Application of Air-Coupled Ultrasonic Arrays for Excitation of a Slow Antisymmetric Lamb Wave

Abstract

Air-coupled excitation and reception of ultrasonic guided waves is already used for non-destructive testing and evaluation (NDT & E). Usually for air-coupled NDT & E purposes the lowest zero-order antisymmetric Lamb wave mode A0 is used, because it is most sensitive to internal defects and thickness variations. The velocity of the A0 mode is reduced with a reducing frequency and at low frequencies may become slower than the ultrasound velocity in air. Such a wave is named a slow Lamb wave. The objective of this research was the development and investigation of an air-coupled excitation method of the slow zero-order antisymmetric Lamb wave based on application of a piezoceramic ultrasonic array. We have proposed to excite the A0 mode by a planar air-coupled phased array with rectangular elements. The array is matched to the wavelength of the A0 mode in the film. Performance of such an excitation method was investigated both theoretically and experimentally. Two excitation methods of the array were analysed: when all array elements were excited simultaneously or one by one with a proper delay. In order to reduce crosstalk between array elements via the air gap, we have proposed an optimization procedure based on additional shifts of electric excitation impulses of the array elements. For experimental verification of the proposed approach a prototype of the air-coupled eight element array made of Pz-29 piezoceramic strips was manufactured. Experimental validation confirmed the possibility of exciting the slow A0 Lamb wave mode through the air gap in thin plates and films.

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

“…The evanescent wave in the surrounding air was created by normal film displacements; therefore, knowledge of the spatial distributions of the normal displacements of the film surface was required. We have shown [35,36] that air also propagates a direct wave radiated by the transmitting array which affects the guided wave propagating in the film. As a result, these two waves interfere with each other in the PVC film and cause periodic oscillations of the amplitude of normal displacements in space.…”

“…To overcome this limitation, we used linear air-coupled ultrasonic arrays to excite the slow A 0 Lamb wave mode. This method has been analyzed in detail in our previous papers [35,36]. However, the air-coupled reception of such waves encounters significant problems because the lack of ultrasonic wave leakage into the surrounding air has not yet been solved.…”

“…To evaluate the efficiency of this approach, theoretical modelling was performed to obtain 2D spatial distributions of the normal displacements in the area corresponding to the geometry of the receiving air-coupled array. In our previous work devoted to the excitation but not the reception of the slow A 0 mode, only displacements along one line corresponding to the symmetry axis of the transmitting array were calculated [35]. However, real arrays possess finite dimensions because the spatial distributions in the plane of the analyzed film are not uniform, and the achievable efficiency of the air-coupled array is not clear.…”

“…The dimensions of the radiating aperture of a single array element were 7 × 1 mm. The coordinates of the centers of the exciting array elements were the same as those in [35]. The distance between the radiating air-coupled array and the PVC film was 1 mm.…”

“…The simulations were performed in the frequency range 10-40 kHz. To obtain an efficient excitation of the slow A 0 mode, the delay times of the excitation signals of the transmitting array were optimized according to our proposed algorithm [35].…”