• Korean Journal of Crystallography
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• v.13 no.2
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• pp.69-72
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• 2002

The interlaminar problems near the free edge of composite laminates are analyzed in this paper. CFRP specimen (［+40/-40］s) and interleaved specimen (［+40//-40］s) with non-woven carbon tissue (NWCT) are discussed under tensile loading condition. The symbol “//”means that the NWCT is located between the CFRP interfaces. The NWCT has carbon short fibers which are discretely distributed with the in-plane random orientation. It was reported/sup 3)/ that the Mode Ⅱ interlaminar fracture toughness of CFRP laminates with NWCT is increased largely and the Mode I interlaminar fracture toughness is not changed significantly. Mode Ⅲ interlaminar fracture toughness is also an important factor in composite structures. But it is not easy to experimentally investigate the Mode Ⅲ interlaminar fracture toughness. The objective of this work is to study the effect of the NWCT and to fundamentally understand the Mode Ⅲ interlaminar shear characteristics of laminated composites with NWCT in the vicinity of a free edge by using finite element method analysis.

• Journal of the KSME
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• v.22 no.1
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• pp.9-14
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• 1982

복합재료를 이용하여 부품제작을 한 후 어떠한 결합방법을 채택할 것인가를 결정할 때 고려해 야할 점을 기계적 결합법과 접착결합법을 비교하여 검토하였다. 기계적 결합은 하중을 많이 받고 분해 및 결합이 자주 예상되는 부품에 채택해야 할 것이며 복합재료의 특성을 고려하여 보강 시에 부착하는 평판의 섬유방향은 가급적 드릴구멍주위를 부드럽게 하여 응력집중을 낮출 수 있으며 하중의 종류에 따라 적층의 섬유방향을 조절함으로서 응력집중을 조절할 수 있다. 드릴 구멍 주위인 파손은 평판의 폭과 구멍의 직경등이 크게 작용함으로 강도해석을 할 경우에 응력 해석을 한 후 허용응력등을 결정해야할 것이다. 접착졀합법은 작업이 간단하나 신뢰도가 떨어지 므로 하중을 많이 받는 구조물에의 사용에 주의를 요하며 설계방법도 매우 다양하게 제안되어 있어 선택함에 있어 하중 환경조건등을 점검해야할 것이다. 접착결합법은 드릴구멍같은 불연속 성을 갖지 않기 때문에 응력집중이 생기지 않으나 접착층의 길이등 기하학적 형상에 따라 다르게 나타남으로 잡착층의 분리가 일어나지 않도록 설계되어야 한다. 특히 복합재료의 이방성인 성 질을 감안하여 접착층에 이웃하는 피접착층의 섬유방향에 주의해야 하며 층간응력이 파손에 미 치는 영향을 고려하여 설계에 임해야 한다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.89-89
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• 2010

발전기 회전자 권선 층간절연지는 대부분 글라스-에폭시(Glass-Epoxy) 및 페놀수지 제품이 사용되고 있다. 이 층간절연지는 과도하게 열적, 기계적 응력을 받게 됨으로 높은 기계적 특성이 요구된다. 실제 장기간 사용된 층간절연지에 대해 기계적 인장특성을 분석하였으며 시험결과 운전년수와 기계적 특성 저하와는 뚜렷한 상관관계가 없는 것으로 판단되었다.

• 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.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.2002-2014
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• 1991

A refined two-dimensional analysis method, taking into account the crack opening deformation, is proposed for the evaluation of stress distributions in transverse cracked cross-ply laminates. The interlaminar stresses which play an important role in laminate failure are evaluated using the concept of interface layer. A series expansion of the displacements is employed and the thermal residual stresses and Poisson's effects in the laminated are taken into consideration in the formulation. The stress distributions are compared with finite element results. The proposed method represents well the characteristics of the stress distributions. The through-the-thickness variation of the stress distribution is remarkable near the transverse crack due to the crack opening deformation. The interlaminar stresses have significant values at the transverse crack tip and the proposed analysis can be applied as a basis for the prediction of the induced delamination onset by using appropriate failure criteria.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.8
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• pp.2139-2157
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• 1994

