Fatigue limit of circumferentially notched metastable austenitic stainless steel under positive/negative stress ratios: Synergistic effects of notch-root mechanistic singularity and phase transformation

•Effects of circumferential notch on fatigue strength of Type304 steel were studied.•Stress concentration factor, Kt, and stress ratio, R, were each varied in two types.•Improvement/deterioration of fatigue limit was decided by combination of Kt and R.•Key factors were cyclic plasticity and phase transformation at the notch root.•Clarifications for the phenomena were provided via microscopy and simulation. Fatigue tests of circumferentially notched Type 304 austenitic stainless steel (ASS) illuminated the unique dependences of fatigue limit on the stress concentration factor, Kt, and stress ratio, R. Fatigue limit under negative R was superior to that under positive R, a result was unexpected from the empirical behavior of smooth ASS. Moreover, switching R from negative to positive apparently diminished the effect of Kt escalation on the deterioration of the fatigue limit. Consolidation of microhardness tests, microstructural observation, finite element analysis revealed that these anomalies stem from the presence/absence of cyclic plasticity and martensitic transformation, which dynamically strengthen the notch-root.