Manufacturing of 3D woven composites and mechanical modeling using fiber-based continuum finite element analysis = 3차원 직조 복합재료의 제조 및 섬유 기반 연속체 유한요소 해석을 이용한 기계적 모델링

국문 초록 (Abstract)

새로운 3차원(3D) 직조 프리폼 제작 기법이 제안되었다. 본 방법은 이차원(2D) 라미네이트 복합재에서 빈번히 발생하는 박리 및 섬유 좌굴 문제를 근본적으로 해소한다. 고도화된 3D 직조 시스템은 다양한 3D 구조를 연속적·고속으로 생산할 수 있으며, 이렇게 제작된 프리폼과 에폭시 수지를 진공 보조 수지 전달 성형(VARTM) 공정으로 복합재화하였다. 마이크로 컴퓨터 단층촬영(µ-CT)을 통해 내부 구조를 평가한 결과, 직조 기계의 매개변수를 최적화함으로써 설계한 3D 프리폼 아키텍처가 정확히 구현됨을 확인하였다. 기존 제조 공정을 확장하여 장(長)형 TTAL(through-thickness all-layer interlaced) 구조 프리폼 제조법을 확립하고, 이를 대상으로 수직형 수지 전달 성형(vertical-inlet RTM) 공정을 적용하였다. 에폭시 수지를 이용해 성형된 3D 직조 TTAL 복합재의 내부 구조를 µ-CT로 평가한 결과, 연장된 프리폼에도 수직 유입 방식이 최적의 수지 집중 침투를 보임을 확인하였다. 이어 해당 복합재를 워프 및 위프트 방향으로 절단하여 인장, 압축, 3점 굽힘 시험을 실시하였으며, 시험편별 응력-변형률 곡선을 그래프로 나타내었다. 그 결과, TTAL 구조의 3D 직조 복합재에서는 Z-바인더 실이 기계적 거동에 지배적인 영향을 미쳐 2D 복합재와 전혀 다른 인장·압축·전단 특성을 나타냄을 확인하였다. 한편, 전통적 2D 모델링 기법으로는 Z-바인더 실의 두께 방향(through-thickness) 거동을 반영할 수 없어 정확한 해석이 어려우므로, 본 연구에서는 섬유 기반 연속체 해석 모델(fiber-based continuum analysis)을 적용하였다. 이 모델은 섬유와 매트릭스의 물성만을 입력하여 X, Y, Z 축 방향 물성을 계산하며, 대형 구조물 해석에 적합한 연속체 모델이다. 인장·압축·3점 굽힘 시험 결과와 수치 해석 결과를 비교한 바, 모든 시험 모드에서 실험 거동을 성공적으로 재현하였으며 특히 Z축 물성의 중요성을 검증하였다. 그러나 국소적 이질성과 복잡한 Z-바인더 형상으로 인해 일부 예측 성능이 저하됨을 확인하고, 이를 개선하기 위해 탄소섬유에 현장 성장(in-situ) 방식으로 탄소나노튜브(CNT)를 접목하는 공법을 연구하였다. 이러한 CNT 접목 프리폼을 바탕으로 다양한 3D 직조 CNT 복합재를 성공적으로 제작·평가하였다.

다국어 초록 (Multilingual Abstract)

