• Journal of Theoretical and Applied Mechanics
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• v.1 no.1
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• pp.69-88
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• 1995

The paper attempts to estimate the incubation time of a cavity in the interface between a power law creep particle and an elastic matrix subjected to a uniaxial stress. Since the power law creep particle is time dependent, the stresses in the interface relax. Through previous stress analysis related to the present physical model, the relaxation time is defined by ${\alpha}$2 which satisfies the equation $\Gamma$0 ｜1+${\alpha}$2k｜m=1-${\alpha}$2 [19]. $\Gamma$0=2(1/√3)1+m($\sigma$$\infty$/2${\mu}$)m($\sigma$0/$\sigma$$\infty$tm) where $\sigma$$\infty$ is an applied stress, ${\mu}$ is a shear modulus of a matrix, $\sigma$$\infty$ is a material constant of a power law particle, $\sigma$=$\sigma$0 $\varepsilon$ and t elapsed time. the volume free energy associated with Helmholtz free energy includes strain energies associated with Helmholtz free energy includes strain energies caused by applied stress anddislocations piled up in interface (DPI). The energy due to DPI is found by modifying the results of Dundurs and Mura[20]. The volume free energies caused by both applied stress and DPI are a function of the cavity size(${\gamma}$) and elapsed time(t) and arise from stress relaxation in the interface. Critical radius ${\gamma}$ and incubation time t to maximize Helmholtz free energy is found in present analysis. Also, kinetics of cavity fourmation are investigated using the results obtained by Riede[16]. The incubation time is defied in the analysis as the time required to satisfy both the thermodynamic and kinetic conditions. Through the analysis it is found that [1] strain energy caused by the applied stress does not contribute significantly to the thermodynamic and kinetic conditions of a cavity formation, 2) in order to satisfy both thermodynamic and kinetic conditions, critical radius ${\gamma}$ decreases or holds constant with increase of time until the kinetic condition(eq.40) is satisfied. Therefore the cavity may not grow right after it is formed, as postulated by Harris[11], and Ishida and Mclean[12], 3) the effects of strain rate exponent (m), material constant $\sigma$0, volume fraction of the particle to matrix(f) and particle size on the incubation time are estimated using material constants of the copper as matrix.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.25 no.11
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• pp.878-885
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• 2012

In this study we aims to examine the co-doping effects of 1/3 mol% $Mn_3O_4+Co_3O_4$ (1:1) on the reaction, microstructure, and electrical properties such as the bulk defects and grain boundary properties of $ZnO-Bi_2O_3-Sb_2O_3$ (ZBS; Sb/Bi=0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Mn,Co-doped ZBS, ZBS(MCo) varistors were controlled by Sb/Bi ratio. Pyrochlore ($Zn_2Bi_3Sb_3O_{14}$) was decomposed and promoted densification at lower temperature on heating in Sb/Bi=1.0 by Mn rather than Co. Pyrochlore on cooling was reproduced in all systems however, spinel (${\alpha}$- or ${\beta}$-polymorph) did not formed in Sb/Bi=0.5. More homogeneous microstructure was obtained in $Sb/Bi{\geq}1.0$ In ZBS(MCo), the varistor characteristics were improved drastically (non-linear coefficient, ${\alpha}$=30~49), and seemed to form $Zn_i^{..}$(0.17 eV) and $V_o^{\bullet}$(0.33 eV) as dominant defects. From impedance and modulus spectroscopy (IS & MS), the grain boundaries have divided into two types, i.e. the one is tentatively assign to $ZnO/Bi_2O_3(Mn,Co)/ZnO$ (0.47 eV) and the other ZnO/ZnO (0.80~0.89 eV) homojunctions.

