Fracture Behavior and Safety Assessment of Plain Woven CFRP/Al Bonded Joints

Adhesively bonded structures offer significant advantages over traditional joining methods and are widely used in aerospace composite-metal hybrid joints. However, stiffness discrepancies at the heterogeneous interface often lead to competitive failures such as debonding and delamination. To address interfacial failure in these structures, Mode I Double Cantilever Beam and Mode II End Notched Flexure fracture toughness tests and numerical simulations were conducted. The Mode I and Mode II fracture energy parameters of Al/CFRP bonded joints were experimentally determined. Subsequently, a finite element model incorporating a bilinear Cohesive Zone Model was established and effectively validated using experimentally derived parameters. The results indicated that under Mode I loading, the property mismatch at the dissimilar interface induces a competitive failure mechanism, causing a failure mode transition from initial cohesive failure within the adhesive layer to interlaminar delamination within the composite. Under Mode II loading, the failure mode exhibits interfacial failure characteristics accompanied by shear failure of the CFRP surface resin. Finally, the study revealed that Non-Pre-Cracked (NPC) structures exhibit macroscopic blunting toughening and high crack initiation resistance due to the absence of sharp initial cracks. Furthermore, the feasibility of predicting the lower bound of the load-carrying capacity of NPC structures using Pre-Cracked (PC) parameters was verified. The predicted limit load remains conservatively about 4% lower than the most unfavorable experimental case, thereby providing a safe and conservative assessment strategy and engineering reference for the damage tolerance design of heterogeneous bonded structures.