• 한국세라믹학회지
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• 제28권1호
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• pp.11-18
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• 1991

A precipitation method, one of the most effective liquid phase reaction methods, was adopted in order to prepare high-tech Al2O3/ZrO2 composite ceramics, and the effects of stress-induced phase transformation of ZrO2 on thermal shock behavior of Al2O3-ZrO2 ceramics were investigated. Al2(SO4)3.18H2O, ZrOCl2.8H2O and YCl3.6H2O were used as starting materials and NH4OH as a precipitation agent. Metal hydroxides were obtained by single precipitation(process A) and co-precipitation(process B) method at the condition of pH=7, and the composition of Al2O3-ZrO2 composites was fixed as Al2O3-15v/o ZrO2(+3m/o Y2O3). Critical temperature difference showing rapid strength degradation by thermal shock showed higher value in Al2O3/ZrO2 composites(process A : 20$0^{\circ}C$, process B : 215$^{\circ}C$) than in Al2O3(175$^{\circ}C$). The improvement of thermal shock property for Al2O3/ZrO2 composites was mainly due to the increase of strength at room temperature by adding ZrO2. The strength degradation was more severe for the sample with higher strength at room temperature. Crack initiation energies by thermal shock showed higher values in Al2O3/ZrO2 composites than in Al2O3 ceramics due to increase of fracture toughness by ZrO2.

• Geomechanics and Engineering
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• 제10권6호
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• pp.775-791
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• 2016

The indirect tensile strengths (ITSs) of different cemented paste backfill mixes with different curing times were determined by considering crack initiation and fracture toughness concepts under different loading conditions of steel loading arcs with various contact angles, flat platens and the standard Brazilian test jaw. Because contact area of the ITS test discs developes rapidly and varies in accordance with the deformability, ITSs of curing materials were not found convenient to determine under the loading apparatus with indefinite contact angle. ITS values increasing with an increase in contact angle can be measured to be excessively high because of the high contact angles resulted from the deformable characteristics of the soft paste backfill materials. As a result of the change of deformation characteristics with the change of curing time, discs have different contact conditions causing an important disadvantage to reflect the strength change due to the curing reactions. In addition to the experimental study, finite element analyses were performed on several types of disc models under various loading conditions. As a result, a comparison between all loading conditions was made to determine the best ITSs of the cemented paste backfill materials. Both experimental and numerical analyses concluded that loading arcs with definite contact angles gives better results than those obtained with the other loading apparatus without a definite contact angle. Loading arcs with the contact angle of $15^{\circ}$ was found the most convenient loading apparatus for the typical cemented paste backfill materials, although it should be used carefully considering the failure cracks for a valid test.

• 한국터널지하공간학회 논문집
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• 제15권5호
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• pp.523-536
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• 2013

해저터널은 육상의 지하구조물과 달리 시공 및 운영 중의 안전성을 확보하기 위해서는 기존의 육상터널에 적용된 응력과 변위계측과 더불어 해저터널의 특성을 고려한 계측이 필수적으로 요구된다. 따라서 해저터널의 안정성을 평가하고자 NATM에 적용되는 개념을 정리하고, 계측계획 수립을 위한 관측 시스템(observation system), 터널의 안전관리를 위한 임계변형률 이론과 국부적으로 발생하는 암석과 지보재의 파괴나 균열의 생성을 파악할 수 있는 미소파괴음 계측을 소개하고, 이러한 방법들을 사용한 해저터널 계측 및 관리방안을 제시하였다.

