ASME section VIII division II 2

[020100]

Hi,
I am wondering whether anybody in the forum has experience for how to use design by analysis in ASME section VIII division II. The real question is how to select the location to use the rule for priminary, secondary and so on.

Thanks

LZ

 

[020100]

My question might be too general and got no answer. I would like to be more specific.

(1). What is the failure mode behind of each safety factor.
such as 1.0*Sm for membrane (primary), 1.5*Sm for lcoal membrane and bending, 3*Sm for Q.
Is it because the primary membrane is related to catastrophic, the rest of them is related to just slow leak.

(2). Local membrane SL (or pl) includes discontinuity, but bending Sb (Pb) excludes discontinuity. So both of them will not happen at the same location. Does it sound right.

(3). If the vessel has flat heads (flanges) with two cylinders and stress on the inner cylinder needs to be examined, where will the location from the corner be used to examine the SL?
right on the corner or a few elements from the corner if FE is used.

I went through old threads. There are quite a few great discussions, but I am looking for more information or stimulate more discussions.

Many thanks.

LZ

 

[prex]

1)No. Primary membrane is limited by yielding, hence failure, as a load causing primary stresses will lead to immediate failure after yielding. Local membrane is a secondary stress that's limited more than other secondaries to avoid excessive distortion. Secondaries are limited by shake down: the limit insures there will be no progressive accumulation of plastic strain by load cycles.
2)No. Local membrane is a secondary stress by nature, Pb is a primary stress.
3)Too few details to judge.
No offense intended: you need to deepen your understanding of stress classification starting from theory (or put your hands on a worked example done by an expert).

prex

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[TGS4]

You will find some of the information that you seek in the 2007 Edition of ASME Section VIII, Division 2, Part 5.

After you have read the ENTIRE Part 5, including the Appendices, please feel free to come back here and ask some specific question. I would suggest that you upload pictures to help illustrate your question.

 

[020100]

Many thanks for your great input, Prex and TGS4.
I have a little confused here with Prex's comments.
(1). I was asking about physics for how the code was developed.
I think that you are talking about mechanics.
I think that what you meant is that if the primary stress reaches yielding, the entire cross section will yield while if bending stress reaches yield strength, it will take more load to yield entire cross section. Therefore, the consequence for memebrene is much higher than bending. As, it has higher SF.

Do I understand it correctly?

(2). I think that I am confused with your definition of primary stress and secondary stress. The code refers primary load and secondary load. In the code, the definition of local membrane is the average of primary stress across any solid section. My understanding is that primary stress is stress due to primary load. The stress could be either membrane or /and bending.

Well, I thought that I could take avantage of the forum to get more information.

Many thanks again.

 

[TGS4]

020100, you are going down the right path, and I think that you are starting to think about the right questions. Well done.

In an effort to short-circuit some of these questions, have you read a document called "Criteria of the ASME Boiler and Pressure Vessel Code for Design By Analysis in Sections III and VIII, Division 2". If it's background information that you seek, then this is THE document.

Otherwise, for your above questions, please reference you questions to specific paragraphs in the 2007/2008 Code - a little clarification may be all that's needed.

 

[prex]

Agree with TGS4's first sentence (the rest too of course).
Note: of course, when speaking of stress limits, when we say bending we mean membrane+bending, and when we say secondary we mean primary+secondary.
020100,
1)The factor 1.5 in the limit for primary bending is simply the ratio of plastic to elastic modulus for a rectangular section (the wall of the vessel); so, as you correctly imply, the limit for both membrane and membrane+bending stresses is the full yielding of section (with a safety factor of course)
2)Don't see what you exactly mean by primary load. A load may generate both primary and secondary stresses: e.g. pressure gives rise to primary stresses of course, but also secondary ones (discontinuity).
Local membrane can't be in its very nature a true primary stress, otherwise its limit would violate the limit above and gross failure would be almost certain. It is in fact a primary+secondary (membrane) stress, that the code classifies specially as a local primary membrane with its associated limit, to avoid excessive distortion in the transfer of load to other portions of the structure.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[020100]

Many thanks, Prex and TGS4 for being patient.

