• 한국수자원학회논문집
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• 제52권4호
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• pp.255-264
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• 2019

산지하천 만곡부의 홍수피해를 줄이고자 사다리꼴 단면의 돌출줄눈을 하천옹벽에 설치하였다. 본 연구에서는 사다리꼴 형상에 의한 흐름저항 효과를 파악하고자 개수로 측벽에 사다리꼴 돌출줄눈을 설치하여 수리실험을 수행하였다. 벽면조도가 k형에 해당하는 ${\lambda}_{nv}$가 6, 9, 12인 경우에 대해 유량에 따른 흐름특성을 파악하였다. 흐름저항은 돌줄줄눈의 설치간격이 멀어짐에 따라 전반적으로 증가하였다. 고유량 조건에서 최대마찰계수는 ${\lambda}_{nv}$가 9일 때 발생하였다. 사다리꼴 돌출줄눈은 정사각형 돌출줄눈과 비교해 흐름저항은 상대적으로 작았지만, 사다리꼴의 형상저항은 전체 흐름저항의 평균 62%를 차지했다. 벽면조도 증가에 따른 흐름저항 효과를 극대화하기 위한 사다리꼴 돌출줄눈의 최적 설치간격은 돌출줄눈 높이의 9~12배 범위임을 확인하였다.

• Geomechanics and Engineering
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• 제25권1호
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• pp.49-58
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• 2021

The resistance of soil to the tractive force of flowing water is one of the essential parameters for the stability of the soil when directly exposed to the movement of water such as in rivers and ocean beds. Biopolymers, which are new to sustainable geotechnical engineering practices, are known to enhance the mechanical properties of soil. This study addresses the surface erosion resistance of river-sand treated with several biopolymers that originated from micro-organisms, plants, and dairy products. We used a state-of-the-art erosion function apparatus with P-wave reflection monitoring. Experimental results have shown that biopolymers significantly improve the erosion resistance of soil surfaces. Specifically, the critical shear stress (i.e., the minimum shear stress needed to detach individual soil grains) of biopolymer-treated soils increased by 2 to 500 times. The erodibility coefficient (i.e., the rate of increase in erodibility as the shear stress increases) decreased following biopolymer treatment from 1 × 10-2 to 1 × 10-6 times compared to that of untreated river-sands. The scour prediction calculated using the SRICOS-EFA program has shown that a height of 14 m of an untreated surface is eroded during the ten years flow of the Nakdong River, while biopolymer treatment reduced this height to less than 2.5 m. The result of this study has demonstrated the possibility of cross-linked biopolymers for river-bed stabilization agents.

• Journal of Welding and Joining
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• 제35권2호
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• pp.58-62
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• 2017

A319 aluminum alloy containing 6.5% Si and 3.5% Cu as major alloying elements has been widely used in machinery parts because of its excellent castability and crack resistance. However it needs more wear resistance to extend its usage to the severe wear environments. It has been known that hyper-eutectic Al-Si alloy having more than 12.6% Si contains pro-eutectic Si particles, which give better wear resistance and lubrication characteristics than hypo-eutectic Al-Si alloy like A319 alloy. In this study, it was tried to clad hyper-eutectic Al-Si alloy on the surface of A319 alloy. In the experiments, Al-36%Si alloy powder was mixed with organic binder to make a fluidic paste. The paste was screen-printed on the A319 alloy surface, melted by pulsed Nd:YAG laser and alloyed with the A319 base alloy. As experimental parameters, the average laser power was changed to 111 W, 202 W and 280 W. With increasing the average laser power, the melting depth was changed to $142{\mu}m$, $205{\mu}m$ and $245{\mu}m$, and the dilution rate to 67.2 %, 72.4 % and 75.7 %, and the Si content in the cladding layer to 16.2 %, 14.6 % and 13.7 %, respectively. The cross-section of the cladding layer showed very fine eutectic microstructure even though it was hyper-eutectic Al-Si alloy. This seems to be due to the rapid solidification of the melted spot by single laser pulse. The average hardness for the three cladding layers was HV175, which was much higher than HV96 of A319 base alloy. From the block-on-roll wear tests, A319 alloy had a wear loss of 5.8 mg, but the three cladding layers had an average wear loss of 3.5 mg, which meant that an increase of 40 % in wear resistance was obtained by laser cladding.

