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Proceedings Paper

Local strain field measurement using phase gradient estimation from holographic speckle correlation
Author(s): Mikael Sjödahl; Yiling Li
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Paper Abstract

This paper introduces a new approach to process holographic specklegrams that gives direct access to local phase gradients. For a full in-plane strain mapping at least three different sensitivity vectors have to be used, if only one principal strain component is of interest two sensitivity vectors suffices. The fields in two defocused planes are calculated by propagating the focused field a known distance numerically. Based on the fields in the three planes a correlation function is formulated that is used to estimate the local phase gradients generated by stressing the sample. The phase and phase gradients for the different sensitivity vectors are then further combined to give the in-plane deformation and strain fields, respectively. The technique was demonstrated on a plate stressed uniaxially. Two illumination directions were used symmetrically oriented in the horisontal plane in relation to the surface normal to the plate. A significant reduction in phase noise was demonstrated. In addition the in-plane strain was estimated to be 0.8 mstrain with a standard deviation of 10 μstrain using an evaluation window size of 15x15 pixels. The average speckle correlation for this case was found to be 0.92. This technique promises to be a valuable tool whenever high quality measurements of strain fields or surface tilts are of interest.

Paper Details

Date Published: 24 August 2015
PDF: 12 pages
Proc. SPIE 9660, SPECKLE 2015: VI International Conference on Speckle Metrology, 966004 (24 August 2015); doi: 10.1117/12.2196979
Show Author Affiliations
Mikael Sjödahl, Luleå Univ. of Technology (Sweden)
Yiling Li, Luleå Univ. of Technology (Sweden)

Published in SPIE Proceedings Vol. 9660:
SPECKLE 2015: VI International Conference on Speckle Metrology
Fernando Mendoza Santoyo; Eugenio R. Mendez, Editor(s)

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