Quantified evaluation of the influence of testing frequency on the fatigue behaviour of unalloyed steels for implementation in resource-efficient fatigue life prediction methods

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The fatigue life prediction methods (LPV) developed at the department of Materials Science and Materials Testing (WWHK) at the University of Applied Sciences Kaiserslautern have so far been limited to the transition between LCF (Low Cycle Fatigue) and HCF (High Cycle Fatigue) as well as to the HCF range (approx. 3 to 5x10^5 cycles). This has led in part to scientific questions regarding the exploitation of the potential of LPV, which is why systematic investigations are to be carried out in this research project. The reason why it has not yet been possible to extend the procedures and methods to the VHCF (Very High Cycle Fatigue) range is that the WWHK did not have the necessary testing infrastructure to achieve cycle numbers up to 10^8 within a reasonable time frame. Through the approval of the large-scale equipment application, a resonance testing machine was procured and taken into operation, which makes it possible that cycle numbers in the VHCF regime can also be achieved in a technically justifiable time frame. Two factors must be considered here. Relevant damage mechanisms can change from the HCF to the VHCF range. For example, crack initiation points shift into the volume in the area of defects, such as inclusions and precipitates, especially in the case of high-strength material conditions. In the case of ductile materials, damage development can also be detected below the critical threshold value for the activation of persistent slip bands, which can be attributed to the high cycle numbers and the associated accumulation processes. For this reason, it is necessary to check which of the measurement techniques used so far can also be used for the high cycle regime. The deformation rate has a significant influence on the damage development per cycle. It is essential that the relationship between frequency and damage contribution and the resulting fatigue life can be quantified and thus made usable for LPV. The aim is therefore to develop LPV for the use in the VHCF regime and to implement influences regarding deformation rate and load-related mechanisms in mathematical approaches and to validate the prediction quality through experiments. Within the work program, the comprehensive mechanical tests to extend the LPV at WWHK will be supported by detailed microstructural characterization at the Chair of Materials Test Engineering (WPT) at TU Dortmund University. The steel SAE 1045 (C45E, 1.0503) is to be used as test material, since a database for the test frequency 5 Hz is already available from completed investigations. Thus, within this research project, the frequency range of 60 to 260 Hz and thus the use of the resonance test system can be focused on.