Out-of-plane static loading performance of lightweight aluminum/composite FML structures for retrofitting applications: Effectiveness of bonded, bolted and hybrid bonded/bolted joining techniques under hydrothermal aging environment

This study evaluates the mechanical response for different flexural loading direction scenarios of the fiber metal laminates (FMLs), an aluminum structure retrofitted with basalt fiber-reinforced epoxy-based composites for aircraft applications. The static bending load-carrying effectiveness under hydrothermal aging of FMLs joining with bolted, bonded, and hybrid bolted/bonded (HBB) techniques were investigated, and damage formations were discussed through post-fracture morphological analyses. Besides, the adhesive was modified by adding halloysite nanotubes (HNTs) to increase the bond durability, especially against the aging environment. Test results showed that HBB structures generally provided superior aging durability compared to other connection types. It was also concluded that the basalt/aluminum FMLs exhibit a more effective load-carrying performance when the aluminum component is positioned under the neutral axis, preventing abrupt load drops. For untreated, wet, and dried conditions, the load-carrying capacities of HBB (nano-adhesive and four bolts) specimens exhibited enhanced load-bearing performance by 24-34%, 25-42%, and 29-30% compared to neat-adhesively bonded samples. Moreover, HBB structures improved performance for the same conditions by 114-141%, 127-155%, and 102-146%, respectively, compared to only-bolted (two bolts) specimens. This study provides insights into the potential use of basalt fiber-reinforced polymer composites BFRPCs in retrofitting aluminum for aerospace applications.