Obliquely incident EMAT for high-order Lamb wave mode generation based on inclined static magnetic field

Abstract

Lamb waves have the characteristics of long propagation distance, and they can be used to realize rapid and wide-scale nondestructive testing of plates. The detection sensitivity can be improved by using a high-order Lamb wave mode with a short wavelength. However, Lamb waves have multimode characteristics. The number of Lamb wave modes greatly increases at high frequencies, and it is difficult to obtain a pure single high-order Lamb wave mode with electromagnetic acoustic transducers (EMATs); this increases the difficulty of signal processing and analysis. In this study, an obliquely incident EMAT based on the inclined static magnetic field was designed to solve this problem. The mode selectivity of the proposed obliquely incident EMAT is superior to that of the traditional EMAT. A new parameter is proposed to improve the mode selectivity of EMATs: the polarization angle of the permanent magnet. The incident angle of incident ultrasonic waves is controlled by selecting the polarization angle of permanent magnet. A rectangular permanent magnet is obliquely polarized to provide a biased inclined static magnetic field. The obliquely incident Lorentz force is formed through the coupling between the eddy current and the inclined static magnetic field by an obliquely polarized permanent magnet. The obliquely incident Lorentz force is the source of the obliquely incident ultrasonic wave. Simulation and experimental results show that the developed obliquely incident EMAT can reduce the amplitude of the unwanted modes and enhance the mode selectivity. It was also verified that the developed obliquely incident EMAT had better frequency-response characteristics and sound-field directivity.