The body construction is increasingly characterized by the use of different materials in multi-material-design, which has led to the application of a variety of mechanical joining processes. To enhance the mechanical joining processes in their efficiency, numerical simulation can be used as an effective tool to reduce the number of experiments and shorten the product development cycles. In addition to the description of the plasticity, the damage and the failure behavior of material must also be taken into account. In self-pierce riveting simulations, the rivet penetrates perpendicular into the sheet surface and produces a three-dimensional stress state. Hence, it is essential to describe the material failure as a function of a three-dimensional stress state. A simple approach to describe the separation of upper sheet in the simulation of the joining process is based on a geometric separation criterion. Such a criterion is not predictive und cannot be used in case of variations in tool configurations, sheet thickness, and material combinations. In this project, the damage model GISSMO (Generalized Incremental Stress State dependent damage Model) is used to describe the evolution of ductile damage and predict the onset of fracture during the self-piercing riveting and shear-clinching. The stress state during the process simulation is studied and the variety of damage specimens are experimental examined to characterize the failure curves. The failure curves are defined in the GISSMO damage model. To ensure the accuracy of the model, the verification of the model using simulation of damage specimens with damage model is performed. For the validation of model, the simulation of the joining process using the damage model is carried out and the results of simulation and experiment are compared. Furthermore, sensitivity analyses are performed to identify the influences of manufacturing processes, the evaluation method, and the degree of discretization on the damage behavior of material.

In modern lightweight designs, it is important to find a compromise between the strength and the weight of the construction detail. Hence, hybrid structures made of aluminum and steel materials are increasingly being used in automotive applications. Due to limitations in the quality of resistance spot welding, self-piercing riveting can be used as an alternative process to join sheets from different material groups. The aim of this project is to develop a computational method to assess the self-piercing riveted components subjected to the cyclic loads. To achieve this goal, two approaches are followed: Evaluation unsing internal forces: A substitute model is developed to describe the stiffness of self-piercing riveted joints subjected to different loading conditions. The parameters of the substitute model are identified and the internal force components acting on the joint are evaluated. The model provides the basis for the subsequent fatigue life estimation of self-piercing riveted components. For joints subjected to low bending moments, the fatigue life of components can be estimated accurately. Due to lack of specimen geometries producing pure bending and the combination of tension-bending forces, it is not possible to estimate the fatigue life of complex components subjected to high bending moments. Based on the results of [Mesc 16], the methodology is further developed to determine the stresses acting on the joint and to characterize the joining point with the use of simulations. The local concept proposed in the FKM guideline nonlinear provides the basis for the analytical assessment of self-piercing riveted components. In this regard, the cyclic behavior of the material and the local stresses are required as input data. The cyclic behavior of the aluminum EN AW-6181A-T6 and steel HX340LAD sheets were already determined in the previous project. Subsequently, in this project the properties of the rivet made of 38B2 steel are identified. The finite element analysis using elastic-plastic material behavior is used to determine the stresses in the joint subjected to the cyclic loads. To verify the model, the results of simulations and experiments are compared concerning the crack initiation zone as well as the determined number of cycles. To determine the stresses that can be used for the analytical assessment, the damage relevant load components need to be identified. In this regard, it is recommended to use the normal stress perpendicular to the crack propagation direction, the stress of crack opening mode I. Using the damage parameter PRAM and considering the support factors according to the FKM guideline nonlinear, a reliable estimation of the crack initiation zone within the joint is possible. Regarding the joint made of aluminum sheet EN AW-6181A, the methodology is able to provide promising results. However, regarding the joints made of aluminum EN AW-6181A and steel HX340LAD sheets, there is still potential to improve the results. The reasons for this are described in chapter 7.2.5 and 7.2.6. An analytical fatigue assessment is relatively easy to achieve with procedure 1. However, contrary to the objective formulated above, expensive fatigue tests are necessary to determine the failure conditions (strength values). This disadvantage can be circumvented by determining the strength information of individual joining points under different load types using procedure 2. The latter, in return, is not suitable for the assessment of complex components with several joining points. Due to the increasing calculation times of the simulation, the application in this case is not economically reasonable. By the described combination of method 1 and 2, the disadvantages of the two individual concepts can be compensated. An analytical fatigue assessment of self-piercing riveted components can be carried out based on the cyclic material behavior. The objective of the project was achieved.