PREFACE

Kwansoo Chung,Hoon Huh,Frederic Barlat,Young Hoon Moon,Heung Nam Han 한국소성가공학회 2011 기타자료 Vol.2011 No.8

ACKNOWLEDGEMENT

Kwansoo Chung,Hoon Huh,Heung Nam Han,Young Hoon Moon,Frederic Barlat 한국소성가공학회 2011 기타자료 Vol.2011 No.8

Invariance of Plastic Strains with Respect to Imposed Rate at Boundary

Chung, Kwansoo,Wagoner, Robert H 대한금속재료학회(대한금속학회) 1998 METALS AND MATERIALS International Vol.4 No.1

The concept of material $quot;formability$quot; is often related to imposed boundary velocity in the literature and in discussions of practical forming operations. While material strain-rate sensitivity is frequently cited, without proof, as the source of this dependence, other factors may contribute, including contact and friction conditions, temperature changes, and inertial forces in the workpiece and tooling, all of which may depend on boundary velocity. The authors showed in 1986 that strain localization in a tensile test (and other operations not involving friction and contact) was theoretically independent of boundary velocity (and material strength) for certain rate-sensitive constitutive equations, contrary to the common perception [1]. In the present paper, we generalize the principle of invariance to operations involving contact and friction and state the conditions under which the invariance is valid. These conditions are closely met for many forming operations. This expands the usefulness of the idea, which can therefore help interpretation of the role of imposed boundary velocity (and material strength) in altering material formability. The generalized principle is presented and proven using a variational formulation for rigid plasticity and the principle is discussed with respect to several analyses of common forming operations.

Neck Formation in Drawing Processes of Fibers

Chung, Kwansoo,Yoon, Hyungsop,Youn, Jae Ryoun The Korean Fiber Society 2001 Fibers and polymers Vol.2 No.1

To better understand the formation of necking in drawing processes of fibers, strain distributions during drawing processes have been analyzed. For simplicity, one-dimensional incompressible steady flow at a constant temperature was assumed and quasi-static model was used. To describe mechanical properties of solid polymers, non-linear visco-plastic material properties were assumed using the power law type hardening and rate-sensitive equation. The effects of various parameters on the neck formation were matematically analyzed. As material property parameters, strain-hardening parameter, visco-elastic coefficient and strain-rate sensitivity were considered and, for process parameters, the drawing ratio and the process length were considered. It was found that rate-insensitive materials do not reach a steady flow state and the rate-sensitivity plays a key role to have a steady flow. Also, the neck formation is mainly affected by material properties, especially for the quasi-static model. If the process length changes, the strain distribution was found to be proportionally re-distributed along the process line by the factor of the total length change.

Nonsteady Plane-strain Ideal Forming without Elastic Dead-zone

Chung, Kwansoo,Lee, Wonoh,Kang, Tae Jin,Youn, Jae Ryoun The Korean Fiber Society 2002 Fibers and polymers Vol.3 No.3

Ever since the ideal forming theory has been developed for process design purposes, application has been limited to sheet forming and, for bulk forming, to two-dimensional steady flow. Here, application for the non-steady case was made under the plane-strain condition. In the ideal flow, material elements deform fellowing the minimum plastic work path (or mostly proportional true strain path) so that the ideal plane-strain flow can be effectively described using the two-dimensional orthogonal convective coordinate system. Besides kinematics, schemes to optimize preform shapes for a prescribed final part shape and also to define the evolution of shapes and frictionless boundary tractions were developed. Discussions include numerical calculations made for a real automotive part under forging.

Ground Penetrating Radar Imaging of a Circular Patterned Ground near King Sejong Station, Antarctica

( Kwansoo Kim ),( Hyeontae Ju ),( Joohan Lee ),( Changhyun Chung ),( Hyoungkwon Kim ),( Sunjoong Lee ),( Jisoo Kim ) 대한지질공학회 2021 지질공학 Vol.31 No.3