In order to understand the delamination between leadframe and epoxy molding compound in an electronic packaging of surface mounting type, the stress intensity factor, T-stress and J-integral in fracture mechanics are obtained. The effects of geometry, material properties and molding process temperature on the delamination are investigated taking into account the temperature dependence of the material properties, which simulates as more realistic condition. As the crack length increases the J-integral increases, which suggest that the crack propagates if it starts growing from the small size. The effects of the material properties and molding process temperature on stress intensity factor, T-stress is and J-integral are less significant than the chip size for the practical cases considered here. The T-stress is negative in all eases, which is in agreement with observation that interfacial crack is not kinked until the crack approaches the edge of the leadframe.

• Composites Research
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• v.12 no.1
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• pp.28-36
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• 1999

Thermal stress-induced failure in the free edge region of various thin carbon/epoxy composite laminates(1mm thick) has been investigated using the three-dimensional finite-element stress analysis, ultrasonic C-scan and microscopic observations. High thermal in-plane and interlaminar stresses were predicted in the interior layer near the free edge boundaries of the laminates. In the interior lamina, not in the skin lamina, of the thin laminates with lay-up of $[0_2/90_2]_s,\;[45_2/-45_0]_s,\;[0_2/60_2]_s$ treated by liquid $N_2$ immersion, many transverse matrix cracks took place due to thermal stress concentration, which agreed qualitatively with the above predictions.

• Composites Research
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• v.14 no.1
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• pp.8-14
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• 2001

A simplified method for determining the three mode(I, II, III) components of the strain energy release rate of free-edge delaminations in composite laminates is proposed. The interlaminar stresses are evaluated using the interface moment and the interface shear forces which are obtained from the equilibrium equations at the interface between the adjacent layers. Deformation of an edge-delaminated laminate is analysed by using a generalized quasi-three dimensional classical laminated plate theory. The analysis provides closed-form expression for the three components of the strain energy release rate. The analyses are performed for composite laminates subjected to uniaxial tension, with free-edge delaminations located symmetrically and asymmetrically with respect to the laminate midplane. The analysis results agreed with a finite element solution using the virtual crack closure technique.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.2
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• pp.198-207
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• 1986

This study investigates interlaminar fracture characteristics of Graphite/Epoxy composite (HFG Graphite/Epoxy) under mode I (opening mode), mode II (sliding mode) and mixed mode loading conditions. The effects on interlaminar fracture toughness due to different fiber orientations on the crack surface are also investigated. The antisymmetric test fixture proposed by M. Arcan is used for this test. Both critical stress intensity foctors and critical energy release rates were determined and several mixed mode fracture criteria were compared to the experimental data. Also fracture surfaces were investigaed to obtain informations on the fracture behaviors of Graphite/Epoxy composite by means of a scanning electron microscope(SEM).

• Composites Research
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• v.19 no.1
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• pp.1-8
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• 2006

Prediction of damage caused by low-velocity impact in laminated composite plate is an important problem faced by designers using composites. Not only the inplane stresses but also the interlaminar normal and shear stresses playa role in estimating the damage caused. But it is well known that the conventional approach based on the homogenization has the limit in description of damage. The work reported here is an effort in getting better predictions of dynamic behavior and damage in composite plate using DNS approach. In the DNS model, we discretize the composite plates through separate modeling of fiber and matrix for the local microscopic analysis. In the view of microscopic mechanics with DNS model, interlaminar stress behaviors in the inside of composite materials are investigated and compared with the results of the homogenized model which has been used in the conventional approach to impact analysis. Also the multiscale model based on DNS concept is developed in order to enhance the effectiveness of impact analysis, and we present the results of multiscale analysis considering micro and macro structures simultaneously.