• 한국농공학회지
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• 제44권2호
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• pp.117-126
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• 2002

Concrete containing discontinuous discrete steel fiber in a normal concrete is called steel fiber reinforced concrete(SFRC). Tensile as well as flexural strengths of concrete could be substantially increased by introducing closely spaced fibers which delay the onset of tension cracks and increase the tension strength of cracks. However, many properties of SFRC have not been investigated, especially properties on repeated loadings. Thus, the purposes of this dissertation is to study the flexural fatigue characteristics of SFRC considering cumulative damage. A series of experimental tests such as compressive strength, splitting tensile strength, flexural strength, flexural fatigue, and two steps stress level fatigue were conducted to clarify the basic properties and fatigue-related properties of SFRC. The main experimental variables were steel fiber fraction (0, 0.4, 0.7, 1, 1.5%), aspect ratio (60, 83). The principal results obtained through this study are as follows: The results of flexural fatigue tests showed that the flexural fatigue life of SFRC is approxmately 65% of ultimate strength, while that of plain is less than 58%. Especially, the behavior of flexural fatigue life shows excellent performance at 1.0% of steel-fiber volume fraction. The cumulative damage test of high-low two stress levels is within the value of 0.6 ∼ 1.1, while that of low-high stress steps is within the value of 2.4 ∼ 4.0.

• Structural Engineering and Mechanics
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• 제55권1호
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• pp.79-92
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• 2015

In this study, free vibration analyses of symmetric laminated cantilever and simply supported damaged composite beams are investigated by using finite element method (FEM). Free vibration responses of damaged beams are examined using Euler Bernoulli beam and classical lamination theories. A computer code is developed by using MATLAB software to determine the natural frequencies of a damaged beam. The local damage zone is assumed to be on the surface lamina of the beam by broken fibers after impact. The damaged zone is modeled as a unidirectional discontinuous lamina with $0^{\circ}$ orientations in this study. Fiber volume fraction ($v_f$), fiber aspect ratio ($L_f/d_f$), damage length ($L_D$) and its location (${\lambda}/L$), fiber orientation and stacking sequence parameters effects on natural frequencies are investigated. These parameters are affected the natural frequency values significantly.

• 한국농공학회논문집
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• 제49권2호
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• pp.27-36
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• 2007

This study, the present study wishes to develop a model that estimates shear strength characteristics of steel fiber reinforced concrete using artifical neural network models. Neural network models, developed as mathematical models, are being widely used not only in its original purpose of pattern recognition, but also in application fields by the function's nonlinear loaming and interpolar ability Neural network has a repetitive rotation algorithm that can cyclically and repeatedly estimate system conditions and parameter ideal values, and it can be used in the modeling of the nonlinear system by nonlinear characteristic functions that construct the system.

• Composites Research
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• 제19권6호
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• pp.16-22
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• 2006

다양한 조건 하에서 최상의 관통 특성을 발휘하기 위한 최적 조건에 대해 연구하였다. 섬유 종횡비(AR: 섬유 길이/섬유 직경), 계면 조건 그리고 섬유 함유량을 관통 저항력과 마찰력에 지대한 영향을 미치는 변수들로 고려하였다. 단섬유 강화고무의 관통 저항력은 기지에 비해 최대 3.4배 증가하였다. 동일한 섬유 종횡비와 섬유 함유량에서 계면 조건이 우수할수록 더 높은 관통 저항력을 보였다. 기지와 섬유 종횡비가 155이하인 일부 단섬유 강화고무의 마찰력은 존재치 않았다. 우수한 계면과 높은 섬유 종횡비를 갖는 단섬유 강화고무의 마찰력은 기지의 관통 저항력보다도 더 높았다. 전체적으로 계면 조건과 섬유 종횡비 그리고 섬유 함유량이 단섬유 강화고무의 관통 특성에 지대한 영향이 미침을 확인하였다.