• Journal of Microbiology
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• 제35권2호
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• pp.103-108
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• 1997

To improve the detection of methicillin resistant staphylococci, lowered incubation temperature (30.deg.) and inclusion of sodium chloride in media have been empirically recommended. However, in this study, we found that sodium chloride in Peptone-Yeast Extract-K$\_$2/HPO$\_$4/ (PYK) medium decreased methicillin minimum inhibitory concentrations. Divalent cations were shown to restore the expression of staphylococcal methicillin resistance. However, when it was determined by efficiency of plating, sodium chloride increased methicillin resistance expression on agar medium in which higher divalent cations were contained in the agar medium. The decrease of minimum inhibitory concentrations at 30.deg.C by sodium chloride occurred in Brain Heart Infusion but did not occur in other media investigated. Interestingly, both PYK and Brain Heart Infusion media had peptone, which contain cholic acids having detergent activities. Inclusion of sodium chloride in PYK caused a higher rate of autolysis. Penicillin binding protein 2a that has a low affinity to beta-lactam antibiotics, was highly inducible in methicillin resistant Staphylococcus epidermidis strains. In this study, we found that autolysins that are activated by the sodium chloride decreased the minimum inhibitory concentration at 30.deg.C, and peptidoglycan is weakened due to the presence of methicillin. Peptone in the media may aggravate the fragile cells. However, stabilization due to the presence of divalent cations and production of penicilin binding protein 2a increase the survival of staphylococci.

• Structural Engineering and Mechanics
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• 제62권6호
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• pp.663-674
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• 2017

This paper presents an approach for evaluating performance of prestressed concrete (PC) bridge girders exposed to fire. A finite element based numerical model for tracing the response of fire exposed T girders is developed in ANSYS. The analysis is carried out in three stages, namely, fire temperature calculation, cross sectional temperature evaluation, and then strength, deformation and effective prestress analysis on girders exposed to elevated temperatures. The applicability of the computer program in tracing the response of PC bridge girders from the initial preloading stage to failure stage, due to combined effects of fire and structure loading, is demonstrated through a case study, and validated by test data of a scaled PC box girder under ISO834 fire condition. Results from the case study show that fire severity has a significant influence on the fire resistance of PC T girders and hydrocarbon fire is most dangerous for the girder. The prestress loss caused by elevated temperature is about 10% under hydrocarbon fire till the girder failure, which can lead to the increase in deflection of the PC girder. The rate of deflection failure criterion is suggested to determine the failure of PC T girder under fire.

• 한국표면공학회지
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• 제37권4호
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• pp.234-241
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• 2004

Cavitation erosion properties of the Cu-9Al-4.5Ni-4.5Fe alloys (Al-bronze) surface-modified by flame quenching process have been investigated. After flame quenching at above $T_{\beta}$, the surfaces of Al-bronze with $\alpha$ + $\textsc{k}$ structure have been changed into the $\alpha$ + $\beta$' martensite phases by the eutectoid reaction of $\alpha$ + $\textsc{k}$\longrightarrow$\beta$ followed by the martensite transformation of $\beta$\longrightarrow$\beta$'. As a result of cavitation test, the measured incubation time and erosion rate of the $\alpha$ + $\beta$' alloy was 1.2 times higher and 1.5 times lower, respectively, compared to those of the conventional $\alpha$ + $\textsc{k}$ alloys, showing a remarkable increase of cavitation resistance with the formation of $\beta$' martensite. This is attributed to a preferential erosion of the $\textsc{k}$ precipitates that show the lowest resistance among the $\alpha$, $\textsc{k}$, $\beta$' phases under cavitation loading.ases under cavitation loading.

