• Steel and Composite Structures
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• 제46권3호
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• pp.319-334
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• 2023

Localizing damages is an essential task to monitor the health of the structures since they may not be able to operate anymore. Among the damage detection techniques, non-destructive methods are considerably more preferred than destructive methods since damage can be located without affecting the structural integrity. However, these methods have several drawbacks in terms of detecting abilities, time consumption, cost, and hardware or software requirements. Employing artificial intelligence techniques could overcome such issues and could provide a powerful damage detection model if the technique is utilized correctly. In this study, the crack localization in flat and folded plate structures has been conducted by employing a Backpropagated Artificial Neural Network (BPANN). For this purpose, cracks with 18 different dimensions in thin, flat, and folded structures having 15⁰, 30⁰, 45⁰, and 60⁰ folding angle have been modeled and subjected to free vibration analysis by employing the Classical Plate Theory with Finite Element Method. A Four-nodded quadrilateral element having six degrees of freedom has been considered to represent those structures mathematically. The first ten natural frequencies have been obtained regarding healthy and cracked structures. To localize the crack, the ratios of the frequencies of the cracked flat and folded structures to those of healthy ones have been taken into account. Those ratios have been given to BPANN as the input variables, while the crack locations have been considered as the output variables. A total of 500 crack locations have been regarded within the dataset obtained from the results of the free vibration analysis. To build the best intelligent model, a feature search has been conducted for BAPNN regarding activation function, the number of hidden layers, and the number of hidden neurons. Regarding the analysis results, it is concluded that the BPANN is able to localize the cracks with an average accuracy of 95.12%.

• 터널과지하공간
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• 제34권4호
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• pp.413-420
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• 2024

도심지 터널에 적용성이 우수한 Gripper TBM 계획시 굴착공법 특성을 반영하지 않고 기존 NATM 터널의 지반이완 개념의 보강계획을 준용하여 굴착안정성 및 시공성이 저하되는 문제를 발생시키고 있다. 본 연구에서는 도심지 근접시공에 따른 보강계획시 Gripper TBM 굴착공법 특성을 고려한 지보계획의 합리화 방안을 제안하고자 하였다. 이를 위해 굴착공법별 시공조건을 고려한 3차원 안정성 검토를 수행하여 굴착에 따른 주변지반에 미치는 영향을 분석하였다. NATM공법은 굴착과 동시에 무지보 자립시간이 발생되어 굴진면에 지반이완이 집중되었으나, Gripper TBM은 커터헤드와 스킨플레이트를 통해 굴진면 주변지반을 지지하므로 스킨플레이트 후방에 지반이완이 발생되었다. 이러한 굴착공법 특성을 고려하여 연구대상현장의 근접시공에 따른 보강계획의 문제점을 지적하고 Gripper TBM 굴진안정성 및 시공성 등을 고려한 합리적인 지보패턴 개선방안을 제안하였다.

• Composites Research
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• 제18권5호
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• pp.21-26
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• 2005

본 연구에서는 복합재의 등가 물성치를 예측하는 방법중 하나인 Mori-Tanaka의 평균장이론이 결합된 Eshelby 이론의 적용한계에 대해 유한요소해석을 통하여 강화재의 크기와 배치 측면에서 고찰하였다. 모델 복합재로 일정 체적비의 강화재를 포함하는 2차원 평판 복합재를 사용하였으며, 강화재의 크기를 변화시키고 또한 강화재를 규칙적 및 불규칙적으로 배치하였다. 이 복합재에 유한요소해석을 적용하여 수치적으로 복합재의 등가 물성치를 구하였으며, 수치해석결과를 Eshelby 이론으로 구한 등가 물성치와 비교하였다. Eshelby 이론으로 예측되는 복합재의 등가 물성치는 시편의 크기에 비해 강화재의 크기가 0.03이하가 되면 강화재의 배치와 관계없이 유한요소해석으로 구한 복합재의 평균 영계수와는 잘 일치하나, 평균 프와송비는 약 $20\%$의 차이를 보였다.

