• Earthquakes and Structures
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• 제12권1호
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• pp.13-21
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• 2017

Recent seismic events occurred in Italy (Emilia-Romagna 2012, Abruzzo 2009) and worldwide (New Zealand 2010 and 2011) highlighted some of the weaknesses of precast concrete industrial buildings, especially those related to the connecting systems traditionally employed to fasten the cladding panels to the internal framing. In fact, one of the most commons fails it is possible to observe in such structural typologies is related to the out-of-plane collapse of the external walls due to the unsatisfactory behaviour of the connectors used to join the panels to the perimeter beams. In this work, the strengthening of a traditional industrial building, assumed as a case study, made by precast reinforced concrete is proposed by the adoption of a dual system allowing the reinforcement of the structure by acting both internally; by pendular columns and, externally, on the walls. In particular, traditional connections at the top of the walls are substituted by devices able to work as a slider with vertical axis while, the bottom of the walls is equipped with two or more hysteretic dampers working on the uplift of the cladding panels occurring under seismic actions. By means of this approach, the structure is stiffened; obtaining a reduction of the lateral drifts under serviceability limit states. In addition, its seismic behaviour is improved due to the additional source of energy dissipation represented by the dampers located at the base of the walls. The effectiveness of the suggested retrofitting approach has been checked by comparing the performance of the retrofitted structure with those of the structure unreinforced by means of both pushover and Incremental Dynamic Analyses (IDA) in terms of behaviour factor, assumed as a measure of the ductility capacity of the structure.

• 한국재료학회지
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• 제18권7호
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• pp.373-378
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• 2008

$Ba(Ti,Sn)O_3$ thin films, for use as dielectrics for MLCCs, were grown from Sn doped BaTiO3 sources by e-beam evaporation. The crystalline phase, microstructure, dielectric and electrical properties of films were investigated as a function of the (Ti＋Sn)/Ba ratio. When $BaTiO_3$ sources doped with $20{\sim}50\;mol%$ of Sn were evaporated, $BaSnO_3$films were grown due to the higher vapor pressure of Ba and Sn than of Ti. However, it was possible to grow the $Ba(Ti,Sn)O_3$ thin films with {\leq}\;15\;mol%$ of Sn by co-evaporation of BTS and Ti metal sources. The (Ti＋Sn)/Ba and Sn/Ti ratio affected the microstructure and surface roughness of films and the dielectric constant increased with increasing Sn content. The dielectric constant and dissipation factor of $Ba(Ti,Sn)O_3$ thin films with {\leq}\;15\;mol%$ of Sn showed the range of 120 to 160 and $2.5{\sim}5.5%$ at 1 KHz, respectively. The leakage current density of films was order of the $10^{-9}{\sim}10^{-8}A/cm^2$ at 300 KV/cm. The research results showed that it was feasible to grow the $Ba(Ti,Sn)O_3$ thin films as dielectrics for MLCCs by an e-beam evaporation technique.

• Composites Research
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• 제30권3호
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• pp.202-208
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• 2017

RTM은 작은 크기에서 큰 크기의 복합재료를 생산할 수 있는 대량 생산 공정이다. 최근 경화 시간이 약 10분 이내인 속경화 수지가 자동차 및 항공우주 산업에 사용되고 있다. 수지의 점도는 경화 정도와 관련이 있으며, 금형 내부로 충진되는 과정에서 급격히 점도가 변할 수 있다. 따라서 속경화 수지의 유동 특성과 경화도를 해석적으로 예측하는 데 많은 노력이 필요하므로 실험적으로 측정하고 평가하는 것이 유리하다. 복합재료의 경화도를 측정하는 방법은 대표적으로 DMA, 유전기법 등이 있다. 본 논문에서는 다채널 경화 모니터링 시스템을 이용하여 속경화 수지의 유동과 경화도를 측정하였다. 총 8채널의 유전센서가 사용되었으며, 다양한 압력 조건에 따른 금형 내부의 유동과 경화도를 측정하고 상호 비교하였다.

