• 대한토목학회논문집
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• 제28권4A호
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• pp.529-536
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• 2008

구조물의 파괴 거동은 하중의 재하 속도에 따라 달라지는 특성을 보이는데, 이는 재료의 속도 의존성으로부터 비롯된다고 할 수 있다. 이러한 현상은 공학의 여러 분야에서 관심사였지만, 파괴 메커니즘이 명확히 규명되지 않았기 때문에 수치 해석을 통한 연구에는 그 한계점과 어려움이 상존하였다. 본 연구에서는 파괴 거동의 속도 의존성을 이해하고자, 취성재료를 대상으로 입자동역학을 이용한 수치해석을 수행하였다. 직접 인장 시험 시뮬레이션을 위해 노치가 있는 시편을 모델링하고, 취성재료가 갖는 특성을 표현하기 위해 Lennard-Jones 포텐셜을 사용하였다. 6가지의 다른 하중 속도에 따른 균열의 거칠기, 균열의 후퇴와 멈춤, 분기 현상과 같은 동적 파괴 특성을 관찰하였다. 해석 결과를 통해 하중 속도에 따른 파괴 거동의 변화 원인을 에너지 유입-소모율의 개념을 도입하여 설명하고, 재료의 파괴 메커니즘이 갖는 속도 의존성에 대해 이해할 수 있는 단초를 마련하였다. 또한, 기존 실험과의 비교를 통해 실제적인 현상과의 유사성을 밝힘으로써 입자동역학의 공학적 적용 가능성을 제시하였다.

• 대한기계학회논문집A
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• 제22권3호
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• pp.540-544
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• 1998

Compliance calibration method is frequently used to determine $G_IC$ from the DCB composite specimen. However, the method requires at least 4 to 5 fracture test (loading-unloading) records. In this study, $G_IC$ of unidirectional graphite/epoxy DCB composites was determined from the elastic work factor approach which uses a single fracture test record. In order to inspect the validity of the elastic work factor approach, $G_IC$ determined from the elastic work factor approach was compared to that of determined from the compliance calibration method. It was shown that $G_IC$ determined from the elastic work factor approach was comparable to that determined from the compliance calibration method. That is, the elastic work factor approach can be used to determine $G_IC$ of unidirectional graphite/epoxy DCB specimen from a single fracture record.

• Composites Research
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• 제13권3호
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• pp.48-57
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• 2000

앞 동반논문의 이론에서 기술된 기재위에 입혀진 코팅크랙킹의 파괴역학 분석을 4점 굴곡시험을 이용하여 실증하였다. 파괴역학 접근에 의해서 코팅의 다중크랙킹을 예측하여 코팅층에서 새로운 크랙이 생길 때의 변위에너지 방출량(G)을 구하였다. 여러 건조시간과 건조온도의 변화에 따른 금속 및 고분자 기재위에 입혀진 코팅의 변위에 대한 코팅 크랙밀도의 실험데이타가 in-situ 코팅의 파괴인성 값을 구하기 위해 사용되었다. 건조온도가 올라가고 건조시간이 길어짐에 따라 $G_c$는 감소하였다. 본 논문은 코팅의 파괴인성 평가에 있어 4점 굴곡시험이 얼마나 유용한지를 보여주며 in-situ 코팅인성을 구하는 방법을 제시하였다.

• 한국지구물리탐사학회:학술대회논문집
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• 한국지구물리탐사학회 2007년도 공동학술대회 논문집
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• pp.291-296
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• 2007

We measured potential waveform of load, displacement, micro electric signal generated by rock and mortar fracture using PXI A/D Converter. The rock type used for measurement was used granite, limestone and sandstone, and mortar specimen. we made measuring equipment of physical properties to confirm basic information of physical properties, measured physical properties of rock engineering, electric resistivity and seismic velocity. Potential waveform system was built using PXI A/D Converter and measured potential waveform of load, displacement, micro-electric signal generated using this during uniaxial compressive test by the specimen finished such test of physical properties. Using the saturated rock and mortar specimen, micro electric signal increased, and It didn't increase a signal in dried rock and mortar specimen according as load and strain rate increases. But signal also increased in saturated or dried specimen in case of sandstone. It was possible to check the close correlation relationship the signal and fracture behavior by a compressive load as the signal of fracture position was increased bigger than the other position. It was also possible to check the correlation relationship between physical properties and micro geo-electric signal.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2000년도 추계학술발표대회 논문집
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• pp.102-105
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• 2000

Recently, applications of integrated large composite structures have been attempted to many structures of vehicles. To improve the cost performance and reliability of the integrated composite structures, it is necessary to judge structural integrity of the composite structures. For the judgement, we need fracture simulation techniques for composite structures. Many researches oil the fracture simulation method using FEM have been reported by now. Most of the researches carried out simulations considering only matrix cracking and fiber breaking as fracture modes, and did not consider delamination. Several papers have reported the delamination simulation, but all these reports require three-dimensional elements or quasi three- dimensional elements for FEM analysis. Among fracture mechanisms of composite laminates, delamination is the most important factor because it causes stiffness degradation in composite structures. It is known that onset and propagation of delamination are dominated by the strain energy release rate and interfacial moment. In this study, laminated composite has been described by using 3 dimensional finite elements. Then impact behavior of the laminated composite is simulated using FEM(ABAQUS/Explicit) with progressive failure mechanism. These results are compared with experimental results.

