Modeling of component failure due to notch effects in press-hardened steel caused by mechanical and thermo-mechanical joints under crash load

The increasing application of press-hardened strength steel in combination with aluminum sheets in the construction of car bodies results in the use of mechanical joining techniques such as self-piercing riveting and thermo-mechanical joining techniques such as friction and resistance element welding. These joints generally represent a notch within the component. The cause of the notch effect is different for the investigated joining techniques. Riveted joints result in a pierced hole with high plastic strains at the edge. Thermo-mechanical joints in press-hardened steel result in a softening zone around the weld due to the applied heat during the joining process. All of these notch effects can be critical for the structural integrity of a component under crash load. Due to limited computational resources, a detailed modeling of joints within a component simulation is not possible. Thus, simplified modelling approaches such as the *CONSTRAINED_INTERPOLATION_SPOTWELD (model 2) are applied in order to represent the load bearing capacity and failure behavior of the joints. These simplified models are usually used with shell elements for the sheet metals, whereby no notch effects such as pre-damage or differences in the material behavior are taken into consideration. Therefore, crack initiation in plane of the sheet metal due to these notch effects cannot be described in the simulation. Subject of this research project is the development of a modeling approach, which takes material failure due to notch effects at mechanical and thermo-mechanical joints into consideration. T wo distinct approaches are developed for the two investigated notch effects. Both of the approaches are based on an explicitly modeled notch zone at the joint. For the mechanical joining techniques, a hole in the press hardened steel is modeled, with an additionally defined pre-damage for the shell elements surrounding the hole. For the thermo-mechanical joints, a softening zone with modified flow and failure behavior is modeled. The modelling approaches are calibrated based on experimental results of tensile and punch tests, which have been conducted with notched specimens. Additionally *CONSTRAINED_INTER_POLATION_SPOTWELD (model 2) models are calibrated based on the results of LWF-KS-2-specimens in order to include the load bearing capacity and failure behavior of the joint itself. The generated models are validated based on experimental results of component tests, which have failed because of notches due to the investigated joining techniques. It can be shown, that the suggested modeling approach is able to reproduce the load bearing capacity of the joint as well as material failure due to the imposed notch effect under complex loading conditions. Thus, the prediction quality of the simulation can be significantly improved, when the observed notch effects are considered in the setup of the simulation model.