Abstract
Fatigue tests were carried out at frequencies of 20 kHz, 600 Hz and 100 Hz for low-temperature-tempered SNCM439 steel. Ultrasonic, high-speed servohydraulic and electromagnetic fatigue test machines were used for 20 kHz, 600 kHz and 100 Hz tests, respectively. The fatigue tests showed that fatigue properties were independent of the frequencies, while fatigue lives were distributed in wide range on the S-N diagram. Almost all specimens were broken from internal Al2O3 inclusions of various sizes between 9 and 82 μm. Fatigue lives were found to depend on the inclusion sizes. ODA, i.e. optically dark area, was observed in all specimens broken over 107 cycles. ODA sizes were also independent of the frequencies. The scattered S-N diagram was found to be expressed by an unique curve by normalizing stress amplitudes σa with fatigue limits σ'w estimated by Murakamis equation. The modified S-N diagram showed a clear fatigue limit between 109 and 1010 cycles.
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