the weird practice related to the PL+Pb+Q evaluation. 4

[YuJie_PV]

Dear all,

I just want to talk about a phenomena in the field of FEA that i encountered.
i am reviewing a FEA report submmitted by vendor, and see it again.
I know that paragraph 5.5.6 in ASME VIII-2 requires to evaluate PL+Pb+Q in case of cyclic loading for elastic analysis, which intend to protection from failure of ratcheting.
I find a lot of engineers evaluate PL+Pb+Q even there is no cyclic loading, but under design loadings instead of operating loadings as specified in ASME VIII-2.
such manner has been performed for so many years, i think it must have some reason that i don't understand.
do you have ever seen such thing?
is it necessary?

thanks in advance.

 

[DriveMeNuts]

What allowable stress are they applying to PL+Pb+Q? 1.5xS?
The european code says that for certain geometries such as nozzles, the bending stresses due to external loads 'should' be considered Primary. However, it has a note saying that if a small amount of plastic deformation is acceptable, then the bending stresses can be considered secondary.
Perhaps, Engineers just want to add a bit more robustness to their design. Or perhaps they don't understand the rules.

 

[YuJie_PV]

Thanks, DriveMeNuts .
the acceptance criterion they apply is: PL+Pb+Q≤3.0xS.
i think you are right. The method lots of people perform doesn't automatically prove it is correct.

i just have another confusion about calculation of PL+Pb+Q. According to "Hopper diagram", the PL+Pb+Q shall be a range. so i am more confused with these text from 5.5.6.1(b)of ASME VIII-2 :
(b) The primary plus secondary equivalent stress range, ΔSn,k, is the equivalent stress range, derived from the highest value across the thickness of a section, of the combination of linearized general or local primary membrane stresses plus primary bending stresses plus secondary stresses (PL + Pb + Q), produced by specified operating pressure and other specified mechanical loads and by general thermal effects.
is it a range or just a "highest value across the thickness of a section"?

 

[TGS4]

There is simply a widespread misunderstanding of the ratcheting failure mode. The range of P+Q is limited to S_ps. The words on the Code are correct. The Hopper Diagram is marginally helpful, at best.

Just as a heads-up, the ASME Code Committee responsible for Part 5 is working on a change to the S_ps limit that would entirely drop the dependency on the allowable stress, S, and leave only the dependency on the yield stress. I am hopeful that this change will assist in disabusing engineers of this misapplication and misunderstanding of this particular failure mode.