Linearized Triaxial Stress Limit

[LS_SMS]

Folks, I am looking at 2023 ASME Sec. VIII, Div. 2, Part 5.3.2 for the triaxial stress limit. It says...

The algebraic sum of the three linearized primary principal stresses from Design Load Combination (1) of Table 5.3 shall be used for checking this criterion.

Why do they put the word "linearized" in there? I understand the term's usage in the context of linearized through-wall stress for the checks in Part 5.2.2 (i.e., membrane, bending, etc.). But I don't understand its relevance in a triaxial stress check.

Interestingly, Sec. III, Div. 5, HBB-3227.4 leaves the word out, which makes more sense to me...

The algebraic sum of the three primary principal stresses (σ1 + σ2 + σ3 ) shall not exceed four times the tabulated value of Smt.

Can anyone shed some light?

 

[TGS4]

This is discussed in ASME PTB-1. What part of that explanation is not clear to you?

 

[LS_SMS]

I don't have access to that. In the absence of it, could you shed some light to help me understand?

 

[TGS4]

Long and short of it is that the paragraph in HBB is similar to what's in VIII-3 - essentially stress-at-a-point, whereas VIII-2 uses a stress linearization process, where one needs to use a valid and validated SCL. And it only uses the primary stresses - therefore we are essentially looking at primary membrane stresses only (except for flat plates, where there actually exist primary bending stresses). Since Local Failure is related to Plastic Collapse, the BPV VIII Code Committee decided to continue with the same approach as Plastic Collapse. Then, later, the BPV VIII Code Committee, determined that this elastic approach is conservative, as compared to the elastic-plastic approach.

Do you understand the purpose of a tri-axial stress check? It is even touched upon in the original Criteria Document: "A limit on the sum of the principal stresses is provided for completeness."