(1). It is clear for safety factor.

(2). I am ordering the book as TGS4 suggested.
I am pretty sure that I will have questions after I read it.
I was trying to clarify one thing:

All of stress are related to loading conditions.
The loading conditions could be pressure, temperature change and so on.
In the code or some reference book, they often state that the stress caused by pressure is called primary stress while stress caused by temp change is called secondary stress.
It is definitely different from what Prex said. If I understand correctly, the pressure could generate primary stress and secondary stress. If I extroplate what you said, temp change could generate primary stress and secondary stress as well. And there is no definition about primary loading and secondary loading.
If I extroplate a little further, the primary stress and secondary stress both exists in the vessel and just might be at different location.

Sorry to ask many questions.

 

[prex]

Yes, a temperature differential may well generate primary stresses. An example that comes to mind is the stress in the supports of a pipe generated by the thermal expansion of the pipe itself: it is normally classified as a primary stress, because a basic property of a secondary stress is that it is self limiting, but a stress generated in a structure by the expansion of another structure is obviously not necessarily self limiting.
As you see stress classification is not an immediate concept: the origin of the load and the behavior of the structure(s) involved both count with various degrees of importance.
And of course primary and secondary stresses generally both exist everywhere at all locations in a vessel.

prex

http://www.xcalcs.com

: Online engineering calculations

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: Magnetic brakes for fun rides

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[davefitz]

there is being offered an instructor led , web based course on the newly revised sect VIII div 2 by ASME- first session starts 8 Sept, next session starts in Nov. See <

http://www.asme.org>

 

[020100]

Hi, Prex,
Thanks for your great explanation. It makes perfect sense as well.
I have two more questions

(1). Why do we need to have more than one criterion (primiary, local membrane, bending, secondary, peak have different safety factor)if all of loads will generate both primary, secondary, membrane, peak stress.
assume that there is no fatigue involved ( I know it will be affected by peak stress).
Can I assume that all of criteria will not be equivalent?
For engineering design, they try to evaluate all of possibilty and choose worst case.

(2). Self-limiting is very good term for mechanician. But I don't see that it is a tool to differentiate primary and secondary in practice or beginner. As an example, stress for all of location on cross section will be calcuated in terms of FEA, e.g ANSYS. The stress intensity will be plotted in terms of membrane, bending, total, peak. How would you use the self-limiting to define which one is primary and secondary.

Mnay thanks.

I have not got my books yet.

 

[prex]

1)Different stress classes have different limits because they are associated with a different failure criterion. Secondaries are allowed to produce yielding (just because they are self limiting) and the failure criterion is ratcheting, that is the accumulation of plastic strain due to load cycles, till rupture occurs.
2)Self limiting is the basic concept. A primary stress is not self limiting: think of a cylinder under pressure, when the membrane stress in the thickness reaches yield, a further increase of pressure will tend to increase the stress, that is not released in any way, and failure is immediate (apart from other phenomena as stress stiffening and section reduction). A secondary stress is self limiting: think of a constrained heated bar, when the stress reaches yield, it won't go any further even if the temperature differential increases.
Stress classification cannot be done by elastic FEA, whilst elastoplastic FEA can simulate shake down, and allow to distinguish between primary and secondary stresses (but not between secondary and peak BTW). The normal procedure is:
-determine primary membrane and bending stresses by formula: they are normally calculated by simple equations of equilibrium (and indeed a definition of primary stress that I love is that it is the minimum distribution of stress that satisfies the equations of equilibrium, discarding the compatibility equations)
-the stresses calculated by FEA are primary+secondary (far from localized peaks)

prex

http://www.xcalcs.com

: Online engineering calculations

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[TGS4]

Great explanation prex. i would highly recommend that 020100 read through the 2007 edition of Division 2. It breaks everything down by "Protection Against Failure Mechanisms".

