• 대한기계학회논문집A
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• 제24권3호
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• pp.676-684
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• 2000

In this study, behavior of $C_t$ which is a well-known fracture parameter characterizing creep crack growth rate, is investigated for weld interface cracks. Finite element analyses were per formed for a C(T) specimen under constant loading condition for elastic-plastic-creeping materials. In modeling C(T) geometry, an interface was employed along the crack plane which simulated the interface between weld and base metals. The $C_t$ versus time relations were obtained under various creep constant combinations and plastic constant combinations for weld and base metals, respectively. A unified $C_t$ versus time curve is obtained by normalizing $C_t$ with $C^*$ and t with $t_T$ for all the cases of material constant variations.

• International Journal of Aeronautical and Space Sciences
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• 제2권2호
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• pp.65-72
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• 2001

This paper demonstrates an explicit-implicit, finite element analysis for linear as well as nonlinear hygrothermal stress problems. Additional features, such as moisture diffusion equation, crack element and virtual crack extension(VCE) method for evaluating J-integral are implemented in this program. The Linear Elastic Fracture Mechanics(LEFM) Theory is employed to estimate the crack driving force under the transient condition for an existing crack. Pores in materials are assumed to be saturated with moisture in the liquid form at the room temperature, which may vaporize as the temperature increases. The vaporization effects on the crack driving force are also studied. The ideal gas equation is employed to estimate the thermodynamic pressure due to vaporization at each time step after solving basic nodal values. A set of field equations governing the time dependent response of porous media are derived from balance laws based on the mixture theory. Darcy's law is assumed for the fluid flow through the porous media. Perzyna's viscoplastic model incorporating the Von-Mises yield criterion are implemented. The Green-Naghdi stress rate is used for the invariant of stress tensor under superposed rigid body motion. Isotropic elements are used for the spatial discretization and an iterative scheme based on the full Newton-Raphson method is used for solving the nonlinear governing equations.

• 터널과지하공간
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• 제16권5호
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• pp.413-421
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• 2006

발파에 의한 암반의 동적 파괴 과정을 설명하기 위해서는 발생한 폭굉압과 가스압의 작용을 동시에 연구해야 한다. 발파 과정에서 폭굉압과 가스압의 발파공 벽면에의 작용을 동시에 모델화하여 이에 따른 암반과의 상호 작용을 수치해석하는 연구에 앞서, 본 연구에서는 단일 발파공에서 생성된 단일 균열망에서의 가스 유동에 미치는 가스압 이력, 균열 길이 그리고 가스압을 산정하기 위해 적용한 상태 방정식의 영향에 대해 분석 하였다. 이를 위하여 단일공 발파에 의해 길이 0.01 m, 간극 0.001 m으로 동일한 5개의 균열로 구성된 단일 균열망이 생성되었다고 가정하였다. 또한 지름이 45 mm인 발파공에 지름이 36 mm인 PEIN을 장약하였다고 가정하여 수치해석을 수행하였다. 그 결과, 균열망을 구성하는 개별 균열에 작용하는 최대 가스압력과 그 도달시간은 사용 폭약의 특성과 균열망의 기하학적 특성에 의해 결정되는 것으로 나타났다.

• 터널과지하공간
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• 제6권3호
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• pp.209-222
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• 1996

Shut-in pressure, reopenting pressure and fracture orientation are very important parameters to be evaluated precisely in in-situ stress measurement by hydraulic fracturing. Graphical methods on pressure-time curves have been conventionally used, even though these are seriously dependent on subjectivity of interpreters. So there have been many demands on new method to objectivity in determining parameters. We have developed integrated hydrofracturing data processing program (HYDFRAC), based on nonlinear regression analysis and can be invoked under the Window graphical user interface. HYDFRAC consiste of three routines, that is shut-in pressure routine, reopening pressure routine, and fracture delineation routine. Each of routines include independent modules according to parameter determination methods. Its application to field tests ensured both objectivity and facility in determining of hydraulic fracturing parameters. Determining shut-in pressures at each pressurization cycles, we adopted the exponential pressure-decay method(EPD method), the bilinear pressure-decay-rate method (PDR method), and the tangent intersection method in order to find the pressurization-cyclic tendency of shut-in pressures. The estimated pressure by PDR method exists in the range of the upper and lower values by EPD method, and lies near to the upper value more than the lower. Being the pressurization cycle increased, the range of upper and lower limits come to be stabilized gradually. By graphical superposition method and bilinear pressure-accumulated volume method, reopening pressures were determined. Vertical and inclined fracture attitudes were determined by applying the directional statistics and sinusoidal curve fitting, respectively. The results of evaluation of hydrofracturing parameters showed that statistical methods could enhance the objectivity better than graphical methods.

