Papers by Author: Soon Il Moon

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: The objective of this study is to evaluate the mechanical properties of static, quasidynamic and dynamic fracture toughness of glass-filled ceramic as promising structural material for a dome port cover of a ramjet engine system. Static and quasi-dynamic tests were carried out using SEPB (Single Edge Pre-cracked Beam) specimens. Static and dynamic fracture toughness tests were also performed using ASTM and strain gage methods with SENB (Single Edge Notched Beam) specimens machined with various notch radii. The critical notch radius was evaluated. Below the critical notch radius, the static fracture toughness of the SENB specimen well agreed with that of the SEPB specimen.

Abstract: In this study, the intrinsic static/dynamic fracture toughness of Al 7175-T74 is evaluated from the apparent static/dynamic fracture toughness of a notched specimen. A critical average stress fracture model is suggested to establish the relationship for predicting the intrinsic fracture toughness from the apparent fracture toughness of a notched specimen. The critical average stress fracture model is established using the relationship between the notch root radius and the effective distance calculated by finite element analysis. The effective distance is the major characteristic describing stress distributions ahead of the notch tip. Therefore, the effective distance can be used to evaluate the behavior of structures containing notches. In this study, effective distance is applied to estimate the failure criterion for the combustion chamber with a notch. It is concluded that the true fracture toughness can be estimated from test results of apparent fracture toughness measured by using a notched specimen. Also, the effective distance can be used to evaluate the failure criterion of structures with notches.

Abstract: By contrast with static fracture toughness determination, the methodology for dynamic fracture toughness characterization is not yet standardized and appropriate approaches must be devised. The accurate determination of the dynamic stress intensity factors must take into account inertial effects. Most methods for dynamic fracture toughness measurement are experimentally complex. However, dynamic fracture toughness determination using strain measurement is extremely attractive in terms of experimental simplicity. In this study, dynamic fracture toughness tests using strain measurement are performed. High rate tension and charpy impact tests are carried out for titanium alloy, maraging steel and Al alloys. In the case of evaluating the dynamic fracture toughness using high rate tension and charpy impact tests, load or energy methods are used commonly. The consideration about inertial effects is essential, because load or energy methods are influenced by inertia. In contrast, if the position for optimum response of strain is provided, dynamic fracture toughness evaluation using strain near crack tip is more accurate. To obtain the position for optimum response of strain, a number of gages were attached at angles of 60°. Reliability for experimental results is evaluated by Weibull analysis. The method presented in this paper is easy to implement in a laboratory and it provides accurate results compared to results from load or energy methods influenced by inertia.

Abstract: Graphite has been developed as heat resistant material. To apply a reliable structural design using graphite, it is very important to investigate thermal shock characteristics. The common experimental methods of thermal shock fracture toughness are quenching and arc discharging heating methods. This paper describes experimental technique to evaluate the thermal shock fracture toughness using laser irradiation and proposes that a critical value of laser power can be a measurement to evaluate heat resistant materials. The laser source is CO₂ laser having maximum power of 4.0kW. The range of laser beam is from 1.0 to 2.7 kW and the beam duration is fixed at 1sec. K and C type thermocouples were used to measure the temperature distribution of a thermal shock fracture toughness specimen. In this study, the temperature distribution of specimen surfaces and critical laser power was investigated. After test, the surface phenomenon of specimen is examined using radiography and SEM. It is concluded that the critical laser power causing fracture can be the major factor of thermal shock fracture toughness of ATJ graphite.