It is worth to mention that it would be more appropriate to have a "stress-strain" analysis of the flange joint.

A "Stress-Strain" analysis is not banned by ASME VIII, since Div 1 describes it in Appendix S: "Such analysis is one that considers the changes in bolt elongation, flange deflection and gasket load that take places with the application of internal pressure, starting from the prestressed condition".

Or in an ASME book, under the "Recommended approach" title: "The most efficient approach for evaluating leakage is to evaluate the flange as a structural system. The flexibility of the flange gaskets and bolts must be modeled to accurately predict the gasket and bolt loads resulting from pressure and external loads”.

Not really interested to have such approach?

Best regards,