• 한국건축시공학회지
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• 제6권2호
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• pp.73-79
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• 2006

The purpose of this study is to enhance the development of construction waste-recycling technologies and its economical efficiency by developing environment-friendly tetrapod, precast concrete, where recycled aggregate is used in order to promote recycling of waste concrete. The results of concrete mechanic characteristics experiments by the circulation coarse aggregate-replacement ratio are as the following. The circulation aggregate is lower and higher than natural aggregate in specific gravity and absorption ratio, respectively so that in case of mix proportioning, unit volume increases, while unit aggregate amount decreases. From the result, sufficient experiments of physical characteristics of circulation aggregate are required to get proper mix proportioning. When circulation aggregate-replacement ratio increases, compressive strength tends to decrease comprehensively, but 50% of replacement ratio is good enough to use. When circulation coarse aggregate's replacement ratio is 0%, drying shrinkage, which causes cracks in concrete and deteriorates durability, shows the minimum length change and the higher the ratio, the larger the length change. Thus. when using circulation coarse aggregate, drying shrinkage should be fairly examined. In freezing-and-thawing resistance, weight loss tends to comprehensively increase its loss at the circulation aggregate-mixed site. And the examination of surface aggregate-omission ratio is further needed and dynamic elastic modulus and durability factor(DF) require more study as well. In order to use circulation aggregate to tetrapod, a clear standard for strength should be first prepared and at the same time, more study about durability is needed.

• Advanced Composite Materials
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• 제17권2호
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• pp.157-166
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• 2008

In this study, multiwalled carbon nanotubes (MWCNTs) were compounded with the poly($\varepsilon$-caprolactone) (PCL) matrix at the solution state using chloroform. For homogeneous dispersion of MWCNTs in polymer matrix, oxygen-containing groups were introduced on the surface of MWCNTs. The mechanical properties of the PCL/MWCNTs composites were effectively increased due to the incorporation of MWCNTs. The composites were characterized using scanning electron microscopy in order to obtain information on the dispersion of MWCNT in the polymeric matrix. In case of 1.2 wt% of MWCNTs in the matrix, strength and modulus of the composite increased by 12.1% and 164.3%, respectively. In addition, the dispersion of MWCNTs in the PCL matrix resulted in substantial decrease of the electrical resistivity of the composites as the MWCNTs loading was increased from 0 to 2.0 wt%. Furthermore, thermal stability of the PCL and PCL/MWCNTs-COOH composites were investigated using the data acquired from the thermogravimetric analysis. The detailed kinetics of the thermal degradation of the composites was investigated by analyzing their thermal behavior at different heating rates in a nitrogen atmosphere. Activation energy of thermal degradation was determined by using the equations proposed by Kissinger and Flynn-Wall-Ozawa. The apparent activation energy of PCL/MWCNTs-COOH composite was considerably higher than that of neat PCL.

• 대한조선학회논문집
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• 제50권4호
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• pp.224-231
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• 2013

The concept of the natural frequency is useful for understanding the characters of oscillating systems. However, when a circular cylinder floating horizontally on the water surface is heaving, due to the hydrodynamic forces, the system is not governed by the equation like that of the harmonic one. In this paper, in order to shed some lights on the more correct use of the concept of the natural frequency, a problem of the heaving circular cylinder is analyzed in the time domain. The equation of motion, an integro-differential equation, was derived following the fashion of Cummins (1962), and its coefficients including the retardation function were obtained using the numerical solution of Lee (2012). The equation was solved numerically, and the experiment was also carried out in the CNU flume. Using our numerical and experimental results, the natural frequency was defined as its average value given by the motion data excluding those of the initial stage. Our results were then compared with those of the existing investigations such as Maskell and Ursell (1970), Ito (1977) and Yeung (1982) as well as the newly obtained results of Lee (2012). Comparison showed that the natural frequency obtained here agrees well with that of Lee (2012), which was found through the frequency domain analysis. It was also shown that the approximation of heaving motion by a damped harmonic oscillation, which was regarded as suitable by most previous investigators, is not physically suitable for the reason that can be clearly shown through comparing the shape of MCFRs(Modulus of Complex Frequency Response). Furthermore, we found that although the previous approximations yield the damping ratio significantly different from our result the magnitude of natural frequency is not much different from our result.

