To investigate the effect of surface work hardened layer on the completely reversed bending fatigue strength, a 0.17% carbon steel (JIS SS41) was tested. The specimens were machined and their surfaces were ground, and vacuum annealed at 650°C for 45 minutes. One group of specimens was electrolytically polished to remove the surface work hardened layer, and the other group was shot peened and the surface roughness was smoothed, to some extent, by emery paper and electrolytic polishing. The fatigue tests were carried out with the Schenck type machine and the change of residual stress was measured during the fatigue process by the X-ray method.
The S-N diagram shows that the shot peened specimens have higher fatigue limit, but at the high stress amplitude the fatigue lives are shorter than the specimens without the work hardened layer. The surface compressive residual stresses of the shot peened specimens are decreased to 2/3 of the initial value by application of only one cycle of stress amplitude. After that, however, the decrease of residual stress is not so drastic but linearly proportional to the logarithm of number of stress cycles. But when the stress amplitude is below a critical value the residual stress is changed only by the first stress cycle, and remain unchanged thereafter.