• Coupled systems mechanics
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• 제7권2호
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• pp.197-209
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• 2018

Fiber-reinforced concrete (FRC) is a material with increasing application in civil engineering. Here it is assumed that the material consists of a great number of rather small fibers embedded into the concrete matrix. It would be advantageous to predict the mechanical properties of FRC using nondestructive testing; unfortunately, many testing methods for concrete are not applicable to FRC. In addition, design methods for FRC are either inaccurate or complicated. In three-point bending tests of FRC prisms, it has been observed that fiber reinforcement does not break but simply pulls out during specimen failure. Following that observation, this work is based on an assumption that the main components of a simple and rather accurate FRC model are mechanical properties of the concrete matrix and fiber pullout force. Properties of the concrete matrix could be determined from measurements on samples taken during concrete production, and fiber pullout force could be measured on samples with individual fibers embedded into concrete. However, there is no clear relationship between measurements on individual samples of concrete matrix with a single fiber and properties of the produced FRC. This work presents an inverse model for FRC that establishes a relation between parameters measured on individual material samples and properties of a structure made of the composite material. However, a deterministic relationship is clearly not possible since only a single beam specimen of 60 cm could easily contain over 100000 fibers. Our inverse model assumes that the probability density function of individual fiber properties is known, and that the global sample load-displacement curve is obtained from the experiment. Thus, each fiber is stochastically characterized and accordingly parameterized. A relationship between fiber parameters and global load-displacement response, the so-called forward model, is established. From the forward model, based on Levenberg-Marquardt procedure, the inverse model is formulated and successfully applied.

• 대한기계학회논문집A
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• 제38권9호
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• pp.943-951
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• 2014

마이크로/나노 압입시험에 사용되는 각뿔 혹은 원뿔형 압입자의 선단 형상은 제작한계 및 사용 중 마모 등으로 인해 필수적으로 곡면 형태를 띄게 된다. 많은 압입시험 관련 연구에서 각뿔형 압입자의 선단 형상은 편의상 구형으로 가정한 후, 얕은 압입에 대한 구형압입 이론식을 적용하고 있다. 이러한 가정에는 근본적으로 두 가지 문제점이 있는데, 첫 번째로 이론해의 정확성은 재료 물성치 및 압입자 형상에 따라 변화한다는 점이며, 두 번째로 각뿔형 압입자의 실제 선단 형상은 이상적인 구형이 아니라는 점이다. 본 연구에서는 유한요소해석에 기반하여 압입시험에 미치는 이 두 요소의 영향을 분석한다. 먼저 탄성 구형 압입시험에 대해 푸아송비와 하중-변위 곡선의 상관관계를 살펴보고, 이를 기반으로 수정된 구형 탄성 압입 관계식을 제시한다. 이어 가정된 Berkovich 선단 형상의 3차원 유한요소해석으로부터 압입깊이에 따른 하중-변위 곡선의 특성을 분석한다.

• 대한기계학회논문집A
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• 제24권3호
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• pp.567-580
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• 2000

This paper proposes an integrated approach, which is independent of specimen geometry and loading type, for predicting the fatigue life of spot welded specimens. We first establish finite element models reflecting the actual specimen behaviors observed on the experimental load-deflection curves of 4 types of single spot welded specimens. Using finite element models elaborately established, we then evaluate fracture parameter J-integral to describe the effects of specimen geometry and loading type on the fatigue life in a comprehensive manner. It is confirmed, however, that J-integral concept alone is insufficient to clearly explain the generalized relationship between load and fatigue life of spot welded specimens. On this ground, we introduce another effective parameter $J_e$ composed of $J_I$, $J_{II}$, $J_{III}$, which has been demonstrated here to more sharply define the relationship between load and fatigue life of 4 types of spot welded specimens. The crack surface displacement method is adopted for decomposition of J, and the mechanism of the mixed mode fracture is also discussed in detail as a motivation of using $J_e$.

• 한국공간구조학회:학술대회논문집
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• 한국공간구조학회 2008년도 춘계 학술발표회 논문집
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• pp.31-36
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• 2008

건축용 막재는 자유로운 형태와 가벼움 때문에 전 세계적으로 대공간 구조물에 많이 사용되어 왔다. 건축용 막재가 바람이나 눈하중에 의해서 하중 조건하에서 구조적 문제를 발생하여 막재가 파손되는 경우가 발생하였다. 본 논문에서는 유리섬유 막재는 대한 1축인장시험 및 반복하중 시험을 수행하여 응력 변형도 곡선을 분석하고자 한다. 반복하중 시험에서 얻은 응력 변형도 곡선에서 유리섬유 막재는 반복하중이 작용할 시에 인장강도 이하에서 파단한다는 것을 알 수 있고, 폴리에스터 막재는 반복하중에 의해서 변위가 점차적으로 증가한다는 것을 보여주었다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2001년도 추계학술대회 논문집
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• pp.255-259
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• 2001

