EMAT phased array probe for detecting surface cracks

Abstract

Electromagnetic-acoustic transducers (EMATs) consist of a magnet and a coil. They are desirable for non-destructive evaluation (NDE) because no direct contact with the specimen is required. However, most EMATs use excitation peak powers greater than 1 kW and hence the driving electronics and the EMAT coils have to be bulky. This has hindered the development of EMAT phased arrays that have functionality and dimensions similar to those of conventional piezoelectric phased arrays, which are widely used in NDE because they offer superior defect characterization in comparison with single-element transducers. In this paper, an EMAT phased array that performs similarly to conventional piezoelectric arrays, uses excitation peak powers not greater than 4.8 W (24 Vpp and 200mA) and have racetrack coils as narrow as 3 mm is reported; this is possible due to the utilisation of coded excitation. The racetrack coils are laid out overlapping 1/3 of their area in their shortest dimension to reduce the crosstalk between the coils to less than −15 dB. An 8-element prototype that operates at a central frequency of 1 MHz is shown to detect defects which have a cross-section area of 0.2×0.8mm2 and are located on the surface opposite to the array.