The Effect of Thermomechanical Strain Cycling on the Fatigue Life of Gas Turbine Material In-738LC: A Life Prediction Model

Experimental and analytical method is developed to address the effect of thermomechanical fatigue (TMF) on a nickel base superalloy which is a γ’/ (Ni₃Al) strengthened material used primarily for land based gas turbine blading. A life prediction model is developed on the basis of a tensile hysteresis energy approach which incorporates the effect of creep, environment, and temperature. The developed life prediction model has shown a good predictive capability to correlate most of IN-738 out-of-phase TMF data from numerous researchers into a 3x scatterband. The TMF tests were carried out on two sets of cyclic temperatures of 482–816°C (900–1500°F) and 482–871°C (900–1600°F). In addition, isothermal tests were carried out at 816°C (1500°F) under strain ranges from 0.3% to 0.8% for life comparison between isothermal and TMF. The results show that the TMF out-of-phase type cycle is the most damaging cycle for the IN-738LC material when compared to both in-phase and isothermal cycles. All experiments were strain-controlled with a triangular waveform and an A-ratio (A = ε_amp/ε_mean) = ∞.