Papers by Author: Hirohiko Takuda

Abstract: In the present study, deformation behavior upon two-step loading in a rolled AZ31 Mg alloy sheet was investigated. The experimental procedure was as follows: (1) a sheet was subjected to in-plane compression, (2) small samples were cut from the compressed sheet along various directions, and (3) uniaxial tension was imparted to small samples. The angle between the first and second loading directions was set to either 0, 30, 60, or 90°. During the second loading, a strong in-plane anisotropy occurred in the stress-strain curve: a sigmoidal curve occurred during the second loading for the angles of 0 and 30°, while it did not arise for the angles of 60 and 90°. From microstructural observations, it was presumed that the aforementioned results could be explained in terms of the activity of detwinning.

Abstract: The objective of this study was to understand the work-hardening behavior of a rolled AZ31 magnesium alloy sheet upon reverse loading. Firstly we carried out an in-plane compression-tension test of a rolled AZ31 magnesium alloy sheet with various compressive strains. The sigmoidal curve was exhibited during tension regardless of the amount of compressive strain, but a shape of the curve was clearly different depending on the compressive strain. To understand the mechanism of this difference, a crystal plasticity finite-element simulation was carried out. The simulation result showed that the above difference in the shape was owing to the difference in the activities of slip and twinning systems during tension depending on the compressive strain.

Abstract: In the present study, the deformation behavior of a cast Mg alloy sheet that had random crystallographic orientations was studied both experimentally and numerically. Although the crystallographic orientations were random, the stress-strain curve was asymmetric between tension and compression: the flow stress under tension was higher than that of compression. Moreover, the stress-strain curve exhibited a strain path dependency: a slightly sigmoidal curve occurred under tension following compression, while it did not occur under compression following tension. Clearly, such tendencies were similar to those observed in rolled Mg alloy sheets although the tendencies were less pronounced in the cast Mg alloy sheet. A crystal plasticity finite-element method was used to understand the mechanism of these results. Simulation results showed that the asymmetry and the strain path dependency in the stress-strain curves occurred in the cast Mg alloy sheet because of the asymmetry in the activity of twinning between tension and compression as in the case of rolled Mg alloy sheets.

Abstract: Magnesium (Mg) alloys are the lightest metals that can be used for structural components, and the press forming of Mg alloy sheets has recently attracted attention in automobile and electrical industries. To increase the number of applications of the press forming, it is crucial to understand mechanical properties of Mg alloy sheets. Because Mg alloys are hexagonal close-packed (hcp) materials, mechanical properties of Mg alloy sheets are significantly different from those of conventional structural sheet metals that have cubic structures. Crystal plasticity models can analyze numerically the interaction between mesoscopic crystalline and macroscopic deformation in metals; thus the models are powerful tools to further understand the mechanical properties of Mg alloy sheets. In the present study, the nonlinear response that arose during unloading under in-plane compression of a rolled magnesium alloy sheet was investigated using a crystal-plasticity finite-element method, focusing on the effects of twinning and detwinning. The mechanism that the nonlinear response was more pronounced under in-plane compression than that under in-plane tension was also discussed. In the simulation, a twinning and detwinning model that has originally been proposed by Van Houtte (1978) and recently extended to the detwinning process by the authors [Hama and Takuda, 2012] was employed. From numerical experiments, it was confirmed that, as already pointed out in literature, the activity of detwinning played an important role on the nonlinear response during unloading. On the other hand, it was also found that the basal slip systems could be very active during unloading because of the dispersion of crystal orientations owing to the activity of twinning during loading, which increased the nonlinear response. It was concluded that the nonlinear response during unloading was more pronounced under in-plane compression than that under in-plane tension owing to these two factors that did not present under in-plane tension.

Abstract: This paper describes the effect of tool modeling accuracy on the accuracy of springback simulation in sheet metal forming. Simulations of a two-dimensional draw-bending process are carried out by using the polyhedral tool model and the model whose surface is smoothed by quadratic parametric surfaces proposed by Nagata [1]. It is found that the tool modeling, especially the normal vector accuracy in the present model, plays an important role in the prediction not only of deformation but also of stress distribution. The simulated results show that the tool model based on the so-called Nagata patch enables a more accurate and efficient simulation.