| Aircraft dope can make aircraft work in execrable environment.However,the thickness of coating is not even during the preparation of the coating,which seriously affects the service life of the aircraft.In order to achieve a wide range detection of coating thickness,ultrasonic guided wave technology is applied to the coating thickness detection to improve the detection efficiency.Firstly,in this paper,the propagation theory of Lamb wave in the aluminum-polyurethane bi-layer structure,and the influence of the thickness of polyurethane coating on the group speed through theoretical equation is presented.Secondly,the dispersion curve of aluminum-polyurethane bi-layer structure is obtained based on vibration mode analysis.Based on the dispersion curve,we found the relationship between coating thickness and guided waves.The result of correlation analysis and error analysis show the interpolation of the linear behavior of the Lamb wave group velocity at specific frequency with regard to coating thickness.The influence of excitation frequency on the amplitude of Lamb wave is analyzed theoretically.Finally,the finite element simulation and experimental verification of electromagnetic ultrasonic Lamb wave are carried out.The arrival time of Lamb wave is extracted by the short-time Fourier transform(STFT)method.The simulation analysis of the change of material property and the position of the excitation source affects the detection of Lamb wave signals.The proposed method is verified by the ultrasonic guided wave experiments on the Aluminum plate with different thickness of polyurethane coating. |
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