• 한국지반환경공학회 논문집
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• 제13권5호
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• pp.13-24
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• 2012

국토의 효율적 이용과 기간시설 수용을 위한 대단위 부지 확충 및 대규모 항만배후부지 개발이 계획 중에 있으며, 이에 따라 해안지역의 준설매립은 지속적으로 이루어지고 있다. 슬러리 상태의 초연약 상태인 준설매립 지반은 오랜 시간에 걸쳐 침강 및 자중압밀 과정을 거치게 되며 투기한 준설토를 압밀 촉진시켜 침하시킬 수 있다면 기존 처분지의 수명을 연장시킬 수 있으므로 경제적으로 유용하리라 판단된다. 재하중 없이 공기를 단축시키고 매립 물량을 증대시키기 위하여 small column test와 medium column을 보완, 개발한 medium column test를 실시하여 PBD와 침투압이 동시에 작용하였을 PBD의 효과 및 침투압의 침강 및 자중압밀 촉진 효과를 검토하였다. 연구 결과, 기존의 자중압밀 공법에 비하여 침투압과 PBD를 적용한 지반은 배수재의 배수거리 단축에 따른 투수 상승 효과와 침투압에 의한 침투 압밀의 효과가 활발히 이루어지는 것으로 나타났다. 이를 통해 매립 직후나 매립 도중에 연직드레인을 직접 타설하여 침강 및 자중압밀 과정을 단축시키면 전체 매립공사 공기를 단축시킬 수 있으며 압밀촉진에 따라 확보된 여유고 만큼 준설토 투기가 가능해져 투기 준설토량을 경제적으로 증대시킬 수 있을 것으로 기대된다.

• Structural Engineering and Mechanics
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• 제53권5호
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• pp.997-1016
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• 2015

The contribution of tensioned concrete between cracks (tension-stiffening) cannot be ignored when analysing deformation of reinforced concrete elements. The tension-stiffening effect is crucial when it comes to adequately estimating the load-deformation response of steel reinforced concrete and the more recently appeared fibre reinforced polymer (FRP) reinforced concrete. This paper presents a unified methodology for numerical modelling of the tension-stiffening effect in steel as well as FRP reinforced flexural members using the concept of equivalent deformation modulus and the smeared crack approach to obtain a modified stress-strain relation of the reinforcement. A closed-form solution for the equivalent secant modulus of deformation of the tensioned reinforcement is proposed for rectangular sections taking the Eurocode 2 curvature prediction technique as the reference. Using equations based on general principles of structural mechanics, the main influencing parameters are obtained. It is found that the ratio between the equivalent stiffness and the initial stiffness basically depends on the product of the modular ratio and reinforcement ratio ($n{\rho}$), the effective-to-total depth ratio (d/h), and the level of loading. The proposed methodology is adequate for numerical modelling of tension-stiffening for different FRP and steel reinforcement, under both service and ultimate conditions. Comparison of the predicted and experimental data obtained by the authors indicates that the proposed methodology is capable to adequately model the tension-stiffening effect in beams reinforced with FRP or steel bars within wide range of loading.

• Structural Engineering and Mechanics
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• 제68권3호
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• pp.299-312
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• 2018

In this paper, an extended finite element method is proposed to analyze both geometric and material non-linear behavior of general Functionally Graded Material (FGM) plate-shell type structures. A user defined subroutine (UMAT) is developed and implemented in Abaqus/Standard to study the elastoplastic behavior of the ceramic particle-reinforced metal-matrix FGM plates-shells. The standard quadrilateral 4-nodes shell element with three rotational and three translational degrees of freedom per node, S4, is extended in the present study, to deal with elasto-plastic analysis of geometrically non-linear FGM plate-shell structures. The elastoplastic material properties are assumed to vary smoothly through the thickness of the plate-shell type structures. The nonlinear approach is based on Mori-Tanaka model to underline micromechanics and locally determine the effective FGM properties and self-consistent method of Suquet for the homogenization of the stress-field. The elasto-plastic behavior of the ceramic/metal FGM is assumed to follow Ludwik hardening law. An incremental formulation of the elasto-plastic constitutive relation is developed to predict the tangent operator. In order to to highlight the effectiveness and the accuracy of the present finite element procedure, numerical examples of geometrically non-linear elastoplastic functionally graded plates and shells are presented. The effects of the geometrical parameters and the volume fraction index on nonlinear responses are performed.

