Recent work by the ASME Code Committees established a new Nonmandatory Appendix Z to Section III, Division 5 of the ASME Boiler and Pressure Code that provides constitutive models for use with the Section III, Division 5 Class A design by inelastic analysis rules. These models promote the use of the more accurate design by inelastic analysis methods by alleviating the designer or owner from the burden of developing and validating a model themselves. Models for four materials — Grade 91 steel, 316H stainless steel, the nickel-based Alloy 617, and the nickel-iron-chromium Alloy 800H — have either been approved or are under consideration at the cognizant ASME Code Committees. All four models have significantly different mathematical forms. While differences in material behavior may justify or require treating each material with a bespoke model, the mathematical exposition and detailed explanation required to describe and document the models in the Code is a significant logistical challenge. That challenge carries over to implementing the models for use in design calculations, as the computer implementations of the models are likewise significantly different. This paper explores the feasibility of a universal high temperature inelastic constitutive model, general enough to accurately capture the key material behavior of these four materials. If such a model exists, it would greatly simplify the new Appendix Z and further reduce the investment required by designers needed to apply the design by inelastic analysis rules. This work therefore attempts to define a generalized constitutive model, fits the model form for the four materials using an extensive experimental test database, collected as part of the effort to develop the original constitutive models, and evaluates the accuracy of the universal models by comparing model predictions against both the experimental data and the current models endorsed or under consideration by the ASME Code Committees.

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