Terahertz Wave Propagation In Marine Protective Coatings And Nondestructive Testing Signal Analysis

Terahertz(THz)nondestructive testing technology provides the capability of non-contact inspection,high precision,and good penetration for non-polar materials.It has become a powerful tool for biomedical,communications and national defense applications etc.However,the research on terahertz technique testing for marine organic protective coatings is just getting started.Currently,there are no systematic studies on terahertz wave propagation in protective coatings and on the mechanism of terahertz nondestructive detection.In addition,the traditional analysis method for terahertz signal of multilayer structure has the problem with poor precision in structure analysis.It has low resolution for thinner coating thickness in early spray stage and has low reliability on detection of hidden defects in coatings,which cannot meet the requirement of industrial applications.Thus this dissertation is focused on the propagation mechanisms of terahertz wave in multilayer marine protective coatings and signal analysis of THz nondestructive testing.The main work and conclusions are follows:A theoretical model based on the finite difference time domain method was established to characterise the propagation of terahertz radiation in multilayer marine protective coatings.The simulation researches were carried out on terahertz pulse detection of both intact and defective coating system below the ship waterline.Subsquently,the quantitative analysis of each paint layer was done to calculate the thickness,as well as the diagnostic analysis for the structures with defects beneath the coating,such as blister defect,detachment defect and oxide corrosion etc.Furthermore,experimental analysis was also carried out on the home-made pulsed terahertz imaging system in reflective mode.Various coating samples with different multilayer paint layers were fabricated to study on the pulsed terahertz detection of intact and defective coating structure.The results showed that the simulated analysis based on the proposed theoretical model could agree well with the experimental results,which indicated that the finite difference time domain model could provide theoretical basis for terahertz nondestructive testing and evaluation of marine protective coatings.During the traditional Fourier deconvolution processing,a bandpass filter is typically incorporated into the inverse filtering to suppress the noise,which would result in over-smoothing and lead to poor detection precision.The stationary wavelet decomposition was proposed to apply on the terahertz detected signal to obtain the detail coefficients which were used for coating structure analysis.When the proposed method was applied on the processing of both simulated and experimental signal,the paint layer thickness was calculated and then compared with the real thickness values.The relative errors were less than 3%,which is smaller than the results obtained by Fourier deconvolution processing.It greatly improved the thickness detection accuracy,and would provide a method for terahertz detection of multilayer marine coatings.The detectable minimum depth is constrained to be half the coherent length of the terahertz pulse.A novel approach based on a neural network-based hybrid signal procession approach was proposed to enhance the capability of resolving thinner layer coating using terahertz pulse imaging system.When compared to the results obtained from multiple regression analysis,the proposed method provided a more accurate estimate for either the antifouling paint layer or anticorrosive paint layer,which is important for in-line painting application.For the problem that it is difficult to locate exactly of the early defect inside the coating system by the faint feature due to the interference of background noise,the method using Hilbert transform combined with wavelet transform modulus maximum on terahertz signal was proposed.The noise could be separated out of the signal and the weak feature caused by early defects would be enhanced and identified according to the relationship between wavelet transform modulus maximum and signal singularity.Combined with the reflection peaks strength,position of defect,and the causes of coating defects,it would help to distinguish different types of defects in the coating.It could be used to evaluate the occurrence of defects beneath the coatings and would benefit the effective maintenance to avoid coating failure.The analysis and conclusions in this dissertation was of great significance for in-line painting monitoring application and paint performance estimation during coatings service lifetime.It could provide some theoretical basis for terahertz nondestructive testing and evaluation of marine protective coating from experimental stage to practical stage.