Research On Optimal Design Technique Of Guided Wave Emats For Aluminum Plate Inspection

Aluminum plates are fundamental materials for national defense and industry, whose quality directly determines that of numerous downstream products. As a necessary quality-control method for aluminum plates, the piezoelectric ultrasonic detecting method is greatly confined in terms of its accuracy, efficiency and environmental adaptability due to its over-dependence on couplant. Electromagnetic ultrasonic guided wave (EUGW) technique, possessing many advantages such as high detection efficiency, strong environmental adaptability and being free of couplant, is of great significance for the improvement of aluminum plate inspection level. However, as the key component of EUGW technique, electromagnetic acoustic transducers'(EMATs) output signal is weak, and they are influenced by the multi-modes and dispersion effects of guided waves, which limit the further development of the technique. In order to improve the performance of EMATs, this paper extensively studies the optimal design technique of two kinds of guided wave EMATs, including surface wave EMATs and Lamb wave EMATs for aluminum plate inspections.This paper proposes a novel 3-D modeling method for meander-line-coil surface wave EMATs with special attention to the influence of dynamic magnetic field. The modeling method combines finite element method with an analytical calculation method which is based on"point force". A 3-D model is established based on the proposed method, and it covers all physical fields involved in EMAT transduction process, including eddy current field, magnetic field, Lorentz force field and sound field. Compared with previous EMAT models, the accuracy and the integrity of the established model are significantly improved, which serves as a good foundation for the study of the optimal design technique of EMATs. Experiment shows that the accuracy of the model is comparatively high, and the largest variation between the model calculation and the experimental value is about 4.3%.Aiming at enhancing the signal strength of electromagnetic surface waves, this paper proposes an optimal design method for surface wave EMATs based on 3-D simulation analysis and orthogonal test method. Through this method, the relationship between EMAT parameters and the signal strength is obtained, the optimal EMAT parameters are acquired, and the basic designing principle for surface wave EMATs is established. Experiment indicates that, after optimization, the surface wave amplitude has increased to 262%. Aiming at minimizing the influence of multi-modes and dispersion effect of Lamb waves, this paper puts forward a"two intersection points"method for decreasing the multi-modes influence and proposes a"zero-slope"principle for minimizing dispersion phenomenon by establishing electromagnetic Lamb wave excitation equations. Combining the"two intersection points"method with the"zero-slope"principle, the optimal operating frequency and spacing interval of coil are obtained. Moreover, with the purpose of enhancing the signal strength of electromagnetic Lamb waves, this paper proposes an optimal design method based on orthogonal test method, with which, the relationship between EMAT parameters and signal strength is obtained, the optimal values of other EMAT parameters are acquired, and the basic designing principle for Lamb wave EMATs is established. Experiment reveals that, after optimization, the multi-modes and dispersion phenomena have been obviously suppressed, and the amplitude of Lamb waves has increased to 216%.Finally, according to the background of the aluminum plate inspection, this paper develops an electromagnetic-ultrasonic-guided-wave experimental and testing device. This device chiefly consists of a high frequency and large power transmitting circuit, high gain and low noise receiving circuits, an 8-channel 12-bit high-speed data acquisition circuit based on USB, and a PC software based on LabWindows CVI. Experiment indicates that the device can effectively verify the validity of the theories proposed in this paper, and it possesses a preliminary defect detection ability, which establishes a good foundation for the further research on the aluminum plate automatic inspection technique based on electromagnetic ultrasounds.