Abstract
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.
Highlights
Plastic forming of sheet metal keeps an important role in modern industrial production and is widely applied to the aerospace industry, automobile manufacturing, equipment manufacturing, and other fields [1,2,3,4]
◦, and 90◦ with the rolling direction, tensile tests were carriedand out safety in the directions strainUniaxial data distributed in the fracture zones, andofvery few strain data were located in the and three sets of true stress–strain curves with little differences were obtained
Model was proposed to predict the forming limit curve (FLC) of an aluminum alloy sheet, and the predicted FLC in this model was proposed to predict the FLC of an aluminum alloy sheet, and the predicted FLC in this method was compared with that of the M–K model based on power functions and punch-stretch tests
Summary
Plastic forming of sheet metal keeps an important role in modern industrial production and is widely applied to the aerospace industry, automobile manufacturing, equipment manufacturing, and other fields [1,2,3,4]. Hou [27] et al used the curve-fitting method to calculate the strain hardening exponent n and hardening exponent m of Al5083-O aluminum alloy sheet and predicted the FLC of the material. Barata Da Rocha [29] carried on the FLC prediction of anisotropic sheets by power approximations, and found that the initial groove angle had a significant effect on the forming limits under different strain paths. The FLC was used to analyze the stretch-forming process of sheet metal, the related tensile test for stress–strain curve and the punch-stretch experiment for FLC were carried out under the temperature and strain rate similar to those of the stretch-forming process. The predicted FLC was applied to analyze the fracture of the curved part in the stretch-forming process, which provided the theoretical basis for judging the fracture defects generated in the stretch-forming process
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