Material (say steel) Remaining Fatigue Life 3

[PersonalProfile]

Hello,

I try searching for myself (online and on this forum) though could not find a suitable answer.

My question is if you had a piece of material (say steel) which had been subjected to fatigue loading (though for the sake of this question you have no way of quantifying its use – i.e measuring of frequency or magnitude is not possible) can / how do you go about assessing the remaining fatigue life of the said piece of material in a know application?

I found the articles on bridges, though to this layperson, they seem to focus on validating / improving the original loading assumptions to validate earlier predications. Again I may have missed the point.

Any information (links etc) will be appreciated.

Thanks and regards,
Lyle

 

[metengr]

lylebrown00;
Impossible to answer because without knowing or understanding the prior load history all you can do is perform rudimentary fatigue testing and determine if the service-exposed material falls below expected values on the S-N curve.

You should Google fatigue testing, there are lots of university lecture notes and industry technical paper links on fatigue (testing) of metals.

 

[CoryPad]

If you are using stress-based analysis, then you really don't have a way forward. However, you can use Damage Tolerance methods (fracture toughness, allowable flaw size) to measure for defects and compare to a maximum limit that would cause fracture.

 

[PersonalProfile]

Again, I have looked for myself, however rightly or wrongly, I could not find a suitable answer in the absence of "verifying" actual service loading.
This is purely hypothetical, though could I treat the material to provide "as new" material condition. I.e. remove a certain depth (although the available CSA would obviously be reduced - maybe this could be restored by another process).
I understand due to the nature of the issue heat treatments (recovery etc) would not assist.
How would the depth to be removed be calculated?

 

[SJones]

Fatigue doesn't just happen from the surface! If you do not know the past loading history, you only have the option of a fracture mechanics based approach to life extension. That means inspecting the object for existing flaws and calculating the tolerable flaw size for the projected future loading spectrum. Google 'failure assessment diagram,' 'engineering critical assessment' and 'BS 7910.'

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Guest102023]

Only hypothetical answers can be provided for hypothetical questions.

You need to start with the design philosophy, which defines the point of failure; i.e., it could be the moment a crack is formed, or it could be a defined crack length based on an appropriate fracture mechanics analysis and following applicable codes.

 

[PersonalProfile]

Thanks again all.

Brimstoner - my question (less hypothetical, if you like) is if you had a second hand piece of steel which had either been subject to some degree of fatigue loading or you could not confirm that it had not been and wanted to use it for what ever reason - break down etc (fabricate with it, as you would a new piece) can you provide any confidence in its life span (i.e. assess its used life - though without analysing its previous duty)?

Or is the rule, if you do not have a mill certificate and or know it's history it is not really suitable for fatigue applications?


Thanks and regards,
Lyle

 

[metengr]

Lyle;
Your question -

if you had a second hand piece of steel which had either been subject to some degree of fatigue loading or you could not confirm that it had not been and wanted to use it for what ever reason - break down etc (fabricate with it, as you would a new piece) can you provide any confidence in its life span (i.e. assess its used life - though without analysing its previous duty)?

No, not without engineering analysis involving mechanical and nondestructive testing. For the "second hand" piece of steel you would need to know/verify material composition - heat treatment - current mechanical properties and perform nondestructive testing to determine if it contains flaws. In its previous life service-related damage could have altered the original properties or induced flaws.