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Full-Field High-Strain Gradient Evaluation from Wrapped ESPI Data Using Phasors

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Electronic Speckle Pattern Interferometry (ESPI) is a sensitive optical method commonly used for making full-field measurements of surface displacements. It would be very desirable to be able to extend the… Click to show full abstract

Electronic Speckle Pattern Interferometry (ESPI) is a sensitive optical method commonly used for making full-field measurements of surface displacements. It would be very desirable to be able to extend the technique also to determine 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 determine 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.

Keywords: high strain; full field; field high; espi data; strain

Journal Title: Experimental Mechanics
Year Published: 2017

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