• Tribology and Lubricants
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• v.37 no.3
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• pp.106-111
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• 2021

There are several studies on reducing the friction that occurs on the relative sliding contact surface of moving parts under extreme environments. In particular, a solid lubricated bearing is studied to solve the tribological problem with friction reduction and durability parts using solid lubricants (lead or silver) in a vacuum atmosphere. Galinstan is mainly used as a liquid metal lubricant, but it is inevitable to have limited tribological applications owing to its high coefficient of friction. Many researchers work on surface texturing for surface modification and precision processing methods. To increase durability and low friction, DLC coating with hydrophobicity is applied on the contact surface texture. Therefore, using an untextured specimen, a dimple specimen, and a DLC-coated dimple specimen under liquid metal lubrication, this paper presents the following experimental sliding friction characteristics in the sliding friction test. 1) The average coefficient of friction of the DLC-coated dimple specimen and dimple specimen are lower compared to that of a non-patterned specimen. 2) In the DLC-coated dimple specimens, the average coefficient of friction changes according to the change in the dimple density. 3) DLC-coated dimple specimens with a density of 12.5 have the lowest average coefficient of friction under 41.6 N of normal load and 143.3 RPM.

• International Journal of Highway Engineering
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• v.13 no.2
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• pp.167-174
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• 2011

This research was conducted to propose the design methodologies of post-tensioned concrete pavements (PTCPs) for specialized section pavements. The specialized sections considered in this study included bus stop sections in urban bus-only lanes and culvert existing highway sections that often showed severe failures. The PTCP designs of those specialized sections were performed based on both the stresses and the fatigue failures, and more conservative design results were selected. In the stress based design, the maximum tensile stress under extreme load conditions was obtained using finite element analyses first. Then, the number of tendons and the tendon spacing were determined so that the tensile stress was less than the allowable flexural strength. The AASHTO fatigue failure equations were used in the fatigue failure based design. From this study, the design methodologies of PTCP for bus stop sections in bus-only lanes and culvert existing highway sections were successfully suggested.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.3A
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• pp.209-216
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• 2009

The structural stability as well as economical efficiency of the wind sensitive structures are strongly dependant on accurate evaluation of the design wind speed. Present study demonstrates a useful wind data obtained at the wind monitoring tower in the Kwangyang Suspension Bridge site. Moreover the Measure-Correlate-Predict (MCP) method has been applied to estimate the long-term wind data at the bridge site based on the wind data at the local weather station. The measured data indicate that the turbulent intensities and roughness exponents are strongly affected by the wind direction and surrounding topography. The new design wind speed based on MCP method is 20m/s lower than that at the original estimation, and the resulting design wind load is only 36% of the old prediction. The field measurement of wind data is recommended to ensure the economical and secure design of the wind sensitive structures because the measured wind data reveal much different from the estimated one due to local topography.

• Earthquakes and Structures
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• v.5 no.4
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• pp.395-416
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• 2013

The magneto-rheological (MR) damper contributes to the new technology of structural vibration control. Its developments and applications have been paid significant attentions in earthquake engineering in recent years. Due to the shortages, however, inherent in deterministic control schemes where only several observed seismic accelerations are used as the trivial input and in classical stochastic optimal control theory with assumption of white noise process, the derived control policy cannot effectively accommodate the performance of randomly base-excited engineering structures. In this paper, the experimental and analytical studies on stochastic seismic response control of structures with specifically designed MR dampers are carried out. The random ground motion, as the base excitation posing upon the shaking table and the design load used for structural control system, is represented by the physically based stochastic ground motion model. Stochastic response analysis and reliability assessment of the tested structure are performed using the probability density evolution method and the theory of extreme value distribution. It is shown that the seismic response of the controlled structure with MR dampers gain a significant reduction compared with that of the uncontrolled structure, and the structural reliability is obviously strengthened as well.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.8
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• pp.77-84
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• 2003

Structural analyses of a turbine bladed-disk for a liquid rocket turbopump are peformed to investigate the safety level of strength and vibration at design point. Due to the high rotational speed of the turbopump, effects of centrifugal forces are carefully considered in the structural analysis. Thermal load caused by extreme temperature distribution is also considered as an external force applied to turbine bladed-disk. A three dimensional finite element method (FEM) is used for cyclic symmetry structural analyses with the MSC/NASTRAN DMAP Alter. Interblade phase angles are considered to investigate structural dynamic characteristics as a function of rotational speed. Through the numerical analysis, effects of centrifugal and thermal loads on the turbine bladed-disk are examined.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.1358-1363
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• 2003

Due to the application of Integrated Head Assembly (IHA) in KSNP+ reactor design, an investigation of reactor internals integrity is carried out to assure that the adoption of IHA does not affect the safety of reactor operation. One of the postulated accident events is the R.V. closure head fall from 5.5m high directly above the reactor vessel that may occur during the refueling operation. The analysis model consists of lumped mass elements of the entire reactor vessel and internals. Because of extreme load, separate elastic-plastic analyses are done for the members that undergo plastic deformation. The analysis verified that the stresses of the reactor internals and the fuel assemblies are within the bound of allowable stress limits and the integrity of the fuel assemblies is maintained.

