An Approach to Errors Introduced in the Mesh-Free Full-Field Strain Measurement Method
One major advantage of digital imaging is that one can measure the placement of a sampled spot in sub-pixel accuracy. This means that if the set-up has a field capable of sampling X length units per pixel, the location of any gray-level spot can be measured with accuracy of X/N, where N is a number mainly depending on the color depth, beyond the overall quality of the imaging set-up [1, 2]. The present work refers to the errors imposed by the recently introduced full-field strain measurement method . Beyond systematic errors of the digital camera, the method itself is not an error-free procedure. A not so obvious source of errors is the sub-spot displacement of each spot-center. It is assumed that the center is invariant relative to the spot boundaries. This is not exactly true in the case of large gradient of deformation. We analytically derive those errors and define the limits where it is kept in affordably low values. A solution to eliminate this systematic error is, also, proposed.
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