Experimental characterisation and numerical analysis of increasing fatigue strength by residual stresses for cross rolling parts

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Within the framework of SPP 2013, research is being conducted into the use of residual stresses induced by forming technology to improve component properties. The aim of the proposed project is the targeted setting of residual stress states in cold forming processes to increase fatigue strength. The project is focused on a hybrid component in which the shaft and hub are joined by a transverse rolling process. This results in a process-related circumferential notch which can have an unfavourable effect on the expected service life. This disadvantage can be compensated by selectively inducing residual compressive stresses in the notch during the rolling process.In the first two funding periods, a stable process was established and the performance of the approach for improving fatigue strength was demonstrated. Within this context, different numerical models were developed to simulate the process and fatigue life. The ability to positively influence the service life through different process variants was demonstrated.In the third and final funding period, the focus is now on the real component, taking into account the influence of a joining partner (hub) under service conditions. For this purpose, the focus will be on the methodical component and process design with regard to the residual stress states to be set in the notch and the joining zone. For practical implementation, a very flexible rolling jig is being set up which also allows complex process variants with a greater variety of geometries in order to meet the requirements for the desired residual stresses profiles.Furthermore, the fatigue behaviour of the component is again considered and gains in importance due to the application to real components under operating conditions. The stability of the compressive residual stresses in the failure-relevant notch and at the joint is critical for the service life. Therefore, the change in residual stresses under cyclic load is measured and simulated by means of an elastic-plastic model. The crack propagation model will be extended to the 3D case and thus, as part of a coherent simulation strategy, the fatigue life of the components will be analysed taking into account the variable residual stresses.At the end of the third funding period, the rolled component will be specifically designable with respect to its joint connection and notch geometry by means of a transferable design methodology. The fatigue strength was analysed under consideration of the residual stress stability and the findings on the residual stress changes were included in the design methodology. The flexible new process kinematic will enable the residual stresses in the notch and the joining zone to be set independently, and long-term tests on real components will provide proof of the overall approach. In addition, transferable statements on residual stress stability in solid components will be available.