The EIFS-based fatigue life prediction approach of nickel-based single crystals with film cooling holes at elevated temperature

Abstract

In this study, a new framework for the fatigue life prediction of nickel-base single crystal (SX) superalloys with different drilling film cooling holes (FCHs) at high temperatures (900 ℃ and 980 ℃) is investigated based on surface integrity quantification and fracture mechanics. For all the tested SX superalloys with anisotropy (smooth and FCHs specimens), the initial damage state is regarded as the equivalent initial flaw size (EIFS) that is independent of the specimens and hole geometry in the same drilling, and the rationality of EIFS is verified for the first time by conducting numerous fatigue tests and comprehensive surface integrity analysis. Subsequently, the fatigue crack path and microstructure of different specimens at different temperatures are analyzed to reveal the crack initiation mechanism and propagation modes, and a new equivalent stress intensity factor, ΔKeq, is proposed to describe the crack propagation driving force. The EIFS and ΔKeq are combined, and a fatigue crack growth rate (FCGR) with better fit is obtained to comprehensively reflect the different fracture modes (the mixture of Stage I and Mode I). Finally, based on the experimental observation and FCGR description, the fatigue life of the FCHs structure at room and high temperature is predicted to be 3–5 times the dispersion zone of the total fatigue life, and the ultimate defect length is proposed to guide the engineering practices.