• 디지털산업정보학회논문지
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• 제6권1호
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• pp.69-81
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• 2010

Physical manifestations of aging due to the lack of exercise include the slowing down of motor learning, cardiopulmonary degradation, and the increasing difficulty to adapt to the environment. Aging is manifested with the lack of aerobic exercise work, decrease in muscular endurance, decline in skeletal and muscular strength, flexibility and agility, and the decrease in reaction speed and balance. Added to those are aging-related physiological changes, including the reduction of muscle bulk, increased body fat, decrease in total body water and basic metabolic rate as activities are reduced, and a decrease in cell and Lean Body Mass (LBM). These changes are known to cause problems. Interest and participation in appropriate physical activities among the elderly is needed to help them increase stamina, avoid diseases, maintain a clear intellect, and basically enable the elderly to live their daily lives as easy as possible. Therefore, physical activities are necessary for the elderly to enhance health-related factors. Special exercises should be performed for the enhancement of muscle function, muscle endurance, flexibility, agility, and balance. An accurate measurement of cardio-respiratory endurance and stamina through basic physical and cognitive characteristics of older adults is also required to ensure safety. Also, the development of a more scientific resistance exercise prescription system for the elderly is desperately needed.

• 반도체디스플레이기술학회지
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• 제21권1호
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• pp.119-126
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• 2022

MAS-based glass, which has been studied to replace the ceramic material used in the plasma etching chamber, has problems such as forming and processing due to its high melting temperature. To solve this problem, in this study, fluoride was added to the existing MAS-based glass to increase the workability in the glass manufacturing and to improve the chemical resistance to CF₄/Ar/O₂ plasma gas. Through RAMAN analysis, the structural change of the glass according to the addition of fluoride was observed. In addition, it was confirmed that high-temperature viscosity and thermal properties decreased as the fluoride content increased and plasma resistance was maintained, it showed an excellent etching rate of up to 11 times compared to quartz glass.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2022년도 가을 학술논문 발표대회
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• pp.107-108
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• 2022

In recent years, the construction industry has a tendency to increase of high-rise builidngs. High rise buildings can use limited space efficiently. But High rise buildings have problem that have extremely heavy weight. Various studies are being conducted to reduce the weight of buildings. Although lightweight aggregate is a meterial that can effectively reduce the weight of buildings, the strength of the aggregate itself is weak and the absorption rate is high, so the strength of the ITZ(Interfacial Transition Zone) area is weak. Therefore, it is essential to improve the interfacial area when using lightweight aggregates. In this study, an experiment was conducted to improve the adhesion between the aggregate and cement paste and to strengthen the interfacial area by coating the surface of the lighteight aggregate with Blast Furnace Slag. To confirm the improvement, compressive strength and EIS(Electrochemical Impedance Spectroscopy) measurements were perfromed. Using EIS, the change in electrical resistance of the cement hardened body was confirmed. As a result, it was confirmed that the lightweight aggregate coated on the surface showed highter compressive strength and electrical resistance than the non-coated lightweight aggregate, and that the coating material was filled in the interfacial area and inside the aggregate that helped to strengthen the compresssive strength and higher electrical resistance.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2003년도 춘계학술대회논문집
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• pp.719-724
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• 2003

The present experimental study deals with noise reduction and improvements in cooling performance in a plasma display panel(PDP) television (TV). To reduce the noise, the effects of installation parameters are studied. The experimental parameters under investigation are the distance between the fan and the rear case of a PDP TV, position of the strut on the fan, and the fan RPM. The variance of RPM is the most significant facto., and a 250 RPM decrease from 910 RPM causes about 4㏈(A) reduction in the system noise. To increase performance, flow characteristics are investigated by using a visualization technique and measuring the volume flow rate. The visualized results show that a radial direction flow due to large system resistance is significant, and an axial velocity oscillation is observed from the measurement of the volume flow rate. To prevent both a radial direction flow and an axial velocity oscillation, sponges are inserted in the space between f3n and the rear case. Inserted sponges improve the volume flow rate of cooling fans up to 32% since they convert a radial direction flow to an axial direction flow. Also an axial velocity oscillation with large amplitude and low RPM disappears. Increasing volume flow rate causes the PDP TV to improve its cooling performance. Additionally the same volume flow rate can be obtained with a decreased fan speed due to the inserted sponge. Noise reductions of 4.2 ㏈(A) at the rear and 1.1 ㏈(A) at the front of the TV are obtained by the decreased RPM. An increase of 10% of the volume flow rate is also achieved by inserting sponges.