The Valid Stress Range for the MPC Omega Model in API 579-1/ASME FFS-1 2016 and the Creep Rupture Model in WRC 541
C.H. Panzarella, Ph.D., E. Gassama, Ph.D., J. Cochran, D.A. Osage, ASME Fellow, P.E.
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The MPC Omega creep model used in API 579-1/ASME-FFS-1 (API 579) and the creep rupture model in WRC 541 has material dependent coefficients of cubic polynomials that are determined through linear regression of initial strain rate and rupture life data. These correlations sometimes exhibit non-physical behavior when extrapolated to the limit of very small and very large stresses. Such anomalous limiting behavior poses problems when used in finite element analysis and other numerical analysis.
The intent of this WRC publication is to document a method for checking the proper physical behavior of the initial strain rate and Larson-Miller parameters across the entire range of stress. If the existing coefficients do not satisfy these checks, then the method described herein is used to extend the range of validity of these correlations. This is done by determining an alternate set of coefficients that matches the original correlations as close as possible within a stress range where valid test data exists, while also having the correct limiting behavior outside of this range.
The validity of all the coefficients of the API 579-1/ASME FFS-1 and WRC Bulletin 541 Evaluation of Material Strength Data for Use in API Std 530, 7th Edition, were checked in this report and new coefficients determined for those materials which do not meet the criteria.