The spring constant of a micro coil spring was measured by uniaxial tensile test. The inner diameter of it is $35{\mu}m$ and the pitch size is about $23{\mu}m$. A suing constant measurement system was developed. It consists of control units, load cell units, linear stages and several specially designed jigs and fixtures. Load and displacement are measured using a commercial load cell of 1000g capacity and a magnetic scale of $0.5{\mu}m$ resolution. In this study, a method to measure the spring constant of micro coil spring is presented and the relationship between misalignment of specimen and measurement error is discussed.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1998년도 봄 학술발표회논문집(II)
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• pp.737-742
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• 1998

The tension stiffening effect is defined as the increase in stiffness in reinforced concrete member due to the stiffness provided by concrete between cracks. If this is disregarded in analysis of reinforced concrete members, especially at the level of service loads, member stiffnesses may be underestimated considerably. This paper presents on the failure behavior and tension stiffening of RC tension test with main variables such as concrete strength, rebar diameter and strength. The tension stiffening was analyzed from the load-displacement relationship by ACI code and the proposed by Collins & Mitchell. In summary, the effect of tension stiffening decrease rapidly as the rebar diameter increase, rebar strength increase, and concrete strength increase. The effect of tension stiffening on RC member is the biggest near the behavior of concrete cracking and decrease as the load close to the breaking point. Thus, the tension stiffening should be considered for the precise analysis near the load of concrete cracking.

• Steel and Composite Structures
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• 제1권3호
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• pp.341-354
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• 2001

This paper provides a state-of-the-art review on advanced analysis models for investigating the load-displacement and ultimate load behaviour of steel and composite frames subjected to static gravity and lateral loads. Various inelastic analysis models for steel and composite members are reviewed. Composite beams under positive and negative moments are analysed using a moment-curvature relationship which captures the effects of concrete cracking and steel yielding along the members length. Beam-to-column connections are modeled using rotational spring. Building core walls are modeled using thin-walled element. Finally, the nonlinear behaviour of a complete multi-storey building frame consisting of a centre core-wall and the perimeter frames for lateral-load resistance is investigated. The performance of the total building system is evaluated in term of its serviceability and ultimate limit states.

• Advances in aircraft and spacecraft science
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• 제3권1호
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• pp.45-59
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• 2016

In this paper, the condensation method which is based on the Rayleigh-Ritz method, is described for the free vibration analysis of axially loaded slightly curved beams subject to partial axial restraints. If the longitudinal inertia is neglected, some of the Rayleigh-Ritz minimization equations are independent of the frequency. These equations can be used to formulate a relationship between the weighting coefficients associated with the lateral and longitudinal displacements, which leads to "connection coefficient matrix". Once this matrix is formed, it is then substituted into the remaining Rayleigh-Ritz equations to obtain an eigenvalue equation with a reduced matrix size. This method has been applied to simply supported and partially clamped beams with three different shapes of imperfection. The results indicate that for small imperfections resembling the fundamental vibration mode, the sum of the square of the fundamental natural and a non-dimensional axial load ratio normalized with respect to the fundamental critical load is approximately proportional to the square of the central displacement.

• 전산구조공학
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• 제2권1호
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• pp.71-78
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• 1989

보의 비선형 해석프로그램의 개발에 있어서 변형연화현상을 고려해 주기 위하여 한점의 변위를 점증시키면서 구조물의 나머지 변위와 가해지는 하중들을 구하는 변위제어법을 사용하였으며, 신속한 결과의 도출을 위하여 단면의 성질인 모멘트-곡률 곡선을 이용하였다. 이때 변형연화현상으로 인하여 요소길이에 따라 같은 구조물일지라도 해석결과가 다르게 나타나는데 이 점을 보완해 주기 위해서 파괴에너지 개념을 도입하여 모멘트-곡률 곡선을 보정하였으며, 비선형 해석을 보다 단순화시키기 위하여 과다철근보에 대해서는 탄성-연화, 과소철근보에 대해서는 탄성-소성-연화로 선형화된 모델을 사용하였다. 이러한 본 해석프로그램을 이용하여 실험된 철근콘크리트 보들을 해석한 결과 보의 하중-처짐 곡선은 실험결과와 거의 일치함을 보였다.

• Composites Research
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• 제30권5호
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• pp.280-287
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• 2017

복합재 후퇴익은 비행 중 굽힘 하중 외에도 비틀림 하중을 받아 복잡한 변형이 발생할 수 있다. 따라서 복잡한 변형이 나타나는 복합재 후퇴익의 구조 건전성 평가를 위해 복합재 후퇴익 구조물의 변위 예측이 요구된다. 날개의 변위 예측은 변위와 변형률 관계를 통해 예측할 수 있지만 후퇴익의 복잡한 변형으로 고정단 부근의 변형률 분포는 복잡하게 나타나기 때문에 일부 위치의 변형률 센서만으로 변형률을 측정하면 변위 예측의 오차가 발생할 수 있다. 본 논문에서는 복잡한 변형률 분포를 고려한 평판 형태의 복합재 후퇴익의 변위를 예측하는 연구를 수행하였다. 유한요소 해석을 통해 변형률 측정 지점을 선정하였다. 측정 지점의 변형률을 이용하여 예측한 변위는 해석에서 계산된 변위와 잘 일치하였으며, 실험을 통하여 검증하였다.