• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.1
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• pp.51-61
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• 1993

A systematic approach of the energy method is proposed for analysis of axisymmetric deep drawing in which the total deforming region is divided into five sections by the geometric characteristic. The corresponding solution is found through optimization of the total energy dissipation with respect to some parameters assumed in the kinematically admissible velocity field defined over each region. The sheet blank is divided into three-or five-layers to consider the bending effect. For the evaluation of frictional energy, it is assumed that the blank holding force acts on the outer rim of the flange and that the contact pressure acting on punch shoulder or die shoulder has uniform distributions, respectively. The computed results by the present method are compared with the experiment and the computed results by the elastic-plastic finite element method for the distribution of thickness strain and the relation between the punch stroke and punch load. The results for the case of multi-layers show better agreements than for the case of a single layer in load vs. stroke relation and strain distribution. It is thus shown that the multi-layer technique can be effectively employed in analyzing axisymmetric deep drawing in connection with the energy method.

• Journal of the Korean Magnetics Society
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• v.2 no.1
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• pp.15-21
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• 1992

Acicular ${\gamma}-Fe_{2}O_{3}$ particles were heated at $90^{\circ}C$ in alkaline solution containing mixed solution of dyadic metal with $Co^{+2}/Fe^{+2}$ ratio of 0.5. When cobalt content was increased, the coercivity of resultant product increased linearly, and surface area decreased. The cobalt ferrite was grown epitaxially on the surface ${\gamma}-Fe_{2}O_{3}$ crystal, and the increase of coercivity was attributed to the crystalline magnetic anisotropy of the cobalt ferrite which is conform to coating layer. We can expect superior magnetic properties above normal ratio of 2. The progress of reaction has an effect on coercivity of cobalt ferrite epitaxial iron oxide. The stability of temperature and the change om standin& of $Co-{\gamma}-Fe_{2}O_{3}$ was largely influenced by the composition of coating layer.

• Journal of the Korean Magnetics Society
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• v.4 no.4
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• pp.313-318
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• 1994

We have investigated the effects of deposition angle on magnetic and magneto-optic properties in Co/Pt multilayer thin films. which were prepared bye-beam evaporation on tilted substrates. with varying tilt angle from $0^{\circ}$ to $60^{\circ}$. The structure of the specimens was examined by x-ray diffractometer and scanning electron microscope. and the magnetic and magneto-optical properties were measured by VSM, torque magnetometer, and Kerr loop tracer. X-ray diffractometry revealed that all of the specimens had multilayer structure and growth orientation of column followed the tangent rule but the crystallograpic orientation, <111>, was slightly deviated from the substrate normal even though the deposition angle was increased up to $60^{\circ}$. A decrement of the magnetization and Kerr angle with the deposition angle was related with that of the film density due to increasing porosity. The perpendicular mag¬netic anisotropy was also decreased with increasing the deposition angle.

• Journal of the Korean Ceramic Society
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• v.37 no.1
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• pp.21-25
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• 2000

Gas pressure sintered silicon nitride based composites with 5 wt% $\beta$-Si3N4 whiskers were prepared, and the variations depending on sintering additives and sintering temperature were studied. Sintering additives were 6 wt% Y2O3-1 wt% MgO(6Y1M), 6 wt%Y2O3-1 wt% Al2O3(6Y1A), 6 wt% Y2O3-1 wt% SiO2(6Y1S), and whiskers were unidirectionally oriented by a modified tape casting technique. Samples were fully densified by gas pressure sintering at 2148 K and 2273 K. As the sintering temperature increased, the size of large elongated grains was increased. Three point flexural strength of 6Y1M and 6Y1M samples was higher than that of 6Y1S sample, and the strength decreased as the sintering temperature increased. The indentation crack length became shorter for the sample sintered at higher temperature, and the difference between the cracks length parallel to and normal to the direction of whisker alignment was decreased. In case of cracks 45$^{\circ}$off the whisker alignment direction, the crack length anisotropy disappeared.

• Journal of the Korean Geotechnical Society
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• v.23 no.3
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• pp.89-100
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• 2007

In the companion paper, we provided the novel elastic-plastic constitutive model based on the micromechanics theory. Herein, the elastic and elastic-plastic deformation of granular soils is meticulously analyzed. To guarantee high accuracy of the microscopic parameter, the systematic procedure to evaluate the parameters is provided. The analysis of the elastic response during the isotropic and triaxial compression shows that the stress-level dependency of cross-anisotropic elastic moduli is induced by the power relationship of the contact force in the normal contact stiffness, while the evolution of fabric anisotropy is more pronounced during triaxial compression. The micromechanical analysis indicates that the plastic strains are likely to occur at very small strains. The plastic deformation of tangential contacts has an important role in the reduction of soil stiffness during axial loading.

• Geophysics and Geophysical Exploration
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• v.9 no.1
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• pp.10-18
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• 2006

We measured the changes in P-wave velocity that occur when injecting $CO_2$ in gaseous, liquid, and supercritical phases into water-saturated anisotropic sandstones. P-wave velocities were measured in two cylindrical samples of Tako Sandstone, drilled along directions normal and parallel to the bedding plane, using a piezo-electric transducer array system. The velocity changes caused by $CO_2$ injection are typically -6% on average, with maximum values about -16% for the case of supercritical $CO_2$ injection. P-wave velocity tomograms obtained by the differential arrival-time method clearly show that $CO_2$ migration behaviour is more complex when $CO_2$ flows normal to the bedding plane than when it flows parallel to bedding. We also found that the differences in P-wave velocity images were associated both with the $CO_2$ phases and with heterogeneity of pore distribution in the rocks. Seismic images showed that the highest velocity reduction occurred for supercritical $CO_2$ injection, compared with gaseous or liquid $CO_$ injection. This result may justify the use of the seismic method for $CO_2$ monitoring in geological sequestration.

