| With the metal plate in the production and long-term use of the process, there is a small amount of defects, such as cracks, delamination, scratch, etc. At the same time external loading and environmental impact will also cause the further expansion of small defects inside the plate, which may lead to destructive accidents, so it is very important to carry out regular testing of the metal plate. Electromagnetic ultrasonic testing technique has the advantages of non contact, non required the presence of the coupling agent, no need to treat the test surface with a complicated and tedious pretreatment, which can be used for large area, quick and comprehensive assessment of the health status of sheet metal, with great economic and academic significance. Phase control technique is applied in ultrasonic field, which can overcome the shortcomings of energy dispersion and small signal amplitude, and improve the ability to detect tiny defects. It is one of the new direction of the future development of detection technology.In this paper, the propagation characteristics of ultrasonic wave in metal plate are studied, and the multi modal characteristics and dispersion characteristics are analyzed. The basic principle and key technology of phase controlled ultrasound were studied, and the time delay of phase control focusing and deflection was calculated. The energy transfer mechanism of the electromagnetic ultrasonic transducer is studied theoretically, and the structure of the Lamb wave EMAT and SH wave EMAT is analyzed by the combined action of the static bias magnetic field and the dynamic induction magnetic field. And the traditional back into the fold on the basis of coil type change lines with a certain radian, designs a new type of bends back to the fold type EMAT. And the effect of curved line degree on ultrasonic amplitude and range of detection was studied. Combined with phase control technology, electromagnetic ultrasonic testing method of metal plate is studied. Dual array excitation method is used, The two excitation array element and the receiving transducer are arranged in a straight line and the acoustic path difference of the two excitation array to the receiving transducer is made up by time delay to achieve Interference enhancement of acoustic wave. The longitudinal placed three array element electromagnetic ultrasonic transducer array excitation, through precise time delay to each transducer excitation of ultrasonic beam can also phase of the arrival time at the receiving end, focused ultrasound, echo signal amplitude is increased, and experimental validation of micro defects, improve the detection resolution.Discrete exciting and receiving structure is used to carry out the experimental verification in Q235 steel plate. The results show that, compared with the straight line EMAT, the bending line EMAT has a wide range of detection range, but with the increase of the bending degree of the line, the amplitude of the echo signal is gradually reduced, and the energy efficiency of EMAT is gradually reduced. The dual excitation method is used for ultrasonic wave superposition, and the signal amplitude is 2 times of the single transducer when the receiver is in the same phase, and the echo amplitude is 0. Three array element excitation transducer array is used in steel plate detection The three element array excitation transducer array plate detection, when the excitation voltage is equal, at 658 mm distance from transducer array, the amplitude of the receiving echo signal is increased to 2 times of the single central array excitation, which can improve the amplitude of echo signal. Ultrasonic focusing of defect can be realized, the amplitude of the defect signal in 520 mm is increased to 1.83 times of the single central array excitation, which can improve the detection ability of small defects. |
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