• Journal of the Korean Society of Safety
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• v.31 no.6
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• pp.6-11
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• 2016

CFRP composite laminates are widely used as structural materials for airplanes, automobile and aerospace vehicles because of their high strength and stiffness. This study aims to examine an effect of curvature on the penetration fracture characteristic of an orthotropic composite laminated shell. For the purpose, we manufactured orthotropic CFRP shell specimen with different curvatures, and conducted a penetration test using an air-gun. Those specimens were prepared to varied curvature radius(${\infty}$, 200mm, 150mm and 100mm)and were stacked to $[O^{\circ}{_3}/90^{\circ}{_3}]_s$. When the specimen is subjected to transverse impact by a steel sphere(${\Phi}10$), the velocity of steel sphere was measured both before and after impact by determining the time for it to pass two ball-screen sensors located a known distance apart. As the curvature increases, the absorption energy and the critical penetration energy increased linearly because the resistance to the bending moment. Patterns of cracks caused by the penetration of CFRP laminated shells included fiber breakage, lamina fracture, matrix crack interlaminar crack and intralaminar crack.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2004.10a
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• pp.274-279
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• 2004

Currently, carbon-fiber reinforced plastics(CFRP) are widely used in both space and civil aircraft due to their superior stiffness and strength to weight ratios compared to conventional metallic materials. This paper is to study the effects of curvature and stacking sequence on the penetration characteristics of composite laminated shell. And were performed to investigate the penetration characteristics of composite laminated shells by the oblique impact. They are stacked to [0$_3$/90$_3$]s, [90$_3$/0$_3$]s and [0$_2$/90$_3$/0]s, [90$_2$/0$_3$/90]s their interlaminar number two and fore. They are manufactured to varied curvature radius (R=100, 150, 200mm and $\infty$). When the specimen is subjected to transverse impact by a steel ball, the velocity of the steel ball was measured both before and after impact by determining the time for it to pass two ballistics-screen sensor located a known distance apart. In general, the critical penetration energy interface decrease and slope angle on the impact surface increased. [0$_3$/90$_3$]s and [0$_2$/90$_3$]s specimens higher than [90$_3$/0$_3$]s and [90$_2$/0$_3$/90]s specimens.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2010.04a
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• pp.729-732
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• 2010

본 논문에서는 이산 변수 최적화에 적합한 유전 알고리듬을 이용하여 복합재 적층 패치의 최적강도설계를 수행하였다. 기저판(substrate)와 접착제(adhesive), 그리고 복합재 적층 패치로 이루어진 구조물에서 패치의 강도를 효율적으로 구하기 위해서 응력 함수 기반의 해석적 방법을 도입하였다. 면외 방향의 응력 함수를 가정하여 가상 공액일의 법칙(complementary virtual work principle)에 적용하였으며, 복합재 패치의 자유 경계조건으로부터 면내 방향의 응력함수를 결정하였다. 응력 함수를 통하여 구한 층간 응력 값은 자유 경계 효과를 잘 나타내었고, 이를 이용하여 패치의 강도 해석을 수행하였다. 강도 해석 시, 복합재 패치의 파괴 기준은 면내 응력들에 대해서는 최대 응력 척도를 사용하였으며, 층간 응력들에 대해서는 quadratic delamination 척도를 사용하였다. 유전 알고리듬을 이용한 최적강도설계 과정에서는 임의의 염색체가 주어진 적층 구속 조건을 만족할 수 있게 수정(repairing)하는 과정을 도입하였다. 또한 다수의 전역해(global optima)를 효과적으로 찾기 위해서 multiple elitism 기법을 도입하였다. 응력 함수 기반의 강도 해석방법과 유전 알고리듬과의 연계를 통한 복합재 적층 패치의 강도최적설계 기법은 패치 구조물의 해석 및 설계에 있어서 효율적인 도구로서 사용할 수 있을 것이라 사료된다.

• Applied Chemistry for Engineering
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• v.9 no.5
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• pp.715-718
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• 1998

In this work, bisphenol type vinyl ester was impregnated into the three-dimensional glass fabrics fabricated from different thickness changes. Their mechanical properties of the specimens have been investigated by three-point bending and flatwise compression tests. Also, interlaminar shear strength (ILSS) has been determined through short-beam test for the evaluation of interfacial adhesion at interfaces between fibers and matrix of the composites. The effect of thickness changes in three-dimensional glass fabric-reinforced composites have been described in this work.

• Structural Engineering and Mechanics
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• v.35 no.3
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• pp.335-348
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• 2010

Cutouts are often provided in structural and aircraft components for ventilation, for access, inspection, electric lines and fuel lines or sometimes to lighten the structure. This paper addresses the effects of cutout shape (i.e., circular, square, diamond, elliptical-vertical and elliptical-horizontal) and size on buckling and postbuckling response of quasi-isotropic (i.e., $(+45/-45/0/90)_{2s}$) composite laminate under uni-axial compression. The finite element method is used to carry out the investigation. The formulation is based on first order shear deformation theory and von Karman's assumptions are used to incorporate geometric nonlinearity. The 3-D Tsai-Hill criterion is used to predict the failure of a lamina while the onset of delamination is predicted by the interlaminar failure criterion. It is observed that for the smaller size cutout area there is no significant effect of cutout shape on load-deflection response of the laminate. It is also concluded that the cutout size has substantial influence on the buckling and postbuckling response of the laminate with elliptical-horizontal cutout, while this effect is observed to be the least in case of laminate with elliptical-vertical cutout.

