Reason for the transition of fatigue crack initiation site from surface to subsurface inclusions in high-strength steels

The fatigue crack initiation in high-strength steels at surface inclusions is linked to high stress amplitudes. Low stress amplitudes are associated with a crack initiation at subsurface inclusions. The aim of the paper is to gain insight regarding the factors that lead to the transition of the crack initiation site from the surface to the interior. On the basis of fatigue tests and fracture mechanics evaluation, it was determined that the transition of the crack initiation site is effected by a constant threshold value for crack initiation at the surface and different crack propagation rates at the surface as opposed to the interior. It is shown that with lower stress amplitudes no critical inclusions with stress intensity factors above this threshold value are positioned at the specimen's surface. Therefore, the cumulative number of inclusions on the surface is calculated based on the cumulative number of inclusions in a metallographic section.