Analytical modelling and analysis of the meander-line coil EMATs.

The meander-line coil electromagnetic acoustic transducer (EMAT) is widely used in the field of ultrasonic nondestructive testing due to its convenience to generate specific mode of guided waves. Some design methods of the meander-coil EMATs are developed in the frequency-wavenumber domain while others in the time-space domain. In this paper, a unified theoretical framework is developed by proposing an analytical model from the system perspective. Signal transfers between different physical fields in EMAT excitation, wave propagation and EMAT reception are represented as linear time-space-invariant systems. Taking the Rayleigh wave EMAT detection as an example, the analytical model for the transfer functions of these systems is established. The analytical model is experimentally verified by different Rayleigh wave detection techniques: the conventional EMAT, the spatial pulse compression (SPC) EMAT, temporal-spatial pulse compression (TSPC) EMAT and detection cases employing the same receiving EMAT. From the system perspective, the received signal of EMAT is interpreted as the response of the filter system to the input signal. It is found that the meander-coil EMAT can be regarded as the frequency domain expression during the detection. And the frequency domain expression plays different roles in different techniques.