Comparaison of temporal and frequency methods applied to ultrasonic nonlinear signals
Résumé
Since microcracks are considerably shorter than the acoustic wavelength, linear acoustic measurements are often not sensitive to such microscopic imperfections. Thereby, alternative methods can be applied to detect the nonlinearities present in a temporal signal as evidence of microdamage. Most methods rely on the analysis of the signal in the frequency domain. However, despite the diversity of the existing methods, the amplitudes of the higher order harmonics, generated by microcracks, are often small and consequently submerged within the noise. As an alternative, the scaling subtraction method (SSM) can be used, working in the time domain. The use of SSM increases the signal to noise ratio compared to Fourier based analysis methods. Simulation data corresponding to nonlinear ultrasonic waves propagating in hysteretic elastic media are used. Data are analyzed in both time (SSM) and frequency (FFT) domains. Results of the analysis are compared and the efficiency of the two approaches is estimated when equipment and environmental noises contaminate the signal. Furthermore, the kind of nonlinearity present in the sample is studied when the intensity of nonlinearity increases. The sensitivity of the above-mentioned approaches is discussed as a function of the excitation frequency, as well as the strength and the position of the nonlinear scatterer.
Domaines
Acoustique [physics.class-ph]
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