• Corrosion Science and Technology
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• 제3권5호
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• pp.198-208
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• 2004

Although there has been no general agreement on the mechanism of primary water stress corrosion cracking (PWSCC) as one of major degradation modes of Ni-base alloys in pressurized water reactors (PWR's), common postulation derived from previous studies is that the damage to the alloy substrate can be related to mass transport characteristics and/or repair properties of overlaid oxide film. Recently, it was shown that the oxide film structure and PWSCC initiation time as well as crack growth rate were systematically varied as a function of dissolved hydrogen concentration in high temperature water, supporting the postulation. In order to understand how the oxide film composition can vary with water chemistry, this study was conducted to characterize oxide films on Alloy 600 by an in-situ Raman spectroscopy. Based on both experimental and thermodynamic prediction results, Ni/NiO thermodynamic equilibrium condition was defined as a function of electrochemical potential and temperature. The results agree well with Attanasio et al.'s data by contact electrical resistance measurements. The anomalously high PWSCC growth rate consistently observed in the vicinity of Ni/NiO equilibrium is then attributed to weak thermodynamic stability of NiO. Redox-induced phase transition between Ni metal and NiO may undermine the integrity of NiO and enhance presumably the percolation of oxidizing environment through the oxide film, especially along grain boundaries. The redox-induced grain boundary oxide degradation mechanism has been postulated and will be tested by using the in-situ Raman facility.

• 한국안전학회지
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• 제31권3호
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• pp.16-21
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• 2016

Carbon Fiber Reinforced Plastic(CFRP) composite with a higher specific strength and rigidity is more excellent than conventional metallic materials or other organic polymer of FRP. It has been widely used in vehicles, aerospaces and high technology industries which are associated with nuclear power fields. However, CFRP laminated composite has several disadvantages as like a delamination, matrix brittleness and anisotropic fibers that are the weak points of the crack initiation. In this present work, the reinforced silicon carbide(SiC) particles were added to the interlayer of CFRP laminates in order to mitigate the physical vulnerability affecting the cracking and breaking of the matrix in the CFRP laminated composite because of excellent specific strength and thermal shock resistance characteristics of SiC. The 1wt% of SiC particles were spread into the CFRP prepreg by using a spray coating method. After that, CFRP prepregs were laminated for the specimen. Also, the twill woven type CFRP prepreg was used because it has excellent workability. Thus the mechanical and fracture behaviors of the twill woven CFRP laminated composite reinforced with SiC particles were investigated with the acoustic emission(AE) method under a fracture test. The results show that the SiC particles enhance the mechanical and fracture characteristics of the twill CFRP laminate composite.

• 터널과지하공간
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• 제26권6호
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• pp.493-507
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• 2016

본 연구에서는 축소모형실험을 통해 석회암 공동 상부에 존재하는 구조물 기초의 안정성을 검토하였다. 공동의 형상은 단축장축비율 1/3인 타원형으로 가정하고, 공동의 심도, 위치, 경사, 크기, 개수를 변화시킨 5가지 그룹, 12개 모형들을 실험하였다. 실험결과로서 모형별 균열개시압력, 최대압력, 변형거동, 파괴양상, 침하곡선을 구하였으며, 공동의 제반 조건들이 기초의 안정성에 어떠한 영향을 미치는지를 알아보았다. 무공동 모형은 전단파괴를 보였으나, 공동 포함 모형들은 관입파괴만 발생한 경우, 전단파괴와 관입파괴가 함께 발생한 경우, 전단파괴가 이중으로 발생한 경우 등의 다소 복잡한 파괴형식을 보였다. 공동의 심도가 작을수록, 크기가 클수록, 개수가 많을수록 기초의 안정성은 감소하였다. 공동의 일부가 기초저면의 직하부에 놓일 때는 부등침하가 관찰되었고, 공동들의 분포상태에 따라 침하곡선은 다른 형태를 보였다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2005년도 하계학술대회 논문집 Vol.6
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• pp.369-370
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• 2005

LTCC (Low Temperature Cofired Ceramic) technology is one of technologies which can realize SIP (System-In-a-Package). In this paper realization of gas sensor using LTCC technology was described. In the conventional gas sensor structure, wire bonding method is generally used as an interconnection method whereas in the LTCC sensor structure, via was used for the interconnection. As sensing materials, $SnO_2$ was adopted. The effect of frit glass portion on the adhesion of the sensing material to the LTCC substrate and the electrical conductivity of the sensing material were analyzed. AgPd, PdO, Pt was added to the sensing material as an additive for improving the gas sensitivity and electrical conductivity and the effect of the amount of additives in the sensing material on the electrical conductivity was investigated. The effect of the amount of frit glass in the termination on the sensor performance, especially mechanical integrity, was considered and the crack initiation and propagation in the boundary between the sensing material and the termination was studied.

