• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.9
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• pp.2242-2251
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• 1993

The mesh-based frictional contact model has been developed which does not rely on the spatial derivatives of the tool surface. Only points on the surface are evaluated from the description. which can then be simplified because of the relaxed demands placed on it. The surface tangents, normals, and corresponding derivatives at each finite-element node are evaluated directly from the finite-element mesh, in terms of the connecting nodal positions. The advantages accrue because there is no longer a need for a smooth tool surface to assure reasonable normals and derivatives. Furthermore, it can be shown that the equilibrium equations can only be properly written with a special normal derived from the mesh itself. The validity, accuracy, computation time, and stability of mesh-based contact model were discussed with the numerical examples of rounded flat-top and rough, flat-top rounded punch forming operations. Also, the forming process of a automobile inner panel section was simulated for testing the robustness of new contact model. In the discussion, the superiority of new model was examined, comparing with tool-based contact one.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1992.03a
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• pp.201-212
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• 1992

The plane strain analysis for simulating the stretch/draw forming operation of arbitrarily-shaped tool profiles and arbitrarily draw-in conditions is introduced. An implicit, incremental, updated Lagrangian formulation is employed, introducing a rigid-viscoplastic constitutive equation. Contact and friction are considered through the mesh-normal, which compatibly describes arbitrary tool surfaces and FEM meshe without depending on the explicit spatial derivatives of tool surfaces. The FEM formulation is tested in the sections automotive inner panel and two-side draw-in. Not only the excellent agreement between measured and computed strains in the stretched section is obtained, but also the numerical stability of current formulation is verified in the two-side draw-in section.

• Transactions of Materials Processing
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• v.1 no.1
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• pp.95-103
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• 1992

The plane strain analysis for simulating the stretch/draw forming operation with an arbitrarily-shaped tool profile is introduced. An implicit, incremental, updated Lagrangian formulation with a rigid-viscoplastic constitutive equation is employed. Contact and friction are considered through the mesh-normal, which compatibly describes arbitrary tool surfaces and FEM meshes without depending on the explicit spatial derivatives of tool surfaces. The linear line elements are used for depicting the formed sheet, based on membrane approximation. The FEM formulation is tested in the sections of automotive inner panel and two-side draw-in. Not only the excellent agreement between measured and computed strains is obtained in the stretched section, but also the numerical stability of formulation is verified in the draw-in section.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1998.06a
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• pp.65-73
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• 1998

The design analysis of axisymmetric, multi-stage deep drawing dies was performed using the rigid-viscoplastic finite element formulation. In the formulation, the axisymmetric CFS algorithm was employed. Hill's non-quadratic normal anisotropic yield criterion and isotropic hardening rule were considered. For trial initial displacements and tool contact points, the geometric force equilibrium method was adopted. In order to see the validity of the formulation, the multi-stage deep drawing processes of shell-cylinder front part of hydraulic booster were simulated. The simulation showed good agreements with measurements and PAM-STAMP.

• Transactions of Materials Processing
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• v.7 no.6
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• pp.594-602
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• 1998

The design analysis of axisymmetric, multi-stage deep drawing dies was performed using the rigid-viscoplastic finite element formulation. In the formulation the axisymmetric CFS algorithm was employed. Hill's non-quadratic normal anisotropic yield criterion and isotropic hardening rule were considered. For trial initial displacements and tool contact points. the geometric force equilibrium method was adopted. In order to see the validity of the formulation, the multi-stage deep drawing processes of shell-cylinder front part of hydraulic booster were simulated. The simulation showed good agreements with measurments and PAM-STAMP results.

• Transactions of Materials Processing
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• v.9 no.6
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• pp.590-597
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• 2000

A finite element formulation lot the simulation of axisymmetric sheet hydroforming is proposed, and an implicit program is coded. In order to describe normal anisotropy of steel sheet, Hill's non-quadratic yield function (Hill, 1979) is employed. Frictional contacts among sheet surface, rigid tool surface, and flexible hydrostatic pressure are considered using mesh normal vectors based on finite element of the sheet. Applied hydraulic pressure is also considered as a function of forming rate and time and treated as an external loading. The complete set of the governing relations comprising equilibrium and interfacial equations is approximately linearized for Newton-Raphson algorithm. In order to verify the validity of the developed finite element formulation, the axisymmetric bulge test is simulated. Simulation results are compared with other FEM results and experimental measurements and showed good agreements. In axisymmetric hydroforming processes of a disk cover, formability changes are observed according to the hydraulic pressure curve changes.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.8
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• pp.2113-2122
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• 1994

