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)

• Subject Area: Materials Science and Engineering ; Mechanical and Industrial Engineering
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 523648868

Technical solutions for the reduction of carbon dioxide emissions and for the sustainable use of renewable energy sources are the essential building blocks for adherence to the internationally agreed climate targets. The design against fatigue damage plays a central role in improving the energy efficiency of all cyclically stressed components, be it small bearings in electric scooters, injection systems in commercial vehicles, or large gears in wind turbines. As yet, however, there are still some open questions regarding the crucial failure mechanisms. For example, in the case of high-strength steels, the occurrence of locally limited, dynamic fine-grain formation is observed with very long fatigue stresses leading to so-called fine-grained areas or optical dark areas (FGA or ODA). In contrast to fine grain formation, which is often intentionally enhanced by processes of massive plastic deformation, the FGA are undesirable and often cause premature failure of the components below the classic fatigue strength. Even in the event of early failures of highly stressed bearings, which have increasingly occurred in recent years for example in wind turbines, fine grain formation is found in the area of damage initiation, which is associated with crack networks below the bearing treads. These cracks are referred to as white etching cracks (WEC) because of the etching behaviour of the direct crack environment, which is also mentioned as white etching area (WEA), during nital etching. The mechanisms underlying the locally occurring fine grain formation have not yet been fully understood: there are several, partly very different explanations. However, there are also similarities if you compare the results of investigations in the electron microscope and in the atom probe for the different nanocrystalline areas of FGA and WEA/WEC. The aim of the research group is therefore to clarify the mechanisms underlying fine grain formation in the development of FGA and WEA/WEC. To complement conventional methods, the applicants have developed new methods, such as in situ testing in a scanning electron microscope, the generation of fine-grained areas at artificial defects in a vacuum, and new simulation approaches, which for the first time will make it possible to record microstructure changes at FGA and WEA/WEC exactly and to compare them systematically (first funding period). The second funding period will be used to transfer the gained knowledge into concrete concepts to reach fine grain formation that can be specifically controlled or suppressed under cyclical loading in order to be able to implement sustainable, more powerful components for more energyefficient and at the same time safer operation with these new concepts.

• DFG Programme: Research Units

Projects

• Analysis of fine grain formation in local multidimensional stress fields during cyclic tension-compression and superimposed cyclic compression-torsion loading (Applicant Kerscher, Eberhard )
• Analysis of the multiaxial stress state during crack initiation by the fracture phase field method (Applicant Müller, Ralf )
• Analytical and high resolution electron microscopy characterisation of microstructures in WEA/WEC- and FGA-regions (Applicants Mayer, Joachim ;  Schwedt, Alexander )
• Coordination Funds (Applicant Kerscher, Eberhard )
• Cross-scale modeling of the formation of FGA and WEC and the associated crack propagation processes (Applicants Kreins, Marion Cornelia ;  Krupp, Ulrich )
• Influence of mean stress, microstructure and temperature on the formation of FGA at VHCF loadings (Applicants Beck, Tilmann ;  Blinn, Bastian )
• Nanoscopic an nanomechanical characterisation of the hydrogen influence on FGA and WEA/WEC formation (Applicants Demchenko, Anna ;  Kerscher, Eberhard )
• Parallelisms between WEA/WEC and FGA under rolling fatigue conditions with variation of pressure, slip and hydrogen loading (Applicant Broeckmann, Christoph )
• Quantification of the influences of composition and microstructure on the mechanisms of FGA formation in quenched and tempered steels by in situ testing and high-resolution analysis (Applicants Krupp, Ulrich ;  Song, Wenwen )

• Spokesperson: Professor Dr. Eberhard Kerscher