Influence of cyclic aging on adhesive mode mixity in dissimilar composite/metal double cantilever beam joints

The adhesive layer in the adhesive joints can experience different modes of loading. Although the fracture energy of adhesive is generally considered to be a material parameter, it is found to be a function of the joint configuration too. Thus, to accurately simulate the behaviour of bonded joints, it is recommended to obtain the fracture energy of the joints using the same substrate(s) as in real applications. In some applications, it is necessary to join dissimilar substrates using adhesives. However, for pure mode fracture tests it is essential to reach the desired loading mode even in a dissimilar joint. Not only the joint configuration but also the environmental conditions need to be considered in fracture tests. In this condition, due to the aging, the stiffness of substrates and adhesive layer might change, and as a result, the adhesive may experience a mixed-mode loading condition. The current study aims to investigate the variation of the mode mixity for dissimilar double cantilever beam adhesive joints with composite/metal substrates subjected to cyclic aging. At different stages of the aging cycles, the mode mixity was calculated during the test using displacement fields obtained by digital image correlation and based on the Williams series expansion. In addition, the variation of flexural stiffness of polymer matrix composite substrates after cyclic aging was investigated using a three-point bending test Finally, based on the variation of composite substrate flexural stiffness and using the finite element method, the variation of the mode-mixity ratio was calculated numerically and compared to the experimental results. The obtained results show that during the cyclic aging the moisture diffusion decreases flexural stiffness of polymer matrix composite substrates significantly, but the variation of substrate flexural stiffness deviates the mode mixity in the aged double cantilever beam specimens.