Monitoring fatigue damage accumulation with Rayleigh wave harmonic generation measurements

The use of nonlinear acoustics, and in particular harmonic generation, continues to gain interest as a means to characterize microstructural changes in engineering materials as a result of processing and in-service conditions. Typical measurements involve the propagation of a monochromatic longitudinal wave toneburst through the bulk of a sample in a through-transmission arrangement. This arrangement is not well suited for field applications, where two-sided access is limited and parallel surfaces are rare. Harmonic generation measurements to monitor fatigue damage in aero engine alloys subject to fatigue will be presented. The use of Rayleigh waves has allowed one-sided access and accommodates a small amount of surface curvature while restricting the probing wave to the surface where fatigue damage is typically initiated. Early results using uncalibrated receiving transducers on Ni-based alloys demonstrate a sensitivity to damage accumulation in low cycle fatigue, where damage is spread over the sample surface. Damage accumulation in high cycle fatigue is concentrated in small regions on the sample surface and so has not yet been revealed by changes in the generation of harmonics using the uncalibrated probes.