Proposal of a Thermo-Mechanical Fatigue Life Prediction Method of Nickel Based Superalloy IN738LC

Abstract

New biaxial thermo-mechanical fatigue testing machine, which can perform thermo-mechanical fatigue (TMF) tests under any selected conditions of axial loading, torsional loading and temperature history with arbitrary amplitude and phase, was developed. TMF tests were performed under complicated axial/torsional loading and temperature waveform (Blade waveform) that simulates strain and temperature histories imposed in actual gas turbine blades, as well as fundamental in-phase and out-of-phase tests. Test temperature was selected between 450-850°C. In case of without strain hold, failure life of the out-of-phase is comparable to that of the Blade waveform, while failure life of the in-phase is longer than that of the out-of-phase and the Blade waveform due to occurrence of negative mean stress. On the other hand, the largest life reduction was observed in the in-phase condition by introducing 6 minutes strain hold at the maximum temperature. Thermo-mechanical fatigue lives under various conditions were well correlated to uniaxial isothermal fatigue data by the equivalent total strain range, Δε_eq proposed in this study. Then, time dependent TMF life prediction model was proposed based on the nonlinear damage accumulation model incorporating with Δε_eq and failure lives of the Blade waveform with strain hold were accurately predicted by the TMF life prediction model.