• Advanced Composite Materials
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• 제17권4호
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• pp.373-407
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• 2008

This paper will examine the possibilities of the virtual tests of composite structures by simulating mechanical behaviors by using supercomputing technologies, which have now become easily available and powerful but relatively inexpensive. We will describe mainly the applications of large-scale finite element analysis using the direct numerical simulation (DNS), which describes composite material properties considering individual constituent properties. DNS approach is based on the full microscopic concepts, which can provide detailed information about the local interaction between the constituents and micro-failure mechanisms by separate modeling of each constituent. Various composite materials such as metal matrix composites (MMCs), active fiber composites (AFCs), boron/epoxy cross-ply laminates and 3-D orthogonal woven composites are selected as verification examples of DNS. The effective elastic moduli and impact structural characteristics of the composites are determined using the DNS models. These DNS models can also give the global and local information about deformations and influences of high local in-plane and interlaminar stresses induced by transverse impact loading at a microscopic level inside the materials. Furthermore, the multi-scale models based on DNS concepts considering microscopic and macroscopic structures simultaneously are also developed and a numerical low-velocity impact simulation is performed using these multi-scale DNS models. Through these various applications of DNS models, it can be shown that the DNS approach can provide insights of various structural behaviors of composite structures.

• 항공우주시스템공학회지
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• 제3권2호
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• pp.1-11
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• 2009

A structural modelling for study on dynamic characteristics of tapered composite aircraft wings in the form of thin-walled beam is presented. The proposed structural model includes effects of transverse shear flexibility exhibited by the advanced composite materials and warping restraint characterizing elastic anisotropy and induced structural couplings. The complex effects of these factors could have a role in more efficient analysis on those structural models.

• Advanced Composite Materials
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• 제18권4호
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• pp.395-413
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• 2009

Impact tests on high-strength membrane materials under biaxial loads were experimentally conducted in order to evaluate influence of biaxial loads on impact fracture of the membrane materials for the inflated applications. Cruciform specimens of the membrane materials were fabricated for applying biaxial loadings during the impact test. A steel ball was shot using a compressed nitrogen gas gun, and struck the membrane specimen. Impact tests on uniaxial strip specimens were also conducted to obtain the effect of specimen configuration and boundary condition on the impact fracture. The results of the measured crack length and the ultra-high speed photographs indicate the impact fracture properties of the membrane fabrics under biaxial loadings. Crack length due to the impact increased with applied tensile load, and the impact damages of the cruciform membrane materials under biaxial loadings were smaller than those of under uniaxial loadings. Impact fracture of the strip specimen was more severe than that of the cruciform specimen due to the difference of boundary conditions.

• Composites Research
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• 제31권5호
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• pp.246-250
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• 2018

본 연구에서는 곡면 복합재 구조물의 모서리에 위치한 결함을 검사하고 가시화하기 위해 제안된 레이저 초음파 기반 회전식 검사 기법을 소개한다. 모서리 부위에 결함이 위치한 L 형 복합재 구조물을 레이저 초음파 기반 회전식 검사 기법을 이용하여 검사하였다. L 형 시편은 층간분리 손상을 모사하기 위해 각기 다른 세 가지 깊이에 위치한 인공 결함을 포함하고 있다. 모든 인공 결함이 각각의 위치한 깊이에 따라 다른 시간대에 선명하게 탐지되었다. 검사결과는 제안된 방법이 어떠한 특별한 도구 없이도 곡면 복합재 구조물의 모서리 부위의 검사에 적합하다는 것을 보여준다.

• Composites Research
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• 제31권6호
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• pp.404-411
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• 2018

복합재료는 주어진 경화 사이클(cure cycle)로 오토클레이브 공정이 진행됨에 따라 수지의 화학수축, 열팽창계수 등에 의한 제품 내 잔류응력(residual stress)이 발생한다. 이로 인해 spring-in, warpage와 같은 열 변형이 발생하고 최종 제품의 수치 정확성이 감소한다. 구조물의 정밀한 제작이 요구되는 항공우주분야에서는 열변형으로 인한 문제를 해결하는 것이 중요하다. 따라서 복합재료의 경화과정을 예측하고 이해하기 위한 연구가 활발히 이루어지고 있다. 본 연구에서는 공정과정에 따른 복합재료의 경화메커니즘을 유한요소해석을 통해 예측하였고, 공정에 의해 발생하는 열변형에 대한 화학수축의 영향을 열팽창계수와 비교하여 분석하였다.

