Analysis of speckle patterns for deformation measurements by digital image correlation

Digital Image Correlation (DIC) – also referred to as white light speckle technique – is an optical-numerical full-field measuring technique, which offers the possibility to determine in-plane displacement fields at the surface of objects under any kind of loading. For an optimal use of the method, the object of interest has to be covered with a speckle pattern. The present paper studies the efficiency of a random speckle pattern and its influence on the measured in-plane displacements with respect to the subset size. First a randomly sprayed speckle pattern is photographed three times. Each picture is taken with a different zoom, yielding three speckle patterns, which are different by the size of the speckles. Secondly a number of speckle patterns are generated numerically using a given speckle size and image coverage. Subsequently, each speckle pattern image undergoes a numerically controlled deformation, which is measured with digital image correlation software. Both imposed and measured displacements are compared and it is shown that the size of the speckles combined with the size of the used pixel subset, clearly influences the accuracy of the measured displacements. Furthermore it is shown that it is possible to create an optimal speckle pattern when a given subset size is chosen.