FRETTING FATIGUE

Fretting is the relative oscillatory tangential movement of small amplitude which may occur between contacting surfaces subjected to vibration, or, as in the case of this review, experiencing cyclic stressing, i.e. fatigue. The surface damage is a function of the contact conditions such as normal load, amplitude of slip, and frequency. In fatigue the additional alternating stresses in the contact region give rise to early initiation of fatigue cracks which may or may not propagate out of the fretted region. An important factor is the coefficient of friction which in turn is influenced by the condition of the surfaces, i.e. their composition, surface roughness, residual surface stresses, and oxide or other surface films. The environment is also a contributory factor to the initiation and propagation of cracks. High temperature oxidising atmospheres may promote the formation of a protective oxide with low friction properties on certain high temperature alloys. Analytical studies have produced a criterion for predicting the site of fatigue crack initiation in practical applications. Based on these analyses and general observations, some guidance is given to measures that can be taken to obviate this type of failure.