Elastic-Plastic Analysis for brittle materials 1

[MG83]

Hi,
I have a project which is a part of pipeline equipment (insulation joint) that contains a brittle filler between metallic parts.
How can i perform elastic-plastic analysis?
Is it possible to use perfect plastic for that material?

 

[TGS4]

No.

An elastic-plastic analysis assumes that a material has adequate/good ductility. This is not an appropriate assumption for a brittle material.

More details (including a picture/sketch) may yield some suggestions.

 

[SJones]

It would be helpful if the OP were to elaborate upon the specific objectives of the “analysis’

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[MG83]

As you can see, green parts are brittle.
Please help me for E-P analysis.

 

[TGS4]

I agree with SJones on his - what is the purpose of this Elastic-Plastic Analysis? What failure mode are you trying to demonstrate Protection Against? Plastic Collapse? Local Failure? Buckling? Ratcheting? Fatigue? Leakage? Creep?

 

[NRP99]

IMO for analysis purpose keep the filler material elastic since it will be in "compressed" state assuming the upper half and lower half with seal slots are pressed against each other and also assuming filler material is not supposed to be pressure containing. I would either leave out the evaluation of this brittle material for above case or stick to Su/high factor as allowable stress (not sure though) Whats the purpose of this filler material? To fill the gap?

Pressure containement would need very high thickness of this brittle material to match with pressure containment capacity of the ductile materials of vessel.

 

[SJones]

The purpose of the nonmetallic filler is to provide electrical isolation between the two metallic sides of the fitting. Pressure retention will come from the series of o-ring seals along the channel for the filler.

There would appear to be an incorrect weld placement in the uploaded sketch: the grey to amber joint is generally located beyond the limits of the filler.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[MG83]

The purpose of EP analysis is to evaluate protection against plastic collapse.

What do you think if i assign ep material for that brittle parts and run up to convergence and approving metallic parts.
Then
Check stresses for that brittle material separately in simple elastic analysis with that brittle specification (coulomb mohr safety factor and su/max-principal)

Note that, as sjones said, brittle materials are epoxy resin and epoxy adhesive provide electrical isolation between the two metallic sides of the fitting.

 

[TGS4]

What is your Code of Construction?

If the brittle material is not structural, then you could argue that it’s engineer’s judgement. But, from your diagram, if the brittle material “fails”, then you have a leak path, and you’re relying on this brittle material to not “break”. I don’t know how you define the strength of the brittle material.

You’re going to need more than an elastic-plastic analysis to demonstrate Protection Against Plastic Collapse (the analytical method for which were developed for ductile materials). More likely, you’ll also need to use an approach analogous to Protection Against Local Failure, but where your strain limits in the brittle materials are very low regardless of the state of triaxiality.

Do you have any experimental data that you can use to validate whatever approach you take?

 

[MG83]

Good question.
Generally, I design pipeline equipment which are made of steel. So the code is viii-2. Preparing Elastic-plastic analysis, local failure check, ratcheting check etc.
But in this case i have brittle parts inside the steel parts.
Brittle part is Astm D709 G10 with Su=275 and Sc=414 and E=17237 Mpa.
Maybe it would be better to forget elastic-plastic analysis. Only run elastic analysis and design steel parts with classification lines. Then use maximum stress theory for that brittle parts.

 

[MG83]

Know
If you are agree
Is the maximum stress theory sufficient for the brittle material (max principal stress <Su), or should a more conservative theory coulomb mohr safety factor be used?

 

[TGS4]

I get that you are attempting to perform an analysis according to ASME Section VIII, Division 2, Part 5. Bu that is not your Code of Construction. ASME Section VIII, Division 2 does not permit non-SA materials, so what you are asking for, in the strict confines of ASME Section VIII, Division 2, is clearly just not permitted. At all.

Now, if your Code of Construction is ASME B31.4, then you may have a little more leeway, but you should refer/defer to paragraph 404.10. However, if by "pipeline" you actually mean B31.3 Process Piping, then you should refer to 304.7.2(d).

Note that in ASME Section VIII, Division 1 Code, when brittle materials (such as cast iron) are permitted, the allowable stress is typically Su/10 - see paragraph UCI-23 and Table UCI-23.

All of Part 5 was only intended for ductile materials. that includes the elastic portion, as well.

 

[MG83]

Thank you TGS4
I got it.
But some common used ductile materials such as A694 (for high pressure fittings) are not present in asme.
You mean I can't perform FEA for them according to VIII-2 part5?

 

[MG83]

The allowable stress in UCI-23 (Su/10) is for cast iron and conservative.
I have been using Coulomb-mohr safety factor for years for above epoxy insulator and the equipment has been working without problems for a long time in gas distribution piping system.

 

[TGS4]

MG83 - you're going to need to be much more specific when you say things like "some common used ductile materials such as A694 (for high pressure fittings) are not present in asme". Is this material available/permitted for use in ASME Section VIII, Division 2 (i.e. Section II, Part D, Tables 4 or 5)? How about Division 1 (Section II, Part D, Table 1A)? How about in ASME B31.3? ASME B31.4? Each ASME Code permitted/prohibits specific materials themselves - so a higher level of specificity is required rather than just saying "ASME".

I'm not saying that you can't perform some description of FEA (other other calculation), using a criteria that you have developed. I'm just saying that ASME Section VIII, Division 2, Part 5 is not for brittle materials.

 

[MG83]

You are right.
Thank you so much

Do you know why A694 group (as i khow) are not exist in part D table 4/5 but exist in B31 3?
Does div2 comeetee have a plan to add them?

 

[TGS4]

Do you know why A694 group (as i khow) are not exist in part D table 4/5 but exist in B31 3?
Does div2 comeetee have a plan to add them?

It's likely not been addressed by the Subgroup on Materials (BPV VIII and BPV II) because no one has asked for it.