Fatigue threshold at high stress ratio under periodic underloads in turbine runner steels

The aim of this experimental work is to determine the effect of turbine start/stop sequences (periodic underloads) on the fatigue threshold of small cycles at high stress ratio (baseline cycles) in two steels used in turbine runners, i.e. AISI 415 and 304L steels. Keeping Kmax constant, a first load procedure is conducted with decreasing ΔK to measure fatigue thresholds at 2 × 10−7 mm/cycle under both constant amplitude loading and periodic underloads at various frequencies. Then a second load procedure is conducted to measure fatigue thresholds at 6.7 × 10−9 and 2 × 10−7 mm/cycle under periodic underloads at one frequency with increasing ΔK of baseline cycles from zero. The periodic underloads applied above a certain frequency decrease the fatigue threshold measured at crack growth rate of 2 × 10−7 mm/cycle. The decrease in fatigue threshold due to periodic underloads is about five times higher when it is measured at 6.7 × 10−9 mm/cycle.