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Open AccessArticle

Air-Coupled Reception of a Slow Ultrasonic A0 Mode Wave Propagating in Thin Plastic Film

Ultrasound Research Institute, Kaunas University of Technology, K. Baršausko St. 59, LT-51423 Kaunas, Lithuania
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Sensors 2020, 20(2), 516; https://doi.org/10.3390/s20020516
Received: 20 December 2019 / Revised: 13 January 2020 / Accepted: 15 January 2020 / Published: 16 January 2020
(This article belongs to the Special Issue Sensors for Nondestructive Testing)
At low frequencies, in thin plates the phase velocity of the guided A0 mode can become slower than that of the ultrasound velocity in air. Such waves do not excite leaky waves in the surrounding air, and therefore, it is impossible to excite and receive them by conventional air-coupled methods. The objective of this research was the development of an air-coupled technique for the reception of slow A0 mode in thin plastic films. This study demonstrates the feasibility of picking up a subsonic A0 mode in plastic films by air-coupled ultrasonic arrays. The air-coupled reception was based on an evanescent wave in air accompanying the propagating A0 mode in a film. The efficiency of the reception was enhanced by using a virtual array which was arranged from the data collected by a single air-coupled receiver. The signals measured at the points corresponding to the positions of the phase-matched array were recorded and processed. The transmitting array excited not only the A0 mode in the film, but also a direct wave in air. This wave propagated at ultrasound velocity in air and was faster than the evanescent wave. For efficient reception of the A0 mode, the additional signal-processing procedure based on the application of the 2D Fourier transform in a spatial–temporal domain. The obtained results can be useful for the development of novel air-coupled ultrasonic non-destructive testing techniques. View Full-Text
Keywords: air-coupled ultrasound; contactless ultrasonic sensors; guided ultrasonic wave; subsonic A0 Lamb wave; air-coupled ultrasonic array; evanescent wave; air-coupled reception of subsonic guided wave air-coupled ultrasound; contactless ultrasonic sensors; guided ultrasonic wave; subsonic A0 Lamb wave; air-coupled ultrasonic array; evanescent wave; air-coupled reception of subsonic guided wave
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Kazys, R.J.; Vilpisauskas, A. Air-Coupled Reception of a Slow Ultrasonic A0 Mode Wave Propagating in Thin Plastic Film. Sensors 2020, 20, 516.

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