| Summary: | 博士 === 國立臺灣大學 === 機械工程學研究所 === 100 === Advanced high strength steel sheets have been widely used in automobile structural parts due to their high strength. However, the technical difficulties are experienced in the stamping of the advanced high strength steel sheets, such as the presence of springback, sidewall curl, and distortion. The finite element analysis also could not render satisfactory results in the springback prediction for the advanced high strength steel sheets even with the optimum simulation parameters adopted. Therefore, in order to examine the forming characteristics of advanced high strength steel sheets and enhance the accuracy of the finite element analysis in springback prediction, the study of material model becomes necessary for the forming of advanced high strength steel sheets.
The material model discussed in this thesis included work hardening rule and yield criterion. In the aspects of work hardening rule, the Bauschinger effect exhibited in the advanced high strength steel sheets was examined by conducting cyclic tension-compression tests with a novel constraint jig developed in the present study. The constraint jig could prevent the sheet specimen from being buckled during the compression test. The finite element analysis was also performed to derive the various characteristic dimensions of the constraint jig to make it applicable to cyclic tension-compression tests with different sheet thicknesses. From the results of repeated tension-compression deformation tests on sheet specimens with different strengths, the Bauschinger effect of advanced high strength steel sheets was confirmed to be indeed significant, and their cyclic hardening characteristics were difference from those of low strength steel sheets. It is also found from the test results that the elastic modulus of steel sheets shows a nonlinear change during the reverse loading path that may affect the springback of sheet metal forming. The microstructure observation made in the present study reveals that the coefficient of dislocation density change during the cyclic tension-compression deformation had the same changing trend as that of flow stress of the sheet metal.
In order to apply the tension-compression test results to the finite element simulations, the material parameters adopted in the Yoshida,-Uemori model that takes the Bauschinger effect into the work hardening rule were constructed. The simulation results showed that during the forming process, if the sheet metals were subjected to cyclic tension-compression deformation, the material model with Bauschinger effect considered was found capable of improving the accuracy of springback prediction significantly.
In yield criteria aspects, in order to explore the best description on yield criteria of plastic deformation of advanced high strength steel sheets, as well as the relationship between yield criteria and material anisotropy, a set of biaxial tension mechanism was especially designed on the clamping jig for the uniaxial tensile testing machine by this thesis. In the design of biaxial tensile mechanism, a finite element analysis was used by this study to conduct a simulation analysis and was successful to design the mechanical drive and two-axis tensile stress ratio, as well as calculated the coefficient of friction during the mechanism slide motion. Meanwhile, the finite element analysis was used by this study to design the central point of test specimen that found to be the best geometrical shape of the specimen in biaxial principal stress, and the range of such region was sufficient to laminate with two strain gauges to measure the supply changes. From biaxial tensile test results, the Barlat yield criteria was discovered to be more appropriate to describe the plastic deformation behavior of advanced high strength steel sheet (590Y) under the biaxial stress. In addition, the basic carrier and industrial carrier (door beam component of vehicle) were also used by the study to conduct finite element springback simulation analysis and experimental verification. From the simulation analysis and experimental verification results, the Barlat yield criteria used by this study plus the consideration of work hardening criteria of Bauschinger effect had proved to be able to improve the predictive accuracy of simulation of springback of the finite element effectively.
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