• Journal of Welding and Joining
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• v.33 no.6
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• pp.6-12
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• 2015

Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of $200^{\circ}C$ to $900^{\circ}C$ due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, $M_6C$ with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.19 no.6
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• pp.447-456
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• 2007

Suspended particles behavior when they go through a vertical riser with heat transfer is of significant concern to system designers and operators in pneumatic transport, various processes such as in chemical, pharmaceutical and food industries. When it comes with the energy system, that knowledge is critical to the reliable design practices of related equipment as heat exchangers, especially in the phase of system scale-up. Without haying a good understanding of the related physics, many scale-up practices based on their pilot plant experience suffer from unexpected behaviors and problems of unstable fluidization typically associated with excessive pressure drop, pressure fluctuation and even unsuccessful particle circulation. In the present study, we try to explain the observed phenomena with related physics, which may help understanding of our unanswered experiences and to provide the designers with more reliable resources for their work. We selected hot exhaust gas with solid particle that goes through a heat exchanger riser as our model to be considered. The effect of temperature change on the gas velocity, thermodynamic properties, and eventually on the particles motion behavior is reviewed along with some heat transfer analyses. The present study presents an optimal riser length at full scale under given conditions, and also defines the theoretical limiting length of the riser. The field data from the numerical analysis was validated against our experimental results.

• Journal of the Korean Geotechnical Society
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• v.28 no.5
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• pp.85-94
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• 2012

Recently, growing interests in frozen ground have stimulated us to advance fundamental theories and systematic researches on soil behavior under freezing conditions. Unlike the well-established soil mechanics theory, temperature variation and phase change of pore-water cause water migration to cold side, ground heaving, sharp increase in earth pressure, etc., which bring about serious problems in frozen geotechnical structures. Elasto-plastic mechanical constitutive model for frozen/unfrozen soil subjected to fully coupled THM phenomena is formulated based on a new stress variable that is continuous in frozen-unfrozen transitional regions. Numerical simulations are conducted to discuss numerical reliability and applicability of the developed constitutive model: one-dimensional heaving pressure, tri-axial compression test, and one-side freezing tests. The numerical results show that developed model can efficiently describe complex THM phenomena of frozen soil, and they can be utilized to analyze and design the geotechnical structures under freezing conditions, and predict their long-term behavior.

• Journal of Powder Materials
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• v.22 no.4
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• pp.271-277
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• 2015

As wrought stainless steel, sintered stainless steel (STS) has excellent high-temperature anti-corrosion even at high temperature of $800^{\circ}C$ and exhibit corrosion resistance in air. The oxidation behavior and oxidation mechanism of the sintered 316L stainless was reported at the high temperature in our previous study. In this study, the effects of additives on high-temperature corrosion resistances were investigated above $800^{\circ}C$ at the various oxides ($SiO_2$, $Al_2O_3$, MgO and $Y_2O_3$) added STS respectively as an oxidation inhibitor. The morphology of the oxide layers were observed by SEM and the oxides phase and composition were confirmed by XRD and EDX. As a result, the weight of STS 316L sintered body increased sharply at $1000^{\circ}C$ and the relative density of specimen decreased as metallic oxide addition increased. Compared with STS 316L sintered parts, weight change ratio corresponding to different oxidation time at $900^{\circ}C$ and $1000^{\circ}C$, decreased gradually with the addition of metallic oxide. The best corrosion resistance properties of STS could be improved in case of using $Y_2O_3$. The oxidation rate was diminished dramatically by suppression the peeling on oxide layers at $Y_2O_3$ added sintered stainless steel.

