• 한국소음진동공학회논문집
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• 제19권9호
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• pp.899-906
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• 2009

In the present paper, dynamic characteristics and vibration control performance of a cylindrical shell structure are experimentally investigated and results are presented in the air and underwater conditions. End-capped cylindrical shell structure is manufactured and macro-fiber composite(MFC) actuators are attached on the inside-surface of the structure. Modal characteristics are studied in the air and under the water conditions and then equation of motion of the structure is derived from the test results. Structural vibration control performances of the proposed structure are evaluated via experiments with optimal control algorithm. Vibration control performances are presented both in the frequency and time domains.

• 한국전산구조공학회논문집
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• 제23권1호
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• pp.9-15
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• 2010

본 논문에서는 압전복합재 작동기가 표면에 부착된 Hull 구조물의 유한요소모델을 구성하여 동적 특성을 고찰하였으며, 구조물의 진동제어 특성을 평가하였다. Hull 구조물은 양 끝이 닫혀있는 실린더형 쉘 구조물을 고려하였으며, 항공기 동체나 잠수함과 같은 수중 구조물 등의 간단한 모델로 사용될 수 있다. 구조물의 진동제어를 위해 최근 NASA Langley 연구소에서 개발된 압전복합재인 Macro-Fiber Composite(MFC)를 적용하였다. MFC는 압전세라믹 섬유를 이용하여 유연성을 향상시키고, 맞물림 전극을 적용하여 면내 방향에서 큰 압전효과를 구현할 수 있도록 하였다. 유한요소모델을 바탕으로 구조물의 지배방정식을 도출하였으며, 동적 특성을 해석하여 실제 제작된 구조물의 실험결과와 비교 검증하였다. 최적제어 알고리즘을 구성하여 구조물의 진동제어 성능을 평가하였으며, 효과적으로 구조물의 진동을 제어할 수 있음을 확인하였다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2006년도 춘계학술대회논문집
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• pp.650-655
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• 2006

Dynamic modeling and active vibration control of smart hull structure using Macro Fiber Composite (MFC) actuator is conducted. Finite element modeling is used to obtain equations of motion and boundary effects of smart hull structure. Modal analysis is carried out to investigate the dynamic characteristics of the smart hull structure, and compared to the results of experimental investigation. Negative velocity feedback control algorithm is employed to investigate active damping of hull structure. It is observed that non-resonant vibration of hull structure is suppressed effectively by the MFC actuators.

• 한국소음진동공학회논문집
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• 제16권8호
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• pp.840-847
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• 2006

Dynamic modelingand active vibration control of smart hull structure using Macro Fiber Composite (MFC) actuators are conducted. Finite element modeling is used to obtain equations of motion and boundary effects of smart hull structure. Modal analysis is carried out to investigate the dynamic characteristics of the smart hull structure, and compared to the results of experimental investigation. Negative velocity feedback control algorithm is employed to investigate active damping of hull structure. It is observed that non-resonant vibration of hull structure is suppressed effectively by the MFC actuators.

• 한국해양공학회지
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• 제26권1호
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• pp.64-69
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• 2012

Dissimilar friction welds were produced using 15-mm diameter solid bars of chrome molybdenum steel (KS SCM440) and carbon steel (KS SM20C) to investigate their mechanical properties. The main friction welding parameters were selected to ensure good quality welds on the basis of visual examination, tensile tests, Vickers hardness surveys of the bond area and HAZ, and macro-structure investigations. The specimens were tested as-welded and post-weld heat treated (PWHT). The tensile strength of the friction welded steel bars was increased to 100% of the SM20C base metal under the condition of a heating time of more than four seconds. Optimal welding conditions were n = 2,000 (rpm), HP = 60 (MPa), UP = 100 (MPa), HT = 5 (s),and UT = 5 (s), when the total upset length was 7.8 (mm). The hardness distribution peak of the friction welded joints could be eliminated using PWHT. The two different kinds of materials were strongly mixed to show a well-combined structure of macro-particles, with no molten material, particle growth, or defects.

• 한국과학교육학회지
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• 제35권4호
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• pp.773-790
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• 2015

Conducted as an ethnographic examination of science teaching and learning in an Innovative Middle School in Korea, this study employs sociocultural theory to examine how structures afford and limit student participation in an innovative school designed to promote student-centered learning. Data includes teacher and student interviews, student responses to a questionnaire, classroom observations, and analysis of video recordings of ten lessons in two in two 8th grade science classes. Using structure|agency dialectic theory, we identify and describe some structures that afford and limit teacher and student agency at the micro (science classrooms), meso (school), and macro (Korean society) levels to raise some questions about current reform measures, such as innovation schools, that seek to position classroom teachers as agents for change in science education reform in Korea. Findings suggest that while teachers and school administrators play an essential role in structuring learning opportunities at the meso and micro levels, they have limited agency to address structural constraints originating at the macro-level, which can negatively impact teaching and learning in the science classroom. We offer implications for policy and practice and argue the need for more qualitative research, informed by sociocultural theory, to inform science education reform efforts in Korea.

