Experimental investigation and constitutive modeling for the hardening behavior of 5754O aluminum alloy sheet under two-stage loading

The two-stage loading tests of 5754O aluminum alloy sheet were carried out. In the first loading stage, the uniaxial tensile tests of big sheets were carried out. Then small specimens were cut off from the pre-strained big sheets along different directions. And in the second loading stage, the uniaxial tensile tests of the small specimens were performed. From the experimental results, it is found that the initial yield stress of each specimen in the second loading stage decreases when the strain path changes. In addition, when the strain path changes the transient effect appeared and no obvious permanent softening was observed. In this study, in order to describe the hardening behavior of 5754O aluminum alloy sheet under two-stage loading, the Chaboche type combined isotropic–kinematic hardening models were adopted with Yld2000-2d and Hill48 as yield functions. It is proven that no permanent softening can be described with Chaboche type model in two-stage loading. Three methods for determining the parameters of the hardening models were developed in order to establish accurate isotropic–kinematic hardening model describing the hardening behavior of 5754O aluminum alloy sheet under two-stage loading. The established constitutive models were implemented into the commercial FEM code ABAQUS as a user material subroutine (UMAT) for numerical simulations. By comparing the experimental and simulated results of the two-stage loading tests, the isotropic–kinematic hardening models describing the hardening behavior of 5754O aluminum alloy sheet under two-stage loading were accurately determined. Also, the influences of the characterization method of Hill48 yield function on the accuracy of the resulting hardening models were discussed. It is also found that the established isotropic–kinematic hardening model describing the hardening behavior under two-stage loading can describe reasonably the springback profile of the three point bending of the pre-strained specimen.