The Research Of Terahertz Spectrum Detection Technology Applied In Nondestructive Testing Of Composite Material

The terahertz radiation(1 THz=1012 Hz) refers to electromagnetic wave in 0.1-10 THz, THz wave is with low energy(4.1Me V), high SNR(signal to noise ratio), and high resolution, widely applied in nondestructive field. Composite material is kind of non-metallic materials commonly used in special equipment, It effectively overcome the shortcomings of source material, while its reduction of hardness and strength on surface caused various defects appearness, which is not common in metal material, and composite material defects is difficult by traditional nondestructive detection techniques. THz spectrum is able to inspect the delamination, inclusion, mechanical damage etc. in the composite material accurately. In recent years, the research on THz wave was forced, THz spectroscopy and THz imaging technology are important research aeras in the development of THz technology in the future.This dissertation focuses on the composite material commonly used in special equipments, combine the inspection code and the engineering application requirements. Based on THz-TDS system, BWO continue wave THz imaging system and frequency modulated continuous wave THz imaging system, the common composite material including polyethylene, carbon fiber reinforced plastic and glass fiber reinforced plastic. 18 materials sample with different defects were inspected, the major contribution and innovation points are as follows:(1) The refractive index and absorption coefficient of the different defects in polyethylene pipe samples were detected through the THz-TDS system. In THz frequency band, the spectrums of different samples differ in the changing rule of refractive index, the location of absorption coefficient etc. And by the time domain waveform, the thickness of the delamination in the PE pipe can be calculated without the refractive index. Experimental results can discriminate detect defects in PE pipe, and provide reference to calibrate the THz-TDS system.(2) The frequency modulation CW THz imaging technology was applied to image the PE pipe(Zmax=21mm), the tomography of PE pipe was processed to benefit to the human eye recognition. The result show that images with different vertical depths(2mm-7mm) express different defects, and the maximum of imaging intensity appear in different position. Metal impurity, the mechanical damages of pipeline inwall, surface damage and buried defects(Zmax=3.2mm) all can be detected. Maximum intensity imaging and the left view of PE pipe contribute to each other, determine the shape, location, and continuous conditions of various defects, etc.(3) BWO continue wave THz imaging technology was used in GFRP, the imaging results indicate transmission CW imaging can detect the defects from the back side of the GFRP(Thickness=5mm) with heat damage(T=300?C and above), impact defect, scratch defect and hole defects(?=1mm), while eliminate the effect of epoxy resin in the GFRP determine the shape, position, size of the defects.(4) The result of reflection imaging of CFRP show that the voids size, scratch damage, the area of the friction defect, heat damage can observe directly. The imaging data combine with the imaging can judge the heat history of the CFRP sample.Some suggestions are put forward for the further study of THz spectroscopy applied to the detection of composite material in the last part of the dissertation.