| Electromagnetic Acoustic Transducer (EMAT) is a global leading technology in the field of non-destructive testing (NDT). The features of EMAT, such as high accuracy, contactlessness, need-no-couplant, make them particularly suitable for the non-destructive inspection under high temperature, moving objects and phased-array-controlled operation.They also can easily generate ultrasonic waves at different modes. All these merits lead EMAT a broad application prospects, and they have attracted more and more attention from researchers. EMAT technique is emphatically researched in this thesis.Firstly, based on the review of EMAT technology development history and its application actuality, this thesis discusses the basic theory of electro-acoustic transduction phenomenon of an EMAT. EMAT consist of magnet, coils and the tested specimen. Mathematical methods are used to analyze field-effect between the three parts, and simulations of magnets and coils are done by finite element methods. All of these laid theoretical basis for design of electromagnetic.The structure and circuit model of EMAT test system is designed based on the theoretical study and model simulation. The snake-liked coils and U-AC electromagnetic were selected to constitute the EMAT. Based on the MAX038 chip, the burst-type pulse bunch signal generator and the power amplifier system are designed. Receiving circuit is designed by the diodes limiter circuit, differential amplifiers and frequency-selective amplification circuit. The received signal is displayed on the TDS5034B oscilloscope.Experimental that detect the surface crack of tested-steel is conducted by EMA test system. The received signal is filtrated by average algorithm of oscilloscope. For different excitation frequency, magnet-excitation voltage and the lift-off distance, the test results were analyzed to study the effects of various parameters on the impact of testing. The preceding theoretical analysis and simulation results are verified by the experimental. |
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