Soo-Ik Oh

Recent development and applications of three-dimensional finite element modeling in bulk forming processes

Abstract In this paper, the methodology of a finite element method (FEM)-based three-dimensional bulk forming modeling program is described using DEFORM™ as an example. The FEM formulations are first reviewed, followed by discussions on the many considerations such as geometry representations, element selections, volume constancy, equation solvers and meshing methods. Since the updated Lagrangian method is employed in the FEM calculation, an automated remeshing procedure must be used so as to continue the simulation when the mesh is severely distorted. Although DEFORM™ was originally developed for metal forming simulation, due to its unique remeshing capability the use of the code has been extended to other manufacturing fields, such as welding, machining and glass forming. To demonstrate the methodology, some successful applications of the program to various forming processes such as ingot breakdown, extrusion and orbital forging are presented in this paper.

Design sensitivity analysis and optimization of the hydroforming process

Abstract Tube hydroforming has recently been drawing the attention of the automotive industries due to its several advantages over conventional methods. It can produce a wide range of products such as sub-frames, engine cradles, and exhaust manifolds with cheaper production cost by reducing the overall number of processes. The tube hydroforming process is based on the use of internal pressure combined with axial load. Successful tube hydroforming depends on the reasonable combination of the internal pressure and the axial load at the tube ends. This paper deals with the optimal process design (internal pressure and axial load) of the hydroforming process using numerical simulation by the explicit finite element code combined with an optimization tool. An optimization technique is used in order to minimize the tube thickness variation by determining the optimal loading path in the tube expansion forming and the sub-frame forming process. The optimization is performed by means of a gradient-based method including sensitivity analysis.

The tube bending technology of a hydroforming process for an automotive part

Abstract This paper presents the simulation results on prebending and hydroforming processes that are used to form an automotive part, a tie bar. Two prebending simulations, by a rotary draw bending machine and a bend die, are carried out to obtain the shape change of the cross-section and the thinning of the tube and the results are compared with each other. In prebending simulation with a rotary draw bending machine, to avoid wrinkling in compressive area of the tube, a wiper die is included. A parametric study is carried out to obtain the effect of the forming parameters such as bend radius and tube thickness. The hydroforming process is simulated considering the results of the prebending simulation.

Cylindrical tube optimization using response surface method based on stochastic process

Abstract This paper presents the optimization result in the crashworthiness problem for maximizing absorbing energy of cylindrical tube. To simulate a complicated behavior of this kind of crash problem, a self-developed explicit finite element code is used. The response surface method based on stochastic process is used, that is especially good at modeling the non-linear, multi-modal functions that often bring about in engineering. The main characteristics of using response surface for global optimization lies in balancing the need to exploit the fitting surface for improving the approximation. It can be shown that how these approximating functions can be used to construct an efficient global optimization algorithm. Especially, with the comparison of result by classical optimization method, it can be shown that presented optimization method is independent of noise factor and existence of local minimum.

Evaluation of heat transfer coefficient during heat treatment by inverse analysis

Abstract Reliable prediction of the properties of a heat-treated workpiece requires accurate data of the heat transfer coefficient during heat treatment process. In the present investigation, inverse heat transfer formulation using a two-dimensional finite element method was developed as a tool to evaluate heat transfer coefficient during heat treatment. The formulation has a function of providing a time profile of heat transfer coefficient on various surface locations with measured temperature at proper locations within workpiece under heat treatment. By the present formulation, heat transfer coefficients were evaluated for fan and water cooling of heated solid cylindrical carbon steel specimens. Comparison of the obtained heat transfer coefficients against the existing results on heat treatment shows that the obtained values of coefficients have reasonable accuracy, enough to predict the properties of the heat-treated workpiece.

A multiplicative finite elasto-plastic formulation with anisotropic yield functions

Abstract In this article, a finite elasto-plastic formulation with anisotropic yield functions is developed by applying a multiplicative decomposition of the deformation gradient. Fulfilling the Clausius–Duhem inequality a flow rule is derived via postulate of maximal dissipation. With restriction to small elastic strains this flow rule yields the commonly used normality rule of additively decomposed models and its description of the evolution of the anisotropy axes. On the basis of the presented multiplicative formulation, corresponding assumptions and restrictions of the additively decomposed models are discussed.

