Research On Nondestructive Testing Technology Of Composite Materials Based On Terahertz Time Domain Spectroscopy

The non-destructive testing technology of composite materials is widely used in aerospace,nuclear industry,automobile industry and other fields.Terahertz(THz)is a cutting-edge non-destructive testing technology,which is widely used in many testing scenarios.THz has good penetrability(non-polar materials),non-contact,and lowenergy radiation.However,on the one hand,the application of THz radiation source and detector technology is not mature,and the speed of imaging detection is limited;on the other hand,the wavelength of THz is in the range of 0.03?3mm,and the imaging resolution is limited,even during the imaging process.Diffraction phenomenon affects the quality of detection.In addition,the propagation mechanism of THz in conductive materials is more complicated,and the detection results need to be further analyzed and evaluated.Based on the principle of THz imaging and non-destructive testing technology of composite material,this thesis establishes the relationship between the measured signal and the internal topography of the experimental sample in THz testing,improving the spatial resolution of THz images and the correlation degree of defect morphology in non-conductor and weak conductor.The contents and innovations studied in this thesis are as follows:1.Using THz to detect the thickness of Glass fiber reinforced plastics(GFRP),and form a method for measuring the thickness of GFRP based on linear fitting.The detection effect of THz time-domain spectroscopy(THz-TDS)system on the material thickness or depth is verified by linear fitting,and the corresponding relationship between the material thickness and THz propagation time is obtained,which proves that the linear fitting results have little influence from different positions of THz detector.2.Studying the improvement methods of THz imaging quality and resolution.Due to the limitation of resolution,THz non-destructive testing methods often have diffraction phenomena and even cause blurred boundaries.Therefore,the improvement of imaging quality and resolution becomes a key point.The processing method based on the Lucy-Richardson algorithm has a certain effect on the image quality.After the original THz image is processed by the Lucy-Richardson algorithm,gray value adjustment and filtering,the grayscale mean gradient(GMG)is used to quantify the sharpness and compare the results.It is shown that this method can improve the imaging quality and resolution of THz images to a certain extent.3.Analyzing the detection effects of THz scanning imaging and vibration thermal imaging(VT)on impact defects of carbon fiber composite,and comparing the advantages and disadvantages of these two detection technologies.Carbon fiber composite material is a kind of polar material,which is widely used in the aerospace field and has a certain degree of conductivity.Therefore,the detection principle of THz for carbon fiber composite material is also more complicated.Energy impact defect is a common damage of carbon fiber composite materials.Under the background of expanding applications of composite materials,qualitative and even quantitative analysis of impact defect detection results is particularly necessary.4.Research on the internal defect detection method of GFRP based on onedimensional convolutional neural network(1D-CNN).This thesis uses convolutional neural network(CNN)to process the experimental results of THz on composite materials,establish a THz defect detection model based on 1D-CNN and realize the effective identification and reconstruction of composite material defects.This thesis takes the one-dimensional time series of each pixel in the image as input,and use a1D-CNN to determine whether the input is a defective pixel,and export the type of defect at the corresponding position of the pixel to realize the combination of THz nondestructive testing technology and deep learning.