Masato Takamura

Investigation on Twisting and Side Wall Opening Occurring in Curved Hat Channel Products Made of High Strength Steel Sheets

High strength steel sheets are becoming increasingly important for the weight reduction of automotive bodies to meet the requirements for reduced environmental impact. However, dimensional defects resulting from springback are serious issues, and effective methods of predicting and reducing such defects are necessary. In this study, we numerically and experimentally analyzed the mechanisms of dimensional inaccuracies caused by springback occurring in curved hat channel deep drawing products. The analysis was based on the static explicit FEM software “TP-STRUCT” (the solver part is known as “STAMP3D”). The results of the experiments and simulations similarly show that the twist angle is positive (right-hand system) when the drawing height is relatively large. We calculated the twist torque around the longitudinal axis using the stress distributions obtained by FE analysis. Through the investigation of twist torque and its transition during the drawing and die removal processes, we found that the negative torque generated by side wall opening occurring in the die removal process is the dominant factor of the positive twist. Knowing such mechanisms of twist in cases with a relatively large drawing height, we attempted to explore methods of reducing side wall opening by giving the side wall a stepped shape with the eventual aim of reducing twist. Consequently, we concluded that the stepped shape on the side wall has marked effects of reducing side wall opening, mainly through the elimination of bending-unbending effects on die shoulders, which was verified by observing the stress distribution obtained by FE analysis.

Reduction of Springback in Hat Channel with High-Strength Steel Sheetby Stroke Returning Deep Drawing

This study investigated the twisting phenomenon in curved hat channel products made of dual-phase 980-MPa-class high-tensile-strength steel sheets. The stroke returning deep drawing (SRDD) method was proposed to deal with twisting. In this new method, after the punch reaches the bottom dead point, it returns to a certain drawing height without the blank holder being removed. With the application of the SRDD method, twisting hardly occurred, but sidewall curl increased. A two-step SRDD was then proposed to reduce the sidewall curl of SRDD products. In the two-step SRDD method, a stroke returning process is carried out in two steps under different conditions. The results showed that the two-step SRDD method reduced the sidewall curl and twist simultaneously.

Effect of Tool Modeling Accuracy on Sheet Metal Forming Simulation

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.

Sheet Forming Simulation Using a Static FEM Program and Considering the Elastic Deformation of Tools

In sheet forming simulations using the finite‐element method (FEM), the elastic deformation of tools during the forming process can play an important role, particularly when accurate analysis is required to predict complex defects such as springback phenomena. However, in simulations of stamping parts with complex geometries, such as automotive body parts, it would be necessary to use hundreds of thousands or even more elements for a tool model to take into account its elastic deformation within a coupled FEM analysis. Therefore, in most simulations based on incremental FEM, tools are regarded as non‐deformable bodies to avoid excessive computation times. In this study, the authors propose an efficient algorithm to deal with the contact between the sheet and the deformable tools, which is specialized for the coupling of the effects between nonlinear‐elastoplastic and linear‐elastic bodies in the framework of a static explicit time integration scheme. Stamping experiments of square cup deep drawing with a ...

The influence of strain rate and strain on the behavior of stress relaxation in 980 MPa-grade dual phase steel sheets

The specimens (980 MPa-grade dual phase steel sheets) were stretched until the pre-defined strain was obtained. Then the specimens were held at the pre-defined strain and measured the change of stress durng holding. We investigated the effects of strain rate and strain at the starting time of holding and whether the stress change during holding could be described by Krempl model. The following results were obtained. First, the stress drop increased with increase of strain rate and the holding time. On the other hand, the stress drop was not affected by strain change at the starting time of holding. Second, initial stress relaxation rate increased with increase of strain rate. However, this strain rate dependency to stress relaxation rate diminished as the holding time became long enough roughly more than 100 s. Third, the stress change during holding obtained by Krempl model accurately agreed with experimental result. It was found that the stress change during holding could be well described by using Krempl model. This suggests that dislocation moves viscously. In addition, the strain rate dependency on stress change during holding could be described by change of the parameter A.

Investigation on Effect of Slide Motion Control on Stamping of High Strength Steel Sheets

High strength steel sheets are increasingly used in automotive body parts with the aim of weight reduction, but their use urgently requires further improvement in sheet forming technology to overcome difficulties such as poor formability, dimensional inaccuracy, etc. On the other hand, servo press facilities are becoming increasingly used in industry and many attempts are being made to bring out their characteristic features for enhancing the formability of high strength steel sheets. Although some of these attempts have been successful in finding the advantages of servo presses for improving formability and dimensional accuracy, the mechanisms of such improvements have yet to be clarified in conjunction with the mechanical properties of the materials used. One of the most remarkable features of the servo press lies in its flexibility in slide motion control. It is thus effective to investigate the relevance of strain rate sensitivity of a material to the mechanism of improvement in formability enabled by the flexible slide motion of the servo press. However, very few studies have been carried out with material testing, material modeling, and numerical analyses combined with experimental verifications. In this study, Norton’s creep model was implemented in the FEM solver in order to take into account visco-elasto-plastic deformation including stress relaxation behavior. Parameters for the visco-elasto-plastic material model were identified through physical measurements and FEM simulations of uniaxial tension and crosshead displacement dwell tests, as shown in Fig. 1. The identified material model was applied to sheet forming simulations of an automotive body part and validity of the model was examined by comparing with stamping experiments using a servo press with a variety of slide motions. Numerical results with the identified material model showed the same tendency with respect to the slide motions as the experimental results. Stress relaxation behavior was found to be an important factor for improving formability enabled by modifying the slide motion.