Papers by Author: Fusahito Yoshida

Abstract: In recent years, warm forming of magnesium sheets has been investigated by many researchers since the ductility of the sheets becomes considerably higher due to low CRSS (critical resolved shear stress) at high temperatures (e.g., [1]-[3]). In the present research, the springback of AZ31 magnesium sheet was investigated by performing a draw-bending experiment at several temperatures from 20 °C (room temperature) to 200 °C at drawing speeds ranging from 0.01 to 1.0 mm/s. From the experiment, it was found that the springback was remarkably reduced at 200 °C, especially at a low forming speed, since the flow stress was very low under such a forming condition, and furthermore, the stress relaxation effect was dominant. The effects of temperature and forming speed on springback were discussed.

Abstract: Warm temperature biaxial stretching equipment was developed in order to observe equi-plastic work locus at elevated temperature. A thermostatic bath is installed in the conventional biaxial stretching machine, and a heat gun and an electric heater in the bath enable to heat the specimen up to 260°C. Since flow stress is affected by strain rate at warm temperature, the equipment enables stress ratio and strain rate controls at the same time. Equi-plastic work loci of AA5182-O was obtained by the developed apparatus at R.T., 170 and 260°C, and temperature dependence on biaxial deformation was discussed.

Abstract: This paper proposes a cyclic plasticity model to describe the closure of a cyclic stress-strain hysteresis loop based on the Y-U model. In this model, the backstress moves in a cyclic memory surface following a newly proposed kinematic hardening law. For this model just the same Y-U parameters can be used, and no additional material parameters are needed. By using a supplementary rule, this model is also able to describe ratcheting.

Abstract: During the last few decades, the enhancement of prediction capability of the sheet metal forming have been increasing dramatically. High accurate yield criteria and wokhardening model (especially, non-linear kinematic hardening model) have a great importance for the prediction of the final shapes of sheet metal. However, the predicted springback accuracy of aluminum alloy sheet metal is not still good due to their complicated plastic deformation behaviors.In the present research, the springback deformation of aluminum alloy sheet metals were investigated by finite element calculation with consideration of initial anisotropy and the Bauschinger effect. In order to examine the effect of the initial and deformation induced anisotropy on the springback deformation, several types of high accurate yield function and hardening rules are utilized in the present research. The calculated springback by Yoshida 6th yield function [1] and Yoshida-Uemori model [2] shows an excellent agreement with the corresponding experimental data, while the other models underestimate the springback.

Abstract: In this study, the circular hole expansion process of high-strength steel sheet is numerically simulated using FE analysis with Hill48 quadratic, Gotoh’s fourth order, Yld2000-2d and Yoshida’s sixth order polynomial yield function. The effects of anisotropic yield functions on local reduction of thickness are evaluated. The thickness distribution around the circular hole edge at just before necking depends on the initial hole diameter. When the initial hole diameter is relative large, the simulation results give almost same thickness distribution among different yield functions. While the initial hole is relative small, individual characteristics of yield function becomes clear and the sixth order yield function gives the best prediction.

Abstract: In this study, the effect of the material anisotropies of hot-rolled high-strength steel sheet on localized deformation behavior in hole expansion test has been investigated experimentally. First, the hole expansion test with the circular hole has been conducted to investigate the effect of anisotropies of material properties on the localized deformation behavior around the hole edge. Next, the hole expansion test with the oval hole has been conducted to investigate the effect of the major axis direction of the oval hole on the localized deformation behavior around the hole edge. As a result, it was clarified that the effect of anisotropies of r-value and n-value on the localized deformation behavior is strong, especially the anisotropy of n-value.

Abstract: Springback of a high strength steel (HSS) sheet of 980 MPa grade was investigated at elevated temperatures ranging from room temperature to 973 K. From U-and V-bending experiments it was found that springback was decreased with increasing temperature at temperatures of above 573 K. Furthermore, springback was decreased with punch-holding time because of stress relaxation. In this work, the stress relaxation behavior of the steel was experimentally measured. By using an elasto-vicoplasticity model, the stress relaxation was described, and its effect on the springback of sheet metals in warm forming was discussed theoretically.

Abstract: The present study aims to determine stress-strain curves at large strains of sheet metals under the uniaxial stress state by using the in-plane stretch-bending test. The combined Swift-Voce model, which describes the large-strain work-hardening of materials by means of a weighting coefficient μ, was used for FE simulation of the stretch-bending. The coefficient μ was determined by minimizing the difference in punch stroke vs. bending strain responses between the experimental data and the corresponding experimental results. By using this inverse approach, stress-strain curves of two levels of high-strength steel sheets of a precipitation hardening type, 590R and 780R, in three sheet directions (0, 45 and 90^o from rolling direction), were determined.

Abstract: Springback is one of the most serious problems in high-strength steel-sheet forming to produce automotive body parts. A springback-root-cause analysis method was developed to identify the areas of stresses at the bottom dead point, which are the most influential in springback. The analysis method of using springback driving stresses, that is, the difference between stresses before and after springback, is more accurate to eliminate the effect of the residual stresses on springback. This analysis method was applied to both a simple model and the forming of an actual part to verify this analysis method. The areas of stresses that have a major impact on springback were identified. A countermeasure against the actual part springback based on this result was devised and it was clarified that the countermeasure is effective on the springback.

Abstract: A new springback cause analysis method was developed for sheet press forming with measured 3-dimentional shapes of dies and a formed part. In conventional springback cause analysis, a stress distribution is calculated by elasto-plastic FEM analysis of press forming. The newly developed method can be directly applied to a press formed part and its forming dies: an elastic FEM analysis of press forming is conducted using measured shapes of both a pressed part and forming dies, and a stress distribution which causes springback is obtained; and then a springback cause analysis is carried out based on the stress distribution. This calculation method is easier than that of conventional press forming simulation because plastic material properties and friction parameters are not required in its procedure. The philosophy of the new method and an example of application to an automotive part are described in this paper.