Further observations on the shape of theS/Ncurve of a ferritic steel

The effect of testing frequencies at levels of 50 and 100 Hz on the shape of the S/N curves of an EnlA steel has been investigated. Axial loadings of the zero to peak (R = 0) and completely reversed (R = −1) types were applied to plain testpieces and the results are compared with rotating cantilever testing. At 50 Hz two discontinuities were observed in the S/N curves for both R=O and R=−1 loadings. At 100 Hz there was no lower discontinuity and the fatigue limit was significantly higher than that obtained at 50 Hz. The role of strain-ageing effects on these observations is discussed.