An electromagnetic acoustic transducer for generating shear horizontal guided waves at two different wavelengths

Shear horizontal (SH) waves are efficiently generated with periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs) which consist of an array of alternating polarity magnets and a racetrack coil. The pitch of the PPM array defines the wavelength of the generated SH waves. Depending on the application, different wavelengths can be necessary, which consequently requires several transducers. Here, a new EMAT that generates SH waves at two wavelengths is presented. The new EMAT consists of a combined array of magnets from two elemental PPM arrays which are assembled in a single structure. A pair of the new dual-wavelength EMAT was fabricated at 24 mm and 12 mm nominal wavelengths and experimentally assessed in a 3 mm-thick aluminium plate. Both the SH0 and the SH1 guided wave modes were effectively generated with fair signal levels at either nominal wavelength. The SH0 is preferably generated at 24 mm since it presents higher amplitude and one operates at a single-mode frequency, whilst the SH1 mode, is preferred at 12 mm since it presents much lower dispersion and higher group speed. The new EMAT allows greater flexibility, since the most suitable wavelength for generating each mode can be obtained through frequency selection.