A new production technique has been introduced for creating three‐dimensional (3D) woven fabric preforms. This approach eliminates the delamination and fiber‐buckling problems typically encountered in two‐dimensional (2D) laminated fabric‐reinforced composites. The advanced 3D weaving system enables continuous, high-speed production of woven preforms in a variety of three-dimensional architectures. Using these preforms and epoxy resin, we produced carbon fiber-reinforced composites via vacuum-assisted resin transfer molding. Micro-computed tomography was employed to assess the internal structure of the composites, confirming that precise 3D preform architectures can be achieved by fine-tuning the weaving parameters of our novel loom.
A new weaving approach incorporating an enhanced z-binder yarn system was developed to enable efficient, continuous, and high-speed fabrication of diverse 3D woven architectures. A through-thickness all-layer interlaced (TTAL) preform was produced. And by extending the existing manufacturing process, we established a method for producing long preforms with a TTAL structure. Next, an RTM approach tailored for the long preform was implemented. It was confirmed that the vertical‐inlet resin transfer molding method is well suited for such extended preforms. Epoxy resin was used to fabricate the composites, and the resulting 3D woven TTAL‐structure composites were evaluated using micro‐CT.
The 3D woven composite with the TTAL architecture was then sectioned into test specimens. Specimens oriented in both the warp and weft directions were prepared for mechanical characterization. Tensile, compression, and three‐point bending tests were performed on these specimens. The results for each orientation were collected and plotted as graphs. As a result, in the 3D woven composite with the TTAL architecture, the z-binder yarn significantly influenced the mechanical behavior, producing tensile, compressive, and shear responses that differed markedly from the trends observed in conventional 2D composites.
The 3D material, which exhibits behavior entirely distinct from that of 2D composites, cannot be accurately modeled using conventional approaches. In 2D composite modeling, through-thickness properties are typically neglected. However, because the z-binder yarn influences the overall performance, the model must also incorporate through-thickness behavior. The fiber-based continuum analysis model is a numerical framework that explicitly accounts for through-thickness behavior. By using only the constituent fiber and matrix properties, it computes effective material responses along the x, y, and z axes, and—being a continuum representation—is well suited for modeling large‐scale components. To validate its capability, simulation results from this model were compared against the mechanical test data for the TTAL-structured 3D woven composite. The comparison demonstrated that the model accurately reproduces the observed behavior in all test modes and confirmed the critical importance of incorporating z-axis properties.
The failure behavior of the TTAL-structured 3D woven composite differed from the predicted response, owing to local heterogeneities and the intricate geometry of the z-binder yarn. Because the measured properties fell short of expectations, we investigated a CNT-grafting methodology to reinforce the composite. This work focused first on establishing an in-situ CNT growth process on the carbon fibers and then on translating that grafted preform into a CNT-reinforced 3D woven composite. By presenting a composite fabrication method, we were able to produce a variety of composites.

목차 (Table of Contents)

• Abstract i
• Contents iv
• List of figures viii
• List of tables xii
• Chapter 1. Introduction 1
• 1.1. 3D woven composite 1
• 1.2. Mechanical behavior of 3D woven composite 4
• 1.3. Multiscale modeling of 3D woven composite 7
• 1.4 research objects 10
• Chapter 2. Manufacturing of 3D woven composite 13
• 2.1. Weaving process and preparation of fiber yarns 14
• 2.1.1. 3D weaving machine and its capability 14
• 2.1.2. The state of the preform at each weaving step. 17
• 2.2. Weaving process and preparation of fiber yarns 20
• 2.2.1 Pattern design of preform 20
• 2.2.2 Fabrication of TTAL structure 23
• 2.2.3 Compact 3D woven composite manufacturing 29
• 2.3 Methodology of Resin Transfer Method (RTM) process for large preform 35
• 2.3.1 Selection of composite manufacturing method and preparation procedure 35
• 2.3.2. 3D woven composite adjusting Vertical RTM 39
• 2.4 Summary 43
• Chapter 3. Mechanical testing of 3D woven composite 45
• 3.1 Preparation Procedures for Mechanical Testing 46
• 3.1.1 Materials and characterization 46
• 3.1.2. Specimen preparation of 3D woven composite 47
• 3.1.3. Testing Equipment and Measurement Instruments 50
• 3.2. Mechanical test of 3D woven composite and its behavior 52
• 3.2.1. Tensile test procedure and the test results 52
• 3.2.2. Compression test procedure and the test results 56
• 3.2.3. 3 Point bending test of 3D woven composite 60
• 3.3. Discussion of 3D woven composite mechanical test results 62
• 3.3.1 Discussion of failure mode for 3 different mechanical test 62
• 3.3.2. Additional Data of TTAL 3D Woven Structure and Comparison of Mechanical Properties with 2D unidirectional composite 72
• 3.4. Summary 81
• Chapter 4. Finite element method via Fiber based continuum analysis 83
• 4.1 Fiber based continuum analysis model 84
• 4.2. Comparison of simulation results and experimental results 92
• 4.2.1. Comparison of tensile test simulation 92
• 4.2.2. Comparison of compression test simulation 97
• 4.2.3. Comparison of 3-point bending test simulation 101
• 4.3. Approaches for improved fiber based continuum modeling 108
• 4.4. Summary 109
• Chapter 5. Applications: Manufacture of 3D woven CNT grafted carbon fiber composite 110
• 5.1. CNT grafted carbon fiber and 3D preform 112
• 5.1.1. CNT grafting method using CVD process 112
• 5.1.2. CNT grafting on the surface of 3D woven preform 116
• 5.1.3. Morphology of CNT grafted 3D woven preform 120
• 5.2. Fabrication of 3D CNT grafted carbon fiber composite 127
• 5.2.1. RTM process for 3D CNT grafted carbon fiber composite 127
• 5.2.2. Expected Functional Enhancements of 3D Woven Composites with CNT grafted preform 130
• 5.3. Summary 133
• 6. Concluding remarks 134
• Reference 138
• Korean abstract 143