Sub-pixel displacement algorithm in temporal sequence digital image ID correlation based on correlation coefficient weighted fitting

Digital image correlation (DIC) is a non-contact technique that is used widely for deformation measurement, but it has problems due to the large amount of calculations required, which make it time consuming. Sub-pixel displacement methods are usually based on spatial correlations, which only consider the spatial continuity of the deformation and ignore temporal continuity. This study proposes a DIC algorithm to calculate sub-pixel displacement combining spatial and temporal correlations. A fast integer-pixel displacement algorithm is employed to calculate full field integer-pixel displacement at different deformation times. Then the moving least squares fitting technique with a weighted function based on the correlation coefficient is used to fit each point's integer-pixel displacement along the time axis, allowing the full field sub-pixel displacement to be calculated for every moment. Experimental results demonstrated the accuracy and efficiency of the proposed algorithm. This achieved the same accuracy as tradition spatial correlation algorithms. Computation efficiency was improved almost 8 fold compared with the IC-GN algorithm, largely due to the integer-pixel displacement calculation; sub-pixel displacement computation only accounted for approximately 2.7% with 41 pixel subsets. Computational efficiency could be further enhanced if a faster integer-pixel displacement calculation method was developed or parallel processing was incorporated.