Thank you very much Mr. Breen you have been very helpfull, but from these explanation another question came up.

According to these, the SIFs are related to the fatigue effects in pipe accesories due to their geometry.

The FEA methods used to determinate the SIFs on complexes geometries, like the one used in the software FE/PIPE, computes SIF´s by the ratio of (PL+PB+Q+F) and a nominal stress.

The primary membrane stress (PL), primary bending stress (PB), secundary stress (Q) and Peak stress (F), acording to ASME Sec. VIII Div. 2, are not related to fatigue effects.

Then the "SIFs" calculated by FEA methods seems more like stress concentration factors.

Are the SIFs calculated by FEA reliable?, I´m missing somethig?

Thanks and Regards