Experimental Mechanics @ Michigan State University
EM Basics: Automated Photoelasticity
Automated photoelasticity has developed as a topic in the last ten to fifteen years during which time major advances have been made, partly as a result of the availability of new technology in computing and image processing. For a review of the subject see Ajovalasit et al^1 or Patterson^2 for work prior to 1988.
The major techniques of automated photoelasticity are: § Spectral contents analysis § Fourier processing § Phase-stepping methods
Early efforts to automate photoelastic analysis involved collection of monochromatic images followed by some form of fringe thinning, with the operator required to identify all the fringes and interpolation used to obtain values between the locations of fringes. The grey field polariscope falls across the boundaries between Fourier analysis and phase-stepping. Fourier analysis requires large numbers of images and so is often impractical. Spectral analysis can provide the absolute fringe order but no information about isoclinic angle. Thus its use in isolation produces significant drawbacks. The maximum fringe order that can be recognised is approximately equal to the number of wavelengths at which intensity information is collected. Generally monochromatic light is used in phase-stepping to produce maps of isoclinic angle and isochromatic fringe order from a theoretical minimum of three images. The disadvantage of phase-stepping is that, whilst multiple fringes can be dealt with by phase unwrapping, the fringe order must provided at a pair of points in order to fix the absolute value of the fringe order map.
Ajovalasit, A., Barone, S., Petrucci, G., 1998, ‘A review of automated methods for the collection and analysis of photoelastic data’ J. Strain Analysis, 33(2):75-91. Patterson, E. A., 1988, 'Automated photoelastic analysis', Strain, 24(1): 15 - 20.
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