A New Omni-directional Dual-mode Electromagnetic Acoustic Transducer for Ultrasonic Lamb Waves Inspection of Plate Structure

Abstract

In recent investigations, omni-directional transducers have been developed for ultrasonic Lamb waves (ULW) imaging of defects in plate structure. This paper proposes an omni-directional dual-mode electromagnetic acoustic transducer (OD-EMAT) for generating and receiving pure single A0 mode and S0 mode ULW respectively in aluminum plate. The developed EMAT consists of a permanent magnet and a double layer coils with a circular meander coil in the upper and a spiral coil in the lower. The coils were integrated into the flexible printed circuit (FPC). Employing the developed EMAT, two ultrasonic Lamb modes can be realized under the condition of the same excitation source position. The performance of the developed EMAT was studied through the experiments conducted in an aluminum plate of 1mm thick. Firstly, it was verified that the A0 mode and S0 mode ULW could be effectively generated and received independently with the developed EMAT. Secondly, the omni-directivity of the developed EMAT was verified due to the even radiation pattern of 360 degree. Thirdly, the frequency response characteristic of the developed EMAT was investigated. The results show that the new OD-EMAT has good frequency response characteristics and validate the quantitative relationship between the center frequency and geometric parameter of the coils. Finally, three types of defects including bonded defect, through-hole defect and corrosion defect were detected. It is also shown that the measurements which were obtained from defects detection experimental appeared to have superior accuracy.