Clarification of ASME VIII-2 5.5.3.2(b) and 5.5.6.2

[IdanPV]

Hello all.

ASME VIII-2 clause 5.5.3.2(b) stated that for Ke,k values greater than 1.0, the simplified elastic-plastic criteria of 5.5.6.2 shall be satisfied.

The case of which Ke,k is greater than 1.0 is applicable for ΔSn,k>Sps.
For example, ΔSn,k=392MPa and Sps=345MPa.
Material is Austenitic stainless steel, per Table 5.13 m=1.7 and n=0.3.
Therefore, Ke,k=1.458>1.0.

So, the criteria of 5.5.6.2 shall be satisfied, but it does not make sense.
5.5.6.2(a) stated that

The range of primary plus secondary membrane plus bending equivalent stress, excluding thermal stress, is less than Sps.

If it was true - I wasn't needed to satisfy 5.5.6.2 at the first place.

I must be missing something, but as far as I understand it, there is no way to satisfy this requirements and clauses 5.5.3.2(b) and 5.5.6.2(a) contradict each other

 

[TGS4]

The important part to pay attention to is the "excluding thermal stress" part.

Otherwise, you fail the ratcheting criteria.

 

[IdanPV]

That is clear.
My problem is with the first part of the sentence.

The range of primary plus secondary membrane plus bending equivalent stress

- this is ΔSn,k isn't it?
If yes so it won't by possible to satisfy this.

If no, I would like to understand what does "primary plus secondary membrane plus bending" means?
Assuming that the bending stress exhibited is fall under the category of secondary stress; Q.
Does it mean I need to check Membrane (Pm or Pl) versus Sps (Pm or Pl<Sps)?

 

[TGS4]

It's all in the "excluding thermal stress" part.

You can have primary plus secondary membrane plus bending equivalent stress that includes thermal stress and some that excludes thermal stress.

 

[IdanPV]

@TGS4,

If I don't have any thermal stress and the bending stress is fall under the category of secondary stress; Q.
Does "range of primary plus secondary membrane plus bending equivalent stress" means that I need to check that the membrane stress is less than Sps?

 

[TGS4]

Yes. You addresses need to pass the ratcheting check.

 

[BJI]

Does "range of primary plus secondary membrane plus bending equivalent stress" means that I need to check that the membrane stress is less than Sps?

Just for my clarification, since 5.5.6.2(a) either doesn't delineate between primary and secondary bending stress, or specifically includes primary plus secondary bending stress (depending on how you read it); bending equivalent stress in my understanding would include primary plus secondary, correct? Therefore, it is not membrane stress less than Sps, it is total stress (excluding thermal stresses) less than Sps.

 

[IdanPV]

That's exactly what I thought.
Assuming you are right, and it shall include primary plus secondary bending stress so you will be ended up with ΔSn,k, which is exactly Pl+Pb+Q.
But then you will enter an "infinite loop", let us go back to my example:

A case of which Ke,k is greater than 1.0 is applicable for ΔSn,k>Sps.
For example, ΔSn,k=392MPa and Sps=345MPa and thermal stresses are not applicable.

Material is Austenitic stainless steel, per Table 5.13 m=1.7 and n=0.3.
Therefore, Ke,k=1.458>1.0 and the criteria of 5.5.6.2 shall be satisfied.
As you suggested, I need to check if the total stresses - Pl+Pb+Q (ΔSn,k) is less than Sps but if it was true, the criteria of 5.5.6.2 should not be satisfied.
On the other hand, it will not be possible to satisfied 5.5.6.2 with ΔSn,k>Sps, and this clause does not make any sense.

That's what bring me back to the conclusion that I need to check if the membrane stress less than Sps,

 

[BJI]

Let ΔSn,k = Pl+Pb+Q including thermal, applicable to the 5.5.6.1 criteria. ΔSn,k is not used in 5.5.6.2, but rather it includes a definition of Pl+Pb+Q excluding thermal. You can only exceed the 5.5.6.1 criteria if the latter is less than Sps, if not you have failed the simplified EP analysis and you would need to run the detailed EP analysis or redesign. So the ΔSn,k value (which is including thermal) is the one that is used for 5.5.3.2 Step 4(b), if you don't have any thermal loading then you would be limited to equation (5.31), for Ke,k=1 (no plasticity correction) and therefore ΔSn,k < Sps or you fail ratcheting.

