• 한국정밀공학회지
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• 제23권4호
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• pp.116-123
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• 2006

During the last two decades, many researchers investigated influences of stress triaxiality on ductile fracture for various specimens and structures. With respect to a transferability issue, the local approach reflecting micro-mechanical specifics is one of effective methods to predict constraint effects. In this paper, the applicability of the local approach was examined through a series of finite element analyses incorporating modified GTN (Gurson-Tvergaard-Needleman) and Rousselier models as well as fracture toughness tests. To achieve this goal, fracture resistance (J-R) curves of several types of compact tension (CT) specimens with various crack length, with various thickness and with/without 20% side- grooves were estimated. Then. the constraint effects were examined by comparing the numerically estimated J-R curves with experimentally determined ones. The assessment results showed that the damage models might be used as useful tool for fracture toughness estimation and both the crack length and thickness effects should be considered for realistic structural integrity evaluation.

• 한국세라믹학회지
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• 제25권3호
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• pp.201-206
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• 1988

Mo 입자의 첨가가 Al_2O_3$ 세라믹스의 미세구조와 기계적 성질에 미치는 영향을 알아보기 위하여 평균입경이 2-micron인 Mo와 6-micron인 Mo를 Al_2O_3$에 각 분산시켜 1$600^{\circ}C$, $H_2$ 분위기에서 5시간 소결하였다. Mo는 Al_2O_3$의 입자성장을 억제시켰으며 Mo의 입자가 작을 때 그 효과는 크게 나타났다. 2-micron Mo를 분산한 경우 꺽임강도와 파괴인성은 크게 증가하여하였으며, 6-micron Mo를 분산한 경우 강도는 증가하지 않았으나 파괴인성은 다소 증가하였다. Al_2O_3$-Mo계의 인성증진기구는 균열편향에 의한 파단면의 증가와 미세균열에 의한 균열전파에너지의 분산에 의한 것으로 보인다.

• 한국세라믹학회지
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• 제49권1호
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• pp.72-77
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• 2012

In the preparation of a hydroxyapatite [HAp]/gelatin [GEL] nanocomposite, the GEL matrix in aqueous solution of $H_3PO_4$ was modified by the introduction of aspartic acid [Asp], asparagine [Asn], and glycine [Gly]. The addition of Asp, Asn and Gly greatly affected the slurry formation of HAp/GEL nanocomposite and the resulting dry body showed variations in toughness with the addition of the different amino acids. The introduction of Asn into HAp/GEL nanocomposite was effective for producing the organic-inorganic interaction between HAp and GEL, and caused the increase of toughness. The formation reaction of the modified HAP/GEL nanocomposites was investigated by using XRD and FT-IR. The organic-organic interaction between the GEL matrix and the additives of Asp, Asn and Gly was confirmed from FT-IR analysis, and the organic-inorganic interaction between HAp nanocrystallites and the modified GEL matrix was also discussed, using FT-IR spectra patterns. Nanocrystallites of HAp were covalently bound with the GEL macromolecules and differently influenced by the modification species of Asp, Asn, and Gly.

• International Journal of Concrete Structures and Materials
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• 제11권1호
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• pp.115-133
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• 2017

To investigate the effectiveness of using fiber reinforced polymer (FRP) sheets in confining heat-damaged columns, 15 circular RC column specimens were tested under axial compression. The effects of heating duration, stiffness and thickness of the FRP wrapping sheets were examined. Two specimen groups, six each, were subjected to elevated temperatures of $500^{\circ}C$ for 2 and 3 h, respectively. Eight of the heat-damaged specimens were wrapped with unidirectional carbon and glass FRP sheets. Test results confirmed that elevated temperatures adversely affect the axial load resistance and stiffness of the columns while increasing their ductility and toughness. Full wrapping with FRP sheets increased the axial load capacity and toughness of the damaged columns. A single layer of the carbon sheets managed to restore the original axial resistance of the columns heated for 2 h yet, two layers were needed to restore the axial resistance of columns heated for 3 h. Glass FRP sheets were found to be less effective; using two layers of glass sheets managed to restore the axial load carrying capacity of columns heated for 2 h only. Confining the heat-damaged columns with FRP circumferential wraps failed in recovering the original axial stiffness of the columns. Test results confirmed that FRP-confining models adopted by international design guidelines should address the increased confinement efficiency in heat-damaged circular RC columns.

• Advances in concrete construction
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• 제16권5호
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• pp.255-267
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• 2023

The production of ultra-early-strength concrete (UESC) traditionally involves complexity or necessitates high-temperature curing conditions. However, this study aimed to achieve ultra-early-strength performance solely through room-temperature curing. Experimental results demonstrate that under room-temperature (28℃) curing conditions, the concrete attained compressive strengths of 20 MPa at 4 hours and 69.6 MPa at 24 hours. Additionally, it exhibited a flexural strength of 7.5 MPa after 24 hours. In contrast, conventional concrete typically reaches around 20.6 MPa (3,000 psi) after approximately 28 days, highlighting the rapid strength development of the UESC. This swift attainment of compressive strength represents a significant advancement for engineering purposes. Small amounts of steel fibers (0.5% and 1% by volume, respectively) were added to address potential concrete cracking due to early hydration heat and enhance mechanical properties. This allowed observation of the effects of different volume contents on ultra-early-strength fiber-reinforced concrete (UESFRC). Furthermore, the compressive strength of 0.5% and 1% UESFRC increased by 16.3% and 31.3%, respectively, while the flexural strength increased by 37.1% and 47.9%. Moreover, toughness increased by 58.2 and 69.7 times, respectively. These findings offer an effective solution for future emergency applications in public works.

