Dimensioning with high-strength steels relies on the knowledge of Wöhler-type S/N data and the assumption that no failure occurs for load levels below the fatigue limit for applications where the number of load cycles exceeds 10 7 .Very-high-cycle fatigue (VHCF) experiments applied to a 0.5C-1.0Cr-Mo tempered steel (German designation: 50CrMo4) revealed surface crack initiation at prior austenite grain boundaries in medium strength condition (37HRC) and internal crack initiation at nonmetallic inclusions at high strength condition (57HRC). Despite the formation of small cracks during cycling up to 10 9 cycles, it seems that these are nonpropagating cracks in the case of the medium strength condition and therefore a real fatigue limit exists. Application of automated electron back-scatter diffraction (EBSD) within the shallownotched area of electro-polished fatigue specimens had shown that prior austenite grain boundaries act as effective obstacles to crack propagation.High-resolution thermography during cycling of the specimens allowed the identification of local plasticity, which led to crack initiation at a later stage of the fatigue life. It was found that Cr segregation rows play a decisive role in the crack initiation process. By means of high-resolution electron microscopy in combination with focused ion beam milling (FIB), evolution of cyclic plasticity and crack initiation was correlated with the material 0 s microstructure. The results are discussed in terms of the completely different crack initiation mechanisms of medium and high strength variants of the same steel. EBSD microstructure and crack propagation data are used to adapt a numerical modeling tool to predict microcrack propagation in the VHCF regime. shear strain interval; ΔK FGA , stress intensity range at FGA; ΔK incl , stress intensity range at inclusion; ΔK th , threshold stress intensity range; ΔK th, nano , nanograin threshold stress intensity; dε/dt, strain rate; √area, square root of projection area of a defect
Dimensioning with high‐strength steels relies on the knowledge of Wöhler‐type S/N data and the assumption that no failure occurs for load levels below the fatigue limit for applications where the number of load cycles exceeds 107. Very‐high‐cycle fatigue (VHCF) experiments applied to a 0.5C‐1.0Cr‐Mo tempered steel (German designation: 50CrMo4) revealed surface crack initiation at prior austenite grain boundaries in medium strength condition (37HRC) and internal crack initiation at nonmetallic inclusions at high strength condition (57HRC). Despite the formation of small cracks during cycling up to 109 cycles, it seems that these are nonpropagating cracks in the case of the medium strength condition and therefore a real fatigue limit exists. Application of automated electron back‐scatter diffraction (EBSD) within the shallow‐notched area of electro‐polished fatigue specimens had shown that prior austenite grain boundaries act as effective obstacles to crack propagation. High‐resolution thermography during cycling of the specimens allowed the identification of local plasticity, which led to crack initiation at a later stage of the fatigue life. It was found that Cr segregation rows play a decisive role in the crack initiation process. By means of high‐resolution electron microscopy in combination with focused ion beam milling (FIB), evolution of cyclic plasticity and crack initiation was correlated with the material's microstructure. The results are discussed in terms of the completely different crack initiation mechanisms of medium and high strength variants of the same steel. EBSD microstructure and crack propagation data are used to adapt a numerical modeling tool to predict microcrack propagation in the VHCF regime.
The important role of inclusions for the fatigue behavior of Nitinol and the related service lifetime for medical devices is stated by numerous studies. Besides the well-known size effect on the fatigue limit, the corresponding crack initiation was observed preferably at particle-void-combinations. However, the detailed relationship of several geometrical inclusion properties and the resulting fatigue load remains not clear. To shed a light on this effects relationship, a numerical investigation was performed with a superelastic material behavior on a macroscopic framework. In the scope of this study, two-dimensional unit cells with fully embedded particles or particle-void-assemblies of different shapes and different relative orientations with respect to the load direction were evaluated. Additionally, those unit cells were subjected to different global strain amplitudes and mean strain levels. The careful evaluation of the results revealed a hierarchy of parameter effects on the fatigue strain. Besides the trivial relationship between global applied and local resulting fatigue load, the inclusion shape and the orientation were observed to show a strong effect on the local fatigue strain.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.