Nonlinear Ultrasonics: Signal Processing Considerations and a Nonlinear Parameter for Rayleigh Waves

Abstract

An effective way to describe changes in the microstructure of a material or to assess fatigue damage at an early stage in fatigue life is by measuring the acoustic nonlinearity parameter beta. The nonlinearity parameter is defined for harmonic longitudinal plane waves and it depends on the ratio of the amplitudes of the first harmonic of the exciting signal and the second harmonic. A reliable measurement of the amplitudes of these harmonics is crucial since their amplitude of the second (higher) harmonic is much smaller than the amplitude of the first harmonic. This research investigates the influence of the apparent nonlinearity that can occur due to the signal processing and shows how this influence can be quantified and minimized to enable a more accurate evaluation of the acoustic nonlinearity parameter. Furthermore the concept of the nonlinear parameter is extended to Rayleigh surface waves by developing a connection between the harmonic amplitudes and the third order elastic constants, using the approximate model by Zabolotskaya. Finally the problem of modeling the influence of fatigue damage on the nonlinear parameter and the elastic constants is discussed. The reduction of the processing nonlinearity combined with Rayleigh surface waves - Rayleigh surface waves are more efficient in the detection of fatigue damage initiated and concentrated at the surface - helps improve the prediction of fatigue damage and the remaining life of a sample.