• Journal of Biomedical Engineering Research
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• v.19 no.3
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• pp.285-290
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• 1998

There are various reports on the fracture of mechanical heart valves implanted in human or animal, and they are pointed out that problems are induced by an erosion of disk surface, due to a cavitation effect. We have been investigating this mechanism using accelerated fatigue tester, and it was found that erosion was enhanced by a compliance effect in the test circuit. In this study, effects of compliance value and location on erosion were discussed, while disk closing velocity was measured by a high speed video camera. It was clarified that faster closing velocity was resulting in a enhancement of erosion on the disk surface.

• Journal of the Korean institute of surface engineering
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• v.49 no.4
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• pp.344-348
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• 2016

Stainless steel is widely applied in many industrial fields due to its excellent anti-corrosion and durability characteristics. However, stainless steel is very vulnerable to cavitation attack caused by high speed flow of fluid in the chloride environments such as marine environment. These conditions promote intergranular corrosion and cavitation-erosion, leading to degradation of the structural integrity and service life. In order to prevent these problems, the stabilized stainless steel is applied to the offshore and shipbuilding industries. In this study, Ti was added to 19%Cr-9%Ni as the stabilizer element with different concentrations (0.26%, 0.71%), and their durabilities were evaluated with cavitation-erosion experiment by a modified ASTM G32 method. The microstructural change was observed with the stabilizer element contents. The result of the observation indicated that the amount of carbide precipitation was decreased and its size became finer with increasing Ti content. In the cavitation-erosion experiment, both weight loss and surface damage depth represented an inverse proportional relationship with the amount of Ti element. Consequently, the stainless steel containing 0.71% of Ti had excellent durability characteristics.

• Corrosion Science and Technology
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• v.19 no.6
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• pp.310-317
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• 2020

This study investigated the damage to the specimen due to liquid droplet impingement erosion corrosion, which improved the corrosion resistance and durability via hard anodization of 5083-H321 aluminum alloy, which is widely used for small ships and marine structures. The experiment combined liquid droplet impingement erosion and electrochemical equipment with the flow rates in natural seawater solution. Subsequently, Tafel extrapolation of polarization curves was performed to evaluate damage due to the liquid droplet impingement erosion corrosion. The damaged surface was observed using a 3D microscope and a scanning electron microscope. The degree of pitting damage was measured using the Image J program, and the surface hardness was measured using the micro-Vickers hardness tester. The corrosion current density, area, depth, and ratio of the damaged areas increased with the increase in flow rate. The grain size of the damaged area at a flow rate of 20 m s-1 showed fewer and minor differences in height, and a smooth curved shape. The hardness of the damaged surface tended to decrease with increase in flow rate.

• Journal of the Korean Geotechnical Society
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• v.29 no.1
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• pp.49-59
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• 2013

Heavy rainfall intensity may cause shallow slope failures and debris flow by rill erosion and scour on land surface. The paper represents the difference between native soil (weathered soil) and reinforced soil, which is mixed by hardening agent with flyash as main material, for investigating experimental findings of rill erosion and erosion. Results obtained from artificial rainfall simulator show that erosion rate of reinforced soil mixed with hardening agent is reduced by 20% because an amount of eroded soil on slope surface is inversely proportional to the increase of soil strength. For example, rainfall of 45mm (at the elapsed time of 25mins in rainfall intensity of 110mm/hr) triggers rill erosion on native soil surface, but the rill erosion on reinforced soil surface does not even occur at 330mm rainfall (at the elapsed time of 3hrs in rainfall intensity of 110mm/hr). As a result of slope stability analysis, it was found that the construction method for reinforced soil surface would be more economical, easy and fast construction technology than conventional reinforcement method.

• Journal of Forest and Environmental Science
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• v.33 no.2
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• pp.136-146
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• 2017

We selected 6 plots ($100m^2$) located 2.2-3.8 km from Minamidake Crater on the north flank of Sakurajima Volcano. We conducted a field study to investigate the effects of volcanic activity on vegetation succession and surface erosion rate. The results showed that trees growing in plots further from the crater had a greater diameter at breast height (DBH), total height, and age. In addition, these plots had a greater number of woody plants and species, as well as a greater total cross-sectional area at breast height. The Fisher-Williams index of diversity (${\alpha}$) and the proportion of evergreen broad-leaved trees were higher in plots located further from the crater. Vegetation succession in these plots was not at the level of a climax forest. From 1972 to 2015, the timing for active volcano, the depth of volcanic ash layer, the dry density, and the pH of ground surface were lower for plots located further from the crater. Furthermore, the average annual sheet erosion from 1972 to 2015 was also lower for plots located further away from the crater. Overall, plots further away from the crater have a better environment for vegetation growth and a lower dry density of the volcanic ash surface layer. It is thought that lower dry density results in increased soil permeability, which impedes surface flow. In order to prevent debris-flow disasters, caused by mud and rock flow resulting from impaired soil penetrability, it is essential to promote soil development and restore penetrability by artificial vegetation restoration.

