How to locate primairy and secondary stresses with FEMAP

[ronski1]

We want to analyse pressure vessels with finite elements by using FEMAP. We analyse also the alternative way but that is not the isue.

We have problems to locate the primairy and secundary stresses according to ASME. I need to find the following stresses:
Pm membrane stress
Pb bending stress
PL local membrane stess
Q secundary stresses

In Femap I can generate the following stresses:
Principe stress
mean stress
Normal stress in X and Y
Von Misses

I use plate elements

Can anyone tell how to locate these stresses

 

[TGS4]

Not to discourage you - but you are in WAY over your head. I recommend getting your hands on WRC Bulletin 429, and the ASME "Criteria" document, and then, after you've done all that, hire a consultant for the first couple of times that you need to perform these evaluations. Have them "mentor" you in how this is done.

Also, take a look through the eng-tips archives for discussions of this nature...

 

[ronski1]

We already ordered the bulletin. We also asked a mentor but they cannot explain how to locate these stresses. There is a API tool voor midplane membrane stress, however this gives more than one membrane stress situtation. I also used principal stress and von misses stress, but that ain't enough. These are all combined stresses.
ASME requires a separation of primairy stresses and so called secondary stresses.

It would be a lot easier if ASME gave a criteria in principal and von misses stress. But that ain't the case, so i 'll hope to get some straight answers here.

 

[TGS4]

For starters, you are going to be dealing in component stresses, so I would forget about using any of the invariants. Have you had the opportunity to peruse the 2007 Edition of Division 2? In it, Annex 5A has a good discussion about stress linearization.

The classification of stresses - primary or secondary, have everything to do with the source of the stress - primary satisfies equilibrium, while secondary satisfies compatibility. The techniques for linearization are in either the WRC Bulletin or Annex 5A (the WRC Bulletin has good explanations, so I use that extensively).

I take it from your questions that you are reasonably well versed in FEA, but not so much in the ASME/pressure vessel world. I would try to find a mentor that excels in BOTH fields. Good Luck!

 

[TGS4]

Oh - and there are numerous technically valid reasons why the ASME rules are written as they are. I suppose it would be a lot easier if the rules/criteria were written in a way that your general FEA program could apply them, but then the rules would be technically incorrect... Ah the joys of pressure vessels...

 

[ronski1]

Thanks
We have 2007 Edition of Division 2. It is very difficult to apply this to femap. I can now locate the primairy membrane stress with a midplane stress tool. But the next question is how to find the local stress and how to recognize it. In other words, what is the definitation of a local area, how large are these areas and when is a stress spot peak and when local.

I'm a little bit confused.
I'll hope that you can help me futher

 

[sdra2]

Hi Ronski

This may already be known to you, but it contains a good discussion around primary and secondary stresses along with non-linear inelastic FEA approaches. I'm guessing that the consultant/mentors that TGS4 refers to would have practical experience in both of these approaches, and how applicable they are to ASME code.

Cheers.

http://www.nafems.org/resources/knowledgebase/