A Novel Multiscale Intra- and Interlaminar Fatigue Life Prediction Framework for CFRP Laminate Composites = CFRP 적층판 복합재의 피로 수명 예측을 위한 멀티스케일 층내·층간 피로해석 프레임워크

다국어 초록 (Multilingual Abstract)

This dissertation presents a novel multiscale fatigue life prediction framework for carbon fiber reinforced polymer (CFRP) laminate composites that unifies intra- and interlaminar damage mechanisms. To describe constituent-level degradation, Mori–Tanaka’s mean-field micromechanics theory was employed, and a modified Chaboche fatigue damage model was formulated to capture nonlinear stiffness and strength reduction throughout the primary, secondary, and tertiary stages of cyclic damage evolution. Fiber–matrix interfacial degradation, which is difficult to quantify experimentally, was directly evaluated using interrupted fatigue test based X-ray computed tomography (CT) image processing, and the resulting interfacial damage behavior was cross-validated with molecular dynamics (MD) simulations. Manufacturing-induced voids were characterized through CT image analysis, and their agglomeration effects on homogenized stiffness and strength were quantified using a finite element based representative volume element (RVE) approach. These results were further incorporated into the stochastic multiscale fatigue framework to account for microstructural variability. The complete framework was implemented in ABAQUS via user subroutines, UMAT for intralaminar behavior and UEL for interlaminar delamination, allowing direct coupling between micromechanical degradation, cohesive damage evolution, and macroscopic fatigue response. Validation was performed through fatigue experiments on T700/Epoxy laminates with various lay-ups ([0]7, [90]14, [±45]4s, [0/90/±45]2s) and stress ratios (R = –1, 0.1, 10). The simulation results showed excellent agreement with experimental S–N curves and failure morphologies, confirming the predictive accuracy of the proposed multiscale intra- and interlaminar fatigue model. Furthermore, the framework was extended to incorporate freeze–thaw (FT) environmental effects based on experimental observations. The FT cyclic environment was found to weaken the fiber–matrix bond, as confirmed by SEM analyses across all tested specimens. However, the fatigue life of CFRP laminates is predominantly governed by the dominant failure mechanisms, which depend on the applied stress ratio and laminate configuration. In shear specimen (SS) configuration, where fiber–matrix debonding is the primary failure mechanism, the FT environment directly accelerates interfacial degradation, leading to a pronounced reduction in fatigue resistance. By incorporating temperature-dependent thermal strain differentials, the proposed framework successfully reproduces the experimentally observed fatigue-life reduction under FT conditions, demonstrating its capability to capture environmental effects on multiscale fatigue behavior. Overall, the proposed multiscale framework provides a physically consistent and experimentally validated approach for predicting fatigue behavior of CFRP laminates, accounting for constituent degradation, fiber-matrix interface damage, void agglomeration, and environmental influences. The results demonstrate its potential as a robust tool for the design, reliability assessment, and lifetime prediction of advanced composite structures operating under complex service conditions.

국문 초록 (Abstract)

