Innovative approaches for mechanical characterizationAnd failure analysis in complex and functional compositeStructures: a fusion of shm, ndt, and fea techniques

:Fiber-reinforced polymer (FRP) composites have been extensively employed in advancedengineering applications owing to their improved specific strength, good fatigueperformance, corrosion resistance and simplicity of manufacturing for attaining thedesired final shape in a cost-effective manner. This great potential encourages thescientific community to use tailored FRPs structures to solve complex engineeringproblems. However, due to the complex microstructure of FRPs, predicting theirmechanical properties, investigating their failure behavior and developing effectivestructural health monitoring (SHM) techniques are essential for their adoption in theindustry. Hence, in this thesis, two examples of complex composite structure, i.e., threedimensionalwoven composites (3DWCs) and adhesively bonded composite repairstructures, are investigated. The study begins with introducing a novel multiscale analysisfor predicting mechanical behavior of 3DWCs. This method utilizes computer aideddesign for generating representative unit cell models and finite element analysis (FEA),thereby simplifying meso-modeling without requiring computed tomography (CT) scans.The accuracy of meso-modeling of the representative unit cell is proven by comparingthe obtained results with the experimental ones in the literature. Furthermore, thevalidation of the proposed methodology with respect to tensile and flexural tests confirmsits accuracy and applicability to complex composite structures like sandwich materials.Additionally, the impact of glass/Kevlar hybridization on the tensile and shear properties of 3DWCs, manufactured via vacuum infusion, is examined. By comparing inter-ply andintra-tow hybridizations using digital image correlation and acoustic emission analysis,this thesis clarifies how these methods affect the stiffness and strength of the composites.Building on this, the thesis evaluates the effectiveness of E-glass hybridization of 3DWCsunder flexural and low-velocity impact tests. Post mortem analysis, carried out viascanning electron microscopy (SEM) and Lock-in Thermography (LIT), sheds light onthe influence of hybridization on damage evolution. Finally, the thesis presents aninnovative in-situ acoustic emission-based methodology for SHM in adhesively bondedcomposite repair structures. This methodology effectively detects and identifies damageas well as provide predictions of failure modes at early loading stages. This novelapproach is verified in two repair structures with distinct patch systems. This thesispresents advanced SHM techniques and a novel modeling approach to improve theunderstanding of mechanical and failure behavior in complex structures, including 3Dwoven and repaired composites.