Comparison of the efficiency of magnet arrangements in polarised, normal incidence SH wave EMATs on aluminium and steel samples

Electromagnetic acoustic transducers (EMATs) are non-contact ultrasonic transducers that are capable of generating a wide variety of ultrasonic wave modes in electrically conductive or magnetic materials. This article documents the modelling, construction and comparison of four EMATs designed for generating normal incidence, linearly polarised shear horizontal (SH) waves. In all four designs, a butterfly coil is used to produce a region in which the eddy current in the sample surface is linear. The difference in the four designs is the configuration of the permanent magnet arrangement, which should provide a strong magnetic field predominantly oriented normal to the sample surface over the region where the coil is linear. This combination of coil and permanent magnets generates an SH wave with its polarisation oriented normal to both the permanent field and the driving current via the Lorentz force. Two such coils are stacked with one rotated by 90 degrees so that both polarisations can be generated and detected individually, which is useful for ultrasonically measuring stress or crystallographic texture in metal components via acoustic birefringence. Via finite element modelling of the static field and experimental measurements of ultrasonic signals in pulse-echo mode, it is found that the magnet arrangements using a high permeability steel core provide a ∼ 70 % larger signal than a simple, single magnet with its magnetisation pointed towards the sample surface, when operating on a mild steel sample. For aluminium samples, the performance of the arrangements with laminated steel cores is shown to be approximately the same as the single magnet used in the measurements. • Butterfly coil EMATs generate linearly polarised, normal incidence shear waves. • Stacked butterfly coils allow orthogonal polarisations without probe rotation. • Four magnet arrangements tested and compared on steel and aluminium samples. • High-permeability core designs outperform single magnets on steel samples. • High-permeability core designs match single magnet performance on aluminium samples.