Fatigue Strength of Low Carbon Steels under the Variations of Stress and Temperature

Abstract

Several fatigue tests were conducted on the low carbon steel at elevated temperatures. First of all, the fatigue strengths under rotating bending stress and the torsional fatigue stress were compared. Then the rotating bending fatigue tests were carried out with stepwise varying stress at 480°C, and the effect of the varying stress on the fatigue life was discussed. Finally, the torsional fatigue tests were carried out to see the effect of the variation of temperature on the fatigue strength.
The mains conclusions from this study are as follows:
(1) The fatigue strength of low carbon steel shows an approximately similar trend against the temperature ranging from room temperature to 500°C, irrespective of whether it is subjected to the rotating bending stress or to the torsional fatigue stress. But the temperature giving the peak of strength is somewhat different in each case.
(2) When the low carbon steel is subjected to the stepwise varying stress at 480°C, Miner's rule is acceptable to hold as the first approximation. There is no significant difference in strengths obtained in the tests with increasing stress amplitude and with decreasing stress amplitude.
(3) Miner's rule also holds in respect to temperature, when it varies cyclically between the two temperatures, in the range higher than 420°C, during the fatigue test. However, when the lower temperature approaches the temperature of blue brittleness, a large deviation from Miner's rule may occur, and a prolonged fatigue life will be obtained perhaps due to the strain aging.