| Organic coating is an excellent anti-corrosion material,which is widely used in automobile,shipbuilding and other manufacturing industries.The thickness and uniformity of the coating determine its performance and life,so it is necessary to find a perfect thickness nondestructive testing technology.Compared with other common nondestructive testing methods,terahertz nondestructive testing technology is more excellent in terms of safety,high efficiency,anti-interference and non-contact.In the detection process,factors such as thin coating thickness and low refractive index will cause the accuracy of thickness detection to decrease.Therefore,this paper establishes a simulation model of terahertz propagation in the coating based on studying the optical parameters of materials.Combined with the thickness optimization method of the correlation coefficient,the simulation of the terahertz transmission characteristics and the high-precision thickness detection of the film coating are realized.The main tasks are as follows:(1)Optimal extraction of the optical parameters of the substrate and coating materials.First,analyze the terahertz spectrum characteristics of five matrix materials: high-resistance silicon,high polymer polypropylene,high-density polyethylene,polytetrafluoroethylene,and polymethyl methacrylate.The optical parameters are extracted based on the simplified optical parameter model,taking into account materials' absorption,signal-to-noise ratio and effective frequency range in the terahertz band,6mm high-density polyethylene was finally selected as the coating substrate for the follow-up study;Then,four common coating materials were selected: colored coating,anti-corrosion coating,thermal insulation coating and wear-resistant coating.The simplex method is used to optimize the optical parameter model,and the spectral characteristics and optical parameters of the coating are explored.Finally,considering the dispersion phenomenon of terahertz waves in the coating,the Debye model and particle swarm optimization algorithm are used to fit and optimize the optical parameters of the coating.The results show that the variance of the optical parameters after fitting is smaller than that before fitting.After eliminating the interference of abnormal data,the root mean square error Between 0.0062 and 0.2444.(2)The simulation method of terahertz propagation in the coating based on the optical transmission matrix is studied.Based on the optical transmission matrix,a simulation model for the propagation of terahertz waves in the coating was established,and the fitted optical parameters of the coating were used for reflection simulation of 165?m colored coating,85?m colored coating,135?m anti-corrosion coating,and 60?m anti-corrosion coating samples.The results showed that the root mean square error was between 0.1063 and 0.16,and the correlation coefficient was between 0.945 and 0.9766.(3)The optimized detection method of coating thickness based on correlation coefficient is studied.Use Wiener deconvolution to reduce the pulse width of the terahertz wave to obtain higher resolution,and then propose a correlation coefficient optimization method to detect the thickness of the coating samples,compare it with the value of eddy current thickness measurement.The results show that the relative error of the colored coating sample is between 1.05?5.97%,the relative error of the anti-corrosion coating sample is between0.66?5.90%,and the relative error of the multilayer film coating sample is between0.87?8.05%.The thickness uniformity of the coating samples is visually analyzed,the thickness values of each interval conform to the normal distribution,and the dispersion coefficient is between 0.0325 and 0.0442.(4)A platform for terahertz simulation and thickness testing of film coatings has been established.The platform is divided into two modules: coating transmission /reflection simulation analysis and coating reflection thickness detection and imaging,which solves the problems of large amount of waveform data and cumbersome operation steps in the terahertz simulation and thickness detection of the coating.Through this platform,the transmission characteristics of the terahertz wave in the film coating can be easily simulated and analyzed,and the thickness of the film coating sample can be optimized and detected efficiently.Studies have shown that through the simulation of the terahertz transmission characteristics and thickness detection technology of film coatings,the simulation of the terahertz waveform and high-precision thickness nondestructive testing of the film coating with a small refractive index and a thin thickness can be completed,and get the corresponding quantitative analysis results.It has important research value and broad application prospects. |
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