Development of a mode-tuning magnetic-concentrator-type electromagnetic acoustic transducer.

In the traditional electromagnetic acoustic transducer (EMAT) based on Lorentz force mechanism, to meet the principle of constructive interference, the coil center distance is generally set to be half of the wavelength of the specified mode. The fixed center-to-center coil produces a Lorentz force under the action of a uniform static magnetic field provided by the magnet, thereby producing a specified mode signal that satisfies the constructive interference. In the above principle, the center distance of the coil is fixed, and applied with a uniform static magnetic field, which the coils with different center distances are combined with the dispersion curve to control the mode of the generated signal; that is, tuning the signal mode by changing the center distance of the coil. Another way to tune the signal mode is by changing the configuration of the magnet. Adopting appropriate waves for the identification of individual types of defects facilitates faster and more accurate detection. When using EMAT, some specifications of EMAT need to be changed, which can be inefficient and costly. To solve the problem, a mode-tuning magnetic-concentrator-type electromagnetic acoustic transducer (MT-MC-EMAT) is proposed in this study. This type of EMAT controls the mode of the generated signal by controlling the center distance of the static magnetic field provided by the magnet; that is, designing a new type of double-layer variable-pitch meander coil and different magnetic concentrators to select each coil. This method can tune the mode of the excitation signal by replacing the magnetic concentrator without changing a series of parameters, such as the coil, magnet, and excitation frequency. Different types of magnetic concentrators were added to a traditional EMAT to guide and concentrate the magnetic field of the permanent magnet, thereby changing the distribution of the magnetic flux density. These magnetic concentrators corresponded to meander coils with different pitches to satisfy constructive interference and achieve signal mode tuning. Both finite element simulation and experiment proved that the mode generated by this transducer was tunable after adding the different types of magnetic concentrators. Furthermore, experiments were conducted to examine the transducer characteristics. Finally, the configuration of the MT-MC-EMAT was optimized through orthogonal experiments. The influence of each parameter on the transducer efficiency of the proposed MT-MC-EMAT was studied, and the optimal parameter combination was confirmed.