• Transactions of Materials Processing
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• v.3 no.2
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• pp.167-177
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• 1994

Most of bulk metal forming processes may be limited by ductile fracture such as surface or internal cracks developing in the workpiece. It is important to identify the conditions within the deforming workpiece which may lead to fracture, and then it is possible to modify the forming processes to produce sound and reliable product. This paper suggests the scheme to simultaneously accomplish prediction of fracture initiation and analysis of deformation in metal forming processes. The Cockcroft-Latham criterion which is successfully applied to a variety of loading situations is used in the present investigation to estimate whether fracture occurs during the deformation process. The numerical predictions and experimental results of two types of metal forming process are compared, axisymmetric extrusion and simple upsetting. The proposed scheme has successfully predicted the fracture initiation found experimentally.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.389-392
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• 2003

The deterioration of dimensional accuracy, caused by springback, is one of problems to always occur in sheet metal forming processes. As the demand for lighter and stronger metals increases, the development of improved forming processes settling the springback problem becomes more important. In this work, springback phenomena are investigated which occur in the press forming process with the anisotropic sheet metal and axisymmetric tools. The improvement possibility of dimensional accuracies, mainly, flatness, will be examined by applying blank holding forces as a method of springback control.

• Transactions of Materials Processing
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• v.9 no.1
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• pp.17-26
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• 2000

A series of parametric study was performed for the investigation on the influence of analysis parameters to the solution behavior in the elastic-plastic-static analysis of several sheet metal forming processes, such as deflection by a point force under plane strain and axisymmetric conditions, plane strain bending by a punch, axisymmetric stretching by a punch, axisymmetric bulging by hydraulic pressure, and axisymmetric deep drawing by a punch. The parameters considered are kind of element, number of elements, integration scheme for elemental equation and friction coefficient. Results obtained for different selections of those parameters were compared with each other, experimental measurements and analytical solution.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.03b
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• pp.245-248
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• 1999

Explicit dynamic finite element analysis has been used widely in the field of sheet metal forming. However in using the analysis technique there are some parameters which are not clearly defined so that engineers may obtain inaccurate solutions In the present study parameters such as time step damping ratio penalty constant and punch speed were investigated on their influence to the solution behavior. Considered forming processes are plane stain bending by a punch and axisymmetric deep drawing.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1995.03a
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• pp.124-130
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• 1995

An upper-bound elemental stream function technique(UBST) is proposed for solivng forging and backward extrusion problems that are geometrically complex or need a forming simulation . And in the forging problems, this study investigates that layer of elements effects dissipation of total energy and load. The element system of UBSTuses the curve fitting property of FEM and the fluid incompressiblity of the stream function . The foumulated optimal design problems with constraints ae solved by the flixible toerance method. In the closed-die forging and backward extrusion, the result of layer of element by this study produces a lower upper-bound solution than that fo UBET and conventional layer of element . And the main advantage of UBST program is that a computer code, once written , can be used for a large variety problems by simply changing the input data.

• Journal of the Korean Society for Precision Engineering
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• v.2 no.2
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• pp.31-42
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• 1985

The metal flow and the strain distribution is investigated for the steady state and non-steady state bulk deformation processes by using an improved visioplasticity method which includes the effective smoothing scheme. The comparison of various smoothing schemes leads to the selection of the five- point least square smoothing method which is employed to reduce the measurement errors. As a steady state forming process experiments are carried out for axisy- mmetric forward extrusion through conical and curved dies of various area reduc- tions using Aluminum and steel billets. Axisymmetric backward extrusion is chosen for a nonsteady state forming process. In axisymmetric forward extrusion the results from visioplasticity show that the curved die of a fourth-order polynomial renders more uniform distribution of strain rates and strains. Higher reduction leads to greater strain rates at the outer side of the billet. The visioplastic observation for axisymmetric backward extrusion as a non-steady state deformation process shows the concentration of higher strain at the inner wall of the extruded product. The visioplastic results in forward extrusion are in agreement with the computed results by the finite element method. It is thus shown that the visio- plasticity combined with a smoothing technique is an effective method to determine the pattern and the distribution of strain rates and strains.

• Transactions of Materials Processing
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• v.14 no.4 s.76
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• pp.392-398
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• 2005

The deterioration of dimensional accuracy, caused by springback, is one of problems to always occur In sheet metal forming processes. As the demand for lighter and stronger metals increases, the development of improved forming processes settling the springback problem becomes more important. In this work, asymmetric springback phenomena are investigated, which occur in the shallow-drawing process with the anisotropic sheet metal and axisymmetric tools. The improvement possibility of dimensional accuracies, mainly, flatness, was examined by applying segmented blank holding forces as a method for springback control.

• Transactions of Materials Processing
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• v.6 no.4
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• pp.300-310
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• 1997

A finite element inverse method is introduced for direct prediction of blank shapes, strain distributions, and reliable intermediate shapes from desired final shapes in axisymmetric multi-step deep drawing processes. This mothod enables the determination of process disign. The approach deals with the Hencky's deformation theory. Hill's second order yield criterion, simplified boundary conditions, and minimization of plastic work with constraints. The algorithm developed is applied to motor case forming, and cylindrical cup drawing with the large limit drawing ratio so that it confirms its validity by demonstrating resonably accurate numerical results of each problem. Numerical examples reveal the reason of difficulties in motor case forming with corresponding limit diagrams.

• Journal of the Korean Society for Precision Engineering
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• v.10 no.3
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• pp.198-204
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• 1993

The study is concerned with finite element analysis and design of axisymmetric deep drawing by local heating. When the bottom shape of a cup is not flat but in complex-shaped, i.e., hemispherical, the cup cannot be drawn in one or two processes in the conventional deep drawing process and the limit drawing ratio is limited as well. By introducing local heating selectively with regards to the heating position, the formability of the sheet metal can be greatly increased with the reduced number of processes. In the Process analysisthe rigid- viscoplastic finite element method is employed and the temperature effect is incorporated. Bishop's step-wise decoupled method is employed to analyze the thermomechanical interaction between deformation and heat transfer. Axisymmetric deep drawing of a hemisphere-bottomed cup has been analyzed for various combinations of heat application in the punch and the die. At the first stage of deep drawing stretch forming is practically carried out by firmly pressing the blankholder with the punch and the die heated at various levels of temperature. Then at the second stage the same cup is drawn for the saame or different combination of temperature. From the computation, it has thus been shown that the fromability of a cup is greatly increased in two-stage deep drawing with increased limet drawing ratio.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.12
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• pp.2936-2948
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• 1993

Multi-stage deep drawing is an important sheet metal forming process. The deformation mechanisms of sheet metals during forming processes are complicated mainly due to the geometry and the lubrication of tools involved, the formability and the anisotropic behaviour of the material. The multi-stage deep-drawing processes including normal-drawing, reverse-drawing, and re-drawing are analyzed by use of the rigid-plastic finite element method. The anisotropic behaviour represented by r-value can be incorporated into the formulation. Punch/die loads and thickness distributions were obtained as results of simulating axisymmetric deep drawing processes. The computed results showed good agreements with experiments.