PSI - Issue 19

Jan Presse et al. / Procedia Structural Integrity 19 (2019) 423–432 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

The importance of lightweight design is more and more increasing. Not only to fulfill reduced emission targets from legislation for cars with internal combustion engines (ICEs), but also to consider different requirements in the design of battery electric vehicles (BEVs). BEVs need light but strong materials to fulfill requirements regarding the safety of battery packs and the weight of the body in white. Since emission targets not only affect premium car manufactures, existing solutions for lightweight design need to be optimized to adopt them in the volume segment as well. An affordable and promising multi-material combination is aluminum and high strength steel. Mechanical joining such as Self-Piercing Riveting (SPR) in combination with structural adhesive allow assembling dissimilar material combinations. Besides higher manufacturing costs for mechanical joining compared with conventional welding, the developing process is still very expensive. Most of the existing approaches to estimate fatigue behavior of self-piercing riveted connections are usually only applicable for a small range of configurations or they need complex simulated models, which leads to a loss of calculation performance [1]. A commonly used fatigue damage parameter in such approaches is the structural stress, developed for resistance spot welds (RSW). Rupp et al. [2] calculated structural stress values for RSW based on cross sectional forces and moments obtained from the finite element analysis. A regression through the calculated structural stresses and fatigue life leads to the reference master SN-curve. Several studies investigated the behavior of structural adhesives and hybrid joined connections under quasi-static [3, 4, 5] and cyclic loads [6, 7, 8, 9]. Especially for configurations under lap shear loads the adhesively bonded and hybrid joined connections show superior performance compared to welded or riveted connections. A promising approach was developed by Schmidt [9], who used an effective stress approach to generate an SN-curve as a reference to estimate the fatigue life of connections with the structural adhesive Betamate 1496V from Dow. This study wants to extend the expertise regarding the fatigue performance of hybrid joined multi-material thin sheet connections. With the insights from the experiments, new methods should be developed that are flexible, robust and easy to use (e.g. for use in transient full vehicle durability simulations). The methods should enable a good estimation of fatigue life to gain optimized solutions according to lightweight critical parameters such as sheet thicknesses and material properties, amount of joints, etc. 2. Nomenclature rivet diameter sheet thickness T temperature

frequency load ratio exponent, characterizes slope of SN-curve equivalent structural stress radial stress , internal forces in x- and y-direction , internal moments in x- and y-direction effective stress , maximum major principal stress 1, maximum von Mises stress scatter Number of cycle to failure