The Effect of Stress Amplitude below the Fatigue Limit on Cumulative Fatigue Lives in Perforated Round Specimens

The fatigue lives under the service load are usually determined by the stress amplitude over the fatigue limit, but several experiments performed hitherto have showed that the stress amplitudes below the fatigue limit, which take the greater part of the service load, influence the fatigue life. But there is not any systematic experiment reported on this problem. Therefore authors have performed program fatigue tests by rotating bending with perforated round specimens of normalized 0.35% carbon steel under the load spectra, which include a stress amplitude below fatigue limit. To clearify the effect of these low stress amplitudes, the length of the fatigue crack is measured. As for the program fatigue lives in this experiment, modified Miner's law can be used for the estimation. According to the results of the measurement of fatigue cracks, the effect of the stress amplitude below fatigue limit changes depending on the crack length at the time the stress is applied, the magnitude of the stress amplitude, and the number of stress cycles in a program cycle.