• Journal of the Korean Ceramic Society
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• v.35 no.8
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• pp.850-856
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• 1998

The dynaamic fatigue behavior of alumina ceramics was observed at room temperature using four-point bending method. Dynamic fatigue fracture strength was observed as function of down speed and notch length. The crack growth exponent of the specimens was calculated from the fracture strength and lifetime in dynamic fatigue test. After loading the stresses in the range of 0% to 105% compared with the average in-ert strength the value of residual fracture strength was measured for unnotched and 0.5mm notched speci-mens at the 0.001 and 0.0005 mm/min down speed respectively. After the 95% stress of the average inert strength was applied repeatedly the value of rsidual fracture strength was measured for 0.5mm notched specimens at the 0.001 and 0.0005 mm/min down speed respectively. The material constant A was found to be almost the same and not to depend on the loading mode or the down speed for unnotched and notched specimen. The value of fracture strength with time calculated from the constants n and A was in good agreement with the measured value.

• Tribology and Lubricants
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• v.9 no.1
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• pp.62-69
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• 1993

Friction and wear behavior of a carbon/carbon composite material for aircraft brake material was experimentally investigated. Friction and wear test setup was designed and built for the experiment. Friction and wear tests were conducted under various sliding conditions. Friction coefficients were measured and processed by a data acquisition system and amount of wear measured by a balance. Stainless steel disk was used as the counterface material. Temperature was also measured by inserting thermocouple 2.5 mm beneath the sliding surface of the carbon/carbon composite specimen. Wear surfaces were observed by SEM, and analyzed by EDAX. The experimental results showed that sliding speed and normal force did not have significant effects on friction coefficient and wear factor of the composite. Temperature increase just below the surface was not large enough to cause any thermal degradation or oxidation which occurred at higher temperature when tested by TGA. Wear film was generated both on the specimen and on the counterface at relatively low sliding speed but cracks, grooves, and wear debris were observed at high sliding speed. Friction coefficient remained almost constant when the sliding speed or normal load was varied. It is believed that the adhesive and abrasive components contributed mainly to the friction coefficient. Wear behavior at low sliding speed was governed by wear film formation and adhesive wear mechanism. At high speed, fiber orientation, ploughing by counterface asperities, and fiber breakage dominated wear of the carbon/carbon composite.

• Tribology and Lubricants
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• v.27 no.6
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• pp.351-355
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• 2011

Al/SiC composites were fabricated by thermal spray process, and the dry sliding wear tests were performed using the various sliding speed of 10, 30, 60 and 90 RPM through 1000 cycles. The applied load was 10 N and radius of wear track was 15 mm. Wear tracks on the Al/SiC composites were investigated using scanning electron microscope(SEM) and energy dispersive X-ray spectroscopy (EDS). In the case of sliding speed of 10 RPM, adhesive wear behavior caused by plastic deformation of composits surface was observed. In the cases of sliding speed of 30, 60, 90 RPM, abrasive wear behavior on the adhered layer formed by debris were observed. Through this study, it was found that the wear behavior of Al/SiC composites was mainly influenced by the sliding speed.

• Proceedings of the KSR Conference
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• 2009.05b
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• pp.409-417
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• 2009

This paper is focused on a relationship between the noise and vibration of a wheelset and the railway irregularity of a high speed railway using a time-varying frequency transform for a preliminary research of the railway condition monitoring by an in-service high-speed railway vehicle. Generally, the monitoring has been performed by a special railway inspection vehicle or industrial engineers for railway maintenance. However, they have been limited at night due to the in-service high-speed railway vehicles, and too slow to monitor all of the section. To solve this problem, the monitoring should be performed by an in-service high-speed railway vehicle. For the research, the noise and vibration of a wheelset are utilized, because they are closely related to the railway condition. They are measured by using some microphones and accelerometers, and stored in an on-board data acquisition system. The signals are post-processed by a time-varying frequency analysis and compared with the result of a railway geometry and profile measurement system. From the comparison, it is able to observe the relationship between the noise and vibration of a wheelset and the irregularity of a high-speed railway. Also, some distinct frequency components are observed, which are not observed in the railway geometry and profile.

