• Restorative Dentistry and Endodontics
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• v.33 no.4
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• pp.377-388
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• 2008

The aim of the present study is to compare the corrosion tendency using two kinds of NiTi files in the various environmental conditions through the visual examination and electrochemical analysis. ProTaper Universal S2, 21 mm (Dentsply Maillefer, Ballaigues, Switzerland) and Hero 642, 0.06 tapers, size 25, 21 mm (Micromega, Besancon, France) rotary instruments were tested. The instruments were randomly divided into eighteen groups (n = 5) by the immersion temperature, the type of solution, the brand of NiTi rotary instrument and the presence of mechanical loading. Each file was examined at various magnifications using Scanning Electron Microscope (JEOL, Akishima, Tokyo, Japan) equipped with energy dispersive X-ray microanalysis (EDX). EDX was used to determine the components of the endodontic file alloy in corroded and noncorroded areas. The corrosion resistance of unused and used NiTi files after repeated uses in the human teeth was evaluated electrochemically by potentiodynamic polarization test using a potentiostat (Applied Corrosion Monitoring, Cark-in-Cartmel, UK). Solution temperature and chloride ion concentration may affect on passivity of NiTi files. Under the conditions of this in vitro study, the corrosion resistance is slightly increased after clinical use.

• Restorative Dentistry and Endodontics
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• v.33 no.4
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• pp.323-331
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• 2008

Flexibility and fracture properties determine the performance of NiTi rotary instruments. The purpose of this study was to evaluate how geometrical differences between three NiTi instruments affect the deformation and stress distributions under bending and torsional conditions using finite element analysis. Three NiTi files (ProFile .06 / #30, F3 of ProTaper and ProTaper Universal) were scanned using a Micro-CT. The obtained structural geometries were meshed with linear, eight-noded hexahedral elements. The mechanical behavior (deformation and von Mises equivalent stress) of the three endodontic instruments were analyzed under four bending and rotational conditions using ABAQUS finite element analysis software. The nonlinear mechanical behavior of the NiTi was taken into account. The U-shaped cross sectional geometry of ProFile showed the highest flexibility of the three file models. The ProTaper, which has a convex triangular cross-section, was the most stiff file model. For the same deflection, the ProTaper required more force to reach the same deflection as the other models, and needed more torque than other models for the same amount of rotation. The highest von Mises stress value was found at the groove area in the cross-section of the ProTaper Universal. Under torsion, all files showed highest stresses at their groove area. The ProFile showed highest von Mises stress value under the same torsional moment while the ProTaper Universal showed the highest value under same rotational angle.

• Restorative Dentistry and Endodontics
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• v.22 no.1
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• pp.209-219
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• 1997

The research of the dentin bonding system was mainly on the chemistry and bonding strength. And in vitro assessement of biocompatibility of dentin bonding system was not completely developed. The purpose of this study was to evaluate the cytotoxic effect of several dentin primers. Scotchbond Multi-Purpose (3M Dental Products. USA). Gluma (BayerDental. Germany). All-Bond (Bisco. USA). ProBond (CaulkDensply, USA) and VeridonFil (Dongyang Nylon. Korea) were included. Cytotoxicity was tested using MTT cell viability test. 0.5 ul. 1 ul. 2 ul and 10 ul of each primer were added to the 96 well plate of incubated L929 cell lines. After 30-minute. 1. 4. 24 and 72-hour exposures. absorbance of L929 cells was observed with ELISA reader. All data were analyzed using t-test. All primers showed cytotoxicity on L929 cells under every conditions used in this study. Absorbance of L929 cells was decreased by time. Scotch bond group exhibited the lowest absorbance value in all exposure time and value.

• 한국사회정책
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• v.25 no.4
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• pp.163-198
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• 2018

This study analyzed the change of the Korean welfare regime during the conservative government. It is clear that the welfare expanded during the conservative government, but this expansion was the process of realizing the constraint of expansion that reduces future welfare expenditure in Korea. In addition, as the public welfare centered on social insurance expanded after 10 years of liberal government, the backward selectivity of the Korean welfare regime strengthened in the conservative administration. Expanding social insurance itself does not reinforce the backward selectivity of the Korean welfare regime. However, the industrial structure created by the export-led growth system has intensified the fragmentation of the labor market, and expanding social insurance designed based on regular workers under these conditions has forced the backward selectivity of the social security system. It is for this reason that the backward selectivity has been reinforced during the conservative government.

• Steel and Composite Structures
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• v.43 no.3
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• pp.341-351
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• 2022

This paper presents an inelastic buckling behavior analysis of rectangular hollow steel tubes with geometrical imperfections under elevated temperatures. The main variables are the temperature loads, slenderness ratios, and exposure conditions at high temperatures. The material and structural properties of steels at different temperatures are based on Eurocode (EN 1993-1-2, 2005). In the elastic buckling analysis, the buckling strength decreases linearly with the exposure conditions, whereas the inelastic buckling analysis shows that the buckling strength decreases in clusters based on the exposure conditions of strong and weak axes. The buckling shape of the rectangular steel column in the elastic buckling mode, which depicts geometrical imperfection, shows a shift in the position at which bending buckling occurs when the lower section of the member is exposed to high temperatures. Furthermore, lateral torsional buckling occurs owing to cross-section deformation when the strong axial plane of the model is exposed to high temperatures. The elastic buckling analysis indicates a conservative value when the model is exposed to a relatively low temperature, whereas the inelastic buckling analysis indicates a conservative value at a certain temperature or higher. The comparative results between the inelastic buckling analysis and Eurocode 3 show that a range exists in which the buckling strength in the design equation result is overestimated at elevated temperatures, and the shapes of the buckling curves are different.

