A Study of Bauschinger Effect in Stamping of Advanced High Strength Steel Sheets
Date Issued
2011
Date
2011
Author(s)
Hong, Ying-Jhih
Abstract
The advanced high strength steel has been widely applied to the car body structural components. However, the high flow stress makes the sheet metal forming process even more difficult. In addition to the defects of fracture and wrinkling, which are commonly present in the stamping of conventional steel sheet, the occurrence of springback, side-wall curl and distortion is the main issue to be solved. In order to cope with this dilemma, the finite element analysis was employed to help the tooling design aiming to eliminate the springback in the sheet forming process. The efforts were endeavored to establish the optimum simulation parameters, such as element size, number of integration point, and punch velocity. However, the accuracy in the prediction of springback is yet to be improved even with the optimum simulation parameters adopted. Hence, the material model including the yield criteria and the work hardening rules has been considered as the other possible reasons that may cause the inaccuracy of the finite element simulations in the sheet forming of advanced high strength steel.
In the present study, the Bauschinger effect that induces anisotropic work hardening properties was examined by both the experimental approach and the finite element analysis. The tension-compression tests were first conducted for the specimens made of conventional 270 and DP590Y steel sheets with the special experimental apparatus designed by Heng-Kuang Tsai in our lab. The Bauschinger effect was clearly observed in the test results of DP590Y steel sheets rather than those of 270 steel sheets. The test results were then used in the finite element simulations as part of the material properties characterizing the kinematic hardening behavior of the sheet metal. Both the Hill 48 yield criterion with isotropic hardening rule and Yoshida-Uemori model (Y-U model) were adopted to simulate the V-bend, U-bend, and U-hat bend of DP590Y steel sheets. The simulation results were compared with the experimental data obtained from the tests conducted in our lab. It is found that the simulation results with the Bauschinger effect considered are more consistent with the experimental data either in springback or in side wall curl than those of simulations using an isotropic hardening rule. The springback occurs in the stamping of an automotive structural part made of DP590Y steel sheets were also examined in the present study by both the finite element analysis and the actual production process. It is noted that the deformation mechanism of these automotive parts is significantly affected by the Bauschinger phenomenon as the advanced high strength steel is adopted. The comparison of the finite element simulation results and the actual production parts also reveals that the simulation results with the use of the Yoshida-Uemori model renders to a more accurate prediction in springback in the stamping of the DP590Y steel sheets. It thus concludes that the Bauschinger effect plays an important role in the stamping of advanced high strength steel sheets and the employment of the kinematic hardening rule in the finite element simulations is necessary.
Subjects
Bauschinger effect
advanced high strength steel
stamping
springback
kinematic hardening rule
experiment
finite element analysis
Type
thesis
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