Two-dimensional digital image correlation (2D-DIC) has the advantages of high computation efficiency and simple experimental setup compared to three-dimensional DIC (3D-DIC). However, 2D-DIC is sensitive to out-of-plane motion, which leads to rather poor strain results. Recently, optical extensometers realized by dual-reflector imaging have been proposed, which can self-compensate the strain errors introduced by out-of-plane motion of the specimen. We will extend strain measurement from an extensometer to full-field deformation measurement. To this end, first, two corresponding areas of interest (AOIs) are created based on the symmetrical axis of the reference image. Second, the displacement and strain fields of these AOIs are computed with a common 2D-DIC algorithm, respectively. Subsequently, high-accuracy deformation results are obtained by taking the average of the deformation of the corresponding calculation points in two AOIs. Two types of uniaxial tensile tests that correspond to uniform and nonuniform strain fields were conducted to validate the feasibility of the proposed self-compensation method. The first experiment indicates that the strain results obtained using the proposed method are in good agreement with those with strain gauges and that the proposed method can achieve higher strain accuracy than 3D-DIC with a small stereo-angle. The results of the second experiment are basically in agreement with those of the ANSYS numerical simulation, which demonstrates the feasibility of the proposed method for the measurement of nonuniform strain fields. Both of the experiments demonstrate that rigid out-of-plane motion will lead to a global strain field error for common 2D-DIC. With no external compensation device, the proposed self-compensation method has the potential in 2D-DIC for accurate strain measurement due to its high level of accuracy.
The strain errors in stereo-digital image correlation (DIC) due to camera calibration were investigated using precisely controlled numerical experiments and real experiments. Three-dimensional rigid body motion tests were conducted to examine the effects of camera calibration on the measured results. For a fully accurate calibration, rigid body motion causes negligible strain errors. However, for inaccurately calibrated camera parameters and a short working distance, rigid body motion will lead to more than 50-μϵ strain errors, which significantly affects the measurement. In practical measurements, it is impossible to obtain a fully accurate calibration; therefore, considerable attention should be focused on attempting to avoid these types of errors, especially for high-accuracy strain measurements. It is necessary to avoid large rigid body motions in both two-dimensional DIC and stereo-DIC.
The bulge test has been utilized extensively in determining the mechanical properties of thin film materials. We develop a bulge-test system that consists of three main components, including an optical system, a loading system, and a control system. A telecentric lens is adopted as an imaging camera in an optical system to provide high imaging quality along with a constant angle of view across the entire field of view. The out-of-plane deformation is obtained by projecting a sinusoidal fringe pattern onto a specimen surface and analyzing recorded images with a digital image correlation algorithm. A least-squares polynomial fitting is presented to solve the problem that the out-of-plane calibration coefficient is not a constant. Experiments were performed to validate the availability and reliability of this proposed bulge-test system in the measurement of a dynamic microscopic deformation of membrane. The measuring range of deformation is from several microns to hundreds of microns with an accuracy of 0.6 μm.
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