Effects of Incremental Values on the Computation Accuracy of Springback in Static Explicit FEM Analysis of Stretch-Bending Process

(Received on February 2, 1995)

Toshihiko KUWABARA, Hidekazu OKA, and Susumu TAKAHASHI

The magnitude of springback of a sheet metal subjected to a stretch-bending process using a cylindrical die is predicted with static explicit FEM and is compared with experimental results. The effects of incremental values on the computation accuracy of springback are examined. The results obtained in this study are as follows. 1) In order to accurately predict the magnitude of springback, it is crucial to reduce the imbalance of moment BM, which is retained at the end of the bending process, to as small a value as possible. BM can be reduced by decreasing the computation error of the elongation L of the sheet strip obtained in the initial stretching process. 2) In order to reduce the computation error of L, it is suggested that the incremental values used in the initial stretching process, i.e., the increment of equivalent stress B in the elastic region and the increment of equivalent plastic strain B , should be as small as possible. 3) In the present analysis, the optimum incremental values that give accurate springback prediction are as follows'. B =0.0002 and B =O.l (y: yield stress) in the initial stretching process-I B =0.002 and the increment of rotating angle Aw is 0.2' in the bending process; A@' =0.002 and Aw=0.01' in the unloading process.

Keywords:stretch-bending process, static explicit elastic-plastic FEM, optimum incremental values, springback, A5182-0



Plastic Deformation Analyses and Simulation System Using Finite Element Polycrystal Model

(Received on March 31, 1995)

Hajime MOTOHASHI, Hiroshi TAKAHASHI, and Shin TSUCHIDA

The aim of this work is to develop a FEM simulation system which relates crystallographic textures to plastic deformation analysis. The employed model is a finite-element polycrystal model (FEPM) proposed by Takahashi where each element in FEM is assumed to be a crystal having different orientation. As example of texture development, deformation textures due to strip rolling are predicted for FCC metal. For a pre-textured sheet, crystal orientations are assigned on the basis of the pole figures obtained by X-ray diffraction. Then the plastic anisotropy can be predicted in terms of flow-stress or -value. Moreover, prediction of ears caused by deep drawing is also possible.

Keywords:finite element method, rolling, deep drawing, texture, anisotropy, polycrystal model



Evaluation of Interface Heat Transfer Coefficient for Thermal Analysis in Forging

(Received on March 31, 1995)

Koukichi NAKANISHI, Fumio NONOYAMA, Masatoshi SAWAMURA and Atsushi DANNO

The accuracy of numerical simulation in forging processes depends on physical parameters used in the analysis. In particular, the interface heat transfer coefficient between workpiece and die is very important for the thermal analysis in forging processes. In this study, a practical method is proposed to measure the interface heat transfer coefficient quantitatively. The interface heat transfer coefficient can be determined by comparing calibration curves with temperature measurements in a simple compression test. The calibration curves are derived from the thermal analysis using a finite difference method and the temperature near the die surface is measured with thermocouples. The effects of lubricants (graphite, B 203)' initial weight of lubricant, temperature, and contact pressure on the interface heat transfer coefficient are evaluated. The interface heat transfer coefficient obtained by this method is applied to a rigid plastic finite element analysis coupled with thermal analysis of a warm forward rod extrusion. The die temperature obtained by numerical analysis shows good agreement with that obtained from the experiment.

Keywords:orging, extrusion, numerical analysis, experimental analysis, FDM, rigid plastic FEM



Extrusion of Rectangular Sections, Angles and Channels
-Three-Dimensional Analysis by Rigid-Plastic FEM -

(Received on March 31, 1995)

Manabu KIUCHI, jun YANAGIMOTO, and Victor MENDOZA

Results of a study on the characteristics of metal flow during three-dimensional extrusion are presented. Numerical simulations are performed using a newly developed 3-D rigid-plastic FEM code. The extrusion processes of products with rectangular, angular and channelcross section through flat-faced dies are chosen as computational examples to demonstrate the simulation capability of the code. Values of average extrusion pressure and metal flow features are obtained as a function of the geometrical complexity of extruded product cross section and eccentricity of the die. Presented results are discussed mostly by intuitive reasoning and comparison with other results is not presented. Future work will include the extrusion simulation of more complex cross sections including non-symmetric shapes and the study of the velocity field in the bearing section.

Keywords:FEM simulation, three-dimensional extrusion, rectangular section, angle, channel.



Determination of Flow Stress by Inverse Analysis Using FEM

(Received on April 4, 1995)

Masahiro MICHINO, Mitsuyuki TANAKA, and Takeshi KITAOKA

An inverse method, named FC-method, has been proposed to obtain flow stress of material beyond the strain limit of measurement by conventional methods. This procedure employs, as the object function of inverse analysis, the balance of external and internal powers which are supplied to a material during a deformation process and used for the deformation, respectively. The balance is an explicit scalar functional of flow stress which is a function of some unknown material constants. Friction coefficient between material and tools for deformation is also one of the unknown constants. Optimizing the balance is equivalent to solving the stationary problem of the functional and the solution gives the values of material constants and the friction coefficient simultaneously. Ring compression of commercial pure aluminum was selected, as an example, to apply FC-method and the results demonstrated that the accuracy of this procedure is sufficient for practical use.

Keywords: material testing, compression test, FEM, inverse method, flow stress, friction coefficient,@non-linear least squares problem, optimization, stationary problem, scalar functional.



Penalty Method Contact-Friction Algorithm and Expression of Tools by B-Spline Function
-Penalty Method Contact Algorithm and Rigid Plastic FEM I

(Received on April 25, 1995)

Toru SHIMIZU, Toshio SANO

FEM analysis is becoming an important tool in the metal forming industry. Many structure analysis codes are applied to the simulation of metal forming processes. In these codes, contact treatment between tools and workpiece is an important problem. We developed a penalty-method contact algorithm which can treat contact and friction problems at the same time. This algorithm is incorporated into a 2-dimensional rigid plastic FEM code. In this code, the tool surface is expressed by a B-spline function, which can describe a smooth surface in terms of a few control vertices. The penalty method is applied to this tool surface. Using this code, the forging process is analyzed, and efficiency of the contact algorithm and the method of expression by B-spline function is confirmed.

Keywords:rigid plastic FEM, forging, contact algorithm, faction, B-spline function.



Volumetrically Elastic and Deviatorically Rigid-Plastic Finite Element Method with Incorporation of Thermal Strain

(Received on May 11, 1995)

Yasuto YOKOUCHI, Yong-Ming GUO, and Hideo SUZUKI

The effects of thermal strain are taken into consideration in the volumetrically elastic and deviatorically rigid-plastic finite element method(FEM) which was previously proposed by the authors. It is shown that the formulation of this method is easier than that of other rigid-plastic FEMs where the effects of thermal strain are also incorporated. The present method is applied to hot upsetting of a ring. Comparisons are made between results obtained with and without consideration of the thermal strain. From the numerical example, it is concluded that 1) the thermal strain strongly affects the distribution of the volumetric strain rate, 2) it weakly affects the distribution of other components of strain rate or stress, and 3) relative differences in punch load and dimensions of unconstrained portion of the work are of the same order as the volumetric thermal strain roughly estimated from the average change of temperature.

Keywords:rigid-plastic FEM, volumetrically elastic material, thermal strain, upsetting, numerical analysis, thermal analysis