PSI - Issue 47

J.B.S. Nóbrega et al. / Procedia Structural Integrity 47 (2023) 408–416 Nóbrega et al./ Structural Integrity Procedia 00 (2023) 000–000

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1. Introduction Adhesive bonding is nowadays industrially used in many areas over mechanical joints, due to advantages such as more uniform stress distributions, cost reduction or the possibility to join different materials (Adams et al., 1997). Nonetheless, particular attention should be paid to surface preparation, environmental degradation, or geometry induced stress concentrations. Although peel loads do not favour the strength of the adhesive bond, in cases where these loads cannot be avoided, joints are designed to resist these solicitations (Cañas et al., 2018). However, peel loads in adhesive joints are particularly critical, since adhesives have a low capacity to withstand this type of effort (Petrie, 2007). Peel tests are characterized in the first instance by the materials used as adherends, which can be flexible or stiff. There are two main variants of peel joint type, these joints may have the purpose of pulling off only one end/fixed arm, such as the floating roller peel test, or pulling both ends like in the T-peel test. The wedge peel test is an impact test aiming to aims to define the peel strength of an adherend, not through a continuously applied force, but through the absorption of energy in a short period of time (Back et al., 2019). The peel test at 90 or 180º is normally used to characterize adhesion between stiff and flexible adherends. The test is carried out using a tensile testing machine, and the loading consists of applying a constant displacement (Sugizaki et al., 2016). The T-peel configuration is often chosen to characterize adhesion between two flexible adherends, leading to the total transmission of loads to the adhesive joint. Thus, it is one of the methods that require lower applied forces to induce peel failure. In the T-peel test, the two adherends are joined by a layer of adhesive and the ends are subsequently pulled in opposite directions (Li et al., 2019). To carry out the test, no special apparatus is needed since, in most situations, the adherends’ thickness leads to easy bending to be fastened to the grips. The floating roller peel test is regulated by the ASTM D3167 standard for stiff and flexible adherends bonded by means of an adhesive. The standard indicates that, as a result of its high peel angle, the test is more severe compared to the climbing drum test, producing a high number of repeatable results. The test consists of pulling the end of the flexible adherend off the surface of the stiff adherend, with an angle controlled by an accessory with a particular geometry defined in the standard (Arouche et al., 2018). The flexible adherend is attached to the lower grip, while the stiff adherend rests on the two floating rollers to start the test. After fixing the specimen, the test is started at a speed of 152 mm/min. The climbing drum test is used when the adherends are not flexible enough to reproduce the more common peel tests. In this test, the most flexible adherend is placed around a rigid cylinder, called a “climbing drum”. This test is useful to estimate peel loads in sandwich structures, when tested under specific conditions and is commonly used in the aeronautical industry to assess the peel strength between skins and honeycomb cores or foams, in ultralight sandwich panels (Yuan et al., 2008). Work is available in the literature regarding the application of peel tests to adhesive, either experimental or numerically based, leading to the peel characterization of the adhesive under different material and geometrical configurations (Mozelewska and Antosik, 2022, Gohl et al., 2021). Pereira et al. (2022) evaluated experimentally the behaviour of the structural adhesive Araldite ® AV138 under peel loads in aluminium-composite, composite composite and aluminium-aluminium joints using the floating roller peel test. The comparison of the behaviour of the various materials involved the analysis of the peel loads not only, but also the analysis of the failure modes. Data analysis showed significant differences in peel strengths as a function of adherend materials. Moreover, composite interlaminar failures were found for the joint configurations with flexible composite adherends. Overall, the best peel strengths were found for the aluminium-aluminium joints and the worst for the aluminium-composite configuration. Liprandi et al. (2020) presented a new theoretical-numerical model, using a three-dimensional cohesive zone model, capable of simulating the delamination of elastic membranes from a stiff adherend. This approach could be applied in the study of complex problems with heterogeneous membranes, complex geometries, or textured surfaces. It was proved that, for the use of a membrane with known mechanical properties, it is possible to establish a direct relationship between the peel load and the peeled length. In this work, a methodology to model the FRPT was developed and validated with experimental data. The proposed numerical model employs the FEM and CZM and accurately represents the ASTM D3167 standard, including the interactions with the experimental device. In this case, a two-dimensional approach was followed. Furthermore, an explicit solver was employed due to the inherent non-linearities of the FRPT. As a case study, the adhesive Araldite® 2015 was modelled and tested experimentally.