Effect of long-term stress aging on aluminum-BFRP hybrid adhesive joint's mechanical performance: static and dynamic loading scenarios

Composite-aluminum hybrid adhesive joints represent an ideal solution for designing lightweight structures for the marine industry. However, seawater aging is a serious concern, limiting the safe service life of the joint. Notably, efforts to understand the impact of aging have largely focused on the short-term periods without considering actual operating conditions. Here, we report the mechanical performance of hybrid joints subjected to the long-term stress aging. Besides, we modified the epoxy adhesive with halloysite nanotubes (HNTs) to limit the aging driven adhesive degradation and improve the adhesive's rigidity. We evaluated mechanical performances of hybrid joints by performing tensile, flexural, and drop-weight impact tests. While we increased the load-carrying capacity by over 25% with the HNTs modification before the stress aging process, modified adhesive withstood almost 55% higher tensile load than the neat epoxy adhesive after six-month stress aging. The modified adhesive also absorbed 41% less impact energy, indicating the efficiency of HNTs on limiting the degradation due to the stress aging. Furthermore, the damage mode transformed from adhesion to cohesion, thanks to the improved adhesive-composite interface performance. We envisage that these exciting results will pave the way for designing robust hybrid joints for the marine industry

Fracture and dynamic mechanical analysis of seawater aged aluminum-BFRP hybrid adhesive joints

Adhesively bonded hybrid FRP-aluminium structures have recently become an efficient solution for marine engineering applications. However, polymer adhesives' bond performance is sensitive to the marine environment due to polymer and interfacial degradation. This study aims to develop mode I, mode II delamination toughness, and Tg data as a comprehensive design guideline for hybrid BFRP-aluminum modified-adhesively bonded joints subjected to seawater aging. The hybrid joints were exposed to long-term seawater aging (for 6 months) to reveal their fracture and thermomechanical performances. Besides, the adhesive was reinforced with HNTs to increase fracture resistance with additional nano-scale toughening mechanisms and to delay the water absorption. After the long-term aging, reinforced adhesively bonded joints exhibited ∼36% higher fracture toughness than neat adhesively bonded joints. Moreover, DMA was conducted on miniaturized SLJ samples, which revealed that HNT modified adhesive joints showed ∼11.5 °C higher Tg. The calculated aging rates also proved the effectiveness of HNTs modification on the epoxy adhesive's aging performance since the HNT reinforced adhesive represented 43% lower aging rates in terms of storage modulus. It is considered that experimental results will help comprehend long-term aging influences on the composite-aluminum hybrid designs’ fracture and thermomechanical performances. These exciting findings will pave the way for the safe use of high stiffness and cost-effective aluminum-BFRP hybrid structures for the marine industry