• 콘크리트학회논문집
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• 제18권3호
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• pp.361-370
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• 2006

본 연구에서는 국내외 강섬유보강 콘크리트보의 실험값 및 이론식을 분석하여 강섬유의 보강효과 및 부재의 역학적 거동을 규명하였고, 기존의 휨강도식보다 더 정확하고 합리적인 휨강도식을 제안하였다. 제안된 휨강도식은 실험값과 좋은 일치를 보여주었고, 강섬유보강 콘크리트보의 휨강도식은 강섬유보강효과뿐만 아니라 철근의 변형률 경화효과도 함께 고려하는 것이 적절함을 알 수 있었다.

• Computers and Concrete
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• 제29권 6호
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• pp.375-391
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• 2022

This paper examined the impact of the cross-sectional structure on the structural results under different loading conditions of reinforced concrete (RC) members' management limited in Carbon Fiber Reinforced Polymers (CFRP). The mechanical properties of CFRC was investigated, then, totally 32 samples were examined. Test parameters included the cross-sectional shape as square, rectangular and circular with two various aspect rates and loading statues. The loading involved concentrated loading, eccentric loading with a ratio of 0.46 to 0.6 and pure bending. The results of the test revealed that the CFRP increased ductility and load during concentrated processing. A cross sectional shape from 23 to 44 percent was increased in load capacity and from 250 to 350 percent increase in axial deformation in rectangular and circular sections respectively, affecting greatly the accomplishment of load capacity and ductility of the concentrated members. Two Artificial Intelligence Models as Extreme Learning Machine (ELM) and Particle Swarm Optimization (PSO) were used to estimating the tensile and flexural strength of specimen. On the basis of the performance from RMSE and RSQR, C-Shape CFRC was greater tensile and flexural strength than any other FRP composite design. Because of the mechanical anchorage into the matrix, C-shaped CFRCC was noted to have greater fiber-matrix interfacial adhesive strength. However, with the increase of the aspect ratio and fiber volume fraction, the compressive strength of CFRCC was reduced. This possibly was due to the fact that during the blending of each fiber, the volume of air input was increased. In addition, by adding silica fumed to composites, the tensile and flexural strength of CFRCC is greatly improved.

• 비파괴검사학회지
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• 제17권2호
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• pp.89-99
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• 1997

단섬유강화금속복합재료는 최근 항공기, 자동차산업에 있어서 관심의 대상이 되고 있는 재료중의 하나이나 재료의 제조 및 성형중에 재료내의 기지재 및 강화재의 열팽창계수의 차이로 인해 재료 내부에 발생되는 열적잔류응력으로 인한 재료 특성의 변화로 실제적인 재료 적용상에 많은 문제점들이 보고되고 있다. 이와 같은 금속복합재료의 잔류응력의 평가에는 몇가지 비파괴적 방법이 적용되고 있으나 그 측정에 많은 어려움이 보고되고 있다. 따라서 금속복합재료의 보다 실제적인 응용을 위하여는 이와 같은 열적잔류응력을 평가하기 위한 이론적모델의 확립이 요구된다. 본 연구에 있어서는 비방향성을 가진 강화재가 2차원 평면 상태로 기지재내에 존재하는 단섬유강화금속복합재료에 있어서 재료에 균일한 온도 변화가 주어질 때 기지재와 강화재의 열팽창계수의 차로 인해 재료 내부에 발생하는 열적잔류응력을 평가, 예측하기 위한 이론적 탄성 모델을 확립하고자 한다. 본 연구에서 해석하고자 하는 이론 모델은 Eshelby의 등가 개재물 방법을 토대로 하고 있으며 과거 제안되고 있는 이론모델을 포함하는 보다 일반성을 가지는 해석 모델로서, 이 해석 모델을 이용하여 열적잔류응력에 미치는 강화재의 체적률, 종횡비, 분포 상태, 분포 cut-off 각도들에 대한 각 인자의 영향을 검토하였다. 그 결과 강화재의 체적률, 종횡비, cut-off 각도들이 강화재의 분포 상태보다도 금속복합재료의 열적잔류응력에 미치는 영향이 현저함을 알 수 있었다.

