The Effect of Torsional Deformation on the Fatigue Strength of Carbon Steel

Abstract

Hereunder is presented a report of studies made on the influence of preliminary workings on the fatigue strength of carbon steel. The effects of residual stress and structural change that are produced by cold-workings on the fatigue strength are discussed. For the increase of fatigue strength, in the case of uniaxial deformation like tension or compression, as previously reported, the structural change is considered to be more responsible than residual stress. In the case of biaxial deformation like torsion, however, these fatigue behaviours are not clarified yet.In this paper, the effect of torsional preliminary working on the fatigue strength of S45C steel is discussed in comparison with that of tensile and compressive preliminary workings. The results obtained are summarized as follows:(1) The residual stress produced by torsional deformation assumes a shearing state in the direction counter to the twisting direction that has been previously given.(2) The materials subjected to preliminary workings, in plastic deformation by three kinds of strain, twisting, stretching and compressing, each to equal level of degrees, have shown increase in fatigue strength quantitatively in the descending order as follows;twisting strain>stretching strain>compressing strain(3) The fatigue limit of twisted specimen continues to increase until such a straining as e*=0.400, and then it tends steeply to decrease. Considering the stress strate of twisted specimens, it is concluded that the residual stress due to the preliminary working does not affect the fatigue limit, although the apparent stress amplitude increases with the shearing residual stress. The fractured surface is considered as apt to be preliminary influenced by the shearing residual stress.(4) From the results of the tension test and the compression test of each strained specimen, it is apparent that the yield point for reversal direction of the deformed direction is lower than that of the annealed specimen because of Bauschinger's effect in the case of stretched and compressed specimen. On the other hand, the yield points in both the tensile and the compressive directions in the case of twisted specimen increase over that of the annealed specimen. These anisotropy of work-hardening is considered to contribute to the result that the fatigue strength of twisted specimen is largest among the preliminary strained specimens in the same amount of equivalent plastic strain by the three kinds of straining processes.