• Transactions of Materials Processing
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• v.21 no.8
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• pp.479-484
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• 2012

The objective of this study is to compensate the elastic recovery in the flexible forming process using the predictive models. The target shape was limited to two-dimensional shape having only one curvature radius in the longitudinal-direction. In order to predict the shape error the regression and neural network models were established based on the finite element (FE) simulations. A series of simulations were conducted considering input variables such as the elastic pad thickness, the thickness of plate, and the objective curvature radius. Then, at sampling points in the longitudinal-direction, the shape errors between formed and objective shapes could be calculated from the FE simulations as an output variable. These shape errors were expressed to a representative error value by the root mean square error (RMSE). To obtain the correct objective shape the die shape was adjusted by the closed-loop using the neural network model since the neural network model shows a higher capability of estimating the shape error than the regression model. Finally the experimental result shows that the formed shape almost agreed with the objective shape.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.4
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• pp.1290-1300
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• 1996

The finite element method, based on rigid-plastic formulation, is widely used to simulate metal forming processes. In order to improve the computational efficiency of the rigid-plastic FEM, one-point integration is used to evaluate the stiffness matrix with four-node rectangular elements and eight-node brick elements. In order to control the hourglass modes, hourglass strain rate components were introduced and included in the effective strain rate definition, Numerical tests have shown that the proposed one-point integration scheme reduces the stiffness matrix evaluation time without deteriorating the convergence behavior of Newton-Raphson method. Simulations of a ring compression, a plane-strain closed-die forging and the three-dimensional spike forging processes were carried out by using the proposed integration method. The simulation results are compared to those obtained by applying the conventional integraiton method in terms of the solution accuracy and computational efficiency.

• Transactions of Materials Processing
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• v.14 no.4 s.76
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• pp.384-391
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• 2005

The nickel-based alloy Nimonic 80A possesses the excellent strength, and the resistance against corrosion, creep and oxidation at high temperature. Its products are used in aerospace engineering, marine engineering and power generation, etc. Control of forging parameters such as strain, strain rate, temperature and holding time is important because change of the microstructure in hot working affects the mechanical properties. Change of the microstructure evolves by recovery, recrystallization and grain growth phenomena. The dynamic recrystallization evolution has been studied in the temperature range of $950\~1250^{\circ}C$ and strain rate range of $0.05\~5s^{-1}$ using hot compression tests. The metadynamic recrystallization and grain growth evolution has been studied in the temperature range of $950\~1250^{\circ}C$ and strain rate range $0.05,\;5s^{-1}$, holding time range of 5, 10, 100, 600 sec using hot compression tests. Modeling equations are proposed to represent the flow curve, recrystallized grain size, recrystallized fraction and grain growth phenomena by various tests. Parameters in modeling equations are expressed as a function of the Zener-Hollomon parameter. The modeling equation for grain growth is expressed as a function of the initial grain size and holding time. The modeling equations developed were combined with thermo-viscoplastic finite element modeling to predict the microstructure change evolution during hot forging process. The grain size predicted from FE simulation results is compared with results obtained in field product.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.1
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• pp.1-12
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• 1990

The study is concerned with the analysis of combined forging of non-axisymmetric shapes with inclined protrusions by UBET technique. Work hardening is considered for the given range of strain rate during the forging process. A complex shape with inclined cavities is analyzed by subdividing the workpiece into finite UBET elements for which simple velocity fields are applicable. An experimental set-up was designed and manufactured for the experiment, and experiments are carried out with lead billets. The devised set-up can be used for closed-die forging of complex shapes with protrusions in which the dies can be separated automatically for easy removal of the forged products. Based on the derived kinematically admissible velocity fields for corresponding UBET elements, general computer programs have been developed. Since the energy dissipation rate for each elemental region is provided by subprograms (Subroutine or Function), the developed program can be applied to the forging problems of various shapes. The present study has shown that the method developed can be effectively applied to forging of non-axisymmetric shapes with complicated protrusions.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.8
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• pp.938-945
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• 2005

The microstructure evolution in hot forging process is composed of dynamic recrystallization during deformation as well as grain growth during dwell time. Therefore, the control of forging parameters such as strain, strain rate. temperature and holding time is important because the microstructure change in hot working affects the mechanical properties. Modeling equations are developed to represent the flow curve. grain size. recrystallized volume fraction and grain growth phenomena by various tests. The developed modeling equations were combined with thermo-viscoplastic finite element modeling to predict the microstructure change evolution during hot forging process. The large exhaust valve spindle (head diameter of 512mm) was simulated by closed die forging with hydraulic press and cooled in air after forging. The preform was heated to each 1080 and 1150$^{\circ}C$. Numerical calculation was performed by DEFORM-2D. a commercial finite element code. Heat transfer can be coupled with the deformation analysis in a non-isothermal deformation analysis. In order to obtain the fine and homogeneous microstructure and good mechanical properties in forging. the FEM would become a useful tool in the simulation of the microstructure development. In forging, appropriate temperature, strain and strain rate and rapid cooling are required to obtain the fine grain microstructure The optimal forging temperature and effective strain range of Nimonic 80A for large exhaust valve spindle are about 1080$\∼$l120$^{\circ}C$ and 150$\∼$200$\%$.

