Qualification of a fatigue test for the investigation of the behavior of high-strength tool materials under realistic conditions by elastomer compression

• Applicant: Professorin Dr.-Ing. Marion Merklein
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: from 2022 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495554034

Tools and tool failure have major influence on the economic efficiency of cold forging processes. In order to minimize losses due to machine downtimes, it is necessary to accurately predict the service life. A challenge in the prediction of tool life is that there is little data on the fatigue behaviour of the used high strength steel materials. In addition, the quality of the available data is limited, as they do not reflect the multi-axial stress conditions of cold forging.The objective of the research project is therefore to investigate a new fatigue test, which can realistically model the stress state occurring in forming technology considering both the multi-axiality and the application of compressive prestresses. For this purpose an elastomer made of polyurethane is used as a pressure medium in a tool for fatigue testing. A cyclic hydrostatic pressure is applied by the repeated compression of the elastomer. By comparing the numerically determined stresses on the inner wall of the fatigue specimen with the tool load during the forming of steel, fundamental knowledge is obtained regarding the stress state in cold forming. In particular, the comparability of the effect of the hydrostatic internal pressures with the contact stresses in forming technology and the suitability of high-strength elastomers as a medium for generating these pressures will be demonstrated. A challenge in the application of polyurethanes occurred in the preliminary tests in the form of burr formation, which leads to wear on the elastomer. The analysis of influencing variables on the burr formation is used to identify process limits regarding the achievable internal pressures and load cycles. In addition, the crack propagation and the fatigue life of the steel specimens and their correlations to the numerically determined stress state will be analyzed.In order to illustrate realistic load cases, the transferability of the test results to systems under preload is demonstrated. In a combined numerical-experimental approach, both conventional reinforcements and new concepts for local prestressing will be investigated. This enables the identification of cause effect relationships between the stresses and strains caused by the reinforcement and the service life of the tools under realistic loads. Finally, the obtained results will be used to evaluate the potential of the test and the benefit compared to conventional fatigue tests.

• DFG Programme: Research Grants