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Technical Brief: Fretting Effects on Fatigue Strength

 

Fretting occurs between two mating surfaces, which are oscillating and under load, and reveals itself as pits in the surface surrounded by oxidation debris.

 

We all know that fretting substantially reduces the fatigue strength of carbon steels.  What is the mechanism by which this strength reduction occurs? 

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. 

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.)  

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.

 

References

"Failure Analysis and Prevention", ASM Metals Handbook Vol. 10, 8th Edition.

"Fretting Fatigue", R.B.Waterhouse, Applied Science Publishers, 1981

"Fretting Corrosion", R.B.Waterhouse, Pergamon Press, 1972.

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