An EMAT for improving the purity and amplitude of S0 Lamb waves

Abstract

Abstract Different waves of the Lamb waves may be superimposed in practical tests due to their multimodal characteristics and dispersion characteristics, thus significantly increasing the difficulty of later signal analysis and creating a massive limitation to their application in non-destructive testing. Only a single magnet was used to generate a static magnetic field in the traditional S0 mode Lamb wave (S0 wave) electromagnetic acoustic transducer (EMAT), which resulted in a less pure S0 wave. Additionally, the accompanying A0 mode Lamb wave (A0 wave) will decrease the accuracy of the results in defect detection and increase the difficulty of the signal processing. To obtain a single-mode S0 wave, a periodic magnet structure EMAT structure was proposed in this study. Finite element models were established, and the effect of single and periodic magnet structure on the static magnetic field was analyzed. The periodic magnet structure EMAT can substantially enhance the S0 mode Lamb wave amplitude by increasing the flux density of the local magnetic field using periodically arranged magnets. However, it also causes enhancement to the A0 wave and decreases the detection accuracy. An improved EMAT was further proposed to increase the S0 wave amplitude with no enhancement to the A0 wave amplitude. Experiments were conducted, and two types of EMAT were comparatively investigated, which agreed with the simulated results. The S0 wave amplitude can be increased by seven times with the improved EMAT compared to traditional EMAT, and the A0 wave is completely submerged in the noise. The improved EMAT proposed in this study can provide guidance for single-mode S0 wave defect detection.