An alternative approach for improving DIC by using out-of-plane displacement information

Abstract

A new approach to perform 2D Digital Image Correlation (DIC) on objects that are subjected to large out-of-plane displacements is presented. It employs out-of-plane information to distort the speckle images of the reference and the deformed surface, increasing their similarity and therefore their sub-set cross-correlation peaks. The new approach is implemented using the combined Fringe Projection and 2D-DIC technique on two experiments: The buckling of an initial flat plate, and the rotation of the reference surface towards the camera from 0° to 60°. In both cases large speckle distortions are produced due to two issues, the lens magnification and the surface tilting. The first speckle distortion is applied to the deformed image to perform the lens magnification correction, and the second speckle distortion is applied to the reference image. The obtained distorted speckle images become very similar, allowing to successfully compute corresponding pixels between both images in a simple way (i.e. without nonlinear minimization processes). For this propose, an analytical iterative equation is derived to determine pixel displacements due to out-of-plane surface tilting. The resulting displacements are compared with those obtained by the conventional procedure, applying 2D-DIC (using open source Ncorr software) on the non-distorted speckle images and then making the lens magnification correction. The novel procedure proposed makes it possible to successfully find the corresponding pixels at the reference surface for out-of-plane rotations of 55° and 60° for which Ncorr failed. 3D-DIC (using commercial VIC software) is also employed to validate the results.