Primary Membrane Stress Range for Thermal Stress Ratchet

[LS_SMS]

Folks, I am looking at 2023 ASME Sec. VIII, Div. 2, Part 5.5.6.3 for the Thermal Stress Ratchet check. This equation is provided...



P_m is referred to as "primary membrane stress." Is it supposed to be a primary membrane stress range? Everything in the fatigue and ratchet assessment up to this point uses stress ranges from the two peaks that form the stress pair. If I'm dealing with two peaks for this calculation of X, it is unclear which primary membrane stress should be used? My intuition tells me it should be the range for the two peaks in the pair. Can anyone share some feedback?

 

[LS_SMS]

Bump.

A friend said, "Pm is definitely not treated as a range in this situation. Rather, it is the Pm from Part 5.2, Protection Against Plastic Collapse." And they went on to say that it may be confusing because Pm used to be a range prior to the 2007 edition.

Does any of that ring a bell with the folks here?

 

[TomLee777]

ASME Sec. VIII, Div. 2 uses stress amplitude for fatigue life assessment, not stress range as people normally do.

 

[LS_SMS]

Thank you @TomLee777. But my question pertains to Pm in the ratchet calculation. And specifically, the question is, "is Pm a singular value (e.g., Pm from the design conditions in Part 5.2), or a range?"

 

[GD2]

Folks, I am looking at 2023 ASME Sec. VIII, Div. 2, Part 5.5.6.3 for the Thermal Stress Ratchet check. This equation is provided...

View attachment 9470

P_m is referred to as "primary membrane stress." Is it supposed to be a primary membrane stress range? Everything in the fatigue and ratchet assessment up to this point uses stress ranges from the two peaks that form the stress pair. If I'm dealing with two peaks for this calculation of X, it is unclear which primary membrane stress should be used? My intuition tells me it should be the range for the two peaks in the pair. Can anyone share some feedback?

No, it's not a stress range. By definition, P_m is derived from the average value across the thickness of the section from internal pressure and other mechanical loads, excludes secondary and peak stresses. Both P_m and S_y are to be determined at the average temperature of the cycle.