• 한국진공학회지
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• 제21권4호
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• pp.193-198
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• 2012

플라즈마화학기상증착 장치를 이용하여 플라즈마 폴리머와 다이아몬드상 탄소(diamond-like carbon, DLC) 박막을 합성하였다. 플라즈마 폴리머 박막 위에 패시베이션 층으로 DLC 박막을 두께에 따라 합성하였고, DLC/플라즈마 폴리머 박막의 구조적, 물리적, 전기적 특성들을 고찰하였다. 기존 플라즈마 폴리머는 누설 전류 특성이 좋고 낮은 유전상수 값을 가지고 있다. 그러나 실제 반도체 공정에 적용되기 위해서는 물리적 특성도 만족되어야 하기 때문에 플라즈마 폴리머 박막 위에 DLC 패시베이션을 적용하여 플라즈마 폴리머의 물리적, 전기적 특성들을 향상시키고자 하였다. DLC 박막의 두께가 증가함에 따라 플라즈마 폴리머의 경도와 탄성계수 값은 증가하였고, root-mean-square 표면거칠기 값은 감소하고 접촉각은 증가하였다. DLC 패시베이션 되어진 플라즈마 폴리머의 경우 패시베이션이 없는 폴리머보다 유전상수 값이 증가하였지만 전기적 누설전류 특성은 향상되었다.

• 한국진공학회지
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• 제19권2호
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• pp.105-113
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• 2010

일차원 나노튜브는 구조는 높은 비표면적, 내부의 빈 공간 및 특유의 물리적 특징을 제공한다. 1차원 산화물 나노튜브 구조물의 합성 방법에 따라 나누어 정리하여 논의하였다. 나노 기판을 이용한 나노튜브 합성은 고른 물리적 구조를 가지는 나노튜브를 대량으로 합성하는데 있어서 이상적인 방법으로서 기판의 형태를 상대적으로 용의하게 조절함으로써 1차원 나노튜브 구조물의 특성을 극대화하였다. 극대화된 특성을 이용한 여러 응용 분야에 대한 연구를 정리하여 제시하였다.

• International Journal of Fluid Machinery and Systems
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• 제3권4호
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• pp.342-351
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• 2010

During the operation of a hydro turbine the fluid mechanical pressure loading on the turbine blades provides the driving torque on the turbine shaft. This fluid loading results in a structural load on the component which in turn causes the turbine blade to deflect. Classically, these mechanical stresses and deflections are calculated by means of finite element analysis (FEA) which applies the pressure distribution on the blade surface calculated by computational fluid dynamics (CFD) as a major boundary condition. Such an approach can be seen as a one-way coupled simulation of the fluid structure interaction (FSI) problem. In this analysis the reverse influence of the deformation on the fluid is generally neglected. Especially in axial machines the blade deformation can result in a significant impact on the turbine performance. The present paper analyzes this influence by means of fully two-way coupled FSI simulations of a propeller turbine utilizing two different approaches. The configuration has been simulated by coupling the two commercial solvers ANSYS CFX for the fluid mechanical simulation with ANSYS Classic for the structure mechanical simulation. A detailed comparison of the results for various blade stiffness by means of changing Young's Modulus are presented. The influence of the blade deformation on the runner discharge and performance will be discussed and shows for the configuration investigated no significant influence under normal structural conditions. This study also highlights that a two-way coupled fluid structure interaction simulation of a real engineering configuration is still a challenging task for today's commercially available simulation tools.

• Korean Chemical Engineering Research
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• 제51권3호
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• pp.382-387
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• 2013

압출 컴파운딩 공정 및 케스팅 필름 공정을 이용하여 생분해성 폴리우레탄(PU)/클레이 나노복합 필름을 제조하였다. PU 수지와의 강한 결합 형성을 위해 유기적으로 개질되어 그 표면에 많은 양의 히드록시기를 갖는 MMT 나노클레이(C30B)를 사용하였다. 압출 공정 중 발생된 높은 전단 응력에 의해 발현된 복합체 내 나노판상체의 삽입/박리 구조 및 분산 상태를 XRD 분석 및 TEM 관찰을 통해 확인하였다. 또한 제조된 나노복합체의 유변물성, 인장물성, 투명성, 산소투과도의 변화를 첨가된 나노클레이 함량에 따라 조사하였으며, 이로부터 나노복합체 내 나노판상체의 박리 및 분산 구조와 물성과의 상관 관계를 제시할 수 있었다. 일정수준의 함량으로 첨가된 나노클레이는 복합 필름의 인장 탄성율, 연신율, 투명성, 산소차단성 등의 성능 향상에 뚜렷하게 기여하였으나, 그 이상의 함량으로 첨가되면 불완전한 박리 및 불균질한 분산성으로 인하여 오히려 성능이 감소하거나 또는 그 증가 폭이 매우 작은 것으로 나타났다. PU/clay 나노복합 필름의 생분해성은 퇴비화 실험을 통한 분해시간에 따른 필름의 산소투과도 및 인장물성의 변화를 관찰함으로써 확인하였다.

