Papers by Author: Young Suck Chai

Abstract: A pressure plate, a component of the clutch system, was analyzed by two-dimensional and three-dimensional finite element method under three major load conditions of thermal loading due to temperature distribution, centrifugal force and contact pressure of diaphragm spring. The results show that the effects of thermal loading and contact pressure of diaphragm spring are important, which suggests the direction of design improvement. A hub plate was also analyzed with finite element modeling. A stress concentration occurs around the fillet region around the window in the plate where the fracture analysis is executed to find the stress intensity factors. These results will be used as the fundamental data for design improvement of the shapes of pressure plates and hub plates.

Abstract: Inconel 690 alloy is a Ni-Cr-Fe austenite solid solution alloy with a wide range of applications such as steam generator tubes in nuclear power plant due to its good mechanical properties and corrosion resistance. In this study, the effects of heat treatment on the microstructural properties of Inconel 690 alloy were investigated. Inconel 690 alloy was annealed at solid solution temperature of 1150°C for 20 min, and cooled down to 800°C with different cooling rates, and then aged at 800°C for 100 min in vacuum, and then cooled down to room temperature by water quenching. The optical microscopy, scanning electron microscopy, and transmission electron microscopy results indicated that in case of the cooling rate of 0.5°C/min, discontinuous carbides along the grain boundaries were formed and when the cooling rate was 10°C/min, continuous carbides were formed. In both cases, many annealing twins were also formed. These results can help to improve the understanding of microstructural properties of Inconel 690 alloy.

Abstract: Numerical analysis by influence function method (IFM) is demonstrated in this study in order to investigate the fretting wear problems on the secondary side of the steam generator, caused by flow induced vibration. Two-dimensional numerical contact model is developed in terms of Cauchy integral equation. The distributions of normal pressures, shear stresses and displacement fields are derived between two contact bodies which have similar elastic properties. The work rate model is adopted to find the wear amounts between two materials. The results are compared with the solutions by finite element analyses, which validates the application of the present method to fretting wear problems.

Abstract: Fracture toughness calculation by Movable Cellular Automata (MCA) method is demonstrated in this study in order to verify the validity of application of MCA method to the fracture problems. Two different geometries: edge-cracked strip and center cracked plate tension specimen have been considered to simulate numerically, which make place in real experiments. Soda-Lime silica glass for brittle material, AISI 1040 Steel and Aluminum Alloy 2024 for ductile materials were used, respectively. Fulfillment of basic principles of fracture mechanics has been checked. In MCA method, special fracture criterion is used to describe the process of crack propagation and critical stress and displacement was obtained, which are used as input data for evaluating the fracture toughness. In comparisons with table data, the results showed the sufficient agreement, which states the facility of the MCA method in application to the fracture problems.

Abstract: Mechanical breakdown often comes from the fatigue in many structural parts and nuclear power plants. Among the fatigue phenomenon, especially fretting fatigue occurs in mechanical joints showing small relative movements between contact surfaces. Although the research was developed for one hundred years, occurrence mechanism is not clearly identified yet. INCOLOY alloy 800 is a iron-nickel-chromium alloy having excellent resistance to many corrosive aqueous media and high-temperature atmospheres. This alloy is 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 INCOLOY alloy 800 was studied. Also, various kinds of mechanical tests such as tension and plain fatigue tests are performed. Fretting fatigue tests were carried out with flat-flat contact configuration using a bridge type contact pad and plate type specimen. Through these experiments, it is found that the fretting fatigue strength decreased about 50% compared to the plain fatigue strength. In fretting fatigue, the oblique micro-cracks at an earlier stage are initiated. These results can be used as basic data in a structural integrity evaluation of heat and corrosion resisting alloy considering fretting damages.

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.

Abstract: Analysis of structures including periodically arrayed in-homogeneities, whether they are microscopic or macroscopic, can be simplified by using homogenized properties. If the unit structure is repeated in a longitudinal direction, the whole structure may be regarded as a beam with effective flexibility. This study proposes a kinematical periodicity constraint to be imposed on the FE model of the unit structure for beams, which improves the accuracy of the effective flexibility along with reduction of the model size. The method is applied to a beam-in-space containing repetitive un-symmetric holes, and it is demonstrated that the deformation behavior of the homogenized beam agrees well with that of the real structure.

Abstract: The initial crack under fretting condition occurs at lower stress amplitude and at lower cycles of cyclic loading than that under plain fatigue condition. INCONEL alloy 600 and 690 are high–chromium nickel alloy having excellent resistance to many corrosive aqueous media and high-temperature atmospheres. In this paper, the effect of fretting damage on fatigue behavior for INCONEL alloy 600 and 690 were studied. Also, various kinds of mechanical tests such as hardness, tension and plain fatigue tests are performed. Fretting fatigue tests were carried out with flat-flat contact configuration using a bridge type contact pad and plate type specimen. Through these experiments, it is found that the fretting fatigue strength decreased about 40~70% compared to the plain fatigue strength in two materials. In fretting fatigue, the wear debris is observed on the contact surface, and the oblique micro-cracks at an earlier stage are initiated. These results can be used as basic data in a structural integrity evaluation of heat and corrosion resisting alloy considering fretting damages.