Research On Characterization Method Of Coating Curing State Based On Bispectrum Analysis

Coating bonding technology is widely used in aerospace applications,defence and new energy sectors,because of its thermal,antioxidant and corrosion resistance characteristics.In the production process,the bonding performance between the coating and the matrix material directly affects the product performance,and the adhesive performance is closely related to the curing degree of the coating,so it is very important to monitor the curing state of the coating on-line.The nonlinear Lamb wave detection technology is characterised by its long propagation distance,low attenuation and suitability for large sheet areas and bonded structures,enabling it to meet the technical requirements and enable real-time detection of the cured state of the coating.This paper we take the steel/coating double-layer composite bonding structure as the object,and study the non-linear Lamb wave-based coating curing state real-time monitoring method and state characterization technology.The main achievements of the thesis include the following:(1)The propagation of nonlinear Lamb waves in steel/coating double-layer composite bonded structures was investigated experimentally based on the non-linear properties of Lamb waves during the curing process of the coating.By exploring the nonlinear fluctuation equation,the physical meaning of the nonlinear coefficients and related parameters are analysed,and a nonlinear ultrasonic monitoring method using relative third-order nonlinear coefficients to characterise different curing states of coatings is proposed,providing theoretical guidance for the design of nonlinear Lamb wave monitoring systems.the changes in coating thickness,sound attenuation coefficient and sound velocity during the whole curing process are analysed.(2)Non-linear Lamb wave monitoring system has been built to monitor the curing process of the coating in real time,to cope with the easy flow characteristics of the coating.Using the nonlinear effect of ultrasonic lamb waves during material propagation process,ultrasonic tests were conducted on the curing process of coatings in steel/coating double-layer composite bonding structures.The changes of coating thickness,acoustic attenuation coefficient and sound velocity throughout the curing process are analysed,which paved the way for the subsequent characterisation of the curing state of the coatings using relative thirdorder non-linear coefficients.(3)In order to improve the accuracy of nonlinear Lamb wave NDT and to excitation of high-energy,nonlinear harmonic signals,a response surface model was developed to optimize the parameters of the detection system,in order to optimize the parameters of the nonlinear ultrasonic detection system.(4)To address the problem of weak and difficult extraction of non-linear harmonics,the diagonal slice spectrum extraction method based on bispectral analysis and a solid state characterization method with relative third-order non-linear coefficients are proposed.The non-linear harmonic spectra and modulated waves are extracted using the direct method of bispectral estimation and qualitative characterization of the curing state of the coating,using diagonal slice spectra and calculations to extract the amplitude of the non-linear harmonics.The relationship between fundamental,third harmonic and thickness is established by the proposed relative third-order non-linear coefficients to characterise the curing process of the coating.The results show that the nonlinear Lamb wave detection technique is feasible and effective in detecting the curing state of coatings in real time,and that the relative third-order non-linear coefficients characterising the curing state of coatings are more sensitive than the linear ultrasonic parameters,which provides a new solution for detecting the curing state of polymers in related fields.