PSI - Issue 28

D. Weiß et al. / Procedia Structural Integrity 28 (2020) 2335–2341 D. Weiß et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction In many areas of product manufacturing, constructions are made from individual components to build up complex structures. Today many areas of applications have these more or less complex structures like in the appliances und electrical industry or most common the car manufacturing, Liebig (1991) and Geoffrey (2012). Especially in the field of car manufacturing, lightweight constructions are very important in order to reduce harmful pollutants, Friedrich (2017). Focusing on carbon dioxide emissions the EU tries to reduce these emissions with legal regulations. According to the current regulation (EU) 2019/631, the target for the average carbon dioxide emissions of the new passenger car fleet in 2030 is a 37,5 % reduction compared to 2021 , EC-European Commission (2019). To achieve this aim, car manufacturers try to reduce the weight of the structure by using multi material design to get the best mechanical properties out of the materials, see Barnes and Pashby (2000). However, these different material combinations make the joining process more difficult and limit the choice of the joining method. The complexity will even increase in the future because of the versatility of assemblies due to different materials and combined joining processes, Gude et al. (2018). Since mechanical joining processes have so far been rigidly configured for the respective material-geometry combination, the Transregio 285 (TRR285) deals with mechanical joining in versatile process chains. In this project, mechanical joints are used as an example to illustrate scientific methods for the adaptability in the sphere of action of the three areas of material (joining suitability), design (joining safety) and production (joining possibility) as well as for the reliable prognosis and design of the joinability. A joining method that can join different and coated materials without additional heat input is clinching. Clinching is a mechanical joining technique for point joining of sheet metal components. It is suitable for ductile metal sheets with thickness between 0.2 mm and 4 mm. Clinching requires no consumables or pre-drilled holes and is performed in a single step, making it an inexpensive and simple technique combined with lower costs and weight reduction, Eshtayeh et al. (2015). A typical process cycle of this technique is shown in Fig. 1. The sheets are initially clamped between the blankholder and the die assembly (stage 1). The punch is then forced onto the sheets, and locally pushes them into the die (stage 2). As the deformed sheets touch the bottom of the die, further downward movement of the punch forces the material to flow radially and to form a button. This material flow provides the mechanical interlock which holds the sheets tightly together (stage 3). Finally, the punch is retracted (stage 4), Pietrapertosa et al. (2003) and Böllhoff (2015).

Fig. 1. Process cycle of clinching according to DVS EFB 3420 (2012).

There are many factors influencing the joint like among other things the material, the die, the join parts, the punch or joining force and the blankholder force, Böllhoff (2013). Due to all these influencing factors during the joining process and due to the service load, cracks can occur in the area of the joint. Within the framework of the subproject "Crack growth in joined structures" of TRR285, the cause-and-effect relationships are analyzed from a fracture mechanical point of view with regard to the safety and the load-bearing capacity of the joined parts. Hence, the subproject deals with crack growth and tries to answer the following questions with regard to the mechanically joined structures:  Does a crack grow?  How fast does a crack grow and in which direction does it grow?  When does the unstable crack growth start?