• Journal of the Korean Solar Energy Society
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• v.31 no.6
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• pp.132-138
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• 2011

Inducing the natural lighting into building has been commonly applied to the spaces where artistic objects are displayed to create better visual environment for the appreciation. The direct natural lighting, especially through top lighting system may, however, bring forth discoloing of highly light-sensitive materials such as paintings or demanding additional cooling load. In addition, it causes to create glare against appreciators or to yield in visual distraction due to the extreme contrast on the inner surfaces of the spaces. Shading device such as louver is capable of preventing such voidable cases with careful manipulation. This study aims to provide basic design guide-line when the flat-form louver has to be applied to top lighting system. Interaction between the daylight and louver was simulated by Reluxpro program with variation of the attachment location, the angle of the lover, surface reflectivity, to obtain the illumination of the floor surface. This study yields the louver just under the top lighting window with 90 degree angle gives the most desirable way of employment when the uniform illumination on the floor surface is required.

• Magazine of the Korean Society of Agricultural Engineers
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• v.36 no.3
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• pp.113-121
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• 1994

Since it is difficult to expect the normal production of plants in greenhouses during hot summer season in Korea, certain provisions on the control of extreme environmental factors in summer should be considered for the year-round cultivation in greenhouses. This study was carried out to find a method to suppress the temperature rising of nutrient solution by cooling, which is able to contribute to the improvement of the plant growth environment in hydroponic greenhouse during hot summer season. A mechanical cooling system using the counter flow type with double pipe was developed for cooling the nutrient solution efficiently. Also the heat transfer characteristics of the system was analysed experimentally and theoretically, and compared with the existing cooling systems of nutrient solution. The cooling capacities of three different Systems, which used polyethylene tube in solution tank, stainless tube in solution tank, and the counter flow type with double pipe, were evaluated. The performance of each cooling system was about 41 %, 70% and 81 % of design cooling load in hydroponic greenhouse of 1 ,000m$^2$ on the conditions that the flow rate of ground water was 2m$^3$/hr and the temperature difference between two liquids was 10 ˚C According to the results analysed as above, the cooling system was found to have a satisfactory cooling capability for regions where ground water supply is available. Fer the other regions where ground water supply is restricted, more efficient cooling System should be developed.

• International Journal of Ocean System Engineering
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• v.2 no.2
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• pp.82-91
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• 2012

Fillet welding is widely used in the assembly of ships and offshore structures. The T-joint configuration is frequently reported to experience fatigue damage when a marine structure meets extreme loads such as storm loads. Fatigue damage is affected by the magnitude of residual stresses on the weld. Recently, many shipping registers and design guides have required that the fatigue strength assessment procedure of seagoing structures under wave-induced random loading and storm loading be compensated based on the effect of residual stresses. We propose a computational procedure to analyze the residual stresses in a T-joint. Residual stresses are measured by the X-ray diffraction (XRD) method, and a 3-D finite element analysis (FEA) is performed to obtain the residual stress profile in the T-joint. The proposed finite element model is validated by comparing experiments with computational results, and the characteristics of the residual stresses in the T-joint are discussed.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.6
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• pp.75-83
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• 2004

Ice rubble pieces broken by the bow impact load and side hull of an icebreaking vessel usually pass along the ship's bottom hull and may hit the propeller/rudder or other stern structures causing serious damage to ship's hull . Therefore it is important to estimate the size of broken ice pieces during the icebreaking process. The dynamic interaction process of icebreaker with infinite ice sheet is simplified as a wedge type beam of finite length supported by elastic foundation. The wedge type ice beam is leaded with vertical impact forces due to the inclined bow stem of icebreaking vessels. The numerical model provides locations of maximum dynamic bending moment where extreme tensile stress arises and also possible fracture occurs. The model can predict a failure length of broken ice sheet given design parameters. The results are compared to Nevel(1961)'s analytical solution for static load and observed pattern of ice sheet failure onboard an icebreaker. Also by comparing computed failure length with the characteristic length, the meaning of ice rubble sizes is discussed.