Microscopic Damage Evolution During Very High Cycle Fatigue (VHCF) of Tempered Martensitic Steel

Abstract

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.

Crack growth rates and microstructure feature of initiation region for very‐high‐cycle fatigue of a high‐strength steel

“…1 The topic of very-high-cycle fatigue (VHCF) which is defined as the fatigue failure beyond 10 7 loading cycles has attracted the interest of researchers, and an increasing number of investigations on VHCF have been carried out to satisfy the needs of social development in recent decades. [2][3][4][5][6][7] The behavior of crack initiation and early growth for VHCF is different from that of low-cycle fatigue (LCF) and high-cycle fatigue (HCF). For LCF and most of HCF cases of metallic materials, fatigue cracks initiate from the specimen surface due to persistent slip bands, whereas for VHCF, cracks usually initiate from the interior of material at non-metallic inclusions or other inhomogeneities.…”

On the Influence of Control Type and Strain Rate on the Lifetime of 50CrMo4

“…The results were obtained at testing frequencies of about 19 kHz for the USP, 700 Hz for the EMF and 20-200 Hz for the VHF. For further validation, test data were provided by Osnabrück University of Applied Sciences [8,115], obtained at 20 kHz with an ultrasonic testing system (USPO) type BOKU Vienna and at about 95 Hz, obtained with a electromechanic resonance testing machine, type Rumul Testronic (RTTO). The investigated material was quenched and tempered to an ultimate tensile strength of 1096 MPa.…”

“…Strain-based models are best calibrated using strain-controlled fatigue tests. In the 1980s and 1990s, component failures in the gigacycle range kick-started very high cycle fatigue (VHCF) research [2][3][4][5][6][7][8][9][10][11][12]. In VHCF fatigue initiation, no macroscopically plastic material behavior is observed, therefore stress based models are employed.…”