ASME Section VIII-Div2 P5 - DBA

[Svk-Mn]

Hello All,

I am new to ASME VIII Div 2. I am trying to understand part 5 DBA procedure.
This is what I understand from the code:

There are two ways to perform the analysis as per part-5:
1. Non-linear method - Elastic-Plastic
2. Linearized method - Elastic Stress

We perform these two analysis to protect the vessel from 4 failure modes:
1. Plastic Collapse
2. Local Failure
3. Buckling
4. Cyclic Loading

My Question is, how do you decide which type of analysis to perform? (Elastic or Elastic-Plastic)
And do we always need to perform analysis for all 4-failure mode?

Thanks.

 

[IdanPV]

a. "how do you decide which type of analysis to perform?" - see ASME VIII-2, 5.2.1.2
b. "do we always need to perform analysis for all 4-failure mode?" - must of the times, yes. Unless your vessel is not subjected to cycle loading, see ASME VIII-2, 5.5.1.

 

[Svk-Mn]

Thank you IdanPV.

 

[TGS4]

You're looking at the situation the exact opposite way that you should. Your first focus should be on ensuring that you are protecting against the 5 failure modes (plastic collapse, local failure, buckling, ratcheting, and fatigue). Within each failure mode, you have two or three options for demonstrating Protection against the individual failure mode. The choice of analysis in each case is independent of the choices that you made for the other failure mode(s).

Does that clarify things?

 

[Svk-Mn]

Thank you TGS4 for the comment. Its been a week since I started to analyze the code in detail. I can now understand that after reading and researching more about part5.
As for now lets consider there are 4 different failure modes that we must protect the vessel against. Each point has its own options:

1. Plastic Collapse
-Elastic Stress
-Limit Load Analysis method
-Elastic plastic stress
2. Local Failure
-Elastic Analysis-Triaxial Stress Limit
-Elastic-Plastic Analysis-Local strain limit
3. Protection against collapse from buckling
-Design Factor
-Numerical Analysis
4. Cyclic Loading
-Elastic Stress Analysis
-Elastic-Plastic Stress Analysis
.
.
.

But the question is HOW DO YOU DECIDE WHICH FAILURE MODE TO ANALYZE? From 5.5.1 its kind of clear that you don't need a cyclic loading analyze performance if vessel is not subjected to it.
What about the other modes? Do you always need to perform the other three? If you perform lets say at least one of them, which type do you perform?
For example you are analyzing the vessel for first failure mode (plastic collapse) and you have Elastic stress, Limit Load and Elastic-Plastic. Which type of analysis do you choose? On what criteria?

Thanks.

 

[Svk-Mn]

TGS4, Please take a look at the attached file. I found it from a Youtube webinar video. From the chart we can see that Linearization method is done only in case of Elastic stress Analysis. Does this mean that for Elastic Plastic and Limit load analysis we can only use non-linear method?

 

[3DDave]

Non-linear problems typically require non-linear solutions. In linear problems a small delta input produces a small, exactly proportional, delta output. If there is any plastic deformation then a small delta input won't affect the stress in the plastic regions even though the strain is changing; instead it will convert more of the elastic region to plastic behavior. Worse, removing that input won't see a removal of the plastic strain.

 

[TGS4]

I'll answer your questions in order:

HOW DO YOU DECIDE WHICH FAILURE MODE TO ANALYZE?

You analyze all of them. In the order a presented in the book: Plastic Collapse, Local Failure, Buckling, Ratcheting, and (if you don't otherwise pass a fatigue screening assessment) fatigue.

Does this mean that for Elastic Plastic and Limit load analysis we can only use non-linear method?

Stress linearization, classification, and categorization are all used in the linear elastic methods for demonstrating Protection against the failure modes of: Plastic Collapse, Local Failure, Ratcheting, and Fatigue. It is not used in any of the non-linear methods. Please note the last sentence in 5.2.1.4 for Protection Against Plastic Collapse.

Do you have a mentor that you can learn this stuff with? Have you considered a training class on the topic?

 

[Svk-Mn]

Thanks 3dDave and TGS4 for your comments.
I neither have a mentor nor the budget for participating in a training(for now). But even before taking any course, I would like to learn all these stuff first.

 

[TGS4]

With neither a mentor nor training, I hope that you're not trying to practice on any real equipment. And don't expect to get trained for free here, either.