Papers by Author: Yeong Moo Yi

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Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi
Abstract: The surface oxidation behavior was investigated over a range of solid state bonding condition of the Ti-6Al-4V ELI alloy. Since the oxides at the bonding interface may prevent the materials from complete bonding, it is important to understand the oxidation behavior at solid state bonding condition. The activation energy of oxidation of Ti-6Al-4V ELI is estimated to be 318 KJ/mol in an environment of solid state bonding process. For Ti-6Al-4V ELI alloy, strucutral integrity of bonding interface without oxides have been obtained at 850°C applying pressure of 3MPa for 1 hour. Solid state diffusion bonding of Ti-15V-3Cr-3Sn-3Al alloy was also obtained under a pressure of 6MPa for 3 hours at 925°C.
Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi
Abstract: The objective of this study is to fabricate lightweight structural panels and pressurized tanks by blow forming of solid-state bonded Ti-6Al-4V sheets. The solid-state bonding process was conducted in selective areas of the sheets and gas pressure was applied to the core sheet in order to form center cells. The microstructure of bonded region shows no indication of any discontinuous or heterogeneous microstructure at interface. It is shown that the blow forming of a lightweight sandwich structural panel and a sphere vessel was successfully performed with solid-state bonded multiple sheets of titanium alloy. The result of configuration and thickness distribution measurement indicate that the FEM analysis can predict the forming behavior during blow forming process of solid-state bonded sheets.
Authors: Jong Woong Lee, Cheol Won Kong, Se Won Eun, Jae Sung Park, Young Soon Jang, Yeong Moo Yi, Gwang Rae Cho
Abstract: Composite materials are used in aerospace structures due to their considerable bending stiffness and strength-to-weight ratio. A composite sandwich is composed of a face-sheet and an aluminum core. The face-sheet of the sandwich takes the bending stress and core of sandwich takes the shear stress. A compression test and FEM analysis accomplished about composite sandwich panels that have curvature. The FEM analysis was performed using a commercial code, ANSYS and the compression test was performed until failure occurred in the sandwich panel. A strain gauge and a displacement gauge were used to acquire the data. In this paper, the failure strength and failure mode was checked. Additionally, the results of the test and analysis were compared.
Authors: Cheol Won Kong, Jong Hoon Yoon, Young Soon Jang, Yeong Moo Yi
Abstract: Lightweight composite pressure vessels were designed with thin metallic and plastic liners. This paper discusses the structural design, the fabrication, and the experiment of the composite pressure vessels with metallic and plastic liners. Both liners were non-load sharing types, therefore only the composite sustained an internal pressure. The liner provided a gas seal. The materials of the liners were Al 6061 and CLPE (cross linked polyethylene) each. The composite pressure vessels were winded using T700 carbon/epoxy on the liners. The multiple cure cycles prevented the aluminum liner from losing a circular shape and the concentrated force at the metallic boss was spread to the composite fiber by the boss design. In case of the plastic liner, the boss design considered that CLPE had no adhesive property. Although the plastic liner has the advantage for the elongation, some environmental tests for temperature and vibration effects are needed in order to use in space applications.
Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi, Dong Hyuk Shin
Abstract: It is well known that the utilization of superplastic characteristics in manufacturing process makes many of aerospace components lighter and stiffer. The weight saving is vitally important especially for aerospace application and even more weight saving is possible when the superplastic forming is carried out with diffusion bonding. In this study, the lightweight sandwich structure was fabricated with superplastic forming(SPF) process from diffusion bonded(DB) Ti-6Al-4V sheets. The solid state diffusion bonding process was conducted in non-vacuum environment under a pressure of 4MPa for 60 minutes at 875°C and the superplastic forming process was followed for another 40 minutes. Good solid state bonding interface have been observed in microstructure observation and the sandwich structure was successfully manufactured. It is important to note that the forming conditions of present study are more practical for application than the previously published conditions, which require vacuum environment, higher temperature and/or pressure.
Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi
Abstract: There are numerous applications of pressure vessels in aerospace field for storing liquid or gaseous media. Generally the metallic vessels have been manufactured by welding two hemispheres, which are machined or spin-formed. In this study, the solid state bonding method was utilized with blow forming to manufacture aerospace titanium tanks. This solid state joining technology replaced the welding process and without any secondary material or liquid phases in bonding process, homogeneous microstructure was obtained at bonding interface. Using this method, a titanium tank of a space vehicle was manufactured and during a hydraulic pressurizing test, the strain and acoustic emission signals are observed to investigate the effect of solid state bonding method on the failure mode and performance of the tank.. This result was compared with the one made by conventional method of spin forming and welding. The result shows that the pressurization rate and the acoustic emission signal increasing rate provide a similar tendency for a vessel of integrity, while the signal increasing rate is much higher than the pressurization rate for a vessel with welding defects. It is clear that the failure mode of the solid state bonded tank is different from the welded tank due to the completely united interface by diffusion process.
Authors: Jong Hoon Yoon, Ho Sung Lee, Yeong Moo Yi, Young Soon Jang
Abstract: Superplastic blow forming with diffusion bonded sheet is an effective forming technology for the production of multi-cell structures which should have light weight and high stiffness for aerospace purpose. In the current study, finite element analysis on superplastic blow forming process has been carried out in order to improve the forming process when manufacturing axi-symmetric multi-cell structures using diffusion bonded Ti-6Al-4V multi-sheets. The simulation focused on the reduction of forming time and obtaining finally required shape throughout investigating the deformation mode of sheet according to the forming conditions, which are diffusion bonding pattern and die geometry. To reduce forming time, a preforming die was required, and to obtain the final shape the bonding pattern should be also modified within allowable geometrical margin, so that the sheet is easy to deform. Moreover, an intermediate simulation result, which was forming pressure profile, was employed in real forming test to check if the prediction was reasonably on progress. In the future, a study on the thickness ratio between each sheet should be followed to obtain optimum process parameters.
Authors: Jong Hoon Yoon, Yeong Moo Yi, Ho Sung Lee
Abstract: In this paper, mould configurations are studied by finite element simulation for superplastic blow forming of combustion chamber outer jacket. One concave and two convex mould configurations are basically considered to determine which type would be advantageous in terms of forming time and thickness distribution. For the simulation, the flow stress equation of duplex stainless steel was determined from free bulging test. The simulation results showed that the concave type was quit a bit different from the other two cases. The concave mould configuration produced shorter forming time and thicker thickness distribution than the others, and it seems to be more effective for superplastic forming of outer jacket. The obtained pressure profile for the concave mould configuration was employed in actual forming and the thickness distribution was compared for the verification.
Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi
Abstract: Titanium alloys have been widely used in aeronautics and aerospace industries due to their high strength, good corrosion resistance and low density. Since many aerospace vehicle systems require high performance lightweight pressurized vessel for storage of propellant, nitrogen, oxygen, or other medium, the titanium alloy is one of the excellent candidates for this purpose. Conventionally spin forming and TIG welding process have been applied to manufacture titanium spherical vessel. In this work, an innovational method of blow forming and solid state bonding technology has been developed to save manufacturing cost and reduce weight of titanium vessel. High temperature behavior of titanium alloy was characterized and according to this result, solid state bonding process was established with demonstration of manufacturing spherical and hollow cylinder pressure vessel. The optimum condition for solid state bonding of this alloy was obtained by applying hydrostatic pressure of 4MPa at 1148K for 1 hour. For blow forming, the pressure profile was developed using MARC software and the maximum pressure of 30MPa was applied. The structural integrity of the vessel was demonstrated by performing a hydraulic pressurization test.
Authors: Jong Woong Lee, Cheol Won Kong, Se Won Eun, Jae Sung Park, Young Soon Jang, Yeong Moo Yi
Abstract: Many methods can be used to assemble large mechanical structures. Using rivets is one of these methods. Rivets are lighter than bolts and allows for efficient assembly. The cylindrical structure has a discontinuous rivet assembled region. So concentration load operates in the edge of discontinuous assembled region. This paper evaluates the force placed upon rivets used to assemble cylindrical structures and strength of structures assembled with rivets. Plastic deformation of rivets occurs at 4 kN and rivets are destroyed at 7 kN. However, rivets are designed to transfer load to neighboring rivets when they are overloaded. Structural test were accomplished using tension and bending load condition. The structural tests showed that plastic deformation first occurs in the fastening where the rivet meets the outside of the cylinder and a number of rivets exhibited this type of deformation. The secondary phase of deformation occurs in the fastening where the rivet meets the inside of the cylinder. Despite the plastic deformation, the structural tests showed that the rivets succeeded in redistributing load to neighboring rivets and as a result, no rivets were destroyed.
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