• 대한기계학회논문집A
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• 제39권10호
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• pp.1011-1019
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• 2015

Insulated gate bipolar transistor (IGBT) 소자는 전동차, 항공기 및 전기 자동차에 가장 많이 사용되는 고전압, 고전력용 전력 반도체이다. 그러나 IGBT 전력소자는 동작 시 발열 온도가 매우 높고, 이로 인해, IGBT 소자의 신뢰성 및 성능에 큰 영향을 미치고 있다. 따라서 발열 문제를 해결하기 위한 IGBT 모듈 패키지의 방열 설계는 매우 핵심적인 기술이며, 특히, 소자가 동작 한계 온도에 올라가지 않도록 방열 설계를 적절히 수행하여야 한다. 본 논문에서는 전동차에 사용되는 1200 A, 3.3 kV 급 IGBT 모듈 패키지의 열 특성에 대해 수치해석을 이용하여 분석하였다. IGBT 모듈 패키지에 사용되는 다양한 재료 및 소재의 두께에 대한 영향을 분석하였으며, 실험계획법을 이용한 최적화 설계를 수행하였다. 이를 통하여 열 저항을 최소화하기 위한 최적의 방열 설계 가이드 라인을 제시하고자 하였다.

• Wind and Structures
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• 제31권2호
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• pp.85-102
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• 2020

Accurate load evaluation is essential in any performance-based design. Design wind speeds and associated wind loads are well defined for synoptic boundary layer winds but not for thunderstorms. The method presented in the current study represents a new approach to obtain design wind speeds associated with thunderstorms and their gust fronts using historical data and Monte Carlo simulations. The method consists of the following steps (i) developing a numerical model for thunderstorm downdrafts (i.e. downbursts) to account for storm translation and outflow dissipation, (ii) utilizing the model to characterize previous events and (iii) extrapolating the limited wind speed data to cover life-span of structures. The numerical model relies on a previously generated CFD wind field, which is validated using six documented thunderstorm events. The model suggests that 10 parameters are required to describe the characteristics of an event. The model is then utilized to analyze wind records obtained at Lubbock Preston Smith International Airport (KLBB) meteorological station to identify the thunderstorm parameters for this location, obtain their probability distributions, and utilized in the Monte Carlo simulation of thunderstorm gust front events for many thousands of years for the purpose of estimating design wind speeds. The analysis suggests a potential underestimation of design wind speeds when neglecting thunderstorm gust fronts, which is common practice in analyzing historical wind records. When compared to the design wind speed for a 700-year MRI in ASCE 7-10 and ASCE 7-16, the estimated wind speeds from the simulation were 10% and 11.5% higher, respectively.

• 한국지반신소재학회논문집
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• 제11권1호
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• pp.47-56
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• 2012

PBD의 통수능력은 지반조건 빛 두께에 비례하여 영향을 크게 받는 성질이 있으며, 배수재 타입 시 시용되는 맨드럴, 타입기기 및 방식에 의한 주변지반의 교란영향으로 배수성능이 저하되는 문제점 등이 있다. 또한, 여러 기지 조건에 따라 통수능의 편차가 크게 나타나며 원지반 조건에 따라 배수재의 통수능 저하, 간극수압소산의 시간지연 등의 분석이 미흡하다. 따리서 본 연구에서는 원지반 조건을 고려한 연직배수재의 복합 통수능 시험을 실시하여 지반개량 및 통수능 평가를 하였다. ${\phi}\;38{\times}70cm$ 크기의 원통형 실린더에 배수재를 고정한 후 교반한 시료를 투기하고 원지반 조건 CL, ML, SM과 준설토 CL을 매립한 후 가입장치를 사용하여 30, 70, 120 kpa의 하중을 가해 단계별로 복합통수능 실험을 수행하였다. 시험결과, 통수능의 크기는 원지반 조건이 SM>ML>CL>CL(준설토)인 순으로, 세립분 함량이 적을수록 통수량은 크게 나타났다.

