A Critical Comparison of Some Metrological Parameters Characterizing Local Digital Image Correlation and Grid Method
The main metrological performance of two full-field measurement techniques, namely local digital image correlation (DIC) and grid method (GM), are compared in this paper. The fundamentals of these techniques are first briefly recalled. The formal link which exists between them is then given (the details of the calculation are in Appendix 1). Under mild assumptions, it is shown that GM theoretically gives the same result as DIC, since the formula providing the displacement with GM is the solution of the minimization of the cost function used in DIC in the particular case of a regular marking. In practice however, the way the solution is found being totally different from one technique to another, they feature different metrological performance. Some of the metrological characteristics of DIC and GM are studied in this paper. Since neither guideline nor precise standard is available to perform a fair comparison between them, a methodology must first be defined. It is proposed here to rely on three metrological parameters, namely the displacement resolution, the bias and the spatial resolution, to assess the metrological performance of each technique. These three parameters are thoroughly defined in the paper. Some of these quantities depend on external parameters such as the pattern of the surface of interest, so the same set of grid images is processed with both techniques. Only the contribution of the camera sensor noise to the displacement resolution is considered in this study. The displacement resolution, the bias and the spatial resolution are not independent but linked. These links are therefore studied in depth for DIC and GM and compared. In particular, it is shown that the product between the displacement resolution and the spatial resolution can be considered as a metric to perform this comparison. The extension to speckled patterns of the lessons drawn from grids is finally addressed in the last part of the paper. As a general conclusion, it can be said that for the value of the bias fixed in this study, the additional cost due to grid depositing offers GM to feature a better compromise than subset-based local DIC between displacement resolution and spatial resolution.
KeywordsDigital image correlation Displacement Full-field measurement Grid method Metrology Strain
The GDR CNRS ISIS is gratefully acknowledged for its partial financial support of this study (TIMEX project).
- 20.Hack E, Lampeas G, Mottershead J E, Patterson E A, Siebert T, Whelan M P (2011) Progress in developing a standard for dynamic strain analysis. In: Experimental and applied mechanics, volume 6 of conference proceedings of the society for experimental mechanics series, pp 425–429Google Scholar
- 21.Sebastian C, Lin X, Hack E, Patterson E (2015) A reference material for establishing uncertainty for static and dynamic displacements. In: Proceedings of the SEM conference. Costa Mesa, to appearGoogle Scholar
- 23.Standard guide for evaluating non-contacting optical strain measurement systems, ASTM standard E2208-02 (2010)Google Scholar
- 24.JCGM 200 (2008) International vocabulary of metrology basic and general concepts and associated termsGoogle Scholar
- 25.Doumalin P, Bornert M, Caldemaison D (1999) Microextensometry by image correlation applied to micromechanical studies using the scanning electron microscopy. In: Japanese Society for Experimental Mechanics (ed) Proceedings of the international conference on advanced technology in experimental mechanics. Ube City, pp 81–86Google Scholar
- 27.Sutton M, Orteu J J, Schreier H (2009) Image correlation for shape, motion and deformation measurements. Basic concepts, theory and applications. SpringerGoogle Scholar
- 35.Badulescu C, Grédiac M, Mathias J-D (2009) Investigation of the grid method for accurate in-plane strain measurement. Measur Sci Technol 20(9):20:095102. doi: 10.1088/0957--0233/20/9/095102. IOP
- 40.Surrel Y (2000) Photomechanics, topics in applied physic, vol 77, chapter Fringe Analysis, pp 55–102Google Scholar
- 44.ISO 5725. Accuracy (trueness and precision) of measurement methods and results, 1994. the International Organization for StandardizationGoogle Scholar
- 52.Grafarend E W (2006) Linear and nonlinear models: fixed effects, random effects, and mixed models. Walter de GruyterGoogle Scholar
- 56.Chrysochoos A, Surrel Y (2012) Chapter 1. Basics of metrology and introduction to techniques. In: Grédiac M, Hild F (eds) Full-field measurements and identification in solid mechanics. Wiley, pp 1–29Google Scholar