Development of Micro Punching System

Abstract A micro hole punching system was developed and micro holes of 100μm in diameter were successfully made on brass sheets of 100μm in thickness. A micro punch made of tungsten carbide was designed to withstand the punch load, considering the buckling and the bending moment due to possible misalignment error. The punch was fabricated by the grinding process with diamond wheel. The die was designed considering the punch load and fabricated by micro electrodischarge machining process. In this system the stripper is designed to guide punch tip to minimize the possible misalignment. The punch was installed on a vertical stepper and the die was mounted on an X-Y translation unit. The precision motion controller controlled all motions of the micro hole punching system. In this study technical difficulties and solutions in the micro hole punching process were also discussed.

Finite element analysis of grain-by-grain deformation by crystal plasticity with couple stress

Abstract Rigid–plastic crystal plasticity with the rate-sensitive constitutive behavior of a slip system has been formulated within the framework of a two-dimensional finite element method to predict the grain-by-grain deformation of single- and polycrystalline FCC metals. For that purpose, individual grains are represented by several numbers of finite elements to describe the sub-grain deformation behavior, and couple stress has been introduced into the equilibrium equation to be able to describe the size effect as well as to prevent mesh-dependent predictions. A modified virtual work-rate principle with an approximate interface constraint has been suggested to use a C 0 -continuous element in the finite element implementation, and the couple stress work-rate has been formulated on the basis of an assumed constitutive behavior. Simulated plane-strain compressions of a single crystal cube show that the shearing and the deformation load are closely related to the imbedded lattice orientation of the crystal grain, and that the sub-grain deformation and the load magnitude can be controlled by the couple stress hardening. It is also confirmed that almost the same predictions are obtained for different mesh systems by considering the couple stress hardening. Simulated plane-strain compressions of a bi-crystal show considerably curved grain-by-grain surface profiles after large reduction for several combinations of the imbedded lattice orientation. The high couple stress hardening predicted around grain boundaries is supposed to be related to the grain size effect. It is also supposed that consideration of couple stress is necessary to predict the sub-grain or the grain-by-grain deformation, and the couple stress hardening may be used to describe the state of microstructures in grain.

Forming of Micro Channels with Ultra Thin Metal Foils

Abstract The objective of this paper is to investigate the feasibility of producing micro scale structures by forming ultra thin metal foils. During this investigation, flat rolled foils of AISI 304 stainless steel (2.5μm in thickness) and pure copper (3.0μm in thickness) were formed into channels of varying shapes. The shapes of these channels were straight lines, concentric circles, crosses, and other curved shapes. The cross sections of the channels ranged from 10∼20μm wide and 5-10μm deep. Forming was done by cold isostatic pressing. Two types of micro dies were used. One was made of SU-8 photo resist on a Si wafer, the other of dry etched (DRIE) Si wafer. The die and metal foil were vacuum packed in a bag made of multilayered film. The forming was conducted with a 240MPa cold hydrostatic press. The formed structures were examined in terms of their dimensions, surface qualities, and potential for defect. The fabrication results show that the sheet metal forming process can be applied to the manufacturing of micro scale structures.

Punching of Ultra Small Size Hole Array

Abstract This paper presents a method by which multiple holes of ultra small size can be punched simultaneously. Silicon wafers were used to fabricate punching die. Workpiece used in the present investigation were the rolled pure copper of 3um in thickness and CP titanium of 1.5um in thickness. The metal foils were punched with the dies and arrays of circular and rectangular holes were made. The diameter of holes ranges from 2-10um. The process set-up is similar to that of the flexible rubber pad forming or Guerin process. Arrays of holes were punched successfully in one step forming. The punched holes were examined in terms of their dimensions, surface qualities, and potential defect. The effects of the die hole dimension on ultra small size hole formation of the thin foil were discussed. The optimum process condition such as proper die shape and diameter-thickness ratio (d/t) were also discussed. The results in this paper show that the present method can be successfully applied to the fabrication of ultra small size hole array in a one step operation.