No abstract

This paper aims at investigating the aging of a single lap joint (SLJ) in water at 70 1C. 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.

Experimental studies were performed on an industrial rubber toughened epoxy adhesive to characterise its diffusion behaviour in water at various temperatures. Gravimetric experiments and density measurements were carried out at 25, 40, 60 and 70 1C on bulk specimens. Non-classical mass uptake and swelling were observed. Differential scanning calorimetry and surface fracture analyses were undertaken to understand this specific behaviour. Experiments showed that adhesive glass transition was crossed during aging, and that cavities developed during sorption. Part of the water diffused in the matrix causing the decrease of the adhesive T g while the other part entered into cavities. To model the water uptake and swelling of the adhesive, it had been chosen to separate the water mass fraction into two parts. A thermo-diffuso-mechanical modelling based on a thermodynamic framework was proposed to solve the diffusion problem. The model was implemented in a user subroutine for ABAQUSTM and a 2D finite element modelling was designed to obtain the simulated diffusion behaviour of the adhesive. Mass uptake and swelling were simulated for different aging conditions and confronted to experimental results. Although a slight deviation was observed between experimental and modelled swellings at 60 and 70 1C, numerical results were satisfactory at mass uptake at 25, 40, 60 and 70 1C so confirming previous hypotheses.

The aging in water of steel/composite glass polyester specimens bonded with an epoxy adhesive has been studied at 25° and 40°C. The damage processes are controlled by water diffusion. The main damage is due to the adhesive swelling but, when specimens are loaded during immersion, the stress gradient at the extremities of the joint accelerates the water diffusion and then the failure of specimens. A simple logarithmic relationship between the specimen life time and the applied load makes easy the life prediction.