• 콘크리트학회논문집
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• 제25권6호
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• pp.631-639
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• 2013

이 연구에서는 기존에 수행된 시공성과 경제성이 향상되고 중진 지역에서 사용 가능한 새로운 프리캐스트 콘크리트 보-기둥 접합상세의 비선형 유한요소해석이 수행되었다. 해당 상세는 복합구조를 기반으로 함으로써 기둥 내에는 각관을 보유하고 있으며 보에는 강판이 매입되어 있는 복잡한 단면을 보유하고 있다. 또한 콘크리트와 강재뿐만 아니라 ECC라는 새로운 재료를 사용함에 따라 요소의 선택 및 재료모델의 결정에 대한 방법론을 제시하였다. 비선형 유한요소해석은 상용 유한요소해석 프로그램인 ABAQUS를 통해 수행되었으며 요소 및 재료 모델은 ABAQUS에서 제공하는 모델들을 사용하였다. 비선형 유한요소해석 결과 기 수행되었던 실험 결과에 유사하거나 보수적으로 평가함으로써 변수분석에 사용할 수 있을 것으로 판단하였다. 구축된 유한요소해석 모델을 통해 해당 상세의 성능에 대한 압축력의 영향, 휨강도비의 영향에 대해 분석하였다. 압축력의 경우 기둥 압축강도의 10~20%에서 가장 좋은 성능을 발휘할 수 있었으며, 휨강도비 1.2 이상에서 기둥의 항복 없이 보의 소성힌지를 유도할 수 있음을 확인하였다.

• Wind and Structures
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• 제27권4호
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• pp.269-282
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• 2018

In this paper, a new displacement field based on quasi-3D hybrid-type higher order shear deformation theory is developed to analyze the static and dynamic response of exponential (E), power-law (P) and sigmoïd (S) functionally graded beams. Novelty of this theory is that involve just three unknowns with including stretching effect, as opposed to four or even greater numbers in other shear and normal deformation theories. It also accounts for a parabolic distribution of the transverse shear stresses across the thickness, and satisfies the zero traction boundary conditions at beams surfaces without introducing a shear correction factor. The beam governing equations and boundary conditions are determined by employing the Hamilton's principle. Navier-type analytical solutions of bending and free vibration analysis are provided for simply supported beams subjected to uniform distribution loads. The effect of the sigmoid, exponent and power-law volume fraction, the thickness stretching and the material length scale parameter on the deflection, stresses and natural frequencies are discussed in tabular and graphical forms. The obtained results are compared with previously published results to verify the performance of this theory. It was clearly shown that this theory is not only accurate and efficient but almost comparable to other higher order shear deformation theories that contain more number of unknowns.

• Smart Structures and Systems
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• 제25권5호
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• pp.605-617
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• 2020

The existence of damages in structures causes changes in the physical properties by reducing the modal parameters. In this paper, we develop a two-stages approach based on normalized Modal Strain Energy Damage Indicator (nMSEDI) for quick applications to predict the location of damage. A two-dimensional IsoGeometric Analysis (2D-IGA), Machine Learning Algorithm (MLA) and optimization techniques are combined to create a new tool. In the first stage, we introduce a modified damage identification technique based on frequencies using nMSEDI to locate the potential of damaged elements. In the second stage, after eliminating the healthy elements, the damage index values from nMSEDI are considered as input in the damage quantification algorithm. The hybrid of Teaching-Learning-Based Optimization (TLBO) with Artificial Neural Network (ANN) and Particle Swarm Optimization (PSO) are used along with nMSEDI. The objective of TLBO is to estimate the parameters of PSO-ANN to find a good training based on actual damage and estimated damage. The IGA model is updated using experimental results based on stiffness and mass matrix using the difference between calculated and measured frequencies as objective function. The feasibility and efficiency of nMSEDI-PSO-ANN after finding the best parameters by TLBO are demonstrated through the comparison with nMSEDI-IGA for different scenarios. The result of the analyses indicates that the proposed approach can be used to determine correctly the severity of damage in beam structures.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2004년도 춘계학술대회 논문집 반도체 재료 센서 박막재료 전자세라믹스
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• pp.68-71
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• 2004

