Parallelisms between WEA/WEC and FGA under rolling fatigue conditions with variation of pressure, slip and hydrogen loading

• Applicant: Professor Dr.-Ing. Christoph Broeckmann
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 523648868

The focus of the project is to understand the formation of damage caused by white etching areas (WEA), which often occur together with a crack network (white etching cracks, WEC) in rolling bearings and fine-grained (dark) areas (FGA) under very long fatigue loads. To date, the formation mechanism of WEC/WEC has not been clearly clarified and is subject of controversial debate. There is consensus that, in addition to the cyclic loading caused by Hertzian pressure, additional loads are required to initiate WEA/WEC. In the proposed project, the time-dependent evolution of WEA/WEC formation and propagation will be investigated with the aim of identifying the phenomenon over all time scales from LCF to VHCF range. For the first time, the influence of the steel composition is systematically investigated. An advanced multi-scale FE model addresses, for the first time on a mesoscopic scale, significant effects of WEC formation that are accessible through a coupled simulation of mechanical, thermal and metal-physical models. A four-disk test rig will be used to prove that coupled WEA/WEC formation can be identified in lubricated rolling contact even at high cycle numbers in the VHCF range. The aim is to prove the analogy of WEC formation in rolling contact and FGA formation under axial fatigue. Interrupted tests and high-resolution characterization are used to identify pre-stages of WEA formation and to understand WEA formation and growth. By varying the steel composition (C content), Hertzian contact pressure, slip, hydrogen loading and electric current, the significant parameters for the time-dependent development of WEA/WEC are to be identified. The multi-scale model for the rolling contact developed in preliminary work is to be extended in the mesoscopic sub-model so that heat conduction and diffusion of C and Cr are taken into account. Furthermore, integration of the mechanisms depicted in the model across many load cycles should allow the prediction of WEA/WEC development over time. The extended FE model will be used to investigate the influence of contact pressure, slip and material in numerical studies by varying the parameters for matrix hardening and transformation behavior and, in combination with experiments, to identify critical conditions of the material and stress states with regard to WEC formation. The project will extend the knowledge on WEA/WEC and FGA phenomena by understanding the influence of stress state and supporting parameters (diffusible hydrogen, electric current) on the time evolution of material damage. In particular, findings on the stabilizing effect of carbon on WEC formation are expected, which will enable a critical review of the established but contradictory hypotheses on WEC and FGA formation.

• DFG Programme: Research Units
• Subproject of: FOR 5701: Identification of the formation mechanisms of white etching cracks and fine granular dark areas during fatigue loading – parallels and differences (White and Dark)