• Journal of Chest Surgery
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• v.31 no.8
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• pp.770-775
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• 1998

Background: Atrial septal defect (ASD) is the most common congenital cardiac anomaly, accounting for 30 percent of congenital heart disease detected in the adult. Many patients with ASD are well tolerated and reach adult without significant symptoms. The patients with ASD die 4th and 5th decades, but prolonged survival is not uncommon. In general, the survival depends on whether pulmonary hypertension develops during adulthood or not. The most common cause of death in the patients with ASD is right ventricular failure or arrhythmias. Materials and methods: From January 1988 to June 1997, 33 cases of ASD underwent open heart surgeries in our hospital. Among them, 31 cases were adult ASD, and 2 tricuspid regurgitation, 1 pulmonic stenosis, 1 mitral regurgitation, 1 tricuspid regurgitation, and 1 coronary artery disease were combinded. All of the patients underwent surgical repair using autologus pericardial patch or direct closure. Results: The postoperative course was smooth and uneventful. Most of the patients showed significant improvement in ECG finding, hemodynamic profile, radiologic finding, and echocardiography, after surgery. Conclusions: Conclusively, most of the ASD should be closed even in patients over the age of 60 years, and early surgical repair must be done to prevent pulmonary hypertension, right ventricular failure, and arrythmias.

• Korean Journal of Materials Research
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• v.10 no.1
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• pp.69-76
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• 2000

Despite the improved formability and processing advantages, the use of semi-solid metals is greatly limited due to the difficulties in controlling the optimum forming parameters. In the present study, the tensile properties of closed die, pressure formed semi-solid A356 alloy were examined. It was demonstrated that the tensile strength of thixoformed A356 alloy could be greatly reduced when the forming parameters were not rigorously controlled. The reduced strength of unappropriately formed products appeared to be related to the coarsening of the primary phases. The possibility of improving tensile properties of as-formed products by simple post heat treatment was also assessed.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.1A
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• pp.104-112
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• 2005

Some design techniques for high speed and low power pipelined 8-bit ADC are described. To perform high-speed operation with relatively low power consumption, open loop architecture is adopted, while closed loop architecture (with MDAC) is used in conventional pipeline ADC. A distributed track and hold amplifier and a cascading structure are also adopted to increase the sampling rate. To reduce the power consumption and the die area, the number of amplifiers in each stage are optimized and reduced with proposed zero-crossing point generation method. At 500-MHz sampling rate, simulation results show that the power consumption is 210mW including digital logic with 1.8V power supply. And the targeted ADC achieves ENOB of about 8-bit with input frequency up to 200-MHz and input range of 1.2Vpp (Differential). The ADC is designed using a $0.18{\mu}m$ 6-Metal 1-Poly CMOS process and occupies an area of $900{\mu}m{\times}500{\mu}m$

• Advanced Composite Materials
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• v.17 no.1
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• pp.1-23
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• 2008

Robotized filament winding technology involves a robot that winds a roving impregnated by resin on a die along the directions of stresses to which the work-piece is submitted in applications. The robot moves a deposition head along a winding trajectory in order to deposit roving. The trajectory planning is a very critical aspect of robotized filament winding technology, since it is responsible for both the tension constancy and the winding time. The present work shows two original rules to plan the winding trajectory of structural parts, whose shape is obtained by sweeping a full section around a 3D curve that must be closed and not crossing in order to assure a continuous winding. The first rule plans the winding trajectory by approximating the part 3D shape with straight lines; it is called the discretized rule. The second rule defines the winding trajectory simply by offsetting a 3D curve that reproduces the part 3D shape, of a defined distance; it is called the offset rule. The two rules have been compared in terms of roving tension and winding time. The present work shows how the offset rule enables achievement of both the required aims: to manufacture parts of high structural performances by keeping the tension on the roving near to the nominal value and to markedly decrease the winding time. This is the first step towards the optimization of the robotized filament winding technology.

• The Korean Journal of Zoology
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• v.36 no.2
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• pp.181-192
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• 1993

The spawning and growth of eightspine stickleback,Pungitius sinensis kaiborae (Tanaka, 19151, were investigated in the Chavang Stream from March 1990 to february 1991 as a part of study on its life history. Nest building, spavming and parental behaviors were similar to those known on the sticklebacks of Europe, North America and Japan. Bht f sinensis kaibarae in the Chayang Stream, peculiarly, showed only superficial gluing behavior in nest building phase and made two nurseries iust behind the nest in Parental Phase. The individuals with fully mature eggs were found from late February to late June. The fecundity of an adult female was about 21 to 110 (mean 47.07) eggs and the diameter of kllv matured ovarian eggs ranged 1.3 to 1.5 mm. The smallest female with fully mature eggs was 25.8 mm in body length and had 21 eggs. The relationship between the number of mature eggs in ovary and body length was No. of eggs = 2.857BL -55.134 (r = 0.890). The number of deposited eggs in a nest was 27 to 637 and thew were composed of several clusters of various developmental stages. The distribution pattern of gonadsomatic index (GSl) against the body length shown that in females 1+ year-old fishes mainly participate in spawning but in males 11 and 2+ year-old fishes equally participate. From the fluctuat지n pattern of GSI and the number of mature eggs in ovary, it was assumed that the spawning season was from February to June and the peak of spawning was early March to late April. The newly hatched vouns was found at late April and their body length was about 10 mm. The vouns (O+) grew rapidly until late August. Then their srowth was slowed down and finallY ceased in midautumn. The young reached about 35 mm BL in their first year of life. The halting of growth was lasted to next Sune when the spawning season would be nearly closed. They (1+) began to grow asain from late june, grew rapidly until tate september and reached about 415 mm BL. Then there was no more grouvth until to die as 2+ yearold fish. Therefore it was assumed that the life span of P. sinensis kaibarae in the Chavang Stream would be about ko years. The relationship between the body length and the body weight was logBW = 2.9541ogBL -4.802 (r = 0.998).