Method Of Plane Stress Measurement Based On Multi-frequency Terahertz Spectroscopy

Terahertz(THz) is the electromagnetic wave in the frequency range of 0.1 to 10 THz, which belongs to far-infrared wave between the microwave and infrared the electromagnetic spectrum. It has lots of advantages such as low energy, high coherence, good permeability and narrow pulse. THz time domain spectroscopy(THz-TDS) technology is widely applied to many fields such as three-dimensional imaging, nondestructive testing, safety inspection and biomedical field. During the recent years, this technology has already been used in the stress measurement gradually. In this paper, the method of plane stress measurement is studied based on TDS system, detailed in the following aspects:The traditional THz-TDS system is improved, where two polarizers are introduced into the optic path. The polarization state of THz wave can be modulated well so that this system can change the polarization direction of the THz radiation. Based on the TDS system the general principle of stress components in the loaded specimen can be obtained, which explain the relationship between the phase shift of THz radiation and stress components.The two methods of stress measurement is proposed based on the relationship between the phase shift of THz wave and each variables. The first one is that frequency is determined and the directions of the polarizers are changed. Another one is that the directions of the polarizers are determined and the frequencies are changed. With the optimized method, the stress components from TDS system can be obtained.Both of load and frequency can influence the results of the experiment. After increasing the load, the relationship between the phase shift and frequency from TDS system is nonlinear. Comparing the results and the data of the strain gauge, the errors can be below 6%. Similarly, the accuracy of the convergence can be improved when increasing the frequency range.Experiment results indicate that this method based on TDS system can obtain the stress components in opaque materials well. During the future study, the two dimensional translation stage can be introduced into the system, which can obtain the information of stress field. Moreover, the stress measurement of opaque materials also has a broad prospect of applications such as ceramic. This method is expected to be a new stress measurement in experimental mechanics.