Experimental Mechanics @ Michigan State University
		Abstracts on Moiré
PROJECTION MOIRÉ FOR OPTICAL PATH LENGTH CORRECTION IN REFLECTION PHOTOELASTICITY M. Heredia and E. A. Patterson Accepted for publication at the BSSM Conference on 'Strain Measurement in the 21st Century', Lancaster, 5-6 September 2001 Abstract: Reflection photoelasticity is a quick and accurate method for obtaining full-field measurements of the surface strains in real components of arbitrary size and geometry. In order to derive correctly the strains from the photoelastic data, an accurate estimate of the optical path length is necessary. For any given point, the optical path length depends on the thickness of the birefringent coating and on the orientation of the surface relative to the observation axis. In this paper a technique based on projection moiré is introduced that allows performing full-field automatic measurements of the 3-D shape of an object. Subsequently, the surface orientation at each point can be obtained, which in turn allows performing an estimation of the optical path length. The technique is designed to integrate with existing methods for automated strain measurement from photoelastic data. Emphasis has been placed on the robustness, simplicity and usability of the procedure and the interface. The apparatus and experimental procedure for data collection are described, including a discussion of aspects such as system calibration. The moiré images can then be processed to obtain the 3-D shape of the surface, using fringe pattern analysis algorithms, which are outlined in the paper. The accuracy of the method is evaluated using moiré simulation with artificial data, and the results of a series of experiments to validate the technique using real photoelastic data are also presented and discussed. Finally, an industrial application is described as a case study to illustrate the combined technique. Vibration tests of turbine blades are routinely performed in industry using reflection photoelasticity alone, and the strains determined by means of automated photoelastic analysis assuming that the optical path length is constant. A series of experiments with turbine blades were performed using the combined technique. The results are discussed in the paper, and the effect that correcting the optical path length with the surface orientation has on the calculated strains is investigated.
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