• 터널과지하공간
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• 제24권6호
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• pp.440-448
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• 2014

수압파쇄 기술은 가스나 석유, 지열 등 에너지자원의 회수율을 향상시키기 위해 다양한 분야에서 응용되고 있으며 수압파쇄 메커니즘 규명 및 응용분야에 대한 연구는 꾸준히 진행되어 왔다. 본 연구에서는 효과적인 수압파쇄를 위하여 실제 현장과 유사한 상황을 구현할 수 있는 축소모형실험을 통해 유도홈의 개수에 따른 수압파쇄시 균열발생 압력을 분석하기 위하여 수압파쇄 시험 장치를 구축하여 수압파쇄 시험을 실시하였다. 또한 그 결과를 토대로 물성과 역학적인 특성을 취득하여 3차원 개별 요소 프로그램인 3DEC을 이용한 수치해석적 모델링 값과 비교 분석함으로써 신뢰성 있는 결과를 도출하고자 하였다. 그 결과 유도홈을 이용할 경우 효과적인 균열 발생이 가능할 것으로 사료된다.

• 한국도로학회논문집
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• 제11권2호
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• pp.229-238
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• 2009

본 연구에서는 콘크리트지하차도 시공 시 수화열에 의한 열응력 분포 특성을 분석하여 시공 재료와 시공 과정에 따른 균열 발생 여부를 분석하여 설계 시에 균열을 억제할 수 있는 재료 특성과 시공 단계를 제시하는 방법에 대하여 연구하였다. 이러한 분석을 위해 열전달 이론을 도입하여 지하차도의 3차원 유한요소해석 모델을 개발하여 구조해석을 수행하였다. 1회 타설하는 콘크리트 부재의 부피가 지나치게 크면 매스콘크리트가 되기 때문에 수화열에 의한 균열이 발생하기 쉬우며 이러한 균열을 억제할 수 있는 방법으로는 크게 시공 단계를 적절하게 배치하는 것과 또는 이러한 균열을 방지 할 수 있도록 재료 특성을 바꾸어 시공하는 것으로 구분할 수 있다. 따라서 본 연구에서는 이를 위해 콘크리트 재료 성질의 시간에 따른 변화 특성 시멘트 종류 및 첨가제 유무에 따른 수화열 발생 특성, 시공 단계에 따른 구조물의 크기, 외부 환경조건 등을 고려하여 분석을 수행하였다.

• 대한기계학회논문집A
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• 제26권11호
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• pp.2390-2398
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• 2002

For integrity analysis of nuclear reactor pressure vessel, including the Pressurized thermal shock analysis, the fast and accurate calculation of the stress intensity factor at the crack tip is needed. For this, a simple approximation scheme is developed and the resulting stress intensity factors for axial semi-elliptical cracks in cylindrical vessel under various loading conditions are compared with those of the finite element method and other approximation methods, such as Raju-Newman's equation and ASME Sec. Xl approach. For these, three-dimensional finite-element analyses are performed to obtain the stress intensity factors for various surface cracks with t/R = 0.1. The approximation methods, incorporated in VINTIN (Vessel INTegrity analysis-INner flaws), utilizes the influence coefficients to calculate the stress intensity factor at the crack tip. This method has been compared with other solution methods including 3-D finite clement analysis for internal pressure, cooldown, and pressurized thermal shock loading conditions. The approximation solutions are within $\pm$2.5% of the those of FEA using symmetric model of one-forth of a vessel under pressure loading, and 1-3% higher under pressurized thermal shock condition. The analysis results confirm that the VINTIN method provides sufficiently accurate stress intensity factor values for axial semi-elliptical flaws on the surface of the reactor pressure vessel.

