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

25

2

1. Introduction At the present time, adhesive joints are widely used in the industry, replacing mechanical and welding joints in many applications. By looking to aircraft industry, one of the major persecutors of the usage of adhesively-bonded technology, one may notice a large increase over time of bonded components. Since the sixties, adhesive bonding finds an increasing use in the assembly of secondary aircraft structural parts. More recently, the structural parts of the Airbus A380 already contain bonded joints, which is indicative of the present status of the extensive use of adhesive bonding in airliners (Tserpes 2020). In addition, the experimental Lockheed Martin X-55 Advanced Composite Cargo Aircraft program demonstrated the possibility of designing and manufacturing large bonded structures featuring low temperature and out-of-autoclave curing (Tang et al. 2014). The reason for the increased use of adhesive bonding is the numerous advantages it offers over conventional methods namely, the ability to join dissimilar materials such as composites with metals and thermosets with thermoplastics, more uniform distribution of stresses, fast and cheap joining process, better vibration damping and fatigue resistance behavior, sealing properties, improved aesthetics, and aerodynamically smoother surfaces (Petrie 2007) . Nonetheless, the drawbacks include the need of adherends’ surface treatment to improve adhesives’ wettability, difficult disassembly of the joined parts and low resistance to temperature and humidity, i.e., durability limitations (Adams et al. 1997). A number of joint architectures have been made available to designers, e.g., butt, single-lap, double-lap, scarf, step, tubular and T-joints, allowing to choose the most suitable one taking into account the expected load cases, environmental conditions, fatigue, along with other factors, bearing in mind that the joint should be designed with the objective of minimizing concentration of stress (Ebnesajjad and Landrock 2014). T-joints are joints in which the parts typically form a 90° angle. They are used in applications such as automotive, marine, and aerospace industries. T-joint is a typical configuration in aircraft structures, and can be found at the interfaces between the fuselage skin and bulkhead or fuselage skin and ribs (Masoudi Nejad et al. 2022). Until few decades ago, one of the main obstacles to a widespread use of adhesively-bonded joints was the lack of models that could correctly predict the joints’ strength, which resulted in strongly oversized structures, leading to a high manufacturing cost. Strength prediction of adhesive joints started around 80 years ago with a simple analytical approach proposed by Volkersen (1938), introducing the concept of differential shear to analyze single-lap joints (SLJ). Later, some researchers, aiming to provide a more general model, evaluated the inclusion of dissimilar adherends (varying the thicknesses and material properties) or composite materials, e.g. Adams and Mallick (1992). Nevertheless, as the model gets more general, the equations become increasingly complex and require the use of a computer (Campilho 2017). Even so, it is important to notice that strength prediction by analytical methods is still useful in both the research and industrial contexts, whenever a quick analysis of a joint is needed. The Finite Element Method (FEM) arose in the mid twentieth century and soon become a popular method for numerical solving (Babuska et al. 2001). As far as adhesive bonding was concerned, FEMmodelling appeared soon to be the typical tool to analyze the joints’ behavior, overcoming the difficulties of the analytical solving method. CZM have been used as an add-on to FEM analysis to perform damage growth simulations. Currently, CZM is widely implemented in commercial finite element codes. In this method, fracture is assumed to occur by progressive separation of the crack surfaces ahead of the crack tip, and by assuming that the material along the crack path follows the specified traction – separation law of an appropriate cohesive zone model. The cohesive law is represented by three parameters, i.e., the critical energy release rate ( G C ), the critical cohesive failure strength ( t 0 ) and the shape of the traction – separation law. The triangular and trapezoidal CZM law shapes are most used for strength prediction of structural materials. In addition to the strength and failure prediction, the evaluation and implementation of techniques that promote the strength improvement of joints is of most importance to promote a widespread usage of adhesively-bonded joints. Several methodologies have been proposed to reduce the stress concentration at bonding ends and to obtain a more uniform stress distribution along the bondline, such as optimization of the L O or the adhesives’ thickness (Oliveira et al. 2019), adding outer or inner chamfers (Moya-Sanz et al. 2017), using adhesive fillets (Luo et al. 2016) and dual adhesives. The dual-adhesive joining technique was initially proposed by Raphael (1966). The procedure includes a flexible adhesive at the ends of the overlap, which is a region of high stresses, and a stiff adhesive in the middle of the overlap, which is a region of low stresses. Few works can be found in the literature related with dual-adhesive method. Pires et al. (2003) evaluated the SLJ strength performance using the dual-adhesive method. To bond aluminum adherends, two epoxy adhesives were used. The stiffer Permabond ESP110 was applied in the middle portion of the overlap, while the softer adhesive, DP490 from 3M, was applied towards the edges, therefore providing the desired