Enhancement of the fatigue limit at HCF- and VHCF-loadings with the help of thermomechanical treatment at the temperature of maximum dynamic strain ageing

• Applicants: Dr.-Ing. Stefan Guth, since 1/2021; Professor Dr. Eberhard Kerscher
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 408139037

In the past fatigue experiments were carried out only until an ultimate number of cycles of 106 or 107 because people thought that there is no further decrease of the cyclic strength at longer lifetimes. Nowadays, we know that a lot of especially high-strength materials can fail even after more than 107 loading cycles. There are several ideas why a failure might happen even after very high cycle fatigue (VHCF). The proposer’s opinion is that the stress concentration at inclusions and the corresponding localized plasticity are the reason for a local grain refinement, which leads to a so called fine grained area (FGA), and the subsequent crack initiation during VHCF. Approaches to improve the VHCF-behaviour or to avoid the said failure mechanisms are not available in literature. So, this is the main question of the current proposal.To increase the VHCF-fatigue limit a stabilized dislocation structure with increased dislocation density has to be realized by a thermomechanical treatment (TMT) at the temperature of maximum dynamic strain ageing. Former investigations of the proposers showed that such a TMT can improve the HCF-fatigue limit of the high-strength bearing steel 100Cr6 but it was never investigated whether this TMT is also useful for other steels and for avoiding VHCF-failure.The goal of the project consequently is the increase of the loadability of high-strength steels at VHCF-loadings. Thereby the responsible underlying mechanisms have to be understood from a materials science and technology point of view.Therefore, the experiments have to show whether the TMT increases the VHCF-fatigue limit and whether or not it modifies and stabilizes dislocation structure. To that end, before testing the possible increase by HCF and VHCF loadings, it is necessary to clarify the boundary conditions of the TMT for the steels 42CrMo4 and 100Cr6. Whether or not TMT causes a change of the failure mechanism will be determined with the means of detailed fractographic investigations of the fracture surfaces. Thereby, the focus will be on the crack initiation region at non-metallic inclusions in order to detect the typical structure FGA on fracture surfaces after VHCF loading. If no such FGA on the fracture surfaces are identified after TMT this would strongly point to the view that the dislocation structure is stabilized, which means that the fatigue limit has increased by microstructural reasons.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Dr.-Ing. Karl-Heinz Lang, until 1/2021