Measurement of Low Level Stress by Integrating Photoelasticity and Spectrometry

Recently, by integrating the photoelasticity and spectrometry, the authors proposed a new stress measurement method by analyzing the transmissivity spectrum (TS) of the periodic extinction phenomenon of component colors of an isochromatic fringe pattern with the increase of the level of stress difference in photoelasticity. An empirical relationship between the wavelength of the extinct component colors of the isochromatic fringe pattern and stress difference can be obtained by regression. In contrast to the traditional photoelasticity, not only the intrinsic measurement error is eliminated but also the determination of the refractive index and stress optic coefficient of the test specimen is unnecessary. However, for the low level stress, e.g. the unavoidable residual stresses produced in the manufacturing process in the glass plate of the thin film transistor liquid crystal display (TFT-LCD), the accuracy of the aforementioned proposed method needs to be improved. With the help of the spectrometer, the variation of TS at different levels of stress difference was analyzed and the light intensities of the plane polarized fringe patterns obtained by the three step phase shift technique were investigated. Furthermore, various regression models between the fringe order and the light intensity were attempted to find the optimum one. A commercially available photoelastic material, PSM-1, was used to verify the feasibility and accuracy of the improved method.