Effects of Temperature Levels and Straining Frequency on the Rate of Crack Propagation in a Low Carbon Steel during Low Cycle Fatigue

Abstract

In the experiment, length of crack, which had started from a notch hole, was measured by a microscope, under the basically tested conditions of Δε=1.0%, ν=5cpm. Additional test conditions are some combinations of Δε=0.3, 0.5, 1.5%, ν=0.1, 1.0, 10, 30, 60cpm. The temperature level is between R. T. and 600°C. The results obtained are summarized as follows:(1) During a strain controlled low cycle fatigue test at 5cpm, the rate of crack propagation is the largest at the blue-brittleness temperature range (200-300°C) and the smallest at the high temperature range (500-600°C). Temperature dependence of the rate of crack propagation seems to be opposite of that of tensile ductility. The period of crack initiation becomes shorter as a temperature increases.(2) At higher temperatures (≥400°C) and low frequencies, numerous micro cracks are initiated in the vicinity of a main crack tip. The main crack propagates rather rapidly by coalescing these micro cracks. Temperature dependence of the rate of crack propagation is effectively influenced by the change of test frequency.(3) The increase in the rate of crack propagation due to coalescing process is estimated qualitatively with the assumption that the micro cracks prohibit the decrease of ductility of a material in the vicinity of a main crack tip.