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Simultaneous microstress and strain measurement with an optical microscope

In this paper, a quantitative method to measure simultaneously micro-stress and strain is proposed. A compact optical transmitting microscope is reconstructed by developing a loading and recording system, and white light source can be replaced with a laser source. The choice of laser source or white light source depends on the thickness of specimen. For strain measurement, grating diffraction method is presented. With the help of a Bertrand lens, the Fourier spectrum of the grating is formed on the CCD sensor plane or with high image quality. Software for precise, fast and automatic determination of the diffraction spot centroids is developed. Local strains are thus measured with high resolution. Stress measurement is carried out to the zero order of grating diffraction spots following the conventional method for retardation measurement with tilting compensator. The combination of the grating diffraction method and the photoelasticity method in the same test provides simultaneous measurement of strain and stress.