Very high cycle fatigue crack initiation mechanism according to a 3D model of persistent slip bands formation in α‐ferrite

Abstract

AbstractMany studies on the behaviour of very high cycle fatigue (VHCF) showed that there appear some distinguishable features compared with the low and high cycle fatigue. The understanding of mechanism of fatigue crack initiation in VHCF is very important. In the present work, 1 mm thin sheet specimens on commercial pure iron were employed to perform tests under tension–compression cyclic loading at 20 kHz ultrasonic frequency. In situ observation was carried out with thermal and optical imaging. Local deformation marks on the specimen and the fracture surface near the crack initiation were investigated by scanning electron microscope. A micron abreast pipes 3D model has been constructed to illuminate the damage accumulation of irreversible deformation. It was found that the micron abreast pipes develop during previous cyclic loadings and remain unaffected at the fracture surface. A mechanism was established to explain the relationship between micron abreast pipes and fatigue crack initiation.