Abstract
Electronic Speckle Pattern Interferometry (ESPI) is a sensitive optical method commonly used for full-field measurements of surface displacements. It would be very desirable to be able to extend the technique also to indicate surface strains. This would provide a full-field, non-contact strain measurement method that avoids the substantial installation burden of strain gauges. A mathematical approach is described where the ESPI data from an in-plane interferometer are numerically differentiated to indicate surface strains. This is a challenging process because numerical differentiation is very sensitive to the presence of noise and ESPI data are inherently noisy. In addition, the phase information from ESPI data are wrapped modulo-2π. The resulting phase discontinuities make it difficult to use local averaging to smooth the data. A technique is described here where phasors are used to avoid the need for phase unwrapping. The effect of noise is reduced by a localized multiple smoothing technique that is effective in preserving spatial resolution, even near very high strain concentrations. Example measurements are shown and the effectiveness of the proposed method is illustrated.
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Acknowledgments
The authors sincerely thank Stresstech Oy, Finland, and the Natural Science and Engineering Research Council of Canada (NSERC) for their kind financial support of this project.
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Heikkinen, J., Schajer, G.S. (2017). Full-Field High-Strain Evaluation from Wrapped ESPI Data Using Phasors. In: Yoshida, S., Lamberti, L., Sciammarella, C. (eds) Advancement of Optical Methods in Experimental Mechanics, Volume 3. Conference Proceedings of the Society for Experimental Mechanics Series. Springer, Cham. https://doi.org/10.1007/978-3-319-41600-7_2
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DOI: https://doi.org/10.1007/978-3-319-41600-7_2
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