Experimental Estimation of Subsurface Crack Initiation Lives and Intrinsic Fatigue Limit in a High Strength Steel

Abstract

Fatigue tests under two-step variable amplitude loading were conducted on a high carbon chromium steel, JIS SUJ 2, in order to clarify the crack initiation lives and the intrinsic fatigue limit in subsurface fracture. Particular attention was paid to the granular area around the non-metallic inclusion in fish-eye. The granular area does not appear in short life region at high stress levels, but appears in long life region at low stress levels. Using this property, low-high sequences with a different cycle ratio at low stress level were applied to shot-peened specimens. If the granular area is observed after testing, then it is assumed that the granular area is generated in the cycle ratio examined. It was found that subsurface crack nucleation took place at the early stage of fatigue life, i.e. 5% or so. Similar low-high sequence tests with 5×10⁸cycles and 10⁹cycles at low stress levels were performed. Provided that the granular area is not observed after testing, the low stress level applied could be an intrinsic fatigue limit for subsurface fracture. After testing for 5 x 10⁸ cycles, the granular area was observed at a low stress level of 800 MPa, but not at 750 MPa. After 10⁹cycles, the granular area was not seen at 650 MPa, but seen at 700 MPa. Therefore, it is concluded that the intrinsic fatigue limit would exist around 650 MPa in the present material.