A Modified Power Law for Creep Analysis

The creep strengths for an annealed Ni-33Fe-25Cr alloy, UNS N08120 (HAYNES^® HR-120^® alloy) are presented from 1100°F to 2200°F at every 100°F for lives up to 30,000 hours. The creep data is used as a basis for converting the power law method (log stress vs. log time at a defined temperature) to the usefulness of the Larson-Miller method for design creep analysis. A new empirical relationship was developed between stress, temperature and creep rupture life. Power Law equations were generated at each temperature using linear regression of the transformed data to obtain the best-fit constants. The constants were then plotted and a best-fit polynomial of the constants as a function of temperature was determined. The polynomial then replaced the constants in the original power law equation to obtain a single expression of stress as a function of time and temperature. Using this new Modified Power Law (MPL) equation, interpolation or extrapolation to obtain stress becomes possible for any temperature and time.