Study On Propagation Mechanism Of Terahertz Wave In Composite Layered Structure

Composite layered structures are widely used in automotive,marine and aerospace applications due to their ability to meet a variety of different mechanical properties.Terahertz nondestructive testing and assessment,as an important means to ensure the function of composite materials,lacks theoretical guidance in the actual operation process.In this paper,the propagation mechanism of terahertz wave in composite layered structure is studied,and the propagation mechanism of terahertz in composite layered structure is analyzed by numerical simulation.Combined with practical engineering application problems,a high-precision nondestructive testing method for composite layered structures is proposed,which provides theoretical support for the engineering application of terahertz nondestructive testing.The research of this paper mainly includes the following aspects:The traditional terahertz transmission matrix method is obtained based on Maxwell’s equations and the electromagnetic wave fluctuation equations to provide a theoretical basis for the propagation simulation of composite layered structures.The limitations of the traditional transmission matrix method are analyzed according to the terahertz time-domain spectral system used for practical terahertz NDT and the characteristics of composite layered structures.In view of the problems of defocusing due to material thickness,oblique terahertz incidence and signal anomalies due to material boundary diffraction in terahertz NDT,the study of terahertz propagation characteristics under non-ideal conditions in three practical engineering applications,namely defocusing,oblique incidence and boundary diffraction,is carried out.Based on the Gaussian beam propagation theory,Fresnel matrix theory and broadband Huygens-Fresnel diffraction theory,the terahertz signal variations before and after the focal plane,at different incidence angles and at material geometrical boundaries are analyzed respectively.The results show that the effective detection range of terahertz nondestructive testing is within 5 mm from the focal length and 2 degrees of inclination angle.In addition,the inverse filtering algorithm and the light inversion method are used to recover out-of-focus images within 15 mm of the focal distance and to correct the terahertz images of diffraction from material geometry boundaries,respectively.Aiming at the problem of inaccurate extraction of optical parameters of composites caused by spectral oscillation caused by Fabry-Perot effect,an adaptive spectral oscillation cancellation method based on variational modal decomposition is proposed to eliminate the spectral oscillations in thin samples.The effectiveness of spectrum oscillation elimination is evaluated by calculating the QS value in quasi spatial(QS)domain.Where,the QS values for the refractive index and extinction coefficient of silicon were 1.5089 and 1.4016 respectively,which reduce the errors in the extraction of optical parameters caused by spectral oscillations of thin samples without iteration.To address the problem of optical parameter inaccuracy caused by the narrow spectral range of the system,a method of optical parameter extraction based on a non-linear optimization algorithm is proposed to extend the effective spectral width to 5 THz,so as to realize the expansion of the effective spectral range of optical parameters and provide accurate optical parameters for the construction of the material characteristic matrix.Based on the high-precision extraction of optical parameters,the research on the construction of the characteristic matrix of complex electromagnetic interfaces is carried out.Combined with the Kirchhoff approximation method,the generalized transmission matrix method is used to realize the construction of the characteristic matrix of the rough interface of composite materials,and the terahertz signal compensation of the single-layer rough surface and the roughness inversion of the multi-layer rough surface are realized.The refractive index change caused by permeation is modelled by the effective medium theory,and an inversion method of the glue layer thickness based on an optimized propagation model is proposed to reduce the gel layer thickness detection error.Of which,the mean value of thickness error of glue layer(I)is reduced from 84.9133 μm to 31.1473 μm.The mean value of thickness error of glue layer(II)was reduced from 72.4463 μm to 26.3764 μm.The research on the high-precision identification and quantification of defects in composite layered was carried out,and the defect identification method based on dynamic time warping algorithm and simulation analysis was proposed to achieve the effective identification of different layered structure and different types of defects.To address the problem of high-precision quantification of defect thickness and area,a defect identification method based on multiple echo analysis and multi-feature parameter fusion algorithm is proposed to improve the defect detection accuracy and provide a high-precision quantitative detection method for the detection of defects in composite layered.In which,the defect thickness measurement error reaches 4% and the area measurement error reaches 6.78% for500 μm thickness defects.The research in this paper is of great significance in explaining the mechanism of terahertz propagation in composite layered structure and the mechanism of terahertz nondestructive detection in layered,and fundamentally analyses the propagation of terahertz in composite layered,which is of high theoretical research value for realizing the combination of terahertz simulation analysis and practical engineering applications.