• Proceedings of the Korean Society for Agricultural Machinery Conference
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• 1993.10a
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• pp.1053-1062
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• 1993

The effect of loading speed on soil failure was studied by using a high speed triaxial compression test. Tests were conducted at 0.35-6.2m/s loading speed to compress soil specimens of sandy loam at different moisture contents. The axial stress at fracture increased with increase in loading speed up to certain critical speeds, however they decreased as the speed up to certain critical speeds, however they decreased as the speed increased further. Experiments were also conducted in the field of sandy loam soil with the vibrating tillage tool. Tests were done at 0.33-0.85m/s tractor speed oscillating frequency 13.7hz and oscillating amplitude 59mm. The maximum oscillating velocity of tillage tool was 2.5m/s. It was observed that for the oscillating operation, initially draft slightly increased with increase in forward speed and then it decreased .For the non-oscillating operation, draft increased continuously with increase in forward speed. Approach of studying soil failure in the laboratory test can be related to the field experiments.

• Atmosphere
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• v.18 no.3
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• pp.171-182
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• 2008

In this study, we analyzed the spatial and temporal distribution of wind resources over Korea based on hourly observational data recorded over a period of 5 years from 457 stations belonging to Korea Meteorological Administration (KMA). The surface and 850 hPa wind data obtained from the Korea Local Analysis and Prediction System (KLAPS) and the Regional Data Assimilation and Prediction System (RDAPS) over a period of 1 year are used as supplementary data sources. Wind speed is generally high over seashores, mountains, and islands. In 62 (13.5%) stations, mean wind speeds for 5 years are greater than $3ms^{-1}$. The effects of seasonal wind, land-sea breeze, and mountain-valley winds on wind resources over Korea are evaluated as follows: First, wind is weak during summer, particularly over the Sobaek Mountains. However, over the coastal region of the Gyeongnam-province, strong southwesterly winds are observed during summer owing to monsoon currents. Second, the wind speed decreases during night-time, particularly over the west coast, where the direction of the land breeze is opposite to that of the large-scale westerlies. Third, winds are not always strong over seashores and highly elevated areas. The wind speed is weaker over the seashore of the Gyeonggi-province than over the other seashores. High wind speed has been observed only at 5 stations out of the 22 high-altitude stations. Detailed information on the wind resources conditions at the 21 stations (15 inland stations and 6 island stations) with high wind speed in Korea, such as the mean wind speed, frequency of wind speed available (WSA) for electricity generation, shape and scale parameters of Weibull distribution, constancy of wind direction, and wind power density (WPD), have also been provided. Among total stations in Korea, the best possible wind resources for electricity generation are available at Gosan in Jeju Island (mean wind speed: $7.77ms^{-1}$, WSA: 92.6%, WPD: $683.9Wm^{-2}$) and at Mt. Gudeok in Busan (mean wind speed: $5.66ms^{-1}$, WSA: 91.0%, WPD: $215.7Wm^{-2}$).

• Korean Journal of Materials Research
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• v.21 no.7
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• pp.396-402
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• 2011

In this study, by using tin chloride solution as a raw material, a nano-sized tin oxide powder with an average particle size below 50 nm is generated by a spray pyrolysis process. The properties of the generated tin oxide powder depending on the inflow speed of the raw material solution are examined. When the inflow speed of the raw material solution is 2 ml/min, the majority of generated particles appear in the shape of independent polygons with average size above 80-100 nm, while droplet-shaped particles show an average size of approximately 30 nm. When the inflow speed is increased to 5 ml/min, the ratio of independent particles decreases, and the average particle size is approximately 80-100 nm. When the inflow speed is increased to 20 ml/min, the ratio of droplet-shaped particles increases, whereas the ratio of independent particles with average size of 80-100 nm decreases. When the inflow speed is increased to 100 ml/min, the average size of the generated particles is around 30-40 nm, and most of them maintain a droplet shape. With a rise of inflow speed from 2 ml/min to 5 ml/min, a slight increase of the XRD peak intensity and a minor decrease of specific surface area are observed. When the inflow speed is increased to 20 ml/min, the XRD peak intensity falls dramatically, although a significant rise of specific surface area is observed. When the inflow speed is increased to 100 ml/min, the XRD peak intensity further decreases, while the specific surface area increases.

