Fatigue life extension by surface machining

[cjkelly83]

I'm looking for any guidance or papers on how fatigue lives can be extended for carbon steel through the machining of the surface to remove surface defects as they develop.

I have found a comment in Fatigue of Metallic materials by Mirk Klesnil and Peter Lukas that refers to work done with copper that showed surface machining after 80% of standard fatigue life elapsed increased life by a factor of 10.

 

[mp87]

Hi,

Do you mean a carbon steel that is already operating? Wouldn't it be easier to shot peen it instead?

 

[kingnero]

Railway track is constantly machined to remove the upper layer that suffers from rolling contact fatigue. No doubt the benefit of this application is much, much higher than 10x (but to be honest, here we're talking about machining several times as well).

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[metengr]

Why would you consider intermittent machining of surface where material is removed resulting in less material to sustain loads and increase stresses? The goal is to introduce a compressible stress from burnishing or shot peening which increases fatigue life with no loss of material.

 

[kingnero]

In certain cases, e.g. the one I gave above, it's certainly the best solution.
However without more information from the OP, it's all guesses at best. The OP asked about machining, maybe he has his reasons for this...

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[cjkelly83]

Hi,

apologies for the delayed response, hopefully the info below is informative and not patronising. It is not the intent to machine the surface to reduce fatigue, it is more that in service the surface has been machined and I want to know if I can make any account of that to reduce fatigue calculations.

The particular application is of an offshore well conductor within its guide frames. The conductor is a vertical pipe that goes from seabed to the platform above water and it protects the oil pipelines from the hydrodyanmic loads. To stabilise the conductor there are guides at various levals which allow a degree of movement +/-39mm.

The particular platform is quite old and is exhibiting material loss within the guides due to fretting. Some of the conductors have already failed at the guides but many are still 'intact' albeit with lost material.

There is an intended repair regime and part of that is a measuremnt of material loss and then a decision on whether the conductor is at risk of failure in the near future. The two criteria for this assesment are ULS and FLS. The ULS is quite clear and easy to define - we have determined the minimum remaining wall thickness that is sufficiently strong to resist the 100yr storm. Based on this thickness a fatigue analysis has been performed to give guidance on the remaining fatigue life. This analysis which uses conventional design S-N curves shows that there is in effect zero fatigue life left at the ULS minimum thickness

A risk based decision is therefore being made on whether to worry about fatigue - it was posited that because the low fatigue life is due to the fact the material has been gradually worn away, the surface material that is accumulating fatigue damage is constantly renewed and therefore faticue life is extended.