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

At low frequencies, in thin plates the phase velocity of the guided A 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 A mode in thin plastic films. This study demonstrates the feasibility of picking up a subsonic A mode in plastic films by air-coupled ultrasonic arrays. The air-coupled reception was based on an evanescent wave in air accompanying the propagating A 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 A 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 A 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.