failure mode for criteria 3Sm

[tigny]

Hi,

I'm working on a pressure vessel that I try to assess according to ASME VIII div 2 fith FEA elastic analysis.

For 4-134 case the calculated maximum shear stress is 197MPa, away from local discontinuities; the calculated stress (2 times 197) 394Mpa is > 3Sm=345 MPa

I would like to know which kind of failure can be expected with this result, i.e. excessive elastic deformation, excessive platic deformation, ...?

yours truly,

 

[prex]

The 3S_m limit is, for a secondary stress, against ratcheting, that is accumulation of plastic deformation in repeated load cycles.
What stress situation are you examining? Such a shear stress is unusual.

prex

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[tigny]

Thank prex.

For 3Sm, these are stresses due to pressure and temperature on a flat plate.

There are parts of the equipement at cryogenic temperature and other parts at steam temperature.

Furthermore the thickness of the plate is low for the pressure I calculated. (primary stresses are already 13% higher than Sm).

Stresses at the center of the plate are lower than 1.5Sm.

Regions of local primary stresses higher than 1.1Sm are difficult to assess since the 1.0sqrt(Rt) cannot be calculated for this flat plate. Do you know if this has to be verified for a flat plate?

Thanks again for your answers.

yours,

tigny

 

[prex]

Yes it has to, but I would be surprised to find a local membrane stress in excess of Sm in a flat plate, and also am surprised of a (general?) primary membrane in excess of Sm. Only bending is normally of concern for a flat plate.

prex

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[tigny]

Prex,

thank you for your answer, which makes me think of going back to the basics.

I'll have a check on finding the membrane stresses (I use I-deas; thin shell elements model for primary membrane stresses; a 3D parabolic tetrahedrons for the 3Sm limit) then I'll come back to give you the right numbers.

yours truly,

tigny

 

[prex]

OK, thin shell elements will give you membrane and bending stresses, but these are not necessarily primary.
Primary and secondary stresses are recognized by their behavior and are normally associated with the type of action the causes them (external load, thermal or differential expansion,...); by no means a solid element will display primary+secondary stresses and a plane one the primaries, if the loads are the same for both!

prex

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Online tools for structural design

 

[tigny]

Prex,

I'm still working on this project.

I'm coming up with another question: chapter 4-137 triaxial stresses; "the algebraic sum fo the three primary principal stress..."

Does it mean this check is to be done only with primary stresses (pressure in my case)?

yours,

tigny

 

[prex]

Yes, code wording is self explanatory.
Personally never met a situation where that criterion was relevant.

prex

http://www.xcalcs.com

Online tools for structural design