Local strain field measurement using phase gradient estimation from holographic speckle correlation

Mikael Sjödahl; Yiling Li

doi:10.1117/12.2196979

24 August 2015 Local strain field measurement using phase gradient estimation from holographic speckle correlation

Mikael Sjödahl, Yiling Li

Proceedings Volume 9660, SPECKLE 2015: VI International Conference on Speckle Metrology; 966004 (2015) https://doi.org/10.1117/12.2196979
Event: SPECKLE 2015: VI International Conference on Speckle Metrology, 2015, Guanajuato, Mexico

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.

Citation Download Citation

Mikael Sjödahl and Yiling Li "Local strain field measurement using phase gradient estimation from holographic speckle correlation", Proc. SPIE 9660, SPECKLE 2015: VI International Conference on Speckle Metrology, 966004 (24 August 2015); https://doi.org/10.1117/12.2196979

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