Extended Geometry Sequence for Evaluating Sheet Formability Under Uniaxial Loading

This work introduces an extended diagonal-cruciform (EDC) geometry sequence for evaluating the formability of sheet metal using standard uniaxial testing machines. The approach is advantageous compared to other testing setups, such as those by Marciniak and Nakajima, which require specialized equipment. Building on the original diagonal-cruciform (DC) geometry, the paper utilizes finite element simulations, and experimental testing with digital image correlation (DIC) on AA6082-O aluminum sheets to confirm that the EDC geometry sequence can generate a wide range of strain loading paths in the first and second quadrants of principal strain space, ranging from pure shear to near equibiaxial tension. The ability of the EDC geometry sequence to identify the onset of diffuse and localized necking through the strain acceleration and the strain-rate peak methods, respectively, and the onset of fracture through thickness reduction measurements and determination of the gauge length strains is also comprehensively analyzed. Overall, the EDC geometry sequence provides a robust, cost-effective, and sustainable alternative for determining the formability limits in principal strain space and in the effective strain versus stress triaxiality space. By combining innovative specimen design with accessible experimental procedures, the new proposed EDC geometry sequence broadens the adoption of biaxial testing in laboratories equipped with only standard uniaxial machines, supporting advances in industrial process design and academic research in sheet metal forming.