• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.11
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• pp.762-769
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• 2017

The extensional stiffness in quasi-isotropic laminates is uniform in the radial direction, but the bending stiffness varies radially due to the stacking sequence. This paper addresses the directional dependency of the bending stiffness and its radial variation in three types of quasi-isotropic laminate reflectors consisting of unidirectional fiber composite materials (UDM) and randomly distributed composite materials (short fiber, RDM). The extensional stiffness and bending stiffness in optical reflectors using RDM are uniform, while the bending stiffness in those using UDM varies radially from 11% to 26%. Also, the stiffness sensitivity, such as the bend-twist or bend-torsion effect, due to the differences in the stiffness value in the composite, is large. These factors are problematic in the optical field requiring precision surfaces. Utilizing RDM might be one way to eliminate the presence of bending stiffness in composite mirror substrates.

• Polymer(Korea)
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• v.30 no.2
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• pp.129-134
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• 2006

Nanocomposites of polyurethane were prepared from inorganic nano particles, $Na^+-montmorillonite$ (MMT), silica, $CaCO_3$, and surface modified MMT and their properties were investigated. It was shown that the molecular weight and polydispenity of nanocomposites of polyurethane were 20000 to 28000 and 1.0 to 2.0, respectively. d-Spacing for nanocomposites of MMT were increased than that of pure MMT. Initial degradation temperature of nanocomposites were 250 to $280^{\circ}C$. And also, the range of weight loss for nanocomposites were decreased and the end of thermal degradation was observed at higher temperatures about $50^{\circ}C$. The elongation at break for $CaCO_3$ filled nanocomposites were the highest among the nanocomposites used in this study. studied. It was found that the tensile strength increased with increasing the filler contents while the silica nanocomposite exhibited the lowest increase and the $CaCO_3$ nanocomposite the highest.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.412-412
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• 2012

갈륨비소(GaAs)는 수직공진표면방출레이저, 발광다이오드, 태양전지 등과 같은 광전소자에 널리 사용되는 물질이다. 그러나 높은 굴절률을 갖는 갈륨비소는 표면에서 30% 이상의 반사율을 갖기 때문에 광손실로 인해 소자의 성능이 저하된다. 따라서 표면 Fresnel 반사율을 낮출 수 있는 효율적인 반사방지막이 필요하다. 최근, 열적 불일치, 물질 선택, 접착력 저하의 단점을 가지고 있는 기존 다중박막을 대체하는 생체모방 서브파장 나노구조가 활발히 연구되고 있다. 이러한 구조는 공기(air)부터 갈륨비소까지 선형적인 유효굴절률 분포를 갖는 유효 단일박막과도 같기 때문에 소자 표면에서의 광손실을 줄일 수 있다. 더욱이, 자연계의 나방의 각막과 나비의 눈의 구조 형태를 모방한 반도체 생체모방 복합 눈(compound eye)은, 즉 마이크로 렌즈모양과 서브파장 나노격자구조의 복합적 형태, 표면에서 우수한 반사방지 특성을 나타낸다. 본 연구에서는, 포토리소그래피와 유도결합플라즈마 식각법을 이용하여 GaAs 기판 표면에 마이크로 렌즈 모양의 패턴을 형성한 후, 스핀코팅을 이용하여 나노 크기를 갖는 실리카 구를 도포하여 건식 식각함으로써 복합 눈 구조를 갖는 갈륨비소 반사방지막을 제작하였다. 제작된 샘플의 표면 및 식각 형상은 전자현미경(scanning electron microscope)을 사용하여 관찰하였으며, UV-vis-NIR spectrophotometer를 사용하여 반사율을 측정하였다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.7
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• pp.634-641
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• 2009

In this work, the behavior of vibratory hub loads induced due to the uncertainties of composite material properties for each of the participating rotor blades is investigated. The random material properties of composites available from the existing experimental data are processed by using the Monte-Carlo simulation technique to obtain the stochastic distribution of sectional stiffnesses of composite blades. The coefficients of variation (standard deviation divided by the mean) obtained from the sectional stiffness constants are used as an input to the comprehensive aeroelastic analysis code that can evaluate the hub loads of a rotor system. It is found that the uncertainty effects of composite material properties inevitably bring a dissimilarity to the rotor system. The influence of hub vibration response with respect to the individual stiffness (flatwise bending, chordwise bending and torsion) changes is also identified.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.11a
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• pp.439-442
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• 2009

In this study, the residual strength of CFRP filament winding pressure vessel after low velocity impact was evaluated quantitatively. After impact test, the pressure vessel was sectioned to produce 25 mm-wide ring specimen and the bursting pressure of this specimen was measured. A finite element model was also fabricated to investigate the deformation and stress distribution characteristics of the impacted CFRP vessel. The degradation of the residual strength along with the increase of impact energy was successfully measured and reviewed.

