Triaxial Stresses 1

[engrdon]

Paragraph 4-137 of ASME Section VIII, Division 2, Appendix 4 states that "the algebraic sum of the the three primary principal stresses (sigma1+sigma2+sigma3) shall not exceed four times the tabulated value of Sm."

I have searched the internet extensively and found papers and aticles on the triaxiality factor, but not about the requirement in the above paragraph.

Does anyone have the background of this relationship and how it relates to pressure vessel design?

Regards,
engrdon

 

[GregLamberson]

Engrdon

The failure mode to be considered is overall material breakdown due to overstress. In a pipe system a triaxial state of stress exists. The three stresses acting on a piece of the pipe wall are the circumferential stress , the longitudinal stress and the internal pressure.

For loading conditions where no shear exists, these three stresses are equal to the principal stresses. The equation used for triaxial stress applies for a no-shear condition.

For other than straight pipe, shear conditions due to torsion from offset must also be evaluated, and the stated stress relationship may not be valid.

The formula indicates that failure can occur for any condition of loading where the combined principal stresses equal or exceed the yield stress of the material.



Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[engrdon]

"The formula indicates that failure can occur for any condition of loading where the combined principal stresses equal or exceed the yield stress of the material."

In the formula, the sum of the principal stresses must be less than 4 x Sm. If one uses SA-105 (when dealing with pipe), at ambient temperature, Sm = 23.3 ksi. The yield of this material at ambient temperature is 70 ksi. So we have 4 x 23.3 ksi = 93.2 ksi; this is much higher than the yield stress.

It seems that there might be an expectation that stress redistribution will take place. Is this so?

Regards,

engrdon

 

[GregLamberson]

I may be mistaken, but isn't SA 105 (generally used for flanges) rated at 70 ksi?

It might be helpful if you provide the details of what it is you are trying to calculate/determine? I know your original post asked about the relationship of triaxial stress to pressure vessle design, I may have gotten you off track in discussing simply the piping angle, if so I apologize.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com