Inferring Post-Necking Strain Hardening Behavior of Sheets by a Combination of Continuous Bending Under Tension Testing and Finite Element Modeling

Abstract

This paper presents a combined experimental and simulation approach to identify post-necking hardening behavior of ductile sheet metal. The method is based on matching a measured load-displacement curve from a continuous bending under tension (CBT) test with the curve simulated using the finite element method (FEM), while adjusting an input flow stress curve into the FEM. The CBT test depletes ductility uniformly throughout the gauge section of a tested sheet, and thus, stretches the sheet far beyond the point of maximum uniform strain in a simple tension (ST) test. Having the extended load-displacement curve, the calibrated flow stress curve is extrapolated beyond the point of necking. The method is used to identify the post-necking hardening behavior of an aluminum alloy, AA6022-T4, and two dual-phase (DP) steels, DP 980 and DP 1180. One measured load-displacement curve is used for the identification of a flow stress curve per material, and then the flow stress curve is used to simulate two additional measured load-displacement curves per material for verification. The predictions demonstrate the utility of the developed CBT-FEM methodology for inferring the post-necking strain hardening behavior of sheets. Furthermore, the results for AA6022-T4 are compared with the hydraulic bulge test data. Unlike the hydraulic bulge test, the CBT-FEM method can predict increasing as well as decreasing anisotropic hardening rate in the post-necking regime. The latter is associated with probing stage IV hardening. The proposed methodology along with the results and these key advantages are presented and discussed in this paper.