• 전산구조공학
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• 제10권4호
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• pp.229-238
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• 1997

본 논문에서는 반복하중에 의해 손상을 입은 아스팔트 콘크리트의 점탄성 구성모델에 대한 역학적 접근방법을 제시하였다. 모의변수로 나타낸 탄성-점탄성 일치원리는 아스팔트 콘크리트의 점탄성과 시간의존 손상의 증가를 별도로 평가하도록 적용되었다. 선형-점탄성 파괴역학에 사용되고 있는 미소균열의 증가법칙이 물체내 손상증가를 나타내는데 성공적으로 사용되었다. 응력과 모의변형도로 나타나는 구성방정식은 먼저 변형도조절에 대해 세워졌으며, 응력과 모의변형도를 모의응력과 변형도로 간단하게 대체함으로써 응력조절 구성방정식으로 변형되었다. 모의응력으로 나타낸 변형된 구성방정식은 응력조절모드에서 파괴에 이르는 아스팔트 콘크리트의 모든 역학적 거동을 충분히 예측하고 있다.

• 대한기계학회논문집
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• 제19권9호
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• pp.2133-2141
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• 1995

The thermo-mechanical fatigue tests were performed on the specimens extracted from 1.5Cr-0. 67Mo-0.33V alloy. The characteristics of thermo-mechanical fatigue crack propagation were examined and reviewed in view of fracture mechanics. The results obtained from the present study are summarized as follows : (1) The propagation characteristics of isothermal low-cycle fatigue crack are dominated by .DELTA.J$_{f}$ in case of PP waveform, and .DELTA.J$_{c}$ in case of CP waveform. (II)The propagation characteristics of thermo-mechanical fatigue crack are dominated by .DELTA.J$_{c}$ for in-phase case, and by .DELTA.J$_{c}$ for out-of-phase. The present results were in good agreement with the equation of propagation law for isothermal low-cycle fatigue crack in case of thermo-mechanical fatigue.tigue.e.

• 대한토목학회논문집
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• 제14권2호
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• pp.269-279
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• 1994

본 논문은 콘크리트 구조물의 재료 비선형과 시간의존적 거동을 고려한 유한요소 해석에 관한 것으로 회전균열모델에 토대를 둔 면내균열모델을 사용하여 하중에 따른 균열의 영향을 모사하였으며 콘크리트는 직교이방성 거동을 한다고 가정하였다. 특히 휨거동 시의 인장보강효과(tension stiffening effect)를 고려하기 위해 파괴에너지 개념에 토대를 둔 한계식을 제안하였고 이를 통해 수치해석시 나타나는 유한요소의 크기에 따른 수치해석 오차의 최소화를 도모하였다. 또한 embedded model을 사용하여 철근의 거동을 모사할 경우 콘크리트의 변위장에 따라 효율적으로 대처할 수 있도록 구성방정식을 체계화 하였으며 비균열단면, 균열단면 등 여러 상황에서 구조물의 시간에 따른 거동을 살펴보기 위해 평면응력 상태에서 재령보정 탄성계수법에 따른 구성방정식을 유도하고 이를 토대로 예제해석을 수행하였다.

• 대한기계학회논문집A
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• 제26권1호
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• pp.171-178
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• 2002

An effect is made in this study to deepen understanding of small punch(SP) creep testing which has been a round for about 10 years as a substitute for the conventional uniaxial creep testing. Even though considerable numbers of SP creep test program have been performed, most of the tests were aimed at measuring creep rupture lives only. Very flew studies showed interest on the meaning of what we were really measuring during the SP creep tests. In this paper meanings of the parameters measured during the SP creep testing, such as punch load and punch displacement rate are investigated using finite element analysis. It was shown that the measured parameters must represent the stress and strain rates of the material at the annular region located at about 0.65 mm from the center of the SP specimen. The material in this location would go through constant maximum stress and strain rate during the testing. Experimental verification is also discussed.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.511-516
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• 2001

Recently small punch creep testing (or miniature disc bend creep test) has received much attention through European collaborative research projects. This method was considered as a substitute for the conventional creep rupture testing by which the residual creep life is measured from the specimen taken out from serviced components of high temperature plants. It would be beneficial if the material creep properties such as power law creep constants as well as the creep rupture life can be measured from the small punch creep test. In this paper a method of assessing creep constants from the small punch creep testing is proposed. Finite element analyses were performed to investigate evolution of stress and strain rate at the weakest locations of the small punch creep specimen. Elastic-plastic-secondary creep analyses were carried out. The estimation equations for creep constants by the small punch creep testing are proposed based on the finite analysis results. Small punch creep tests were also performed with 9Cr steel and the accuracy of the proposed equation was verified by the experimental results.

• 대한기계학회논문집A
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• 제25권9호
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• pp.1493-1500
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• 2001

Recently small punch creep testing (or miniature disc bend creep test) has received much attention through European collaborative research projects. This method was considered as a substitute for the conventional creep rupture testing by which the residual creep life is measured from the specimen taken out from serviced components of high temperature plants. It would be beneficial if the material creep properties such as power law creep constants as well as the creep rupture life can be measured from the small punch creep test. In this paper a method of assessing creep constants from the small punch creep testing is proposed. Finite element analyses were performed to investigate evolution of stress and strain rate at the weakest locations of the small punch creep specimen. Elastic-plastic-secondary creep analyses were carried out. The estimation equations for creep constants by the small punch creep testing are proposed based on the finite analysis results. Small punch creep tests were also performed with 9Cr steel and the accuracy of the proposed equation was verified by the experimental results.