• Steel and Composite Structures
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• 제26권4호
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• pp.513-531
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• 2018

In this article, static, buckling and free vibration analyses of a sinusoidal micro composite beam reinforced by single-walled carbon nanotubes (SWCNTs) with considering temperature-dependent material properties embedded in an elastic medium in the presence of magnetic field under transverse uniform load are presented. This system is used at micro or sub micro scales to enhance the stiffness of micro composite structures such as bar, beam, plate and shell. In the present work, the size dependent effects based on surface stress effect and modified strain gradient theory (MSGT) are considered. The generalized rule of mixture is employed to predict temperature-dependent mechanical and thermal properties of micro composite beam. Then, the governing equations of motions are derived using Hamilton's principle and energy method. Numerical results are presented to investigate the influences of material length scale parameters, elastic foundation, composite fiber angle, magnetic intensity, temperature changes and carbon nanotubes volume fraction on the bending, buckling and free vibration behaviors of micro composite beam. There is a good agreement between the obtained results by this research and the literature results. The obtained results of this study demonstrate that the magnetic intensity, temperature changes, and two parameters elastic foundations have important effects on micro composite stiffness, while the magnetic field has greater effects on the bending, buckling and free vibration responses of micro composite beams. Moreover, it is shown that the effects of surface layers are important, and observed that the changes of carbon nanotubes volume fraction, beam length-to-thickness ratio and material length scale parameter have noticeable effects on the maximum deflection, critical buckling load and natural frequencies of micro composite beams.

• 한국지구과학회지
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• 제23권2호
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• pp.194-206
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• 2002

This article describes the major themes to change in historical and philosophical perspectives of science education that lead the US and Korean science curriculum reform movements since 1957. Inquiry teaching and criticism of teaching science as inquiry in the late 1950s and the 1960s, Science-Technology-Society (STS) Curricula, and Science Literacy and the 1980s science literacy crisis are discussed. In the US, three major curricular projects as responses to the scientific literacy crisis are exemplary such as the Project 2061 sponsored by the American Association for the Advancement of Science, the Project on Scope, Sequence, and Coordination (SS&C) initiated by the National Science Teachers Association (NSTA), and the National Science Education Standards (NSES) published by the National Research Council. To identify how each set of national content standards differ, we compared specific content standards related to the theory of plate tectonics in Earth and Space science in grades 9-12 over the three national standards: Benchmarks of AAAS, NSES of the NRC, and SS&C of the NSTA. Against this historical background of the US science education reform movements, the curriculum reform movements in Korea is briefly discussed. In general, Korean science curriculum reform movements have reflected and resembled the recommendations of the US reform movements. In addition, it is important to note that throughout the history of curriculum revision in Korea, there have been continuing pendulum swings between a theoretical, discipline-centered curriculum and a liberal, humanistic, and student-centered curriculum, which pays more attention to students in terms of their interest and psychological preparedness. In conclusion, the sixth and seventh national science curriculum revisions reflect rather a student-centered movement by reducing technical and sophisticated topics, taking constructivism learning theory into consideration, and adding more STS related topics.

• Composites Research
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• 제25권5호
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• pp.136-140
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• 2012

본 논문에서는 복합재료 IM rod가 적용된 골절부의 세포 분화과정을 모사하기 위해 유한요소해석을 실시하였다. 세포의 골화과정을 해석하기 위해 편향 변형률을 이용한 메카노 규제 이론을 사용하였으며, 반복 계산을 위해 Python 코드를 이용하여 서브루틴을 구현하였다. 치료에 가장 적절한 복합재료 IM rod의 강성을 찾기 위해 직물 탄소섬유/에폭시 복합재료 (WSN3k)의 적층각도를 바꾸어 해석을 실시하였다. 골절부에 가해지는 기계적 자극에 따른 치료효율을 비교하기 위해 두 가지 초기 하중 조건을 적용하였다. 그 결과 치료효율은 강성의 차이보다 하중에 의해 큰 영향을 받았으며, 초기 하중이 몸무게의 10%이고, 적층순서가 $[{\pm}45]_{nT}$일 때 치료효율이 가장 높았다.

