• 한국건설순환자원학회논문집
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• 제7권3호
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• pp.279-286
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• 2019

본 연구에서는 기준압축강도 180MPa의 강섬유 보강 초고강도 콘크리트(UHSC)의 인장 특성에 관한 실험 연구를 수행하였다. 부피비 1%의 강섬유를 혼입하여 직접인장강도 시편과 3점 하중재하 휨 실험을 위한 프리즘 시편을 제작하였다. 제작된 시편은 균열 유도를 위하여 시편 중앙에 노치를 설정하였으며, 각 평가방법에 따라 실험을 수행하였다. 우선, 콘크리트의 균열 후 거동 특성을 파악하기 위하여 직접인장강도 실험을 수행하여 응력-변형률 곡선을 분석하였으며, 3점 하중재하 휨 실험을 통하여 하중-CMOD 곡선을 얻고, 역해석을 수행하여 응력-변형률 곡선을 분석하였다. 직접인장강도 실험과 3점 하중재하 휨 실험의 인장거동 특성은 유사하게 나타났다. 또한, SC 구조설계지침에서 제시하고 있는 인장응력-변형률 곡선 모델링을 수행하고, 측정값과 예측값의 비교분석을 수행하였다. 재료감소계수가 1.0일 때, 변형률이 0.02일 때까지 예측값은 측정값과 유사하게 나타나지만, 재료감소계수가 0.8일 때, 예측값은 측정값의 최소값에 근접한다. 또한, 변형률이 0.02를 초과하는 구간에서는 SC 구조설계지침에 의한 예측값이 측정값을 과소평가한다.

• Computers and Concrete
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• 제7권4호
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• pp.387-402
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• 2010

From typical stress-axial strain curve and stress-volume strain curve of a concrete under uniaxial compression, the initiation and localization of microcracks within the interior of the specimen can be identified. The occurrence of random microcrack indicates the end of the linear elasticity, and the localization of microcrack implies formation of major crack, which triggers the onset of unstable crack propagation. The interval between initiation and localization of microcracks is characterized by a stable microcrack growth. Based on fracture behavior observed from a uniaxial compressive test of a concrete cylinder, a model has been developed to extract fundamental fracture properties of a concrete, i.e. the equivalent fracture toughness and the size of fracture process zone. The introduction of cracking Poisson's ratio accounts for tensile failure characteristics of concrete even under uniaxal compression. To justify the validity of the model proposed, tests on three-point bending have been performed to obtain the fracture toughness in accordance with two parameter fracture model and double-K fracture model. Surprisingly, it yields favorably comparable results and provides an encouraging alternative approach to determine fracture properties for concretes.

• Computers and Concrete
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• 제3권1호
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• pp.17-28
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• 2006

Replacement of actual stress distribution in a reinforced concrete (RC) flexural member with a simpler geometrical shape, which maintains magnitude and location of the resultant compressive force, is an acceptable conceptual trick. This concept was originally perfected for normal strength concrete. In recent years, high strength concrete (HSC) has been introduced and widely used in modern construction. The stress block parameters require updating to account for special features of HSC in the design of flexural members. In future, more varieties of concrete may be developed and a corresponding design procedure of RC flexural members will be required. The usual practice is to conduct large number of experiments on various sizes of specimen and then evolve an empirical relation. This paper presents a numerical procedure through which the stress block parameters can be numerically derived for a given strain ratio variation. The material model for concrete is presented and computational procedure is described. This procedure is illustrated with several variations of strain ratio. The advantages of numerical procedure are that it costs less and it can be used with new material models for any new variety of concrete.

