Life Prediction Modeling of Combined High-Cycle Fatigue and Creep

Industrial gas turbine blades are subjected to high temperatures and an array of mechanical and dynamic loads, making creep and high-cycle fatigue critical aspects of turbine blade design. The combination of creep and high-cycle fatigue produces a synergistic interaction effect whose explicit consequence to turbine life has been the subject of very little research. This interaction remains unaccounted for by current, decoupled life prediction models, which traditionally incorporate such interactions into conservative design safety factors. Improved lifing models capable of capturing these effects are now needed in order to maintain current reliability standards in next-generation operating conditions. This research identifies the life-limiting aspect of a combined high-cycle fatigue and creep response in conventionally cast Alloy 247 LC, and captures the interaction of the two loads in a novel life prediction model. The proposed model is created from a comprehensive collection of experimental data obtained using an unconventional two-part test method, where test specimens pre-deformed to a prescribed creep strain are fatigue loaded at an elevated temperature and high frequency until failure. A variety of temperatures, creep strains, and fatigue loading conditions are explored to ensure that the resulting model is applicable to the myriad of potential turbine blade operating conditions. Rigorous metallographic assessments accompanying each test are leveraged to create a microstructurally-informed combined life prediction model.