Such a high stress range without thermal loading is an unlikely situation since the design would typically be rectified when it fails the protection against plastic collapse assessment. Do you have displacement controlled loading other than thermal?

 

[DriveMeNuts]

The bending stresses around nozzles are secondary and they are not displacement controlled.

 

[BJI]

The bending stresses around nozzles are secondary and they are not displacement controlled.

Agreed, no one has said anything contrary to this.

 

[IdanPV]

As per 5.5.3 Step 4 (b) a fatigue penalty factor (Ke,k) shall be used.
As I said, there are no thermal stress.
So ΔSn,k=Pl+Pb+Q and it is greater than Sps but less than mSps.
So, I need to use Ke,k=1.458>1.0.

Now, back to step 4 (b) – the criteria of 5.5.6.2 shall be satisfied:
(a)The range of primary plus secondary membrane plus bending equivalent stress, excluding thermal stress, is less than Sps.

In other words – where thermal stress are not exist (which is the case here) Pl+Pb+Q (which is ΔSn,k actually, doesn't it?) need to be less than Sps.
But it is impossible to satisfy this requirement since ΔSn,k>Sps, and that is why I was forced to check 5.5.6.2 at the first place.
And that is the "infinite loop" I was talking about.

As for the load – only internal pressure.

 

[BJI]

Yes, it would need to be less than Sps. Under internal pressure only, the component is not well proportioned. You have an increased risk of gross plastic deformation (more likely) or ratchet (less likely). What is the level of local membrane stress?

 

[TGS4]

If you do not meet the ratcheting limit, then you are required to change the geometry or the load. You have, effectively, failed to demonstrate Protection Against one of the applicable failure modes. Which means that the fatigue analysis is not applicable because you have progressive plastic deformation and not just cyclic plasticity.

 

[IdanPV]

For me, it seems very unlikely that the intent of "The range of primary plus secondary membrane plus bending equivalent stress" in 5.5.6.2(a) is ΔSn,k.
Because:
1. If it was ΔSn,k, clause 5.5.6.2(a) were simply written as "ΔSn,k<Sps".
2. Take a look at 5.5.3.2 step 4 (b), for (Ke,k>1.0)<=> Sps<ΔSn,k<mSps, I cannot find any scenario in which ΔSn,k>Sps and ΔSn,k<Sps at the same time. And that is exactly what 5.5.6.2(a) asks to check.
3. Assume that the code "Knows" that Sps<ΔSn,k because it forces me to calculate Ke,k, why does he ask me to check if ΔSn,k<Sps?
4. The definition of ΔSn,k can be seen in 5.5.6.1(b), and it is not the same as in 5.5.6.2(a).

 

[BJI]

I cannot find any scenario in which ΔSn,k>Sps and ΔSn,k<Sps at the same time

I feel this has been answered a number of times already. I could see a scenario when ΔSn,k(including thermal)>Sps and ΔSn,k(excluding thermal)<Sps. This is supported by the difference in definitions you noted in point 4.

 

[IdanPV]

As I wrote - there are no thermal stress in this case.
And yet ΔSn,k>Sps so I shall check 5.5.6.2(a). If 5.5.6.2(a) was referring to ΔSn,k I guess it was clearly stated in the Code.

I just want to understand if the bending stress which 5.5.6.2(a) is referring to is Primary Bending stress.

 

[TGS4]

If you think that the Code rules and definitions are unclear, then please submit a Request for Interpretation to the ASME Code. Once you have done so, please advise of the ASME tracking number assigned to your inquiry.

 

[IdanPV]

TGS4,

I submited a Request for Interpretation to the ASME Code, tracking number is 21-1316.