• Steel and Composite Structures
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• 제19권2호
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• pp.369-384
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• 2015

This work presents a free vibration analysis of functionally graded metal-ceramic (FG) beams with considering porosities that may possibly occur inside the functionally graded materials (FGMs) during their fabrication. For this purpose, a simple displacement field based on higher order shear deformation theory is implemented. The proposed theory is based on the assumption that the transverse displacements consist of bending and shear components in which the bending components do not contribute toward shear forces and, likewise, the shear components do not contribute toward bending moments. The most interesting feature of this theory is that it accounts for a quadratic variation of the transverse shear strains across the thickness, and satisfies the zero traction boundary conditions on the top and bottom surfaces of the beam without using shear correction factors. In addition, it has strong similarities with Euler-Bernoulli beam theory in some aspects such as equations of motion, boundary conditions, and stress resultant expressions. The rule of mixture is modified to describe and approximate material properties of the FG beams with porosity phases. By employing the Hamilton's principle, governing equations of motion for coupled axial-shear-flexural response are determined. The validity of the present theory is investigated by comparing some of the present results with those of the first-order and the other higher-order theories reported in the literature. Illustrative examples are given also to show the effects of varying gradients, porosity volume fraction, aspect ratios, and thickness to length ratios on the free vibration of the FG beams.

• 대한조선학회논문집
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• 제58권5호
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• pp.294-302
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• 2021

In this study, the 3D printing technique called design for additive manufacturing (DfAM) that is widely used in various industries was applied to marine leisure ships of equipment. The DfAM for the stanchion for crew safety was applied to the equipment used in an actual recreational craft. As design constraints, the design alternatives were not to exceed the safety and weight of the existing stainless steel material, which were reviewed, and the production of a low-cost FFF-type 3D printing method that can be used even in small shipyards was considered. Until now, additive manufacturing has been used for manufacturing only prototypes owing to its limitations of high manufacturing cost and low strength; however, in this study, it was applied to the mass production process to replace existing products. Thus, a design was developed with low manufacturing cost, adequate performance maintenance, and increased design freedom, and the optimal design was derived via structural analysis comparisons for each design alternative. In addition, a life-cycle assessment based on the ISO 1404X was conducted to develop sustainable products. Through this study, the effectiveness of additive manufacturing was examined for future applications in the shipbuilding industry.

• 대한금속재료학회지
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• 제49권6호
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• pp.474-487
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• 2011

Bars of OFHC Cu with the diameter of 45 mm were processed by equal channel angular pressing up to 16 passes via route $B_c$, and homogeneity of their microstructures and mechanical properties was examined at every four passes which develop the equiaxed ultrafine grains. In general, overall hardness, yield strength and tensile strength increased by 3, 7, and 2 times respectively compared with those of unECAPed sample. Cross-sectional hardness exhibited a concentric distribution. Hardness was the highest at the center of bar and it decreased gradually from center to surface. After 16 passes, overall hardness decreased due to recovery and partial recrystallization. Regardless of the number of passage, yield strength and tensile strength were quite uniform at all positions, but elongation showed some degree of scattering. At 4 passes, coarse and ultrafine grains coexisted at all positions. After 4 passes, uniform equiaxed ultrafine grains were obtained at the center, while uniform elongated ultrafine grains were manifested at the upper half position. At the lower half position, grains were equiaxed but its size were inhomogeneous. It was found that inhomogeneity of grain morphology and grain size distribution at different positions are to be attributed to scattering in elongation but they did not affect strength. The present results reveal the high potential of practical application of equal channel angular pressing on fabrication of large-sized ultrafine grained bars with quite homogeneous mechanical properties.

• Advances in nano research
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• 제13권1호
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• pp.97-111
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• 2022

In the present article, silica nanoparticles (SNPs) were exploited to improve the tribological and mechanical properties of vinyl ester/glass fiber composites. To the best of our knowledge, there hasn't been any prior study on the wear properties of glass fiber reinforced vinyl ester SiO₂ nanocomposites. The wear resistance is a critical concern in many industries which needs to be managed effectively to reduce high costs. To examine the influence of SNPs on the mechanical properties, seven different weight percentages of vinyl ester/nano-silica composites were initially fabricated. Afterward, based on the tensile testing results of the silica nanocomposites, four wt% of SNPs were selected to fabricate a ternary composite composed of vinyl ester/glass fiber/nano-silica using vacuum-assisted resin transfer molding. At the next stage, the tensile, three-point flexural, Charpy impact, and pin-on-disk wear tests were performed on the ternary composites. The fractured surfaces were analyzed by scanning electron microscopy (SEM) images after conducting previous tests. The most important and interesting result of this study was the development of a nanocomposite that exhibited a 52.2% decrease in the mean coefficient of friction (COF) by augmenting the SNPs, which is beneficial for the fabrication/repair of composite/steel energy pipelines as well as hydraulic and pneumatic pipe systems conveying abrasive materials. Moreover, the weight loss due to wearing the ternary composite containing one wt% of SNPs was significantly reduced by 70%. Such enhanced property of the fabricated nanocomposite may also be an important design factor for marine structures, bridges, and transportation of wind turbine blades.

