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 (like balanced biaxial tension) by theoretical compensation of the strains.
Journal: SAE International Journal of Materials and Manufacturing
Citation: Agha, A., "Effectiveness of 2D Digital Image Correlation in Capturing the Fracture Behavior of Sheet Metal Alloys," SAE Int. J. Mater. Manf. 16(2):2023, https://doi.org/10.4271/05-16-02-0009.
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One of the main technical challenges faced in the design and development of stamping dies and manufacturing is the complex springback response of sheet metal alloys. Springback is the elastic recovery in the material when unloaded after the forming operation. And the introduction of advanced materials like high-strength aluminum alloys and advanced high-strength steels (AHSS) exhibiting high strength and ductility makes it even more challenging. The magnitude of springback is directly proportional to the ratio of flow stress to Young's modulus of the material, which makes it typically high for such high-strength materials. Moreover, the anisotropic behavior of aluminum alloys and the multi-phase microstructures of AHSS result in strong tension-compression asymmetry leading to complex springback behavior.
FADI-AMT participated in the NADDRG 2022 Spring Symposium with a presentation titled: “From Standard to High-Speed DIC: Investigating Rate Effects on Formability and Fracture"
NADDRG 2022 Spring Symposium focused on Digital Image Correlation as applied to sheet metal forming and fracture, and covered an important range of DIC-related topics:
This event was of interest to technical specialists (stamping, modeling), manufacturing and materials engineers, as well as those with less exposure looking to gain knowledge regarding this innovative technology.