• 한국산학기술학회논문지
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• 제16권2호
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• pp.921-926
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• 2015

엠보싱된 알루미늄 판재는 표면적이 증가하여 방열효과가 뛰어나고 가공경화에 의해 굽힘강성이 증가하여 자동차 열차단기에 널리 사용된다. 하지만 판재의 특성상 주름발생률이 높아 프레스 성형에 많은 제약이 따른다. 본 연구에서는 3차원 구조 알루미늄 판재의 프레스 성형성 평가를 위한 기초연구로 3차원 구조 엠보싱 콘 형상 판재의 기계적특성을 평가하고 굽힘실험을 통해 프레스 가공 후 발생하는 스프링백을 정량적으로 평가하였다. 엠보싱 판재는 패턴의 방향에 따라 인장특성이 상이하다. 특히 평행 엠보싱 시편의 경우 항복응력이 감소하며 대각 엠보싱 시편의 경우 항복응력이 증가하게 되고 영률의 감소가 크게 나타난다. 그 결과 굽힘 성형가공 후에 스프링 백에 영향을 미친다.

• 한국도로학회논문집
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• 제18권1호
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• pp.63-72
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• 2016

PURPOSES : The use of roller-compacted concrete pavement (RCCP) is an environmentally friendly method of construction that utilizes the aggregate interlock effect by means of a hydration reaction and roller compacting, demonstrating a superb structural performance with a relatively small unit water content and unit cement content. However, even if an excellent structural performance was secured through a previous study, the verification research on the environmental load and long-term durability was conducted under unsatisfactory conditions. In order to secure longterm durability, the construction of an appropriate internal air-void structure is required. In this study, a method of improving the long-term durability of RCCP will be suggested by analyzing the internal air-void structure and relevant durability of roller-compacted concrete. METHODS : The method of improving the long-term durability involves measurements of the air content, air voids, and air-spacing factor in RCCP that experiences a change in terms of the kind of air-entraining agent and chemical admixture proportions. This test should be conducted on the basis of test criteria such as ASTM C 457, 672, and KS F 2456. RESULTS : Freezing, thawing, and scaling resistance tests of roller compacted concrete without a chemical admixture showed that it was weak. However, as a result of conducting air entraining (AE) with an AE agent, a large amount of air was distributed with a range of 2~3%, and an air void spacing factor ranging from 200 to $300{\mu}m$ (close to $250{\mu}m$) coming from PCA was secured. Accordingly, the freezing and thawing resistance was improved, with a relative dynamic elastic modulus of more than 80%, and the scaling resistance was improved under the appropriate AE agent content rate. CONCLUSIONS : The long-term durability of RCCP has a direct relationship with the air-void spacing factor, and it can be secured only by ensuring the air void spacing factor through air entraining with the inclusion of an AE agent.

• Bulletin of the Korean Chemical Society
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• 제2권4호
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• pp.142-147
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• 1981

A blend polymeric system composed of poly(methyl methacrylate) (PMMA or PM) and polystyrene (PS) dissolved in chloroform was rheologically studied. The viscosities ${\eta}_{bl}$ of the blend system with various blending ratios ${\chi}$ changing from zero (pure PS solution) to unity (pure PMMA solution) were measured at $25{\circ}C$ as a function of shear rates ${\dot{s}}$ by using a Couette type viscometer. ${\eta}_{bl}$ at a given ${\dot{s}}$ decreased exponentially with ${\chi}$ reaching asymptotic constant value of ${\eta}_{bl}$ ; ${\eta}_{bl}$ at a given ${\chi}$ is greater at a smaller ${\dot{s}}$. These results are explained by using Ree-Erying's theory of viscosity, ${\eta}_{bl}=(x_1{\beta}_1/{\alpha}_1)_{b}_1＋ (x_2{\beta}_2/{\alpha}_2)_{bl}[sinh^{-1}{\beta}_2(bl) {\dot{s}}]/{\beta}_2(bl){\dot{s}}$. The Gibbs activation energy ${\Delta}G_i^\neq$(i = 2 for non-Newtonian units) entering into the intrinsic relaxation time ${\beta}$ is represented by a linear combination ${\Delta}G_i^\neq(bl) ={\chi}{\Delta}G_i^{\neq}_{iPM}+(1-{\chi}){\Delta}G_i^{\neq}_{iPS}$;the intrinsic shear modulus$[[\alpha}_i]^{-1}$ is also represented by $[{\alpha}_i(bl)]^{-1}={\chi}[{\alpha}_{iPM}]^{-1}+(1-{\chi})[{\alpha}_{iPS}]^{-1}$ and the fraction of area on a shear surface occupied by the ith flow units $x_i(bl)$ is similarly represented, i.e., $x_i(bl) = {\chi}x_{iPM}+(1-{\chi})x_{iPS}$. By using these ideas the Ree-Eyring equation was rewritten which explained the experimental results satisfactorily.