• Korean Journal of Materials Research
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• v.21 no.6
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• pp.347-351
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• 2011

Hydroxyapatite (HAp) powders with different crystallinities were synthesized at various calcination temperatures through the co-precipitation of $Ca(OH)_2$ and $H_3PO_4$. The degradation behavior of these HAp powders with different crystallinities was assessed in a simulated body fluid solution (SBF) for 8 weeks. Below $800^{\circ}C$, the powders were nonstochiometric HAp, and the single HAp phase was successfully synthesized at $800^{\circ}C$. The degree of crystallinity of the HAp powders increased with an increasing calcination temperature and varied in a range from 39.6% to 92.5%. In the low crystallinity HAp powders, the Ca and P ion concentrations of the SBF solution increased with an increasing soaking time, which indicated that the low crystallinity HAp degraded in the SBF solution. The mass of the HAp powders linearly decreased with respect to the soaking time, and the mass loss was higher at lower crystallinities. The mass loss ranged from 0.8% to 13.2% after 8 weeks. The crystallinity of the HAp powders increased with an increasing soaking time up to 4 weeks and then decreased because of HAp degradation. The pH of the SBF solution did not change much throughout the course of these experiments. These results suggested that the crystallinity of HAp can be used to control the degradation.

• Macromolecular Research
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• v.16 no.2
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• pp.139-148
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• 2008

Biodegradable and elastic poly(L-lactide-co-$\varepsilon$-caprolactone) (PLCL) was electrospun to prepare nanofibers, and N-isopropylacrylamide (NIPAAm) was then grafted onto their surfaces under aqueous conditions using $^{60}Co-{\gamma}$ irradiation. The graft yield increased with increasing irradiation dose from 5 to 10 kGy and the nanofibers showed a greater graft yield compared with the firms. SEM confirmed that the PLCL nanofibers maintained an interconnected pore structure after grafting with NIPAAm. However, overdoses of irradiation led to the excessive formation of homopolymer gels on the surface of thc PLCL nanofibers. The equilibrium swelling and deswelling ratio of the PNIPAAm-g-PLCL nanofibers (prepared with 10 kGy) was the highest among the samples, which was consistent with the graft yield results. The phase-separation characteristics of PNIPAAm in aqueous conditions conferred a unique temperature-responsive swelling behavior of PNIPAAm-g-PLCL nanofibers, showing the ability to absorb a large amount of water at < $32^{\circ}C$, and abrupt collapse when the temperature was increased to $40^{\circ}C$. In accordance with the temperature-dependent changes in swelling behavior, the release rate of indomethacin and FITC-BSA loaded in PNIPAAm-g-PLCL nanofibers by a diffusion-mediated process was regulated by the change in temperature. Both model drugs demonstrated greater release rate at $40^{\circ}C$ relative to that at $25^{\circ}C$. This approach of the temperature-controlled release of drugs from PNIPAAm-g-PLCL nanofibers using gamma-ray irradiation may be used to design drugs and protein delivery carriers in various biomedical applications.

• Journal of the Korean institute of surface engineering
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• v.54 no.5
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• pp.230-237
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• 2021

In this study, the influence of silicon contents on the microstructure, mechanical and tribological properties of Ti-Al-Si-N coatings were systematically investigated for application of cutting tools. The composition of the Ti-Al-Si-N coatings were controlled by different combinations of TiAl₂ and Ti₄Si composite target powers using an arc ion plating technique in a reactive gas mixture of high purity Ar and N₂ during depositions. Ti-Al-Si-N films were nanocomposite consisting of nanosized (Ti,Al,Si)N crystallites embedded in an amorphous Si₃N₄/SiO₂ matrix. The instrumental analyses revealed that the synthesized Ti-Al-Si-N film with Si content of 5.63 at.% was a nanocomposites consisting of nano-sized crystallites (5-7 nm in dia.) and a three dimensional thin layer of amorphous Si₃N₄ phase. The hardness of the Ti-Al-Si-N coatings also exhibited the maximum hardness value of about 47 GPa at a silicon content of ~5.63 at.% due to the microstructural change to a nanocomposite as well as the solid-solution hardening. The coating has a low friction coefficient of 0.55 at room temperature against an Inconel alloy ball. These excellent mechanical and tribological properties of the Ti-Al-Si-N coatings could help to improve the performance of machining and cutting tool applications.

