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# Section VIII Div 2 Compressive Loads design (EPC's vs 3-D.5 "Prager" curves)

## Section VIII Div 2 Compressive Loads design (EPC's vs 3-D.5 "Prager" curves)

(OP)
Which approach does compress use for compressive load design in Section VIII Division 2 when materials other than carbon and low alloy steel are considered?

Does compress use External Pressure Charts from Section II subpart 3 or does compress use Section VIII Div 2 part 3-D.5 to calculate tangent modulus?

If the method from VIII-2 part 3-D.5 is used then how are the calculations made?

I am following the rules in VIII-2 part 4.4.3, (and 4.4.5) for nickel alloy shells under external pressure and the instructions are to calculate tangent modulus "based on a stress equal to Fhe." So when I did that the result of using equation 4.4.6 was an allowable stress that was way lower than the Div 1 allowable stress. I am pretty sure I know where the problem is but I want to ask around in case there is something I am missing.

It seems like the instructions are wrong and that they should tell the user to calculate tangent modulus iteratively based on a STRAIN equal to Fhe / Ey. They would have to try out multiple values of stress to calculate Et until Stress / Et = Fhe / Ey.

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