Attenuation of a Slow Subsonic A0 Mode Ultrasonic Guided Wave in Thin Plastic Films

Abstract

The ultrasonic testing technique using Lamb waves is widely used for the non-destructive testing and evaluation of various structures. For air-coupled excitation and the reception of A0 mode Lamb waves, leaky guided waves are usually exploited. However, at low frequencies (<100 kHz), the velocity of this mode in plastic and composite materials can become slower than the ultrasound velocity in air, and its propagation in films is accompanied only by an evanescent wave in air. To date, the information about the attenuation of the slow A0 mode is very contradictory. Therefore, the objective of this investigation was the measurement of the attenuation of the slow A0 mode in thin plastic films. The measurement of the attenuation of normal displacements of the film caused by a propagating slow A0 mode is discussed. The normal displacements of the film at different distances from the source were measured by a laser interferometer. In order to reduce diffraction errors, the measurement method based on the excitation of cylindrical but not plane waves was proposed. The slow A0 mode was excited in the polyvinylchloride film by a dry contact type ultrasonic transducer made of high-efficiency PMN-32%PT strip-like piezoelectric crystal. It was found that that the attenuation of the slow A0 mode in PVC film at the frequency of 44 kHz is 2 dB/cm. The obtained results can be useful for the development of quality control methods for plastic films.

Development of Ultrasonic Techniques for Measurement of Spatially Non-Uniform Elastic Properties of Thin Plates by Means of a Guided Sub-Sonic A0 Mode

“…In this sample the sub-sonic A0 mode was excited and the spatial distributions of the of-plane displacements of the mica paper were measured by the laser interferometer using the experimental setup presented in Figure 19. The experimental setup was the same as used for investigation of PVC samples [29]. From the presented spectrum it follows that it is possible to identify a few A 0 modes propagating with different velocities in the case of multiple circular non-uniformities as well.…”

“…In this sample the sub-sonic A 0 mode was excited and the spatial distributions of the of-plane displacements of the mica paper were measured by the laser interferometer using the experimental setup presented in Figure 19. The experimental setup was the same as used for investigation of PVC samples [29]. For determination of elastic properties of the mica paper a slow sub-sonic A0 mode guided wave is excited by a contact or by an air-coupled PMN-32%PT ultrasonic transducer (Figure 19).…”

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

“…At lower frequencies at which the sub-sonic A 0 mode may propagate this attenuation is lower and in the case of PVC at 44 kHz is 2 dB/cm. [38]. Hence, one advantage of the sub-sonic A 0 mode is the lower attenuation.…”

“…During the manufacturing of PVC films, various defects such as wrinkles, holes, a rough surface and thickness variations arise. Some of these are detected by optical methods, but such defects as holes and, especially, thickness variations, could be found by ultrasonic methods using the discussed A 0 mode guided waves [38,39]. Taking into account that the velocity of the A 0 mode can be measured with a high precision such a method can be used for measurements of thickness of moving plastic ribbons.…”

Comparison of Ultrasonic Non-Contact Air-Coupled Techniques for Characterization of Impact-Type Defects in Pultruded GFRP Composites

“…However, the frequency dependence of attenuation losses of guided waves is much more complex and depends not only on the radiation of leaky waves, but also on guided wave modes. For example, it was shown that, in the case of the slow subsonic antisymmetric A0 Lamb wave mode, when a leaky wave is not radiated, the attenuation coefficient can still reach 2 dB/cm even at low 44 kHz frequency [ 20 ].…”