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Stress Linearization 2nd thread

Stress Linearization 2nd thread

Stress Linearization 2nd thread

(OP)
Hello Prex/TGS4,

I red your previous posts you wrote regarding ASME SECTION VIII, DIVISION 2 ANNEX 5.A.

I am still not 100% sure how to calculate it

The first two steps of ANNEX 5.A are clear. I am confused how to calculate Bending Stress and Membrane + Bending Stress:

My questions refer to the forum and the following sources:

http://www.eng-tips.com/viewthread.cfm?qid=412084
http://fsefea.com/stress-linearization-asme-sec-vi...
https://pveng.com/home/fea-stress-analysis/lineari...

Membrane Stress
Sm= VonMisesStress of (Snm, Stm, Shm, Tntm, Tnhm, Tthm) = PmV =sigma_ eqvm
All right, all three sources are calculating Membrane Stress with the same formula.

Bending Stress:
In a post here in the forum user Soave posted on Mar 17 calculated the Bending Stress with the formula:
Sb = VonMisesStress (Sbxx, Sbyy, Sbzz, Sbxy, Sbxz, Sbyz)

At pveng.com
PbV = VonMisesStress(Snb, 0, Shb, 0, Tnhb, 0)

First question: According ANNEX 5.A bending stresses should be calculated only for the local hoop and meridional (normal) component Stresses, therefore I think pveng.com used the right formula?

Membrane + Bending Peak Stress:
Suave used sxx at node 1 and subtracted it with the formula that fits to the formula of Annex 5.A. In example min= sxx(0)-(Smxx+Sbxx).

At pveng.com the total Vmises Stress is used.

Second question: If I compare Step 3 of ANNEX 5.A I would say I have to calculate it with the formulas 5.A.3 and 5.A.4. Here Suave is right?

Membrane + Bending Stress:
Suave calculated the Bending Stress with the formula:
Sm+b = VonMisesStress of (Smxx+Sbxx,Smyy+ Sbyy, Smyy+Sbzz, Smxy+Sbxy, Smxz+ Sbxz,Smyz+ Sbyz)

At pveng.com and fsefea.com it is done by:
- Membrane + Bending Stress - end i (+ bending) [->sigma_ eqvmb0]
PmbVi = VonMisesStress of (Snm + Snb, Stm, Shm + Shb, Tntm, Tnhm + Tnhb, Tthm)
- Membrane + Bending Stress - end j (- bending) [ ->sigma_ eqvmbt]
PmbVj = VonMisesStress of (Snm - Snb, Stm, Shm - Shb, Tntm, Tnhm - Tnhb, Tthm)

Here the maximum of one of these two values is the value for Membrane + Bending Stress. I guess this difference is also related to the hoop and meridional component issue.

Last Questions: TGS4 wrote on 2 Aug 16 a tensor, where I can not find + Tnhb. I do have to include Tnhb right?
If I want to visualize Membrane + Bending Stress I draw a line from PmbVi to the Point L=half thickness/height membrane stress and from this point to PmbVj?

Any help would be greatly appreciated. I am a Student from Germany and tried my best to explain. I know my English is not perfect, but I hope anyone can understand.

Best regards

RE: Stress Linearization 2nd thread

This is the right way.


This is the wrong way.


These are from my training class (hence the reference to "slides").

RE: Stress Linearization 2nd thread

TGS4, What is the motive for not including the bending effects of the through thickness stresses?
I know this is the procedure as provided by the code but I do not know what the reason is. My guess is the following:
For the through thickness shear stresses since they have a distribution like a parabola the moment around the mid-point is very close to zero.
For the through thickness normal stress I do not have any idea why bending effects are neglected.
How much does that neglecting change the results by the way?

RE: Stress Linearization 2nd thread

TGS4, there is also 5.A.4.1.2.b.2, and as 'needs to be considered only' to me implies 'but may be considered in all cases', I would say that, to promote standardization of procedures (and also because I don't know how to check whether a stress distribution results in torsion...), it would be simpler to include in your first table the σyz,b component. This is what pveng.com does, as reported by Robse (I didn't personally check) and I think it is a correct procedure in all cases.
Robse, concerning the peak stress, only the tensor is defined (at wall surfaces) in STEP 3. If you refer to Step 6 in pveng site, then that's not the same; they define the peak stress as the highest stress resultant on the SCL, whilst STEP 3 in App.5.A defines the single components of the peak stress as the increment of the total stress components at wall surface with respect to the previously calculated m+b stress components (where some b components are zero).
Concerning your second question, I don't see your point: STEP 3 refers exclusively to the peak stress tensor (and not to a 'Membrane + Bending Peak Stress', as you say; such a quantity is not defined in the code).
Your last question is already answered above. But you don't need to visualize the m+b stress along the SCL, as the code only requires to determine the stress resultants (VonMises here) at wall surfaces, the rest doesn't matter.

prex
http://www.xcalcs.com : Online engineering calculations
https://www.megamag.it : Magnetic brakes and launchers for fun rides
https://www.levitans.com : Air bearing pads

RE: Stress Linearization 2nd thread

(OP)
Thank you all for bringing light into the darkness.

I will calculate the Bending stress with VonMisesStress(Snb, 0, Shb, 0, Tnhb, 0)

Thank you also for clarifying peak stress and Membrane + Bending Peak Stress! I completely overlook the difference.






RE: Stress Linearization 2nd thread

Quote (prex)

there is also 5.A.4.1.2.b.2, and as 'needs to be considered only' to me implies 'but may be considered in all cases',
There is no implication. The rules are as-written. If you disagree, you are free to request an interpretation from the Code Committee.

paulettea- the explanation in WRC429 (and mostly repeated in PTB-1) is thorough.

RE: Stress Linearization 2nd thread

The implication, for me, comes from the fact that 'needs be considered only if' isn't the same as 'must be considered if', but rather 'can be considered always, but is required to only if'. Words count. That wording should be clarified.

prex
http://www.xcalcs.com : Online engineering calculations
https://www.megamag.it : Magnetic brakes and launchers for fun rides
https://www.levitans.com : Air bearing pads

RE: Stress Linearization 2nd thread

Code Committee meetings are this coming week, and WG-DBA meets on Monday. I'll discuss to see if we ought to correct/clarify. We're working on big changes to the elastic rules, anyways, but not sure if they'll work their way through all of the relevant committee in time for the 2019 edition publication deadlines.

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