Effectiveness of 2D Digital Image Correlation in Capturing the Fracture Behavior of Sheet Metal Alloys

It is a consensus in academia and the industry that 2D digital image correlation (2D-DIC) is inferior to a stereo DIC for high-accuracy material testing applications. It has been theoretically established by previous researchers that the 2D-DIC measurements are prone to errors due to the inability of the technique to capture the out-of-plane motion/rotation and the calibration errors due to lens distortion. Despite these flaws, 2D-DIC is still widely used in several applications involving high accuracy and precision, for example studying the fracture behavior of sheet metal alloys. It is, therefore, necessary to understand and quantify the measurement errors induced in the 2D-DIC measurements. In this light, the presented work attempts to evaluate the effectiveness of 2D-DIC in mechanical testing required for the generation of fracture strain vs. triaxiality curve for sheet metal. This work presents a direct comparison of fracture strains obtained by 2D-DIC and stereo DIC for four loading conditions (uniaxial tension, plane strain, shear, and balanced biaxial tension) on two materials with very diverse mechanical and fracture properties—CR4 and DP800 steel. The comparisons are done for full-field strain contours, fracture strains, and strain paths/triaxialities generated using the two DIC systems. A simple technique is proposed to compensate for the effects of out-of-plane motion in the 2D measurements. It is shown that 2D-DIC can capture the material deformation with sufficient accuracy not only for planar specimens but also for certain scenarios involving out-of-plane motion (such as balanced biaxial tension) by theoretical compensation of the strains.