Measurement of Plate Thickness Based on the Optimal Unidirectional Generation of Ultrasonic Waves Using Dual Electromagnetic Acoustic Transducer

Abstract

Unidirectional generation of shear horizontal (SH) ultrasonic guided waves can be achieved using dual-coil electromagnetic acoustic transducers (EMATs), through the interference mechanism when appropriately phased signals are used to drive each EMAT coil. Optimal unidirectional generation requires precise knowledge of the intended wave mode dispersion curve, which in turn, depends on the plate’s thickness. In this paper, we exploit optimal unidirectional generation with a dual-EMAT in order to measure plate thickness. The proposed thickness measurement method uses a dual periodic permanent magnet (PPM) EMAT, acting as transmitter, and two conventional PPM EMAT receivers on opposite sides of the transmitter, in order to receive the forward (enhanced) and backward (weakened) generated SH ultrasonic guided waves. Then, two consecutive frequency sweeps are used to generate the SH0 and SH1 modes, in order to seek the maximum ratio of the forward travelling wave amplitude to the backward travelling wave amplitude. The first sweep determines the optimum operating frequency for the first wave mode, which is a necessary parameter for the next sweep. The second frequency sweep determines the plate thickness. The new method was initially evaluated with a frequency-wavenumber-domain analytical model. The proposed method showed a much higher frequency-specificity, which was assessed by a quality factor figure of merit defined by the ratio of the centre frequency to the -3 dB frequency bandwidth, when compared to that obtained using the conventional method that is based solely on the amplitude of the generated wave, and is therefore less susceptible to error in the thickness estimation. The method was experimentally evaluated with a dual-PPM EMAT on aluminium plates, showing good agreement with simulations and providing more than ten-fold higher specificity than the conventional method, and accurate thickness measurements, with an error lower than 0.8%.