Papers by Author: Chang Sung Seok

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Authors: Seung Baek, Sung Kuen Cho, Chang Sung Seok
Abstract: In this study, we investigated indentation behavior by varying the tip radius in a finite element model. We measured the tip radius and shape of the nano-indenter by SPM (scanning probe microscope) and compared them with the simulation results. The tip radius of cube corner indenter was measured to be 39nm, in agreement with the common tip radius of such indenters, which range from 20~50nm.
Authors: Jae Sil Park, Chang Sung Seok
Abstract: In order to analyze the elastic-plastic fracture behavior of a structure, the fracture resistance curve of the material should be known. However, it is difficult to evaluate the fracture characteristics with an experiment on the piping system. Instead, the fracture toughness obtained from standard specimen tests is used to analyze the structure and assess the fracture characteristics of the total structure. It is known that toughness data from the standard specimen test are conservative to predict fracture behavior of the real piping. Thus the fracture resistance curve by the fracture test of the real scaled pipe specimen would be applied to the integrity evaluation for the piping system. However, it is not only difficult to perform but also very expensive to perform full-scale pipe tests. The objective of this thesis is to propose a method to estimate the fracture resistance curve of a pipe from the result of standard specimen fracture test. To estimate the fracture resistance curve for a pipe specimen, load – load-line displacement records of a standard specimen were transformed to those of the pipe specimen. The load ratio method was proposed in order to calculate the crack length from load – crack mouth opening displacement records for the pipe specimen. To prove the validity of this estimation results, fracture tests for pipe specimens were performed. Consequently the applicability of the proposed method was investigated by comparing estimated results with tested results.
Authors: Byeong Choon Goo, Jae Hoon Kim, Jung Won Seo, Chang Sung Seok
Authors: Jeong Pyo Kim, Bong Kook Bae, Chang Sung Seok
Abstract: It's required that mechanical properties of in-service facilities maintain the safety of operation in power plants as well as chemical plants. In this study the four classes of thermally aged 1Cr-1Mo-0.25V specimens were prepared using an artificially accelerated aging method at 630°C. Ultrasonic tests, tensile tests, KIC tests and hardness tests were performed in order to evaluate the degree of the material degradation. The mechanical properties deteriorated as the specimen degraded, but the attenuation coefficient and the harmonic generation level of the ultrasonic signal increased. The non-linear parameter derived from the harmonic generation level is especially sensitive and will be a good parameter to evaluate the material degradation.
Authors: Chang Sung Seok, Jae Sil Park, Young Jin Kim
Authors: Hyung Ick Kim, Yong Huh, Jeong Pyo Kim, Chang Sung Seok
Abstract: The mechanical properties of in-service facilities under harsh environment a decrease as materials of the facilities degrade. This decrease of mechanical properties can affect the safety operation of the facilities. Therefore, the extent of degradation due to prolonged service exposure must be estimated. Nondestructive evaluation method is a good technique for monitoring the change of mechanical properties of in-service facilities. The most widely used nondestructive methods are the ultrasonic method and the indentation test, which is advantageous with respect of applicability to in-service facilities. The modified theoretical Vary's equation, considering nonlinear response due to material degradation, was proposed for obtaining the correlations between ultrasonic parameters and fracture toughness. Experimental results showed that ultrasonic attenuation, velocity, and nonlinear parameters have significant correlation with fracture toughness and yield strength. The nondestructive evaluation system can be used to obtain the yield strength and ultrasonic parameters simultaneously, and this information can be used to predict the fracture toughness. The predicted results produced good correlations with the experimental results, indicating that the nondestructive evaluation system can be effective in evaluating material properties and degradation, and the life time of facilities.
Authors: Jong Hyun Baek, Young Pyo Kim, Woo Sik Kim, Chang Sung Seok
Abstract: Pipelines for natural gas transmission may be subjected to plastic deformation by the outside force such as ground subsidence, ground liquefaction, cold bending and mechanical damage. Plastic deformation affects the tensile properties and fracture toughness. Tensile test, Crack tip opening displacement test and Charpy impact test were conducted on an API 5L X65 pipe to investigate the mechanical properties of pipeline subjected to plastic deformation. Axial tensile pre-strain up to approximately 10% was applied to plate-type tensile specimens cut from the API 5L X65 pipe prior to mechanical testing. Tensile test revealed that the yield strength and the tensile strength increased with increasing tensile pre-strain. However, the CTOD and Charpy impact energy values for crack initiation decreased with increasing tensile pre-strain.
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.
Authors: Yong Huh, Hyung Ick Kim, Chang Sung Seok
Abstract: In this study, we performed the static test of nuclear piping materials by the unloading compliance method and the normalization data reduction technique and obtained two fracture resistance curves (J-R curves). The two curves were similar, which proves that the normalization data reduction technique can be adopted in the static test. Then we performed the dynamic fracture toughness test for welding part of nuclear piping. The J-R curves were obtained from the dynamic test by the normalization data reduction technique and were compared to those of the static test results.
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