• Tribology and Lubricants
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• v.18 no.5
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• pp.351-356
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• 2002

The surfaces of machine components in sliding contact such as bearing, gears and pistons etc. frequently operate under the condition of mixed lubrication due to high load, high speed and slip. These machine components often undergo the inception of scuffing in practical application. The scuffing failure is a critical problem in modern machine components, especially for the requirement of high efficiency and small size. However, it is difficult to find a universal mechanism to explain all scuffing phenomena because there are so many factors affecting the onset of scuffing. In this study, scuffing experiments are conducted using Acoustic Emission(AE) measurement by an indirect sensing approach to detect scuffing failure. Acoustic Emission(AE) signal has been widely utilized to monitor the interaction at the friction interface. Using AE signals we can get an indication about the state of the friction processes, about the quality of solid and liquid layers on the contacting surfaces in real time. The FFT(Fast Fourier Transform) analyses of the AE signal are sued to understand the interfacial interaction and the relationship between the AE signal and the state of contact is presented.

• Journal of the Society of Naval Architects of Korea
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• v.29 no.1
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• pp.55-60
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• 1992

A study has been made on the hydrodynamic forces on and the motion response of a sliding block in a bay within the framework of linear potential theory. To simplify the problem, following assumptions are made : The configuration of the bay is a long channel with narrow width, constant depth and straight coastline. Incident waves are long compared to the depth. We applied matched asymptotic expansion techniques. The flued domain is subdivided into three regions ; ocean, bay entrance, bay regions. Boundary-vague problems are solved first in each region. Then unknown coefficients are determined by matching individual solutions at the intermediate region between two neighboring legions. It is found that the motion of the block is greatly amplified at the resonant frequencies, in particular at the quarter wavelength mode. We examined the mechanism of negative added mass, which results from the localized hydrodynamic resonance.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.281-284
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• 2007

The effect of Ca addition on the microstructure evolution and deformation behavior of AZ31 magnesium alloy produced by hot extrusion was investigated. For this purpose, Ca was added into AZ31 melts to the level of 0.7 and 2.0 wt.% Ca. Then, AZ31 base alloy and Ca modified AZ31 alloys were extruded at $383^{\circ}C$. Ca added alloys showed finer grain size and increased hardness value rather than AZ31 base alloy. After isothermal hot compression, the shape of tested specimen exhibited a noticeable anisotropy due to the crystallographic texture effect. The ratio of major and minor axes of ovality was not directly related to test condition and Ca amount. Flow stress level increases with the increase of Ca addition at temperature below $300^{\circ}C$ because of fine microstructure. However, at high temperature and low strain rate region ($400^{\circ}C$ and $10^{-3}s^{-1}$), reverse tendency was observed since main deformation mechanism changes from dislocation slip to grain boundary sliding or diffusional process at high temperature.

• Transactions of Materials Processing
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• v.27 no.1
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• pp.18-27
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• 2018

As one of the functional metal parts in steam turbine diaphragm assembly, the hollow-partitioned turbine nozzle (stator) has large and thick geometries, as well as an asymmetric configuration. Therefore it is hard to support a metal blank in the die cavity. To ease this situation and control posture and position of metal blank (workpiece), a blank support structure is newly introduced. The blank support structure is basically composed of enlarged arms from the blank, guide pins and linear bearings. It can help to control the intermediate blank without a critical sliding phenomenon. The operation mechanism of this blank support structure, during thick plate forming for the hollow-partitioned turbine nozzle stator, is first evaluated. A series of FEM-based numerical simulations, with respect to the width of the guide arm as geometric design parameters, are carried out to investigate its applicable range. As the results, it is observed the blank support structure for this thick plate forming can guide the workpiece to have stable posture during the plate forming process.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.994-995
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• 2006

The microstructure of the extruded Al-20Si bars showed a homogeneous distribution of eutectic Si and primary Si particles embedded in the Al matrix. The grain size of ${\alpha}-Al$ varied from 150 to 600 nm and the size of the eutectic Si and primary Si in the extruded bars was about 100 - 200 nm. The room temperature tensile strength of the alloy with a powder size $<26{\mu}m$ was 322 MPa, while for the coarser powder ($45-106{\mu}m$) it was 230 MPa. With decreasing powder size from $45-106{\mu}m$ to $<26{\mu}m$, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultrafine-grained constituent phases. The fracture mechanism of failure in tension testing and wear testing was also studied.

