Thermo-Mechanical Fatigue of Mar-M247: Part 2—Life Prediction

A life prediction model is proposed based on microstructural observations of damage in thermo-mechanical fatigue and isothermal fatigue experiments on Mar-M247 Nickel based Superalloy. The model incorporates damage accumulation due to fatigue, environment (oxidation and γ′ depletion), and creep processes. The model is capable of predicting lives at different temperatures, strain rates and temperature-strain phasing conditions. The model successfully predicted the shorter lives at high strain amplitudes in in-phase thermo-mechanical fatigue cases and the shorter lives at lower strain amplitudes in out-of-phase thermo-mechanical fatigue cases and the associated crossover in life. The prediction of a nonproportional strain-temperature history (diamond shaped) was very satisfactory. A unified constitutive equation was utilized to predict the stresses, which influenced the creep damage term. The oxidation term is a function of mechanical strain range, temperature-strain phasing and incorporated oxidation and γ′ depletion kinetics.