• Journal of Korea Foundry Society
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• v.36 no.3
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• pp.81-87
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• 2016

In this study, the effect of scrap ratio on the mechanical properties of Zr alloys was studied. Oxygen content in the ingot cake increased rapidly with increasing fraction of scrap, which can be attributed to the surface oxide of scrap including small pieces of turning, chips, etc. Iron content did not increase much with the increasing addition of scrap, suggesting scrap materials was well reserved in the iron-free container. As-cast structure of Zr alloy with the scrap:sponge ratio displayed plate/or needle ${\alpha}$ phase and no appreciable change of the cast structure was observed with change of scrap fraction. The strength increases with increasing fraction of scrap, which can be attributed to the increase of oxygen content. The ductility decreased slightly with increase of scrap fraction. Dislocation-oxygen interaction is known to increase the strength at the expense of ductility. Ingot cake with intentionally added $Fe_2O_3$ exhibited the drastic decrease of the formability, even exhibited the brittle fracture behavior during rolling. The oxidation resistance, however, increased with the increase of scrap fraction because of high oxygen content, which may prevent more penetration and diffusion of oxygen into matrix.

• Journal of the Korean institute of surface engineering
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• v.37 no.2
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• pp.80-85
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• 2004

Chromium-carbon (Cr-C) and chromium-carbon-phosphorus (Cr-C-P) alloy deposits using trivalent chromium sulfate baths containing potassium formate were prepared to study their current efficiency, hardness change and phase transformations behavior with heat treatment, respectively. The current efficiencies of Cr-C and Cr-C-P alloy deposits increase with increasing current density in the range of 15-35 A/dm$^2$. Carbon content of Cr-C and phosphorous of Cr-C-P layers decreases with increasing current density, whereas, the carbon content of Cr-C-P layer is almost constant with the current density. Cr-C deposit shows crystallization at $400^{\circ}C$ and has (Cr+Cr$_{ 23}$$C_{6}$) phases at $800^{\circ}C$. Cr-C-P deposit shows crystallization at $600^{\circ}C$ and has (Cr+Cr$_{23}$ $C_{6}$$+Cr_3$P) phases at $800^{\circ}C$. The hardness of Cr-C and Cr-C-P deposits after heating treatment for one hour increase up to Hv 1640 and Hv 1540 and decrease about Hv 820 and Hv 1270 with increasing annealing temperature in the range of $400～^{\circ}C$, respectively. The hardness change with annealing is due to the order of occurring of chromium crystallization, precipitation hardening effect, softening and grain growth with temperature. Less decrease of hardness of Cr-C-P deposit after annealing above $700^{\circ}C$ is related to continuous precipitation of $Cr_{23}$ $C_{6}$ and $Cr_3$P phases which retard grain growth at the temperature.

• Korean Journal of Construction Engineering and Management
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• v.5 no.3 s.19
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• pp.71-78
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• 2004

In conventional method of supporting soil shuttering wall during excavation a system of struts and wales to provide cross-lot bracing is common in trench excavations and other excavations of limited width. This method, however, becomes difficult and costly to be adopted for large excavations since heavily braced structural systems are required. Another expensive and unsafe situations are expected when temporary struts must be removed for the construction of underground structures. This paper introduces innovative strut systems which can be used as permanent underground structures after its role as brace system to resist earth pressure during excavation phase. Underground structural system suggested from architect is checked against the soil lated pressures before the analysis of stresses developed from gravity loads. In this technology, named SPS(Struts as Permanent System), retaining wall is installed first and excavation proceeds until the first level of bracing is reached. Braces used as struts during excavation will serve as permanent girders when buildings are in operation. Simultaneous construction of underground and superstructure can proceeds when excavation ends with the last level of braces being installed. In this paper, construction sequence and the calculation concept are explained in detail with some photo illustrations. SPS technology was applied to three selected buildings. One of them was completed and two others are being constructed Many sensors were installed to monitor the behavior of retaining wall, braces as column in terms of stress change and displacement. Adjacent ground movement was also obtained. These projects demonstrate that SPS technology contributes to the speed as well as the economy involved in construction.