The reflective pulsed terahertz time-domain spectroscopy system with a frequency range of 0.25 ~ 2.5 THz was used to detect the defects of polyethylene pipe hot-melt joints. Using white PE100 plate, normal and inclusion defect samples were made. Through the point-by-point scanning of the terahertz system, the waveform and kurtosis imaging are obtained. According to the difference between the waveform and the image, the presence or absence of defects and the size of defects can be distinguished. The sobel edge detection operator has a good recognition level for defects. The recognition error is below 6.47 %, and the error of metal recognition can reach 3.54%.
A biconical axicon structure that can generate terahertz Bessel beams with long non-diffraction distance is designed. The optical element is centered on the axis of the traditional axicon and grinds the bottom surface of the axicon into a conical surface. The non-diffraction distance and light field distribution expression of terahertz Bessel beam generated by biconical axicon are theoretically analyzed. The light field distribution characteristics of traditional axicon and biconical axicon are numerically simulated. The results show that the terahertz wave can generate a terahertz Bessel beam with a non-diffraction distance of 110.18 mm through a biconical axicon with a bottom angle of 20 ° and 15 °. Under the same parameters, the traditional axicon can only generate a terahertz Bessel beam of 27.55 mm, and the non-diffraction distance is broadened by 82.63 mm.
In this paper, a terahertz axicon (n=1. 55) with a base angle of 10 degrees is designed, introduced into a Terahertz Time domain spectroscopy(THz-TDS) (0. 1-1THz), and a transmission beam scanning experiment is built. At each waveform of the raster scan of each section, the spectrum is calculated using a Fast Fourier Transform (FFT) to obtain the beam cross section for each frequency. The measured central beam width at different frequencies varies between 7mm and 2. 5mm, and the distance between the measurement plane and the tip of the axicon is 80mm; by numerically processing the information of each plane, the beam reconstruction results at different frequencies can be obtained. From the analysis results, it can be seen that the THz-TDS can generate non-diffracting beams with depth of field exceeding 160mm via an axicon with a base angle of 10 degrees, the central spot width does not exceed 7 mm, and there are almost no side lobes. The results show that the Bessel beam can provide a significant depth of field expansion capability for the THz-TDS, which will have a positive impact on the non-destructive testing of samples with large thickness and uncertain defect locations.
PC materials have been widely used in medical equipment, automotive manufacturing, aerospace, electronics and other fields, and its thickness detection has become an important part of its quality control. In this paper, based on the thickness detection requirements of PC materials, the propagation characteristics of THz waves in PC materials are simulated, and a pulsed terahertz reflection system is built. The PC materials of different thicknesses are sampled and their single-point waveforms are obtained as actual detection signals. Using the deconvoluted air signal as input, the optical parameters of PC material extracted by transmissive method, the propagation simulation model of THz wave in different thickness PC materials is established, and the simulation waveform is simulated from three aspects: flight time, waveform and amplitude. Compare and analyze the actual detected waveform.
Multilayer composites assembled flexibly with have important effect on the performance and safety of aircrafts. The nondestructive detection on the adhesion layer is an important index to evaluate the quality of aircraft assembly. Terahertz Time-Domain Spectroscopy (THz-TDS) is a newly developed spectroscopy technique based on femtosecond laser technology which currently applied to qualitative analysis as a means of security detection and material identification. Compared with the traditional tensile testing, the detection of defects in the adhesion layer could be nondestructive, visible, positioning and more accurate. The spectral analysis on the material to be assembled was done respectively. The testing model was established in accord with the extracted optical parameters. With the employment of a reflective THz-TDS device, X-Y spot scanning was done to obtain waveforms of every location on an assembled sample. Layered analysis was done by selecting region of interest in time domain waveforms. Conclusions of Time- Frequency spectrum analysis and scanning imaging performance are relatively satisfying through the experiments. The defects could be located and analyzed accurately and efficiently. The research reveals that THz-TDS (0.1THz~5THz) has good testing performance on the adhesion quality of multilayer composites.
Coating curing curve is one of the most important methods to reflect the coating curing properties. It is of great significance for the coating curing properties. In this paper, by using the reflective Terahertz (THz) time-domain spectroscope technique, the curing properties of coating with different thicknesses are studied. Three different parameters used for studying the properties of coating curing curve are proposed in this paper. They are respectively the differential time of flight, power spectrum and amplitude for reflective THz time-domain waveform. In this paper, two kinds of coating (with different thicknesses) curing properties curves are established and the relative errors from three parameter analysis methods are compared respectively. This study shows that the study on coating curing properties curves by using the power spectrum of reflective THz time-domain waveform is superior to the amplitude parameter method. But for the thick coating, the differential time of flight for the reflective THz time-domain waveform can also better reflect the coating curing properties.
According to nondestructive testing principle for the terahertz time domain spectroscopy Imaging, using digital image processing techniques, through Terahertz time-domain spectroscopy system collected images and two-dimensional datas and using a range of processing methods, including selecting regions of interest, contrast enhancement, edge detection, and defects being detected. In the paper, Matlab programming is been use to defect recognition of Terahertz, by figuring out the pixels to determine defects defect area and border length, roundness, diameter size. Through the experiment of the qualitative analysis and quantitative calculation of Matlab image processing, this method of detection of defects of geometric dimension of the sample to get a better result.
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