Optics and Lasers in Engineering, March, 2015, Vol.66, p.19(15)
To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.optlaseng.2014.08.004 Byline: Paolo Mazzoleni, Fabio Matta, Emanuele Zappa, Michael A. Sutton, Alfredo Cigada Abstract: This paper discusses the effect of pre-processing image blurring on the uncertainty of two-dimensional digital image correlation (DIC) measurements for the specific case of numerically-designed speckle patterns having particles with well-defined and consistent shape, size and spacing. Such patterns are more suitable for large measurement surfaces on large-scale specimens than traditional spray-painted random patterns without well-defined particles. The methodology consists of numerical simulations where Gaussian digital filters with varying standard deviation are applied to a reference speckle pattern. To simplify the pattern application process for large areas and increase contrast to reduce measurement uncertainty, the speckle shape, mean size and on-center spacing were selected to be representative of numerically-designed patterns that can be applied on large surfaces through different techniques (e.g., spray-painting through stencils). Such "designer patterns" are characterized by well-defined regions of non-zero frequency content and non-zero peaks, and are fundamentally different from typical spray-painted patterns whose frequency content exhibits near-zero peaks. The effect of blurring filters is examined for constant, linear, quadratic and cubic displacement fields. Maximum strains between [+ or -]250 and [+ or -]20,000[micro][epsilon] are simulated, thus covering a relevant range for structural materials subjected to service and ultimate stresses. The robustness of the simulation procedure is verified experimentally using a physical speckle pattern subjected to constant displacements. The stability of the relation between standard deviation of the Gaussian filter and measurement uncertainty is assessed for linear displacement fields at varying image noise levels, subset size, and frequency content of the speckle pattern. It is shown that bias error as well as measurement uncertainty are minimized through Gaussian pre-filtering. This finding does not apply to typical spray-painted patterns without well-defined particles, for which image blurring is only beneficial in reducing bias errors. Author Affiliation: (a) Rolls-Royce plc, MV-GF, Victory Road, P.O. Box 31, Derby DE24 8BJ, UK (b) Dept. of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA (c) Dept. of Mechanical Engineering, Politecnico di Milano, via La Masa 1, 20156 Milan, Italy (d) Dept. of Mechanical Engineering, University of South Carolina, 300 Main Street, 29208 Columbia, SC, USA Article History: Received 18 April 2014; Revised 21 July 2014; Accepted 4 August 2014
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