Papers by Author: Jae Hoon Kim

Abstract: The fatigue crack growth rate is the most important factor in predicting the life of a product when applying the damage tolerance design concept. Studies related to pure mode I for structures under fatigue loading have been actively conducted, while not many studies are conducted on the mixed mode. In this study, therefore, mixed mode fatigue crack growth experiments were designed using the Compact-Tension-Shear (CTS) specimens and the loading devices, proposed by Richard. Furthermore, the finite element analysis was used in determining the stress intensity factors of CTS specimen. As the results, the fatigue crack growth rate using the equivalent stress intensity factors proposed by previous researchers was lower than that of pure mode I at the initial stage of crack growth when the load angle increases.

Abstract: Components of gas turbines must be extremely resistant to high temperatures, high stresses, high-temperature corrosion, and erosive environments. The materials used in these environmental conditions are mainly nickel-based superalloys. In this study, the low-cycle fatigue of the nickel-based superalloy Inconel 792 was examined. The total strain range of a gas turbine between 760 °C and 870 °C was considered as the parameter representing the actual gas turbine operation. In addition, tests were performed using a trapezoidal waveform of the total strain to reflect the operation-stop conditions of a gas turbine with frequent shutdowns. The results of the fatigue test were compared with the Coffin–Manson method and energy method. The fractured surface was analyzed using a scanning electron microscope (SEM).

Abstract: The rocket nozzle throat is exposed to combustion gas of a high temperature. Therefore, it is important to select suitable material for the nozzle which can maintain its structural integrity at a high temperature and to evaluate the material for proper rocket nozzle design. ATJ graphite, as the candidate material for rocket nozzle throat, is studied. In this study, an experimental method to evaluate compressive fracture behavior of ATJ graphite is presented. In particular, the effects of specimen size, temperature and heating time on compressive strength were investigated. Three types of specimens were used in uniaxial tests at room temperature, where the ratios of diameter to length are 1/2 of ASTM standard specimen, one of type I specimen and 1/0.8 of type II, respectively. Also, two kinds of specimen, which were coated using antioxidant and non-coated respectively were tested at elevated temperature to investigate temperature dependency of compressive strength.

Abstract: Particulate reinforced composites show viscoelastic behavior under various loading conditions. It is important to estimate mechanical behavior when this material is used for solid fuel on rocket motor. Cracks can be generated and propagated due to high combustion pressure in the service time. These cracks expand the burning area and lead to excessive combustion. Consequently, the rocket motor can cause malfunction or the performance deteriorated. In this study, edge cracked sheet specimen was used to perform the crack propagation tests. These tests were conducted in the range of temperature from -60°C to 60°C under 2.54 and 12.7 mm/min of strain rates. Based on the results, the stress intensity factors and the crack propagation rests were calculated. The crack resistance curves show the transition that stress intensity factors increases as temperature decrease. Also, fracture surfaces were investigated by scanning electron microscope to determine the fracture behavior under various temperatures.

Abstract: An aluminum alloy is used in airplanes and aerospace to reduce the weight of structures. Recently, researchers have studied how to intensify the strength of aluminum alloys. Shot peening is one of method to reinforce strength by hitting the surface of materials to make residual stress. In this study, high cycle fatigue characteristics of aluminum alloy Al 7075-T6 and Al 2024-T4 were analyzed. Fatigue characteristics of before and after the shot peened materials were tested by cantilever-rotary bending fatigue test machine (YRB 200, Yamamoto). Also, fractographic analysis was performed by a Scanning Electron Microscopy (SEM). The results showed that the fatigue life of both shot peened materials increased more than others.

Abstract: High strength nickel-base super alloys have been used in turbine blades for many years because of their superior performance at high temperature. The prediction of fatigue life for superalloys is important for improving the efficiency. In this study, low cycle fatigue tests are performed the variables of total strain range, and room and elevated temperature. The relations between plastic and total strain energy densities and number of cycles to failure are examined in order to predict the low cycle fatigue life of IN738LC super alloy. The fatigue life is evaluated by the Coffin-Manson equation, also the predicted lives by plastic and total strain energy density are compared with experimental results.

Abstract: The rotating components in the hot sections of land-based gas turbine are exposed to severe environment during several ten thousand hours at above 1100 oC operating temperature. The failure mechanism of the hot gas components would be accompanied by material degradation in the properties of high temperature and creep rupture strength. Many hot gas components in gas turbine are made of Ni-based superalloy because of their high temperature performance. In this work, we surveyed the time and temperature dependent degradation of Ni-based superalloy. We prepared the specimens from GTD111 that are exposed at 871 oC and 982 oC in 1,000 ~ 10,000 hours. We carried out the mechanical test and microstructural observation.

Abstract: Welding residual stress has important influence on the performance of engineering components. In this paper, the residual stress and FWHM were measured by X-ray diffraction method to investigate characteristics for P92 steel welds. The aim of the study is to estimate the residual stress and FWHM distribution characteristics. A compressive residual stress distributed as a function of depth has a different pattern in welds and base metal. A large tensile residual stress occurs welds and near the HAZ, but approaches gradually zero as away from the welding center. Residual stress and FWHM undergo rapid relaxation after PWHT. Also, FWHM is a scalar quantity without any directional difference and is proportional to hardness on the whole.

Abstract: The combustor chamber, diffuser and nozzle are the main components of the ramjet engine. In this study, the thermal strength of the combustion chamber of the ramjet engine was evaluated. The combustion chamber consists of an Inconel alloy 718 liner and a 17-4Ph stainless steel housing. The liner is rapidly heated to a high temperature. The heated liner is cooled with a film cooling method that forms a cold boundary layer to separate the hot gas from the surface of the liner. The thermo-structural analysis is evaluated the thermal strength of super alloy structure with various thermal insulation performances by finite element method with code MSC/Nastran. The result of the analysis is compared with accelerated stress rupture test. The experiment is performed to get safety design and estimate actually life-time for combustor chamber under high temperature. In general, the work in this paper is helpful to further improve the understanding and evaluation of thermal strength of the super alloy structure with various thermal insulation performances.

Abstract: In this study, failure behavior of hot gas casing for gas turbine was investigated. The microstructure and damage mechanism of serviced hot gas casing were examined. Also low cycle fatigue tests of the Inconel 617 super alloy is used for structural material of hot gas casing were performed. To predict the low cycle fatigue life, Coffin-Manson and strain energy density methods were used.