• Steel and Composite Structures
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• 제30권1호
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• pp.31-46
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• 2019

In the present study, the various dynamic properties of MWCNT embedded fiber reinforced polymer uniform and tapered composite (MWCNT-FRP) plates are investigated. Various configurations of a tapered composite plate with ply-drop off and uniform composite plate have been considered for the development of the finite element formulation and experimental investigations. First order shear deformation theory (FSDT) has been used to derive the kinetic and potential energy equations of the hybrid composite plates by including the effect of rotary inertia, shear deformation and non-uniformity in thickness of the plate. The governing equations of motion of FRP composite plates without and with MWCNT reinforcement are derived by considering a nine- node rectangular element with five degrees of freedom (DOF) at each node. The effectiveness of the developed finite element formulation has been demonstrated by comparing the natural frequencies and damping ratio of FRP composite plates without and with MWCNT reinforcement obtained experimentally. Various parametric studies are also performed to study the effect of CNT volume fraction and CNT aspect ratio of the composite plate on the natural frequencies of different configurations of CNT reinforced hybrid composite plates. Further the forced vibration analysis is performed to compare the dynamic response of the various configurations of MWCNT-GFRP composite plate with GFRP composite plate under harmonic excitations. It was observed that the fundamental natural frequency and damping ratio of the GFRP composite plate increase approximately 8% and 37% respectively with 0.5wt% reinforcement of MWCNT under CFCF boundary condition. The natural frequencies of MWCNT-GFRP hybrid composite plates tend to decrease with the increase of MWCNT volume fraction beyond 2% due to agglomeration of CNT's. It is also observed that the aspect ratio of the CNT has negligible effect on the improvement of dynamics properties due to randomly orientation of CNT's.

• Structural Engineering and Mechanics
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• 제67권4호
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• pp.403-415
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• 2018

In the present study, nonlinear dynamic response of polymer-CNT-fiber multiscale nanocomposite plate resting on elastic foundations in thermal environments using the finite element method is performed. In this regard, the governing equations are derived based on Inverse Hyperbolic Shear Deformation Theory and von $K{\acute{a}}rm{\acute{a}}n$ geometrical nonlinearity. Three type of distribution of temperature through the thickness of the plate namely, uniform linear and nonlinear are considered. The considered element is C1-continuous with 15 DOF at each node. The effective material properties of the multiscale composite are calculated using Halpin-Tsai equations and fiber micromechanics in hierarchy. The carbon nanotubes are assumed to be uniformly distributed and randomly oriented through the epoxy resin matrix. Five types of impulsive loads are considered, namely the step, sudden, triangular, half-sine and exponential pulses. After examining the validity of the present work, the effects of the weight percentage of SWCNTs and MWCNTs, nanotube aspect ratio, volume fraction of fibers, plate aspect, temperature, elastic foundation parameters, distribution of temperature and shape of impulsive load on nonlinear dynamic response of CNT reinforced multi-phase laminated composite plate are studied in details.

• 한국터널지하공간학회 논문집
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• 제12권1호
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• pp.51-60
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• 2010

최근 새로운 형태의 폴리올레핀 섬유에 대하여 성능향상 개발이 이루어지고 있으며, 기존 섬유와 혼입하여 사용하거나 폴리올레핀 섬유만을 사용한 콘크리트 특성에 대하여 연구와 검증이 필요하다. 본 연구의 목적은 강섬유와 폴리올레핀 섬유를 사용한 콘크리트의 휨 인성 및 휨 인장강도에 대한 성능을 확인하기 위하여 324개의 보 시편을 제작하고, KS F2566과 ASTM C 1399-02규격에서 제시한 4점 휨 실험을 수행하여 휨 인성지수, 등가 휨 인장강도, 평균잔류강도를 분석하였다. 실험결과, 섬유 혼입률, 섬유 형상비에 대한 휨 인성 및 인장강도에 대한 효과가 확인되었으며, 강섬유와 폴리올레핀의 최적의 혼입비율을 제시하였다.