Development of the MPC Omega Method for Life Assessment in the Creep Range

Abstract

A methodology for characterizing and assessing the behavior of materials after service in the creep range has been developed and used on a broad range of materials and components. It incorporates the results of relatively short-term tests and improved databases on materials properties. The essence of the method is the definition of a material performance characteristic which the author refers to by the symbol Ωp. This coefficient effectively describes the rate at which a material’s ability to resist stress is degraded by strain. While Ωp is a function of stress, temperature, and mode of loading, it is amenable to parametric representation and is, therefore, useful in predicting life and strain accumulation. Time to failure and total accumulated strain are shown to be consequences of a characterizing strain rate, as defined herein, and an appropriate Ωp for the operating conditions and geometry of interest. Accumulated strain, future strain, current creep rate, remaining life, total damage, and damage rate are among the quantities which are easily calculated. The development of the method employs and extends the concepts of Larson-Miller, Monkman-Grant, Robinson, Theta Projection, Kachanov, and Norton.