• The Transactions of the Korean Institute of Electrical Engineers B
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• v.55 no.8
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• pp.416-422
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• 2006

With the increasing demand for renewable energy, distributed power included in fuel cells have been studied and developed as a future energy source. For this system, a power conversion circuit is necessary to interface the generated power to the utility. In many cases, a high step-up dc/dc converter is needed to boost low input voltage to high voltage output. Conventional methods using cascade dc/dc converters cause extra complexity and higher cost. The conventional topologies to get high output voltage use flyback dc/dc converters. They have the leakage components that cause stress and loss of energy that results in low efficiency. This paper presents a high boost converter with a voltage multiplier and a coupled inductor. The secondary voltage of the coupled inductor is rectified using a voltage multiplier and series-connected with the boost voltage of primary voltage of the coupled inductor. Therefore, high boost voltage is obtained with low duty cycle. Theoretical analysis and experimental results verify the proposed solutions using a 300W prototype.

• Journal of the Korean Institute of Telematics and Electronics
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• v.27 no.1
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• pp.68-73
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• 1990

GaAs epilayer was grown on Si(100) substrate using the two-step growth method by MBE. The crystal growth mode have been investigated by RHEED. The hydrogenation effects of GaAs epilayer were studied by DLTS and Raman spectroscopy. The four electron traps in GaAs/Si layer were observed and their activation energy ranged from 0.47 eV to 0.81 eV below the conduction band. After hydrogenation at 250\ulcorner for 3 hours, new trap not observed and electron traps at Ec-0.68, 0.54 and 0.47 eV were almost passivated. Whereas the Ec-0.81 eV level showed no significant change in concentration. From Raman measurement, GaAs epilayer is found to be influenced by the tensile stress.

• Smart Structures and Systems
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• v.1 no.4
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• pp.385-404
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• 2005

The paper presents a model to calculate reinforcement strain using measured crack width in members under applied tension, flexure, and/or shear stress. Crack mapping using a new type of distributed coaxial cable sensors for health monitoring of large-scale civil engineering infrastructure was recently proposed and developed by the authors. This paper shows the results and performance of such sensors mounted on near surface of two flexural beams and a large scale reinforced concrete box girder that was subjected to cyclic combined shear and torsion. The main objectives of this health monitoring study was to correlate the sensor's response to strain in the member, and show that magnitude of the signal's reflection coefficient is related to increases in applied load, repeated cycles, cracking, and reinforcement yielding. The effect of multiple adjacent cracks, and signal loss was also investigated. The results shown in this paper are an important step in using the sensors for crack mapping and determining reinforcement strain for in-situ structures.

• Nuclear Engineering and Technology
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• v.42 no.5
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• pp.590-599
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• 2010

Recently, efficient operation and practical management of power plants have become important issues in the nuclear industry. In particular, typical aging parameters such as stress and cumulative usage factor should be determined accurately for continued operation of a nuclear power plant beyond design life. However, most of the major components have been designed via conservative codes based on a 2-D concept, which do not take into account exact boundary conditions and asymmetric geometries. The present paper aims to suggest an effective fatigue evaluation methodology that uses a prototype of the integrated model and its transfer functions. The validity of the integrated 3-D Finite Element (FE) model was proven by comparing the analysis results of individual FE models. Also, mechanical and thermal transfer functions, known as Green's functions, were developed for the integrated model with the standard step input. Finally, the stresses estimated from the transfer functions were compared with those obtained from detailed 3-D FE analyses results at critical locations of the major components. The usefulness of the proposed fatigue evaluation methodology can be maximized by combining it with an on-line monitoring system, and this combination, will enhance the continued operations of old nuclear power plants.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.13 no.4
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• pp.485-500
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• 2000

In this paper the adaptive crack propagation analysis based on the estimated local and global error in the element-free Galerkin (EFG) method is presented. It is possible to keep consistency and accuracy of analysis in each propagation step by adaptive analysis. The adaptivity analysis in crack propagation is achieved by adding and removing the node along the background integration cell that are refined or recovered as estimated error. These errors are obtained by calculating the difference between the values of the projected stresses and original EFG stresses. To evaluate the performance of proposed adaptive procedure, the convergence behavior is investigated lot several examples. The results of these examples show the efficiency of proposed scheme in crack propagation analysis.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.05a
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• pp.207-212
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• 2009

