PSI - Issue 41

T.F.C. Pereira et al. / Procedia Structural Integrity 41 (2022) 14–23 Pereira et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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increasingly used in many applications and industrial fields, from the simplest shoemaker to the high-tech aerospace. In addition, this joining method is also recommended for impact scenarios, with emphasis to collision performance improvement in the automotive industry (Valente et al. 2019). The advantages of adhesive bonding include the integrity conservation of the parent materials, the possibility to join different materials, and also the more uniform stress distributions than riveting or welding (Adams et al. 1997). Adhesive bonds also promote good strength-weight and cost-effectiveness ratios, excellent fatigue behavior and corrosion resistance. Nonetheless, few drawbacks can be appointed, namely the disassembly difficulty without causing damage, low resistance to temperature and humidity, the prerequisite of a surface treatment, and joint design orientated towards the elimination of peel stresses (Petrie 2000). Several joint architectures are available, allowing the designers to choose the most suitable joint depending on the applicability and load cases, although the most common are SLJ, double-lap joints, and scarf joints (da Silva et al. 2011). Reliable strength predictions methodologies applicable to all relevant case of studies, i.e., load cases, adhesive types, environmental conditions, are required to promote an even more extensive use of adhesively bonded joints. The two big methodologies available for strength and failure analysis are the analytical and numerical methods. Several analytical models were proposed and enhanced along the years (Volkersen 1965, Hart-Smith 1973). Analytical methods provide a faster response, nonetheless, the strength prediction of adhesives with a high degree of plasticity, thermal effects issues, and advanced composite materials become a difficult and time-consuming task. Few approaches were introduced for strength prediction, namely continuum and fracture mechanics (da Silva and Campilho 2012). In addition, Barenblatt (1959) proposed the CZM approach to detect the crack growth behavior in perfect brittle materials. Moreover, Dugdale (1960) assumed a process zone at the crack tip making this approach also suitable to analyze perfect plastic materials. This method is based on traction-separation laws between stresses and relative displacements to predict damage initiation and propagation in adhesively-bonded joints and composites along established paths. Accurate fracture toughness and cohesive parameters determination are required to obtain the maximum efficiency of this method. Impact loadings in adhesively-bonded joints are relevant for many industrial applications. Therefore, the simulation of joints’ behavior under impact is a very active discipline, mainly empowered by the automotive industry. Few works addressed the strength prediction of adhesive bonded joints subjected to impact loads. Al-Zubaidy et al. (2011) performed an experimental evaluation on the effect of impact loads on the bond strength, effective bond length and failure modes of adhesively-bonded joints. Two mild A36 steel plates were adhesively bonded with Araldite ® 400 in a double strap joint configuration, using one and three plies’ straps made of carbon -fiber reinforced plastic (CFRP). Static and impact tensile tests results were then compared to evaluate the influence of the loading rate. The authors found that the bond strength could be significantly increased with dynamic loads mainly due to the shear strength improvement promoted by an epoxy adhesive between steel and CFRP. In addition, the strength variation with L O (from 10 to 60 mm) was evaluated and a small influence of the impact loads on the effective bond length was found. Additionally, the failure mode changed with the increase of impact loading, i.e., from CFRP and adhesive interface failure to CFRP delamination and CFRP rupture for specimens with one and three layers, respectively. This failure mode change is due to the shear strength improvement of epoxy adhesive causing an increase in bond strength. To compare the differences in the behavior between ductile and brittle adhesives, Kemiklioglu et al. (2015) tested two adhesives with similar strengths but distinct ductility under axial single and axial repeated (dynamic) conditions for different impact energies (from 5 to 20 J with 5 J intervals). Adherends manufactured with glass fiber reinforced polymer were bonded with a ductile and brittle adhesive (Scotch DP-460 and Loctite-9466, respectively), to produce SLJ for testing. Tests with ductile adhesives showed higher strength in the single impact and repeated impact tests at 15 J. In contrast, the brittle adhesive (Loctite 9466) showed higher values of strength for repeated impact of 5 and 10 J. For both adhesives, the 10 J tests impact provided the highest strength. Morin et al. (2013) proposed a simplified macroscopic three-dimensional cohesive element for collision engineering. The originality was based on a new cohesive element capable of representing an elastic-viscoplastic behavior, with the ability to model the initiation and propagation of cracks in the material. In addition to the reduced computational time, the great advantage lies in the fact that its calibration is simplified compared to existing cohesive elements. The geometry is based on an element developed at the interface between two approaches, namely the continuous cohesive zone and discrete CZM methods. These elements consist of an 8-node hexahedron, which can be seen as a generalized spring between two opposing structural elements. This discrete spring is connected to