• 항공우주시스템공학회지
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• 제10권1호
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• pp.103-110
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• 2016

This study shows the statistical properties of the domestic composite material prepregs test result. During the last three years(2012.5~2015.6) the prepreg specimen tests have been performed by referring to NCAMP developed test procedure which was approved by FAA. The database of (1) Carbon Tape, (2) Glass Fabric, and (3) Carbon Fabric composite material prepregs' characteristics have been established for certified aircraft structures. This qualified materials can be used for aircraft structural design through proper material equivalency procedures.

• 한국분말재료학회지
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• 제19권2호
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• pp.122-126
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• 2012

The Fe-based self-fluxing alloy powders and TiC particles were ball-milled and subsequently compacted and sintered at various temperatures, resulting in the TiC particle-reinforced Fe self-fluxing alloy hybrid composite, and the microstructure and micro-hardness were investigated. The initial Fe-based self-fluxing alloy powders and TiC particles showed the spherical shape with a mean size of approximately 80 ${\mu}m$ and the irregular shape of less than 5 ${\mu}m$, respectively. After ball-milling at 800 rpm for 5 h, the powder mixture of Fe-based self-fluxing alloy powders and TiC particles formed into the agglomerated powders with the size of approximately 10 ${\mu}m$ that was composed of the nanosized TiC particles and nano-sized alloy particles. The TiC particle-reinforced Fe-based self-fluxing alloy hybrid composite sintered at 1173 K revealed a much denser microstructure and higher micro-hardness than that sintered at 1073 K and 1273 K.

• 소성∙가공
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• 제23권8호
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• pp.482-488
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• 2014

Flexible forming technology has advantages in sheet metal forming, because it can be implemented to produce various shaped molds using a single apparatus. Due to this advantage, it is possible to apply it to the manufacture of an aircraft winglet mold. Presently, most aircraft winglets are manufactured from composite materials. Therefore, the mold for the curing process is an essential element in the fabrication of such composite materials. Compared to conventional mold forming, flexible forming has some advantages such as reduced manufacturing cost and uniformity of mold thickness. If the thickness of the mold is consistent, then the heat transfer will occur uniformly during the curing process leading to improved formability of the composite material. In the current study, numerical simulations were performed to investigate the possibility of flexible forming for manufacturing of the winglet mold. In order to match the size of the actual product, the shape of objective surface was divided to fit the dimensions of the apparatus. The results from the numerical simulations are compared with the objective surface to verify the accuracy. In conclusion, the current study confirms the feasibility and the potential to manufacture winglet molds by flexible forming.

• Advanced Composite Materials
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• 제18권1호
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• pp.21-42
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• 2009

The vibration characteristics of post-buckled laminated composite doubly curved shells are investigated. The finite element method is used for the analysis of post-buckling and free vibration of post-buckled laminated shells. The geometric non-linear finite element model includes the general non-linear terms in the strain-displacement relationships. The shell geometry used in the present formulation is derived using an orthogonal curvilinear coordinate system. Based on the principle of virtual work the non-linear finite element equations are derived. Arc-length method is implemented to capture the load-displacement equilibrium curve. The vibration characteristics of post-buckled shell are performed using tangent stiffness obtained from the converged deflection. The code is first validated and then employed to generate numerical results. Parametric studies are performed to analyze the snapping and vibration characteristics. The relationship between loads and fundamental frequencies and between loads and the corresponding displacements are determined for various parameters such as thickness ratio and shallowness.

• Composites Research
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• 제36권5호
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• pp.321-328
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• 2023

본 연구에서는 다관절 로봇 암을 활용한 열 성형 공정을 통해 열가소성 복합재 부품을 개발하였다. 유한요소해석을 기반으로 로봇 암이 적용된 고속 열 성형 공정의 최적 공정 변수를 도출하였고, 이를 기반으로 실제 제작 공정을 통해 공정 변수를 구체화하였다. 제작된 부품과 유한요소해석 간의 두께 균일성, 주름 분포를 비교하였고, 이를 통해 유한요소해석의 타당성을 입증하였다. 또한, 제작된 복합재 부품의 성형성을 평가하기 위해 결정화도와 기공률을 측정하였고, 그 결과 우수한 성형성을 보인 것으로 평가하였다. 이에 따라 본 연구는 로봇 암기반의 고속 열 성형 공정을 통한 공정 확립과 이를 통한 복합재 구조의 제작 가능성을 확인하였다.