• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2002.11a
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• pp.23-32
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• 2002

The torsional forward extrusion is the process that is executed by punch travel and die rotation. The advantages of having the die rotation on this process are that forming load can be reduced and optimal die angle can be increased. This provides a possibility to extrude cold-worded material where a large extrusion force and die angle are required. Also, this process can improve the material properties owing to the high deformation and uniform strain distribution. The forming load and optimal die angle of this process are determined by the upper bound analysis using stream function and the optimization technique. To verify the theoretical result, we have carried out experiments and FE simulations using DEFORM3D.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1995.03a
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• pp.41-53
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• 1995

Forging of spur gears has been investigated by means of upper-bound method. The term forging means forging of spur gears with solid cylindrical billets, hollow billets with flat punch. Kinematically admissbile velocity field for forging of spur gears has been proposed in this study. The 1/2 pitch of spur gear has been divided into seven dieformation regions, wherein, an involute curve has been introduced to represent the shape of die profile. Especially neutral surface has been introduced intor forging of hollow gears from hollow billets. By using the kinematically admissible velocity field, the powder requirements and suitable conditions for forging fo spur gears were successfully calculated with numerical method. According to the analysis , the acceptable number of teeth for forging of spur gears is from 15 to 20.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.03b
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• pp.111-114
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• 1999

The twisting and extrusion process of the product with involute helical fin from the round billet is developed by the upper bound analysis. The twisting of extruded product is caused by the twisted die surface connecting the die entrance section and the die exit section linearly. In the analysis, the internal shear surface is defined as the curved twisted plane from the taisting of die surface and the shear work is calculated by the consumption of shear energy The increase rate of angular velocity is determined by the minimization of plastic work. The results of the analysis show that the angular velocity of the extruded product increases with the die twisting angle, the reduction of area, and decreases with the die length, the friction condition.

• Journal of the Korean Wood Science and Technology
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• v.20 no.4
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• pp.21-30
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• 1992

An activation energy equation for moisture diffusion in wood was developed with an assumption that activation energy is directly proportional to wood specific gravity. Theoretical activation energies obtained from the activation energy equation were revealed to be always lower than actual activation energies, which implies that activation energy isn't affected only by wood specific gravity. The other affecting factors are possibly anatomical structures of wood which determine a ratio of vapor diffusion to bound water diffusion in wood. For the convenience of estimating actual activation energy by using the activation energy equation, thirteen kinds of species were categorized into three groups according to their anatomical structures.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.04a
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• pp.501-506
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• 2000

An iterative numerical computational algorithm is presented to design a plate or shell element subjected to membrane and flexural forces. Based on equilibrium consideration, equations for capacities of top and bottom reinforcements in two orthogonal directions have been derived. The amount of reinforcement is determined locally, i.e., for each sampling point, from the equilibrium between applied and internal forces. Based on nonlinear analyses performed in a hyperbolic cooling tower, the analytically calculated ultimate load exceeded the design ultimate load from 50% to 55% for an analysis with relatively low to high tension stiffening, cases $\gamma$=10 and 15. For these cases, the design method gives a lower bound on the ultimate load with respect to Lower bound theorem, This shows the adequacy of th current practice at least for this cooling tower shell case studied. To generalize the conclusion more designs - analyses should be reformed with different shell configurations.

• 제어로봇시스템학회:학술대회논문집
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• 1987.10b
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• pp.125-129
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• 1987

The stability margin of the LQ regulator is investigated in the time domain. it is shown that the same guaranteed gain margin as that of the frequency domain analysis can be obtained with simple assumptions for the continuous time systems. It is also shown that the allowable modelling error bound can be expressed in terms of system matrices and Riccati equation solution. Guaranteed qain. margin and the allowable modelling error bound for the discrete time systems are also obtained by the similar procedures. In this case, through the some examples, the gain margin is shown to be less conservative than the frequency domain analysis result.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.31A no.9
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• pp.34-43
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• 1994

The unequal symbol error probability of TCM(trellis coded modulation) is analyzed and applied to the derivation of bit error probability of /RS/Trellis concatenated coded-modulation system. An upper bound of the symbol error probability of TCM concatenated with RS code is obtained by exploiting the unequal symbol error probability of TCM, and it is applied to the derivation of the upper bound of the bit error probability of the RS/Trellis concatenated coded-modulation system. Our upper bounds of the concatenated codes are tighter than the earlier established other upper bounds.

• Journal of the Korean Geotechnical Society
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• v.17 no.5
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• pp.165-172
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• 2001

본 연구에서는 하천인접구간에서와 같이 지하수가 풍부한 지반에서 시공되는 터널의 막장 안정성을 평가하기 위하여 터널 막장에 작용하는 힘의 두 가지 요소를 고려하였다. 하나는 극한해석 중 upper bound solution으로부터 산출된 유효응력이며, 또 하나는 지하수의 정상류 흐름조건을 고려한 수치해석으로부터 산출된 침투력이다. 지하수가 풍부한 토사지반에서의 터널 시공시 터널 막장에 작용하는 힘을 구하기 위하여 침투력을 고려한 극한해석의 해를 구한 결과 터널 막장의 안정성을 유지하기 위한 최소 지보력은 터널 막장에 작용하는 유효응력과 침투력의 합으로 나타낼 수 있었다. 또한 터널 막장에 작용하는 평균침투압은 지하수위에 비례하여 작용하는 것으로 나타났으며, 이를 실내모형 실험 결과를 통하여 검증하였다. 지하수의 정상류 조건 하에서의 토사터널에 대한 실내모형 실험 결과, 터널 막장에 작용하는 침투력은 수치해석 결과 비슷한 양상을 보여주어 제안된 이론의 타당성을 입증하였다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.04b
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• pp.42-45
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• 2004

High quality CdTe single crystal for the solar cell fabrication was grown by vertical Bridgman method. The etch pits patterns of {111}surfaces of CdTe etched by Nakagawa solution was observed the {111} A composed of Cd atoms with typical triangle etch pits of pyramid mode. From the photoluminescence measurement on {111}A, we observed free exciton($E_x$) existing only higy quality crystal and neytral acceptor bound exciton($A^0$,X) having very strong peak intensity. Then, the full width at half maximum and binding energy of neutral acceptor bound exciton were 7 meV and 5.9 meV, respectively. By Haynes rule, an actibation energy of impurity was 59 meV. Therefore, the origins on impurity level acting as a neutral acceptor were associated Ag or Cu elements.

• Journal of applied mathematics & informatics
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• v.14 no.1_2
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• pp.185-191
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• 2004

Given a connected graph G, we say that a set EC\;{\subseteq}\;V(G)$ is convex in G if, for every pair of vertices x, $y\;{\in}\;C$, the vertex set of every x - y geodesic in G is contained in C. The convexity number of G is the cardinality of a maximal proper convex set in G. In this paper, we show that every pair k, n of integers with $2\;{\leq}k\;{\leq}\;n\;-\;1$ is realizable as the convexity number and order, respectively, of some connected triangle-free graph, and give a lower bound for the convexity number of k-regular graphs of order n with n > k+1.