Using NB-3200 FEA As An Option To Stress Equations In NB-3600
The ASME B&PV Section III Div.1 Code 2013 Edition is currently approved in 10CFR50.55(a).
The 2013 edition, permits elastic FEA per NB-3200 in place of the stress equations of NB-3600, as follows:
For pressure design – “NB-3611.2 Acceptability When Stresses Exceed Stress Limits. When the stresses as determined by the methods given in NB-3630 exceed the limits thereof, the design can be accepted, provided it meets the requirements of NB-3200.”
For shakedown check Eq. (10)(12)(13) – For the shakedown check, quoting from NB- NB-3653.1 Satisfaction ofFor shakedown check Eq. (10)(12)(13) – For the shakedown check, quoting from NB- NB-3653.1 Satisfaction ofPrimary Plus Secondary Stress Intensity Range. “(b) If for one or more pairs of load sets eq. (a)(10) is not met, thepiping product may still be satisfactory, provided that the conditions of NB-3653.6 are met or provided that therequirements of NB-3200 are satisfied.”
Equations (10), (12) and (13), with the nomenclature in NB-3650, have the form:
Sn=C1 (Po Do)/2t+C2 Do/2I Mi+1/(2(1-ν)) Eα|∆T1 |+C3 Eab |αaTa -αbTb |≤3Sm – Eq. (10)
Se=C2 Do/2I 〖Mi〗* – Eq.(12)
Sn=C1 (Po Do)/2t+C2 Do/2I Mi+C3‘Eab |αaTa -αbTb |≤3Sm – Eq.(13)
Equation (10), and likewise the alternatives Equations (12) and (13), is meant to check shakedown, i.e. the prevention of excessive ratcheting, and is an approximation of Figure NB-3222-1 PL + Pb + Pe + Q ≤ 3Sm.
For peak stress intensity – Quoting from “NB-3653.2 Satisfaction of Peak Stress Intensity Range.”
“NOTE: This simplified analysis is intended to provide a value of Sp that conservatively estimates the sum of PL + Pb + Pe + Q + F as required in Figure NB-3222-1.”
Equation (11) for peak stresses is an approximation of NB-3200 Figure NB-3222-1 PL + Pb + Pe + Q + F, which is then used to evaluate the fatigue usage factor.
Sp= (K1C1)(Po Do)/2t + (K2C2)Do/2I Mi+K3 C3 Eab |αaTa -αbTb |+1/(2(1-ν)) K3 Eα|∆T1 |+1/(1-ν) Eα|∆T2| – Eq.(11)
Which method is more conservative NB-3600 or NB-3200?
The two methods are different, NB-3200 based on FEA and NB-3600 based on closed-form equations. Note that the 3Sm limit is the same for Eq.(10), (12), or (13), as in PL + Pb + Pe + Q; so there is no difference on the 3Sm stress limit.
Also, both NB-3200 and NB-3600 use the same (S,N) fatigue curve. The two methods are different, so there is no quantitative means to conclude which method will always produce a larger or lower result.
In summary, it is not a mathematical foregone conclusion that one method (NB-3600) is always more conservative than FEA (NB-3200); but the equations of NB-3600 were developed on purpose to be more conservative because simpler than the FEA results; and generally, based on past experience, it is most often the case that NB-3600 may fail Eq.(10), (12), and (13), while it may pass PL + Pb + Pe + Q ≤ 3Sm.
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