• Geomechanics and Engineering
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• v.5 no.1
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• pp.37-55
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• 2013

An important issue in the design of soil-nailing systems, as long-term retaining walls, is to assess their stability during seismic events. As such, this study is aimed at simulating the dynamic behavior and failure pattern of nailed structures using two series of numerical analyses, namely dynamic time history and pseudo-static. These numerical simulations are performed using the Finite Difference Method (FDM). In order to consider the actual response of a soil-nailed structure, nonlinear soil behaviour, soil-structure interaction effects, bending resistance of structural elements and construction sequences have been considered in the analyses. The obtained results revealed the efficiency of both analysis methods in simulating the seismic failure mechanism. The predicted failure pattern consists of two sliding blocks enclosed by three slip surfaces, whereby the bottom nails act as anchors and the other nails hold a semi-rigid soil mass. Moreover, it was realized that an increase in the length of the lowest nails is the most effective method to improve seismic stability of soil-nailed structures. Therefore, it is recommended to first estimate the nails pattern for static condition with the minimum required static safety factor. Then, the required seismic stability can be obtained through an increase in the length of the lowest nails. Moreover, placement of additional long nails among lowest nails in existing nailed structures can be considered as a simple retrofitting technique in seismic prone areas.

• Journal of Power System Engineering
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• v.10 no.4
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• pp.119-126
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• 2006

Gerotor hydraulic motor is widely used in hydraulic systems due to its low speed, high torque output and compactness in rotational direct driving of a heavy weight. Gerotor is a Planar mechanism consisted of a pair of rotor and circular teeth of stator assembly which forms a closed space, so called a chamber. The motion of rotor relative to the circular tooth is produced by the pressure difference of hydraulic operating fluid between the adjacent chamber. As all active contact points of rotor and circular teeth are subjected to very high sliding friction, a reduction in the performance of the gerotor hydraulic motor can not be avoided. Therefore, the core design parameters of gerotor profile used in hydraulic motors is to minimize a friction force by high contact stresses. The analytical design method of gerotor profile, based on envelope of a family of curves, is proposed. In this study, the influence of the tip clearances on three critical contact points between rotor and circular teeth of stator assembly has been explored by experimental data in this paper. At the same time a improvement method to reduce the friction force is proposed and the tip clearances on three critical points for getting an optimum gerotor profile are also analyzed.

• KSTLE International Journal
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• v.6 no.2
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• pp.45-50
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• 2005

Plasma ceramic spray that is applied on a machine part under severe work conditions has been investigated for tribological behavior. The application of ceramic coatings by plasma spray has become essential in tribosystems to produce wear resistance and long life in severe conditions. The purpose of this study was to investigate the wear characteristics of $8\%Y_{2}O_3-ZrO_2$ coating, in view of the effect of post-spay heat treatment. The plasma-sprayed $8\%Y_{2}O_3-ZrO_2$ coating was studied to know the relationship between phase transformations and wear behavior related to post-spray heat treatment. Wear test was carried out with ball on disk type on normal loads of 50N,70N and 90N under room temperature. The phase transformation of phase and the value of residual stress were measured by X-ray diffraction method(XRD). Tribological characteristics and wear mechanisms of coatings were observed by SEM. The tribological wear performance was discussed in the focusing of residual stress. Consequently, post-spray heat treatment plays an important role in decreasing residual stress. Residual stress in the coating system has a significant influence on the wear mechanism of coating.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2000.11a
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• pp.332-340
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• 2000

Friction properties of automotive brake pads containing different types of abrasivess were investigated. Five different abrasives, including o-quartz, magnesia, magnetite, alumina, zircon, were employed in this investigation and size effects of the abrasives on friction characteristics were also studied using 1, 50, 140$\mu\textrm{m}$ size zircon. Experimental results showed that the hardness and size of these abrasive particles were strongly related to friction behaviors and wear mechanisms. Harder and smaller abrasives showed higher friction coefficient and more wear. The surfaces of friction materials with different sizes of abrasives showed that two different modes of abrasion (two-body and three-body abrasion) appeared during sliding. Considering the above results, abrasive materials were thought to destroy transfer film and the extent of the destruction depends on the types and sizes of abrasive particles. A mechanism of the wear mode transition (two-body to three body abrasive motion) was suggested considering the binding energy and friction energy in terms of abrasive particle size.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2000.11a
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• pp.406-413
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• 2000

The friction and wear behaviors of Magnetron Sputtered MoS$_2$films were investigated by using a pin on disk type tester which was designed and manufactured for this experiment. The experiment was conducted by using silicon nitride (Si$_3$N$_4$) as a pin material and Magnetron Sputtered MoS$_2$on bearing steel (STB2) as a disk material, under operating conditions that include different surface roughness (Polishing specimen, Grinding specimen)(2types), linear sliding velocities in the range of 22, 44, 66mm/sec (3types), normal loads vary from 9.8N, 19.6N, 29.4N(3types), corresponding to contact pressures of 1.9∼2.7GPa and atmospheric conditions of high vacuum( 1.3${\times}$10$\^$-4/Pa), medium vacuum( 1.3${\times}$10$\^$-l/Pa), ambient air(10$\^$5/Pa)(3types). We investigated fracture mechanism in magnetron sputtered MoS$_2$films with Magnetron Sputtering method in each experiment.