• Tribology and Lubricants
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• v.11 no.5
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• pp.128-135
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• 1995

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales, ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer, and boundary lubrication and for nanofabrication/nanomachining purposes. Micro/nanotribological studies of single-crystal silicon, natural diamond, magnetic media (magnetic tapes and disks) and magnetic heads have been conducted. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurements of atomic-scale friction of a freshly-cleaved highly-oriented pyrolytic graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in friction and those in corresponding topography were displaced relative to each other. Variations in atomic-scale friction and the observed displacement has been explained by the variations in interatomic forces in the normal and lateral directions. Local variation in microscale friction is found to correspond to the local slope suggesting that a ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both micro- and macro scales which results from the surface preparation and anisotropy in surface roughness. Microscale friction is generally found to be smaller than the macrofriction as there is less ploughing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load approaching to the macrofriction at contact stresses higher than the hardness of the softer material. Wear rate for single-crystal silicon is approximately constant for various loads and test durations. However, for magnetic disks with a multilayered thin-film structure, the wear of the diamond like carbon overcoat is catastrophic. Breakdown of thin films can be detected with AFM. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nono scratches. AFM has been modified to obtain load-displacement curves and for nanoindentation hardness measurements with depth of indentation as low as 1 mm. Scratching and indentation on nanoscales are the powerful ways to screen for adhesion and resistance to deformation of ultrathin fdms. Detection of material transfer on a nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thichness with a lateral resolution on a nanoscale have been conducted using AFM. Self-assembled monolyers and chemically-bonded lubricant films with a mobile fraction are superior in wear resistance. Finally, AFM has also shown to be useful for nanofabrication/nanomachining. Friction and wear on micro-and nanoscales have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribological studies may help def'me the regimes for ultra-low friction and near zero wear.

• The Journal of Engineering Geology
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• v.21 no.4
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• pp.323-336
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• 2011

The hydro-mechanical behavior of the Earth's crust is strongly dependent on the fractional volume and geometrical structure of effective pore spaces. This study aims to understand the characteristics of pores using electrical impedance. We measured the electric impedance of core samples (diameter, 38-50 mm; length, 70-100 mm) of three types of granite (Hwangdeung, Pocheon, and Yangsan) and two types of sandstone (Boryung and Berea) with different porosities and pore structures, after saturation with saline water of varying salinities. The results show that resistance decreases but capacitance increases with increasing salinity of the pore fluid. For a given salinity, the resistivity and formation factor are reduced with increasing porosity of the rocks, and the capacitance increases. Berea sandstone shows anisotropy in resistance, tortuosity, and cementation factor, with these factors being highest normal to bedding planes. This result indicates that the connectivity of pores is weakest normal to bedding. In conclusion, the electrical characteristics of the tested samples are related not only to their porosity but also to the pore geometry.

• Journal of the Korean Ceramic Society
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• v.36 no.1
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• pp.43-49
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• 1999

Gas pressure sintered silicon nitride based composites with 0~5wt% $\beta$-Si3N4 whiskers were prepared. The whiskers were unidirectionally oriented by a modified tape casting technqiue and green bodies with various microstructure were formed by changing stacking sequences of sheets cut from the tape. Orientations of the large elongated grains of the sample after gas pressure sintering were the same as the those of the whiskers of green body, and the sintering shrinkage and mechanical properties of sintered sample were consistent with the microstructural characteristics. In case of unidirectional samples, the sintering shrinkage normal to whisker alignment direction was larger than that parallel to the direction. The shrinkage difference inceaed as the whiskercontent increaed. As whisker content increaed, the crack length normal to and parallel to tape casting direction became shorter and larger, respectively. Although the grain size increased by th whisker addition, the flexural strength of unidirectional samples was not lower than that of smaple without the whisker. In case of crossplied and 45$^{\circ}$rotated samples, the anisotropy of mechanical preoperties disappeared.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.41-49
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• 2005

We have developed a random heterogeneous velocity model with bimodal distribution in methane hydrate-bearing Bones. The P-wave well-log data have a von Karman type autocorrelation function and non-Gaussian distribution. The velocity histogram has two peaks separated by several hundred metres per second. A random heterogeneous medium with bimodal distribution is generated by mapping of a medium with a Gaussian probability distribution, yielded by the normal spectral-based generation method. By using an ellipsoidal autocorrelation function, the random medium also incorporates anisotropy of autocorrelation lengths. A simulated P-wave velocity log reproduces well the features of the field data. This model is applied to two simulations of elastic wane propagation. Synthetic reflection sections with source signals in two different frequency bands imply that the velocity fluctuation of the random model with bimodal distribution causes the frequency dependence of the Bottom Simulating Reflector (BSR) by affecting wave field scattering. A synthetic cross-well section suggests that the strong attenuation observed in field data might be caused by the extrinsic attenuation in scattering. We conclude that random heterogeneity with bimodal distribution is a key issue in modelling hydrate-bearing Bones, and that it can explain the frequency dependence and scattering observed in seismic sections in such areas.