PSI - Issue 28

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Vinicius Carrillo Beber et al. / Procedia Structural Integrity 28 (2020) 1950–1962 V.C. Beber and M. Brede / Structural Integrity Procedia 00 (2019) 000–000

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railway industry, especially for the joining of composite structures (Budhe et al., 2017). An adhesive is a “non-metallic substance which can join two substrates together via bonding to the substrate surfaces (adhesion) and via its own internal strength (cohesion)” (DIN EN, 2008). When compared to welding, adhesive bonding (a low heat process) presents several advantages, namely: no heat-affected zone, joining of very thin adherends, and joining of dissimilar materials. In comparison with traditional mechanical fasteners, adhesives have: no hole requirement, continuous load distribution, and lower fabrication costs related to the cost of fasteners and drilling tools (da Silva et al., 2011; Wolter et al., 2020). Engineering components can be classified in primary and secondary components depending on their relevance with regard to structural integrity (Sonsino, 2007): primary components are load-bearing elements in the structure, whereas secondary components are not load-bearing. Adhesively bonded structures in railway vehicles can also be divided following the aforementioned classification, i.e. primary structures (relevant for structural integrity) and secondary structures (not relevant for structural integrity). According to each type of structure, the class of adhesive applied might change. For instance, structural adhesives are usually applied in primary structures (e.g. walls) in which high strength/stiffness and low deformation are required, as well as thin adhesive layers are used (Adams et al., 1997). On the other hand, elastic adhesives are usually applied in secondary structures (e.g. window) in which low strength/stiffness and high deformation are required, as well as thick adhesive layers are used (Habenicht, 2009). During operation, railway structures are exposed to static stresses (e.g. start/stop operation, impacts) or cyclic stresses (e.g. vibration) with the latter making materials susceptible to fatigue failure. Due to the polymeric nature of adhesive materials, the presence of stress multiaxiality can be especially critical (Ward and Sweeney, 2012). Stress multiaxiality, which occurs due to multiaxial loads (e.g. tension/torsion) (Kosmann et al., 2018) or due to adherend constraining (Wang and Chalkley, 2000), can be associated with high levels of hydrostatic stresses (Beber et al., 2017), which have a substantial effect on the mechanical behaviour of adhesive materials (Beber et al., 2019). Therefore, it expected that a proper characterisation of the mechanical behaviour of adhesives under different multiaxial stress levels is a key aspect for understanding of the long-term durability of bonded joints for both static and fatigue loads. 1.2. Aim of the research work In this background, the present research work deals with an investigation of the multiaxial behaviour of elastic (polyurethane-based) and structural adhesives (epoxy-based) for railway applications. Samples with different stress multiaxialities (butt joint, scarf joint, and thick adherend shear test joint) were tested under static and fatigue conditions. Finite Element Analysis was carried out to assess the distributions of stress multiaxiality within the adhesive layer of each type of joint for both adhesive classes. Finally, an explanation for the static and fatigue strength was proposed taking into consideration the stress multiaxiality on the adhesives under investigation. 2. Definition of multiaxiality The state of stress in any point in the adhesive layer can be described by a stress tensor as given in Eq. 1. The stress tensor is made of normal stresses ( ii  ) and shear stresses ( ij  ):

11                12 21 22 ij

    

13

(1)

23



31

32

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By coordinate rotation of the reference system it is possible to derive a stress representation where shear stress components ( ij  ) are zero. In this new tensor, the maximum principal stress tensor ( MP ij  ), has only normal stress