Through-thickness strain field measurement using the mirror-assisted multi view digital image correlation

A low-cost, easy-to-implement and effective mirror-assisted multi-view digital image correlation (multi-view DIC) technique, which uses only a regular two-camera stereo-DIC system and two planar reflective mirrors, is proposed for the full-field through-thickness strain (TTS) measurement of sheet samples. The front and rear surfaces of a plate-shaped sample are reflected by the mirrors and form two virtual surfaces behind the mirrors. The virtual surfaces are simultaneously captured and subsequently reconstructed by the stereo-DIC system. Furthermore, by utilizing the speckle patterns premade on the two mirrors, the 3D shapes of real surfaces can be estimated from the reconstructed virtual surfaces. By subtracting the 3D coordinates of the rear surface from those of the front surface, the full-field thickness distributions and TTS maps of the sample can be calculated. Compared with the regular multi-view DIC, which requires at least four cameras, the proposed technique has the prominent advantages of direct full-field TTS measurement by using only two cameras and simple implementation procedures. Two real experiments, including the full-field TTS measurement of a sheet sample subjected to in-plane translation and the thinning limit estimation of an advanced high-strength steel specimen subjected to uniaxial tensile loading, well validated the efficacy and accuracy of the developed technique.