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Failure Analysis Case Histories
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Technical Brief: Fretting
Effects on Fatigue Strength
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Fretting occurs between two mating surfaces, which are oscillating and under
load, and reveals itself as pits in the surface surrounded by oxidation debris. |
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We all know that fretting substantially reduces the
fatigue strength of carbon steels. What is the mechanism by which this strength
reduction occurs?
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In the fretting area it appears that there is substantial
local plastic deformation of the metal surface layers. This can often be seen
metallographically, either non-destructively, via replicas, or by examining
conventional sections. Small crack-like flaws can be generated in this deformed
material and one or more of these can act as "conventional" fatigue cracking
initiation sites. |
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If fretting fatigue cracking does occur it takes place close
to the boundary of the fretting area. If you have fatigue crack initiation
that is quite close to, but not at a stress concentrator you should always
investigate the possibility of it being of fretting origin. (Typically we will
see one to three cases of fretting fatigue per hundred
fatigue failures, so it is not common. But, if anybody is investigating a large
number of fatigue failures and are not seeing any of fretting origin it might be
that they are not being recognized. It is not glaringly obvious.) |
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One thing that we
have seen with fretting, and others have observed it too, is that sometimes the
resulting cracks remain shallow. They sometimes do not grow beyond a 1 or 2 mm
depth in the immediate fretting region. In those cases repair might be
possible.
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References
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"Failure Analysis and Prevention", ASM Metals Handbook Vol. 10,
8th Edition. |
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"Fretting Fatigue", R.B.Waterhouse, Applied
Science Publishers, 1981
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"Fretting Corrosion", R.B.Waterhouse,
Pergamon Press, 1972.
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