• 한국전산구조공학회논문집
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• 제24권4호
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• pp.413-419
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• 2011

나노 크기의 허니콤 구조물은 거시적 크기의 허니콤 구조물에 비해 보다 높은 부피 대비 표면적 비율을 구현하여 전기적, 화학적인 촉매로써 기능성을 극대화할 수 있다. 나노 크기의 구조물은 거시적 크기의 구조물과 다른 기계적 거동양상을 보이며 이는 표면효과에 기인한다. 이러한 표면효과는 원자 수준 전산모사(atomistic simulation)를 통해 규명할 수 있으나 나노 허니콤 구조물의 거동을 예측하는 것은 현실적으로 과도한 전산자원 및 계산시간으로 인해 불가능한 실정이다. 본 연구에서는 표면응력 탄성모델을 적용한 브리징방법을 균질화기법과 연계하여 나노 크기의 허니콤 구조물의 기계적 거동을 효율적으로 예측하였다.

• Structural Engineering and Mechanics
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• 제59권2호
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• pp.291-312
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• 2016

Seismic performance evaluation of shear wall is essential as it is the major lateral load resisting member of a structure. The ultimate load and ultimate drift of the shear wall are the two most important parameters which need to be assessed experimentally and verified analytically. This paper comprises the results of monotonic tests, quasi-static cyclic tests and shake-table tests carried out on a midrise shear wall. The shear wall considered for the study is 1:5 scaled model of the shear wall of the internal structure of a reactor building. The analytical simulation of these tests is carried out using micro and macro modeling of the shear wall. This paper mainly consists of modification in the hysteretic macro model, developed for RC structural walls by Lestuzzi and Badoux in 2003. This modification is made by considering the stiffness degradation effect observed from the tests carried out and this modified model is then used for nonlinear dynamic analysis of the shear wall. The outcome of the paper gives the variation of the capacity, the failure patterns and the performance levels of the shear walls in all three types of tests. The change in the stiffness and the damping of the wall due to increased damage and cracking when subjected to seismic excitation is also highlighted in the paper.

• 한국재료학회지
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• 제19권6호
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• pp.300-304
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• 2009

The domain structures of annealed (001)-oriented $Pb(Mg_{1/3}Nb_{2/3})O_3-x%PbTiO_3$ (PMN-x%PT) crystals for x = 10, 20, 30, 35, and 40 at% were investigated by Polarized Optical Microscopy (POM) and Scanning Force Microscopy (SFM) in the piezoresponse mode. Both Polar Nano-Domains (PND) and long strip-like domains were clearly observed. The results also showed how the domain structure changed between phases with an increasing x in the PMN-x%PT crystals and the domain hierarchy on various length scales ranging from 40 nm to 0.1 mm. Distorted pseudo-cubic phase (x < 20%) consisted of PNDs that did not self-assemble into macro-domain plates. The rhombohedral phase (x = 30%) consisted of PNDs that began to self-assemble into colonies along preferred {110} planes. The monoclinic phase (x = 35%) consisted of miniature polar domains on the nm scale, whereas, the tetragonal phase (x = 40%) consisted of {001} oriented lamella domains on the mm scale that had internal nano-scale heterogeneities, which self-assembled into macro-domain plates oriented along {001} the mm scale.

• 소성∙가공
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• 제30권4호
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• pp.195-200
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• 2021

In this study, we investigated the dominant processing variables that would influence the microstructural development of AA6061 deformed by differential speed rolling (DSR) at ambient temperature. For this purpose, we carried out a series of DSR on the samples to investigate the effects of roll speed ratio, sample rotation, and number of operation under specific rotation. Among these, the condition with a height reduction of 75% at a speed ratio of 1:4 through rotation along rolling direction (RR) displayed the pronounced results of more homogenous matrix-structure and superior mechanical properties than the others tested in this study. This was mainly due to the cross shearing of macro-shear pattern in segment where dynamic recrystallization took place with ease throughout the sample. Thus, an average microhardness value of 101 Hv was obtained in the present sample deformed by 4-pass DSR with RR where macro cross-shearing was effectively applied.