Combined stress evaluation in piping is typically based on which theory or theories?

• A. Maximum shear or distortion energy theory
• B. Maximum principal stress theory
• C. Cauchy stress theory
• D. Mohr's circle concept

Answer: (A)

The important idea is that combined stresses in piping are evaluated using the distortional energy approach, specifically the von Mises yield criterion. For ductile metals, yield tends to occur due to the energy associated with distortion from the multi-axial stress state, not just the largest single principal stress. The von Mises criterion converts all the normal and shear stresses from pressure, bending, axial loads, and any torsion into one equivalent stress, which is then compared to the material’s yield strength in simple tension. This makes it well suited for predicting when yielding will happen under complex loading in piping systems and aligns with common engineering practice in ASME piping design.

The other ideas aren’t normally used as the primary yield criterion in this context. Maximum principal stress theory centers on the largest principal stress, which is more applicable to certain brittle materials. Mohr’s circle is a valuable graphical tool for analyzing stresses and finding principal or shear stresses, but it isn’t itself a yield criterion. Cauchy stress theory describes how stresses are defined and transformed, not how yielding occurs.