Effect of orientation of small defects on fatigue limit of steels

In order to clarify the effect of defect orientation on the fatigue limit of three types of steels; JIS-S15C, JIS-S45C and JIS-SNCM439, a small semi-circular slit was introduced into the surface of a round specimen. The slits were tilted at 0 degrees, 30 degrees or 60 degrees with respect to the plane normal to the loading axis, but all of them had 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 tilting angles, a non-propagating crack was found at the fatigue limit, i.e. the fatigue limit was determined by the non-propagation condition of crack initiated from the defect. In JIS-S15C and JIS-S45C, the fatigue limit was nearly independent of the tilting angle, which was in good agreement with the predicted value by the root area parameter model. On the other hand, in JIS-SNCM439, the fatigue limit was also in agreement with the prediction at the tilting angle of 0 degrees, but it increased with an increase in the tilting angle. These results indicated that the root area parameter model can predict a conservative fatigue limit for the tilted defects. In this paper, the mechanistic reason for the effect of the tilting angle on the fatigue limit will be discussed by paying special attention to the crack path and length of non-propagating crack.