Spatial Averaging in Porosity Assessment by Ultrasonic Attenuation Spectroscopy
The characterization of porosity in cast aluminum via the frequency dependence of the ultrasonic attenuation yields both the average pore radius and the volume fraction1. For dilute porosity of less than 6% volume fraction it was found that in case of 50 μm or higher average pore size the experimental measurement of the attenuation could be fit by the theoretical model for single scattering by spherical voids, and the resulting porosity parameters are in good agreement with independent optical and density measurements. Although some deviation between the ultrasonic and optical or density results are accepted as fully justified by the different physical nature of the methods, certain deviations of the ultrasonic results taken under different conditions, e.g. measured on samples of different lengths, show the need for a better understanding of the relation between the measured attenuation and the sought porosity parameters. It is shown that the porosity induced attenuation is smaller than the value predicted by the simplified theoretical model, which can result in consequent discrepancies above the measuring error. Spatial averaging before and after the spectral analysis is found to increase the reliability of the ultrasonic results by effectively rejecting the incoherent components in the attenuated signal.
KeywordsSpatial Average Ultrasonic Attenuation Backscatter Signal Aluminum Alloy Casting Coherent Signal
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