PNNL

Abstract

Second harmonic generation using nonlinear ultrasonic waves have been shown to be an early indicator of possible fatigue damage in nuclear power plant components. This technique relies on measuring amplitudes, making it highly susceptible to variations in transducer coupling and instrumentation. Moreover, these type of measurements are relative, requiring a reference to compare the evolving material nonlinearity and making decisions about the remaining useful life. Permanently mounting transducers on the structure allows continuous monitoring and minimizes variation due to transducer coupling. This paper proposes an experimental procedure for in-situ surface wave nonlinear ultrasound measurements on specimen under high cycle fatigue loading without interrupting the experiment. Results show that variability due to transducer bonding is reduced, however the applied loading has a significant effect on the nonlinearity parameter, depending on the used frequency and excitation amplitude. Moreover, relations describing the effects of instrumentation nonlinearity on the measured nonlinearity parameter are derived. It is shown that spurious instrumentation nonlinearities result in variations in the measured nonlinearity parameter even if the instrumentation nonlinearity does not change.