Research On Cyclic Deformation Behaviour Of Sheet Flowing Through Drawbead And Its Subsequent Forming Performance

When sheet flowing through drawbead it is bearing cyclic bending, unload and reverse bending as well as work hardening and Bauschinger Effect. At the same time the thickness of sheet is reduced because of the duplicate bending. So that the subsequent forming performance of the sheet changed a lot after flowing through drawbead. The changes determines whether the forming will go well or not .So the changes of subsequent forming performance are important factors to be taken account when design drawbead.By analyzing the cyclic deformation features of sheet enduring bending and reverse bending when flowing through drawbead proposed two mathematical models in the paper based on HILL yield criterion including kinematic hardening rule and combined hardening rule. In both models the anisotropy, work hardening and Bauschinger Effect of the sheet are considered.Analyzed the deformation features of sheet flowing through semi-cycle drawbead by taking FEM numerical simulation and adopting three hardening models which are isotropic hardening rule based on Mises yield criterion, kinematic hardening rule based on HILL yield criterion and mixture hardening rule based on HILL yield criterion. As a result the tangential stress-strain cyclic relation curve is obtained. And investigated the influence of Bauschinger Effect on the tangential stress-strain cyclic relation and the influence of the neural layer moving inward on the tangential stress of the middle layer of different models. The comparison of the results of simulation and experiment showed that the combined hardening model has the highest precision. The simulation results also showed that the sheet bearing bending and reverse bending twice when flowing through semi-cycle drawbead , bearing bending and reverse bending three times when flowing through trapeziform drawbead and the tangential modulus of different loading branches reduces greatly at the second loading branch by analyzing the stress-strain curve.The influence of structure parameters of drawbead on the pre-deformation of sheet was studied by the pre-deformation test and pointed that the pre-strain increased little when the height of drawbead exceed 10. The curve of sheet subsequent forming performance was obtained from the test of subsequent forming performance. And the influence of structure parameters on the features and the variation law of the subsequent forming performance curve was analyzed. Then the relation between the evolution of sheet subsequent performance curve and sheet pre-deformation was studied. Research indicates that the subsequent yield strength of the three kinds of material in the experiment is bigger than 1.5 times the yield strength because of the hardening when sheet flowing through drawbead. When the equivalent pre-strain is small the subsequent yield strength is less than the limit of intensity and subsequent elongation is less than the elongation of sheet. When the equivalent pre-strain exceeded 0.1 the subsequent elongation is less than 20% and no hardening so the hardening index n and plastic strain ratio r couldn't be determined.Taken orthogonal experimental investigation on the influence of the structure parameters of semi-cycle drawbead and trapeziform drawbead on resistance of drawbead ,sheet pre-deformation and sheet subsequent forming performance. Research showed that the resistance, the pre-deformation and the subsequent yield strength and subsequent limit of intensity increase as the height of the drawbead increase, however the subsequent elongation decrease. As the radii of the drawbead and groove increase, the indexes above changed in the reverse direction. And different settings of parameters of drawbead can produce same resistance, however different sheet pre-deformation and subsequent forming performance. As the equivalent pre-strain increase both the subsequent yield strength and limit of intensity increase, however the ratio of them is close to 1 and the subsequent elongation decrease exponentially.The relations between sheet pre-deformation, subsequent forming performance and the structure parameters of drawbead were analyzed. Then the principles of design and adjustment for the structure parameters of semi-cycle and trapeziform drawbead were proposed based on an overall consideration of sheet resistance and sheet subsequent forming performance.