A study of the impact of humid aging on the strength of industrial adhesive joints

This paper aims at investigating the aging of a single lap joint (SLJ) in water at 70 °C. Experimental observations show an important decrease of deformability from 1 day of aging, while initial behaviour is not impacted. Further investigations reveal that the initial crack occurs at the interface near extremities of overlaps at a load depending from aging time. To predict the behaviour of the aged SLJ submitted to tensile loading, it is necessary to know precisely the state of stress in the adhesive layer and at the interface. The authors develop a finite element model to simulate the diffusion of water in the adhesive layer of the SLJ and the subsequent residual strength testing. The mechanical behaviour of the adhesive is predicted by a coupled elasto-plastic law which is deduced from an experimental work on bulk adhesive aged in water. It can model simultaneously its intrinsic behaviour as a function of temperature and water concentration and allow predicting the tensile tests on aged adhesive specimens and the volume change due to temperature and diffusion of water. Using the finite element software ABAQUS, the authors show that during diffusion of water, stresses can be important in the adhesive layer and can generate small damage. Then, for a load that led without aging to crack initiation and final fracture, stress field in the aged adhesive layer is not that critical. Such results tend to confirm that interface has been mostly impacted and weakened by water.