| With the development and widespread application of coating technology,the detection and testing of manufacturing quality and service performance has gradually become the focus of researchers.Due to the influence of manufacturing stability and other factors,even the coating parameters of the same component have certain fluctuations,multiple parameters of the coating must be tested simultaneously to ensure the accuracy of the non-destructive characterization of the coating parameters.The existing nondestructive testing methods are often limited to the measurement of one single parameter,and it is difficult to realize the simultaneous characterization of multiple parameters of a surface coating components in actual engineering.In this research,a three-parameter inversion method based on the ultrasonic reflection coefficient amplitude spectrum(URCAS)and phase spectrum(URCPS)is proposed.The response of the amplitude spectrum and phase spectrum to the parameters is studied theoretically.The idea of constructing the inversion objective function based on sample correlation coefficient and acoustic impedance constraint is proposed.Finally,the validity of this method is verified by experimental tests on aluminum plates and coating specimens.The main contents are as follows:(1)The URCAS and URCPS are derived based on ultrasonic propagation in the model of layered medium,laying a theoretical foundation for characterization of medium parameters such as thickness,density and acoustic velocity.Then an inversion algorithm is established combining with correlation coefficient as the objective function.For the relief of ill-posedness of inverse problem,material-oriented regularization method is introduced,using acoustic impedance as constraint for objective function.(2)The mechanisms of absorbing attenuation in viscoelastic medium and scattering attenuation in inhomogeneous medium are analyzed.The attenuation in coatings consist of absorbing of epoxy matrix and Rayleigh scattering of second phase particles.The attenuation coefficients in coating specimens are measured by the proposed methods,fitting method and URCPS inversion method.Results provide validation for acoustic attenuation mechanism in coatings.(3)Calibration experiments are carried out on aluminum plates to obtain the precision and accuracy of measuring URCAS and URCPS.The accuracy of URCAS and URCPS increases respectively from 0.036 and 0.046 to 0.021 and 0.016 as main frequency rises from 5 MHz to 10 MHz.The feasibility of inversion is studied by analyzing the variation of URCAS and URCPS with respect to thickness,acoustic velocity,density and attenuation.(4)The sensitivity of URCAS and URCPS to parameters of aluminum plate and coating specimens and inversion errors are calculated.As the frequency increasing,the sensitivities increase while the errors decrease.The extremum increases with the frequency monotonously when the attenuation of medium is negligible.When the attenuation is sufficiently large,the extremum increases to maximum and drops to zero.URCAS and URCPS are obtained from time domain signals acquired on aluminum plates,then thickness,acoustic velocity,density are inversely calculated by the proposed algorithm.The errors of URCAS inversion at 10 MHz are smaller than those at 7.5 MHz and 5 MHz,while the errors of URCPS inversion change slightly with frequency.The probe with main frequency of 7.5 MHz is selected according to the sensitivity analysis results for coating specimens testing.The errors of thickness,acoustic velocity and density inversion results are less than 5.58%. |