본 연구는 싱크로트론 방사광의 단색광 (monobeam)을 이용한 영상을 획득하였다. 영상센서로서 CMOS를 사용하였으며 센서 앞단에는 형광체 (phosphor)를 이용하여 방사광에 대한 빛의 신호로서 영상을 획득하였다. 사용된 싱크로트론 방사광의 beam size는 $5mm{\times}2mm$ 이며 ion chamber를 통한 beam intensity 는 $10{\times}10^{-7}$이다. 형광체는 각각 ZnS(Cu:Al), ZnS(Ag,Al), $BiTiO_3$, $Y_2O_2S(Tb)$로서 4가지를 사용하였으며 여기에 사용된 형광체는 기계식 스크린 프린팅 (Screen Printing) 방식으로 직접 제조하였다. 두께는 모두 동일하게 $10{\mu}m$이며 각각에 대한 PL(Photoluminescence)을 측정하여 분석하였다. object로는 물고기와 20linepair를 사용하였으며 CMOS센서를 이용하여 각각의 phosphor에 대하여 영상을 획득하였다. 영상의 평가는 20line pair 영상의 MTF를 이용하였다. 각각의 형광체에 대한 MTF는 5 lp/mm 에서는 0.5650, 0.2150, 0.7890, 0.3840 이며 10 lp/mm 은 0.4500, 0.0900, 0.2510, 0.1500이고 15 lp/mm 는 0.1900, 0.0300, 0.1430, 0.0500이며 마지막으로 20 lp/mm 은 0.0810, 0.004, 0.0500, 0.0320의 MTF 값을 나타내었다. $10{\mu}m$ 두께에 대하여 ZnS(Cu:Al)이 가장 좋은 MTF의 값을 나타내었다.

• Smart Structures and Systems
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• 제17권6호
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• pp.957-980
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• 2016

Structural damage detection (SDD) is a challenging task in the field of structural health monitoring (SHM). As an exploring attempt to the SDD problem, a hybrid self-adaptive Firefly-Nelder-Mead (SA-FNM) algorithm is proposed for the SDD problem in this study. First of all, the basic principle of firefly algorithm (FA) is introduced. The Nelder-Mead (NM) algorithm is incorporated into FA for improving the local searching ability. A new strategy for exchanging the information in the firefly group is introduced into the SA-FNM for reducing the computation cost. A random walk strategy for the best firefly and a self-adaptive control strategy of three key parameters, such as light absorption, randomization parameter and critical distance, are proposed for preferably balancing the exploitation and exploration ability of the SA-FNM. The computing performance of the SA-FNM is evaluated and compared with the basic FA by three benchmark functions. Secondly, the SDD problem is mathematically converted into a constrained optimization problem, which is then hopefully solved by the SA-FNM algorithm. A multi-step method is proposed for finding the minimum fitness with a big probability. In order to assess the accuracy and the feasibility of the proposed method, a two-storey rigid frame structure without considering the finite element model (FEM) error and a steel beam with considering the model error are taken examples for numerical simulations. Finally, a series of experimental studies on damage detection of a steel beam with four damage patterns are performed in laboratory. The illustrated results show that the proposed method can accurately identify the structural damage. Some valuable conclusions are made and related issues are discussed as well.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.294-299
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• 2004

An assessment of a novel laser-electric hybrid propulsion system was conducted, in which a laser-induced plasma was induced through laser beam irradiation onto a solid target and accelerated by electrical means instead of direct acceleration only by using a laser beam. A fundamental study of newly developed rectangular laser-assisted pulsed-plasma thruster (PPT) was conducted. On discharge characteristics and thrust performances with increased peak current compared to our previous study to increase effects of electromagnetic forces on plasma acceleration. Maximum peak current increased for our early study by increasing electromagnetic effects in a laser assisted PPT. At 8.65 J discharge energy, the maximum current reached about 8000 A. Plasma behaviors emitted from a thruster in various cases were observed with an ICCD camera. It was shown that the plasma behaviors were almost identical between low and high voltage cases in initial several hundred nanoseconds, however, plasma emission with longer duration was observed in higher voltage cases. Canted current sheet structures were also observed in the higher voltage cases using a larger capacitor. With a newly developed torsion-balance type thrust stand, thrust performances of laser assisted PPT could be estimated. The impulse bit and specific impulse linearly increased. On the other hand, coupling coefficient and the thrust efficiency did not increase linearly. The coupling coefficient decreased with energy showing maximum value (20.8 ？Nsec/J) at 0 J, or in a pure laser ablation cases. Thrust efficiency first decreased with energy from 0 to 1.4 J and then increased linearly with energy from 1.4 J to 8.6 J. At 8.65 J operation, impulse bit of 38.1 ？Nsec, specific impulse of 3791 sec, thrust efficiency of 8 %, and coupling coefficient of 4.3 ？Nsec/J were obtained.

