Cold bending and yielding 1

[cyt4]

Consider this:

Couple of T and elbow sections are seamless cold worked tubing(inco625,same filler). T has a hole cut and hole is drawn up to form the collar. How do we account for any of the thinning and residual stresses? Do they matter? If so, is there a good method to account for the residuals?

I am interested in this problem because analysis is predicting plastic straining after each pressure and thermal cycle(displacement based loading) at the t and the elbow. I would like to limit the parts' life to a plastic strain limit. Should we test the T and the elbow to actually get the cycle life data? How would one go about substantiating a yielding part is the more general question...

thanks

 

[LittleInch]

Thinning is accounted for within the design of the item and normally measured.

Residual stress not so common to account for as normally you don't get into plastic yielding other than extreme design case.

Plastic Straining after EACH cycle looks to me like something quite odd and most definitely needs testing IMHO. How much plastic strain aree we talking about here?

What design code are you using / following? They normally provide some guidance.

A bit more background or drawings would help.

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

 

[cyt4]

Hey Littleinch,I am just looking at some of the nuclear codes for guidance. There is no requirement to follow one either(aerospace). Basically, the proposed design is predicted to be yielding for the high temp differential case where the lack of flexibility of the line forces enforced displacements on the Tee and elbows(predicted from a global model). I am getting my relative displacements and using a detailed 3D FEA to find the strain after each cycle. The strains for the proposed T are on the order of 1% after the proof, pressure+thermal cycle, which is a nominal case. I know that the model might not represent reality but it is a start. Do you normally test out your weld or like cold drawn piping? Or codes "just take care of you"?

thanks

ps.Especially this was very helpful as it looks like they care a lot about plastic side of analysis:

http://www.world-nuclear.org/getmed...83/REPORT-Non-Linear-Analysis-Part-1.pdf.aspx

 

[LittleInch]

There are a few nuclear guys on this forum but I don't touch it, so maybe not the best person.

Don't really understand your last sentence.

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

 

[TGS4]

Are you talking about cyclic plasticity or ratcheting?

 

[cyt4]

It looks like plastic strains decrease after each cycle. The unload does not increase the strain. I have also run it for some cycles to see if ratcheting stopped. It became quite small after <100 cycles. I guess it ratchets and then settles to a small plastic strain and cycles. Meanwhile it consumed about 6% strain.

 

[TGS4]

If eventually you shake down to a stable cyclic plasticity hysteresis loop, then you just have cyclic plasticity. You will need to do an elastic-plastic fatigue analysis per VIII-2, Part 5. Residual stress is not part of the consideration, because it will disappear after the first cycle or two. Any "simple" Code will not just take care of you - this is far beyond the realm of most typical construction Codes.