• 콘크리트학회논문집
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• 제16권2호
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• pp.155-162
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• 2004

초기재령 콘크리트의 탄성계수 변화와 크리프 및 건조수축 현상을 고려하는 강 합성 거더의 시간종속적 처짐해석을 수행하였다. 초기재령 콘크리트의 탄성계수 발현과정을 고려한 구성관계는 총 응력-변형률 관계를 Taylor의 선형급수 확장을 이용하여 기준시간에 관하여 확장함으로써 시간종속적 증분형태로 유도하였다. 강 박스거더의 단면형상 변화 위치와 지점부를 기준하여 거더를 분할하고 분할된 구간에서 단면해석을 통해 곡률을 구하여 2차 다항식으로 가정한 처짐곡선에 경계조건을 적용함으로써 처짐곡선의 증분관계식을 유도하였다. 부모멘트 구간의 강 박스 하단에 콘크리트를 타설한 이중합성 박스거더의 초기재령 거동해석을 수행하였으며, 강 박스 하단의 콘크리트 타설두께가 거더의 거동에 주는 영향을 수치해석 결과를 통해 분석하였다. 끝으로, 보 요소를 이용한 유한요소해석 결과와 개발된 단면해석법을 이용한 해석 결과를 비교함으로써 정확성을 검증하였다.

• 대한기계학회논문집A
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• 제30권6호
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• pp.670-677
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• 2006

Vacuum interrupters are used in various switch-gear components such as circuit breakers, distribution switches, contactors. The electrodes of a vacuum interrupter are manufactured of sinter-forged Cu-Cr material for good electrical and mechanical characteristics. Since the closing velocity is 1-2m/s and impact deformation of the electrode depends on the strain-rate at the given velocity, the dynamic material property of the sinter-forged Cu-Cr alloy is important to design the vacuum interrupter reliably and to identify the impact characteristics of a vacuum interrupter accurately. This paper is concerned with the dynamic material properties of sinter-forged Cu-Cr alloy for various strain rates. The amount of chrome is varied from 10 wt% to 30 wt% in order to investigate the influence of the chrome content on the dynamic material property. The high speed tensile test machine is utilized in order to identify the dynamic property of the Cu-Cr alloy at the intermediate strain-rate and the split Hopkinson pressure bar is used at the high strain-rate. Experimental results from both the quasi-static and the high strain-rate up to the 5000/sec are interpolated with respect to the amount of chrome in order to construct the Johnson-Cook and the modified Johnson-Cook model as the constitutive relation that should be applied to numerical simulation of the impact behavior of electrodes.

• 전기전자학회논문지
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• 제7권1호
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• pp.32-42
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• 2003

본 논문에서는 이방성 매질의 덮개층과 기판을 갖는 사각 마이크로스트립 패치 안테나의 방사패턴을 full-wave approach와 모멘트 계산법을 이용하여 연구하였다. 이방성 매질내의 구성관계식을 통하여 제안된 구조에서의 파수영역 다이애딕 그린 함수를 유도하였으며 이 결과로부터 전계 적분 방정식을 수식화하였다. 갤러킨 모멘트법을 사용하여 전계 적분 방정식을 이산화 하여 전류밀도의 계수를 구한 후에 구좌표계의 원거리 전계는 고정 위상법(stationary phase method)을 이용하여 얻을 수 있다. 전사모의 실험 결과의 타당성을 검증하기 위해서 기존의 결과와 비교하여 일치된 결과를 얻었으며 이방성 덮개층의 두께 변화 그리고 기판과 덮개층의 이방성 비에 따른 방사패턴이 제시되고 분석되었다.

• 터널과지하공간
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• 제11권3호
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• pp.225-236
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• 2001

암반 절리면의 거칠기는 전단강도 및 변형거동에 가장 중요한 영향을 미치는 요소 중의 하나이다. 절리면은 전단변형이 발생하는 동안 손상을 받게 되고 이에 따라 거칠기각은 연속적으로 낮아진다. 절리면의 수직팽창성과 강도 경화 및 연화현상도 거칠기의 변화와 관련이 있는 것으로 이해되고 있다. 따라서 절리면의 거칠기의 변화를 효율적으로 반영시킬 수 있는 수치해석 모델의 개발이 중요하다. 이 연구에서는 탄소성이론을 바탕으로 거칠기 변화를 고려할 수 있는 탄소성 응력-변형 증분식을 유도하여 절리면 거질기 변화가 절리면 전단거동에 미치는 영향을 정량적으로 평가하는데 이용하였다. 유도된 탄소성 증분식을 검증하기 위하여 수치 주기전단시험을 실시하였다. 수치시험결과 개발된 탄소성 증분식은 거친 절리면의 주기전단시험에 나타나는 일반적인 현상들을 모사하는데 효과적임을 알 수 있었다.

