VHCF behaviour of case-hardened specimens made of two grades of steel SAE 5120 differing in microstructure

In this investigation notched and fracture mechanics specimens were machined from the steel SAE 5120 (DIN EN 20MnCr5) in two cleanliness conditions. The first material is used for high-performance components and is nearly free of oxides. The second material is standard steel and used for common requests in the field of drive engineering containing non-metallic inclusions in a dimension that is state of the art in steel making. The notched specimens were case-hardened in two different ways. Afterwards, the surfaces were ground and therefore showed compressive residual stresses. Uniaxial VHCF tests were performed up to 200 million cycles under the stress ratios R = −1 and R = 0 using two resonant pulse systems. For this purpose the geometries of the specimens had been optimised so that the achieved test frequency was 190 Hz. Fatigue failure is possible at all applied numbers of cycles meaning that there is no real fatigue limit in this region. Fatigue crack initiation took place from different origins and at different sites. Different analyses are shown for interpretating the type of fatigue crack initiation and its site in the specimen. The fracture mechanics specimens were blank hardened in order to have the core properties of the notched specimens. Crack propagation was measured in the near-threshold region. An extended Kitagawa diagram is shown to demonstrate the influence of stress amplitude and flaw size on the failure probability.