AirCoupled Reception of a Slow Ultrasonic A0 Mode Wave Propagating in Thin Plastic Film 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 aircoupled methods. The objective of this research was the development of an aircoupled 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 aircoupled ultrasonic arrays. The aircoupled 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 aircoupled receiver. The signals measured at the points corresponding to the positions of the phasematched 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 signalprocessing procedure based on the application of the 2D Fourier transform in a spatialxe2x80x93temporal domain. The obtained results can be useful for the development of novel aircoupled ultrasonic nondestructive testing techniques.

Instantaneous Phase Coherence Imaging for NearField Defects by Ultrasonic Phased Array Inspection This paper describes an imaging method for nearfield defect detection in aluminum plates based on Greenxe2x80x99s function recovery and application of instantaneous phase coherence weighting factors. The directly acquired acoustic information of nearfield defects is usually obscured by the nonlinear effects due to the physical limitation of the acquisition system. Using the diffuse field to recover the Greenxe2x80x99s function can effectively retrieve the early time information. However, averaging operations of finite number in this process produces an imperfect imaging result. In order to improve the image quality, two kinds of instantaneous phased coherence weighting factors are used to weight the Greenxe2x80x99s function to reduce the background noise and improve the signaltonoise ratio: the instantaneous phase coherence factor (IPCF), and the instantaneous phase weighting factor (IPWF). Experiments are conducted on two aluminum plates with two and four nearfield defects, respectively. As a result, the background noise of amplitude images weighted by IPCF and IPWF is less than that of the conventional total focusing method (TFM). In addition, the IPCF image achieves a better signaltonoise ratio (SNR) than that of IPWF, and the phase discontinuity in an IPWF image is suppressed through the IPCF.