Unidirectional emission and detection of Lamb waves based on a powerful and compact coils-only EMAT

• Published in: NDT & E international : independent nondestructive testing and evaluation Vol. 122; p. 102492
• Main Authors: Rieger, Kai, Erni, Daniel, Rueter, Dirk
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.09.2021
• Subjects: EMAT; Lamb wave; Particularly compact; Phased array; Ultrasonic; Unidirectional; Lamb wave; MFL; Unidirectional; RF; ECT; Phased array; RMS; EMAT; Ultrasonic; Particularly compact; NDT
• ISSN: 0963-8695; 1879-1174
• DOI: 10.1016/j.ndteint.2021.102492

This study investigated a compact noncontact electromagnetic acoustic transducer (EMAT) without bulky magnets or separate electromagnets for unidirectional transmission and reception of ultrasonic Lamb waves in 1 mm thick steel plates. The transducer head consists of only two, closely adjacent and elongated flat coils. The powerful sensor technology bridged air gaps up to 1.5 mm. Two phase-adjusted 1 MHz pulses at 1 kV amplitude enhanced the sideways-projected ultrasound field in one direction; others were attenuated by destructive interference. A 5 mm ultrasound wavelength with 1 mm thick steel plate was used for testing; wave field calculations were in good agreement with results. As a possible application, an inaccessible edge of a steel plate was scanned, where the difficulty was increased by an additional edge at a similar distance representing competing interfering reflections. More general progress beyond this specific non-contact application is the demonstration of phase-accurate ultrasound transmission and sensitive echo reception with a compact system of mutually coupling inductors at high power and continuous high-bias currents (1 kA), allowing compact and powerful EMAT phased arrays based on single-layer densely arranged coils.