Isochronous stress strain curves instead of direct creep analysis

[M.hmk]

Is it possible to use Isochronous stress strain curves instead of direct creep analysis(with a creep material model) to analyze systems that operate in creep range?

I want to obtain the amount of strains for a system which operate in creep range after 10000 hrs.
Therefore I want to perform static non-linear analysis with use of Isochronous stress-strain curve related to 10000 hrs as material model. Is it right?

Thanks in advance.

 

[mskds545]

If you mean the two sentences:

"Which is a caution that strain limits may also need to be based on the stress state rather than a set of unique values as in [ASME III]. Examples of such an approach are provided by the time independent local strain criteria in [ASME VIII-2 5.3.3] or the multiaxial damage criteria in [API 579]."​

I would interpret that to mean that the time-independent local strain limit is an example of how stress state can be taken into account when formulating a strain criterion, not as suggesting that 5.3.3 is meant for use in the creep regime. Using the time-independent limit would imply that ductility has no rate-dependence -- that is, a low stress and long time failure would have the same ductility as a high stress and short time failure.

-mskds545

 

[r6155]

While you guys are still calculating, who is fixing the cracks in the welds?
I don't know a project manager who can wait so long for the calculators to repair the welds.

Regards

 

[mskds545]

Not sure if you're saying I should decline to answer his questions about elastic follow-up on principle, or that I should pack up my welding gear and go fix his plant instead of answering... but we're under quarantine, so the latter isn't really an option, and nobody should want me welding on anything that is going to contain pressure anyway. From the little he told us about the actual application, I agree that it sounds like a welding problem and said as much. But elastic follow-up is a real thing, I've seen it cause real failures, and it can be characterized by calculations in meaningful ways, so I'm not sure why it isn't worth discussing.

-mskds545

 

[M.hmk]

@r6155
Two action plans has been started together. One short term plan to allow start-up of the plant as soon as possible including complete NDE on all of the line joints and repair of revealed cracks. The first plan has been started at site.

The long term plan will be implemented in the first coming overhaul or as soon as detection of abnormal behaviors. this plan includes new route design with reduction of size and thickness.
Indeed I am investigating the possibility of elastic follow-up and finding a method to evaluate it in old route. Then I will consider it in design of new route.

 

[M.hmk]

@mskds545

How about step 13 of para. 10.5.2.3 of ASME FFS-1 2016 ? There is a criterion for accumulated inelastic strains in table 10.3. But it is independent of stress state.

 

[M.hmk]

@MSKDS
Do you have any idea about my last post?
Your comments will be appreciated.