PSI - Issue 42

D. Weiß et al. / Procedia Structural Integrity 42 (2022) 879–885

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Author name / Structural Integrity Procedia 00 (2019) 000–000

1. Introduction Joining is used in many areas of manufacturing, for example in household appliances or in the electrical industry. In these areas, components or metal sheets are formed into complex assemblies, Geoffrey (2012) and Liebig (1992). One of the most common areas is automotive manufacturing. There are strict regulations to reduce harmful pollutants. By decreasing the weight of the structure, this aim can be achieved, Friedrich (2017). The weight is minimized with help of multi-material design, where the materials are used in the right place, which have specific mechanical properties, Barnes and Pashby (2000). By utilizing different materials in components, the joining process becomes complicated and the joining method is limited. Therefore, clinching can be applied to joining dissimilar and coated materials in industry due to its simplicity and common applicability, Meschut et al. (2014). Moreover, clinching has the advantages of not requiring consumables or pre-drilled holes, causing low costs and weight, Dietrich (2018). A picture of the clinching process is shown in Fig. 1. In the first step of the process cycle, the sheets are arranged on top of each other and clamped between the blankholder and the die assembly. Then the punch is pressed onto the sheets and they are pushed locally into the die in a second step. As soon as the sheets touch the bottom of the die, the material flows radially and a further downward movement of the punch forms a button. This third step ensures mechanical interlocking by material flow. The sheets are held firmly together by this step. At the end of the process, the punch is retracted, Deutscher Verband für Schweißen (2009). This joining method is appropriate for structures made of ductile sheets that have thicknesses between 0.2 mm and 4 mm, Eshtayeh et al. (2015). To save weight, the materials and components are designed to the limit of static strength. However, it is also important to consider fatigue strength, crack sensitivity and the achievable crack growth lifetime, Richard and Sander (2016). Cracking can occur both during the joining process and in service due to fatigue and corrosion. A number of studies have been published in the past on the fatigue behavior of clinched structures. For example, there have been investigations identifying crack initiating positions during the clinching process, Kim and Kim (2014), Lambiase and Di Ilio (2016) and Coppieters et al. (2017). These cracks can affect the fracture mechanical behavior, especially in the vicinity of the joint, and can cause crack propagation due to cyclic loading, which subsequently leads to failure of the entire component. Therefore, further knowledge and investigations on fatigue crack growth behavior in clinched structures is necessary. In this context, a new specimen geometry, the so-called CC-specimen, is taken directly from a clinched connection (see Fig. 1) in order to identify a possible influence of the outer forming area of the clinched joint on the fatigue crack growth rate.

Fig. 1. Illustration of the clinching process, the clinched connection and the fracture mechanical investigation.

2. Geometry of the CC-specimen To examine the influence of the clinched joint area on the crack growth rate, the geometry of the CC-specimen is crucial. It is of particular importance that the specimen is taken directly out of a clinched structure. Furthermore, it should be mentioned that the thickness of the sheets under investigation is only 1.5 mm. Such thin sheets were also used in Weiß et al. (2021) to determine the fracture mechanical parameters of the base material with help of an extended Mini-CT specimen. A schematic illustration of the extraction of the CC-specimen is shown in Fig. 2. For