PSI - Issue 41

P.M.D. Carvalho et al. / Procedia Structural Integrity 41 (2022) 24–35 Carvalho et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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wide difference in stiffness. A FEM analysis was carried out to support the design and identification of effective dual adhesive proportions to maximize the joint strength. In addition, experimental tests were performed to validate the predictions of the numerical analysis. Experimentally, it was concluded that the joint strength is higher (up to 22%) for joints bonded with dual-adhesives compared with those with single-adhesive. This difference occurs due to the reduced peak shear stresses obtained with the dual-adhesive joint (DAJ) leading to higher joint strengths. After evaluating different length ratios, it was found that peak shear stresses decrease with the increase of the length ratio, and reaches a minimum at about 0.5. A good correlation was found between numerical and experimental data. Temiz (2006) investigated the gain in the strength of double strap joints between 2023 T3 aluminum adherends and bonded with dual-adhesive method, subjected to a bending moment. A stiffer adhesive (3M epoxy 2214 Regular) was applied in the middle portion of the overlap, while a flexible adhesive, SBT 9244 from 3M, was applied towards the edges. Non-linear FEM analyses were carried out to predict failure behavior, to assist with the geometric design and to identify effective ratios of a dhesives’ properties to maximize joint strength. It was found that, by applying the flexible adhesive towards the edges of the overlap to reduce the stress concentration at the overlap ends, it was possible to increase the joint strength and delay failure onset when compared with joints bonded with a single-adhesive. In addition, using a flexible adhesive in a DAJ showed that the strains do not significantly increase, when compared with increase in the predicted failure load, since the stiffer adhesive has a constraining effect on the strain of the flexible adhesive. Kumar and Pandey (2010) performed a non-linear 2D and 3D FEM analysis on a dual-adhesive bonded single lap joint, considering both material and geometric nonlinearities. The modulus-graded adhesives were modelled as an elasto-plastic multi-linear material, whereas the adherends as both linear elastic and bi-linear elasto-plastic material. The 3D analysis results showed the existence of a complex multi-axial stress/strain state in the bondline at the overlap ends that cannot be observed in 2D plane strain analysis. Yet, it was observed that the use of a dual adhesive method in the L O region of the adhesively bonded joints increases the failure load of the joints. Ramezani et al. (2020) presented a comprehensive experimental analysis of dual-adhesive SLJ through the application of the digital image correlation (DIC) method. For this analysis, two different adhesives (one stiff and one flexible) were used, and different parameters were considered, namely the thickness of the adhesive and adherends, and the L O . The iteration of these parameters on the strain distribution was also experimentally analyzed by the DIC method. The authors performed a numerical analysis to evaluate the effect of the L O on the stress distribution along the bondline. It was verified that the failure starts at the interface between the rigid and flexible adhesives. DIC results showed that, by increasing the load, the point of maximum compressive peel strain moves to the interface of the stiff and flexible adhesives. Finally, it was noticed that dual-adhesive SLJ with thicker adherends have increased strength, even with higher peel strains at the ends. Akhavan-Safar et al. (2022) reviewed the benefits of DAJ and their manufacturing challenges, evaluating several joint architectures. The main conclusion was that the strength of a joint is significantly enhanced using dual-adhesive compared to the single-adhesive. In addition, several manufacturing and design recommendations are provided, aiding to achieve the optimum performance of DAJ. Aiming to improve the strength of T-joints, in this work the dual-adhesive technique is studied, consisting of using two adhesives, placing a more flexible adhesive at the overlap ends and a stiffer one in the central area. To test this method in T-joints, different adhesive ratios were implemented. To evaluate this concept, a numerical study was carried out using the CZM technique, in the ABAQUS ® software, including a study of failure modes, stresses, strength and dissipated energy.

2. Materials and methods 2.1. Adherends and adhesives

The joints were fabricated using an aluminum alloy, the AW 6082-T651. The mechanical properties of this alloy were obtained experimentally in a previous work, being E =70 GPa (Young’s modulus),  =0.30 (Poisson’s coefficient),  y =261.67 MPa (tensile yield stress),  u =324.0 MPa (tensile strength) and  f =21.70% (tensile failure strain) (Campilho et al. 2011). On the other hand, the adhesives considered in this work were the Araldite ® 2015 and SikaForce ® 7752, whose mechanical properties were also obtained experimentally in a previous work (Campilho et al. 2013, Faneco et al. 2017), and are listed in Table 1. Properties of the adhesives Araldite ® 2015 (Campilho et al. 2013), and SikaForce ® 7752 (Faneco et al. 2017).