Principle And Method Of Stress Measurement Based On Terahertz Time-domain Spectroscopy

Terahertz (THz) refers to the electromagnetic wave in the frequency range of0.1to10THz, which lies on the electromagnetic spectrum between the microwave andinfrared. It has many advantages such as narrow pulse, low energy, high coherenceand good permeability. THz time domain spectroscopy (THz-TDS) technology iswidely applied to nondestructive testing, chemical identification, safety inspection,and other fields, and has huge potential applications in the field of experimentalmechanics measurement. In this paper, based on TDS system, the principle andmethod of stress measurement are studied, detailed in the following aspects.In order to change the polarization direction of the THz radiation transmittingspecimen, polarizers are added in the traditional THz TDS system in our experiment.Polytetrafluoroethylene (PTFE) is chosen as the experimental material and processedinto a dumbbell uniaxial tensile specimen. Then a micro loading device is designedand placed into TDS system, so as to realize the experimental mechanicsimprovement of the TDS system.The relationship model between the phase delay and principal stress is establishedand the air humidity of satisfying experimental requirement is determined byexperiments of TDS stability. By tensile tests, the mechanical parameters of PTFE aredetermined. According to the theory model, the verification experiments ofstress-optic law are designed, and experiment principle and methods are deduced indetail based on the experimental conditions. Experimental results indicate that thecalculated refractive index grows linearly with the applied stress, which can beconsidered a straight verification of the uniaxial stress-optic effect in the terahertzfrequency.By experiments, the stress-optic coefficients A and B are measured. Based onTDS, the principle and method of stress measurement are proposed, and the methodsof principal stress separation are determined. Compared to the method of resistancestrain measurement, the stress measured by TDS is smaller than it with the error of10%~20%.Stress measurement method based on TDS can effectively measure the internalstress of materials, and has a broad prospect of applications in the field of stress measurement of opaque materials such as ceramic. This method is expected to be anew stress measurement in experimental mechanics.