• Smart Structures and Systems
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• 제19권5호
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• pp.487-497
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• 2017

Semi-active devices use the building's own motion to produce resistive forces and are thus strictly dissipative and require little power. Devices that independently control the binary open/closed valve state can enable novel device hysteresis loops that were not previously possible. However, some device hysteresis loops cannot be obtained without active analog valve control allowing slower, controlled release of stored energy, and is presents an ongoing limitation in obtaining the full range of possibilities offered by these devices. This in silico study develops a proportional-derivative feedback control law using a validated nonlinear device model to track an ideal diamond-shaped force-displacement response profile using active analog valve control. It is validated by comparison to the ideal shape for both sinusoidal and random seismic input motions. Structural application specific spectral analysis compares the performance for the non-linear, actively controlled case to those obtained with an ideal, linear model to validate that the potential performance will be retained when considering realistic nonlinear behaviour and the designed valve control approach. Results show tracking of the device force-displacement loop to within 3-5% of the desired ideal curve. Valve delay, rather than control law design, is the primary limiting factor, and analysis indicates a ratio of valve delay to structural period must be 1/10 or smaller to ensure adequate tracking, relating valve performance to structural period and overall device performance under control. Overall, the results show that active analog feedback control of energy release in these devices can significantly increase the range of resetable, valve-controlled semi-active device performance and hysteresis loops, in turn increasing their performance envelop and application space.

• 한국재료학회지
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• 제30권10호
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• pp.502-508
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• 2020

This paper focuses on the electrical properties and stability against DC accelerated aging stress of ZnO-V₂O₅-MnO₂-Nb₂O₅-Bi₂O₃-Co₃O₄-Dy₂O₃ (ZVMNBCD) varistor ceramics sintered at 850 - 925 ℃. With the increase of sintering temperature, the average grain size increases from 4.4 to 11.8 mm, and the density of the sintered pellets decreases from 5.53 to 5.40 g/㎤ due to the volatility of V₂O₅, which has a low melting point. The breakdown field abruptly decreases from 8016 to 1,715 V/cm with the increase of the sintering temperature. The maximum non-ohmic coefficient (59) is obtained when the sample is sintered at 875 ℃. The samples sintered at below 900 ℃ exhibit a relatively low leakage current, less than 60 mA/㎠. The apparent dielectric constant increases due to the increase of the average grain size with the increase of the sintering temperature. The change tendency of dissipation factor at 1 kHz according to the sintering temperature coincides with the tendency of the leakage current. In terms of stability, the samples sintered at 900 ℃ exhibit both high non-ohmic coefficient (45) and excellent stability, 0.8% in 𝚫E_B/E_B and -0.7 % in 𝚫α/α after application of DC accelerated aging stress (0.85 E_B/85 ℃/24 h).

• 한국재료학회지
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• 제5권7호
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• pp.816-819
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• 1995

저압 유기금속 화학 증탁법에 의하여 Pt/Ti/SiO$_2$/Si 기판위에 (Ba$_{1-x}$ , Sr$_{x}$)TiO/$_3$박막이 제조 되었다. 제조된 BST 박막의 결정화도는 증착온도가 증가함에 따라 (100)방향으로 우선 성장하였다. 90$0^{\circ}C$에서 증착한 BST 박막은 100kHz의 주파수에서 유전상수가 365, 유전손실이 0.052를 나타내었다. 인가전계에 따라 축전용량의 변화가 작은 상유전 특성을 보였으며 0.2MV/cm인가 전계에서 축적 전하 밀도(charge storage density)는 60fC/$\mu\textrm{m}$$^2$을, 0.15MV/cm인가 전계 영역에서 누설 전류밀도(leakage current density)는 20nA/$\textrm{cm}^2$을 나타냈다.냈다.

• 한국재료학회지
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• 제11권1호
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• pp.44-47
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• 2001

Ni 전극을 사용하는 $BaTiO_3$계 내환원성 X7R 조성에서 $Er_2$$O_3$ 첨가가 유전특성에 미치는 영향에 대하여 환원성 분위기에서 연구하였다. $MnO_2-MgO$가 첨가된 내환원조성에서 첨가량이 조절된 $Er_2O_3$의 복합첨가로 유전율의 온도안정성이 향상되어 X7R 규격을 만족시켰으며 2,970 이상의 상온 유전상수와 1.0% 이하의 유전손실율이 관찰되었다. $Er_2O_3$가 3.0 mol% 이상으로 과량 첨가되었을 경우 유전체의 온도특성은 향상되었으나 상온 유전상수가 현저히 감소하였다. 다른 첨가조성(1.5 mol% $Er_2O_3$2.0 mol% MgO)이 고정될 때 TCC곡선은 $MnO_2$첨가량이 증가함에 따라 시계방향으로 회전하였으며, 온도안정성을 향상시켰다.