Exact physical significance of Sec VIII Div.2 Annexure 5D

[PVDEngg]

Hello Gentlemen! May I know what is the appropriate significance of Sec VIII Div. 2 Annexure 5D? It is referred in para. 5.5.3.4, which talks about alternative for determining PEAK stresses around a nozzle opening instead of detailed stress analysis as per Para 5.5.3 FATIGUE ASSESSMENT – ELASTIC STRESS ANALYSIS AND EQUIVALENT STRESSES.

Meeting all the requirements described in annexure, it provides stress indices for nozzles; which in turn can be used to determine normal, tangential and radial stress components by multiplying stress indices to computed membrane hoop stress from Para. 4.3. Is my understanding up to here correct?

If yes, how to use those 3 stress components for determining peak stress. And how that peak stress be used as alternative to determination of effective alternating equivalent stress range?

I would also like to get my understanding about PEAK STRESS cleared herewith. From Fig. 5.1- Peak stress is Increment added to primary or secondary stress by a concentration, so is anticipated much less than computed membrane hoop stress using formulas given in Para. 4.3. But since stress indices are greater than 1, values of peak stress calculated will be greater than membrane hop stress. Isn't this contradictory?

However as per definition of Peak Stress given in Para. 5.12, peak stress can be higher than membrane hoop stress due to internal pressure. It can be thermal stress or discontinuity stress as well which sounds like peak stress could be total stress.

Please guide and clear what Peak Stress exactly is and how Stress indices from Annexure 5D can substitute entire elastic stress analysis as per Para. 5.5.3.

Thank you in advance!

 

[NRP99]

Please refer definition section of Part 5- Para 5.12 for definition of peak stress.

To have meaning, peak stress is always an increment added to average stress. For example the peak stress near hole for plate with hole loaded axially case is X.X(it is ALWAYS greater than 1) times the average stress at continuous section. Here the increment above 1 is due to effect of hole and in general cases due to notch/structural discontinuity. The peak stress adds to the average stress at the discontinuity section and are responsible for the crack formation due to cyclic loading. Hence we take total stress (primary +secondary+peak) for calculation of fatigue life in pressure vessel design. In general same approach is used in other applications where total stress including peak stress is used in fatigue life calculation.

 

[PVDEngg]

Thank you so much for your enlightening response NRP99.

I hope somebody enlightens me about Annexure-5D soon.

 

[PVDEngg]

Well.....it seems like either my query is not clear or this is area of learning for most of us....

Please let me know if I am not clear in my query...Waiting for the opinions which will throw some light on this...

 

[TGS4]

Your question confuses me. What is it exactly that you don't understand? Peak stresses will ALWAYS be greater than membrane stresses.

 

[PVDEngg]

Regards TGS4!!

By the time I am clear about 'peak stress' based on research so far.

Now the only question is about VIII-2 Annex-5D. To frame it in single sentence, "How to use Annexure-5D as an alternative to VIII-2 clause 5.5.3" ?

What is its output and how can one use it for fatigue assessment?

 

[NRP99]

PVDRE
If you are clear about concept of peak stress, I think you would easily get what Annex 5.D is all about. Code has given the stress indices to calculate peak stress around nozzle opening. You can use this calculated peak stress for fatigue life calculation as per part 5.5.3.
Annex 5.d Output will be-peak stress.
Annex 5.d Output Used for-fatigue life calculation as per Part 5.5.3

But I would recommend to use numerically calculated(FEA stress analysis) peak stress values for the calculation of fatigue life which is general engineering practice in every industry.

 

[PVDEngg]

Thank you very much for you response NRP99.

Noted your recommendation about using FEA for peak stress evaluation. But many of the times during estimation it is not really feasible to carry out detailed FE stress analysis, because of stringent timelines.
So, was looking for this alternative. Would you please help me filling the disconnects in my understanding about this whole thing based on your reply?

From VIII-2, Clause 5.5.3 fatigue damage factor and thereby fatigue life can be calculated based on EFFECTIVE TOTAL EQUIVALENT STRESS AMPLITUDE, defined as one-half of the effective total equivalent stress range. Effective total stress range ΔSpk being difference between 6 component stresses at event of stress reversal as shown in equation 5.25. So I am trying to figure out how to use Peak Stresses σt, σn and σr from Annex-5D to calculate ΔSpk. Or is it like it is totally different how to calculate fatigue life from stress indices..?

Hope I am clear in my query....

 

[NRP99]

Code restricts use of this annex 5.D for nozzle openings only and these are very easy to model. For complex situations, anyway, you should not be using this section. FEA is the only appropriate way then. There is no alternative.

You should revise your fatigue knowledge. I will give you hint -

The stress range( ΔSpk) is nothing but the total stress(P_L+P_b+Q+F) at event 90 degree minus Total stress(P_L+P_b+Q+F) at event 0 degree of sinusoidal varying load histogram.

Peak stress=Stress concentration factor*Average stress at continuous section=Total stress at peak location

ΔSpk= Difference between Total stress at peak location at 0 degree and 90 degree events of sinusoidal varying load histogram.

You should be knowing how to calculate total stress using σt, σn and σr.

 

[PVDEngg]

NRP99...Thank you for your comprehensible reply.

Thank you TGS4...

 

[olgamsp]

Hi, I have 2 simple questions

1) ΔSpk is the equivalent von misses stress that I can take from my simulation before the stress linearization?

2) Sa (alternating stress amplitude) isn't Salt ? If the answer is no, the way to calculate Sa from Part 5 is calculate the screening criteria factor C1 and then multiply that with S? Like says in part 5.5.2.4 Fatigue Analysis Screening, Method B.

Thanks.

 

[NRP99]

olgamsp

1) ΔS_P,k= Difference between Total stress at peak location(In your FEA model) at 0 degree and 90 degree events of sinusoidal varying load histogram.
2)Yes. From ASME Sec VIII Div 2- Para 5.13,
S_a = alternating stress obtained from a fatigue curve for the specified number of operating cycles
S_alt,k = Alternating equivalent stress [Calculated (either by FEA or analytical) alternating stress with appropriate factors applied to stress range ΔSpk] for K^th cycle.

 

[olgamsp]

Hi, NRP99, thanks for your response. I still have the question how can I calculate Sa if I want to aplly the assesment of part 5.5.3 from ASME VIII div 2?

Thanks

 

[TGS4]

I don't understand your question and/or your confusion. Please provide additional details or perhaps an example.