PSI - Issue 57

Cristian Bagni et al. / Procedia Structural Integrity 57 (2024) 859–871 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The fatigue behaviour of SPR and SPR-adhesive joints with sheets of different materials and thicknesses was investigated by Sun et al. (2007). Both lap shear and cross-tension specimens were tested, and it was found that the hybrid joints showed improved fatigue performance compared to the SPR joints for all the tested combinations of thicknesses and materials. Al-Samhan and Darwish (2003) modelled spot-welded and hybrid (spot-welds + adhesive) lap shear joints confirming that the addition of adhesive to a spot-welded joint increases the strength of the joint. Furthermore, it was found that hybrid joints where the adhesive bond-line has a spew fillet are stronger than hybrid joints with square edge bond-lines. Al-Samhan (2005) through a series of finite element (FE) analysis found that the strength of rivet bonded joints increased with the bond-line thickness; however, it was also recognised that the bond-line thickness should be kept as small as possible to minimise the deformation of the adhesive. Furthermore, it was concluded that when joining sheets with different thicknesses, combining adhesive and rivets helps to balance the stresses and therefore strengthens the joint. In the past twenty years, several FE modelling approaches have been used to study the static and fatigue behaviour of hybrid joints (see for example Al-Samhan and Darwish (2003), Al-Samhan (2005), Sadowski et al. (2011), Sadowski and Zarzeka-Raczkowska (2012) and Fricke and Vallée (2016)). However, most of the modelling approaches proposed or used in the literature are suitable for the analysis of simple specimen-like geometries. In particular, the most common FE analysis of hybrid joints found in the literature are performed either using solid elements or plane stress/plane strain two-dimensional elements. For more complex geometries (such as automotive body-in-white models), the use of two-dimensional plane stress/plane strain elements is not a viable option, while the use of solid elements might lead to finite element models that are computationally too onerous. Heyes et al. (2012) proposed a fracture mechanics-based approach to assess the fatigue performance of adhesively bonded and hybrid joints suitable for the analysis of full-body structures. The authors proposed a FE meshing strategy that required relatively limited changes to the meshing strategy commonly used by automotive manufacturers. However, the proposed method assessed the fatigue performance of adhesive and hybrid joints by calculating a safety factor, which is a function of the maximum value of the J-integral at each calculation point, and the threshold strain energy release rate. Therefore, it was not able to produce finite fatigue life estimations. This approach was mainly a consequence of the significant scatter and shallow slopes of the load-life data, typical of the early generation adhesive tested by Heyes and co-workers, which could produce unrealistic or very inaccurate fatigue life estimations. To optimise the design of structures joined using hybrid techniques and reduce the risk of in-service fatigue failures, the transportation industry needs efficient, robust, and easy-to-use approaches for the modelling and fatigue life prediction of hybrid joints. This work introduces two pragmatic approaches for estimating the fatigue life of hybrid joints that can be easily adopted by companies in the transportation industry. In Section 2, the two approaches are described together with FE modelling guidelines to recover the relevant stresses to be used as an input in nCode DesignLife to estimate the fatigue life of hybrid joints. The proposed methodologies enable the fatigue analysis of hybrid joints to be performed by using standard nCode DesignLife fatigue analysis engines together with bespoke SN curves obtained through testing of hybrid joint specimens, representative of the joints in the production parts, as described in Section 3. 2. Finite element modelling and fatigue analysis In this Section, two practical methodologies for estimating the fatigue life of hybrid joints, easily implementable by companies in the transportation industry, are proposed. FE modelling guidelines to recover the required stresses are suggested for both methodologies. These guidelines provide FE models that are not computationally too onerous, with a good level of mesh insensitivity and that do not require congruent meshes, with reasonably small changes to the typical FE modelling strategies currently used, especially in the automotive industry. Furthermore, it is also briefly described how standard DesignLife fatigue analysis tools can be used to predict the fatigue life of hybrid joints using the results from the FE analysis. Finally, a convergence study of the FE models obtained using the proposed guidelines is also presented.