Characterization and localization of fatigue damage in nickel-based superalloys using nonlinear ultrasonic harmonic method

The early detection and location of fatigue damage is vital in ensuring the reliability of aero-engine turbine blades. In this study, low- and high-cycle fatigue interruption tests were conducted on nickel-based superalloys at room temperature, and the ultrasonic linear parameters (P-wave velocity and attenuation coefficient) and nonlinear parameters (ratio of second harmonic amplitude to square of the fundamental amplitude) were measured in different regions of samples to characterize and locate fatigue damage. The results indicate that the normalized ultrasonic nonlinear parameter β́ exhibits a very strong correlation with the degree of fatigue damage. Furthermore, by combining the ultrasonic P-wave and Rayleigh wave nonlinear methods, the fatigue crack source can be located in two dimensions. Fatigue characteristics such as the ratchet curve, cumulative strain and fracture morphology under the two fatigue modes were studied to clarify the ultrasonic nonlinear response mechanism. Under low-cycle fatigue, β́first increases, then stabilizes before decreasing. Under high-cycle fatigue, β́ initially remains stable and then begins to decline. The evolution of β́ is closely related to the plastic deformation generated during fatigue, and the decrease in β́ in the later stages is due to the propagation of microcracks.