• Journal of Chest Surgery
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• 제44권2호
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• pp.89-98
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• 2011

Background: Although considerable efforts have been made to improve the graft patency in coronary artery bypass surgery, the role of biomechanical factors remains underrecognized. The aim of this study is to investigate the influences of geometric configurations of the bypass graft on hemodynamic characteristics in relation to anastomosis. Materials and Methods: The Numerical analysis focuses on understanding the flow patterns for different values of inlet and distal diameters and graft angles. The Blood flow field is treated as a two-dimensional incompressible laminar flow. A finite volume method is adopted for discretization of the governing equations. The Carreau model is employed as a constitutive equation for blood. In an attempt to obtain the optimal aorto-coronary bypass conditions, the blood flow characteristics are analyzed using in vitro models of the end-to-side anastomotic angles of $45^{\circ}$, $60^{\circ}$ and $90^{\circ}$. To find the optimal graft configurations, the mass flow rates at the outlets of the four models are compared quantitatively. Results: This study finds that Model 3, whose bypass diameter is the same as the inlet diameter of the stenosed coronary artery, delivers the largest amount of blood and the least pressure drop along the arteries. Conclusion: Biomechanical factors are speculated to contribute to the graft patency in coronary artery bypass grafting.

• Smart Structures and Systems
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• 제21권3호
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• pp.279-286
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• 2018

The free longitudinal vibration of a circular truncated nanocone is investigated based on the nonlocal elasticity theory. Exact analytical formulations for tapered nanostructures are derived and the nonlinear differential governing equation of motion is developed. The nonlocal small scale effect unavailable in classical continuum theory is addressed to reveal the long-range interaction of atoms implicated in nonlocal constitutive relation. Unlike most previous studies applying the truncation method to the infinite higher-order differential equation, this paper aims to consider all higher-order terms to show the overall nonlocality. The explicit solution of nonlocal stress for longitudinal deformation is determined and it is an infinite series incorporating the classical stress derived in classical mechanics of materials and the infinite higher-order derivative of longitudinal displacement. Subsequently, the first three modes natural frequencies are calculated numerically and the significant effects of nonlocal small scale and vertex angle on natural frequencies are examined. The coupling phenomenon of natural frequency is observed and it is induced by the combined effects of nonlocal small scale and vertex angle. The critical value of nonlocal small scale is defined, and after that a new proposal for determining the range of nonlocal small scale is put forward since the principle of choosing the nonlocal small scale is still unclear at present. Additionally, two different types of nonlocal effects, namely the nonlocal stiffness weakening and strengthening, reversed phenomena existing in nanostructures are observed and verified. Hence the opposite nonlocal effects are resolved again clearly. The nano-engineers dealing with a circular truncated nanocone-based sensors and oscillators may benefit from the present work.

• The Korean Journal of Physiology and Pharmacology
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• 제2권3호
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• pp.331-343
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• 1998

Sublethal dose of bacterial lipopolysaccharide (LPS) would induce protection against cardiac ischemic/reperfusion (I/R) injury. This study examines the following areas: 1) the temporal induction of the cardio-protection produced by LPS; and 2) the relations between a degree of protection and the myocardial prostacyclin ($PGI_2$) production. Rats were administered LPS (2 mg/kg, i.v.), and hearts were removed 1, 4, 8, 14, 24, 48, 72,and 96 h later. Using Langendorff apparatus, haemodynamic differences during 25 min of global ischemia/30 min reperfusion were investigated. The concentration of $PGI_2$ in aliquots of the coronary effluent was determined by radioimmunoassay as its stable hydrolysis product $6-keto-PGF1_{\alpha}$ and lactate dehydrogenase release were measured as an indicative of cellular injury. LPS-induced cardiac protection against I/R injury appeared 4 h after LPS treatment and remained until 96 h after treatment. $PGI_2$ release increased 2-3 fold at the beginning of reperfusion compared to basal level except in hearts treated with LPS for 48 and 72 h. In hearts removed 48 and 72 h after LPS treatment, basal $PGI_2$ was increased. To determine the enzymatic step in relation to LPS-induced basal $PGI_2$ production, we examined prostaglandin H synthase (PGHS) protein expression, a rate limiting enzyme of prostaglandin production, by using Western blot analysis. LPS increased PGHS protein expression in hearts at 24, 48, 72, 96 h after LPS treatment. Induction of PGHS expression appeared in both isotypes of PGHS, a constitutive PGHS-1 and an inducible PGHS-2. To identify the correlationship between $PGI_2$ production and the cardioprotective effect against I/R injury, indomethacin was administered in vivo or in vitro. Indomethacin did not inhibit LPS-induced cardioprotection, which was not affected by the duration of LPS treatment. Taken together, our results suggest that $PGI_2$ might not be the major endogenous mediator of LPS-induced cardioprotection.