Erdem Özdemir - 030070307 - 9th Week Definitions - Part 3

Fretting

Mechanical FailureNew - better definition

Fretting may be defined as the damage caused by low amplitude oscillary motion between two contacting surfaces. Within this broad definition, fretting wear and fretting fatigue mechanisms are included. In fretting wear, where the damage is measured by the volumetric material losses, there is no lower limit on the contact pressure, or amplitude of oscillation, below which wear damage will not occur. It also occurs whether the mode of motion is micro or macro slip, in the presence or absence of alternating bulk stresses in one of the contacting bodies. Fretting fatigue, on the other hand, is defined in terms of rreduction in fatigue strenght or fatigue life, regardless of the severity of the associated fretting wear damage. This effect tends to be more pronounced for high strenght materials. Unlike fretting wear, fretting fatigue damage requires the presence of body stresses and it has a lower limit, which is defined by the fretting 
fatigue strenght. If the applied body stress is smaller than this, the fatigue life is indefinite and the cracks nucleated by the fretting action will not propagate to failure.


(Fretting Fatigue: Current Technology and Practices, 1367. issue - Editör: David W. Hoeppner,V. Chandrasekaran,Charles B. Elliott p.229)

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Fluid flow or mechanical rubbing can cause removal of or damage to a protective oxide, increasing the proximity of the bare metal and the attacking medium. This can result in increased attack rates because a stable oxide layer is frequently a corrosion rate limiter. An example is erosion corrosion caused by high flow rates of domestic hot water in copper pipes, especially around fittings, which can generate turbulence. Another example involves the press fit of gears, wheels, pulleys, etc., onto shafts that experience elastic torsion, or bending. Small relative motions occur at the contacting surfaces which mechanically break up protective oxide layers. This type of corrosion is known as fretting corrosion.
(Standard Handbook of Machine Design (3rd Edition), Edited by: Shigley, Joseph E.; Mischke, Charles R.; Brown, Thomas H. Jr., p44-28)