Occurrence and evolution of slip markings in a single phase BCC α-iron and a low carbon steel during very high cycle fatigue regime

In the Very High Cycle Fatigue (VHCF) domain, as in Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) domains, the failure of components by fatigue always begins by a damage stage followed by the stages of crack initiation and crack propagation (short and long crack). The duration of each stage depends on the stress amplitude. During the damage stage, the dislocations structure evolves depending on the crystal structure, which leads to the occurrence of Persistent Slip Bands (PSB) in FCC metals or Slip Markings (SM) in BCC metals. However in the VHCF domain, for low enough stress amplitude, no PSB/SM occurs, even after 10⁹–10¹⁰ cycles. In this work, fatigue tests are performed on α-iron (with a 0.008 wt% carbon content) and low carbon steel (with a 0.028 wt% carbon content) flat specimens thanks to a piezoelectric machine running at 20 kHz in continuous mode. The occurrence of SM is followed during the test with an optical camera focused on the central zone of the specimen, where the displacement is close to zero. When the stress amplitude is lower than 40 to 50 % of the lower Yield Stress, no SM is detected. In the initiation and short crack propagation stages, few SM are observed on the specimen surface in well oriented grains. In the long crack propagation stage, SM are visible on all grains near the fracture surface.