• Steel and Composite Structures
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• 제16권6호
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• pp.559-576
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• 2014

A new mathematical model and its finite element formulation for the non-linear stress-strain analysis of a planar beam strengthened with plates bolted or adhesively bonded to its lateral sides is presented. The connection between the layers is considered to be flexible in both the longitudinal and the transversal direction. The following assumptions are also adopted in the model: for each layer (i.e., the beam and the side plates) the geometrically linear and materially non-linear Bernoulli's beam theory is assumed, all of the layers are made of different homogeneous non-linear materials, the debonding of the beam from the side-plates due to, for example, a local buckling of the side plate, is prevented. The suitability of the theory is verified by the comparison of the present numerical results with experimental and numerical results from literature. The mechanical response arising from the theoretical model and its numerical formulation has been found realistic and the numerical model has been proven to be reliable and computationally effective. Finally, the present formulation is employed in the analysis of the effects of two different realizations of strengthening of a characteristic simply supported flexural beam (plates on the sides of the beam versus the tension-face plates). The analysis reveals that side plates efficiently enhance the bearing capacity of the flexural beam and can, in some cases, outperform the tensile-face plates in a lower loss of ductility, especially, if the connection between the beam and the side plates is sufficiently stiff.

• Structural Engineering and Mechanics
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• 제65권4호
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• pp.491-504
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• 2018

Because of sandwich structures with low weight and high stiffness have much usage in various industries such as civil and aerospace engineering, in this article, buckling and free vibration analyses of coupled micro composite sandwich plates are investigated based on sinusoidal shear deformation (SSDT) and most general strain gradient theories (MGSGT). It is assumed that the sandwich structure rested on an orthotropic elastic foundation and make of four composite face sheets with temperature-dependent material properties that they reinforced by carbon and boron nitride nanotubes and two flexible transversely orthotropic cores. Mathematical formulation is presented using Hamilton's principle and governing equations of motions are derived based on energy approach and applying variation method for simply supported edges under electro-magneto-thermo-mechanical, axial buckling and pre-stresses loadings. In order to predict the effects of various parameters such as material length scale parameter, length to width ratio, length to thickness ratio, thickness of face sheets to core thickness ratio, nanotubes volume fraction, pre-stress load and orthotropic elastic medium on the natural frequencies and critical buckling load of double-bonded micro composite sandwich plates. It is found that orthotropic elastic medium has a special role on the system stability and increasing Winkler and Pasternak constants lead to enhance the natural frequency and critical buckling load of micro plates, while decrease natural frequency and critical buckling load with increasing temperature changes. Also, it is showed that pre-stresses due to help the axial buckling load causes that delay the buckling phenomenon. Moreover, it is concluded that the sandwich structures with orthotropic cores have high stiffness, but because they are not economical, thus it is necessary the sandwich plates reinforce by carbon or boron nitride nanotubes specially, because these nanotubes have important thermal and mechanical properties in comparison of the other reinforcement.

• 한국해안·해양공학회논문집
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• 제24권2호
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• pp.89-96
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• 2012

유탄성이론과 고유함수전개법에 기초하여 규칙파중 유연한 그물망의 동적거동과 파랑하중을 살펴보았다. 그물망은 일정한 잠긴깊이를 가지고 수직으로 설치되어 있으며, 상단 끝은 수면에 고정되어 있고 하단 끝은 추에 연결되어 있다. 초기장력이 충분히 크다고 가정하여 장력의 동적 성분을 무시하였다. 유연한 그물망에서의 경계조건식은 투과효과를 나타내는 Darcy의 법칙과 유연성을 나타내는 물체 경계조건식이 결합된 형태이다. 개발된 해석모델은 불투과성/투과성 수직판과 유연막 모델로 확장이 가능하다. 해석모델을 이용하여 여러 설계변수(파랑특성, 공극율, 잠긴깊이, 초기장력)들의 변화가 그물망의 파랑하중과 거동특성에 미치는 영향을 살펴보았다.

• 한국강구조학회 논문집
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• 제10권2호통권35호
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• pp.201-209
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• 1998

강섬유보강 적층복합구조물에서 온도의 변화는 구조물의 응답에 중요한 영향을 미칠수 있다. 온도의 급작스런 변화는 재료의 강도와 성질을 현저히 저하시켜 구조물의 대변형, 좌굴, 고응력상태를 유발하는 중요한 인자가 된다. 본 연구에서는 등분포로 재하된 온도하중에 의한 적층복합판의 온도좌굴에 관한 해석을 수행하였다. 전단변형의 효과를 정확히 고려하기위해 5개의 변수로 구성된 고차전단변형이론을 적용하였다. 적층판의 배열각도, 적층판의 수, 폭-두께비의 변화, 형상비의 변화에 따른 임계좌굴온도를 구하여 1차전단변형이론에 의한 결과와 고전적이론에 의한 결과와 비교분석하였다.