• 한국건설관리학회논문집
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• 제23권5호
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• pp.98-107
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• 2022

본 연구의 목적은 플랜트 건설사업에서 모듈러 설계대안에 대한 위험기반의 평가지수를 도출하는 모델을 제안하는 것이다. 이에 따라 ① 기초사업비와 모듈을 적용하는 대상과 범위를 설정하는 단계, ② 설계, 구매, 제작, 운송, 시공업무에 대한 중요도, 편리성, 효율성 지수를 평가하는 단계, ③ 중요 위험사건의 영향에 따른 경제적 손실을 예방하기 위해서 필요한 예비비를 분석하여 총사업비를 추정하는 단계, ④ 모듈 설계대안별로 평가된 관리업무의 효율성 지수와 총사업비를 비교하여 RVI를 도출하는 단계로 구성하였다. 그리고 모델의 사용성을 검증하기 위해서 Module Plan-A는 Pipe Rack을 모듈로 적용하는 경우, Module Plan-B는 Steel Structure, Pipe Rack, Process Module을 모듈로 적용하는 경우로 설정하여 관리업무의 중요도, 편리성, 효율성 지수를 평가하고 위험예비비를 포함하여 총사업비를 추정하였다. 이와 같은 과정으로 모델을 검증한 결과, 1개의 공종에 모듈을 적용한 Module Plan-A 보다 3개의 공종에 모듈을 적용한 Module Plan-B가 관리업무의 효율성은 낮은 것으로 나타났지만, 기초사업비에서 간접비가 절감되어 종합적인 RVI는 높게 평가되었다.

• 센서학회지
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• 제30권4호
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• pp.267-272
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• 2021

In this study, Ag was deposited to investigate its applicability as a surface-enhanced Raman scattering substrate after forming a grass-type black silicon structure through maskless reactive ion etching. Grass-structured black silicon with heights of 2 - 7 ㎛ was formed at radio-frequency (RF) power of 150 - 170 W. The process pressure was 250 mTorr, the O₂/SF₆ gas ratio was 15/37.5, and the processing time was 10 - 20 min. When the processing time was increased by more than 20 min, the self-masking of Si_xO_yF_z did not occur, and the black silicon structure was therefore not formed. Raman response characteristics were measured based on the Ag thickness deposited on a black silicon substrate. As the Ag thickness increased, the characteristic peak intensity increased. When the Ag thickness deposited on the black silicon substrate increased from 40 to 80 nm, the Raman response intensity at a Raman wavelength of 1507 / cm increased from 8.2 × 10³ to 25 × 10³ cps. When the Ag thickness was 150 nm, the increase declined to 30 × 10³ cps and showed a saturation tendency. When the RF power increased from 150 to 170 W, the response intensity at a 1507/cm Raman wavelength slightly increased from 30 × 10³ to 33 × 10³ cps. However, when the RF power was 200 W, the Raman response intensity decreased significantly to 6.2 × 10³ cps.

• Tribology and Lubricants
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• 제38권5호
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• pp.185-190
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• 2022

Among the operating limits of a heat pipe, the capillary limit is significantly affected by the characteristics of the wick, which is determined by the capillary performance. The major parameters for determining capillary performance are the maximum capillary pressure and the spreading characteristics that can be expected through the wick. A well-designed wick structure improves capillary performance and helps improve the stability of the heat pipe by enhancing the capillary limit. The capillary performance can be improved by forming a porous microstructure on the surface of the wick structure through surface modification techniques. In this study, a microstructure is formed on the surface of the wick by using a surface modification method (i.e., an electrochemical etching process). In the experiment, specimens are prepared using stainless-steel screen mesh wicks with various fabrication conditions. In addition, the spreading and capillary rise performances are observed with low-surface-tension fluid to quantify the capillary performance. In the experiments, the capillary performance, such as spreading characteristics, maximum capillary pressure, and capillary rise rate, improves in the specimens with microstructures formed through surface modification compared with the specimens without microstructures on the surface. The improved capillary performance can have a positive effect on the capillary limit of the heat pipe. It is believed that the surface microstructures can enhance the operational stability of heat pipes.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2020년도 봄 학술논문 발표대회
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• pp.14-15
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• 2020

The need for off-site construction has increased in the construction industry in Korea in recent years due to the enforcement of the 52-hour workweek, the increasing age of workers on construction sites, the deepening dependence on overseas workers, and the stagnation of productivity in the construction industry. Thus, studies on OSC started in April 2020. In addition, as a national policy study advocating the modular construction method, which is one of the OSC methods, was completed in the first half of 2019. 70 housing units in two complexes that satisfied the Housing Act requirements have been supplied to citizens. However, although modular construction methods have been recognized as a dramatic construction method that achieves shortened construction schedules and solves the issues of cost reduction and the shortage of technical workers on sites by combining the advantages of the manufacturing industry and applying the economies of scale, realistically it has issues due to the rising cost of steel and a low pre-fabrication ratio. Moreover, the construction time of core parts, such as those built by pouring concrete, has become a factor that hinders the shortening of construction times. Thus, this study aims to propose a precast concrete(PC) modular construction system, which fuses three-dimensional infill as an interior finish material and a three-surface PC module that can acts as a structure for a construction method that is economical and can shorten construction time.

