Prediction of fatigue thresholds of steels with surface defects

The resistance-curve (R-curve) method was used to assess the effect of the diameter of drilled holes on the fatigue threshold of a carbon steel (JIS S45C) and a spring steel (SUP190). The specimens of the carbon steel with various hole diameters were subjected to tension-compression fatigue tests, and those of the spring steel to rotating bending and torsional fatigue tests. The fatigue crack made near the fatigue thresholds of drill-holed specimens of the carbon steel was modeled by the ring crack around the equator of the semi-cubic hole or by the quarter-circle at the corner of the cylindrical hole. When the diameter of drill holes was d = 0.05 and 0.1mm, the experimental data of the fatigue threshold agreed well with the prediction obtained under the assumption of the propagation threshold of the ring crack. The assumption of the quarter-circle crack gave a better prediction for the diameters of d = 0.3 and 0.5mm. For the case of the spring steel, on the other hand, the threshold value predicted from the crack propagation was far below the experimental data. The critical value of the stress intensity factor for crack nucleation was determined in order to obtain a better agreement with the experimental data. The critical value thus determined was nearly double the threshold stress intensity range for crack propagation. The nucleation condition of fatigue cracks controlled the fatigue threshold of the spring steel. © 2008 The Society of Materials Science, Japan.