The effect of gradient microstructure generated by ultrasonic surface mechanical attrition treatment (SMAT) on the fatigue performance of an austenitic stainless steel has been evaluated in the context of localized and general plasticity by comparing two load ratios: fully reversed tension-compression (TC) R_TC = -1 and tension-tension (TT) R_TT = +0.1, respectively.

After identical SMAT processing conditions, the fatigue limit was enhanced by +17% for R_TC while it was reduced by -7% for R_TT. Under R_TC cyclic loading, self-heating of the specimens and sub-surface martensitic transformation occurred but not for R_TT. The residual stress measurements have also revealed that while the stress gradient was smoothed after the R_TC fatigue loading, it was completely reverted for the R_TT one. In turns, the fatigue crack initiation sites and propagation were modified between the R_TC and R_TT loading conditions.

Considering the stabilized surface residual stress after fatigue loading, the use of a Crossland criterion allowed to explain both the effects of load ratio and SMAT on the high cycle fatigue behavior of the stainless steel. It turns out that under the studied loading conditions the modifications of residual stress state can be considered as the primary factor governing the varying fatigue performances and the observed triggering at different initiation sites.