Experimental study on mixing ratio effect of hybrid graphene nanoplatelet/nano-silica reinforcement on the static and fatigue life of aluminum-to-GFRP bonded joints under four-point bending

In the scenario of nanoparticle-reinforcement, synergistic effect of adding two or more nanoparticles types to the adhesive can be introduced as a solution to achieve more efficient bonded joints. This paper aims to investigate the influence of hybrid insertion of graphene nanoplatelet (GNP) and silica nanoparticle (SNP) additives with various GNP + SNP mixing ratios on the static strength, endurance, and crack initiation life of aluminum-to-GFRP bonded joints under four-point bending. Four specimen groups were prepared: I) neat, II) hybrid-type reinforcements with different mixing ratios of 30 %GNP + 70 %SNP, III) 50 %GNP + 70 %SNP, and IV) 70 %GNP + 30 %SNP. Static tests indicated that, for 0.5 and 1.0 wt%, 70 %GNP + 30 %SNP had the maximum average static failure load, while, for 1.5 wt%, 30 %GNP + 70 %SNP resulted in the highest static strength. Under a fatigue load of 50 %, the longest endurance and crack initiation life was obtained for 70 %GNP + 30 %SNP hybrid-reinforced group. However, under higher fatigue loads of 70 %, variations in hybrid mixing ratio had a negligible effect on crack initiation phase for 0.5 and 1.0 wt%. Under fatigue loads of 70 % and 1.5 wt%, 30 %GNP + 70 %SNP group had 3.4 % and 2.6 %, and 5.1 % longer crack initiation life than 50 %GNP + 50 %SNP and 70 %GNP + 30 %SNP groups, respectively. Finally, major microstructural reinforcing mechanisms were interpreted using scanning electron microscopy (SEM).