Displacement Measurement Errors in Digital Image Correlation Due to Displacement Mapping Function

In the classic subset-based local digital image correlation (DIC) technique, the displacement measurement errors due to the displacement mapping function have been an important factor to improve the measurement accuracy. In general, the first-order displacement mapping function is broadly used. But, in the real experiments, the order of the deformation of the test sample is higher than that of the used displacement mapping function. So the errors due to the undermatched displacement mapping functions are existent. Although the random errors due to the overmatched shape function have been studied, the root-mean square errors (RMSEs) due to the matched or mismatched displacement mapping function are not studied. In this paper, the RMSEs of the measured displacements due to the matched or mismatched displacement mapping function are investigated experimentally by employing the simulated and real experimental speckle patterns. Moreover, the relationships between the RMSE and the subset size or the image noise are thoroughly examined. Finally, the practical implementation of DIC is recommended in terms of the subset size and the image noise.