Fatigue Strength of Notched Specimen of Steel under Pulsating Torsion

In the previous papers, some of the present authors carried out axial fatigue tests about the notched specimens of steel to find the S-N curves for wide ranges of number of stress cycles, and the endurance limit curves for large mean stresses. In this paper, we made torsional fatigue tests about the specimen which had circumferential notch made of 0.36%°C steel, covering the number of stress cycles N=1-107, where the stress conditions were τm/τa=0, 1, 2 and 4, in which τm denotes the torsional mean stress and τa the torsional stress amplitude. The summary of the results is as follows: (1) About the notched specimen, the S-N curves have the following characteristics when plotted on the figure in which the ordinate represents τmax=τm+τa and the abscissa represents number of stress cycles N in logarithmic scale, and each curve is drawn for τm/τa=const.. Each curve declines slowly when the number of stress cycles N is small, then the slope becomes gradually steep with an increase of N, but near the endurance limit each curve becomes again horizontal. This tendency agrees with that of the previous results in axial stress, but the S-N curves are not parallel with each other against the previous results. (2) About the notched specimen, the endurance limit curve in the endurance limit diagram under torsional stress has a tendency rather to increase in stress amplitude for large mean stress. This phenomenon is believed to be based on the strengthening effect of strain-hardening which has occurred in the early stage of fatigue accompanied with the decay of mean stress, and of coaxing effect that the material of notch root undergoes. (3) Using the above results, we found a method of construction of empirical formula representing the τmax-N relation.