6. Frequency

Within a limited frequency range that overlaps, for example, that of conventional testing machines (5 to 200 Hz, i.e. for a rotating machine between 300 and 12,000 rpm), it is considered that, in the absence of heating or corrosion, test frequency has no influence on the endurance limit of steels.

However, when the frequency is increased significantly, an improvement in the endurance limit is generally observed. This strengthening of the metal at higher frequencies is due to the shorter duration of maximum stress. Disregarding this increase in endurance is therefore in the interests of safety. However, Neppiras' proposal [5] to use the S-N curve of vibration fatigue materials has triggered a great deal of research into material endurance. The results show that there is no fatigue limit at 10...