Mistake in Peng's Book?

[Patassa]

Hi,

I'm currently studying Mr. Peng's book on Pipe Stress engineering and on page 52 in the discussions of the different failure theories he states that "ASME has adopted the maximum shear failure theory in its piping and pressure vessel codes", but I'm also reading Mr. Becht's guide to 31.3 where he states that "the pressure design rules in the Code are based on maximum normal stress, or maximum principle stress".

Can someone please clarify this seeming contradiction?

And on that note, is there any significance to the 3D maxiumum stress intensity as calculated by Caesar II? I've seen Mr. Diehl reply in a few forums that he "doesn't worry about non-code stresses", and I suppose at his level of expertise he has good reasons to say that, I just don't know what they are at this point in my young career.

Thank you

 

[BigInch]

This explanation is rather easy to understand,

http://www.roymech.co.uk/Useful_Tables/Mechanics/stress.html

 

[Patassa]

So does the code take both failure modes into consideration? Is that how those two statements are resolved as not being contradictory?

 

[Patassa]

Edit found this from Diehl, "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."

So I suppose it is both, one failure theory for sustained and a different one for pressure design.

 

[XL83NL]

To some extent you can see a parallel between the max shear stress formulation and B31.3 para 319.4.4 (eq. (17)), however I'd be interested if anyone can share a reference to some (old) paperwork which shows how the different theories are worked out to Code formulas.