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Test and Analysis of Triaxially Braided Composite Circular Arch under Three-Point Bending
Biruk F. Nega,우경식,이한솔 한국복합재료학회 2019 Composites research Vol.32 No.5
In this paper, the buckling behavior of triaxially braided circular arch with monosymmetric open section subjected to three-point bending was studied experimentally and numerically. First, test specimens were manufactured using vacuum assisted resin transfer molding (VARTM). Then the specimen was tested under three-point bending to determine the ultimate buckling strength. Before performing the numerical analysis, effective material properties of the braided composite were obtained through micro-meso scale analysis virtual testing validated with available test results. Then linear buckling analysis and geometrically non-linear post buckling analysis, established to simulate the test setup, were performed to study the buckling behavior of the composite frame. Analysis results were compared with experimentally obtained ones for verification. The effect of manufacturing defects of tow misalignment, irregular surface and resin rich region, and uncertainties during test setup were studied using numerical models. From the numerical analyses performed it was observed that both manufacturing defect and uncertainties had effect on the buckling behavior and strength.
Stress Behaviour of Adhesively Bonded Single Lap Joint with Various Spew Geometry
( Biruk F. Nega ),( Tsinuel N. Geleta ),( Kyeongsik Woo ) 충북대학교 건설기술연구소 2016 建設技術論文集 Vol.35 No.2
단일 랩 접착 조인트는 조인트 구조의 제작상의 어려움으로 인하여 관련 산업에서 가장 빈번하게 사용되는 방법이다. 본 논문에서는 다양한 spew 접착 형태를 가지는 단일 랩 접착 조인트의 응력거동에 대한 연구를 수행하였다. 알루미늄과 탄소-에폭시 평판을 에폭시 접착제로 연결된 조인트를 대상으로 유한요소 해석을 수행하고 응력분포와 최대응력값의 변화를 조사하였다. 조인트에서 발생하는 최대 응력의 감소를 다양한 접착 형태에 따른 비교 기준으로 이용하였다. 삼각형, 원형, 타원형 그리고 원호형 spew 조인트를 고려하여 해석을 수행한 결과, 원호형의 spew 조인트 모델에서 최대 응력의 감소가 발생하였다. 또한 접착 두께와 이음 길이에 대하여 응력 분포의 측면에서 조사하였는데, 큰 접착 두께의 모델에서, 그리고 긴 이음 길이의 모델에서 낮은 최대 응력 값이 나타났다. Adhesively bonded single lap joints are among the most common bonding technique used in manufacturing industries. They are used to assemble parts which can not be manufactured from single peace. In this study, various spew geometry of adhesively bonded single lap joint was investigated. Stress distribution from finite element analysis was used to asses the behaviour of joints between aluminum and carbon-epoxy composite substrates. Reduction of maximum stresses induced in the joint was used as a criteria to choose between different spew geometries. The four geometries compared were triangular, circular, elliptical and arc, from which arc spew geometry was found to have maximum reduction in von Mises stress. The effect of adhesive thickness and lap length on stress distribution were also investigated. It was found that the larger the thickness, the lower von Mises stress developed and also the longer the lap length the lower maximum von mises stress value was resulted in.
