• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.4
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• pp.373-379
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• 2010

EMAT has been applied in various fields for flaw detection and material characterization because it has noncontact property in wave generation and a good mode selectivity. Unfortunately, however, EMAT shows low signal to noise ratio relative to commercial contact transducer because of low energy conversion efficiency. If the phase matching through the control of time delay between each coil consisting of the array EMAT is accomplished, it is expected that it will be a solution for the improvement of low signal to noise ratio. In this experiment, the phased array EMATs which consists of 3 or 4 meander coils and one big magnet were fabricated for surface and vertical shear wave generation. Effect of phased delay control on signal directivity and amplitude enhancement was verified. A slit with the depth of 0.5 mm and a side-drill hole of 0.5 mm diameter were clearly detected by fabricated phased array EMATs, respectively.

• Journal of Korea Water Resources Association
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• v.38 no.5 s.154
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• pp.365-377
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• 2005

This paper presents numerical works carried out for developing an advanced computational framework for understanding injury- and mortality-inducing flow phenomena in hydropower facilities. Large-eddy simulation (LES) of a circular jet flow is carried out to help interpret the results of recent experiments that exposed live fish to the shear zone of a turbulent jet. The instantaneous flow field of LES is characterized by intense velocity, pressure, and vorticity fluctuations, which could exert forces and moments on a fish considerably larger than those exerted by the same fish exposed to the corresponding steady, time-averaged flow. In this study, also, unsteady modeling of flow in a hydroturbine draft tubewas carried out using a hybrid unsteady RANS/LES, so-called detached-eddy simulation (DES). Results from DES show that the potential for disorientation and excessive residence times of fish within the draft tube is certainly considerable.

• Magazine of the Korean Society of Agricultural Engineers
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• v.34 no.1
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• pp.41-48
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• 1992

Various soil behaviors usually occurring in the geotechnical problems, such as, cutting and embankments, stability of slope, seepage, consolidations, shearing failures and liquefaction, should be predicted and analyzed in any way. An approach of these predictions may be followed by the development of the constitutive equations as first and subsequently solved by numerical methods. The purpose of this paper is develop the constitutive equation of sands uder monotonic or cyclic loadings. The constitutive equation which is based on elasto-plastic theory, modified anisotropic consolidated stress parameter by Sekiguchi et al and Pender's theory is derived. And the equation is included a new stress parameter, hardening function, Bauschinger's effects and Pender's theory. The model is later evaluated and confirmed the validity by the test data of Ottawa sand, Banwol sand Hongseong sand. The following conclustions may be drawn: 1. The consititutive equation which is based on elasto-plastic theory, modified anisotropic consolidated stress parpameter by Sekiguchi et al and Pender's theory is derived. The equation in included a new stress parameter, hardening function, Bauschinger's effect and Pender's theory. 2. For Ottawa sand, the result of the constitutive equation shows a better agreement than that of Oka et al. The result of axial strain agrees well with the tested data. However, the result of horizontal strain is little bit off for the cyclic loadings or large stress. It is thought that the deviation may be improved by considering Poisson's ratio and precise measurement of shear modulus. 3. Banwol sand is used for the strain and stress tests with different relative densitites and confining pressures. The predeicted result shows a good agreement with the tested data because the required material parameters were directly measurd and determined form this laboratory. 4. For Hongseong sand, the tests under same amplitude of cyclic deviatoric stress shows a similar result with the tested data in absolute strain. It shows the acute shape of turning point because the sine wave of input is used in the test but the serrated wave in prediction.

• Magazine of the Korean Society of Agricultural Engineers
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• v.31 no.1
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• pp.82-95
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• 1989

This study was carried out for the stability analysis of earth dam by the variation of compaction density. The test samples were taken from five kinds of soil used for banking material and the degree of compaction for this samples were chosen 100, 95, 90, 85, and 80 percent. The stability problems were analysed by the settlement and camber( extra banking) of dam, strength parameter and dam slope, and coefficient of permeability and seapage flow through dam body. The results of the stability analysis of earth dam are as follows. 1. The more the fine particle increases and lower the compaction degree becomes, the lower the preconsolidation load becomes but the compression index becomes higher. 2. Sixty to eighty percent of settlement of dam occurs during the construction period and the settlement ratio after completion of dam is inversly proportional to the degree of compaction. 3. The camber of dam has heigher value in condition that it has more fine particle(N) and heigher dam height(H) with the relation of H= e(aN-bH-e). 4. The cohesion(C) decreases in proportion to compaction degree(D) and fine particle(N) with the relation of C= aD+ bN-c, but the internal friction angle is almost constant regardless of change of degree of compaction. 5. In fine soil, strength parameter from triaxial compression test is smaller than that from direct shear test but, they are almost same in coarse soil regardless of the test method. 6. The safety factor of the dam slope generally decreases in proportion to cohesion and degree of compaction but, in case of coarse soil, it is less related to the degree of compaction and is mainly afected by internal friction angle. 7. Soil permeability(K) decreases by the increases of the degree of compaction and fine particle with relation of K=e(a-bl)-cN) 8. The more compaction thickness is, the less vertical permeability (Kv) is but the more h6rzontal permeability (KH) is, and ratio of Kv versus KH is largest in range from 85 to 90 percent of degree of corn paction. 9. With the compaction more than 85 percent and coefficient of permeability less than ${\alpha}$X 10-$^3$cm/sec, the earth dam is generally safe from the piping action.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.9
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• pp.745-751
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• 2014

