Experimental and numerical analysis of forming-induced residual stresses for fatigue strength increase of highly cyclically loaded high-purity copper components

• Applicants: Professor Dr.-Ing. Alexander Brosius; Professor Dr.-Ing. Markus Kästner
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing ; Mechanics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 531874972

With respect to the transformation of transportation and energy policy and the resulting increased usage of electric traction drives and generators, the question of the fatigue behavior of high-purity copper components, which serve as parts for power transmission in the highly cyclically loaded powertrain of battery electric vehicles, for example, is becoming the focus of current research. The multi-stage forming process of the components results in complex and interacting influences on their fatigue life. In particular, the influence of forming induced residual stresses has not been considered to date and, analogous to the utilization in steel components, could have high potential for component improvements. The objective of the project is the application and the transfer of methodological knowledge developed for steel components in the SPP on the use of forming-induced residual stresses for the improvement of fatigue life to components made of high-purity copper. The focus is on the establishment of generalized process-structure-property linkages taking residual stresses into account. A conductor rail produced by bending is used as an example for the investigation. Its service life is to be improved by inducing compressive residual stresses. For this purpose, an additional process step during manufacturing is planned. In addition to the influence on residual stresses, the overall process causes a change in the local material state. This makes it necessary to consider the influence of residual stresses, local hardening and the process-related microstructure on fatigue life. By implementing a targeted, cyclic material characterization, the investigation of defined material states under the influence of residual stresses is carried out. The forming of the reference component is mapped virtually with the aid of suitable process modeling. Using the process model, the process is optimized with a residual compressive stress distribution at the failure-critical point as the target. Accompanying residual stress measurements are used to validate the model. Subsequently, experimental component characterization of selected process variants is carried out to determine the influence of the process variants on the service life. Based on the compiled virtual and experimental data, a computational fatigue life evaluation is developed considering process-related residual stresses and local material state. At the end of the knowledge transfer project, elaborated process-structure-property linkages are available as a basis of the performed process design for lifetime improvement of high-purity copper components. Through the extensive experiments on residual stress influence for different material states, transferable knowledge is developed, which expands the data base for copper materials.

• DFG Programme: Research Grants (Transfer Project)
• Application Partner: ZF Friedrichshafen AG