Low Cycle Reversed Bending Fatigue Curves of 9% Ni Steel and Weld Metal

Abstract

Deflection controlled reversed bending fatigue tests in low cycle fatigue range were carried out at ambient temperature and at-196°C on five 9% Ni steels of ASTM A553 grade A and transverse butt joint specimens made by MIG arc welding and manual arc welding. In addition, notch effects were investigated by introducing a surface notch of the elastic stress concentration factor of approximately 3 into the test section of both base and weld metals. Cyclic amplitude of surface strain at the test section, which was continuously measured by a plastic-range type strain 'gauge at ambient temperature and-196°C, showed a stationary behaviour after a number of imposed cycles corresponding to a few percent of crack initiation life. This stationary strain amplitude ea is used for the evaluation of low cycle fatigue strength which is plotted against number of cycles to crack initiation N_c. In case where measurement of surface strain amplitude failed, this was estimated from a dynamic calibration curve of ε_a versus bending deflection.
The relation between ε_a and N_c was expressed by a modified Stromeyer's formula, providing any given percent probability curve for crack initiation basis. Notch effects are observed at room and low temperatures by showing approximately 1. 7 of fatigue strength reduction factor, while weld metals are insensitive to the effects. Fatigue strength is generally higher at-196 °C than at ambient temperature. Weld metal shows lower fatigue strength than base metal in the whole life range.