An Omnidirectional EMAT Employing Spiral Coil and Annular Magnets for Enhanced Generation of A0 Mode Lamb Wave

As a typical noncontact ultrasonic testing method, electromagnetic ultrasonic guided wave testing has received extensive attention in recent years. However, the low energy efficiency and signal-to-noise ratio is especially obvious in omnidirectional electromagnetic acoustic transducers. To solve this problem for noncontact guided wave positioning detction, this article proposes an omnidirectional electromagnetic acoustic transducer (EMAT) for enhanced generation of A0 mode ultrasonic Lamb waves with pure mode in non-ferromagnetic plates. The proposed EMAT consists of a spiral coil and a group of annular magnets. The annular magnets build a special kind of periodic permanent magnets (PPM). The radius difference between the adjacent annular magnets is set to half the wavelength of the desire wave mode to maximize the excitation efficiency. In 2D axisymmetric FE simulations, the distribution of the bias magnetic fields and eddy currents was analyzed, and the mode of the generated guided waves was determined. In experiments, the omnidirectional EMATs were fabricated to verify the performance of generating omnidirectional A0 mode Lamb waves. The results prove the performance of the omnidirectional EMATs for effective generation of pure antisymmetric mode Lamb waves at specific frequency.