Development Of Shear Horizontal Mode Electromagnetic Acoustic Transducer For Metal Plate Detection

As a kind of ultrasonic guided wave detection technology,electromagnetic guided wave detection has the advantages of no coupling agent,high detection efficiency and strong environmental adaptability.As a key component of electromagnetic ultrasonic technology,electromagnetic acoustic transducers（EMATs）have been widely concerned and developed rapidly with the progress of related technologies.Compared with other modes,SH₀ mode guided wave has the advantages of non-dispersion,but it is difficult to excite single SH₀ mode.This subject takes metal plates（aluminum plates,steel plates）as the research object.Based on the Lorentz force mechanism and Magnetostrictive effect mechanism,a variety of types of electromagnetic acoustic transducers are developed in this paper.The performance tests of the transducers are carried out by experiments,including modal singularity,frequency response characteristics.Using COMSOL multiphysics simulation software to model EMAT and its key components.The influence of the structure parameters of the electromagnetic acoustic transducer on the performance of the transducer is studied,and the optimized design of the transducer is carried out according to the simulation results.The main research contents include:（1）Design of omnidirectional periodic permanent magnet electromagnetic acoustic transducer（OD-PPM EMAT）.Based on the Lorentz force mechanism,on the basis of the traditional periodic permanent magnet electromagnetic acoustic transducer,the fan-shaped magnet and spiral coil are used instead of the rectangular magnet and runway coil to realize the excitation of omnidirectional shear horizontal mode SH₀ wave in the plate structure.The simulation results show that the direction of Lorentz force and the single SH₀ mode are excited.It is verified by experiments that the OD-PPM EMAT can excite a single SH₀ mode in the aluminum plate,and the developed electromagnetic acoustic transducer has better frequency response characteristics.Through omnidirectional test experiment,it is verified that the developed OD-PPM EMAT can excite the SH₀mode propagating in 360-degree direction.（2）Design of directional shear horizontal wave electromagnetic acoustic transducer（D-SH-EMAT）with Electromagnet.Based on the magnetostrictive mechanism,this transducer is aimed at ferromagnetic steel plate.It uses an electromagnet instead of a permanent magnet to provide a bias static magnetic field.Not only overcome the shortcomings of the traditional permanent magnet EMAT adsorption of ferromagnetic materials,but also increase the number of adjustable parameters in the detection process.The simulation of the combined action of Lorentz force and magnetostrictive force verified the excitation of a single SH₀ mode,and the single SH₀ mode was experimentally proven.The frequency characteristics of the transducer and the sound field directivity experiments verify the good characteristics of the transducer.Finally,the relevant structural parameters of the transducer were optimized through experiments,and the degree of influence of each parameter on the signal received by the transducer was studied.（3）Design of an omnidirectional electromagnet type shear horizontal mode electromagnetic acoustic transducer（OSH-EMAT）.In the steel plate test,a sucker-type electromagnet is used to provide the required horizontal magnetic field.The horizontal magnetic field generated by the inner and outer diameter portions of the sucker-type electromagnet has a better effect than the horizontal magnetic field generated by a cylindrical permanent magnet.To reduce the problem of magnet leakage when the permanent magnet provides a horizontal magnetic field.Thus,the strength of the magnetostrictive force generated in the steel plate specimen is increased,and the amplitude of the SH₀ mode is increased.The single SH₀ mode of the transducer was proved by simulation and experiments,and the characteristics of the transducer were tested by experiments.Finally,the relevant structural parameters of the transducer are optimized through experiments.