• Journal of the Korean Recycled Construction Resources Institute
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• v.10 no.3
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• pp.343-350
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• 2022

In recent years, nanomaterials, such as nano-silica, carbon nanotubes, and graphene oxide (GO), have been suggested to improve the properties of the interfacial transition zone (ITZ) between aggregates and cement pastes, which has most adversely affected the strength of quasi-brittle concrete. Among the nanomaterials, GO with superior dispersibility has been reported to be effective in improving the properties of ITZ of normal-strength concrete by forming interfacial chemical bonds with Ca²⁺ ions abundant in ITZ. In this study, the effect of GO on the properties of ITZ in the high-strength mortar was elucidated by calculating the change in hydration heat release, ITZ thickness, and the porosity around ISO sand, which was obtained with isothermal calorimetry tests and scanning electron microscope image analysis, respectively.

• Journal of Korea Foundry Society
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• v.42 no.4
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• pp.218-225
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• 2022

For hydrogen-vehicle applications (air pressure control valve housing, APCVH), an investigation was conducted to determine how micro-defects in a high- pressure die-casted Al alloy (industrial code: ALDC12) could be controlled by means of a post-treatment using an organic-based impregnation solution in order to improve the air- tightness of the die-casted Al sample. Two different impregnation solutions were proposed and its test results were compared to a imported product from Japan with respect to the processing variables used. A structural investigation of the components under study was conducted by means of computer tomography and 3D X-ray micro-CT. These observations revealed that the use of the impregnation treatment to seal micro-defects led to highly significant and beneficial changes which were attributed mainly to interconnections among inherent micro-pores. A leak test after impregnation revealed that the performance improvement rate of the die-casted Al sample was ~70% for INNO-01. Therefore, the developed impregnation solutions offer an effective strategy to control the micro-defects found in various vehicle parts via die-casting.

• Structural Engineering and Mechanics
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• v.89 no.1
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• pp.33-46
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• 2024

The present research delves into the analysis of nonlinear free and forced vibrations of porous functionally graded (FG) shallow shells reinforced with oblique stiffeners, which are embedded in a nonlinear elastic foundation (NEF) subjected to external excitation. Two distinct types of PFG shallow shells, characterized by even and uneven porosity distribution along the thickness direction, are considered in the research. In order to model the stiffeners, Lekhnitskii's smeared stiffeners technique is implemented. With the stress function and first-order shear deformation theory (FSDT), the nonlinear model of the oblique stiffened shallow shells is established. The strain-displacement relationships for the system are derived via the FSDT and utilization of the von-Kármán's geometric assumptions. To discretize the nonlinear governing equations, the Galerkin method is employed. The model such developed allows analysis of the effects of the stiffeners with various angles as desired, in addition to the quantitative investigation on the influence of the surrounding nonlinear elastic foundations. To numerically solve the problem of vibrations, the 4th-order P-T method is used, as this method, known for its enhanced accuracy and reliability, proves to be an effective choice. The validation of the present research findings includes a comprehensive comparison with outcomes documented in existing literature. Additionally, a comparative analysis of the numerical results against those obtained using the 4th Runge-Kutta method is performed. The impact of stiffeners with varying angles and material parameters on the vibration characteristics of the present system is also explored. The researchers and engineers working in this field may use the results of this study as benchmarks in their design and research for the considered shell systems.

• Journal of Powder Materials
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• v.31 no.2
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• pp.152-162
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• 2024

The directed energy deposited (DED) alloys show higher hardness values than the welded alloys due to the finer microstructure following the high cooling rate. However, defects such as microcracks, pores, and the residual stress are remained within the DED alloy. These defects deteriorate the wear behavior so post-processing such as heat treatment and hot isostatic pressing (HIP) are applied to DED alloys to reduce the defects. HIP was chosen in this study because the high pressure and temperature uniformly reduced the defects. The HIP is processed at 1150℃ under 100 MPa for 4 hours. After HIP, microcracks are disappeared and porosity is reduced by 86.9%. Carbides are spherodized due to the interdiffusion of Cr and C between the dendrite and interdendrite region. After HIP, the nanohardness (GPa) of carbides increased from 11.1 to 12, and the Co matrix decreased from 8.8 to 7.9. Vickers hardness (HV) decreased by 18.9 % after HIP. The dislocation density (10^-2/m²) decreased from 7.34 to 0.34 and the residual stress (MPa) changed from tensile 79 to a compressive -246 by HIP. This study indicates that HIP is effective in reducing defects, and the HIP DED Stellite 6 exhibits a higher HV than welded Stellite 6.