For the multi-nozzle propulsion, the rupture pressure of nozzle closure has an effect on the initial strain rate of ignition. Moreover, the deviation of rupture pressure for each nozzle closure leads to side forces which can disturb the attitude control of rocket. When designed, it should be considered whether nozzle closures are ruptured equally and exactly in the intented pressure. In this paper, the rupture behavior is analyzed by analytical and experimental methods for plate and "+" notched nozzle closures. The rupture pressure and deviation for operating temperature, whether notched or not and notched directions are analyzed. This paper provides a comparison between rupture pressure prediction of finite elements method which tool is Abaqus/Explicit and results of the rupture test. Jonson-Cook shear failure model which corresponds to the damage initiation criterion were used in this simulation.

• Journal of Powder Materials
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• v.21 no.3
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• pp.215-221
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• 2014

Translucent alumina is a potential candidate for high temperature application as a replacement of the glass or polymer. Recently, due to the increasing demand of high power light emitting diode (LED), there is a growing interest in the translucent alumina. Since the translucent property is very sensitive to the internal defect, such as voids inside or abnormal grain growth of sintered alumina, it is important to fabricate the defect-free product through the fabrication process. Powder injection molding (PIM) has been commonly applied for the fabrication of complex shaped products. Among the many parameters of PIM, the flowability of powder/binder mixture becomes more significant especially for the shape of the cavity with thin thickness. Two different positions of the gate were applied during PIM using the disc type of die. The binder was removed by solvent extraction method and the brown compact was sintered at $1750^{\circ}C$ for 3 hours in a vacuum. The flowability was also simulated using moldflow (MPI 6.0) with two different types of gate. The effect of the flowability of powder/binder mixture on the microstructure of the sintered specimen was studied with the analysis of the simulation result.

• The Journal of Engineering Geology
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• v.18 no.3
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• pp.245-255
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• 2008

The variation of the electrical property of embankment material was analyzed from laboratory experiments and the result of field survey, in order to enhance the interpretation of electrical resistivity survey frequently used for safety assessment of embankment. At first, the kaolinite, showing similar physical property with core material of embankment, was used to examine the variation of the resistivity value according to degree of consolidation. The test showed that a drop of shear strength induces increase of resistivity value regardless of degree of water content. This result means that porous zones of weak core material in embankment may be appeared as highly resistive part in the electrical resistivity survey. This observation implies that it may fail to detect weak core material by electrical method, if we only try to and zones showing low resistivity value. And, we performed Standard Penetration Test (SPT) to analyze the correlation between electrical property and ground stiffness. Finally, a mechanism to describe the variation of electrical resistivity due to grouting effect was proposed and real field data were analyzed.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.13 no.5
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• pp.2009-2014
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• 2012

It is very important to minimize the die roll height of gear, in order to maximize the shear cutting surface(tooth surface) of fine blanking. In this paper, the fine blanking tool sets were manufactured in order to study the die roll height according to V-ring position and die chamfer shape. They were constituted of the various die inserts and guide plate inserts with different die chamfer shape and different V-ring distance. The fine blanking experiment was conducted with the fine blanking tool sets and the die roll height of the gear samples was measured. It should be noted that die roll height on the fine blanked gear increased with increasing the V-ring distance and die chamfer angle.

• Journal of the Korea institute for structural maintenance and inspection
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• v.20 no.5
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• pp.44-49
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• 2016

Among the deteriorating agents, chloride ion is reported to be one of the most harmful ions due to its rapid diffusion and direct effect on steel corrosion. Cold joint which occurs in mass concrete placing is vulnerable to shear resistance and more severe deterioration. The paper presents an quantitative evaluation of chloride diffusion coefficient in OPC(Ordinary Portland Cement) and GGBFS(Ground Granulated Blast Furnace Slag) concrete containing cold joint. GGBFS concrete shows $6.6{\times}10^{-12}m^2/sec$ which is almost 30% level of OPC concrete results and the trend is repeated in the case of cold joint concrete. Compared with OPC concrete, GGBFS concrete is evaluated to have better resistance to chloride penetration, showing 0.30 times of chloride diffusion coefficient in concrete without cold joint 0.39 times with cold joint, respectively.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.5
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• pp.107-115
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• 2014

In this study, we examined the seismic isolation effects due to the difference between the center of mass of the building and the center of stiffness of isolation layer. Because the base isolation technique is a technique that is highly dependent on the performance of seismic isolation devices installed on the seismic isolation layer, we have to examine the horizontal stiffness of seismic isolation devices after making them. If difference between the design stiffness and the actual stiffness of the seismic isolation device occurred, a big problem may be generated in the upper members on the seismic isolation layer. The analytical results show that the more eccentricity increases, the more maximum response acceleration, story shear and the member forces of the upper part of the structure increases, and the damage is expected to be in excess. Therefore, it is recommended that if possible, isolation devices have to be designed to coincide the center of mass of the building with the center of stiffness of isolation layer. If not after making isolation devices, they need to be relocated to prevent the eccentricity.