• 한국재료학회지
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• 제10권5호
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• pp.321-327
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• 2000

다공성 알루미나 소결체내부로 3Y-TZP 및 12Ce-TZP 전구체를 각각 액상침투시킴으로써 2종류의 $Al_2O_3/TZP$복합체를 제조하였다. 소량의 (~11.0 wt%) TZP의 첨가는 Al2O3소결체 ($1600^{\circ}C$, 2시간)의 강도 (19~59%)와 파괴인성(14~157%)을 증가시켰다. 3Y-TZP의 첨가는 복합체의 강도의 향상에 12Ce-TZP의 첨가는 인성의 향상에 보다 효과적이었다. 침투도니 TZP는 복합체의 내부보다 표면에 집중되었으며, 그 결과 이곳에서의 입성장에 빨랐고 $Al_2O_3$의입성장 억제효과도 상대적으로 뛰어났다. 입계 및 입내균열전파가 일어났으나 $Al_2O_3/12Ce-TZP$의 경우가 $Al_2O_3/3Y-TZP$에 비하여 입계파괴가 우세하였다

• 한국분말재료학회지
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• 제14권2호
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• pp.132-139
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• 2007

In the Ti(CN)-Co/Ni cermet, WC is an effective additive for increasing sinterability and mechanical properties such as toughness and hardness. In this work, WC, (WTi)C and (WTi)(CN) were used as the source of WC and their effects were investigated in the respect of microstructural evolution and mechanical properties. Regardless of the kinds of WC sources, the hard phase with dark core and bright rim structure was observed in the Ti(CN)-Co/Ni cermet under the incorporation of relatively small amount of WC. However, hard phases with bright core began to appear and their frequency increased with the increase of all kinds of WC source addition. The ratio of bright core to dark one in the (TiW)(CN)-Co/Ni cermet was greatest under the incorporation of (WTi)C compared at the same equivalent amount of WC. The mechanical properties were improved with the addition of WC irrespective of the kinds of sources, but the addition of (WTi)(CN) was less effective for the increase of fracture toughness.

• 한국결정성장학회지
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• 제8권4호
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• pp.645-650
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• 1998

계면유도 침전법을 이용, 알루미나와 이트리아를 코팅한 SiC 분말을 사용하여 제조된 탄화규소의 물성을 관찰하였다. $Al_2(SO_4)_3$과 $Y_2(SO_4)_3$의 수용액에서 요소를 분해시켜 수화물 및 탄화물의 전구체로 구성된 석출물을 SiC의 분말 표면에 코팅한 후 하소하여 알루미나와 이트리아가 코팅된 탄화규소 시편을 제조하였다. $1900^{\circ}C$에서 소결한 탄화규소는 약 97.8%의 소결밀도를 나타내었으며, 소결 후 annealing을 행한 시편은 $\beta$상 탄화규소에서 $\alpha$상 탄화규소로의 상전이가 일어나 주상입자가 형성되었다. 균열 전파시 주상입자를 중심으로 입계간 균열(intergranular crack)이 일어났으며, 주상입자의 pullout 효과 의한 균열길이의 증가로 SiC 소결체의 인성이 증진되는 것으로 나타났다. annealing 시간이 3시간 이내인 경우에는 소결조제의 첨가량이 적은 시편의 파괴인성치가 높았으나 그 차이는 미미하였고, annealing 시간이 길어짐에 따라 소결조제의 양에 따른 인성의 차이는 거의 나타나지 않았다. 이러한 결과는 annealing 시간이 길어짐에 따라 입자의 정단축비(aspect ratio)가 커지고 주성입자에 의한 pullout 효과가 인성증진의 주된 인자가 되어, 소결조제의 양과 관련된 영향이 적었기 때문인 것으로 사료되었다.

• 대한기계학회논문집
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• 제16권1호
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• pp.63-69
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• 1992

본 연구에서는 특히 앞서 지적한 표면균열의 진전거동에 관한 연구와도 관련 하여 측면홈을 가진 중앙관통균열(side-grooved center cracked tension) 시험편에 대 해 피로균열 진전실험을 수행하고, 평면변형률 조건의 만족여부 및 응력강도계수 평가 방법에 관하여 검토하여 비교적 새로운 결과를 얻었으므로 여기에 보고하고자 한다.

• 한국결정성장학회지
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• 제8권4호
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• pp.634-639
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• 1998

$Y_2O_3$와 $Nb_2O_5$가 첨가된 지르코니아에 입자 크기가 다른 두종류의 Al2O3를 부피비로 10~30% 첨가한 복합체를 155$0^{\circ}C$ 온도에서 5시간 소결하여 $250^{\circ}C$ 온도의 공기 분위기하에서 1000시간 혹은 $180^{\circ}C$ 온도의 0.3MPa H2O vapor pressure 분위기하에서 5시간 열처리를 하여 X-ray 회절분석 방법을 이용하여 저온 상안정성을 조사하였다. 복합체 전부 열처리 후에 정방정 상에서 단사정 상으로 상전이가 발생하지 않는 우수한 저온 상안정성이 관찰되었다. 복합체의 상아정성에 Nb 첨가가 $Al_2O_3$의 첨가보다 더 효과적이었다. 2.8$\mu$m 입자크기를 가진 Al2O3를 부피비로 20% 첨가한 복합체에서 최적의 조건이 관찰되었으며 강도와 인성값은 각각 670MPa와 7.1MPam1/2이었다. Al2O3 입자 크기가 2.8$\mu$m에서 0.2$\mu$m로 감소함에 따라 강도는 다소 증가하였으나 파괴인성값은 감소하였다.