Durability modelling concepts and tools for the cohesive environmental degradation of bonded structures

This paper presents a framework that has been developed to enable the environmentally degraded response of bonded structures to be assessed. This framework includes both interfacial and cohesive weakening. Numerical tools are outlined which enable the implementation of this framework for structures where cohesive (rather than interfacial) weakening determines the degraded structural response. The application of these tools is illustrated for the case of a single-lap joint. Initially, the effect of the adhesive fillet on the kinetics of moisture diffusion through the joint is quantified and an approximate analytical model presented. This is followed by assessing the residual strength of an aluminium (2024-T3) lap joint bonded with FM1000 that has been exposed to 30 days immersion in water. This modelling approach is based on a coupled mechanical-diffusion analysis which has been solved by using finite element analysis principles. Coupled non-linear finite element analyses of both dry and wet joints are undertaken. Appropriate diffusion data and moisture-dependent adhesive material properties have been used in the wet joint analysis to determine the spatial variation of moisture and hence the variation in material response throughout the adhesive layer. It is shown that the wet and dry joint strengths can be predicted from known adhesive failure strains and further that failure would initiate at the centre of the wet joint. (C) 1997 Elsevier Science Ltd.