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failure mode for criteria 3Sm

failure mode for criteria 3Sm

failure mode for criteria 3Sm

(OP)
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,

RE: failure mode for criteria 3Sm

The 3Sm 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

http://www.xcalcs.com
Online tools for structural design

RE: failure mode for criteria 3Sm

(OP)
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

RE: failure mode for criteria 3Sm

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

http://www.xcalcs.com
Online tools for structural design

RE: failure mode for criteria 3Sm

(OP)
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

RE: failure mode for criteria 3Sm

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

http://www.xcalcs.com
Online tools for structural design

RE: failure mode for criteria 3Sm

(OP)
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

RE: failure mode for criteria 3Sm

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

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