• Geomechanics and Engineering
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• v.36 no.5
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• pp.465-474
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• 2024

Uniaxial compressive strength (UCS) is a critical geomechanical parameter that plays a significant role in the evaluation of rocks. The practice of indirectly estimating said characteristics is widespread due to the challenges associated with obtaining high-quality core samples. The primary aim of this study is to investigate the feasibility of utilizing the gene expression programming (GEP) technique for the purpose of forecasting the UCS for various rock categories, including Schist, Granite, Claystone, Travertine, Sandstone, Slate, Limestone, Marl, and Dolomite, which were sourced from a wide range of quarry sites. The present study utilized a total of 170 datasets, comprising Schmidt hammer (SH), porosity (n), point load index (Is(50)), and P-wave velocity (Vp), as the effective parameters in the model to determine their impact on the UCS. The UCS parameter was computed through the utilization of the GEP model, resulting in the generation of an equation. Subsequently, the efficacy of the GEP model and the resultant equation were assessed using various statistical evaluation metrics to determine their predictive capabilities. The outcomes indicate the prospective capacity of the GEP model and the resultant equation in forecasting the unconfined compressive strength (UCS). The significance of this study lies in its ability to enable geotechnical engineers to make estimations of the UCS of rocks, without the requirement of conducting expensive and time-consuming experimental tests. In particular, a user-friendly program was developed based on the GEP model to enable rapid and very accurate calculation of rock's UCS, doing away with the necessity for costly and time-consuming laboratory experiments.

• Journal of the Korean earth science society
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• v.45 no.4
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• pp.363-376
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• 2024

Alluvial beds are intimately associated with electrical properties related to soil types, including clay mineral content, porosity, and water content. The hydraulic property governs water movement and storage in alluvial beds. This study revealed electrical resistivity and hydraulic properties in space and time in relation to the hydrogeological data, groundwater pumping, and rainfall infiltration into the alluvial bed located in Daesan-myeon, Changwon City. An electrical resistivity survey with electrode spacings of 2 and 4m using a dipole-dipole array indicates that electrical resistivity changes in the alluvial bed depend on groundwater pumping and rainfall events. Additionally, rainfall infiltration varies with hydraulic conductivity in the shallow zone of the alluvial bed. The 2 m electrode spacing survey confirms that electrical resistivity values decrease at shallow depths, corresponding with rainfall and increased water content in the soil, indicating rainfall infiltration approximately 1-2 m below the land surface. The 4m electrode spacing survey reveals that hydraulic conductivity (K) values and electrical resistivity (ρ) values display an inverse relationship from the surface to the water table (approximately 9 m) and at deeper levels than the water table. Notably, ρ values are impacted by pumping around the depth of the water table at 9 m. This study suggests that time-lapsed electrical resistivity surveys in space and time could be effective tools for detecting the impact of rainfall and pumping, as well as hydraulic conductivity in shallow alluvial beds.

• Journal of the Korean Electrochemical Society
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• v.3 no.1
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• pp.49-56
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• 2000

In the present we.k, temperature dependence of oxygen diffusivity in the polyvinyl chloride (PVC) film $D_f$ formed on gold electrode was investigated using steady-state rotating disk electrode (RDE) technique and modulated electrohydrodynamic (EHD) impedance technique. Both the diffusion rate defined as the ratio of oxygen diffusivity in the PVC film to the film thickness $D_f/\delta_f$ and the time constant $\delta_f^2/D_f$ for oxygen diffusion through the PVC film were obtained from plot of the limiting current versus disk rotation speed and from filing the EHD impedance spectra experimentally measured to those theoretically calculated on the basis of the diffusion equation for mass transport through the non-conductive and porous film, respectively. By combining measured $D_f/\delta_f$ with $\delta_f^2/D_f$, we determined $\delta_f\;and\;D_f$ at room temperature separately. As temperature increased, it appeared that the $D_f$ value measured for the PVC film-covered gold RDE was enhanced more rapidly than that $D_s$ value in the solution measured for the PVC film-free gold RDE. This means that the pores glowing with increasing temperature act as effective diffusion paths within the film. The present in-situ steady-state and modulated EHD measurements prove to be effective for determining $\delta_f\;and\;D_f$, separately and at the same time the porosity of the PVC film at temperatures below glass temperature $T_g$ of the film.