• Proceedings of KOSOMES biannual meeting
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• 2017.04a
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• pp.256-256
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• 2017

• Journal of Biomedical Engineering Research
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• v.23 no.4
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• pp.309-315
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• 2002

Since the discovery, in the 1980s, of erosion-pit-induced fractures in implanted mechanical heart valves. cavitation on the surface of mechanical heart valves has been widely studied as a possible cause of pitting. Several factors, including peak dp/dt of the ventricular pressure. maximum closing velocity of the leaflet, and squeeze flow. have been studied as indices of the cavitation threshold. In the present study. cavitation erosion on the surface of a mechanical valve was examined by focusing on squeeze flow and the water hammer phenomenon during the closing period of the valve. In this study, we measures pressure wave forms near a valve and closing velocities of a disk, which were placed in a holder with and without compliance. In case of all holders, pressure drop of below vapor pressure expect at near the surface disk. It was also found that the closing velocity of the disk increased and that cavitation erosion was enhanced too. These results suggest that disk closing velocity during the closing phase has signifiant effects on pitting erosion.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.16-16
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• 2011

Plasmas in saline solutions receive considerable attention in recent years. How the operating parameters influence the plasma characteristics and how the electrode erosion occurs have been topics that require further study. In the first part of this talk, the effect of the frequency on the plasmas characteristics in saline solution driven by 50~1000 Hz AC power will be presented. Two distinct modes, namely bubble and jetting modes, are identified. The bubble mode occurs under low frequencies. In this mode, one mm-sized bubble is tightly attached to the electrode tip and oscillates with the applied voltage. With an increase in the frequency, it shows the jetting mode, in which many smaller bubbles are continuous formed and jetted away from the electrode surface. Multiple mechanisms that are potentially responsible to such a change in bubble dynamics have been proposed and the dominant mechanism is identified. From the Stark broadening of the hydrogen optical emission line, electron densities in both modes are estimated. It shows clearly that the driving frequency greatly influences the bubble dynamics, which in turn alters the plasma behavior. In the second part, the study of the erosion of a tungsten electrode immersed in saline solution under conditions suitable for bio-medical applications is presented. The electrode is immersed in 0.1 M saline solution and is positively or negatively biased using a DC power source up to 600 V. It is identified that when the electrode is positively biased, erosion by the surface electrolytic oxidation is the dominant mechanism with an applied voltage below 150 V. An increase in the applied voltage leads to the formation of the plasma and the damage by the plasma and the thermal effect becomes more prominent. The formation of the gas film at the electrode surface leads to the formation of the plasma and hinders the electrolytic erosion. In the negatively-biased electrode, no electrolytic oxidation is seen and the damage is mostly likely due to the plasma erosion and the thermal effect.

• Journal of Korean Society of Forest Science
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• v.20 no.1
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• pp.51-60
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• 1973

The trend and achievments of soil erosion control research in Japan were investigated through observation tours and reference work and following facts were found to be important aspects which should be considered in the soil erosion control research program in Korea. Experiments on forest and water relations, and ground water phenomena at the water source zone in Tokyo University. Studies on land-slides and erosion control dam in Kyoto University. Studies on mud-flow and snow avalanches in Hokkaido University. Studies on sanddune fixation and disaster damage prevention forests in Kyushu University. Studies on forest denudations in Nagoya University. Studies on Greening-works and soil erosion prevention chemicals in Tokyo Agriculture University. Training on planning of erosion control works and prevention of disaster damages in Forest Research Institute. Experiments on soil erosion phenomena and infiltration in Tohoku Branch, FRI. Experiments on erosion and surface stratum failure of steep slopes and their prevention methods in Railway Technical Research Institute.

• Journal of Korean society of Dental Hygiene
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• v.17 no.4
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• pp.657-667
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• 2017

Objectives: The aim of this study was to evaluate the potential of dental erosion by three different types of commercial fermented milk on the enamel surface of bovine teeth. Methods: Forty bovine teeth (four groups of 10) were immersed in fermented milk (experimental groups: liquid type, condense-stirred type and condense-drink type) or mineral water (control group) for 1, 3, 5, 10, 15, 30, 60, 90 and 120 minutes. Enamel surface microhardness was measured before and after treatment with a microhardness tester, and Scanning Electron Microscope (SEM) was used to assess the enamel surface morphology. Results:Changes in enamel surface microhardness (before-after treatment; ${\Delta}VHN$) were significantly different among the four groups (p<0.05). ${\Delta}VHN$ was highest in the liquid type group, followed by the condense-stirred type, condense-drink type, and control groups. The ${\Delta}VHNs$ of three types of fermented milk groups were higher than that of the control group (p<0.05). The liquid type group showed higher ${\Delta}VHN$ than the other two types of condense fermented milk groups (p<0.05). However, there was no significant difference in ${\Delta}VHN$ between the two types of condense fermented milk (p>0.05). The results of SEM observation have shown the most severe surface damage in bovine teeth immersed in the liquid type of fermented milk. Conclusions: In this study, it appears that liquid type fermented milk causes greater development of dental erosion. The physical properties and pH of fermented milk types must be considered for prevention of dental erosion associated with ingestion of fermented milk.