The digital Image Correlation (DIC) method is implemented to allow measurements of both first and second order displacement gradients. Thirteen unknown parameters, consisting of the components of displacement and their first and second order displacement gradients, as well as the grayscale value offset, are determined through optimization of a correlation coefficient. The previous DIC method assumes that the local deformation in a subset of pixels is represented by a truncated Taylor series including up to the first order displacement gradient terms, so deformations consisting of higher order displacement gradients tend to distort the infinitesimal strain measuremnets. By refining the method to measure both the first and second order displacement gradients, more accurate infinitesimal strain measurements can be achieved in relatively larger deformation situations. The refinement has also provided a method for measurements of second order displacement gradient useful in the investigation of materials sensitive to strain gradient.
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