ASME Sec VIII, Div 2, 5.2.2 - Shell Elements in ANSYS

[mechengm]

I'm a starting junior engineer and fairly new to working with this ASME code, so I wanted to get some clarifications regarding doing the elastic stress analysis with ansys. I'm currently doing the evaluation of the stresses for my linear model according to section 5.2.2 in ASME Sec VIII, Div 2 by doing a load combination from table 5.3. I'm using shell elements in ANSYS, and I understand that I can directly obtain the membrane and membrane + bending stresses. When post processing results in ansys, it only gives me the option to view Von Mises stresses for top/bottom (for bending and local membrane) and middle (membrane). Is this acceptable to use when comparing to my allowable stresses (S, SPL etc...)? My understanding is that when using solid elements, I would use SCL to linearize the stresses and obtain the membrane and membrane + bending stresses (which are NOT Von Mises stresses) and compare these to my material's allowable stresses. But in this case ANSYS would only give me the option to view the three principal stresses and von Mises. If using shell elements Von Mises top/bottom and middle and comparing it to my allowable stresses is acceptable, why? Else why not?

 

[DriveMeNuts]

Have you had a look at Section 5-A.4.2 SHELL ELEMENTS?

ANSYS allows you to create solution plots for 1st, 2nd and 3rd Principle stresses. You then do the calc in 5-A.4.2 and Eqn 5.1 to answer your own question. Do this manual test calc and see if it produces the same result as the ANSYS Von Mises plot.

 

[mechengm]

Thanks for the reply! Maybe I'm misunderstanding what you're saying and correct me if I'm wrong but aren't the stresses mentioned in 5-A.4.2 (σij,in and σij,out) simply just the stress tensors obtained from the inside surface and outside surface of the shell and not the principal stresses? So then using the principal stresses to do the calculations in 5-A.4.2 doesn't work? Even if I plug in those principal stresses in equation 5.1 to obtain Se (Von Mises Stress), what do I do with that? Thanks again!

 

[TGS4]

Your stresses need to first be categorized before they are compared to allowable values. You can’t just compare stresses directly from an FEA output without first doing the engineering judgement work. See 5.2.1.2.

 

[DriveMeNuts]

If σ[sub]ij[/sub] is set to σ[sub]22[/sub], then this is a principle stress.

I am finding that your problem is a lack of understanding of basic engineering terminology and prefixes, which are not specific to Pressure Vessel Engineering. Look at the Wikipedia page for Cauchy stress tensor, and Mohr's circle while you are at it.

But if you don't have a grasp of how stress tensors works, you won't have a clue about stress classification as per TGS4's post.

 

[r6155]

@ mechengm
ASME VIII Div 2 is not for a beginning junior engineer. You need training and experience in Division 1 for at least 10 years (design, fabrication and inspection)
I'm sorry.

Regards

 

[DriveMeNuts]

Stress classification is part of design. Div 1 design is just dumb calculation rules which barely mention the word 'stress'. I didn't understand the fundamentals of the dumb calculation rules until I started using Div 2 FEM projects.
I was doing FEM two years after leaving Uni, but I had a mentor who checked and interpreted the FEM results, which I learned from. Never too early to start.....as long as you have a mentor.