Papers by Author: Jae Mean Koo

Abstract: Thermal barrier coating. Thermal fatigue. Exposure time. Thermal fatigue test is one of the most widely used method to evaluate the durability of thermal barrier coating (TBC). However, thermal fatigue test can be concluded in totally different results according to the test variations. Especially, Exposure time of thermal fatigue test can affect the delamination life cycle of TBC. In this study, using the same test equipment which Kim et al. used, thermal fatigue tests were performed with different holding time at high temperature, and the test results by Kim et al. and those by this study were compared. In addition, delamination map was come to perfection from the test results to define more accurate thermal fatigue life.

Abstract: A gas turbine blade operates under severe conditions including high temperature, high speed rotation, and frequent starts and shutdowns. Under such operating conditions, a blade is exposed to a thermo mechanical fatigue environment. These conditions shorten the life of the turbine parts and reduce the reliability of the gas turbine. Therefore, research on the characteristics of blade material is necessary. In this study, thermo-mechanical fatigue (TMF) tests were conducted for IN738LC, which is base a material for turbine blades. Tests were conducted under both the IP (in-phase) and OP (out of-phase) conditions, and tests results were compared in terms of fatigue life. After obtaining the ε-N curves, additional TMF tests were conducted according to variations in fatigue life. The purpose of the TMF test is to observe the change in mechanical properties of materials damaged during TMF and how these relate to fatigue life. For this purpose, an indentation test was conducted and the relationship between hardness and fatigue life were obtained.

Abstract: Mechanical structures with power sources experience repeated force produced by motors. As a result, the life of the pipes reduces and ultimately, the pipes collapse. In most cases, the U-shape pipe is made from a straight pipe by complicated bending work. During this work process, plastic deformation of the pipe produces residual stress in the pipe. The residual stress significantly affects the fracture behavior of the pipe. In this paper, fatigues tests of U-shape bending pipe are performed by uniaxial testing machine and residual stresses were evaluated by FEM analysis. So we established the relation between residual stress and fatigue life. And we suggested new equation for fatigue life prediction using the residual stress of U-shape bending pipe.

Abstract: Tensile and fracture toughness tests of coolant piping materials were performed at various loading rates and temperatures. The crack measurement method for coolant piping materials consisted of the unloading compliance method for quasi-static loading conditions and condition, while the electric potential drop method and the normalization method for quasi-dynamic loading conditions. Tensile and fracture toughness tests of coolant piping materials were performed at the design temperatures of the primary piping system of nuclear power plants, which are 449K (176°C) and 589K (316°C). Also, the plastic η factor considering strength mismatch of welding part was applied to the fracture toughness test results and the J-R curves by the plastic η factor considering strength mismatch of welding part was compared with the J-R curves by the plastic η factor introduced in ASTM E1820-01.