• 콘크리트학회논문집
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• 제13권6호
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• pp.568-574
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• 2001

콘크리트 시험편에 대한 삼점휨실험을 실시하여 여러 실험에 대해 측정된 반응의 평균값을 계산하곤 계산된 평균거동으로부터 삼점휨 시험편이 완전히 파손될 때까지 연속적으로 성장하는 균열에 대한 파괴거동이 분석되었다. 이 연구에서 사용된 실험변수는 25.4mm와 6.4mm의 초기균열길이 그리고 2000sec와 20sec의 실험기간이며, 초기균열길이는 파괴진행대(FPZ) 크기의 영향과 시험편의 기하학적 특성 및 경계조건에 대한 영향을 분석하기 위한 것이다. 실험기간은 초기 크리프(creep)의 영향을 위해 설정되었으나, 이 연구의 하중점-변위 속도에서는 심각한 영향이 관측되지 않았다. 여러 실험의 평균 외부일로부터 평균하중이 계산되었으며, 변형률 게이지를 사용하여 평균 균열길이를 측정하였다. 최대하중 이전의 저항곡선은 파괴진행대의 크기에 대해 유사한 값을 보였다. 그러나 최대하중 직후에는 초기균열의 크기에 관계없이 0.088~0.154mm의 하중점-변위에서 88mm의 불안정 균열성장이 발생되었으며, 평균 파괴에너지율 $G_{F}$$^{ave}$ = 115N/m은 이 불안정 균열성장 동안에 발생되었다. 균열길이 111mm에서 완전한 파괴진행대가 형성되었고, 25.4mm와 6.4mm의 초기균열길이에 대해 파괴진행대의 크기는 각각 86mm와 105mm이었다. 완전한 파괴진행대의 형성이후 저항곡선의 평균 파괴에너지율은 25.4mm와 6.4mm의 초기균열길이에 대해 각각 229N/m로 284N/m로 $G_{F}$$^{ave}$의 두 배 이상이었다.이었다.이었다.

• 산업기술연구
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• 제20권A호
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• pp.247-256
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• 2000

Precursor gels with the composition of $xZrO_2{\cdot}(100-x)SiO_2$ systems (x=10, 20 and 30 mol%) were prepared by the sol-gel method. Kinetic parameters, such as activation energy, Avrami's exponent, n, and dimensionality crystal growth value, m, have been simultaneously calculated from the DTA data using Kissinger and Matusita equations. The crystallite size dependence on tetragonal to monoclinic transformation of $ZrO_2$ was investigated using XRD, in relation to the fracture toughness. The crystallization of tetragonal $ZrO_2$ occurred through 3-dimensional diffusion controlled growth(n=m=2) and the activation energy for crystallization was calculated using Kissinger and Matusita equations, as about $310{\sim}325{\pm}10kJ/mol$. The growth of $t-ZrO_2$, in proportion to the cube of radius, increased with increasing heating temperature and heat-treatment time. It was suggested that the diffusion of Zr4+ions by Ostwald ripening was rate-limiting process for the growth of $t-ZrO_2$ crystallite size. The fracture toughness of $xZrO_2{\cdot}(100-x)SiO_2$ systems glass ceramics increased with increasing crystallite size of $t-ZrO_2$. The fracture toughness of $30ZrO_2{\cdot}70SiO_2$ system glass ceramics heated at $1,100^{\circ}C$ for 5 h was $4.84Mpam^{1/2}$ at a critical crystaliite size of 40 nm.

• Structural Engineering and Mechanics
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• 제30권2호
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• pp.177-190
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• 2008

Material failure behavior is generally dependent on loading rate. Especially in brittle and quasi-brittle materials, rate dependent material behavior can be significant. Empirical formulations are often used to predict the rate dependency, but such methods depend on extensive experimental works and are limited by practical constraints of physical testing. Numerical simulation can be an effective means for extracting knowledge about rate dependent behavior and for complementing the results obtained by testing. In this paper, the failure behavior of a brittle material under different loading rates is simulated by molecular dynamics analysis. A notched specimen is modeled by sub-million particles with a normalization scheme. Lennard-Jones potential is used to describe the interparticle force. Numerical simulations are performed with six different loading rates in a direct tensile test, where the loading velocity is normalized to the ratio of the pseudo-sonic speed. As a consequence, dynamic features are achieved from the numerical experiments. Remarkable failure characteristics, such as crack surface interaction/crack arrest, branching, and void nucleation, vary in case of the six loading cases. These characteristics are interpreted by the energy concept approach. This study provides insight into the change in dynamic failure mechanism under different loading rates.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 2002년도 proceedings of the second asia international conference on tribology
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• pp.203-204
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• 2002

The wear behaviour of steam generator (SG) tubes (Inconel 600 and 690) against support materials (405 and 409 ferritic stainless steels) has been experimentally studied in room temperature water using reciprocating wear apparatus with tube-an-plate configuration. The results showed that the wear rate of Inconel 690 was lower than that of lnconel 600 with increasing normal loads and sliding amplitudes. Also, plastic deformation layers appear below the surface of both SG tubes, which have a specific thickness and are small compared with their grain size. This means that wear rate of SG tubes in water condition is closely related to the formation and fracture of plastic deformation layers.

• Structural Engineering and Mechanics
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• 제23권2호
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• pp.163-175
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• 2006

In this paper, the dynamic response of a piezoelectric layer with a penny-shaped crack is investigated. The piezoelectric layer is subjected to an axisymmetrical action of both mechanical and electrical impacts. Two kinds of crack surface conditions, i.e., electrically impermeable and electrically permeable, are adopted. Based upon integral transform technique, the crack boundary value problem is reduced to a system of Fredholm integral equations in the Laplace transform domain. By making use of numerical Laplace inversion the time-dependent dynamic stress and electric displacement intensity factors are obtained, and the dynamic energy release rate is further derived. Numerical results are plotted to show the effects of both the piezoelectric layer thickness and the electrical impact loadings on the dynamic fracture behaviors of the crack tips.