• Journal of Korean Society of Steel Construction
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• v.17 no.4 s.77
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• pp.395-405
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• 2005

This paper summarizes the test results of the flexible-stiff mixed system for the effective use of high-strength steel. Steel with a high degree of strength and performance is being increasingly required as buildings get larger and taller. High-strength steels cannot be used for many applications, though, because they have a number of defects. For instance, they have a high yield ratio, a small strain in maximum stress, and equal Young's modulus compared to mild steels. A new structural system is needed to effectively use high-strength steels with some defects. This paper proposes the flexible-stiff mixed system for the effective use of high-strength steels with high yield ratios. The possibility of using the system is discussed through the test of flexible-stiff mixed columns with high-strength steels. The main variable of the specimens is the yield displacement ratio, including both the force ratio and the stiffness ratio. The proper yield displacement ratio is proposed by adopting the flexible-stiff mixed system. The test results showed that the proposed flexible-stiff mixed system has a high capacity for energy absorption and the highest capacity for energy absorption when the yield displacement ratio of the flexible element to the stiff element ranges from 2.7 to 3.3.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.9
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• pp.1027-1032
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• 2012

Two types of very small multiplying gears and arrays have been developed for new dental hand-pieces, and the increased speed ratios, modules, number of teeth, gear diameters, and gear types were calculated based on the dynamics of the machinery. The contacting and bending strengths were evaluated for gear teeth with two design concepts using AGMA equations and finite element analyses, and the contacting stresses on teeth with and without DLC (diamond-like-carbon) coating layers were calculated. Fatigue and tension tests were performed to obtain an S-N curve, the Young's modulus, and the strength of the gear material, and these were utilized in the analyses. Slightly larger stresses were found for 2-axis-type gears than for other types of gears, and the S-N curves showed that a gear lifetime of 109 cycles was satisfied. The contacting stresses in gears coated with DLC were reduced by 30%. A new prototype model of a hand-piece with small gears was successfully fabricated and tested.

• Applied Chemistry for Engineering
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• v.27 no.2
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• pp.221-226
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• 2016

Production of eco-friendly and biologically harmless materials is strongly required in all industries. In particular, reducing volatile organic compounds in coating processes is extremely important to secure worker's safety. During recent two decades, extensive research works on water-borne polyurethane dispersion (PUD) have been continuously developed as an alternative to solvent-borne polyurethane. However, PUD was shown inferior mechanical properties to the organic solvent-borne polyurethane due to a limit to the molecular weight increase, which resulted in the limit of applications. To overcome this drawback, several approaches have been examined such as polymer blends and thermal/radiation induced crosslinking. Among these methods, the radiation curing system was suitable for industrialization because of the high crosslinking density and fast curing speed. In this study, we overcame the drawback for PUD via introducing benzophenone radiation curable units to PUD. We synthesized PUD films which possessed good dispersion in water for 30 days, increased Tg and Td more than $5^{\circ}C$ after UV curing film as well as improved young's modulus more than double.

• Journal of Adhesion and Interface
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• v.12 no.3
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• pp.88-93
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• 2011

Optimal dispersion and fabrication conditions of carbon nanotube (CNT) embedded in phenolic resin were determined by electrical resistance measurement; and interfacial property was investigated between plasma treated carbon fiber and CNT-phenolic composites by electro-micromechanical techniques. Wettability of carbon fiber was improved significantly after plasma treatment. Surface energies of carbon fiber and CNT-phenolic nanocomposites were measured using Wilhelmy plate technique. Since surface activation of carbon fiber, the advancing contact angle decreased from $65^{\circ}$ to $28^{\circ}$ after plasma treatment. It was consistent with static contact angle results of carbon fiber. Work of adhesion between plasma treated carbon fiber and CNT-phenolic nanocomposites was higher than that without modification. The interfacial shear strength (IFSS) and apparent modulus also increased with plasma treatment of carbon fiber.

• Polymer(Korea)
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• v.38 no.1
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• pp.74-79
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• 2014

Microfibrillated cellulose (MFC) was chemically modified with two different silane coupling agents (3-aminopropyltriethoxysilane and 3-mercaptopropyltriethoxysilane) and lauroyl chloride. The surface modification of MFC was confirmed by infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), and contact angle measurements. Composite paper was successfully prepared with surface modified MFC and polyamide (PA) fiber. The surface modification of MFC not only prevented aggregation of MFC but also improved adhesive property between PA fiber and surface modified MFC. It was impossible to prepare papers of only PA fiber because there is no binder to connect PA fibers. That is, surface modified MFC as a binder in PA fiber played a crucial role in making composite paper. Composite paper with silane modified MFC showed higher tensile strength and modulus than composite paper with lauroyl moiety modified MFC. The structure, morphology, and mechanical properties of composite paper were analyzed by scanning electron microscope (SEM) and universal testing machine (UTM).