• Journal of the Korean Ceramic Society
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• v.38 no.1
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• pp.22-27
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• 2001

$\beta$-Si$_3$N$_4$종자입자 첨가가 소결조제로 Y-Mg-Si-Al-O-N계 oxynitride glass를 사용하여 일축가압 소결을 행한 SiC-Si$_3$N$_4$복합재료의 미세구조와 기계적 특성에 미치는 영향을 고찰하였다. 길게 자란 $\beta$-Si$_3$N$_4$입자들과 등방성의 $\beta$-SiC 입자들이 균일하게 분포된 미세구조를 얻었다. $\beta$-Si$_3$N$_4$종자입자 함량이 증가함에 따라 SiC-Si$_3$N$_4$복합재료의 강도와 파괴인성이 증가하였고, 이는 복합화에 기인하는 결함크기의 감소와 길게 자란 $\beta$-Si$_3$N$_4$입자에 의한 균열가교 및 균열회절 등에 기인하였다. SiC-70 wt% Si$_3$N$_4$복합재료의 대표적인 강도와 파괴인성은 각각 770 MPa과 6.2 MPa.m$^{1}$2/ 이었다.

• Composites Research
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• v.13 no.6
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• pp.63-72
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• 2000

In this paper, statistical treatments of effective properties for plain weave textile composites were presented. Configurations up to 32 layers with varied stacking phase shifts were considered. Effective properties were calculated by numerical simulation in which uni-axial tensile and shear load were applied at unit cell. Sample analysis was utilized to consider the inherent randomness in the phase shift and the results were treated statistically. It was found that effective properties were dependent on stacking phase shifts for thin plain weave textile composites. The distribution of $E_{xx}$ and $V_{xy}$ were skewed and the range of possible values was relatively large. As the number of layers increased, however, the distribution width became narrower and mean values converged. In contrast, $G_{xy}$ was not affected by phase shifts and thickness changes.

• Composites Research
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• v.28 no.4
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• pp.204-211
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• 2015

This paper presents a theory related to a two-dimensional linear cross-sectional analysis, recovery relationship and a one-dimensional nonlinear beam analysis for composite wing structure with initial twist. Using VABS including a related theory, the design process of the composite rotor blade has been described. Cross-sectional analysis was performed at cutting point including all the details of geometry and material. Stiffness matrix and mass matrix were linked to each section to make 1D beam model. The 3D strain distributions within the structure were recovered based on the global behavior of the 1D beam analysis and visualize numerical results.

• Composites Research
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• v.35 no.6
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• pp.439-446
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• 2022

In this study, fiber-reinforced composite coil springs for automobiles were manufactured using the braiding method, and mechanical tests and damage evaluation were performed to confirm their safety. Through the analysis of the load-displacement behavior, the stiffness of the springs was evaluated to meet the specifications. In addition, the distribution of voids and the impregnation rate on the spring wire section were analyzed to clearly understand the criteria for the mechanical properties of the composite material. Moreover, the tested springs were visually inspected to confirm the damaged parts, and the failure mode was analyzed by observing crack initiation and propagation behavior of cross-sectional samples taken from the crack and failure adjacent areas of springs using SEM.

• Journal of Labour Economics
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• v.29 no.2
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• pp.67-116
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• 2006

This study directly analyzes the wage distributions rather than indirectly looking at a few of their moments. It also investigates wage distributions using various descriptive and semi-parametric methods. The wage distributions of Korean manufacturing industries can in general be represented by three distinct forms, underdeveloped, advanced and the medium of the two. The discrepancies in these distribution forms are explained by differences in the labor-type distributions and their weights in the composition of wage distribution forms, and further clarified through various descriptive statistics based on them. However, the descriptive statistical analysis has a limit in that it shows mixed outcomes of different categoric variables. Then, this problem is resolved by applying a semi-parametric estimation of hazard function and the marginal effect evaluations of variable changes on estimated distributions not on the function. As a result of this marginal analysis, the common features and differences of categoric variables and their intensities of effects on distributions are revealed.