Constraints on the structure and composition of the active layer are important for understanding permafrost evolution. Soil convection owing to repeated moisture-induced freeze-thaw cycles within the active layer promotes the formation of self-organized patterned ground. Here we present the results of ground penetrating radar (GPR) surveys across a selected sorted circle near King Sejong Station, Antarctica, to better delineate the active layer and its relation to the observed patterned ground structure. We acquire GPR data in both bistatic mode (common mid-points) for precise velocity constraints and monostatic mode (common-offset) for subsurface imaging. Reflections are derived from the active layer-permafrost boundary, organic layer-weathered soil boundary within the active layer, and frozen rock-fracture-filled ice boundary within the permafrost. The base of the imaged sorted circle possesses a convex-down shape in the central silty zone, which is typical for the pattern associated with convection-like soil motion within the active layer. The boundary between the central fine-silty domain and coarse-grained stone border is effectively identified in a radar amplitude contour at the assumed active layer depth, and is further examined in the frequency spectra of the near- and far-offset traces. The far-offset traces and the traces from the lower frequency components dominant on the far-offset traces would be associated with rapid absorption of higher frequency radiowave due to the voids in gravel-rich zone. The presented correlation strategies for analyzing very shallow, thin-layered GPR reflection data can potentially be applied to the various types of patterned ground, particularly for acquiring time-lapse imaging, when electric resistivity tomography is incorporated into the analysis.

Ground Penetrating Radar Imaging of a Circular Patterned Ground near King Sejong Station, Antarctica

Kim, Kwansoo,Hyeontae, Ju,Lee, Joohan,Chung, Changhyun,Kim, Hyoung-kwon 극지연구소 2022 극지연구소 초록집 Vol.- No.-

Numerical Simulation of Time-dependent Spring-back Behavior for Aluminum Alloy 6022-T4 Sheet

Taejoon Park,Kwansoo Chung,Hansun Ryou,Myoung-Gyu Lee,R.H. Wagoner 한국소성가공학회 2010 기타자료 Vol.2010 No.6

In order to analyze the time-dependent spring-back behavior of the aluminum alloy 6022-T4 sheet, the viscoelastic/plastic constitutive law was applied by utilizing a linear viscoelastic/plastic model previously developed. As for the plastic deformation, the combined isotropic-kinematic hardening law was used to represent the Bauschinger behavior and transient hardening, while a non-quadratic anisotropic yield function, Yld2000-2d, was applied to account for anisotropic yield behavior. The numerical formulation was developed based on the incremental deformation viscoelastic/plasticity theory and then, the constitutive law was implemented into the ABAQUS/Standard commercial finite element program using the user-defined material subroutine, UMAT. The viscoelastic behavior was characterized by the creep test above the initial yield stress level, while anisotropic yielding and hardening parameters were obtained by the uniaxial tensile test. The constitutive law and the formulation were successfully validated for time-dependent springback in the draw-bend test.

Three Dimensional FEM Simulation for Spinning of Non-circular Fibers

Kim, Heejae,Chung, Kwansoo,Youn, Jae-Ryoun The Korean Fiber Society 2000 Fibers and polymers Vol.1 No.1

A finite element method is employed fer a flow analysis of the melt spinning process of a non-circular fiber, a PET(polyethylene terephthalate) filament. The flow field is divided into two regions of die channel and spin-line. A two dimensional analysis is used for the flow within the die channel and a three dimensional analysis fur the flow along the spin-line. The Newtonian fluid is assumed for the PET melt and material properties are considered to be constant except for the viscosity. Effects of gravitation, air drag force, and surface tension are neglected. Although the spin-line length is 4.5 m only five millimeters from the spinneret are evaluated as the domain of the analysis. Isothermal and non-isothermal cases are studied fer the flow within the die channel. The relationship between the mass flow rate and the pressure gradient is presented for the two cases. Three dimensional flow along the spin-line is obtained by assuming isothermal conditions. It is shown that changes in velocity and cross-sectional shape occur mostly in the region of 1mm from the die exit.

Stress analysis of rotating annular hyperbolic discs obeying a pressure-dependent yield criterion

Woncheol Jeong,Kwansoo Chung 국제구조공학회 2016 Structural Engineering and Mechanics, An Int'l Jou Vol.58 No.4

The Drucker-Prager yield criterion is combined with an equilibrium equation to provide the elastic-plastic stress distribution within rotating annular hyperbolic discs and the residual stress distribution when the angular speed becomes zero. It is verified that unloading is purely elastic for the range of parameters used in the present study. A numerical technique is only necessary to solve an ordinary differential equation. The primary objective of this paper is to examine the effect of the parameter that controls the deviation of the Drucker-Prager yield criterion from the von Mises yield criterion and the geometric parameter that controls the profile of hyperbolic discs on the stress distribution at loading and the residual stress distribution.