Sliding-mode fracture behavior of friction lap-welded aluminum-GFRP hybrid joints utilizing a thermoplastic interlayer

Abstract

The Mode-II fracture response of friction lap-welded aluminum-to-thermoset glass fiber reinforced polymer (GFRP) composite joints, incorporating an interlayer made of thermoplastic material, was investigated through an integrated experimental and numerical approach. End-notched flexure (ENF) specimens were fabricated and tested under three-point bending. The strain energy release rate and the traction–separation response for the cohesive zone model (CZM) were determined using the compliance-based beam method (CBBM). This data reduction scheme was selected for its ability to account for the equivalent crack length and the presence of a non-negligible fracture process zone (FPZ). To ensure pure Mode-II loading conditions, the bending stiffnesses of the substrates were equalized by thickness adjustment, thereby eliminating contributions from Mode-I fracture energy due to stiffness mismatch. The joining mechanisms were examined using scanning electron microscopy (SEM), revealing mechanical interlocking at the interface, adhesion by the thermoplastic polymer, and resin penetration into the aluminum surface micro-features; these mechanisms collectively contributed to the overall joint integrity. Furthermore, a trilinear with secondary hardening traction–separation law was adopted to accurately simulate the experimental response, including the combined effects of interlayer polymer plastic and strain hardening and fiber bridging observed in the joints. The numerical results demonstrated reasonable agreement with the experimental data.