Out-of-Plane motion evaluation and correction in 2D DIC

2D and stereo Digital Image Correlation (DIC) allows to retrieve complex displacement and strain fields on a specimen’s surface. Although 2D DIC is strongly affected by out-of-plane motions, in many situations, it is preferred over stereo DIC because of its ease to use and because only one camera is required. The out-of-plane movements can be ascribed mainly to three causes: the camera positioning, the imperfections of the used test device, and the camera selfheating. These effects gain importance when the distance between the camera and the specimen is reduced. The positioning of the camera aims to have its optical axis perfectly perpendicular to the specimen to observe. Nevertheless small but effective misalignments can easily happen even if suitable devices are used for the alignment. This contribution concerns the experimental evaluation of these movements considering a cyclic uni-axial tensile test performed on an aluminium specimen. The study is particularly focused to the out-of-plane motions that occur at every cycle because of the tensile bench, which are the more critical ones. Finally a compensation method, based on fixed compensation plates, is presented. The method allows to properly correct the data coming from a 2D DIC set-up.