• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.4
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• pp.1051-1061
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• 1995

In order perform leak-before-break design of nuclear piping systems and integrity evaluation of reactor vessels, full stress-strain (.sigma. - .epsilon.) curves and fracture resistance (J-R) curves are required. However it is time-consuming and expensive to obtain J-R curves experimentally. The objective of this paper is to develop two methods for J-R curve prediction. In the first method, elastic-plastic finite element analyses for a series of crack length / specimen width ratio were performed. Accordingly the load versus load line displacement (P .delta.) curve corresponding to the fracture strain is obtained and the J-R curve based on the generalized locus method is obtained. In the second method, the correlation between .sigma.-.epsilon. curves and J-R curves was statistically analyzed and an empirical equation to predict the J-R curve from the .sigma.-.epsilon. test result is proposed. A good correlation between the predicted results based on the proposed methods and the experimental ones is obtained.

• Journal of Welding and Joining
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• v.14 no.5
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• pp.49-58
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• 1996

In Korea Gas Corporation, as one of the pipeline repairing methods, damaged pipelines are sometimes treated with a temporally employment of split sleeve. On conducting the repair process, circumferential fillet and longitudinal groove welding usually must be included. For the case of groove welding, a considerable amount of R&D have been carried out related to property changes, while few study on the property change in fillet welding has been conducted. In this paper, so as to confirm the specification of fillet welding in terms of safety and reliability, properties changed by fillet welding were investigated for two welding processes. Qualifying tests such as reviewing macrostructure and nick-break tests were performed according to API 1104 and ASME section IX. In addition, tensile properties and hardness were evaluated according to KS B0841 and BS 4515. The fillet weld prepared by the qualified procedure showed melting depth of 0.8∼1.3mm and heat affected zone of 2.8∼3.4mm length. No crack and lack of penetration were observed. And the results of hardness and nick-break tests satisfied code requirements. The area crossed by fillet and groove welding line was found to have minimal tensile strength.

• Journal of Welding and Joining
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• v.15 no.4
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• pp.166-177
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• 1997

The high strength AlZnMgCu0.5 alloy is a light metal with good age hardenability, and has a high tensile and yielding strength. Therefore, it can be used for structures requiring high speciple strength. Even though high strength AlZnMgCu alloy has good mechanical properties, it has a lot of problems in TIG and MIG welding processes. Since lots of high heat absorption is introduced into the weldment during TIG and MIG processes, the microstructural variation and hot cracks take place in heat affected zone. Therefore, the mechanical properties of high strength AlZnMgCu0.5 alloy can be degraded in weldment and heat affected zone. Welding process utilizing high density heat source such as electron beam should be developed to reduce pore and hot cracking, whichare usually accompanied by MIG and TIG welding processes. In this work, electron beam welding process were used with or without AlMg4.5Mn as filler material to avoid the degradation of mechanical properties. Mechanical and metallurgical characteristics were also studied in electron beam weldment and heat affected zone. Moreover hot cracking mechanism was also investigated.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.12
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• pp.1188-1193
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• 2007

This study focuses on the corrosion problems of STS316(stainless steel 316) tube for the cooling system of PEMFC (Proton Exchange Membrane Fuel Cell) operation. Deionized water which is highly corrosive is used especially for cooling agent of PEMFC to eliminate electrical conductivity, The tensile stress analysis was performed to check the change of mechanical strength of cooling line and pH of the water was monitored for the observation of extent of corrosion at simulated PEMFC operating condition. When STS316 tube was exposed to deionized water for 500 hours, substantial cracks were found on the surface and the pH of water was decreased from 6.8 to 5.8. For prevention of corrosion problems, the STS316 was coated by three kinds of fluororesin such as PTFE, FEP and ETFE. Among the coating materials, PTFE was the most protective in corrosive environment and was maintained the mechanical strength. To lower the cost, the same experimental analyses were carried out for iron tubes and the result will be discussed in detail.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.4 s.175
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• pp.858-865
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• 2000

It is well recently recognized that quench is one of the serious problems for the integrity of superconducting magnets, which is mainly attribute to the rapid temperature rising in the magnet due to some extrinsic factors such as conductor motion, crack initiation etc. In order to apply acoustic emission(AE)echnique effectively to monitor and diagnose superconducting magnets, it is essential to identify the sources of acoustic emission. In this paper, an acoustic emission technique has been used to monitor and diagnose quenching phenomenon in racetrack shaped superconducting magnets at cryogenic environment of 4.2K. For these purposes special attention was paid to detect AE signals associated with the quench of superconducting magnets. The characteristics of AE parameters have been analyzed by correlating with quench number, winding tension of superconducting coil and charge rate by transport current. In addition, the source location of quench in superconducting magnet was also discussed on the basis of correlation between magnet voltage and AE energy.

