The Analysis of Adhesive Bonds Using Electromagnetic Acoustic Transducers

Abstract

EMATs have been used in ultrasonic based research for over 20 years [1,2]. Improvements in permanent magnet technology and the required supporting electronic circuitry have made EMATs a realistic alternative to piezoelectric transducers in many applications. EMATs are limited to working on electrical conductors and are roughly three orders of magnitude less sensitive than PZT, so generally there has to be a special reason to use an EMAT in preference to a PZT. The scanning of an adhesively bonded plate is one of these areas where the EMAT should offer an improvement over tests done with conventional PZT transducers. The EMAT is a broadband device (5MHz bandwidth centred on 5MHz), giving it high spatial resolution in the direction of propagation, and is also a totally non-contact device, so no acoustic couplant is required. The most attractive feature of the EMAT used here in this work is that it generates radially polarised SH waves which should be more sensitive than longitudinal waves at normal incidence to a bonded interface. The design and construction of the EMAT will first be discussed and this will be followed by the presentation of various results obtained using the EMAT systems. EMATs have been used to probe bond integrity in the past [3,4], but have to the best of the authors knowledge not been used in a broadband, true pulse-echo mode.