Experimental estimation of crack initiation lives and fatigue limit in subsurface fracture of a high carbon chromium steel

In order to clarify subsurface crack initiation life and fatigue limit, two-step variable amplitude loading tests were conducted using fine-particle bombarded specimens of a high carbon chromium steel, JIS SUJ2. Particular attention was paid to the granular area formed around a non-metallic inclusion in fish-eye. Based on the experimental finding that the granular area was not seen in short life regime at high stress levels, but observed in long life regime at low stress levels, the subsurface crack initiation was estimated by examining whether the granular area was present or not after testing. It was found that the subsurface crack nucleation occurred at the early stage of fatigue life, i.e., 5% or so, as a result of low–high sequence tests with different cycle ratios at low stress level. Furthermore, the results, which were obtained under similar low–high sequences performed until 109 cycles at six low stress levels followed by high stress level applied to failure, revealed that the fatigue limit for N = 109 would be around 650 MPa in the present material.