• Journal of Hydrogen and New Energy
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• v.32 no.1
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• pp.86-91
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• 2021

In order to save the energy in vehicles using renewable energy, it is necessary to reduce the weight of parts with polymer matrix composites. Carbon nanotube (CNT) is the nano-scale reinforcement used to increase the interlaminar strength of fiber reinforced composites or enhance the fracture toughness of polymer. However, since the degree of improvement in mechanical properties varies according to the various experimental conditions such as shape of reinforcement, types of matrix and dispersion of reinforcement, research to find the optimal conditions is essentially needed. In this study, CNT/epoxy composites with different CNT concentration were fabricated under the same conditions, and the optimal CNT content (2 wt%) was found through Mode 1 fracture toughness test. Furthermore, through optical microscopy, it was confirmed that the fracture toughness was rather decreased due to the CNT aggregation when the CNT content exceeded 2 wt%.

• Composites Research
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• v.17 no.4
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• pp.7-17
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• 2004

In this study, the effect of laminate thickness on the compressive behaviour of composite materials is investigated through systematic experimental work using the stacking sequences, $[O_4]_{ns},{\;}[45/0/-45/90]_{ns}$ and $[45_n/0_n/-45_n/90_n]_s$ (n=2 to 8). Parameters such as fibre volume fraction, void content, fibre waviness and interlaminar stresses, influencing compressive strength with increasing laminate thickness are also studied experimentally and theoretically. Furthermore the stacking sequence effects on failure strength of multidirectional laminates are examined. For this purpose, two different scaling techniques are used; (1) ply-level technique $[45_n/0_n/-45_n/90_n]s$ and (2) sublaminate level technique $[45/0/-45/90]_{ns}$. An apparent thickness effect existes in the lay-up with blocked plies, i.e. unidirectional specimens ($[O_4]_{ns}) and ply-level scaled multidirectional specimens ($[45_n/0_n/-45_n/90_n]_s$). Fibre waviness and void content are found to be main parameters contributing to the thickness effect on the compressive failure strength. However, the compressive strength of the sublaminate level scaled specimens ($[45/0/-45/90]_{ns}$) is almost unaffected regardless of the specimen thickness (since ply thickness remains constant). From the investigation of the stacking sequence effect, the strength values obtained from the sublaminate level scaled specimens are slightly higher than those obtained from the ply level scaled specimens. The reason for this effect is explained by the fibre waviness, void content, free edge effect and stress redistribution in blocked $0^{\circ}$ plies and unblocked $0^{\circ}$ plies. The measured failure strengths are compared with the predicted values.

• Applied Chemistry for Engineering
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• v.26 no.4
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• pp.406-412
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• 2015

Unidirectional carbon/carbon (C/C) composites were manufactured using phenolic resins as a precursor of the carbonized matrix throughout a one-step manufacturing process. Also, molybdenum oxide ($MoO_3$) and binder pitches were impregnated with phenolic resins to improve the bulk density and mechanical property of the C/C composites. In this study, the influence of $MoO_3$ and binder pitches on mechanical properties of the C/C composites were investigated by measuring flexural strength (${\sigma}_f$) and interlaminar shear strength (ILSS). The results show that the enhancement of interfacial adhesions between the fibers and matrix resins of the C/C composites with $MoO_3$ and binder pitches which led to the improvement of mechanical properties of the C/C composites. This indicates that the presence of $MoO_3$ and binder pitches in C/C composites can develop the graphite structure and increase the bulk density.

• Polymer(Korea)
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• v.24 no.4
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• pp.499-504
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• 2000

The effect of anodic oxidation on high strength PAN-based carbon fibers has been studied in terms of surface functionality and surface energetics of the fiber surfaces, resulting in improving the mechanical properties of composites. According to FT-IR and XPS measurements, it reveals that the oxygen functional groups on fiber surfaces induced by an anodic oxidation largely influence the surface energetics of fibers or the mechanical interfacial properties of composites, such as the interlaminar shear strength (ILSS) of composites. According to the contact angle measurements based on the wicking rate of a test liquid, it is observed that anodic oxidation does lead to an increase in surface free energy of the carbon fibers, mainly due to the increase of its specific (or polar) component. From the surface energetic point of view, it is found that good wetting plays an important role in improving the degree of adhesion at interfaces between fiber and epoxy resin matrix of the resulting composites. Also, a direct linear relationship is shown between 01s/01s ratio and ILSS or between specific component and ILSS of the composites for this system.

• Composites Research
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• v.15 no.6
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• pp.16-23
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• 2002

In this work, the effect of anodic oxidation on surface characteristics of high strength PAN-based carbon fibers was investigated in mechanical interfacial properties of composites. The surface properties of the carbon fibers were determined by acid-base values, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and contact angles. And their mechanical interfacial properties of the composites were studied in interlaminar shear strength (ILSS) and critical stress intensity factor ($K_{IC}$). As a result, the acidity or the $O_{ls}/C_{ls}$ ratio of carbon fiber surfaces was increased, due to the development of the oxygen functional groups. Consequently, the anodic oxidation led to an increase in surface free energy of the carbon fibers, mainly due to the increase of its specific (or polar) component. The mechanical interfacial properties of the composites, including ILSS and $K_{IC}$, had been improved in the anodic oxidation on fibers. These results were explained that good wetting played an important role in improving the degree of adhesion at interfaces between fibers and epoxy resin matrix.