Hi
I would like to clarify the failure theory used in ASME B31.3.
as per code,sustain stress calculation based on Max Principal stress theory and for expansion stress based on Max shear stress theory.

Is there any specific reason behind using Max principal stress as it is not advisable for ductile material?

Regards
Pt

Please correct me if I'm wrong all stress guru's there but I think both use max shear stress theory...

cheers...

Any Comments...

B31.3 stress evaluation is based on the maximum shear stress (Tresca) theory but it is simplified. Remember that the codes were written long before we all had fast PCs and software. (Up until a few years ago, B31.3 calculation could be performed with a slide rule.)
You could say that the required wall thickness for pressure calculation is a maximum principle stress (Rankine Theory) check based on hoop stress alone.

I agree with Dave...This simplified approach has been adopted for many years ago up until now and it works...