| As a newly emerging technology of non-destructive testing(NDT), due to its many advantages such as no coupling medium, non-contact, adapting to the high- temperature and high-speed environment and easy to produce various types of ultrasonic, electromagnetic ultrasonic detection can achieve different detection intention. So it has a broad application prospect in various fields. And it is especially concerned by researchers on nondestructive testing and evaluation. Because of its advantages,make electromagnetic ultrasonic testing particularly suitable for detecting the thickness of liquid-core in solidification continuous casting billet. The paper aims at being conducive to the automatic online measurement of the liquid-core, the feasibility of quantitative detection using electromagnetic ultrasonic testing was verified by the modeling, simulation and experiment. The results can lay theoretical foundation for applying electromagnetic ultrasonic testing technology to related fields.The paper studied the EMAT based on the Lorentz force mechanism, using the combination of transverse magnetization magnet and track type coils which can extremely improve the excitation efficiency of longitudinal wave. Its working mechanism and relevant parameters optimization was deduced through the two dimensional modeling and finite element simulation analysis. From the perspective of point, line, and surface, the magnetic field, eddy current field, Lorentz force field and particle displacement field distribution were displayed. And the effects of different parameters on the performance of transducer were also compared. The typical values of EMAT design parameters were given through the simulation, which can be used as a guide for the design of the transducer subsequently.According to the characteristics of the solidification slab with liquid-core, the solid-liquid-solid model was abstracted. Both excitation transducer and receiving transducer were regarded as a point source. The propagation of ultrasonic was treated as the refraction and reflection of ray. Considering the actual working condition of the slab, the relation of transmitter installation position, ultrasonic incidence angles and receiver installation range is obtained by theoretical calculation and finite element simulation. And the characteristics of echo signal were extracted through the simulation of transmission on the vibration marks surface which was simulated by Bezier curve and different thickness of solid-liquid-solid model.In terms of echo signal processing, the paper realized the estimation of delay time which mirrored the thickness of liquid-core by using the algorithm of parameter estimation based on the envelope. Through the simulation and experiment, the estimation precision and computing time at different thickness were analyzed. The Gaussian echo model, wavelet denoise, Hilbert envelope extraction, the least squares estimate and gauss-newton iterative were adopted to realize the high estimation precision of time delay parameter. And the influence of the two cases that whether to denoise and whether to extract the envelope were compared. The results validate the feasibility of the signal processing algorithm.To verify the correctness of the theory and simulation model derived in the paper, on account of the higher requirements of excitation signal in electromagnetic ultrasonic testing as well, the pulsed exciting power supply based on the STM32, whose excitation frequency, pulse duty ratio and excitation cycle were all controllable, were designed for measuring the thickness of liquid-core in experiment. And the measurement data achieved higher accuracy, which directed verified the validity and availability of the theoretical model and algorithm. |
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