The 3-D FEM analysis for simulating the stamping operation of planar anisotropic sheet metals with arbitrarily-shaped tools is introduced. An implicit, incremental, updated Lagrangian formulation with a rigid-viscoplastic constitutive equation is employed. Contact and friction are considered through the mesh-normal, which compatibly describes arbitrary tool surfaces and FEM meshes without depending on the explicit spatial derivatives of tool surfaces. The consistent full set of governing relations, comprising equilibrium equation and mesh-normal geometric constraints, is appropriately linearized. The linear triangular elements are used for depicting the formed sheet, based on membrane approximation. Barlat's non-quadratic anisotropic yield criterion(strain-rate potential) is employed, whose in-plane anisotropic properties are taken into account with anisotropic coefficients and non-quadratic function parameter. The planar anisotropic finite element formulation is tested with the numerical simulations of the stamping of an automotive hood inner panel and the drawing of a hemispherical punch. The in-plane anisotropic effects on the formability of both mild steel and aluminum alloy sheet metals are examined.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.03b
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• pp.26-29
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• 1999

The sectional forming analysis program for analyzing the hydroforming processes of axisymmetric sheet parts was tleveloped. The rigid-viscoplastic FEM formulation based on membrane theory was derived, wh~cta simi~ltaneously solve force equilibrium as well as non-penetration condition. Hill's non-quadratic normal anisotropic yield theory(1979) was used for material behaviour. For describing the liquid pressure iaction, the flexible tool concept was introduced. Isotropic hardening law was also assumed. To verify the \,alidity of the formulation, the stepped cup forming process as well as the hydrostatic bulging test were \imnlated. Simulation results agreed well with Finckenstein and experimental ones.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1995.10a
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• pp.60-63
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• 1995

The behavior of the work materials in the chip-tool interface in extremely high strain rates and temperatures is more that of viscous liquids than that of normal solid metals. In these circumstances the principles of fluid mechanics can be invoked to describe the metal flow in the neighborhood of the cutting edge. In the present paper an Eulerian finite element model is presented that simulates metal flow in the vicinity of the cutting edge when machining a low carbon steel with carbide cutting tool. The work material is assumed to obey visco-plastic (Bingham solid) constitutive law and Von Mises criterion. Heat generation is included in the model, assuming adiabatic conditions within each element. the mechanical and thermal properties of the work material are accepted to vary with the temperature. The model is based on the virtual work-stream function formulation, emphasis is given on analyzing the formation of the stagnant metal zone ahead of the cutting edge. The model predicts flow field characteristics such as material velocity effective stress and strain-rate distributions as well as built-up layer configuration

• Earthquakes and Structures
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• v.7 no.5
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• pp.647-665
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• 2014

Squat reinforced concrete walls require enough shear strength in order to promote flexural yielding, which creates the need for designers of an accurate method for strength prediction. In many cases, especially for existing buildings, strength estimates might be insufficient when more accurate analyses are needed, such as pushover analysis. In this case, estimates of load versus displacement are required for building modeling. A model is developed that predicts the shear load versus shear deformation of squat reinforced concrete walls by means of a panel formulation. In order to provide a simple, design-oriented tool, the formulation considers the wall as a single element, which presents an average strain and stress field for the entire wall. Simple material constitutive laws for concrete and steel are used. The developed models can be divided into two categories: (i) rotating-angle and (ii) fixed-angle models. In the first case, the principal stress/strain direction rotates for each drift increment. This situation is addressed by prescribing the average normal strain of the panel. The formation of a crack, which can be interpreted as a fixed principal strain direction is imposed on the second formulation via calibration of the principal stress/strain direction obtained from the rotating-angle model at a cracking stage. Two alternatives are selected for the cracking point: fcr and 0.5fcr (post-peak). In terms of shear capacity, the model results are compared with an experimental database indicating that the fixed-angle models yield good results. The overall response (load-displacement) is also reasonable well predicted for specimens with diagonal compression failure.