Delamination Analysis of Carbon/Epoxy Laminate Using Continuum Damage Mechanics
( Biruk F. Nega ),( Kyeongsik Woo ) 충북대학교 건설기술연구소 2018 建設技術論文集 Vol.37 No.1
본 논문에서는 탄소/에폭시 복합재 적층시편의 층간분리 거동을 연속체파괴역학에 기초한 점진적 파선 해석을 사용하여 연구하였다. 시편의 중앙선을 따라 얇은 기지재료를 삽입하고 이의 파괴거동을 통하여 층간분리를 모사하였다. 재료의 파괴는 연속체파괴역학 이론에 따라 재료의 특성을 저하시키는 방법으로 모델링 하였는데, 이 방법은 유한요소 이산화에 민감한 것으로 알려져 있으므로, 본 논문에서는 먼저 이를 수치적으로 확인하고 정확한 해석적 예측을 위해 두 가지 대안 전략을 사용하였다. 첫 번째 방법은 매칭 결과를 생성하는 유한요소 크기의 기술적 선택이고 두 번째 방법은 일정한 유한요소 크기를 가지는 메쉬에 대해 등가 인터페이스 강도를 사용하는 것이다. 이 전략을 사용하여 DCB, ENF 및 MMB 시편형상에 대해 해석을 수행하였고 이를 별도의 실험결과와 비교하였다. 또한 VCCT와 CZM 방법에 의해서도 해석을 수행하였으며 그 결과도 함께 비교하였다. 해석결과 첫 번째 방법의 경우 각각의 모드에서 동시에 사용할 수 있는 요소크기를 결정하는 것은 가능하지 않은 것으로 나타났다. 두 번째 방법을 사용하여 모드 I 및 II에서 결정된 등가 강도가 혼합 모드 층간분리 예측에 적용한 결과 타 해석방법에 의한 결과와 허용 가능한 차이 이내에서 일치하였다. In this paper, delamination behavior of carbon/epoxy laminates was studied using progressive failure analysis based on continuum damage mechanics (CDM). Thin interface material was inserted along delamination path and the propagation was modeled by the failure of the interface material employing material property degradation. As CDM-based failure prediction is known to be sensitive to finite element discretization used, this issue was first verified and two alternative strategies were employed to get correct prediction. The first approach involves technical selection of finite element size generating matching result and the second determining equivalent interface strength for a given mesh to cope with mesh dependence. Analyses were performed for mode I/II and mixed-mode configurations and predicted results were compared with those by experiment and other numerical methods for validation. From the results obtained from the first approach, it was observed that different element sizes were required to for configurations considered having no clear relationship. On the other hand, using the second approach equivalent strength determined from mode I and II was used for mixed mode delamination prediction. The later technique showed tolerable difference in the prediction and could be applied to more general problems.
Failure Analysis of Triaxially Braided Composite Under Tension, Compression and Shear Loading
Biruk F. Nega,Kyeongsik Woo 한국항공우주학회 2019 International Journal of Aeronautical and Space Sc Vol.20 No.2
In the current study, continuum damage mechanics-based progressive failure analysis was performed to predict the initiation and progression of failure in triaxially braided textile composite unit cell under tensile, compressive and shear loading conditions. Geometric and finite element modeling was done at mesoscale unit cell level, with repeating nature of the unit cell considered through the application of periodic boundary condition. Analysis result was first validated with experimentally obtained stress–strain curve for uniaxial tension loading. Then, failure prediction was extended for tension, compression and shear failure loading in different directions. Depending on the out-of-plane boundary condition, single ply and infinitely stacked symmetric and antisymmetric plies were also considered to investigate the effect of stacking sequence on the in-plane properties. Detailed examination of failure modes at different damage initiation and propagation stages was presented. After full mechanical properties were obtained from mesoscale virtual testing, macroscale failure analysis of laterally loaded triaxially braided composite cylinder was performed using predicted properties. The result was then compared with test result for validation of the method used.
Delamination behavior of L-shaped composite beam with manufacturing defects
Kyeongsik Woo,Biruk F. Nega,Douglas S. Cairns,Jim Lua 대한기계학회 2020 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.34 No.9
Composite structures are more susceptible to manufacturing defects than conventional materials. Fiber misalignment, cracks, and voids are typical types of manufacturing defects in laminated composites. Defects can greatly reduce structural integrity and load carrying capacity, so their effects on material and structural strengths must be understood. In this paper, the effect of manufacturing defects on the progressive delamination behavior was studied for carbon/epoxy composite laminated L-beam under four-point bending test conditions. The defects of wavy plies and pure resin that had flowed out were characterized from surface photography and then transformed into finite element modeling using semi-automatic approach to which pre-delamination and void were included. Next, progressive failure analyses were performed with the interface delamination and matrix direction failure accounted for by cohesive zone modeling. Numerical results were examined focusing on the effects of defects on the peak load reduction and the variation of delamination pattern. The stacking sequence effect was also investigated.