Papers by Author: Jae Do Kwon

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Authors: Jae Do Kwon, Sang Jin Cho, Yong Tak Bae
Abstract: The aged degradation of material is observed when heat-resisting steel is exposed for long periods of time at high temperatures. In the present study, the degraded 1Cr-0.5Mo steel that is used for long periods of time at high temperature(about 515°C) and artificially reheat-treated materials are prepared. These materials were used to study the effect of aged degradation on fretting fatigue behavior. Through this experiment, it is found that the fretting fatigue strength of reheat-treated 1Cr-0.5Mo steel is approximately 46 percent lower than that of the plain fatigue strength of the same material. Furthermore, the fretting fatigue strength of degraded 1Cr-0.5Mo steel was less than 53 percent of the same material™s plain fatigue strength. The maximum value of fatigue strength difference is observed as 57 percent between the fretting fatigue of degraded material and plain fatigue of reheat-treated material. These results can be used as basic data in a structural integrity evaluation of heat-resisting steel considering aged degradation effects.
1221
Authors: J.D. Parks, Jai Sug Hawong, Jae Do Kwon, S.H. Choi
507
Authors: Chang Su Woo, Wan Doo Kim, Jae Do Kwon
Abstract: The fatigue analysis and lifetime evaluation are very important in design procedure to assure the safety and reliability of the rubber components. The interest of the fatigue life of rubber components such as the engine mount is increasing according to the extension of warranty period of the automotive components. In this study, the fatigue lifetime prediction methodology of the vulcanized natural rubber was proposed by incorporating the finite element analysis and fatigue damage parameter determined from fatigue tests. Finite element analysis of 3D dumbbell specimen of natural rubber was performed based on a hyper-elastic material model determined from the tension, compression and shear tests. The Green-Lagrange strain at the critical location determined from the finite element analysis was used for evaluating the fatigue damage parameter of the natural rubber. Fatigue tests were performed using the 3D dumbbell specimens with different levels of maximum strain and various load. The basic mechanical properties test and the fatigue test of rubber specimens under the normal and elevated temperature were conducted. Fatigue life curves can be effectively represented by a following single function using the maximum Green-Lagrange strain. Fatigue lives of the natural rubber are predicted by using the fatigue damage parameters at the critical location. Predicted fatigue lives of the engine mount agreed fairly with the experimental fatigue lives a factor of two.
589
Authors: H. Kitagawa, Jae Do Kwon, S.H. Choi
549
Authors: Dae Kyu Park, Seung Wan Woo, Il Sup Chung, Young Suck Chai, Jae Do Kwon
Abstract: Studies on the strength and fatigue life of machines and structures have been conducted in accordance with the development of modern industries. In particular, fine and repetitive cyclic damage occurring in contact regions has been known to have an impact on fretting fatigue fractures. INCONEL alloy 600, 690 and INCOLOY alloy 800 are iron-nickel-chromium alloy having excellent resistance to many corrosive aqueous media and high-temperature atmospheres. These alloy are used extensively in the nuclear power plants industry, the chemical industry, the heat-treating industry and the electronic industry. In this paper, the effect of fretting damage on fatigue behavior for INCONEL alloy 600, 690 and INCOLOY alloy 800 were studied. Also, various kinds of mechanical tests such as tension and plain fatigue tests are performed. The objective of this study is to guarantee reliability of the facility applied in a power plant by comparing three materials in respect to fretting fatigue.
243
Authors: Jae Do Kwon, Seung Wan Woo, Choon Yeol Lee, Suk Chull Kang, Ho Sang Shin
Abstract: Conventionally, shield metal arc welding (SMAW) process was applied to join pipes of RCL, which caused lot of loss in time and cost due to excessive heat input and defects in joining section. Recently, narrow-gap welding (NGW) process was introduced to overcome the disadvantages of SMAW. However, the application of NGW to nuclear power plant is not yet common because safety of NGW process is not proven. In present paper, the welded coupons are manufactured under different welding processes in carbon steel. Then, microstructure observations and various mechanical tests are performed. It is verified that the mechanical properties of carbon steel are greatly changed after repair welding process due to applied heat flux, and that the effect of post-welding heat treatment is beneficial.
2853
Authors: Jae Do Kwon, Seung Wan Woo, Young Hwan Choi
Abstract: A dissimilar weld zone exists between the pipe and nozzle in a primary reactor cooling system (RCS). Thermal aging is observed in cast stainless steel, CF8M used in a pipe as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330°C. No effect is observed in low-alloy steel. SA508 cl.3 is used in a nozzle. The artificially accelerated aging specimens are prepared to maintain for a temperature of 430°C for 300, 1800, and 3600hrs, respectively. Then, various mechanical tests such as hardness, tension, impact test, are performed in virgin and aged specimens in order to determine the existence of dissimilar weld zones. The specimens for elastic-plastic fracture toughness tests are prepared for one type, where a notch is created in the heat affected zone of CF8M. From the experiments, it was found that J-integral values decrease as age increases.
1691
Authors: Jae Do Kwon, Yong Tak Bae, Sung Jong Choi, Young Suck Chai, Hitoshi Ishii
Abstract: Fretting is a potential degradation mechanism of structural components and equipments exposed to various environments and loading conditions. It is well known that the fatigue life under fretting condition decreases approximately 50-70% compared with that under non-fretting fatigue condition. The specific gravity of titanium alloy is 4.5 which is lighter than steel, however, its specific strength, heat and corrosion resistance are superior to steel. Ti-6Al-4V alloy is a kind of a+b phase titanium alloy, and mechanical properties are changed by alloy elements, shapes and distributions of microstructures. In this study, three different kinds of specimens are prepared under different heat treatments in order to produce different microstructures. Through various kinds of mechanical tests, the following conclusions are observed: 1) The microstructures are observed as equiaxed, bimodal and lamellar microstructures respectively. 2) The elongation percentage is superior for the equiaxed microstructure, and the hardness and tensile strength are superior for the lamellar microstructure. 3) The plain fatigue limit of lamellar structure shows higher value than that of the equiaxed and bimodal structures. 4) The fretting fatigue limit considerably decreases compared with the plain fatigue limit for all materials. 5) The fretting damage of contact surface increases with an increase of cyclic loading amplitude under the constant contact pressure.
1089
Authors: Jae Do Kwon, S.H. Choi, S.G. Kwak, K.O. Chun
537
Authors: Chang Su Woo, Wan Doo Kim, Jae Do Kwon
Abstract: The interest of the fatigue life for rubber components was increasing according to the extension of warranty period of the automotive components. In this study, the fatigue lifetime prediction methodology of the vulcanized natural rubber was proposed by incorporating the finite element analysis and fatigue damage parameter determined from fatigue tests. Finite element analysis of 3D dumbbell specimen and rubber component was performed based on a hyper-elastic material model determined from the mechanical tests. The Green-Lagrange strain at the critical location determined from the finite element analysis was used for evaluating the fatigue damage parameter of the natural rubber. Fatigue tests were performed using the 3D dumbbell specimens and rubber component with different levels of maximum strain and various load. Fatigue life curves can be effectively represented by a following single function using the maximum Green-Lagrange strain. Fatigue lives of the natural rubber are predicted by using the fatigue damage parameters at the critical location. Predicted fatigue lives of the rubber component for automobile vehicle agreed fairly with the experimental fatigue lives.
181
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