Papers by Author: Jae Mean Koo

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Authors: Hong Sun Park, Hyung Ick Kim, Yong Huh, Moon Young Kim, Sang Yeol Park, Jae Mean Koo, Chang Sung Seok
Abstract: The Ni-based superalloy Inconel 718 (IN-718) was developed in the 1950s and is currently used for several critical gas turbine components due to its good balance of mechanical properties. Because of its stable mechanical properties at high temperature, the alloy is used for turbine bolts. The mechanical properties of in-service facilities are required to maintain safety operation in power plants. Ultrasonic tests, tensile tests and hardness tests were performed to evaluate the degree of the material degradation of Inconel 718. The mechanical properties decreased as degraded, but the longitudinal velocity of the ultrasonic signal increased. Also, the microstructure of the degraded Inconel 718 was to study the relationship between the result from ultrasonic test and the results from destructive methods.
523
Authors: Bong Kook Bae, Chang Sung Seok, Jae Mean Koo, Hyung Ick Kim, K.Linga Murty
Abstract: A burst rupture test has been mainly used for evaluating the burst properties of internal pressurized tubes. Although the burst creep test provides accurate results, its complicated and material-wasting process makes it difficult to perform this test. In the burst test, it is known that the hoop stress is a main reason of burst, so it can be expected that the burst rupture properties are strongly related with the hoop creep rupture properties. A ring test is occasionally used for obtaining the hoop directional properties of tube-shaped structures. In this study, the creep rupture properties of Zirlo tubing are investigated at temperature ranging from 365 to 570  using the ring specimens. An analysis of the fractography was performed and the estimation of service life with Larson-Miller parameter was conducted. Finally, In view of the Dorn equation of power-law, the creep mechanism was determined and discussed.
565
Authors: Chang Sung Seok, Bong Kook Bae, Sung Keun Cho, Jae Mean Koo
255
Authors: Seung Baek, Jae Mean Koo, Chang Sung Seok
Abstract: Cast austenitic stainless steel piping pump, valve casings and elbows are susceptible to reductions in toughness and ductility because of long term exposure at the operating temperatures in the LWR (light water reactor). In this study, the three classes of thermally aged CF-8A cast austenitic stainless steel specimens were prepared using an artificially accelerated aging method for 0, 2,679 and 3,572 hours at 400oC. An indentation technique was applied to evaluate of the thermal aging of CF-8A cast austenitic stainless steel. We have performed indentation tests (automated ball indentation and nano-indentation) and EDS (energy dispersive spectroscopy) in order to characterize the micro-structural changes of the phase with aging. The fracture toughness of aged CF-8A cast stainless steel was determined by standard fracture toughness tests and automated ball indentation techniques.
869
Authors: Seung Baek, Jae Mean Koo, Chang Sung Seok
Abstract: In this study, we performed fracture tests on thermally aged DLC thin films by using micro-indentation and nano-indentation and investigated the deformation and fracture characteristics of these films. The aged high hydrogen films have better characteristics such as thermal and mechanical stability than low hydrogen films.
1374
Authors: Seung Baek, Jae Mean Koo, Chang Sung Seok
Abstract: Nano-indentation test is used widely to determine the fracture toughness of brittle materials and to provide information on important material properties such as the Young’s modulus and hardness. In this study, using nano-indentation testing, atomic force microscope (AFM), and finite element method (FEM), we performed the indentation fracture toughness and fracture strength measurement for a (100) single crystalline silicon at different load states. In addition, the loads of the phase transformation events during unloading were estimated by the load-depth curves. The phase transformation load and micro-crack propagation events at pop-out during the unloading process depended on the maximum applied indentation load.
601
Authors: Jung Hun Choi, Jong Hyun Baek, Dae Jin Kim, Jae Mean Koo, Chang Sung Seok
Abstract: In case of large steel water pipe, it have been observed that its fracture mostly occurs due to the complicated outside fatigue load on the pipe in the underground. It is also well known that its damage and leakage happen mainly in a weld zone. In this study we evaluated the fatigue characteristics based on size effect and residual stress by comparing the test results on the standard specimen collected from real pipe with those on full scale pipe.
2419
Authors: Chang Sung Seok, Bong Kook Bae, Jae Mean Koo, Jae Sil Park, Yong Huh
Abstract: In the case of inner pressurized pipe, the hoop stress mainly causes the failure of pipe. For evaluating the hoop tensile properties of pipes, a ring test is used although it has an inevitable shortcoming of disturbance by the bending. Arsene suggested a central piece-inserted ring test for reducing the bending. At high temperatures, the friction between the specimen and the central piece exerts a serious influence on the results, so an effect on friction should be considered in the analysis. In this study, for the evaluation of the hoop properties using the central piece-inserted ring model of Arsene, we performed the ring tensile tests at several temperatures. From the ring tensile tests and numerical analysis the load-displacement conversion relationship of ring specimen (LCRR) was determined. We could obtain the hoop tensile properties by means of applying the LCRR to the results of the ring tensile test. From the result, it was observed that the strength decreased as the temperature increased, and the elongation dropped a little around 400°C.
440
Authors: Hyung Ick Kim, Hong Sun Park, Jae Mean Koo, Sung Ho Yang, Moon Young Kim, Chang Sung Seok
Abstract: The advancement in superalloys permits the hot gas path components to operate for thousands of hours under severe centrifugal, thermal and vibratory stresses. The blade of a gas turbine must withstand the most severe condition combined of temperature, stress, and environment. After a long operation, the damaged blades of a gas turbine used are welded for build-up and repaired. We analyzed and compared the mechanical properties of GMAW(Gas Metal Arc Welding), a manual welding method, a laser cladding method, and an automatic welding method under research and development.
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