The influence of GNP and nano-silica additives on fatigue life and crack initiation phase of Al-GFRP bonded lap joints subjected to four-point bending

As bonded components of many industrial structures are inevitably under bending fatigue loads, it is essential to improve their mechanical behavior under these loads. This paper aims to investigate the influence of adding graphene and silica nano-particles to Araldite 2015 adhesive on the fatigue behavior of aluminum-to-composite bonded single lap joints (SLJs) by a novel fixture under four-point bending. Four adhesive groups were prepared: I) neat, II) nano silica-reinforced, III) nano graphene-reinforced (groups II and III include 0.5 wt%, 1.0 wt%, and 1.5 wt% contents), and IV) a mixture of these particles (0.5 wt% of each particle). Besides, all nano-reinforced groups were prepared with similar processing steps including magnetic stirring (30 min duration, speed of 180 rpm) followed by ultrasonic mixing (1 h duration) and vacuum oven drying. Tensile tests indicated that the tensile strength of the graphene-reinforced bulk specimens was greater than that of silica-reinforced ones for 0.5 wt% and 1 wt% of filler contents. Also, the average static failure load (ASFL) of the graphene-reinforced SLJs under four-point load was greater than that of silica-reinforced joints for 0.5 wt% and 1 wt%, and it was lower than that of the silica-reinforced specimens for 1.5 wt% of particle contents due to the presence of more agglomeration of nano graphene particles in the adhesive. Then, for maximum fatigue loads of 40%-70% of ASFL, and load ratios of R = 0.5, 0, and -1, load vs. life cycle curves were examined for all SLJ groups. The ASFL and fatigue life of group IV SLJs (mixture of 0.5 wt% of each particle) were improved compared to the joints reinforced by separate usage of each particle with 1 wt% content. Using backface strain measurements, the influence of adding nano-particles on retarding the crack initiation phase was discussed under different load conditions. Finally, optical microscope observations indicated that adding nano-particles changes the failure mode from adhesive to cohesive. The major reinforcing mechanisms encountered in this research were interpreted by scanning electron microscopy.