• Korean Journal of Materials Research
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• v.10 no.5
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• pp.321-327
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• 2000

Two kinds of $Al_2O_3/TZP$ composites were prepared using the liquid infiltration of 3Y-TZP and 12Ce-TZP precursors into hte sintered porous $Al_2O_3$. Small TZP additions(~11.0wt%) had increased the strength(19~59%) and fracture toughness(14~157%) of the sintered Al2O3 material($1600^{\circ}C$, 2h). The addition of 3Y-TZP was effective on case of the strength. By the way, in case of the fracture toughness that of 12Ce-TZP was effective. Infiltrated TZP was concentrated on the surface where its grain growth was enhanced and $Al_2O_3$ grain growth was effectively inhibit-ed, when compared to the inner region of the composite. The indentation crack was propagated through both intergranular modes and transgranular and the proportion if intergranular fracture was the larger in $Al_2O_3/12Ce-TZP$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.11
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• pp.1415-1421
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• 2013

Recently, to resolve problems regarding legal liability for accidents and disasters, various simulation techniques such as F.E.M. and F.V.M. have been used in the field of forensic engineering. In this study, we performed mechanical structure analysis using ADINA to investigate the cause of bridge collapse accidents. Such accidents occurred owing to modified and missing processes in comparison with the original design while filling with concrete. Modified and missing processes cause buckling of the upper plate and twisting of the main girder. Through this study, we determine the exact cause of bridge collapse by comparing the evaluation of the structure stability of the original design with the evaluation of the structure stability of the modified and missing process using ADINA structure analysis. Hence, this result indicates that buckling prediction through FEA is the most effective method.

• Journal of the Korean Ceramic Society
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• v.54 no.2
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• pp.141-149
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• 2017

The influence of the co-additive concentration (0 - 45 wt% with an interval of 5 wt%) of MgO-$TiO_2$ on the phase formation, microstructure and fracture toughness of MgO-$TiO_2$-reinforced dental porcelain nanocomposites derived from a one-step sintering technique were examined using a combination of X-ray diffraction, scanning electron microscopy and Vickers indentation. It was found that MgO-$TiO_2$-reinforced dental porcelain nanocomposites exhibited significantly higher fracture toughness values than those observed in single-additive (MgO or $TiO_2$)-reinforced dental porcelain composites at any given sintering temperature. The amount of MgO-$TiO_2$ as a co-additive was found to be one of the key factors controlling the phase formation, microstructure and fracture toughness of these nanocomposites. It is likely that 30 wt% of MgO-$TiO_2$ as a co-additive is the optimal amount for $MgTi_2O_5$ and $Mg_2SiO_4$ crystalline phase formation to obtain the maximum relative density (96.80%) and fracture toughness ($2.60{\pm}0.07MPa{\cdot}m^{1/2}$) at a sintering temperature of $1000^{\circ}C$.

• Korean Journal of Materials Research
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• v.15 no.12
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• pp.796-801
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• 2005

In order to reduce the weight of glass in architecture, automobile, bottles, displays, a new technique that can strengthen glass was developed using various method. Generally, the strength achieved of glass-ceramics is higher as is 1.he fracture toughness by the formation of a crystalline phase inside glass. In this study, $70SiO_2-20Na_2O-10CaO-10TiO_2$ glasses were irradiated to strengthen by heterogeneous phase using femto-second laser pulse. Laser pulse irradiation of samples was analyzed by DTA, TMA, XRD, nano-indenter and SEM. Samples irradiated by laser had lower value$(3\~4\times10^{-3}Pa)$ of nano indentation which related with mother glass$(8\times10-3Pa)$ than values. Microcracks were occurred around laser irradiation area when femtosecond laser with the repetition rate of 1kHz was used as the light source to induced heterogeneous phase.

• Journal of the Korean Ceramic Society
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• v.37 no.1
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• pp.21-25
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• 2000

Gas pressure sintered silicon nitride based composites with 5 wt% $\beta$-Si3N4 whiskers were prepared, and the variations depending on sintering additives and sintering temperature were studied. Sintering additives were 6 wt% Y2O3-1 wt% MgO(6Y1M), 6 wt%Y2O3-1 wt% Al2O3(6Y1A), 6 wt% Y2O3-1 wt% SiO2(6Y1S), and whiskers were unidirectionally oriented by a modified tape casting technique. Samples were fully densified by gas pressure sintering at 2148 K and 2273 K. As the sintering temperature increased, the size of large elongated grains was increased. Three point flexural strength of 6Y1M and 6Y1M samples was higher than that of 6Y1S sample, and the strength decreased as the sintering temperature increased. The indentation crack length became shorter for the sample sintered at higher temperature, and the difference between the cracks length parallel to and normal to the direction of whisker alignment was decreased. In case of cracks 45$^{\circ}$off the whisker alignment direction, the crack length anisotropy disappeared.

• Journal of the Korean Society for Heat Treatment
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• v.36 no.3
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• pp.128-136
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• 2023

In this study, the effects of heat treatment on the nano-scale creep behavior of CoCrFeMnNi high-entropy alloy (HEA) processed by high-pressure torsion (HPT) was investigated through nanoindentation technique. Nanoindentation experiments with a Berkovich indenter were performed on HPT-processed alloy subjected to heat treatment at 450℃, revealing that the hardness of the HPT-processed alloy (HPT sample) significantly increased with the heat treatment time. The heat treatment-induced microstructural change in HPT-processed alloy was analyzed using transmission electron microscopy, which showed the nano-sized Cr-, NiMn-, and FeCo-rich phases were formed in the HPT-processed alloy subjected to 10 hours of heat treatment (HPT+10A sample). To compare the creep behavior of HPT and HPT+10A samples, constant load nanoindentation creep experiments were performed using spherical indentation indenters with two different radii. It was revealed that the predominant mechanism for creep highly depended on the applied stress level. At low stress level, both HPT and HPT+10A samples were dominated by Coble creep. At high stress level, however, the mechanism transformed to dislocation creep for HPT sample, but continued to be Coble creep for HPT+10A sample, leading to higher creep resistance in the HPT+10A sample.