Development of Dual-Wavelength,Large-Diameter,Liquid-Crystal Photoelasticity-Measuring Instrument
Objective One of the main technical difficulties in the measurement of large differential dimensions and oversized components using optical elastometers is that only spatial scanning and splicing measurements are realizable,which significantly reduces the measurement accuracy.According to the basic principle of liquid-crystal displays(LCDs),liquid-crystal panels can be modulated rapidly under the action of the applied voltage and the pixel-by-pixel light polarization state.Moreover,technological advancement has enabled the size of a single liquid-crystal panel to reach 10 m2 or larger.If the liquid-crystal panel can be used as a light source for photoelasticity instruments,then high-speed rail windshields and float-glass assembly lines on flat glass and other large-sized components can be inspected online at high speeds.Thus,the speculative large-diameter measurement problems inherent in existing photoelasticity-measuring instruments can be overcome.However,most liquid-crystal panels emitting elliptically polarized light cannot be used easily to obtain the ideal circularly polarized light,which implies that the existing circularly polarized light cannot be used directly.Therefore,a measurement method for elliptical-light illumination must be developed.Methods Using an LCD instead of a quarter-wave plate enabled the system used in this study to automatically adjust the form of elliptically polarized light,thus significantly reducing costs and providing a theoretical basis for large-sample measurements.The illumination involved two lights of similar wavelengths,and the LCD directed both forms of elliptically polarized light onto the sample vertically.Subsequently,a CCD was used to record an image of a linearly polarized light modulated by a quarter-wave plate and polarizer.Finally,the principal-stress difference was accurately determined from a set of 12 phase-shifted images using the developed algorithm.Results and Discussions To verify the comprehensive performance of the developed instrument,we used a classic counterpressure sheet as the sample to be measured.We measured it using our developed instrument and the classical six-step phase-shift method;subsequently,we compared the measurement results yielded by the two methods.The sample to be measured is a polycarbonate circular plate with a diameter of 120 mm and a thickness of 4 mm,which is counter-pressed in the direction of the diameter.After the power switch was turned on,the control software of the measuring instrument was started,and the system completed the data acquisition and calculation within 10 s.Figs.5(b)-(g)show the six difference plots corresponding to I in Eq.(8),which were obtained by subtracting each pair of wavelengths of the 12 original bright-and dark-field photoelastic stripes from each pair of bright-and dark-field photoelastic stripes obtained by the system.Figs.5(b)-(d)correspond to the wavelength pass and Figs.5(e)-(g)correspond to the wavelength.Using the six intensity-difference plots shown in Fig.5,the sums in Fig.6(a)and 6(b)were calculated using Eq.(11),and the wrapped phase in Fig.6(c)was calculated using Eq.(12).By removing the wrapping and dividing it by the optical-stress constant,the principal-stress difference in Fig.6(c)was calculated based on the elliptically polarized photoelasticity.To facilitate the quantitative measurement of the accuracy of the measuring instrument,we used the classical six-step phase-shift method to measure the sample for comparative measurements,and the measurement results are presented in Fig.7.Figs.7(a1)-(a6)show the intensity images obtained using the classical six-step phase-shift photoelasticity,Fig.7(b)shows the difference in principal stresses computed using the classical six-step phase-shift method,and Fig.7(c)shows the difference between the results measured using the developed instrument and the classical phase-shift method.The result shows that the maximum difference between them is 0.082 MPa and that the relative accuracy error is 2.85％.Fig.8 shows the measurements of the stress-direction angle,where the average deviation of the direction angle is 0.01 rad.Conclusions In this study,an optical instrument that can measure the stress distribution in a sample is developed using advanced light sources,i.e.,dual-wavelength LEDs and liquid-crystal spinning plates,as well as advanced light-modulation devices.Relevant experimental measurements were performed,and experimental comparisons were conducted with the classical six-step phase-shift method to validate the method,which yielded a relative error of 2.85％.The photoelastic liquid-crystal measuring instrument investigated in this study presents a simple structure,can be operated rapidly,and does not require complicated mechanical rotations or other manual interventions.The maximum resolution of the instrument is 0.5 mm,which satisfies the standard for photoelasticity measurement.
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