• Restorative Dentistry and Endodontics
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• v.23 no.1
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• pp.391-400
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• 1998

The aim of this study was to determine the shaping ability of stainless-steel K file (S-S K file), nickel-titanium K file (Ni-Ti K file) and engine driven nickel-titanium file (Quantec file) in resin simulated root canal. Computed tomography was used to evaluate the change of the root canal morphology. Thirty nine resin simulated root canal were divided into four groups (A:12, B:12, C:12, D:3). Resin simulated canals were scanned by computed tomography before instrumentation (1st C-T scan). Canals were instrumented using step back preparation technique with S-S K file in group A and Ni-Ti K file in group B. Group C was prepared with engine driven Ni-Ti file. Group D was uninstrumented to compare the 1st C-T scan images with 2nd C-T scan images of root canal. Instrumented canals were again scanned using computed tomography (2nd C-T scan), and reformated images of the uninstrumented canals were compared with images of the instrumented canals. In the sections of 2mm and 6mm from the apex, Quantec file caused significantly less canal transportation than S-S K file and Ni-Ti K file (p<0.05). Quantec file produced more centered than S-S K file and Ni-Ti K file in the sections of 2mm and 4mm from the apex (p<0.05). There was no significant difference in the removed volume of canals among the each groups (p>0.05). However the removed canal volume from the apex to 5mm were significantly higher than them from 5mm to 1mm (p<0.05) in each groups. Under the conditions of this study, preparation with Quantec file was more effective and produce more appropriate canal shapes than S-S K file and Ni-Ti K file.

• Restorative Dentistry and Endodontics
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• v.39 no.4
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• pp.310-318
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• 2014

Objectives: This study examined the influence of the resin thickness on the polymerization of silorane- and methacrylate-based composites. Materials and Methods: One silorane-based (Filtek P90, 3M ESPE) and two methacrylate-based (Filtek Z250 and Z350, 3M ESPE) composite resins were used. The number of photons were detected using a photodiode detector at the different thicknesses (thickness, 1, 2 and 3 mm) specimens. The microhardness of the top and bottom surfaces was measured (n = 15) using a Vickers hardness with 200 gf load and 15 sec dwell time conditions. The degree of conversion (DC) of the specimens was determined using Fourier transform infrared spectroscopy (FTIR). Scratched powder of each top and bottom surface of the specimen dissolved in ethanol for transmission FTIR spectroscopy. The refractive index was measured using a Abbe-type refractometer. To measure the polymerization shrinkage, a linometer was used. The results were analyzed using two-way ANOVA and Tukey's test at p < 0.05 level. Results: The silorane-based resin composite showed the lowest filler content and light attenuation among the specimens. P90 showed the highest values in the DC and the lowest microhardness at all depth. In the polymerization shrinkage, P90 showed a significantly lower shrinkage than the rest two resin products (p < 0.05). P90 showed a significantly lower refractive index than the remaining two resin products (p < 0.05). Conclusions: DC, microhardness, polymerization rate and refractive index linearly decreased as specimen thickness linearly increased. P90 showed much less polymerization shrinkage compared to other specimens. P90, even though achieved the highest DC, showed the lowest microhardness and refractive index.

• Journal of Electrical Engineering and Technology
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• v.4 no.4
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• pp.566-569
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• 2009

In this paper, we propose a less conservative a linear matrix inequality (LMI) condition for the constrained robust model predictive control of systems with input constraints and polytopic uncertainty. Systems with input constraints are represented as perturbed systems with sector bounded conditions. For the infinite horizon control, closed-loop stability conditions are obtained by using a parameter dependent Lyapunov function. The effectiveness of the proposed method is shown by an example.

• 제어로봇시스템학회:학술대회논문집
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• 2000.10a
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• pp.270-270
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• 2000

Input delay is frequently encountered in the practical systems since measurement delay and computational delay can be represented by input delay. In this viewpoint, this paper deals with the robust control problem of input delayed systems with structured uncertainty. Robust stability conditions are provided in terms of linear matrix inequalities(LMIs) and it is shown that the proposed conditions can give less conservative maximum bound of input delay guaranteeing robust stability.

• Restorative Dentistry and Endodontics
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• v.34 no.2
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• pp.113-119
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• 2009

The purpose of this study was to compare the tensile bond strength of several self-adhesive resin cements bonded to dentin surfaces with different wet conditions. Three self-adhesive resin cements: Rely-X Unicem (3M ESPE, St. Paul, MN. USA). Embrace Wetbond (Pulpdent. Oakland. MA. USA). Maxcem (Kerr. Orange. CA. USA) were used. Extracted sixty human molars were used. Each self-adhesive resin cement was adhered to the dentin specimens (two rectangular sticks from each molar) in different wet conditions. Tensile bond strength were measured using universal testing machine (EZ Test. Shimadzu corporation. Kyoto. Japan) at a crosshead speed of 1.0mm/min. After the testing. bonding failures of specimens were observed by Operative microscope (OPMI pro, Carl Zeiss. Oberkochen, Germany). T-test was used to evaluate the effect of dentin surface wetness. One-way ANOVA test was used to evaluate the tensile bond strength of self-adhesive resin cements in the same condition. Scheffe's test was used for statistical analyzing at the 95% level of confidence. The result showed that wetness of dentin surface didn't affect tensile bond strength of self-adhesive resin cements and Maxcem showed the lowest tensile bond strength.