Development Of Electromagnetic Acoustic Transducers For Guided Wave Inspection Of Metal Plates

Electromagnetic ultrasonic technology is one type of the ultrasonic guided waves technology,which has many advantages such as free coupling agent,high efficiency and strong adaptability.As the key component of the electromagnetic ultrasonic technology,electromagnetic acoustic transducer attracted wide attention and got rapid development.Based on the Lorentz force mechanism and magnetostrictive effect mechanism,several types of EMAT have been proposed for the detection of metal plates in this dissertation.The performances of the transducer were verified by experiments,which include modal singularity,frequency response characteristic and acoustic field radiation characteristics of ultrasonic guided waves.The influence of the structure parameters on the performance of the developed transducers is studied through the simulation modeling of EMAT and its key components.The main research work includes:?1?The development of omni-directional A₀ mode electromagnetic acoustic transducer?OA-EMAT?.A new type of transducer structure has been developed with employing a concentric permanent magnet pairs instead of conventional cylindrical permanent magnet to improve the energy of ultrasonic guided waves excited by the transducer,and the performances of the transducer were tested experimentally.Two-dimensional finite element simulation model of the developed EMAT was established,and the correctness of the model is verified.Based on the finite element model,the geometric size of the concentric magnet pair and the distance between the permanent magnets and the aluminum plate were simulated and optimized.?2?The development of array shear horizontal mode magnetostrictive patch transducers?MPTs?.Based on the magnetostrictive effect mechanism,two kinds of MPTs which can both excite single SH₀ mode were developed by using planar solenoid coil array and flexible printed circuit,and the performances of the transducer were tested experimentally.In order to study the directive property of acoustic radiation field of the transducer,the bias magnetic field distributions were simulated and analyzed by simulation.The results showed that the directive property of acoustic radiation field is mainly determined by the structure of permanent magnet array and the coil array.?3?The development of directional shear horizontal mode electromagnetic acoustic transducer.Based on the Lorentz force mechanism,a new EMAT with a period grating coil was developed which can excite single SH₀ mode in metal plate,and the performances of the transducer were tested experimentally.In order to improve the energy of ultrasonic guided waves exited by the developed transducer,the permanent structure parameters of coil was simulated and optimized.When slightly larger than the effective size of the coil,the size of the permanent magnet is optimal.