Uniaxial Strain Limit in Section VIII Div 2 Part 5 1

[JonOl]

Are the formulas correct in Table 5.7 in BPVC VIII Div 2 Part 5 (Uniaxial Strain Limit for Use in Multiaxial Strain Limit Calculations)? I have a stainless steel with a reported elongation at break of 35%. The uniaxial strain limit is calculated as 3*ln(1+E/100) = 94%, almost three times higher than strain at failure! It even states that the maximum value calculated from m2, elongation at break and reduction of area should be used. I am looking at the 2019 version of the code, maybe something have changed in the new version?

 

[DriveMeNuts]

m2 is true elongation at true ultimate stress.
To convert the material Engineering Elongation specified in the material spec (i.e. E = 35%) to True Elongation, use ln(1+E/100). This is what Table 5.7 does, and if you do this, you get a similar number to m2.
The number before this equation is a material constant. 3x for Stainless, 2x for Ferritic, 1.3x for Aluminium are all material constants dependent on factors such as grain size, cleanliness, inclusion content etc. that contribute to voiding and microcrack initiation.
α_sl is another material constant dependent on metallurgical (crystallographic) structure.
Stainless steel is better for intense cold working and cold stretching, therefore I would expect it to be better for local failure, hence the higher material factors.

It looks like if you set the Uniaxial Strain Limit to m2 as per Note 1, then you get a conservative result. I don't see why you would do this. You would always have a material test certificate with details on elongation, or ASME Part 2 provides minimum acceptable elongation for each material.

ASME PTB-1 provides comprehensive detail on the local failure method, including derivation of the big local failure equation.

 

[NRP99]

I would stick to minimum recommended values instead of unreliable test data. Relying on single test results would be very "unconservative". To use test values, I would have to consider more samples. And it will become a project of its own which is costly. Fortunately, ASME has already established min values based on experience/test results and I could use those values with confidence.

Go ahead, if you have statistical distribution of test data based on number of sample points which can be used with confidence.

Edit:
Reasoning of ASME behind using the values mentioned -

The values in Tables 5A and 5B are established by the Committee only. In the determination of allowable stress values, the Committee is guided by successful experience in service, insofar as evidence of satisfactory performance is available. Such evidence is considered equivalent to test data where operating conditions are known with reasonable certainty. In the evaluation of new materials, it is sometimes necessary to be guided by the comparison of test information with available data on successful applications of similar materials....