The change in the residual stress induced on the surface of unnotched mild steel specimens by turning was investigated under rotating bending fatigue tests. The residual stresses were measured periodically on the surface by the X-ray film method.
The fraction of decrease of residual stress to the original residual stress vs. the logarithm of the number of cycles were found nearly linear, and the slope of these lines depended upon the applied stress-amplitude. At stress amplitude near the yield strength of this material, i.e. σ=40kg/mm², there was remarkable relaxation of residual stress. At this stress amplitude, the initial residual surface stresses of about 35∼40kg/mm² induced by turning was reduced almost to zero after the number of cycles N=10³∼2×10³ (cycle ratio was about 0.1∼0.2).
The S-N curves of the specimens with and without work hardened layer, annealed before and after turning, crossed each other at high stress level. At least, as one of the reasons for this fact it can be considered that the beneficial compressive residual stresses which work efficiently at low stress level faded away in the early stage of the life at high stress level.
Usually we use rotating bending fatigue specimens machined by turning. Then, in multiple stress amplitude fatigue tests, especially with the initial stress level higher than the second stress level, the decrease of residual stress must be taken into consideration.