Effect of small defect orientation on fatigue limit of carbon steels

In order to clarify the effect of defect orientation on the fatigue limit of two types of steels, JIS- S15Cand JIS- S45C, a small semi- circular slit was introduced onto the surface of a round specimen. The slit was tilted at 0 degrees, 30 degrees or 60 degrees with respect to the plane normal to the loading axis, but all of themhad the same defect size, root area= 188 mu m, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. In all the combinations of the materials and tilt angles, a non- propagating crack was found at or just belowthe fatigue limit, that is, the fatigue limitwas determined by the non- propagation condition of a crack initiated from the defect. In both steels, the fatigue limit was found to be nearly independent of the tilt angle for the same value of projected size root area, which was in good agreement with the prediction by root area parametermodel. In this paper, a mechanistic explanation for the insensitivity of the fatigue limit to the tilt angle is proposed.