Frequency Effect on Notch Sensitivity of Low-Cycle Fatigue Test at Elevated Temperatures (Evaluation of ASME Code Case N-47)

Abstract

The frequency effect on the notch sensitivity in low-cycle fatigue life was studied at an elevated temperature. The material used was SUS 316 austenitic stainless steel and the nominal total strain range controlled tests were performed at 600°C in atmosphere with the frequency range from 0.001 to 1Hz. The elastic stress concentration factors K_t employed were 2.6, 4.2 and 6.0. From the tests, the following conclusions were obtained.
The number of cycles to failure of the notched specimen decreased with an increase of K_t. But the rate of decrease in fatigue life with an increase of K_t decreased with decreasing frequency. The above results were discussed from the viewpoints of applied stress cycling and of a model analysis on crack propagation. Also discussions on the life prediction rule of notched specimens were presented in connection with ASME Boiler & Pressure Vessel Code Case N-47. An empirical formula of N_f=N_f0K_t^m ν^l was proposed and shown to predict accurately the life of notched specimens at any condition of K_t and ν. Furthermore, the background of the empirical formula was discussed from the viewpoints of Neuber rule and the frequency modified fatigue life equation. The same type of the formula was derived by combining the frequency modified fatigue life equation with Neuber's rule.