminum-carbon fiber fabric), AL-GF (aluminum-glass fiber fabric), AL-HC (aluminum-carbon/glass hybrid fabric), and AL-HG (aluminum-glass/carbon hybrid fabric). Epoxy adhesive films were interleaved between metal and composite plies to enhance interfacial bonding. Mode-I interlaminar tensile strength (ILTS) and mode-II interlaminar shear strength (ILSS) were measured using curved beam and short beam tests, respectively, while mode-I and mode-II fracture toughness (GIc and GIIc) were obtained from double cantilever beam (DCB) and end-notched flexure (ENF) tests. Across laminates, interlaminar tensile strength (ILTS) values lie in a narrow band of 31.6-31.8 MPa and interlaminar shear strength (ILSS) values in 41.0-41.9 MPa. The mode-I initiation (GIc,init) and propagation (GIc, prop) toughnesses are 0.44-0.56 kJ/m² and 0.54-0.64 kJ/m², respectively, and the mode-II toughness (GIIc) is 0.65-0.79 kJ/m². Scanning electron microscopy reveals that interlaminar failure localizes predominantly at the metal-adhesive interface, displaying river-line features under mode-I and hackle patterns under mode-II, whereas the adhesive-composite interface remains intact. Collectively, the results indicate that, under the present processing and test conditions, interlaminar strength and toughness are governed by the metal-adhesive interface rather than the composite reinforcement type, providing a consistent strength-toughness baseline for model calibration and interfacial design.