020100, the heart of the matter, as prex explains, is that you have to understand the failure mechanisms, and then evaluate the different failure mechanisms. There are different design margins for the different failure mechanisms for good reason.

 

[020100]

Thanks, Prex for still being patient and answering all of my questions.
Especially, I don't think that I could get the answers from any reference book. Your answer is related to theory of the coding instead of searching for tables. I really hate to search for the tables and classify the stress in terms of the table instead of understanding the theory. I think that I have picked up slowly, but more questions if you don't mind.

(1). Criterion with 1.0*Sm, 1.5*Sm fully understood.
But secondary is 3*Sm. I understand how 1.5*Sm was derived now.
Could you please explain how 3 in 3*Sm was derived? I know it could higher, but why it is 3 intead of 2.5? Silly question, I guess.

(2). You mentioned that elastic FE cannot be used to classify primary and secondary stress. Elastic FE will only calcuate combinated primary and secondary. You mentioned that primary stress should be calculated by analytic formula (plate and shell theory, I guess). But I am not sure I understand it completely. The reason is as follows.
(2.1) Analytic solution is derived from very regular geometry. The geometry will be significantly different from the vessel. Are we sure that the analytic solution will always generate worst case?
(2.2). If you could use analytic solution to get stress, why could we use FE to calcuate the stress for the real vessel geometry? I think that the key is which cross section you will choose to get the stress. Is it correct?
Or there is just no procedure in ASME for how to use FE for this problem.
I have not thinked it very carefully, but I would see that the stress on the middle plane for cylinder might be very close.

(2.3). I am getting quite slow with the self-limiting. The reason is not that I don't understand your example for heating bar. It is perfect example. A lot of book refer to it as well. But my problem is that the real applifaction is not simple.
I like what you said that stress classification can be done using elastic-plastic method. But is there any paper or book I could refer? I would like to dig it out by myself since I felt a little guilty to take advantage of your time.

Thank TGS4 as well for your comments. I have the 07's version. I think that it is more friendly to FE approach, but it is not crystal clear to me yet.

Regards

 

[prex]

1)3*Sm is, in simple terms, equivalent to 2*Y (though it may be smaller in many cases). 2*Y is, for a stress calculated by the elastic method, the limit of the variation of stress in a load cycle (called stress range in the code) to avoid accumulation of plastic strain. This is a point you should study more closely in a textbook (and never forget that the limit 3*Sm is always for a stress range, not for the absolute value of the maximum stress).
2)You don't necessarily need plate and shell theory to calculate primaries. All of Div.1 is formula based (and indeed Div.1 deals with and limits primary stresses only) and also Div.2 has the formula based approach: this is what I mean for formula calculated primaries.
2.3)I must insist: self limiting is the basic concept you must understand if you want to understand the underlying theory. I'll give you a less trivial example.
The discontinuity stresses at the junction between e.g. a hemispherical head and a cylindrical shell (that BTW may be calculated by plate & shell theory) are due to a mechanical load (pressure) and of course the stresses calculated by the elastic method are proportional to the load, so that they increase with increasing the load. However they are not required to satisfy the laws of equilibrium (the resultant of those stresses on any section through wall doesn't have to equilibrate a pressure load on the same section), they are only required to satisfy the compatibility of deformations of the two structures, that would deform in different ways if unconnected. As a consequence, when the load is sufficiently high to cause yielding, those stresses will stop from increasing with a load increase, as the compatibility of deformations is already reached: they are self limiting.
Don't forget also a very important point: we are speaking in all this of stresses calculated with the elastic method: various plastic approaches are also allowed by code, but it's a completely different matter.
I think it is really time for you to gain a better understanding of theory in a textbook and to check it with some good examples.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[020100]

Thanks, Prex.
It appears to me that the rule was developed using formulae before FE was discovered. The code is trying to pick up more from FE, but it has not fully accomodated FE approach in the rule if I understood all of replies.
It might be very straight forward for somebody who has been using the code for his life.
It is hard to understand for somebody with pure FE background.

Great discussions with frustration.

Keep forum moving.