• Advances in concrete construction
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• v.8 no.3
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• pp.207-215
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• 2019

Application of nanotechnology can be used to tailor made cementitious composites owing to small dimension and physical behaviour of resulting hydration products. Because of high aspect ratio and extremely high strength, carbon nanotubes (CNTs) are perfect reinforcing materials. Hence, there is a great prospect to use CNTs in developing new generation cementitious materials. In the present paper, a parametric study has been conducted on cementitious composites reinforced by two types of multi walled carbon nanotubes (MWCNTs) designated as Type I CNT (10-20 nm outer dia.) and Type II CNT (30-50 nm outer dia.) with various concentrations ranging from 0.1% to 0.5% by weight of cement. To evaluate important properties such as flexural strength, strain to failure, elastic modulus and modulus of toughness of the CNT admixed specimens at different curing periods, flexural bending tests were performed. Results show that composites with Type II CNTs gave more strength as compared to Type I CNTs. The highest increase in strength (flexural and compressive) is of the order of 22% and 33%, respectively, compared to control samples. Modulus of toughness at 28 days showed highest improvement of 265% for Type II 0.3% CNT composites. It is obvious that an optimum percentage of CNT could exists for composites to achieve suitable reinforcement behaviour and desired strength properties. Based on the parametric study, a tentative optimum CNT concentration (0.3% by weight of cement) has been proposed. Scanning electron microscope image shows perfect crack bridging mechanism; several of the CNTs were shown to act as crack arrestors across fine cracks along with some CNTs breakage.

• Korean Journal of Materials Research
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• v.29 no.2
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• pp.97-105
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• 2019

In the current steel structures of high-rise buildings, high heat input welding techniques are used to improve productivity in the construction industry. Under the high heat input welding, however, the microstructures of the weld metal (WM) and heat-affected zone (HAZ) coarsen, resulting in the deterioration of impact toughness. This study focuses mainly on the effects of fine TiN precipitates dispersed in steel plates and B addition in welding materials on grain refinement of the HAZ microstructure under submerged arc welding (SAW) with a high heat input of 200 kJ/cm. The study reveals that, different from that in conventional steel, the ${\gamma}$ grain coarsening is notably retarded in the coarse grain HAZ (CGHAZ) of a newly developed steel with TiN precipitates below 70 nm in size even under the high heat input welding, and the refinement of HAZ microstructure is confirmed to have improved impact toughness. Furthermore, energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS) analyses demonstrate that B is was identified at the interface of TiN in CGHAZ. It is likely that B atoms in the WM are diffused to CGHAZ and are segregated at the outer part of undissolved TiN, which contributes partly to a further grain refinement, and consequently, improved mechanical properties are achieved.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2001.05a
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• pp.106-109
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• 2001

In this work, we investigate the toughening mechanism of the rubber-modified epoxy resin. The fracture toughness($K_{IC}$) is measured using CT specimens for three kinds of rubber-modified epoxy resin with different rubber content. The damage zone and rubber particles around a crack tip of a damaged specimen just before fracture are observed by a polarization microscope and an atomic force microscope(AFM). Both the fracture energy($G_{IC}$) and the size of damage zone increase with the rubber content below l5wt%. The size of the rubber particles can be qualitatively correlated with the $G_{IC}$ and the size of damage zone. The cavitation of the rubber particles inside the damage zone is observed, which is expected to be main toughening mechanism by rubber particles. the stress which causes the cavitation of rubber particles is estimated by the Dugdale model.

• Journal of the Korean Ceramic Society
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• v.30 no.10
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• pp.785-790
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• 1993

Sialon ceramics are presently seen as promising materials with high hardness, strength, fracture toughness and corrosion resistance for friction and wear applications. The objective of present work is to improve of mechanical properties and wear resistance of $\alpha$-Sialon(x=0.4) by addition of SiC whisker. $\alpha$-sialon(x=0.4)/SiC whisker composites were obtained by hot-isostatic pressing at 173$0^{\circ}C$ for 1 hour under 1757Kg/$\textrm{cm}^2$ N2 pressure after pressureless sintering the mixture of Si3N4, Y2O3, AlN at 1780~180$0^{\circ}C$ for 3~5 hours in N2 atmosphere. As the amount of SiC whisker content increased, relative density and hardness were decreased, however fracture toughness, bending strength and tribological properties were improved. Tribological properties of $\alpha$-Sialon/15 vol% SiC whisker composite were improved in spite of its low mechanical properties.

