S-N fatigue curve deformed pipeline

[usecheijk]

Hi.
Deformational processes, like a dent (15% deep), in a pipeline (API 5LX65) changes the S-N fatigue curve of the deformed material?

Thanks

 

[LittleInch]

Deformation which work hardens the material like dents and gouges, renders S-N curves inapplicable as these are based on stresses in the elastic region.

If you've got a dent that big then it's time to repair it....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TGS4]

LittleInch, you are incorrect. The S-N curves in VIII-2 are actually strain-life, and include strains in excess of yield. In a typical fatigue analysis, a ratcheting analysis is first done to ensure shakedown to elastic action. However, an elastic-plastic fatigue analysis could also be done. The action of deforming/creating the dent did some non-trivial fatigue damage.

However, the S-N data generally includes allowable-stress level mean stress and work-hardened materials anyway.

The geometry of the dent will add some additional stress concentration.

 

[LittleInch]

OK, I'll bow to greater knowledge, however the OP didn't say which code he was using.

I still think a dent of that size is not something which should be allowed.

I'm out of my depth when it comes to fatigue life calcs, but I do know straining the pipe beyond yield takes a big chunk out of the remaining life.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

A dent of 15% of diameter in a pipeline is most likely excessive, probably well over the 2% strain limit in one place or another.

 

[TGS4]

I have performed FFS evaluations on dents of this magnitude in pipelines before. The dent generally doesn't impact Protection Against Plastic Collapse, Protection Against Local Failure, Protection Against Failure From Cyclic Loading: Ratcheting. Based on my experience, it certainly impacts Protection Against Failure From Cyclic Loading: Fatigue. And for carbon steels in sour service, there is often a limit on plastic strain due to concerns about residual stress with respect to stress corrosion.

 

[BigInch]

I tend to agree, however it does affect pig passage, can affect pressure drop, can increase erosion and buckling potential from axial load and external pressure.

 

[usecheijk]

Thanks to all for your answers,
But the question is: ¿can the S-N curves be used in fatigue life analysis of dents where residual stresses and plastic deformation are present?, i.e. ¿S-N curve for this material before and after the dent, is the same?

Thanks,

Jack

 

[TGS4]

S-N curve for this material before and after the dent, is the same?

Yes. But hopefully you understand that there is more to the picture than just that.