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

QUESTION
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.

REPLIES

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.

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