Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 131
closely fitting surfaces when they are subject to slight (usually less than
30 µm) relative oscillatory movement. The surfaces are often badly pitted,
and finely powdered oxide debris is formed. A reddish brown detritus is
often seen exuding from fretting ferrous contacts and this is sometimes
referred to as ‘cocoa’! The situation of a loose joint under varying load may
be susceptible to fretting corrosion and also the case when wire ropes rub
together in service over a protracted period. Fretting-induced damage is
observed widely in contacting assemblies as diverse as bearings, the dovetail
notch turbine blade and mechanically fastened joints. Almost all materials
are susceptible to the process and its incidence in vibrating machinery is
high. It may not only cause dimensional loss of accuracy of closely fitted
components, but may also seriously reduce the fatigue strength of an assembly.
No single mechanism appears to be responsible for the phenomenon.
Initially, metallic debris forms due to an adhesion mechanism; this continuously
oxidizes and is mechanically dispersed by the vibratory motion, so that the
formation of a stable protective oxide film is prevented in the fretted region.
The process thus continues indefinitely and, in the later stages, surface pits
and cracks of the order of 100 µm in length are observed.
As discussed earlier in this chapter, the presence of such damage will be
highly deleterious to the fatigue resistance of the material and it is not
uncommon for the fatigue life to be reduced by up to 90% if fretting damage
occurs. The preventive measures adopted depend on whether or not the
surfaces involved are intended to undergo some relative motion.
If the surfaces are not intended to move, it may be possible to prevent slip
by increasing the friction between the surfaces. Alternatively, a thin sheet of
an elastic material may be introduced between the surfaces to accept the
relative movement without slip. If the surfaces are intended to undergo
relative motion, an improvement in the lubrication should reduce the amount
of intermetallic contact and thus ameliorate the situation.
3.5 Further reading
H.T. Angus, Cast Iron: Physical and Engineering Properties, Butterworth, London, 1976.
K. Easterling, Introduction to the Physical Metallurgy of Welding, Butterworth, London,
1983.
W.F. Gale and T.C. Totemeier (eds), Smithells Metals Reference Book (8 th Edition), Elsevier
Butterworth Heinemann, Oxford, 2004.
R.W.K. Honeycombe and H.K.D.H. Bhadeshia, Steel: microstructure and properties, 2 nd
edition, Butterworth Heinemann, Oxford, 2002.
I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold,
London, 1992.
W.H. Kearns (Ed.), Welding Handbook (Volume 2 – Welding Processes), Seventh Edition,
American Welding Society, Miami, Florida, 1978.
D.T. Llewellyn and R.C. Hudd: Steels – Metallurgy & Applications, 3 rd edition, Butterworth-
Heinemann, London, 1998.