Subsurface crack initiation and propagation mechanisms in very high cycle regime

Rotary bending fatigue test was adopted to study the subsurface fracture property of a low alloy steel in very high cycle regime. As a result, the subsurface crack initiation and propagation with fine granular area (FGA) induced by subsurface incision is the predominant reason for fatigue facture of this low alloy steel in very high cycle regime of N > 10 ⁶, which can be divided into three stages to discuss: (I) small crack propagation inside of FGA, (II) stable crack propagation in the fish-eye region outside of FGA and (III) final catastrophic fracture outside of fish-eye. The crack growth rate in the first stage is lower than 10 ^-11m/cycle, which means that the progress of FGA formation is extremely slow and consumes the great majority of total fatigue life. The interior stress intensity factor range corresponding to subsurface inclusion and the stress intensity factor ranges of FGA and fish-eye (ΔK _int-th, ΔK _FGAand ΔK _fish-eye) can be regarded as the threshold values of controlling subsurface crack propagation in these three stages, respectively.