Hybrid Joining Technologies for Composites: Mechanisms, Performance, Challenges, and Perspectives

Fiber-reinforced polymer (FRP) composites are increasingly used in aircraft structures with their high specific strength, stiffness, corrosion resistance, and lightweight nature. However, reliably joining FRP components is challenging because mechanical joining can create stress concentrations, fiber damage, and delamination, while adhesive bonding can cause peel stress, environmental degradation, and inspection challenges. This review systematically covers hybrid joining approaches for FRPs, emphasizing joint designs, load-sharing mechanisms, mechanical performance, surface treatment, interlayer engineering, aerospace applications, certification concerns, and emerging technologies. This systematic literature review focused on bolted-bonded, riveted-bonded, welded-bonded, and friction-based hybrid connections. The review analyzed the evaluated research for static strength, fatigue behavior, impact resistance, failure mechanisms, joint design parameters, surface modification, interlayer engineering, aerospace implementation, inspection, repair, and certification. Other emerging themes included structural health monitoring, thermoplastic composites, automated joining, and bio-inspired interfaces. Based on the literature, this study shows that hybrid joining provides load-transfer mechanisms and progressive failure behavior comparable to those of single-joining approaches. Reported impact testing suggests that hybrid joints absorb roughly 45% more energy than adhesive joints. Moreover, surface treatments such as abrasion, plasma treatment, laser ablation, and chemical etching, as well as toughening interlayers, can further improve interfacial performance and damage tolerance. Aircraft applications include thermoplastic control surface components, wing-box joints, and fuselage splices. The performance of hybrid FRP joints depends heavily on controlled load sharing between the adhesive and mechanical or friction-based joints. Therefore, adhesive layer thickness, fastener torque, edge distance, overlap length, and fastener spacing are important in reducing stress concentrations and progressive failure. Tailored interlayers and surface activation may further improve fracture resistance and adhesion. However, the following are restricted: certification, non-destructive assessment, long-term durability, repair techniques, and uniformity of repair procedures. Future work should develop new solutions for monitoring, automation, production efficiency, and damage tolerance by combining embedded sensors, machine learning, thermoplastic welding, robotic joining, and bio-inspired interfaces. Hybrid joining methods provide an integrated approach to enhancing the structural performance of FRP composite connections, including damage tolerance, fatigue endurance, and reliability, particularly in aerospace applications. This is achieved through complementary joining techniques that enable gradual load transfer and maintain load-carrying capacity after local damage. Future research should also focus on standardized qualification procedures, standardized processes for reliable non-destructive inspection, environmental durability, automated process control, intelligent health monitoring, and scalable certification procedures to enable the wider application of hybrid composite joining technologies in next-generation aerospace structures.