• Journal of Soil and Groundwater Environment
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• v.13 no.4
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• pp.1-7
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• 2008

This study analyzed for hydrodynamic dispersion characteristics of multi-soil layer (Silt and clay, Find sand, Coarse sand), data of a field tracer test on the multi-soil layer and data of laboratory column experiments on the samples on each soil layers. Through the analysis of permeability and flow, MS (Silt and clay) and FS (Fine sand), which were low effective porosity, were higher average linear velocity while CS (Coarse sand), which was high effective porosity, was higher hydraulic conductivity. Hydraulic conductivity function based on average soil particle diameter was assumed Y=$3.49{\times}10^{-8}e^{15320x}$ and coefficient of determination was 0.90. Average linear velocity function based on average soil particle diameter was assumed Y=$1.88{\times}10^{-7}e^{11459x}$ and coefficient of determination was 0.81. Longitudinal dispersivity function based on average soil particle diameter was Y = 0.00256$e^{5971x}$ and coefficient of determination was 0.98. According to the linear regression analysis of average linear velocity and longitudinal dispersivity, assumed function was Y = 21.7527x + 0.0063, and coefficient of determination was 0.9979. The ratio of field scale/laboratory scale was 54.09, it exhibited scale-dependent effect of hydrodynamic dispersion. Field longitudinal dispersivity (1.39m) was 7.47 times as higher than longitudinal dispersivity estimated by the methods of Xu and Eckstein (1995). Hydrodynamic dispersion on CS layer was occurred mainly by diffusion flow in the test aquifer.

• Korean Journal of Environmental Agriculture
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• v.18 no.1
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• pp.54-60
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• 1999

Three different types of lab-scale anaerobic bioreactors, AF and two-stage ASBF-PR and ASBF-SP, were evaluated in treating swine wastewater by operating at $1{\sim}2$ days of hydraulic retention time with increasing organic loading rate upto 6.3 $kg-COD/m^3{\cdot}d$ at $35^{\circ}C$. Seeding the anaerobic bioreactors with waste anaerobic digester sludge from a municipal wastewater treatment plant was effective and a 40-day acclimation period was required for steady-state operation. Three anaerobic bioreactors were effective in treating swine wastewater with COD removal efficiency of $66.4{\sim}84.9$% and biogas production rate of $0.333{\sim}0.796m^3/kg-COD_{removed}{\cdot}d$. Increases of organic loading rate by increasing influent COD concentration and/or decreasing hydraulic retention time caused decreases in COD removal efficiency and increases in biogas production rate. At relatively high organic loading rate employed in this study, the treatment efficiency of AF and ASBF-PR were similar but superior than that of ASBF-SP, indicating that porosity and pore size of the media packed in the bioreactors are more important factors contributing the performance of to bioreactors than specific surface area of the media. TKN in swine wastewater must be removed prior to the anaerobic processes when anaerobic process is considered as a major treatment process since influent TKN concentration of $1,540{\sim}1,870mg/L$ to the bioreactors adversely affect the activity of methanogenic bacteria, resulting in decreases of treatment efficiency and biogas production rate by 50%.

• Journal of the Korean Ceramic Society
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• v.35 no.5
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• pp.421-428
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• 1998

The effect of silane coupling agents on the water stability of HAC/PVA based MDF cement composites which were modified with urethane and epoxy resin were studied as a function of the functional groups and addition amount of silane coupling agent. According to the composition of polymer matrix the silanes with different functional groups showed the different effectiveness. In case of the only PVA matrix the silane with vinyl functional group was more effective than other silanes. When the epoxy resin was added the silane of epoxy-methodxy group enhanced the flexural strength of dry and wet state more than other. In case of urethane-added MDF cement the silane of diamine group was effective and enhanced the water sta-bility fo MDF cement composite more and more as the addition amount of silane increased, Especially in case of warm-presed composite the effect of silane was enhanced By addition of 2wt% of silane with 야-amine group the flexural strength of urethane-added composites were enhanced by 20% more in dry state 40-70% in wet state in accord with the porosity analysis. The flexural strength of the poxy resin-added MDF cement composite was increased by addition of 1wt% and 2wt% silane of epoxy-methoxy group However the addition of 4wt% of silane decreased the flexural strength of dry and wet state by formation of closed pore in the polymer matrix.