Analysis of fine grain formation in local multidimensional stress fields during cyclic tension-compression and superimposed cyclic compression-torsion loading

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

Rolling bearings enable low-friction operation of moving parts relative to each other and are therefore one of the most important machine elements. They are widely used, particularly in high-quality mechanical engineering. The failure of rolling bearings often results in major economic damage due to repair, replacement, and secondary damage. Due to the complex load conditions and numerous influencing factors from assembly and operation, there are many different causes of failure, e.g. chipping, grey spotting, or pitting. For this reason, great efforts have been and continue to be made to increase the reliability of rolling bearings, which remains an extremely important challenge, especially regarding sustainability. Today's rolling bearings can be well de-signed for their application thanks to increased steel cleanliness levels and improved lubricants, among other things, and can in principle achieve very long running times. For around 20 years, however, there have been repeated early failures of rolling bearings due to so-called white etch-ing cracks (WEC), which cannot be attributed to any of the usual causes. These are groups and networks of cracks on the flanks of which a white etching, altered microstructure, the so-called white etching area (WEA), appears. Similar microstructural changes are also observed in uniaxial very high cycle fatigue (VHCF) testing in the immediate vicinity of non-metallic inclusions. In this case, we speak of fine granular areas (FGA). The two phenomena WEA/WEC and FGA have so far mostly been observed separately and models for possible mechanisms of formation have been developed independently of each other. In preliminary work, cyclic compression-torsion tests have succeeded in simulating the local stress state that can occur in rolling bearings on samples in such a way that fine grain formation could also be observed in the cracked area, which does not differ from fine grain zones in WEA in terms of its mechanical hardness and ion-induced secondary electron image. This fine grain formation only occurred in a specific phase relationship between cyclic compressive swelling stress and alternating torsion, while no fine grain formation could be produced in a second investigated phase relationship. From this, the applicant concludes that the formation of fine grains depends very sensitively on the locally existing multiaxial stress state. This research project aims to investigate whether similar microstructural changes occur in WEA/WEC and FGA formation during the operation of rolling bearings or during VHCF and whether they are based on a common mechanism. The sub-project focusses on the investigation of the stress states relevant for fine grain formation and the influence of hydrogen.

• 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)