The effect of surface pretreatment and moisture on the fatigue performance of adhesively-bonded aluminium

Adhesively-bonded aluminium single lap shear joints were tested to failure under fatigue loading to ascertain the effects of surface pretreatment and moisture in both durability and fracture path. The aluminium substrate used was AA5754 with and without a silicon-based pretreatment. The adhesive used was a commercially available DGEBA-based epoxy resin cured by a modified dicyandiamide. Fatigue tests were performed at 20 Hz using an R-ratio of 0.1. To monitor the effects of moisture, tests were performed in a chamber that generated 96% relative humidity (r.h.). SN curves were generated and several microscopic and surface analysis techniques were employed to characterise the failure mode of the joints. The techniques included simple visual observation, SEM, TEM and high magnification video recordings. The fatigue performance of pretreated and untreated joints were similar when tested in air, but the performance of the untreated joints was greatly reduced when tested in 96% r.h. The differences in fatigue performance between untreated and pretreated joints in 96% r.h. are thought to be due to differences in failure mode of the joints arising from the improved interfacial bonding between the pretreatment and the adhesive. Pretreated joints failed predominantly cohesively through the adhesive, while untreated joints failed mainly interfacially at the adhesive/aluminium interface. High magnification video recordings have also shown that the adhesive fillet is a significant factor in determining the life of a single lap joint.