基于空间域谐波控制的电磁超声Lamb波模态抑制方法

Lamb waves excited by electromagnetic acoustic transducers (EMAT) often have multiple modes, which make waveform analysis difficult and even interfere with the judgment of defect information, leading to misjudgment. Harmonics of conventional EMAT coils in the space wavenumber spectrum are the important cause of the multi-mode phenomena. This paper proposes that the eddy current density distribution (or the Lorentz force distribution) in the specimen is equivalent to the spatial low-pass filtering of the coil current density distribution. By effectively designing the coil, the harmonics in the space domain can be suppressed, and the sinusoidal (that is, single wavenumber) eddy current density distribution can be excited. Combining the characteristics of sinusoidal pulse width modulation (SPWM) technology, it was proposed to apply the time domain SPWM technology to the space domain to design the coil parameters. A finite element simulation model was established, and the simulation proved that the sinusoidal eddy current density distribution can be excited. The experiment was conducted by taking the S0 mode Lamb wave as an example. It is shown that the A0 mode excited by the third harmonic in the wavenumber spectrum is greatly suppressed, and the suppression ratio is 21dB.