PROTECTION AGAINST LOCAL FAILURE ASME SECVIII DIV 2

[lonlyboy]

dear member
I'm going to design a FSE (thick wall expansion joint) and a nozzle to shell junction by ASME SEC VIII DIV 2 .
in addition to an elastic analysis (PROTECTION AGAINST PLASTIC COLLAPSE)our client asked to
check PROTECTION AGAINST LOCAL FAILURE,but I found that principals stress to be extracted
when/where stress are primary. am I right?(I mean primary principal stress)
- if yes ,it seems that protection against local failure is not applicable in FSE, while most of stress
are secondary (PL, PL +Q ),because of discontinuity and temperature specially in load cases (4,5,6,7 see ASME SEC 8 UHX part).
- for analyzing stress in nozzle to shell WRC 107 is not applicable in this case therefore Nozzle-pro (PRG software )is used
but I want to know if I should consider local failure too ? I mean check (S1+S2+S3)option

 

[TGS4]

Protection Against Local Failure is always required to be performed, unless you can be exempted per 5.3.1.1. I would also note the caution for the elastic Local Failure check - 5.3.2 - given in ASME PTB-1 (2014):

Two issues that are apparent is the use of an elastic stress basis for a local criterion and the stress category that is used with this criterion. It is not apparent how pseudo elastic stresses, i.e. elastically calculated stresses that exceed the yield strength can be used to evaluate a local fracture strain of a ductile material with strain hardening. In addition, the type of stress used in the criterion (i.e. linearized or average values verse stress at a point) and stress category (i.e. primary, secondary and peak) needs to be resolved. Since local failure is the failure mode being evaluated, the type of stress and stress category used in VIII-3 would appear to more correct. For ductile materials, a local criterion based on elastic analysis may not meaningful and the elastic-plastic method that follows is recommended for all applications.

For your nozzle example, I suspect that you could exempt yourself as I described above.

In general, I follow the advice provided by PTB-1 (2014) and always use the Elastic-Plastic Analysis method. For your thick-wall expansion joint, I think that you would probably want to use the elastic-plastic analysis method for demonstrating Protection Against Plastic Collapse, as well, since the classification of the stresses could be complicated and/or ambiguous. Remember the caution in 5.2.1.2...

 

[lonlyboy]

thanks for your kind answer .
actually I'm not expert in elastic - plastic analysis .

ASME SEC 8 DIV2 2015
5.3.1.2 Two analysis methodologies are provided for evaluating protection against local failure under applied design loads.When protection against plastic collapse is satisfied by the method in 5.2.3,either method listed below is acceptable.
5.3.2 Elastic Analysis–Triaxial Stress Limit.
5.3.3 Elastic-Plastic Analysis–Local Strain Limit.
according to latest version of ASME it seems this two method are acceptable when limit load analyses is used . is it right?
if ,a local criterion based on elastic analysis may not meaningful, so why software like PRG (nozzle pro)consider it in elastic analysis?

ASME Sec8-1 PART UHX table-uhx 17 help to classify stress for FSE also TEMA 9th
give us a procedure to analyses.

 

[TGS4]

Indeed, the Code says that both methods are acceptable. Suffice it to say that there has been some substantial controversy regarding the elastic method - enough controversy that WG-DBA (VIII) has moved all discussion of it to the end of our agenda, because discussing it takes up too much time from other, more pressing, issues.

The wording currently in VIII-2 was there from the original 2007, which itself was a controversial compromise. Because of the historical aspects of the elastic method - which, as described in the original Criteria of the ASME Boiler and Pressure Vessel Code For Design By Analysis in Sections III and VIII, Division 2

Since the "stress intensity" limit used in these Codes is based upon the maximum shear stress criterion, there is no limit on the "hydrostatic" component of the stress. Therefore, a special limit on the algebraic sum of the three principal stress(sic) is required for completeness.

it was decided for the 2007 Edition to retain the elastic analysis method, despite a complete lack of engineering rigor on its application, as discussed in PTB-1.

Commercial software, such as Nozzle Pro (and others) perform this analysis because it is, strictly-speaking, permitted by the Code. However, my personal engineering judgement, based on the extensive discussion that have been going on for the last 10+ years, as well as the commentary in ASME PTB-1, is that I will not use it and my recommendation is to not use it. You can take that for what it's worth.

Be forewarned that just because you have a classification guide from the likes of UHX or TEMA, that this will allow you to properly classify the stresses from an FEA. Heed the warning in 5.2.1.2.