• 대한소아치과학회지
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• 제49권1호
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• pp.57-64
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• 2022

콘빔 전산화 단층촬영(CBCT)은 치열을 왜곡시켜 추가 치열 이미지가 필요하다. 치열 이미지로 주로 사용되는 석고 모형을 대신해 구강 스캐너로 CBCT의 치열 이미지를 보완할 때의 임상적 활용 가능성을 평가하였다. 만 12 - 18세의 20명에게 상악에 대한 석고 모형, 구강 스캔 이미지, CBCT이미지를 획득하였다. 비교를 위해 두 종류의 구강 스캐너를 이용하였는데, 그 중 하나의 구강 스캐너로는 전악을 3분할하여 각각을 스캔한 후 이들을 병합해 전악 이미지를 획득하는 방법을 추가로 시행하였다. 구강 스캐너를 이용해 얻은 가상 골격-치열 하이브리드 이미지를 석고 모형을 통해 얻은 이미지와 중첩하여 각 기준점에서의 좌표 값의 차이와 거리를 측정하였다. 결과적으로 구강 스캐너로 분할하여 스캔하는 방법을 시행했을 때 평균 거리 2 ㎛를 보여 가장 적게 나타났다. 구강 스캐너를 적절히 이용하면 가상 골격-치열 하이브리드 이미지를 위한 치열 이미지로 훌륭히 활용될 수 있을 것이다.

• Computers and Concrete
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• 제31권5호
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• pp.433-442
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• 2023

This study presents the visually observed behavior of fibers embedded in concrete samples that were subjected to a flexural bending test. Three types of fibers such as macro polypropylene, macro polyethylene, and the hybrid of steel and polyvinyl alcohol were mixed with cement by a designated mix ratio to prepare a total of nine specimens of each. The bending test was conducted by following ASTM C1609 with a net deflection of 2, 4, and 7 mm. The X-ray computed tomography (XCT) was carried out for 7 mm-deflection specimens. The original XCT images were post-processed to denoise the beam-hardening effect. Then, fiber, crack, and void were semi-manually segmented. The hybrid specimen showed the highest toughness compared to the other two types. Debonding based on 2D XCT sliced images was commonly observed for all three groups. The cement matrix near the crack surface often involved partially localized breakage in conjunction with debonding. The pullout was predominant for steel fibers that were partially slipped toward the crack. Crack bridging and rupture were not found presumably due to the image resolution and the level of energy dissipation for poly-fibers, while the XCT imaging was advantageous in evaluating the distribution and behavior of various fibers upon bending for fiber-reinforced concrete beam elements.

• Steel and Composite Structures
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• 제24권3호
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• pp.287-296
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• 2017

The buckling capacity of a radially retractable hybrid grid shell in the closed position was investigated in this paper. The geometrically non-linear elastic buckling and elasto-plastic buckling analyses of the hybrid structure were carried out. A parametric study was done to investigate the effects rise-to-span ratio, beam section, area and pre-stress of cables, on the failure load. Also, the influence of the shape and scale of imperfections on the elasto-plastic buckling loads was discussed. The results show that the critical buckling load is reduced by taking account of material non-linearity. Furthermore, increasing the rise-to-span ratio or the cross-section area of steel beams notably improves the stability of the structure. However, the cross section area and pre-stress of cables pose negligible effect on the structural stability. It can also be found that the hybrid structure is highly sensitive to geometric imperfection which will considerably reduce the failure load. The proper shape and scale of the imperfection are also important.

• 콘크리트학회논문집
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• 제25권2호
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• pp.175-185
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• 2013

최근 다른 압축강도로 타설된 프리캐스트 콘크리트(PC)와 현장타설 콘크리트(CIP)의 복합 부재의 사용이 증가하고 있지만 현행 기준에는 서로 다른 강도로 복합화된 부재의 전단강도에 대한 설계 기준이 없다. 그래서 이번 연구에서 서로 다른 압축강도(24 MPa, 60 MPa)로 분리 타설된 보의 전단강도 실험을 수행하여 복합 부재의 전단강도에 대해 알아보았다. 변수로는 단면형상, 휨철근비, 그리고 전단경간비를 고려하였다. 실험 결과 값과 현행 전단 기준식과 단면적비로 계산한 유효 콘크리트 강도를 이용한 예측 값을 비교하였다. 실험 결과를 분석해보면 철근비가 낮고 압축대에 60 MPa가 사용된 실험체들에 대해 설계 기준식을 과대평가하였다. 실험 결과를 기준으로 PC와 CIP 복합부재의 전단설계 기준을 제안하였다.