Thanks Dorin for that information.

If I'm not mistaken this is also related to smooth bar specimen vs. the buttwelded ASME B31.3 basis for Markl Fatigue Tests for piping and piping components.

Also the difference in Code stress vs. Max Stress Intensity same as sample summary below.

"Highest Stresses: (lb./sq.in.) LOADCASE 14 (EXP) L14=L2-L12
Ratio (%): 225.4 @Node 3608
Code Stress: 67312.5 Allowable Stress: 29862.5
Axial Stress: 1313.8 @Node 3608
Bending Stress: 65990.6 @Node 3608
Torsion Stress: 8109.8 @Node 3619
Hoop Stress: 0.0 @Node 2672
Max Stress Intensity: 80147.3 @Node 3608"

Actually, one of the client from my previous years ask why the Code stress is different than the Max Stress Intensity.

The Code stress as I know is derived from the principal stress out of the longitudinal and shear stress (e.g bending and torsion due to thermal expansion) , while the maximum stress intensity is derived from longitudinal, hoop, radial and shear stress (radial and hoop being a stress related to pressure)

This means that Code stress is pure thermal expansion loads while max stress intensity is a combination of pressure and thermal expansion loads.

And it has been a proven experience from the Code that pressure is considered as primary stress (W+P1) unless there is really a significant amount of pressure cycling in the life of piping component which the designer should be aware about if it's a requirement in the operation of a certain piping system.

But this is rare in the industry, and I myself haven't experienced to do a detailed pressure cycling fatigue calculation of a piping component.

When we use this Max Stress Intensity as a fatigue evaluation, shall we double the stress or lower the allowable by a factor of 2?

I believe yes, because of the presence of buttweld in a piping component comparing it to Smooth Bar Fatigue Curve, which approximately equal to 2 (FSRF).

But how about in a bend where the maximum stress intensity lies in extrados and not on the buttweld itself (but it would be a tedious job to model each weld and input a multiplier of 2 on the stress) so I agree with Dorin to stick to a factor of 2 to simplify the process, which is the original intent of Markl Fatigue Tests.

Your expert opinion is highly appreciated.

Warm Regards,