Multi-Mode Nonlinear Ultrasonic Characterization of Fatigue Crack Evolution in GH4169 Nickel-Based Superalloy

GH4169 nickel-based superalloy is widely used in critical aerospace components, where early fatigue damage detection is essential for structural safety. This study investigates the evolution of nonlinear ultrasonic responses during fatigue damage and crack propagation in GH4169 alloy through laser-fabricated controlled-crack specimens and interrupted fatigue tests on compact-tension specimens. Three wave modes—Rayleigh waves, normal-incidence longitudinal waves, and end-face-incidence longitudinal waves—were employed, and the results were compared with linear ultrasonic parameters. The controlled-crack experiments showed a crack-geometry dependence consistent with that predicted by the nonlinear spring model, with the normalized nonlinear parameter β′ exhibiting excellent linear correlations with a2 and 1/b2. The Rayleigh-wave nonlinear parameter exhibited a non-monotonic three-stage evolution: increasing during 6000–8000 cycles, peaking at 9000 cycles when an optically detectable surface crack was first observed, and decreasing thereafter. All three wave modes showed consistent overall non-monotonic trends despite different peak positions, while linear parameters remained essentially constant during the early fatigue stages. The observed three-stage evolution is interpreted as being associated with progressive changes in fatigue-induced microstructural damage, crack-related nonlinear interactions, and ultrasonic wave propagation as the crack grows. These findings demonstrate that multi-mode nonlinear ultrasonic techniques are sensitive to the evolution of fatigue damage and crack propagation and can provide complementary information for fatigue-damage characterization in aerospace components.