Different waves of the Lamb waves may be superimposed in practical tests due to its multimodal characteristics and dispersion characteristics, thus greatly increasing the difficulty of later signal analysis and creating a huge limitation to their application in non-destructive testing (NDT). 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. In order 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 7 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 the single-mode S0 wave defect detection.
The poor conversion efficiency and obvious lift-off effect of the electromagnetic acoustic transducer (EMAT) are commonly known to be problems for its practical application. For the purpose of enhancing the performance of EMATs, numerical simulations were performed in order to analyze the effect of various parameters. The results indicate that only the magnet-to-coil distance can effectively enhance the conversion efficiency and weaken the lift-off effect at the same time. When the magnet-to-coil distance is 2 mm, the lift-off effect will continue to be weakened as the magnet-to-coil distance increases, whereas the decrease of the lift-off effect is inconspicuous and the conversion efficiency starts to decline at this time. Therefore, to get the best performance of this specific EMAT, the suitable magnet-to-coil distance is 2 mm. The experiment effectively verifies the improvement of EMATs with a magnet-to-coil distance of 2 mm.
This paper provided a method to improve the conversion efficiency and lessen the lift-off effect of spiral coil electromagnetic acoustic transducer (EMAT) for some specific situations, for example, irregular sample surface, online inspection and varying coating. Moreover, a new evaluation method for the performance of EMAT is proposed. To study the effect of the structural parameters on the performance of EMAT, the orthogonal test method with five factors and four levels is employed. Consequently, the degree of five factors on the lift-off performance and the theoretical maximum value of amplitude are obtained. The results show almost all factors have an obvious impact on the theoretical maximum value of amplitude, while only the magnet-to-coil distance can affect the lift-off performance effectively. The experiment results well agree with the numerical results, in which the improvement of the lift-off performance and the theoretical maximum value of amplitude is 31.94% and 78.79% respectively. In addition, the conversion efficiency of EMAT is increased by 125% when the lift-off is 1 mm. These imply the performance of spiral coil electromagnetic acoustic transducer is improved dramatically and the detectable lift-off is increased as well.
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