• Applied Chemistry for Engineering
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• v.2 no.4
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• pp.301-310
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• 1991

Epoxy resins have been widely used for many applications along with good processibility. However, epoxy resin systems have poor hot/wet performance properties and brittleness after resin curing and have limited to apply for environmental resistant materials. In order to improve the toughness of epoxy resin, this review article deals with incorporation method of rubber and high performance thermoplastics into the matrix resin. In addition, molecular design of epoxy resin and modification of thermoplastic have been introduced for improving hot/wet properties of epoxy resin.

• Korean Journal of Materials Research
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• v.18 no.7
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• pp.357-361
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• 2008

Microstructure and mechanical properties of WC-3 wt% Co cemented carbides, fabricated by a spark plasma sintering (SPS) process, were investigated in this study. The WC-3 wt%Co powders were sintered at $900{\sim}1100^{\circ}C$ for 5min under 40MPa in high vacuum. The density and hardness were increased as the sintering temperature increased. WC-3 wt%Co compacts with a relative density of 97.1% were successfully fabricated at $1100^{\circ}C$. The fracture toughness and hardness of a compact sintered at $1100^{\circ}C$ were $21.6 MPa{\cdot}m^{1/2}$ and 4279 Hv, respectively.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2003.10a
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• pp.3-6
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• 2003

In this work, poly(ethylene oxide) nanofibers were fabricated by electrospinning to prepare nanofibers-reinforced composites. And the PEO powders-impregnated composites were also prepared to compare with physicochemical properties of nanofibers-reinforced composites. Morphology and fiber diameter of PEO nanofibers were determined by SEM observation. Mechanical interfacial properties of the composites were investigated in fracture toughness tests and interlaminar shear strength (ILSS) test. As a result, the fiber diameter decreased in increasing applied voltage. However the optimum condition for the fiber formation was 15 ㎸, resulting from increasing of jet instability at high voltage and the prepared PEO nanofibers were useful in fiber reinforced composites. The PEO-based nanofibers-reinforced composites showed an improvement of fracture toughness factors ($K_{IC} and G_{ IC}$) and ILSS, compared to the composites impregnated with PEO powders. These results were noted that the nanofibers had higher specific surface area and larger aspect ratio than those of the powder, which played an important role in improving the mechanical interfacial properties of the composites.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.9
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• pp.2368-2373
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• 1994

The advancement of the mechanical properties of metal materials caused by a recent high technology contributes to the stability and productivity of mechanical structures. However, the advanced mechanical properties depends on the conditions of crystal boundaries and the improvement of the texture. Although the tensile strength and a hardness of a steel would be increased by the adding B, it seems to be a marked decreases of the toughness which caused in the weaken workability. This study is concerned with a characteristics of the B compound which will be mixed with $H_3BO_3$ and the metallic magnesium. What affected in the mechanical property and texture is checked by the strength and the texture test. As a result, it is shown that the improvement of the mechanical property and the texture homogeneity. In addition, it seems that a molten metal which is added by the B compound is deoxized and cleansed.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.670-677
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• 2003

Although Gas Tungsten Arc Welding (GTAW or TIG welding) is considered as high quality and precision welding process, it also has demerit of low melting rate. Narrow-gap TIG welding which has narrow joint width reduces the groove volume remarkably, so it could be shorten the welding time and decrease the overall shrinkage in heavy wall pipe welding. Generally Narrow-gap TIG welding is used as orbital welding process, it is important to select the optimum conditions for the automatic control welding This paper looks at the application and metallurgical properties on Narrow-gap TIG welding joint of heavy wall large austenitic stainless steel pipe to determine the deposition efficiency, the resultant shrinkage and fracture toughness. The fracture toughness depends slightly on the welding heat input.

• Journal of Korea Foundry Society
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• v.8 no.4
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• pp.459-466
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• 1988

This study was carried out in the austempering temperature and time after Ni, Mo addition in purpose of modification of impact toughness of austempered ductile irons. Addition of alloy element and austempering treatment of $900^{\circ}C$ 60 minutes followed by $300^{\circ}C$, $350^{\circ}C$ and $400^{\circ}C$ for 60 minutes, in this case impact value was increased by ideal mixed structure. But impact value was decreased when holing time is 120minutes, this is attributed to segregation and carbide precipitation from high carbon austenite. Highest impact value was obtained by $350^{\circ}C$ (Mo-addition) and $400^{\circ}C$ (Ni-addition). This phenomena was caused by presence of remained austenite. At all austempering temperature,, Ni-added specimen showed higher impact values than that of Mo-added specimen. And hardness property was affected by austempering temperature and holding time rather than amounts of alloying element.