본 논문에서는 탄소섬유 강화 복합재(Carbon Fiber Reinforced Polymer, CFRP) 적층판의 피로 수명 예측을 위한 다중 스케일(multiscale) 피로 해석 프레임워크를 새롭게 제안하였다. 제안된 모델은 적층판 내부의 층내(intralaminar) 및 층간(interlaminar) 손상 거동을 통합적으로 고려할 수 있도록 구성되어 있으며, 구성 성분 수준에서의 물성 열화를 정량적으로 반영할 수 있다. 섬유 및 기지의 피로 거동을 기술하기 위해 Mori–Tanaka 평균장 미소역학 이론을 적용하였으며, 기존 모델에서 한계점인 초기 손상 단계(primary stage)의 비선형 거동을 표현하기 위해 Modified Chaboche 피로 손상 모델을 도입하였다. 또한, 실험적으로 관측이 어려운 섬유–기지 계면 손상(fiber–matrix interfacial debonding)을 정량적으로 평가하기 위해 피로 중단(fatigue interrupt) 시험 기반 X-선 CT 영상처리(image processing) 기법을 적용하였고, 이를 분자동역학(Molecular Dynamics, MD) 시뮬레이션을 통해 상호 검증(cross-validation)하였다. 제조 과정에서 발생하는 내부 기공(void) 분포는 CT 영상을 통해 분석하였으며, 기공 응집(void agglomeration)에 따른 유효 강성 및 강도 저하를 해석적으로 평가하였다. 이렇게 얻어진 결과를 바탕으로 확률론적(multiscale stochastic) 피로 해석 프레임워크에 반영하여 제조 불확실성을 포함한 물성 변동성을 고려하였다. 전체 프레임워크는 ABAQUS 환경에서 구현되었으며, 층내 거동은 UMAT, 층간 박리(delamination)는 UEL 서브루틴을 통해 모델링하였다. 이를 통해 미소역학적 물성 저하, 계면 손상 진전, 그리고 거시적 피로 응답이 유기적으로 연계되도록 하였다. 본 연구에서 제안한 모델은 T700/Epoxy 복합재 적층판([0]7, [90]14, [±45]4S, [0/90/±45]2S)에 대해 다양한 응력비(R=–1, 0.1, 10) 조건에서 수행된 피로 시험과 비교·검증하였다. 그 결과, 실험에서 얻은 S–N 곡선 및 파손 형상과의 높은 일치를 보였으며, 제안된 모델의 신뢰성과 예측 정확성을 입증하였다. 또한, 동결–해동(Freeze–Thaw, FT) 환경에서 발생하는 계면 손상을 모사하기 위해 온도 의존적 열팽창계수 차이에 따른 계면 손상 축적 거동을 모델링하여, FT 환경에서 관찰된 피로 수명 저하 경향을 성공적으로 재현하였다. 결론적으로, 본 연구에서 제안한 다중 스케일 피로 해석 프레임워크는 구성 성분 열화, 계면 손상, 기공 응집, 환경적 요인까지 통합적으로 고려할 수 있는 물리적·실험적 근거 기반의 모델로서, 복합재 구조물의 피로 수명 예측, 설계 신뢰성 향상, 및 내환경성 평가에 효과적으로 활용될 수 있음을 확인하였다.

목차 (Table of Contents)

• Abstract i
• List of Contents iii
• List of Figures vi
• List of Tables xvi
• Chapter 1 . Introduction 1
• 1.1 Background 1
• 1.2 Aims and Scope 10
• 1.3 Outline of Dissertation 11
• Chapter 2 . Constitutive model for CFRP laminate composites 14
• 2.1 Study background 14
• 2.2 Micromechanics-based homogenization model 16
• 2.2.1 Mori-Tanaka Model 17
• 2.2.2 Micromechanics-based model with weakened interface and considering progressive fiber-matrix debonding for CFRP 21
• 2.2.3 Constitutent model for fiber and matrix and progressive interface debonding model 27
• 2.3 Modified Chaboche fatigue damage model 31
• 2.4 Intralaminar damage model 35
• 2.5 Interlaminar damage model 38
• Chapter 3 . Progressive interface damage model 45
• 3.1 Study background 45
• 3.2 Experiment approach based on CT image & image processing 47
• 3.3 Simulation approach based on MD fatigue simulation 58
• Chapter 4 . Void agglomeration effect 70
• 4.1 Study Background 70
• 4.2 Void agglomeration 73
• 4.3 Void distribution utilizing CT image processing results 79
• Chapter 5 . Fatigue experiment 87
• 5.1 Study background 87
• 5.2 Material and Manufacturing Specimen 88
• 5.3 Test procedure and methodology 97
• 5.4 Experiment results 102
• Chapter 6 . Multiscale fatigue analysis framework 121
• 6.1 Overall framework of the multiscale fatigue damage analysis 121
• Chapter 7 . Simulation Results 124
• 7.1 Characterization result of the constituents for intralamnar 125
• 7.1.1 Fiber 125
• 7.1.2 Matrix 127
• 7.1.1 Fiber-matrix interface 129
• 7.2 Characterization result for interlamiar 132
• 7.2.1 Double Cantilever Beam (DCB) 133
• 7.2.2 End Notched Flexure (ENF) 136
• 7.3 Procedure of the multiscale fatigue analysis framework 140
• 7.4 Multiscale fatigue simulation results 146
• 7.4.1 Unidirectional laminate 147
• 7.4.2 Multiaxial laminate 154
• 7.4.3 Qusai-isotropic laminate 158
• Chapter 8 . Freeze-thaw environment 165
• 8.1 Study background 165
• 8.2 Test procedure and methodology 171
• 8.3 material and specimen 176
• 8.4 Experiment results 179
• 8.5 Multiscale fatigue analysis with Freeze-thaw effect 190
• Chapter 9 . Conclusion 198
• 9.1 Summary 198
• 9.2 Contributions of the Present Work 200
• 9.2 Limitations & Future Work 202
• References 206
• Abstract in Korean 220