Aluminum Alloy Sheet-Forming Limit Curve Prediction Based on Original Measured Stress-Strain Data and Its Application in Stretch-Forming Process

(A) new method, by directly utilizing original measured data (OMD) of the stress-strain relation in the Marciniak-Kuczynski (M-K) model, was proposed to predict the forming limit curve (FLC) of an aluminum alloy sheet. In the groove zone of the M-K model, by establishing the relations of the equivalent strain increment, the ratio of shear stress to the first principle stress and the ratio of the second principle stress to the first principle stress, the iterative formula was established and solved. The equations of theoretical forming limits were derived in detail by using the OMD of the stress-strain relation. The stretching specimens of aluminum alloy 6016-T4 were tested and the true stress-strain curve of the material was obtained. Based on the numerical simulations of punch-stretch tests, the optimized specimens' shape and test scheme were determined, and the tests for FLC were carried out. The FLC predicted by the proposed method was more consistent with the experimental results of FLC by comparing the theoretical FLCs based on OMD of the stress-strain relation and of that based on traditional power function. In addition, the influences of anisotropic parameter and groove angle on FLCs were analyzed. Finally, the FLC calculated by the proposed method was applied to analyze sheet formability in the stretch-forming process, and the predicted results of FLC were verified by numerical simulations and experiments. The fracture tendency of the formed parts can be visualized in the forming limit diagram (FLD), which has certain guiding significance for fracture judgment in the sheet-forming process.