• Transactions of the Korean Society of Mechanical Engineers
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• v.1 no.2
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• pp.65-72
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• 1977

Major problems in the design and use of refief valve are (a) chattering because of instability, (b) excessive pressure differential which makes the valves crack far below maximum pressure diminishing useful flow in the system. In this study, A magnet-coupled relief valve is investigated theoretically and experimentally in order to improve the performance of a conventional direct type reliefvalve. A theory is developed to predict the performance, response, and stability of the magnet-coupled valve taking into account the delivery line response. In the experiment, a typical magnet-coupled relief valve is designed on the basis of the analytical results; the discharge rates are measured varying the supply pressure, and both the pressure-time curves and valve displacament-time curves are recorded providing the supply pressures greater than the setting pressure. The measured override characteristic curves are then compared with those of conventional pilot type and direct type releif valves. It is showm that the excessive pressure differential of a magnet-coupled relief valve becomes less than that of a conventional direct type valve. It is also shown that the most important chatacteristic of a magnet-coupled relief valve is to eliminate valve chattering due to instability regardless of the magnitude of setting pressures and discharge rates, which suggests wide applications of the idea of the use of a magnet in the design of hydraulic valves.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.262-267
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• 2004

The unloading compliance method is the most commonly used method to evaluate the fracture resistance characteristics of a material. In dynamic loading condition, the direct current potential drop(DCPD) method has been used because the unloading compliance method can not be applied due to the discontinuity of loading. However, even in the dynamic test using DCPD method, there is a problem that the voltage drops sharply on the initiation of crack. For the reason metioned above, the normalization method was suggested on ASTM E 1820 which is revised recently, as a new method to evaluate the dynamic fracture resistance characteristic. The nomalization method can be used to obtain a fracture resistance curve directly from a load-load line displacement. In this study, we obtained two fracture resistance curves from static test of welding part of nuclear piping both by unloading compliance and nomalization method. The two curves were almost same each other, so the adaptability of the nomalization method has been proved. We conducted a dynamic fracture resistance test for the same material. The fracture resistance curve from the dynamic test was obtained by normalization method and compared to that of the static test result.

• Journal of Biosystems Engineering
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• v.15 no.3
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• pp.177-185
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• 1990

The break of rotary blade is occured from a stress concentration of the inside of blade by the outside impulsive load. In order to examine its inside stress and stress concentration of rotary blade, a epoxy plate which is suitable to applicate by photoelastic system is used to experiment. These results are summarized as follow. 1. Refer to the existence of bolt hole and a size of its of rotary blade, a stress concentration which cause the break of rotary blade is not exposed. 2. It is expected to be break to section of hold of rotary blade and the break of this is due to that there are concentrated by shearing force, bending moment and bending stress. 3. When the crack which caused from processing are set up to any location, the stress concentration taken to the creak point. 4. Without regard to the location of the reaction points of rotary blade, the bending stress which is greated than the bending moment is occured within about 6 em toward the center line of bolt hole and it was possible to break that section.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.12 s.243
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• pp.1586-1596
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• 2005

In this paper, a structural damage identification method (SDIM) is developed to identify the line crack-like directional damages generated within a cylindrical shell. First, the equations of motion for a damaged cylindrical shell are derived. Based on a theory of continuum damage mechanics, a small material volume containing a directional damage is represented by the effective orthotropic elastic stiffness, which is dependent of the size and the orientation of the damage with respect to the global coordinates. The present SDIM is then derived from the frequency response function (FRF) directly solved from the equations of motion of a damaged shell. In contrast with most existing SDIMs which require the modal parameters measured in both intact and damaged states, the present SDIM may require only the FRF-data measured at damaged state. By virtue of utilizing FRF-data, one may choose as many sets of excitation frequency and FRF measurement point as needed to acquire a sufficient number of equations for damage identification analysis. The numerically simulated damage identification tests are conducted to study the feasibility of the present SDIM.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.05e
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• pp.85-88
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• 2003

In this study, porcelain insulator samples which have a different alumina composition were manufactured in order to test electrical and mechanical properties and make an analysis of the propagation phenomena of micro cracks on porcelain body. From XRD quantitative analysis the crystalline phases were different with alumina composition, sample C and D which have about 17wt% Corundum phase without the Cristobalite phase shows better electrical and mechanical properties than sample A and 8 which have the Cristobalite phase. In dielectrics test on porcelain samples with below 17wt% alumina composition, it was found that the amount of glass phase$(SiO_2)$have an main effect to decrease the dielectric loss$(tan{\delta})$, and the dielectric breakdown voltage of aluminous porcelain insulators was largely affected by its relative density. As a micro cracks analysis, HRS were measured, then the intensity of HRS increased with the amount of alumina composition. On the other hand, the propagation behaviors of cracks was fairly influenced by the distribution of pores.