Increasing Behavior of Strength for Fatigue Fracture of Austenitic Stainless Steel during Fatigue Tests at Elevated Temperature

In order to clarify notch effect on fatigue strength of an austenitic stainless steel, rotating bending fatigue tests were carried out for smooth and notched specimens at 573K. Clear endurance limits were recognized and the fatigue strength did not decrease continuously with increasing stress concentration factor K(t), notwithstanding any non-propagating crack was not recognized at notch roots for specimens endured at 10(8) stress cycles. According to hardness tests, an area of notch root hardened during fatigue test by dynamic strain aging. The hardening behavior occurred remarkably with increasing K(t). Effects of K(t) and the hardening behavior on fatigue strength canceled each other, and then fatigue strength became insensitive to K(t). Fatigue strength sigma(SIT) obtained by stress increment fatigue tests became higher than the ordinary fatigue strength for each specimen of K(t) that is, the coaxing effect was recognized clearly. This result indicated that the strength for fatigue fracture increased gradually during fatigue test. sigma(SIT) had a peak value at K(t) of 2.0 and it decreased with increasing K(t) sensitively. The increasing behavior of strength for fatigue fracture depended on K(t) and fatigue fracture was controlled by circumstances of hardened material and stress amplitude considering K(t).