Adhesive joints are frequently exposed to cyclic ageing conditions during their service life, which can have a substantial impact on the mechanical properties of both the adhesive and the substrates. The safe life philosophy, commonly employed in the design of bonded joints, underscores the importance of obtaining an accurate estimate of the adhesive's durability. Therefore, it is essential to enhance the predictive capabilities of the adhesive's mechanical behavior under cyclic ageing conditions. This research aims to expand the use of quasi-static cohesive zone modelling (CZM) for damage and fracture analysis of dissimilar adhesive joints subjected to cyclic ageing environments. The first step involved measuring the mechanical properties of the adhesive through tensile tests on unaged and cyclically aged dogbone specimens, considering their moisture content and ageing cycles. Based on the results, a degradable CZM was developed. To validate the numerical model, dissimilar double cantilever beam specimens (DCBs) of glass fibre reinforced polymer (GFRP) and aluminium were manufactured and tested before and after ageing. The load-displacement curves of the bi-materials bonded joints were successfully predicted using the developed model where the properties of the material are defined as a function of the moisture uptake and ageing cycles at each material element. The obtained results showed that after 4 ageing cycles, the maximum load of DCB specimens decrease considerably.

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.

In some industrial applications, adhesive joints are cyclically exposed to a moist environment, where cyclic moisture absorption and desorption can significantly alter the fracture energy of the bonded joints. Most previous studies are based on monotonic aging conditions, while the performance of bonded joints under cyclic aging is not well explored The aim of the current study is to investigate the effect of cyclic aging on mode I fracture energy of dissimilar DCB (double cantilever beam) adhesive joints. Accordingly, bulk adhesive plates were manufactured and exposed to 4 aging cycles. After the aging process, at different exposure times, the aged adhesive plates were used to bond dissimilar Al/GFRP substrates using a secondary adhesive. Then the prepared DCBs were tested and subsequently the mode I fracture energy of the adhesive was determined. Meanwhile, using gravimetrical tests and numerical simulation, moisture diffusion of the adhesive layer in different exposure times was analysed. Using experimental and numerical results, the variation of fracture energy as a function of moisture uptake was studied. In addition, glass transition temperature (T_g) and chemical bonding of the aged adhesive were analysed in different aging cycles. The results showed that by increasing the number of aging cycles, the reduction rate of mode I fracture energy between the aging cycles decreases.

Nowadays, dissimilar adhesive joints are used widely in different industrial sectors. For this type of joints, the investigation of the fracture behaviour under pure mode I loading is a vital issue. In order to study mode I fracture parameters of adhesive joints, scientists have proposed standard double cantilever beam (DCB) specimens. In dissimilar DCB adhesive joints, because of material asymmetry, obtaining pure mode I is challenging. To obtain pure mode I loading in dissimilar DCB joints, scientists have proposed two criteria based on (1) the flexural stiffness and (2) the distribution of the longitudinal strain along the bondline. However, the achievement of pure mode I in dissimilar DCB joints needs more research. In this paper, the mode-mixity in dissimilar DCB joints designed based on the two mentioned criteria is investigated numerically and experimentally. Metal-composite DCBs were fabricated and tested. During the tensile tests, the horizontal and vertical displacement fields of the adhesive layer were calculated using digital image correlation (DIC) and the finite element method (FEM). Based on the obtained displacement fields and using Williams’ series expansion, the mode-mixities were investigated. The results show that in order to achieve pure mode I loading in steel-composite joints, the criterion based on longitudinal strain is more suitable and in aluminium-composite joints the mode-mixities given by two criteria are almost the same.