• Korean Journal of Optics and Photonics
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• v.31 no.1
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• pp.13-19
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• 2020

To measure underwater acoustics using a fiber-optic Sagnac interferometric sensor, the sensitivities of ring-type probes are investigated by theoretical and experimental studies. A ring-type probe was fabricated by packaging a single-mode fiber wound around an acrylate cylinder of diameter 5 cm with epoxy bond. The probes were prepared as A-type, which was packaged with 46.84 m of sensing optical fiber, and B-type, which was packaged with 112.22 m of sensing fiber. The underwater acoustic test was performed at frequencies of 50, 70, and 90 kHz, and over a range of acoustic pressure of 20-100 Pa, to study the sensitivity. A commercial acoustic generator was located 1 m from the acoustic sensor, such as the ring-type probe or a commercial acoustic sensor. From the experimental test, the acoustic sensitivity of the ring-type probe had different values due to acoustic frequencies, unlike the theoretical prediction. Therefore, the experimental sensitivities were averaged for comparison to the theoretical values. These averaged sensitivities are 25.48 × 10^-5 rad/Pa for the A-type probe and 60.79 × 10^-5 rad/Pa for the B-type probe. The correction coefficient of Young's modulus c was determined to be 0.35.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.4
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• pp.501-518
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• 2019

The rock properties measured under in-situ geological condition can be used to increase the reliability in numerical simulations with regard to the long-term performance of a high-level waste repository. In this study, the change in mechanical properties of KURT (Korea atomic energy research institute Underground Research Tunnel) granite was evaluated under the simulated THM (Thermo-Hydro-Mechanical) coupled condition due to a deep geological formation in the disposal repository. The rock properties such as uniaxial compression strength, indirect tensile strength, elastic modulus and Poisson's ratio were measured under the coupled test conditions (M, HM, TM, THM). It was found that the mechanical properties of KURT granite is more susceptible to the change in saturation rather than temperature within the test condition of this study. The changes in uniaxial compression strength and indirect tensile strength from the rock samples of dried or saturated conditions showed the maximum relative error of about 20% and 13% respectively under the constant temperature condition. Therefore, it is necessary to use the material properties of rock measured under the coupled THM condition as input parameters for the numerical simulation of long-term performance assessment of a disposal repository

• Journal of Conservation Science
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• v.34 no.6
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• pp.459-470
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• 2018

Cellulose nitrates were used for folklore artifacts(ornamental beads and pipes in hatstrings, frames of eyeglasses, ornamental eyeglass cases, headband ornaments, and jeogori buttons) between the 19th and 20th centuries; however, they are susceptible to cracking, crazing, embrittlement, and crumbling due to deterioration. To consolidate and adhere deteriorated cellulose nitrate folklore artifacts, water-soluble acrylic emulsion adhesives were investigated. For consolidation, Plextol D 498, which has the lowest viscosity in low concentrations, was used. In adhesive films whose glass transition temperature(Tg) is lower than room temperature, the tensile stress and modulus decreased and the strain increased; therefore, the flexibility was high. The Plextol D 498 and Plextol D 498 and Dispersion K 52 films maintained their adhesiveness and flexibility after artificial-sunlight-accelerated ageing, and Plextol D 498 and Dispersion K 52 films hardly caused yellowing. Plextol D 498 was the most stable for accelerating ageing. A low concentration of Plextol D 498 emulsion resulted in the best permeability on the surface of cellulose nitrate, compared with other acrylic emulsions. To prevent ornamental hatstrings from cracking, crazing, embrittlement, and crumbling, a Plextol D 498 emulsion was used. After applying low concentrations(1%, 3%) of the emulsion to consolidate the fragments and high concentration to adhere the fragments, the ornamental hatstrings were protected from crumbling by deterioration, and their fragments were well-adhered. To preserve it from deterioration by oxygen and humidity, the treated ornament was sealed with an oxygen-barrier film using a low-humidity oxygen scavenger.