Papers by Author: Choon Yeol Lee

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: 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: Conventionally, shielded-metal arc welding (SMAW) process has been applied to join pipes of reactor coolant loop, which caused defects and lot of loss in time and cost due to excessive heat input 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 commonly used, because safety of NGW process is not fully proven. In the present paper, welded coupons are made of stainless steel. They are manufactured under different processes; general welding (GW), and repair welding after GW. Performed are various mechanical tests to investigate microstructure, tensile strength and so on. It is verified that the mechanical properties of stainless steel are slightly changed after repair welding process. It is also found from stress corrosion cracking tests that the failure time of repair welding is shorter than that of general welding.

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.

Abstract: Vanadium alloys in the composition range around V-4Cr-4Ti have been proposed as candidate materials for fusion reactor applications and structures. These applications will require detailed characterization of constitutive and fracture properties. This study is aimed at understanding the basic constitutive and fracture mechanisms in vanadium alloys. Understanding of the basic constitutive and fracture mechanisms is achieved through a series of mechanical tests. These test results are combined with quantitative models of the underlying macro- and micromechanics. In addition to these experimental studies, finite element analysis (FEA) techniques are used to determine stress and strain fields to verify the constitutive law used in the fracture specimens.