• 한국재료학회지
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• 제22권10호
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• pp.513-518
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• 2012

Recently, automobile parts have been required to have high strength and toughness to allow for weight lightening or improved stability. But, traditional micro-alloyed steel cannot be applied in automobile parts. In this study, we considered the influence of quenching temperature and cooling rate for specimens fabricated by vacuum induction furnace. Directly quenched micro-alloyed steel for hot forging can be controlled according to its micro structure and the heat-treatment process. Low carbon steel, as well as alloying elements for improvement of strength and toughness, was used to obtain optimized conditions. After hot forging at $1,200^{\circ}C$, the ideal mechanical properties (tensile strength ${\geq}$ 1,000 MPa, Charpy impact value ${\geq}\;100\;J/cm^2$) can be achieved by using optimized conditions (quenching temperature : 925 to $1,050^{\circ}C$, cooling rate : ${\geq}\;5^{\circ}C/sec$). The difference of impact value according to cooling rate can be influenced by the microstructure. A fine lath martensite micro structure is formed at a cooling rate of over $5^{\circ}C/sec$. On the other hand, the second phase of the M-A constituent microstructure is the cause of crack initiation under the cooling rate of $5^{\circ}C/sec$.

• 한국재료학회지
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• 제24권5호
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• pp.266-270
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• 2014

Aluminum nitride(AlN) films were grown on the C-face and on the Si-face of (0001) silicon carbide(SiC) substrates using plasma-assisted molecular-beam epitaxy(PA-MBE). This study was focused on first-stage growth manipulation prior to the start of AlN growth. Al pre-exposure, N-plasma pre-exposure, and simultaneous exposure(Al and N-plasma) procedures were used in the investigation. In addition, substrate polarity and, first-stage growth manipulation strongly affected the growth and properties of the AlN films. Al pre-exposure on the C-face and on the Si-face of SiC substrates prior to initiation of the AlN growth resulted in the formation of hexagonal hillocks on the surface. However, crack formation was observed on the C-face of SiC substrates without Al pre-exposure. X-ray rocking-curve measurements revealed that the AlN epilayers grown on the Si-face of the SiC showed relatively lower tilt and twist mosaic than did the epilayers grown on the C-face of the SiC. The results from the investigations reported in this paper indicate that the growth conditions on the Si-face of the SiC without Al pre-exposure was highly preferred to obtain the overall high-quality AlN epilayers formed using PA-MBE.

• 수산해양기술연구
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• 제31권3호
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• pp.272-278
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• 1995

강도가 각각 다른 세 종류의 프리프레그를 사용하여 일방향 CFRP를 적층하였으며, 모드 I과 모드 II 실험을 통하여 층간 파괴인성치를 고찰하였고, 또한 적층 섬유방향을 변화시킨 사교적층판의 그것도 함께 고찰하였으며, 이를 요약하면 다음과 같다. 1) 임계에너지 방출률 G 하(IC)의 값을 컴플라이언스 법, 수정 컴플라이언스 법, 그리고 보이론에 의해 계산하여 비교 검토한 결과 본 연구에서 사용한 수정식에 의한 값들이 거의 일치하였다. 2) G 하(IC) 값은 대체로 프리프레그의 C, B, A재의 순으로 높게 나타났으며, G 하(II C)의 값을 세가지 식에 의해 계산하여 비교 검토한 결과 거의 일치하였다. 3) 사교적층판의 경우 G 하(IC) 값은 [0/90] 하(6s), [0/45] 하(6s), [0/45/90] 하(6s)의 순으로 높게 나타났으며, G 하(II C)는 [0/90] 하(6s), [0/45/90] 하(6s), [0/45] 하(6s)의 순으로 높게 나타났다. 4) 사교적층판과 일방향의 임계에너지 방출률을 비교하였을 때, 모드 I의 경우 일방향의 결과가 다소 높았으며, 모드 II의 경우는 [0/45] 하(6s)의 결과는 거의 일치함을 알 수 있었다.