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Concept
Fretting
Summary
Fretting refers to wear and sometimes corrosion damage of loaded surfaces in contact while they encounter small oscillatory movements tangential to the surface. Fretting is caused by adhesion of contact surface asperities, which are subsequently broken again by the small movement. This breaking causes wear debris to be formed.
If the debris and/or surface subsequently undergo chemical reaction, i.e., mainly oxidation, the mechanism is termed fretting corrosion. Fretting degrades the surface, leading to increased surface roughness and micropits, which reduces the fatigue strength of the components.
The amplitude of the relative sliding motion is often in the order of micrometers to millimeters, but can be as low as 3 nanometers.
Typically fretting is encountered in shrink fits, bearing seats, bolted parts, splines, and dovetail connections.
Fretting damage in steel can be identified by the presence of a pitted surface and fine 'red' iron oxide dust resembling cocoa powder. Strictly this debris is not 'rust' as its production requires no water. The particles are much harder than the steel surfaces in contact, so abrasive wear is inevitable; however, particulates are not required to initiate fret.
Fretting in Aluminium causes black debris to be present in the contact area due to the fine oxide particles.
Fretting examples include wear of drive splines on driveshafts, wheels at the lug bolt interface, and cylinder head gaskets subject to differentials in thermal expansion coefficients.
There is currently a focus on fretting research in the aerospace industry. The dovetail blade-root connection and the spline coupling of gas turbine aero engines experience fretting.
Another example in which fretting corrosion may occur are the pitch bearings of modern wind turbines, which operate under oscillation motion to control the power and loads of the turbine.
Fretting can also occur between reciprocating elements in the human body. Especially implants, for example hip implants, are often affected by fretting effects.
Official source
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