Durability of adhesively bonded joints in humid environment considering viscoelastic behavior

This research is concerned with the viscoelastic effect in durability of adhesively bonded joints at aggressive environmental condition. Structural adhesives due to viscoelastic nature shows rate dependent material behavior, the redistribution of stress-strain curves in adhesive joints during viscoelastic deformation influences the strength of the joints considerably. Moreover, moisture has degrading effect on joint durability. A single lap joint configuration using Aluminum adherents with FM73-M adhesive immersed in water for up to 1 year at room temperature and 500C. Diffusion of moisture to the bulk adhesive was determined by gravimetric experiments, also concentration profile of moisture diffusion found experimentally by previous researchers. Using Creep tests at various stress levels for characterizing viscoelastic parameters as a prony series of FM73-M at room temperature &500C has been carried out by many researchers. In this paper, initially moisture diffusion modeling was undertaken by using the analogy between heat transfer and diffusion analyses for the single lap joint. Then, fully-coupled visco-elastic stress-diffusion analysis with finite element modeling was carried out. The results exhibited a reduction in stress concentration and improved stress distribution due to viscoelastic adhesive behavior for the single lap joint. Finally, progressive damage was modeled using the cohesive zone model with a moisture dependent bi-linear traction-separation law. Furthermore, beside the cohesive elements, continuum elements were employed to incorporate the viscoelastic behavior of the adhesive. A good agreement was found between the predicted residual strength and the experimental results. Moreover, it was found that including visco-elastic behavior caused longer predicted life for the adhesive joint which was in opposite of the deleterious moisture effect on the joint durability in long term exposure.