Hold-Time Effect on Notch Sensitivity in Low-Cycle Fatigue Life of Metal at Elevated Temperature

Evaluations of Both ASME Code Case N-47 and Neuber's Rule

Abstract

The hold-time effect on notch sensitivity in low-cycle fatigue life was studied at 600°C in air by using austenitic stainless steel SUS 316. The total strain controlled tests with the hold-time t_H from 10min. to 60min. were performed by use of the round notched specimens with the stress concentration factor K_t of 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 t_H. But the rate of decrease in fatigue life with an increase of t_H was the most at t_H of 10min. and at a longer hold-time the life was almost constant. Particularly, for the case of K_t of 2.6 the fatigue life increased slightly with an increase of t_H in connection with notch-strengthening of the metal tested. Also, the life prediction method of“Modified both frequency ν and K_t fatigue life rule” of N_f=N_f0K_t^mν^l was applicable in order to predict accurately the life of a notched specimen at any condition of K_t and t_H. The life prediction rule which corresponds to ASME Biler & Pressure Vessel Code Case N-47 was evaluated and the prediction was found inaccurate in comparison with that mentioned above. Furthermore, Neuber's rule was examined from the elasto-plastic solution for a semi-infinite plate with a semi-elliptic notch under anti-plane shear and the modified rule of K_t^b=KσKε was found applicable to the prediction in creep-fatigue interaction, where b takes the numerical value from 1 to 2.