Multi-coil focused EMAT for characterisation of surface-breaking defects of arbitrary orientation

Electromagnetic Acoustic Transducers (EMATs) are a useful ultrasonic tool for non-destructive evaluation in harsh environments due to their non-contact capabilities, and their ability to operate through certain coatings. This work presents a new Rayleigh wave EMAT transducer design, employing geometric focusing to improve the signal strength and detection precision of surface breaking defects. The design is robust and versatile, and can be used at frequencies centered around 1 MHz. Two coils are used in transmission mode, which allows the usage of frequency-based measurement of the defect depth. Using a 2 MHz driving signal, a focused beam spot with a width of 1.3±0.25 mm and a focal depth of 3.7±0.25 mm is measured, allowing for defect length measurements with an accuracy of±0.4 mm and detection of defects as small as 0.5 mm depth and 1 mm length. A set of four coils held under one magnet is used to find defects at orientations offset from normal to the ultrasound beam propagation direction. This EMAT has a range which allows detection of defects which propagate at angles from 16° to 170° relative to the propagation direction over the range of 0-180°, and the setup has the potential to be able to detect defects propagating at all angles relative to the wave propagation direction if two coils are alternately employed as generation coils.