• Nuclear Engineering and Technology
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• 제45권2호
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• pp.223-236
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• 2013

In the present study the behavior of fire and the residual strength of fire-ignited RC slabs are investigated by experimental tests and numerical simulations. The fire tests of RC slabs were carried out in a furnace using the ISO 834 standard fire. The load capacity of the cooled RC slabs that were not loaded during the fire tests was evaluated by additional 3 point bending tests. The influence of the proportion of PP (polypropylene) fibers in the RC slabs on the structural behavior of the RC slabs after the fire loading was investigated. The results of the fire tests showed that the maximum temperature of concrete with PP fiber was lower than that of concrete without PP fiber. As the concrete was heated, the ultimate compressive strength decreased and the ultimate strain increased. The load-deflection relations of RC slabs after fire loading were compared by using existing stress-strain-temperature models. The comparison between the numerical analysis and the experimental tests showed that some numerical analyses were reliable and therefore, can be applied to evaluate the ultimate load of RC slabs after fire loading. The ultimate load capacity after cooling down the RC slabs without PP fiber showed a considerable reduction from that of the RC slabs with PP fiber.

• Structural Engineering and Mechanics
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• 제48권6호
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• pp.833-848
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• 2013

Nine rectangular-section of High Strength Concrete(HSC) beams were designed and casted based on the American Concrete Institute (ACI) code provisons with varying of tensile reinforcement ratio as (${\rho}_{min}$, $0.2_{{\rho}b}$, $0.3_{{\rho}b}$, $0.4_{{\rho}b}$, $0.5_{{\rho}b}$, $0.75_{{\rho}b}$, $0.85_{{\rho}b}$, $_{{\rho}b}$, $1.2_{{\rho}b}$). Steel and concrete strains and deflections were measured at different points of the beam's length for every incremental load up to failure. The ductility ratios were calculated and the moment-curvature and load-deflection curves were drawn. The results showed that the ductility ratio reduced to less than 2 when the tensile reinforcement ratio increased to $0.5_{{\rho}b}$. Comparison of the theoretical ductility coefficient from CSA94, NZS95 and ACI with the experimental ones shows that the three mentioned codes exhibit conservative values for low reinforced HSC beams. For over-reinforced HSC beams, only the CSA94 provision is more valid. ACI bending provision is 10 percent conservative for assessing of ultimate bending moment in low-reinforced HSC section while its results are valid for over-reinforced HSC sections. The ACI code provision is non-conservative for the modulus of rupture and needs to be reviewed.

• 한국구조물진단유지관리공학회 논문집
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• 제15권3호
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• pp.79-87
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• 2011

파형강판 구조물은 기존의 단지간 구조물에 비하여 더 큰 압축력과 활하중에 유연한 휨거동을 나타내며, 시공중에는 활하중에 의한 비대칭 토압분포가 발생하게 된다. 따라서 본 연구에서는 현재 국내에서 생산 및 시공이 이루어지고 있고 휨변형에 탁월한 대골형 파형강판의 볼트 이음부의 극한강도 및 모멘트강도 등을 검토하고자 하였다. 그리고 선행연구 결과를 바탕으로 볼트 이음부의 보강량 증가에 따른 강도 증가와 시험체의 파괴양상을 비교함으로서 대골형 파형강판 시험체의 성능을 평가하고자 하였다.

• Steel and Composite Structures
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• 제19권5호
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• pp.1299-1319
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• 2015

This paper presents a detailed parametric study, conducted using finite element tools to cover a range of several geometric and material parameters, on the behaviour of thin-walled partially encased composite (PEC) columns. The PEC columns studied herein are composed of thin-walled built-up H-shaped steel sections with concrete infill cast between the flanges. Transverse links are provided between the opposing flanges to improve resistance to local buckling. The parametric study is confined to eccentrically-loaded columns subjected to major axis bending only. The parameters that were varied include the overall column slenderness ratio (L/d), load eccentricity ratio (e/d), link spacing-to-depth ratio (s/d), flange plate slenderness ratio (b/t) and concrete compressive strength ($f_{cu}$). The overall column slenderness ratio was chosen to be the primary variable with values of 5, 10 and 15. Other parameters were varied within each case of L/d ratio. The effects of the selected parameters on the behaviour of PEC columns were studied with respect to the failure mode, peak axial load, axial load versus average axial strain response, axial load versus lateral displacement response, moment versus lateral displacement behaviour and the axial load-moment interaction diagram. The results of the parametric study are presented in the paper and the influences of each of the parameters investigated are discussed.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.470-470
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• 2013

