• Earthquakes and Structures
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• 제15권5호
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• pp.453-462
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• 2018

Damages to buildings affected by a near-fault strong ground motion are largely attributed to the vertical component of the earthquake resulting in column failures, which could lead to disproportionate building catastrophic collapse in a progressive fashion. Recently, considerable interests are awakening to study effects of earthquake vertical components on structural responses. In this study, detailed modeling and time-history analyses of a 12-story code-conforming reinforced concrete moment frame building carrying the gravity loads, and exposed to once only the horizontal component of, and second time simultaneously the horizontal and vertical components of an ensemble of far-field and near-field earthquakes are conducted. Structural responses inclusive of tension, compression and its fluctuations in columns, the ratio of shear demand to capacity in columns and peak mid-span moment demand in beams are compared with and without the presence of the vertical component of earthquake records. The influences of the existence of earthquake vertical component in both exterior and interior spans are separately studied. Thereafter, the correlation between the increase of demands induced by the vertical component of the earthquake and the ratio of a set of earthquake record characteristic parameters is investigated. It is shown that uplift initiation and the magnitude of tensile forces developed in corner columns are relatively more critical. Presence of vertical component of earthquake leads to a drop in minimum compressive force and initiation of tension in columns. The magnitude of this reduction in the most critical case is recorded on average 84% under near-fault ground motions. Besides, the presence of earthquake vertical components increases the shear capacity required in columns, which is at most 31%. In the best case, a direct correlation of 95% between the increase of the maximum compressive force and the ratio of vertical to horizontal 'effective peak acceleration (EPA)' is observed.

• 한국초전도ㆍ저온공학회논문지
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• 제14권2호
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• pp.12-15
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• 2012

This paper presents simulation and small-scale experimental tests of a fault current controller. Smart fault controller as proposed and proven conceptually in our previous work is promising technology for the smart power grid where distributed and even stochastic generation sources are prevalent and grid operations are more dynamic. Existing protection schemes simply limiting the fault current to the pre-determined set values may not show best performance and even lead to coordination failures, potentially leading to catastrophic failure. Thus, this paper designs fault current controller with a full bridge thyristor rectifier, embedding a superconducting coil for which the controller is electrically invisible during normal operation because the loss due to the coil is near-zero. When a fault occurs and the resulting current through the superconducting coil exceeds a certain value set intelligently based on the current operating condition of the grid, the magnitude of the fault current is controlled to this desired value by adjusting the firing angles of thyristors such that the overall system integrity is successfully maintained. Detailed time-domain simulations are performed and lab-scale testing circuits are built to demonstrate the desired functionality and efficacy of the proposed fault current controller.

• 대한기계학회논문집A
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• 제38권4호
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• pp.379-384
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• 2014

가스터빈엔진의 가장 핵심 부품인 디스크와 블레이드는 고온, 고압축비, 고속 회전이라는 가혹한 환경에서 지속적으로 운용된다. 이러한 가혹한 환경과 디스크와 블레이드가 가지는 큰 회전 에너지로 인해 디스크 및 블레이드에 의해 유발되는 파손은 항공기 손상 혹은 탑승자의 피해로 이어지는 재해적 고장 혹은 한계 고장으로 이어진다. 그러므로 디스크와 블레이드의 구조적 건전성의 마진을 충분히 확보하기 위해서 본 연구에서는 디스크의 취약 부위인 도브테일의 형상을 최적화하고, 그 해의 강건성을 확인하기 위해 치수 공차와 피로 수명의 산포와 같은 불확실성에 대하여 신뢰도 해석을 수행하고자 한다. 이 결과를 통해 결정론적 방법인 최적설계의 필요성과 함께 한계를 확인하고, 향후 신뢰도 기반 최적설계의 필요성을 인지하고자 한다. 이를 위해 비선형 열-구조 연성해석과 접촉 해석을 포함한 유한요소해석을 수행하였다.

• Steel and Composite Structures
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• 제19권3호
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• pp.569-583
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• 2015

Recent years, explosive welding structures have been widely used in many engineering fields. The bonding state detection of explosive welding structures is significant to prevent unscheduled failures and even catastrophic accidents. However, this task still faces challenges due to the complexity of the bonding interface. In this paper, a new method called dual-tree complex wavelet transform based permutation entropy (DTCWT-PE) is proposed to detect bonding state of such structures. Benefiting from the complex analytical wavelet function, the dual-tree complex wavelet transform (DTCWT) has better shift invariance and reduced spectral aliasing compared with the traditional wavelet transform. All those characters are good for characterizing the vibration response signals. Furthermore, as a statistical measure, permutation entropy (PE) quantifies the complexity of non-stationary signals through phase space reconstruction, and thus it can be used as a viable tool to detect the change of bonding state. In order to more accurate identification and detection of bonding state, PE values derived from DTCWT coefficients are proposed to extract the state information from the vibration response signal of explosive welding structure, and then the extracted PE values serve as input vectors of support vector machine (SVM) to identify the bonding state of the structure. The experiments on bonding state detection of explosive welding pipes are presented to illustrate the feasibility and effectiveness of the proposed method.

