Abstract
The incident second harmonic wave is a problematic issue for the precise measurement of the acoustic nonlinearity parameter. This paper proposes a compensation method to remove the effect of the incident second harmonic component in the measurement of the absolute acoustic nonlinearity parameter using the calibration method. For this, the second harmonic component detected by the receiving transducer is considered as the sum of the component due to material nonlinearity and the component included in the incident signal and a numerical calculation model is developed as a function of the propagation distance. In the model, the factors related to the material nonlinear parameter and the magnitude of the incident second harmonic component are unknown and these are determined by finding a value that best matches the experimental data according to the change in the propagation distance; compensation for the incident second harmonic component is then achieved. The case where the phase of the second harmonic wave due to material nonlinearity is opposite to that of the fundamental wave is also considered. To verify the validity of the proposed method, fused silica and aluminum alloy Al6061-T6 specimens with different thicknesses corresponding to the propagation distance are tested. The experimental results show that the nonlinear parameters changed significantly according to the propagation distance before compensation but were very stable after compensation. Additionally, the average values of the nonlinear parameter are 11.04 in the fused silica, which is within the literature value range (10.1 to 12.4), and that for the Al6061-T6 is 6.59, which is close to the literature value range (4.5 to 6.12).
Highlights
The nonlinear ultrasonic technique has been researched as a nondestructive method to diagnose material degradation [1,2,3]
A method to compensate the effect of the incident second harmonic component in an experimentally obtained second harmonic displacement amplitude was proposed for the precise measurement of the absolute acoustic nonlinearity parameter using a novel calibration method
The second harmonic component detected by the receiving transducer was considered as the sum of the component due to material nonlinearity and the component due to the incident signal and a numerical calculation model considering diffraction was developed as a function of the propagation distance
Summary
The nonlinear ultrasonic technique has been researched as a nondestructive method to diagnose material degradation [1,2,3]. The magnitude of each component according to the change in propagation distance was expressed as a sound field analysis model and by finding the model parameters that best matched the experimental result, the magnitude of the incident second harmonic component was specified. As this method is for Rayleigh waves and can be applied only to relative measurements, it cannot be applied to the calibration method that uses longitudinal waves for absolute acoustic nonlinearity parameter measurements. The attenuation effect was not considered because it was negligible when considering the short propagation distance in the experiment and the low attenuation coefficient in both materials [16,17]
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