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: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495554034

The basic objective of the research project is to provide a test method for the realistic investigation of fatigue in cold forging tools on a laboratory scale. The method is based on the use of elastomers, which are compressed in forging dies and thus exert a high internal pressure on them. In the first phase of the project, it was shown that the experimental principle is suitable for applying high internal pressures generating fatigue failure in cold forging dies. Furthermore, the potential for investigating prestressing concepts was demonstrated based on a difference in fatigue life and crack path of reinforced and non-reinforced dies, which is consistent with the behaviour known from industrial practice. These investigations were carried out on steel dies with a constant cross-section. For that reason, neither an analysis of axial tensile stresses at cross-section transitions nor the investigation of the material behaviour of cemented carbide has been possible so far. The existing test method is therefore to be extended for a holistic investigation in the second project phase. By using the method, knowledge on the fatigue failure depending on the multiaxial stress state and the die material is to be generated. With the use of different reinforcement systems, it is possible to adjust the stress state so that uniaxial or multiaxial tensile stresses occur in the die. The stress state is calculated with the aid of a simulation model and used to understand the occurring fatigue behaviour. In order to achieve the research objectives, a new tool geometry with an oval and non-constant cross-section must first be designed. This generates local axial and tangential tensile stress peaks, which will be investigated regarding their influence on the fatigue behaviour. Subsequently, tests are to be carried out with the new tool geometry and optimized with regard to elastomer wear. Elastomer wear presented a challenge for efficient testing in the first project phase and is therefore to be reduced by suitable punch or elastomer geometries. The fatigue behaviour under multi-axial loading is then to be investigated for steel and cemented carbide tools. A profound analysis of the stress state and its influence on tool fatigue is to be carried out by using different prestressing systems for a targeted adjustment of the tool stresses. Finally, the results of the second project phase will be evaluated in comparison with the first phase and existing conventional fatigue tests. After successful completion of the second project phase, extensive knowledge on fatigue failure depending on the stress state will exist. This can be used in a third project phase for the service life prediction of cold forging tools.

• DFG Programme: Research Grants