• 한국강구조학회 논문집
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• 제27권6호
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• pp.513-523
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• 2015

H형강 철골 모멘트 골조에서는 기둥의 강축 방향 용접 모멘트접합부를 활용하는 것이 구조적으로 가장 이상적이다. 하지만, 모멘트 골조가 직교하는 경우, 기둥의 약축 방향으로도 용접 모멘트접합부를 사용해야 한다. 국내에서는 특히 강축 및 약축 방향 모두를 용접 모멘트접합부를 자주 사용하는 관행이 있다. 대표적인 연성내진상세인 RBS(reduced beam section, 보단면감소)접합부의 경우 국내외적으로 강축접합 위주로 실험연구가 진행되어 약축 RBS 용접모멘트접합부에 대한 실험자료가 매우 희소하다. 본 연구에서는 RBS를 도입한 약축 용접 모멘트접합부의 내진성능을 실험적으로 고찰하였다. Gilton-Uang (2002)의 선행연구를 참고하여 보 플랜지는 연속판과 맞댐용접하고 웨브는 전단이음판과 C-형 필릿용접부로 설계하여 실험을 수행하였다. 보에 사용된 강재가 내진용 강재가 아닌 일반 SS400 강재임에도 불구하고 3% 이상의 소성회전각을 발휘하였다. 제한된 결과이긴 하지만, 전단이음판과 보 웨브 사이의 C-형 필릿용접부 설계에서 보 웨브에 작용하는 휨모멘트와 전단력의 편심의 영향을 구체적으로 고찰할 필요가 있는 것으로 판단된다. 본 실험결과를 토대로 전단이음판 부근의 보 웨브 파단을 방지할 수 있는 C-형 필릿용접 형상 및 설계방안을 제안하였다. 보 플랜지와 연속판의 완전용입 용접부는 맞댄이음(butt joint) 형식이 되어 국부좌굴에 취약하고 이로 인해 피로파괴가 발생하는 경향이 있으므로 용접작업에 지장이 없는 범위에서 용접접근공(스켈럽)의 크기를 최소화할 필요가 있다. RBS형상, 스틱아웃, 트림, 연속판 두께 증가 등과 같이 이미 검증된 약축 모멘트 용접접합부의 내진상세들은 따르는 것이 바람직하다.

• 한국안광학회지
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• 제15권4호
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• pp.361-366
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• 2010

목적: Hollow Cathode Discharge방법을 이용한 HCD 이온플레이팅 장비를 제작하였고, 이 장비를 이용하여 stainless steel 위에 TiN 박막을 질소 가스양을 변화하면서 코팅하였으며, 이때 TiN 박막의 미세색상변화를 분석하였다. 방법: 질소 가스에 대한 TiN박막의 미세색상변화를 광학적으로 관찰하기 위해 분광복사계와 분광광도계를 사용하였고, 질소 가스에 대한 TiN 박막의 성분 변화를 알기위해 XPS로 분석하였다. 결과: 질소가스 120 sccm의 TiN 박막의 CIE 색좌표는 (0.382, 0.372)로 은색과 금색의 혼합색으로 나타나고 질소 가스양이 증가함에 따라 x, y값 둘다 조금씩 커져 금색이 점점 진해지는 것을 알 수 있었다. 또한 분광광도계에 의한 TiN 박막의 반사율에 있어서 질소 가스양이 증가함에 따라 550 nm 파장근처에서의 반사율의 기울기가 대체적으로 점점 커지는 것을 알 수 있었다. 그리고 XPS를 사용하여 Ti scan 한 결과, $N_2$ 성분에서 유래된 458 eV의 조그마한 피크는 조금씩 더 들어가며 TiN 성분에서 유래된 455 eV의 큰 피크는 약간씩 커지는 것을 볼 수 있었다. 결론: 질소 가스양이 120 sccm인 TiN 박막에서는 표면에 있는 TiC 성분과 표면과 내부에 존재하는 $N_2$, TiN 성분으로 인해 은색과 금색의 혼합색으로 나타났으나, 질소 가스양이 증가함에 따라 TiN 성분이 다른 성분에 비해 점점 많아져 금색이 점점 진해진 것으로 유추할 수 있었다.