Research On Nondestructive Testing Of Advanced Ceramic Fiber Composite Based On Terahertz Technology

The new ceramic fiber composite material has been applied in the outer insulation of ultrahigh sound velocity aircraft due to its advantages of high temperature resistance,low density,high specific strength,ablation resistance,etc.Due to the technological level of production and assembly etc,this kind of material may appear defects which include inclusions,holes,even large area debonding etc.Owing to the particularity of the material structure and application scenarios of ceramic fiber composite material,the effect of conventional nondestructive testing methods is not good.As a new nondestructive testing technology,Terahertz(THz)technology has great potential in the field of nondestructive testing of this kind of composite material,which can be complementary with conventional testing technology.Based on Terahertz Time-Domain Spectroscopy(THZ-TDS),this thesis regards ceramic fiber composite material components as the research object,carries out nondestructive testing technology research from the material's influence on the THz wave transmission characteristics,defect location and quantitative characterization of the defect area and other aspects:(1)In view of the influence of ceramic fiber composite material characteristics on energy attenuation during the process of THz wave transmission and the ability of THz-TDS technology to detect the thickness of this type of material,this thesis studies the THz wave transmission model in ceramic fiber composite material,discusses the effect of Mie scattering theory(MST)on the detection of particles by THz wave,researches the penetration ability of THz-TDS detection technology in this kind of material,analyzed the reasons why the density and microbes aperture of ceramic fiber composite material has an effect on the attenuation of THz wave radiation energy and establishes a mathematical model between material properties and the energy integral value.(2)In view of the defect detection of the adhesive layer in ceramic fiber composite material,based on the time-domain waveform of debonding defects and intact bonding position in the samples obtained by the THZ-TDS detection technology,this thesis qualitatively analyzes the existence of defects,measures the average refractive index,determines the debonding depth and thickness.Aiming at the error of defect position analysis caused by the average refractive index,this thesis further analyzes the location of defect by using THz tomography,establishes the assessment model of defect location and realizes the high precision localization of the defects in the adhesive layer of ceramic fiber composite components.(3)In view of the problem that THZ wave source jitter and material anisotropy and other factors have a great impact on the THZ reflection imaging quality and the characterization precision of the defect area,this thesis uses Fuzzy C-means(FCM)mean value algorithm to segment the preprocessed images after adaptive filtering to realize visualization and quantitative characterization of defect area.On this basis,aiming at the problem that the above algorithm has a low quantitative characterization precision for the small defect area,this study improves the characterization precision of small regions based on the regional image segmentation algorithm improved on the basis of energy integral value,establishes corresponding evaluation indexes of image segmentation,and proves the feasibility and accuracy of using the above two algorithms to complement each other to realize the defect area characterization of ceramic fiber composite material through a large number of experiments.