• Journal of Ocean Engineering and Technology
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• v.35 no.6
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• pp.414-425
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• 2021

Overcrowding of high-rise buildings in urban zones change the airflow pattern in the surrounding areas. This causes building wind, which adversely affects the wind environment. Building wind can generate more serious social damage under extreme weather conditions such as typhoons. In this study, to analyze the wind speed and wind speed ratio quantitatively, we installed five anemometers in Haeundae, where high-rise buildings are dense, and conducted on-site monitoring in the event of typhoon OMAIS to determine the characteristics of wind over skyscraper towers surround the other buildings. At point M-2, where the strongest wind speed was measured, the maximum average wind speed in 1 min was observed to be 28.99 m/s, which was 1.7 times stronger than that at the ocean observatory, of 17.0 m/s, at the same time. Furthermore, when the wind speed at the ocean observatory was 8.2 m/s, a strong wind speed of 24 m/s was blowing at point M-2, and the wind speed ratio compared to that at the ocean observatory was 2.92. It is judged that winds 2-3 times stronger than those at the surrounding areas can be induced under certain conditions due to the building wind effect. To verify the degree of wind speed, we introduced the Beaufort wind scale. The Beaufort numbers of wind speed data for the ocean observatory were mostly distributed from 2 to 6, and the maximum value was 8; however, for the observation point, values from 9 to 11 were observed. Through this study, it was possible to determine the characteristics of the wind environment in the area around high-rise buildings due to the building wind effect.

• Journal of Korea Foundry Society
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• v.18 no.6
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• pp.570-577
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• 1998

An investigation has been conducted to describe solidification characteristics in Sn-Zn binary system and Sn-Zn-Ag ternary system added by Ag produced by the continuous casting process using heated mold as a basic study for developing Pb-free solder materials. To obtain the continuous casting rods with mirror surface and near net shape at higher casting speed, water flow rates must be increased and mold temperature must be lowered. However, surface tearing in the casting rods occured at lower continuous casting speed while break out occured at higher continuous casting speed even if optimum conditions such as water flow rate and heated mold temperature are determined. Primary ${\alpha}Sn$ and eutectic structure in unidirectioally solidified Sn-Zn alloys were finer with increased casting speed. But, directionality may not be expected for primary Zn in hypereutectic Sn-Zn alloy. It was found that the addition of $0.2{\sim}0.8%$ Ag promoted the growth of primary ${\alpha}Sn$ dendrites. The changes of tensile strength and elongation in Sn-Zn binary alloys were not observed while the increase of tensile strength and the decrease of elongation in Sn-Zn-Ag ternary alloys were observed with increased casting speed.

• Wind and Structures
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• v.17 no.4
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• pp.419-433
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• 2013

This paper investigates the vertical profiles of horizontal mean wind speed and direction based on the synchronized measurements from a Doppler radar profiler and an anemometer during 16 tropical cyclones at a coastal site in Hong Kong. The speed profiles with both open sea and hilly exposures were found to follow the log-law below a height of 500 m. Above this height, there was an additional wind speed shear in the profile for hilly upwind terrain. The fitting parameters with both the power-law and the log-law varied with wind strength. The direction profiles were also sensitive to local terrain setups and surrounding topographic features. For a uniform open sea terrain, wind direction veered logarithmically with height from the surface level up to the free atmospheric altitude of about 1200 m. The accumulated veering angle within the whole boundary layer was observed to be $30^{\circ}$. Mean wind direction under other terrain conditions also increased logarithmically with height above 500 m with a trend of rougher exposures corresponding to lager veering angles. A number of empirical parameters for engineering applications were presented, including the speed adjustment factors, power exponents of speed profiles, and veering angle, etc. The objective of this study aims to provide useful information on boundary layer wind characteristics for wind-resistant design of high-rise structures in coastal areas.