Smaller element size means higher secondary stress?

[Lirock]

Hi,
I got a question about element size and stress. We know that in the area of geometry discontiguous, boundary condition applying and node force applying, there usually gets secondary stress.According to ASME, if we use Sm for primary stress criteria, then we should use 1.5Sm for secondary stress, which means we use 1.5Sm to check the finite element results.
Here is the problem, I found that the smaller size of element, the higher stress we will get, until it goes to a stable value. In engineering, if we use this stable value to be the maximum stress, to make sure it is below 1.5Sm, we usually use much more thick plates than the reality need. I wonder what is the suitable element size?

Regards!
Rock

 

[TGS4]

Question 1 - what edition of the ASME Code are you using?

Question 2 - you will want to refine your mesh until you have achieved a solution that is independent of the mesh.

More questions to follow once you answer these questions.

 

[Transient1]

I am not experienced in applying ASME code, but in your case are you allowed to exceed the yield stress? Accounting for plasticity will make a difference. Then the next question becomes do you have a fatigue requirement, you may need the thicker plate anyways.

I agree with TGS4's response to question 2. If you see that the value is becoming stable as you reduce the mesh size, then that is the solution.

 

[Lirock]

Hi,
I used ASME Volumn VIII, the chapter " Design based on stress". For the primary stress, I calculated them with the function STRESS = E x STRAIN, and it has allowable stress Sm,which is the yield stress divided by safety factor. In the results of FEM, I can use the displacement of nodes to get the primary stress. For the stresses got from FEM directly, should we have to check them with 1.5Sm? If the results are higher than 1.5Sm, we must increase the thickness of plates or do sth to make them to be checked?

Regards!

Rock

 

[TGS4]

liweisc - unfortunately, you're way out of your league on this one. Your best course of action would be to hire someone that has experience with FEA AND ASME pressure vessels. The questions that you are asking have been asked and answered many time over on the Boiler and Pressure Vessel forum. But, judging from your questions, you're in over your head - ask for some help from somebody that knows. This is more than can be provided for free and anonymously over Eng-Tips.

 

[Pawel27]

Maybe there is no need to refine mesh so much. According to EN standard some plastification is allowed. Generally it is said that EN standard is based on ASME code. Read ASME carefully, mabye there is something more about allowed plastic strain limit. I guess that plastification you have at nozzle area - it is normal. For example EN 13445 allows
5% plastic strain.