In recent years, energy conservation has become more important than ever, especially in the mobility sector. Therefore, mechanical joining technologies have gained importance due to the lower energy requirements compared to welding or soldering. In this way, mechanical joining technologies such as self-piercing riveting (SPR) and clinching can be used to join different material combinations. Mori et al. [
1] compared self-piercing riveting (SPR), clinching and spot-welding of aluminum sheets with respect to the static and fatigue properties. In [
2] it was shown that welding of aluminum and steel sheet materials leads to the formation of brittle intermetallic phases, which significantly influence the joint properties. Compared to self-piercing riveting, clinching does not require any auxiliary parts [
3]. In clinching, two sheets of materials are joined together by cold forming. However, using a laser assistant the process can be realized as a warm forming process [
4]. In clinching process, a punch moves downwards and presses the materials into the die cavity. As a result, the quality-relevant geometric parameters such as interlock, bottom thickness and neck thickness are formed. Bielak et al. [
3] presented a method to study the joinability of material combinations in the numerical simulation of clinching process based on the geometric properties of the joint. Furthermore, Ewenz et al. [
5] investigated the influence of tool geometry on the mechanical and fatigue properties of the clinched joints. Lee et al. [
6] investigated the mechanical properties of the clinched joints subjected to different loading conditions. These studies demonstrate the complexity and versatility of the clinching process and its impact on the structural reliability. However, in the work of Abe et al. [
7] and Lin et al. [
8], it was reported that the characterization of component reliability of clinched structures is primarily based on phenomenological testing and evaluation of individual joint designs. In fact, clinching can be considered a versatile process, as it can be individually adapted to the sheet thickness, the material and, in particular, the tool geometry. This versatility becomes a disadvantage when it comes to design of clinch and considering the complexity of influencing factors. Typical variations in sheet thickness [
9], material [
10] or tool geometry [
11] lead to significant changes in the mechanical properties. In addition, the arrangement of the clinch points influences the mechanical properties [
12]. Su et al. [
10] and Zhang et al. [
13] reported the complexity of failure behavior of clinched joints subjected to the fatigue loads. Consequently, it would not be possible to transfer the results with respect to the characteristics values as well as process window to other joints. In this regard, Krause and Chernenkoff [
14] investigated the fatigue behavior of spot-welded structures compared to mechanically joined structures. However, the results of their investigation cannot be used in case of other clinch geometries. Kim [
15] investigated the fatigue behavior of clinched joints without considering the stresses and strains induced during the clinching process. The results of aforementioned investigations show that it is not possible to optimally design a clinch joint in advance. In addition, various series of experiments have to be carried out and evaluated. In fact, for the safe design of clinch joints, a detailed knowledge of the highly stressed areas is essential. Nevertheless, considering different load ranges (from low to very high cycle fatigue) this seems to be not sufficient due to different failure modes corresponding to the different load ranges. As well, the influence of form versus force closure is not necessarily represented in this way. To achieve this goal, experimental characterization of the fatigue behavior has to be combined with simulation-based analysis of the stress state in the highly stressed region. In this way, the complexity of the joint geometry and its various notch effects should be taken into account.
The motivation of this study is to analyze the location of fatigue crack initiation in clinched joints by considering the generated stress and strains during the joining process as well as the joint strength simulation. In fatigue analysis, the notch effect is determined by comparing the fatigue life of unnotched and notched specimens. For this purpose, experimental investigations are carried out on clinched joints of HCT590X steel sheets to analyze the failure modes under different cyclic loads. In this work, the FEM simulations of a single lap shear test is used to investigate the notch effect and analyze the fatigue strength of the joined structures. An FEM simulation of the joining process is performed to calculate the geometric properties and the plastic strains and stresses. To create the 3D model for the strength analysis of the joint, the results of the joining process are rotated 180° around the axis of symmetry, taking into account the plastic deformations and stresses. The shear tensile model is tested under specific quasi-static loads, and evaluated. Since there is no material modeling for fatigue damage of clinched joints, no crack calculation or prediction is possible. The FE simulation is mainly performed to support the experimental results. The aim is to evaluate the stress peaks as well as contact properties during different loads on the joints and to find a correlation with the experimental results in terms of cracking.