Recently, active materials such as amorphous silicon (a-Si), poly crystalline silicon (poly-Si), transition metal oxide semiconductors (TMO), and organic semiconductors have been demonstrated for flexible electronics. In order to apply flexible devices on the polymer substrates, all layers should require the characteristic of flexibility as well as the low temperature process. Especially, pentacene thin film transistors (TFTs) have been investigated for probable use in low-cost, large-area, flexible electronic applications such as radio frequency identification (RFID) tags, smart cards, display backplane driver circuits, and sensors. Since pentacene TFTs were studied, their electrical characteristics with varying single variable such as strain, humidity, and temperature have been reported by various groups, which must preferentially be performed in the flexible electronics. For example, the channel mobility of pentacene organic TFTs mainly led to change in device performance under mechanical deformation. While some electrical characteristics like carrier mobility and concentration of organic TFTs were significantly changed at the different temperature. However, there is no study concerning multivariable. Devices actually worked in many different kinds of the environment such as thermal, light, mechanical bending, humidity and various gases. For commercialization, not fewer than two variables of mechanism analysis have to be investigated. Analyzing the phenomenon of shifted characteristics under the change of multivariable may be able to be the importance with developing improved dielectric and encapsulation layer materials. In this study, we have fabricated flexible pentacene TFTs on polymer substrates and observed electrical characteristics of pentacene TFTs exposed to tensile and compressive strains at the different values of temperature like room temperature (RT), 40, 50, $60^{\circ}C$. Effects of bending and heating on the device performance of pentacene TFT will be discussed in detail.

• 마이크로전자및패키징학회지
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• 제30권4호
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• pp.50-60
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• 2023

BGA(ball grid array)는 높은 집적도와 우수한 방열 성능을 갖고 있어 널리 이용되는 방식의 패키지이다. BGA에서 솔더볼은 패키지와 PCB를 전기적으로 연결하는 중요한 역할을 하므로, 다양한 기계적 하중 하에서 솔더볼의 비탄성 변형을 이해하는 것은 반도체 패키지의 강건설계에 필수적이다. 본 연구에서는 공정 중 PCB의 휨, die와 substrate 간의 열팽창 계수 차이 등으로 인해 소성변형이 발생한 솔더볼의 초기 형상이 비탄성 변형과 파단에 미치는 영향을 유한요소 해석으로 분석하였다. 시뮬레이션 결과, shear와 bending 하중에서 tilted, hourglass 형상 모두 파단이 발생한 반면, compression 하중이 작용하는 경우는 모두 파단이 발생하지 않았다. Shear와 bending 하중에 compression이 각각 결합될 경우, 응력삼축비가 0보다 작은 값으로 유지되어 파단이 억제되었다. 또한 변형에 취약한 요소의 Lagrangian-Green 변형률 텐서를 이용해 비교한 결과, 동일한 하중 조건이라도 솔더볼의 형상에 따라 변형의 양상에 유의미한 차이가 있음을 확인하였다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1998년도 가을 학술발표논문집(II)
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• pp.489-494
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• 1998

This paper describes the possibility to reuse concrete waste produced by demolition of reinforced concrete structures as aggregate for concrete from the viewpoint of strength. Concrete rubble obtained from the demolished buildings at Taejon were crushing machine to reuse as coarse aggregate. The strength properties, such as compressive strength, splitting tensile strength, bending strength and shear strength, of recycled and normal concrete were examined and compared experimentally when water cement ratio was varied. From the results of this study, it was thought that in case of non-washed aggregate concrete, strength properties of recycled coarse aggregate is similar to that of normal concrete, In W/C 55%~45%, stress-strain curve of recycled concrete shows more stable than that of normal concrete, while in W/C 35%, it shows brittle behavior.