Linearized stress on local zone

[Mm.Kaiser]

Hi guys,
I am really confused about what said in VIII-Div2 Part5 about Local stresses and i need your advices.
I decide to analyze a type of flange that you can see in image below which contains a FEA of a slice of flange.
As you see, stress in edge of teeth is more higher than middle of teeth.I defined the stress classification line (SCL) there and derived membrane and bending stresses but...

1- Should i consider it as a Local stress or General stress? if i consider it as general, all the parts should become thicker although the stress is so localized! if i consider it as local, is it true that Pm is Pl<1.5Sa and Pm+Pb is Pl+Q<3Sa?

2- As you know in ASME, Allowable stress is Sy/1.5 and in B16.5, Hydrostatic pressure is 1.5 times to design pressure. so what should we do in test pressure design? if we consider that SCL as a general and compare the membrane stress to Sy/1.5, membrane stress in test condition will become equal to Sy and isn't admissible.
In brief, the second question is that how we can use test criteria in local zones that i couldn't find any rule for it in ASME.

Best Regards

 

[TGS4]

The geometry is not particularly clear from your attached photo, so I am unable to tell if your SCL is valid or not. Have you confirmed the validity from Annex 5-A?

What I will tell you is that the selection of a particular classification is the responsibility of the design engineer and the ASME rules are not as prescriptive as you may think that they are. However, in the definition of local membrane in 5.2.2.2(b), you will find some additional guidance.

One additional thing that I will mention is that the limit on primary+secondary membrane-plus-bending is not related to and completely different from Protection Against Plastic Collapse; it is related to Protection Against Failure From Cyclic Loading: Ratcheting. So, although you may use the same SCL, you will absolutely be using a different loading scenario: Design Load Case Combination vs operating load ranges.

As far as the hydrostatic test condition, in Division 2 there is no limit on any of the stress classifications other than what is provided in 4.1.6.2.

 

[Mm.Kaiser]

Thanks a lot for your response. I'll attach another image so you can determine what i said. (the model is a kind of quick opening closure)

1-Please just tell me that in relevant SCL, stresses that i derived from ansys (Pm and Pb) are [Pm and Pm+Pb] or [Pl and Pl+Q]?

2-As you mentioned, whenever i use VIII-2 part5, i should not to consider Hydrostatic test load? it means that hydrostatic test is not important if we use design loads of part5?!

3-If we design our parts according to "Elastic Analysis (SCL)", do we oblige to consider limit load or elastic-plastic analysis too? as you know, to calculate the thickness, stress classification line is the better way.

Sincerely

 

[TGS4]

I still don't get much from your pictures. It appears to be a sector model of a flange-like structure with nubbins on the inside. I have no idea how this is loaded, supported, where the pressure acts, etc.

1) It is unlikely that you would classify your stresses in the SCL shown (and I can't even start to address whether or not that is a valid or appropriate SCL) as Pm. In your geometry, a determination of whether or not the bending stresses are primary or secondary would be very difficult. See 5.2.1.2.

2) You are correct. The Code Committee is currently evaluating whether or not the hydrostatic test condition should be included in an evaluation, and what the appropriate stress limits (or load factors) would be. In the meantime, you have 4.1.6.2.

3) The selection of the methodology (elastic, limit load, or elastic-plastic) is left to the designer as described in 5.2.1.1, with the provisos provided in 5.2.1.2, 5.2.1.3, and 5.2.1.4.

as you know, to calculate the thickness, stress classification line is the better way

No, it is not. See 5.2.1.2, 5.2.13., and 5.2.1.4.

 

[Mm.Kaiser]

I'm So sorry because i didn't attach a clear and complete image. Please find new final attached file.
The model is a bayonet type closure which contains a flange and plug(door). also you can see the pressure that used in analysis.
My main question is that which type of stress is on the edge line of teeth in flange and plug; Primary or secondary?
Thanks a lot for the time which you spend.
Best Regards

 

[TGS4]

Your answer is found in 5.2.1.2:

For components with a complex geometry and/or complex loading, the categorization of stresses requires significant knowledge and judgment. This is especially true for three-dimensional stress fields. Application of the limit load or elastic-plastic analysis methods in paragraphs 5.2.3 and 5.2.4, respectively, is recommended for cases where the categorization process may produce ambiguous results.