In order to optimize the press formability of incremental sheet forming for complex shape (e.g human face), a combination of both CAM and FEM simulation, is implemented and evaluated from the histories of stress and strain value by means of finite element analysis. Here, the results, using ABAQUS/Explicit finite element code, are compared with fracture limit curve (FLC) in order to predict and optimize the press formability by changing parameters of tool radius and tool down-step according to the orthogonal array of Taguchi's method. Firstly, The CAM simulation is used to create cutter location data (CL data). This data are then calculated, modified and exported to the input file format required by ABAQUS through using MATLAB programming. The FEM results are implemented for negative incremental sheet forming and then investigate by experiment.

• Fire Science and Engineering
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• v.17 no.4
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• pp.42-56
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• 2003

This research made a survey to 119 EMT laying stress on general contents connected with job in a frame of mutually organic cooperation system between the processes, composing Emergency Medical Care Transportation Policy in Korea, as a step before hospital, of happening emergency patients, 119 first-aid service of the spot, transportation of patients, construction of communication network etc.. As a result of analysis to that, it is found that there must be systematic devices which makes EMT not to be caught on medical dispute, a modernization of emergency equipments, professional first-aid agents, a proper personnel arrangement. Consequently, it suggests policy plan focusing on structural and functional aspect to improve an Emergency Medical Care Transportation system into a realistic one.

• Journal of Institute of Control, Robotics and Systems
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• v.17 no.10
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• pp.1029-1036
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• 2011

An energy-efficient reference walking trajectory generation algorithm is suggested utilizing allowable ZMP (Zero-Moment-Point) region, which maxmizes the energy efficiency for cyclic gaits, based on three-dimensional LIPM (Linear Inverted Pendulum Model) for biped robots. As observed in natural human walking, variable ZMP manipulation is suggested, in which ZMP moves within the allowable region to reduce the joint stress (i.e., rapid acceleration and deceleration of body), and hence to reduce the consumed energy. In addition, opimization of footstep planning is conducted to decide the optimal step-length and body height for a given forward mean velocity to minimize a suitable energy performance - amount of energy required to carry a unit weight a unit distance. In this planning, in order to ensure physically realizable walking trajectory, we also considered geometrical constraints, ZMP stability condition, friction constraint, and yawing moment constraint. Simulations are performed with a 12-DOF 3D biped robot model to verify the effectiveness of the proposed method.

• Composites Research
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• v.28 no.4
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• pp.182-190
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• 2015

The phenomenological evolution laws of damage can be defined either based on residual life or residual strength. The failure of a specimen can be defined immediately after or before fracture. The former is called in this paper by "failure defined by approach I" and the latter "failure defined by approach II." Usually at failure there is a discontinuity of loading variables and, because of this, damage at failure is discontinuous. Therefore the values of damage at failure by two different approaches are not the same. Based on this idea the sequence effects of the phenomenological evolution law of damage given by $dD/dN=g(D)f({\Phi})$ were studied. Thin-walled graphite/epoxy tubes consisting of four of $[{\pm}45]_s$ laminates were used for the experimental study of sequence effects and the effects of mean stress on fatigue life. It was found that the sequence effects in two step uniaxial fatigue for $[{\pm}45]_s$ graphite/epoxy tubular specimen showed that a high-low block loading sequence was less damaging than a low-high one.

• Journal of the Korea Concrete Institute
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• v.14 no.3
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• pp.373-381
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• 2002

The cost on transmission and erection of the precast prestressed concrete members largely depends on the weight of them. Optimum process is performed on a U-beam section to control the prestressing force, to reduce the self-weight, and to meet the required strength and stability. The strength, deflection, and concrete stress at the top and bottom of the section considered are required to check according to each construction step in this process. The weight of the original rectangular concrete beam could be reduced up to 39∼50％ from this method. Two full scale prototype U-beams were proposed and tested in this study. It was found that the U-beams in the test showed good performance in strength and serviceability within the limits of ultimate strength design method.