Study On The Stress Characterization Method Based On Terahertz Time Domain Spectroscopy

Terahertz wave is an electromagnetic wave with a frequency range of 0.1～10 THz.It has the advantages of low energy,good permeability,high coherence and narrow pulse width.Terahertz time domain spectroscopy(THz-TDS)is a widely used terahertz spectrum analysis technology,which has been used in the research of physics,chemistry,biology,medicine and other scientific and technological fields.In recent years,this technique has also been applied to the research of experimental mechanics measurement.Due to the high permeability of terahertz waves to dielectric materials,THz-TDS shows great potential in internal stress characterization.Based on THz-TDS technology,this paper developed the measuring theory,analysis system and experimental method for quantitative characterization of material plane stress state.The main contents of this paper include the following five aspects:1.A polarization-sensitive THz-TDS imaging system for stress analysis was built in this paper.Firstly,wire grid polarizers were added into the space optical THz-TDS system to control and sense the polarization state of terahertz wave.And a mechanical loading device was added to modulate the THz signal by stress.Secondly,based on the all-fiber THz-TDS system,the polarization-sensitive dual-arm THz-TDS imaging system was built,and the transmission and reflection were both used in THz-TDS imaging system.Finally,several key parameters of the system were calibrated and a high-precision signal delay analysis method was developed.2.A method for characterization of plane stress of isotropic materials based on multi-polarization terahertz wave was proposed.Firstly,a stress-terahertz wave modulation model based on stress birefringence of isotropic materials was established.The terahertz phase responses of different polarization states were derived according to the geometric optical Jones matrix method.Thirdly,a recursive search algorithm was established to solve the plane stress state parameters of materials.Finally,the refractive index and stress optical coefficients of typical isotropic materials in THz band were measured,and the plane stress state of isotropic materials was experimentally measured,which verified the proposed stress characterization method.3.A method for characterization of plane stress of anisotropic materials based on THz-TDS system was developed.Firstly,according to the crystal structure characteristics,the transfer model of terahertz wave in different loaded lattice structure materials was summarized,and the stress characterization method for anisotropic materials was proposed.Secondly,based on the crystal structure of different materials,the elastic-optical coefficient matrix was given,and the stress terahertz wave response on different crystal planes of materials with different crystal structures was deduced,and the multi-polarization terahertz wave stress characterization method suitable for widely anisotropic materials was proposed.Finally,based on the crystal structure of single crystal silicon,the proposed stress characterization method was used to study the plane stress state of single crystal silicon.The independent components of the elasticoptical coefficient of single crystal silicon material were measured,and the plane stress state on the crystal plane [100] of single crystal silicon was characterized.4.Stress field imaging based on THz-TDS was developed.Firstly,the THz-TDS stress field imaging system using raster scanning was developed,and the amplitude field imaging method based on terahertz pulse signal was developed.Secondly,a plane stress analysis algorithm based on the amplitude of terahertz pulse signal and a plane stress separation algorithm based on the phase of terahertz signal were developed.Finally,through THz-TDS imaging of the pure bending specimen of typical elastic materials,the stress optical coefficient of the material was calibrated,the stress field distribution of the material on the disk specimen under diameter-compression was measured,and the measurement accuracy of the stress field in the elastic range was given.5.The analytical method of elastic-plastic strain in materials was developed.Firstly,for the elastoplastic deformation of materials,the co-modulation model of refractive index of materials by elastic deformation and plastic deformation was proposed.Experimental research was carried out based on this model.Secondly,the modulation mode of the transmitted THz-TDS signal by the elastoplastic strain was analyzed during the elastoplastic stretching of PTFE specimen.Finally,according to the proposed modulation model,the elastic deformation coefficients and plastic deformation coefficients of the material were measured.