Open Access

Alexander Giertler, Ulrich Krupp

• Herein, the fatigue damage mechanisms of a low‐alloyed 0.5 wt% carbon steel (50CrMo4) are examined in the high cycle fatigue and very high cycle fatigue regime, taking into account different strength conditions. For this purpose, the heat treatment of the material is conducted using two different batches at two different tempering temperatures, which lead to hardnesses of 37HRC and 57HRC, respectively. The fatigue tests are conducted accounting for different test frequencies of f = 95 Hz and f = 20 kHz, as well as size effects. It is found that the fatigue behavior of the tempered steel 50CrMo4 can be subdivided into type I (surface cracks) and type II (internal cracks) VHCF damage behavior depending on the material strength condition. With the aid of in situ test equipment, the influence of microstructural banding on the local fatigue strength is demonstrated by high‐resolution thermography. By implementing an ultrasonic testing machine in a high‐resolution scanning electron microscope, the development of a fine granular area (FGA) at an artificial defect in vacuum atmosphere is demonstrated.