PSI - Issue 54

D.F.T. Carvalho et al. / Procedia Structural Integrity 54 (2024) 398–405 Carvalho et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Recently adhesive bonding has gained the attention of the transportation industries such as aerospace/aeronautics, automotive, rail and maritime (Petrie 2013, Boutar et al. 2016, Suzuki 2018, Hart-Smith 2021). When compared to more traditional bonding processes, such as bolting, riveting or welding, adhesive bonds have several advantages, such as more uniform distribution of stresses, less weight gain, the possibility of joining different materials and with complex shapes, good vibration damping, high resistance to fatigue loading, lower manufacturing costs, and many more (Ramírez et al. 2020, Desai et al. 2023). The most often used geometry is the single-lap joint owing to its relative manufacturing simplicity (Tsai and Morton 1994, Öztoprak and Gençer 2023). But when submitted to a tensile load, the asymmetry of the load transfer results in deflection of the joint, giving rise to considerable peeling stresses (  y ) at the overlapping edges of the adhesive layer that have a negative impact on the joint's performance (Hart-Smith 1973). In order to minimise the disadvantages described above, various geometries of adhesive joints have been proposed, such as the double-lap, scarf, stepped, among others (Adams et al. 1997, Petrie 2008). The stepped-lap joint provides several benefits over the single-lap joint regarding the reduction of peak stresses  y and  xy and present a better visual appearance. The main disadvantage of the stepped lap joint is that its manufacturing process is more complex and involves expensive machining operations. An alternative way to reduce the stress gradients is the use of mixed adhesive technique, where the bond line is formed by two adhesives, ideally a stiffer one is placed at the middle of the adhesive layer and a more flexible one at its extremities (da Silva and Adams 2007). Various methods can be used to predict the strength of adhesive joints, such as analytical methods (Volkersen's pioneering formulations (Volkersen 1938)) or numerical methods, like approaches based on the Finite Element Method (FEM) (He 2011, Tserpes et al. 2021). One of the most widely accepted and used methods is Cohesive Zone Modelling (CZM), which, when combined with FEM and employing fracture mechanics principles, enables accurate prediction of the strength of adhesive joints. The CZM method has been extensively used to simulate the initiation and propagation of delamination’s in composite materials or to analyse cohesive and interfacial flaws (Saeedifar et al. 2017). The dual-adhesive method was first presented by Raphael (1966) in 1966 and has been widely applied to single lap joints (Breto et al. 2017). With this technique it is possible to increase the strength of the joint and also to design safe multi-stage joints for specific operating conditions. In da Silva and Adams (2007), the authors analysed experimentally the dual-adhesive joint (DAJ) concept submitted to high and low temperatures. Additionally, the authors also considered adhesive joints with the same material (titanium alloy) and different materials (titanium and composite). They concluded that DAJs provide a stronger joint when varying from low to high temperatures, in comparison to single adhesive joints (SAJs) suitable for high temperatures. When it comes to dissimilar adherend joints (titanium/composite), the authors found that DAJ joints are more resistant, particularly if materials with a high coefficient of thermal expansion are involved. The stepped-lap adhesive joints have been analysed by several researchers (Kimiaeifar et al. 2013, Sancaktar and Karmarkar 2014, Durmuş and Akpinar 2020) . The work presented in Sawa et al. (2010) investigated experimentally and numerically the stress distribution in step joints, of aluminium and steel adherends bonded with a structural epoxy adhesive, under flexural loading. The authors observed that the stresses are concentrated at the edge of the stiffer adherend interface. Furthermore, decreasing the Young's modulus ratio between the two adherends, as well as the thickness of the adhesive ( t A ), results in a reduction of the maximum stress. In general, adhesive joints made with adherends of different materials have a lower resistance than joints made with adherends of the same material. This study focuses on analysing the strength of both single-adhesive and dual-adhesive single-step joints with aluminium adherends of the AW 6082-T651 alloy. Three commercially available adhesives, spanning from brittle to ductile, were analysed and different overlap lengths ( L O ) were also addressed. Additionally, different combinations of adhesives were explored within the context of the dual-adhesive technique. The preliminary experimental work served mainly to validate the accuracy of the numerical models. In the numerical work, a study was carried out to predict the strength and energy at failure considering a triangular cohesive zone model (CZM). The accuracy of the CZM was effectively confirmed through a comparison with experimental results.