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-S15C and JIS-S45C, a small semi-circular slit was introduced onto the surface of a round specimen. The slit was tilted at 0 , 30 or 60 with respect to the plane normal to the loading axis, but all of them had the same defect size, area?=?188 μm, 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 below the fatigue limit, that is, the fatigue limit was 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 area, which was in good agreement with the prediction by the area parameter model. In this paper, a mechanistic explanation for the insensitivity of the fatigue limit to the tilt angle is proposed.