• 대한기계학회논문집A
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• 제36권10호
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• pp.1241-1248
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• 2012

본 연구의 목적은 CW 레이저 조사에 의한 실리콘 웨이퍼의 손상을 평가하는 것이다. 먼저, 레이저 조사에 의한 온도 및 응력 변화를 3 차원 유한요소해석 모델을 이용하여 예측하였다. 해석 결과, 93 $W/cm^2$의 레이저 빔이 조사되었을 때, 실리콘 웨이퍼의 표면의 응력은 약 140 MPa 까지 증가하였으며 균열이 발생할 것으로 예측되었다. 레이저 강도가 더욱 증가하여 186 $W/cm^2$ 일 때에는 실리콘 웨이퍼의 표면의 온도는 $1432^{\circ}C$까지 증가하였으며 표면부가 용융될 것으로 예상되었다. 실험 결과, 102 $W/cm^2$ 의 레이저 빔이 실리콘 웨이퍼 표면에 조사되었을 때 표면부에 균열이 발생하였고, 레이저 빔의 강도가 더욱 증가하여 140 $W/cm^2$ 일때 표면부에서 용융이 발생하였다. 용융이 발생하는 레이저 빔의 강도는 유한요소해석 결과보다 낮은 값이었으며 이는 표면부에서 생성된 균열에 의해 레이저 빔의 다중반사와 다중흡수가 일어나 레이저 빔의 흡수량이 증가하였기 때문이다.

• Steel and Composite Structures
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• 제35권3호
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• pp.439-447
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• 2020

This paper presents two sets of full three-dimensional thermoelastic finite element analyses of superimposed thermo-mechanically loaded Spar Wingskin Joints made with laminated Graphite Fiber Reinforced Plastic composites. The study emphasizes the influence of residual thermal stresses and material anisotropy on the inter-laminar delamination behavior of the joint structure. The delamination has been pre-embedded at the most likely location, i.e., in resin layer between the top and next ply of the fiber reinforced plastic laminated wingskin and near the spar overlap end. Multi-Point Constraint finite elements have been made use of at the vicinity of the delamination fronts. This helps in simulating the growth of the embedded delamination at both ends. The inter-laminar thermoelastic peel and shear stresses responsible for causing delamination damage due to a combined thermal and a static loading have been evaluated. Strain energy release rate components corresponding to the Mode I (opening), Mode II (sliding) and Mode III (tearing) of delamination are determined using the principle of Virtual Crack Closure Technique. These are seen to be different and non-self-similar at the two fronts of the embedded delamination. Residual stresses developed due to the thermoelastic anisotropy of the laminae are found to strongly influence the delamination onset and propagation characteristics, which have been reflected by the asymmetries in the nature of energy release rate plots and their significant variation along the delamination front.

• 대한기계학회논문집A
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• 제31권2호
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• pp.277-284
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• 2007

The steam generator in a nuclear power plant is a large heat exchanger that uses heat from a reactor to generate steam to drive the turbine generator. Rupture of a steam generator tube can result in release of fission products to environment outside. Therefore, an accurate integrity assessment of the steam generator tubes with cracks is of great importance for maintaining the safety of a nuclear power plant. The steam generator tubes are supported at regular intervals by support plates and rotations of the tubes are restrained. Although it has been reported that the limit load for a circumferential crack is significantly affected by boundary condition of the tube, existing limit load solutions do not consider the restraining effect of support plate correctly. In addition, there are no limit load solutions for circumferential cracks in U-bend region with the effect of the support plate. This paper provides detailed limit load solutions for circumferential cracks in top of tube sheet and the U-bend regions of the steam generator tube with the actual boundary conditions to simulate the restraining effect of the support plate. Such solutions are developed based on three dimensional finite element analyses. The resulting limit load solutions are given in a polynomial form, and thus can be simply used in practical integrity assessment of the steam generator tubes.

• Nuclear Engineering and Technology
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• 제51권1호
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• pp.238-248
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• 2019

In the high-temperature and high-pressure irradiation environments, the multi-field coupling processes of hydrogen diffusion, hydride precipitation and mechanical deformation in Zircaloy cladding tubes occur. To simulate this hydrogen-induced complex behavior, a multi-field coupling method is developed, with the irradiation hardening effects and hydride-precipitation-induced expansion and hardening effects involved in the mechanical constitutive relation. The out-pile tests for a cracked cladding tube after irradiation are simulated, and the numerical results of the multi-fields at different temperatures are obtained and analyzed. The results indicate that: (1) the hydrostatic stress gradient is the fundamental factor to activate the hydrogen-induced multi-field coupling behavior excluding the temperature gradient; (2) in the local crack-tip region, hydrides will precipitate faster at the considered higher temperatures, which can be fundamentally attributed to the sensitivity of TSSP and hydrogen diffusion coefficient to temperature. The mechanism is partly explained for the enlarged velocity values of delayed hydride cracking (DHC) at high temperatures before crack arrest. This work lays a foundation for the future research on DHC.

