Abstract
We describe a small electromagnetic acoustic transducer (EMAT) that is different from traditional large EMATs. The maximum vertical magnetic flux density decreases quickly, and its distribution becomes very uneven with increasing lift-off distance of the permanent magnet. To increase the magnetic field strength of a small magnet without increasing the thickness of the magnet configuration, we propose a different permanent magnet configuration. With the same lift-off distance of the magnet, the increase in the maximum vertical magnetic flux density is about 20% when using this configuration. When this lift-off distance is 1.5 mm, the configuration increases the amplitude of the received signal by 71.4% for an aluminium specimen; in contrast, only a 29.0% increase is achieved with a low-carbon steel specimen. This is because, in the latter, the vertical magnetic flux density is less than 0.66 T. The interplay between the Lorentz force and magnetostrictive force leads to a decrease in the efficiency of both forces in generating ultrasonic waves.
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