• Smart Structures and Systems
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• 제15권1호
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• pp.135-150
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• 2015

Large concrete structures are prone to cracks and damages over time from human usage, weathers, and other environmental attacks such as flood, earthquakes, and hurricanes. The health of the concrete structures should be monitored regularly to ensure safety. A reliable method of real time communications can facilitate more frequent structural health monitoring (SHM) updates from hard to reach positions, enabling crack detections of embedded concrete structures as they occur to avoid catastrophic failures. By implementing an unconventional mode of communication that utilizes guided stress waves traveling along the concrete structure itself, we may be able to free structural health monitoring from costly (re-)installation of communication wires. In stress-wave communications, piezoelectric transducers can act as actuators and sensors to send and receive modulated signals carrying concrete status information. The new generation of lead zirconate titanate (PZT) based smart aggregates cause multipath propagation in the homogeneous concrete channel, which presents both an opportunity and a challenge for multiple sensors communication. We propose a time reversal based pulse position modulation (TR-PPM) communication for stress wave communication within the concrete structure to combat multipath channel dispersion. Experimental results demonstrate successful transmission and recovery of TR-PPM using stress waves. Compared with PPM, we can achieve higher data rate and longer link distance via TR-PPM. Furthermore, TR-PPM remains effective under low signal-to-noise (SNR) ratio. This work also lays the foundation for implementing multiple-input multiple-output (MIMO) stress wave communication networks in concrete channels.

• 한국ITS학회 논문지
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• 제16권2호
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• pp.128-139
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• 2017

철도는 항공기, 선박 등과 더불어 대표적 대중교통 수단으로서 최근 고속 철도의 등장으로 인해 그 비중이 점점 더 높아지고 있으며, 아울러 대형사고의 위험 또한 증가하고 있다. 이중에서 철도 차량의 차축 베어링은 높은 안전성이 요구되는 부품으로서 최근 이의 고장예측을 위한 건전성 관리기술(Prognostics and Health Management, PHM)에 많은 연구가 집중되고 있다. PHM은 센서를 통해 얻은 데이터로부터 결함관련 특징신호를 추출하고 현재의 고장수준 진단과 미래의 고장싯점을 예측하는 기술로서, 이중에서 가장 중요한 부분은 올바른 특징신호를 추출하는 것이다. 그러나 지금까지의 특징신호들은 잡음으로 인한 심한 변동이나 비단조 경향으로 인해 고장예측에 이용하기에 부족한 점이 있었다. 본 연구에서는 이를 극복하기 위해 주파수 에너지 이동현상을 기반으로 정보 엔트로피를 특징신호로 사용하는 새로운 특징신호 추출법을 개발하고 IEEE 2012 PHM 경진대회에서 공개된 FEMTO 베어링 수명시험 데이터를 대상으로 기존의 특징신호들과 고장예측 성능비교를 함으로써 그 우수성을 검증하였다.

• 구강회복응용과학지
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• 제16권1호
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• pp.13-25
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• 2000

Optimal Pressable Ceramic is one of the all-ceramic restorations with a shaded translucent pressed core and layering porcelains. The purpose of this study was to evaluate the marginal fidelity according to margin types and measurement sites, and to evaluate fracture strength according to margin types. Twenty seven OPC crowns made according to 3 types of cervical finishing lines were used in this study. Marginal gaps were measured before and after cementation. A Steromicroscope(SZ-ST(R), Olympus, Japan) was used to measure the space between the margin of OPC crown and the finishing line of metal model. Marginal gaps were measured at the labial, mesial, lingual and distal site, which were demonstrated in advance. Fracture strength testing was carried out using an Instron(Model M100EC, Mecmesin, England) at a cross head speed of 5 mm/min. All crowns were loaded until catastrophic failures occurred. The result were as follow: 1. In comparison according to variable margin before cementation, the marginal gap were increased in chamfer margin($47.50{\pm}18.39{\mu}m$), $120^{\circ}$shoulder margin ($55.21{\pm}14.4{\mu}m$) and $90^{\circ}$shoulder margin($71.18{\pm}13.30{\mu}m$) in ascending order, and there were significant differences between chamfer margin and $90^{\circ}$shoulder margin, $120^{\circ}$shoulder margin and between $120^{\circ}$shoulder margin and $90^{\circ}$shoulder margin respectively(p<0.05). 2. In comparison according to variable margin after cementation, the gap discrepancies were increased in chamfer margin($60.78{\pm}30.37{\mu}m$), $120^{\circ}$shoulder margin($66.67{\pm}11.18{\mu}m$) and $90^{\circ}$shoulder margin($85.78{\pm}17.23{\mu}m$) in ascending order, but there was significant difference only between chamfer margin and $90^{\circ}$shoulder margin(p<0.05). 3. Labio-lingual points showed a better marginal fidelity than that of proximal point(p<0.05). 4. Chamfer margin($48.76{\pm}8.45kgf$) showed higher fracture strength than $120^{\circ}$ shoulder margin($40.57{\pm}7.90kgf$) and $90^{\circ}$ shoulder margin(32.7.90kgf) (p<0.05), but there was significant difference only between chamfer margin and $90^{\circ}$ shoulder margin(p<0.05).