• International Journal of Concrete Structures and Materials
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• 제9권1호
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• pp.89-118
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• 2015

The geometric nonlinearity of off-diagonal bracing system (ODBS) could be a complementary system to covering and extending the nonlinearity of reinforced concrete material. Finite element modeling is performed for flexural frame, x-braced frame and the ODBS braced frame system at the initial phase. Then the different models are investigated along various analyses. According to the experimental results of flexural and x-braced frame, the verification is done. Analytical assessments are performed in according to three dimensional finite element modeling. Nonlinear static analysis is considered to obtain performance level and seismic behaviour, and then the response modification factors calculated from each model's pushover curve. In the next phase, the evaluation of cracks observed in the finite element models, especially for RC members of all three systems is performed. The finite element assessment is performed on engendered cracks in ODBS braced frame for various time steps. The nonlinear dynamic time history analysis accomplished in different stories models for three records of Elcentro, Naghan and Tabas earthquake accelerograms. Dynamic analysis is performed after scaling accelerogram on each type of flexural frame, x-braced frame and ODBS braced frame one by one. The base-point on RC frame is considered to investigate proportional displacement under each record. Hysteresis curves are assessed along continuing this study. The equivalent viscous damping for ODBS system is estimated in according to references. Results in each section show the ODBS system has an acceptable seismic behaviour and their conclusions have been converged when the ODBS system is utilized in reinforced concrete frame.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2000년도 봄 학술발표회논문집
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• pp.389-396
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• 2000

A three-dimensional finite element dynamic analysis was conducted to evaluate the effects of reducing cross beams from a simply supported straight P.S.C girder bridge. Two analyses were performed on the P.S.C girder bridge; one with 7 cross beams which is commonly used as current standard, and the other with 3 cross beams. A frequency analysis was conducted first in order to establish the dynamic characteristics of the bridge and determine an appropriate time step to use in the time history analyses. To assess the function and effectiveness of the cross beams, time history analysis was conducted for aforementioned two analysis cases. In the analysis, the complete model was subjected to a loading condition corresponding to the one passing truck loading. Several results of deflection, bending moment and shear forces were compared for two cases. From the analysis results, reduction of cross beams was found to have only a minimum effect on the response of the bridge. The maximum deck slab bending moment was found to decrease. This decrease should result in smaller flexural crack widths in the deck slab, which may lead to an improved deck performance.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2001년도 추계학술발표대회 논문집
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• pp.87-90
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• 2001

In particle or short-fiber reinforced composites, cracking of the reinforcements is a significant damage mode because the broken reinforcements lose load carrying capacity. This paper deals with elastic stress distributions and load carrying capacity of intact and cracked ellipsoidal inhomogeneities. Three dimensional finite element analysis has been carried out on intact and broken ellipsoidal inhomogeneities in an infinite body under pure shear. For the intact inhomogeneity, as well known as Eshelby(1957) solution, the stress distribution is uniform in the inhomogeneity and non-uniform in the surrounding matrix. On the other hand, for the broken inhomogeneity, the stress in the region near crack surface is considerably released and the stress distribution becomes more complex. The average stress in the inhomogeneity represents its load carrying capacity, and the difference of average stresses between the intact and broken inhomogeneities indicates the loss of load carrying capacity due to cracking damage. The load carrying capacity of the broken inhomogeneity is expressed in terms of the average stress of the intact inhomogeneity and some coefficients. It is found that the broken inhomogeneity with higher aspect ratio still maintains higher load carrying capacity.