Influence of mean stress, microstructure and temperature on the formation of FGA at VHCF loadings

• Applicants: Professor Dr.-Ing. Tilmann Beck; Dr.-Ing. Bastian Blinn
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 523648868

There are various, sometimes contradictory, models for the formation of fine granular areas (FGA) during very high cycle fatigue (VHCF) of tempered steels. It has been and is still discussed extremely controversially, which FGA formation mechanisms dominate in dependency with the material and loading conditions, especially mean stresses, and how the FGA formation influences the fatigue life. Moreover, it is still unclear, which similarities and differences exist to the formation of white etching areas / cracks (WEA/WEC), being observed at cyclic contact loads. These research questions are ad-dressed in the research group. In this subproject, the relevance of, and the conditions for the different FGA formation mechanisms are focused. In this context, it is analyzed if local cyclic plastic deformation and/or a detachment of carbides lead to FGA and hence, crack propagation, or if the FGA is a result of cyclic contacts of the crack surfaces and thus, a consequence of crack propagation. Moreover, there is still a lack of reliable work on the influence of elevated temperatures, which enhance the diffusion of carbon, on the FGA formation. Thus, this project is based on the following research hypothesis: "The formation of FGA at VHCF of tempered martensitic steels depends in a complex manner on (i) the mean stress or the load ratio R, (ii) the strength or the plastic deformability and thus, the tempering temperature, (iii) the carbon content and (iv) the loading temperature. Additionally, the formation of cracks and possibly FGA correlates with the hardening potential of the surrounding material volume". To answer the research questions implied by this hypothesis, low-alloyed tempered steels with C contents of 0.25 to 1.0 wt.% are investigated. These steels are analyzed in different heat treatment conditions with hardnesses from 32 HRC to 62 HRC. The resulting differences in carbide structure, strength and plastic deformability enable the analysis of the relation between FGA formation and cyclic plasticity or carbide detachment. Using cyclic microindentation, it is also investigated, if the FGA formation is influenced by the cyclic hardening potential, and how this potential is changed in the vicinity of the crack initiation site. In the VHCF tests, R from -2 to 0.5 are applied to analyze the impact of cyclic contacts of the crack surfaces on the FGA formation. This is complemented by the investigation of different material conditions, which result in differently pronounced crack closure. The combined influence of mean stress and temperature is analyzed for a C content of 0.5 wt.% and an intermediate hardness in the entire R range. Comprehensive fracture surface and microstructure analyzes, combined with simulations of crack closure effects, both performed in collaboration with other subprojects, allow an in-depth understanding of the FGA formation mechanisms, far beyond the current state-of-knowledge.

• 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)