IDENTIFICATION OF FATIGUE CRACK ZONES IN VERY HIGH CYCLE FATIGUED 42CrMo4 STEEL WITH THE USE OF QUANTITATIVE FRACTOGRAPHY

Very high cycle fatigue (VHCF) is fatigue caused by the growth of an internal fatigue crack in materials under stresses below the standard fatigue limit and number of cycles beyond 10⁷. The fracture surface of steels and alloys after VHCF can be divided into distinct zones, such as the fine granular area (FGA) and the so-called “fisheye”. Differences in the morphology of the crack surface can be numerically estimated by the roughness parameter. Murakami Y. et al. showed that the magnitude of the linear roughness Ra is proportional to the stress intensity factor, whereas Shiozawa K. et al. measured Ra within the FGA and fisheye. Stanzl-Tschegg S. et al. revealed presence of the smooth area (SA) between the FGA and the fisheye. The aim of this work is the quantitative fractographic analysis of this smooth area, which was not reported in the literature so far. Hardened and nitrided specimens of 42CrMo4 steel were used for ultrasonic fatigue testing under symmetric loading conditions ( R =-1) at a resonant frequency of 19.5 kHz. Fracture surfaces after fatigue failure were examined by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Smooth area was experimentally defined as a fatigue crack zone between the FGA and the fisheye. This zone is characterized by (i) visual smoothness and (ii) minimal values of the surface roughness parameters: Sq =4.15 μm (roughness), S'q =0.60 μm (microroughness) and Rs =1.02 (normalized surface area). It has been established that the SA is the result of the propagation of an internal fatigue crack at the stage preceding the stage of a stable crack growth.

very high cycle fatigue, ultrasonic fatigue testing, confocal laser scanning microscopy, fractography, 42CrMo4 steel, fatigue failure, fatigue crack, roughness