Structural integrity of hybrid joints for transport applications

This thesis examines the structural integrity of automotive joints in lightweight vehicle design, focusing on aluminium alloys AA6082 (6xxx series) and AA7075 (7xxx series). With the automotive industry increasingly adopting lightweight materials such as advanced high-strength steels, aluminium alloys, and carbon-fibre-reinforced polymer composites to improve fuel efficiency and reduce emissions, reliable joining methods are essential. Three joining techniques were investigated: self-piercing riveting (SPR), adhesive bonding (SikaPower® 497 epoxy), and hybrid SPR–adhesive bonding. Single lap shear specimens were prepared for similar (AA6082–AA6082) and dissimilar (AA6082–AA7075) configurations. Quasi-static tests were performed under a constant displacement rate, and fatigue tests under tension–tension loading (load ratio R = 0.1, and frequency 10Hz) in both ambient and wet conditions. Quasi-static results showed SPR joints had the lowest failure loads and displacements, while adhesive joints achieved the highest failure loads and greater displacements. Hybrid joints exhibited slightly lower failure loads than adhesive joints but the greatest displacements and energy absorption, with a distinct two-stage failure process. Dissimilar joints showed higher strengths but lower energy absorption than similar joints. Fatigue results indicated SPR joints had the shortest fatigue lives, failing at rivet-induced stress concentrations. Adhesive joints performed best, failing in a mixed cohesive–interfacial fracture in the adhesive layer. Hybrid joints matched adhesive joints at low loads but had improved lives at high loads; however, their fatigue curves were steeper, indicating greater load sensitivity. Dissimilar substrates generally improved fatigue lives but increased load sensitivity. The wet testing conditions had no significant effect on fatigue performance in this research due to the relatively short duration of exposure to water. This study demonstrates the relative advantages and limitations of SPR, adhesive, and hybrid joints for lightweight automotive structures, highlighting how joint type, material pairing, and loading conditions influence strength, energy absorption, and fatigue life.