Account for the Local Notch or Effect of the Weld - ASME Code Fatigue Analysis

[misc]

Code Folks,

In the ASME B&PV Code, Section VIII, Div. 2, Paragraph 5.5.3.2 Step 4(a) (as an example), the sentence reads "(a) If the local notch or effect of the weld is accounted for in the numerical model, then Kf = 1.0 in eqs. (5.30) and (5.36)." What does this mean? What would an adequate representation of a local notch or effect of a weld look like in an FEA model so as to permit the use of a Kf factor of 1?

For example if a fillet weld is explicitly modeled at a joint as an idealized triangle with a straight hypotenuse, does that qualify as accounting for the effect of the weld?

Since it would be very challenging to accurately model an actual notch or actual weld profile, can a condition wherein Kf = 1 applies to FEA results ever be achieved?

Many thanks in advance!

 

[IdanPV]

In an FEA model, notch effect of the weld can only be accounted for if there is no singularity, in this case you will have Kf=1 (FRSF is 1.0).
For as-welded regions, the stresses can not be taken directly from the FEA solution due to singularities.
Fatigue assessment in those areas is calculated using the secondary stress, and an FSRF in accordance with Tables 5.11 and 5.12 shall be use.

Source - Fatigue Design of Process Equipment, Chris Hinnat, the Paulin Research Group.
Fatigue Design of Process Equipment

 

[misc]

Does anyone know the rationale / basis for the low cycle fatigue modification in the structural stress method in the ASME Code Section VIII, Div. 2? The factor is essentially [E / ( 1 - nu^2)] on stress range as given in Eqn. (5-58) of Section 5.5.5.2 of the 2023 Edition of the Code.

Also, there is a note under it that I find a bit confusing. It says to use always use the modification regardless of high cycle or low cycle fatigue and that for high cycle applications this procedure will provide the correct results. Can anyone provide comment on the meaning/intention of this note?

Thank you.

 

[TGS4]

That note is incorrectly written. That is actually a multi-axial correction factor.