Abstract
Ultrasonic non-destructive testing techniques (NDT) based on the application of guided waves are already used for inspection of plate-type structures made of various materials, including composite materials. Air-coupled ultrasonic techniques are used to test such structures by means of guided waves. The objective of this research was development and investigation of air-coupled excitation of a slow A0 Lamb wave mode in thin plastic films by a PMN-32%PT ultrasonic array. It is known that when the velocity of the A0 mode in the film is less than the ultrasound velocity in air no leaky wave is observed in a surrounding air. It opens new possibilities for NDT of composite structures. The influence of the airborne wave may be eliminated by 3D filtering in a wavenumbers-frequency domain. A special filter and corresponding signals processing technique were developed in order to obtain directivity patterns and velocity maps of the waves propagating in all directions. The measured ultrasound velocity values prove that, with the proposed method, it is possible to excite a slow A0 Lamb wave mode and to separate it from other parasitic waves propagating in air. Measurements of the parameters of the slow A0 mode, such as the propagation velocity in the plastic film, may be applied for the material characterization.
Highlights
Ultrasonic guided waves are already widely used for non-destructive evaluation, structural health monitoring, and material characterization
From the obtained ultrasound velocity graphs follows that the velocity of the A0 mode in all directions is approximately the same, which shows that the investigated polyvinyl chloride (PVC) film is isotropic
The performed investigation has shown that the slowA0 mode Lamb wave in thin plastic films may be efficiently excited by air-coupled PMN-32%PT ultrasonic arrays
Summary
Ultrasonic guided waves are already widely used for non-destructive evaluation, structural health monitoring, and material characterization They propagate in objects and structures in at least one dimension of which is comparable with the wavelength of the ultrasonic wave-pipes, rails, ropes, fuel tanks, and composite plates [1,2,3,4,5,6,7,8]. Application of hardware shielding usually does not enable to separate signals created by a slow A0 mode and a wave in the surrounding air For this purpose we propose to apply signal processing based on filtering in the frequency-wavenumber domains [20]. The objective of the performed research was the development and investigation of air-coupled excitation of a slow A0 Lamb wave mode in thin plastic films by a PMN-32%PT ultrasonic array.
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