Effects of adherends recycling on the distribution of hardness in bonded region of adhesively bonded single lap joints

The phenomenon of work hardening in the bonded region of steel adherends has been studied, and the effect of adherends recycling on the shear strength of a single lap joint is analyzed in this paper. The values of hardness on the surfaces of failed joints were measured after being simply treated (removing the adhesive layer by solvent) before the specimen preparation next time. Mild steel or pure copper sheet and an epoxy adhesive were used to prepare the single lap joints and the same couple of adherend pieces underwent three or four times of strength and hardness tests. The results obtained show that the evident hardness change occurred after the adherends passed their first or second strength testing and most of the points tended to increase their values of hardness as the tests were performed. It may be due to the contribution of the stress concentration during loading, and the residual stress occurred after the adherends unloading. The results obtained also show that the increase of hardness occurring in the bonded zone is greater than that on the face opposite to the bonded region. The distribution of micro-hardness was verified by the strain gauges testing method.