Carbon fiber reinforced composites, as a kind of high-performance composite material, possess many advantages like light weight, high specific strength and specific modulus, good resistance to corrosion and chemical stability. They have been widely used in aerospace, automotive industry and sports equipment, etc., and have become a research hotspot in the past few decades. Most carbon fiber reinforced composites are fabricated as laminated structures. Compared to the in-plane directions, there are no fibers in the thickness direction; therefore, the interlaminar properties are relatively poor, which have lower strength and toughness. Delamination and crack propagation damage will easily happen when subjected to load, which directly affects the loading capability. Besides, carbon fiber reinforced composites generally have lower damping properties dur to their high stiffness, resulting in that the tiny cracks in many engineering structures fabricated by carbon fiber reinforced composites, including military equipment, automotive and aircraft parts, etc., will gradually propagate due to the fatigue loading caused by the vibration during daily use, leading to premature failure. Therefore, domestic and foreign scholars have carried out a lot of related research work and also proposed some effective solutions in recent years. However, there is still a lack of methods that can improve the interlaminar and damping properties of carbon fiber reinforced composites at the same time. Based on the existed research, interleaving flexible interlayers (thermoplastic polymer membranes or plant fiber layers) or hybridizing nanoparticles could not only improve the interlaminar properties, but also influence the damping properties. Therefore, the interlaminar hybridization method is used in this thesis to interleave nanoparticles, thermoplastic polymer, nano-fibrous membranes and plant fibers to enhance the interlaminar and damping properties of carbon fiber reinforced composites. For the used thermoplastic polymer, polysulfone is selected to fabricate the thermoplastic interleaves owing to the good miscibility with epoxy. Besides, to well maintain the mechanical properties of the hybrid composites, plant fiber fabrics with better mechanical properties are selected. For the used nanoparticles, cellulose nanocrystals which originate from plants are selected in this thesis. Tongji University Doctor of Engineering Abstract IV Firstly, the effect of cellulose nanocrystals (CNC) and polysulfone (PSF) on the mechanical properties and fracture toughness of epoxy resin are investigated. The CNCs are successfully extracted from microcrystalline cellulose by acid hydrolysis. The extracted CNCs are mixed with epoxy to investigate the effect of different CNC contents on the performance of epoxy matrix. Results show that the homogeneous dispersion of CNCs in the epoxy is the key factor to improve the mechanical properties and fracture toughness of epoxy. The toughening effect of thermoplastic PSF resin is obvious in the epoxy resin. During the curing process, the reaction induced phase separation will occur in the PSF/epoxy blend resin. The phase separation mechanism is closely related to the content of PSF in the PSF/epoxy blend resin. The reinforcing and toughening mechanism of CNC and PSF on the epoxy are revealed by morphological observation and other characterization methods. The mechanical properties and fracture toughness of epoxy can be effectively enhanced after hybridized with CNC and PSF, which provides basic raw materials for the fabrication of interleaves in interlaminar hybridization to improve the interlaminar and damping properties of carbon fiber reinforced composites. Then, based on the above experimental results, the PSF, PSF/CNC nano-fibrous membranes with a certain mass fraction are fabricated by electrospinning technique and interleaved between the interlayers in the unidirectional carbon fiber reinforced epoxy composites. The interlaminar fracture toughness, damping properties and other properties of these interleaved composite laminates are investigated by interlaminar fracture toughness test, vibration damping test and other mechanical tests. Results show that interleaving CNC/PSF nano-fibrous membranes in the carbon fiber reinforced composites effectively enhances the interlaminar fracture toughness and damping properties of carbon fiber reinforced composites. The in-plane mechanical properties are however not affected too much. Based on the obtained test results and microstructure morphological observation, the interlaminar toughening and damping modification mechanism are revealed. Subsequently, the effect of flax fiber hybridization on the interlaminar fracture toughness and damping properties of carbon fiber reinforced composites are investigated. Results show that the interlaminar fracture toughness of carbon fiber reinforced composites is improved greatly after hybridized with flax fiber. Flax fiber surface treatment can further improve the interlaminar properties of the composite laminates. The damping properties of hybrid fiber reinforced composites are greatly Tongji University Doctor of Engineering Abstract V influenced by the stacking sequence of flax fibers. When the flax fibers are at the outmost layers, the damping properties are enhanced significantly compared to carbon fiber reinforced composites. Based on the obtained test results and microstructure morphological observation, the interlaminar toughening and damping enhancement mechanism are revealed, which are related to the multi-layered and hollow microstructure of flax fibers. Lastly, based on the experimental results, the finite element numerical simulation method is used to investigate the interlaminar fracture behavior and vibration process of carbon fiber reinforced epoxy resin hybrid composites from the perspective of calculation. Based on the traditional bilinear cohesive model, a trilinear cohesive model including fiber bridging effect is established to describe the interlaminar fracture behavior of unidirectional fiber reinforced composites. Finite element modal analysis is used to investigate the natural frequency and mode of vibration of unidirectional fiber reinforced composites. Results show that compared to the experimental results, the cohesive model used in this thesis can effectively predict the interlaminar crack propagation and vibration modal characteristics of unidirectional fiber reinforced composites, which provides calculation methods for the optimization design of the interlaminar hybrid composite materials. Conclusively, a new method that can improve the interlaminar and damping properties of carbon fiber reinforced composites simultaneously is introduced in this thesis. The problems of relatively poor interlamianr and damping properties of unidirectional carbon fiber reinforced composites are solved by interlaminar hybridization with electrospun nano-fibrous membranes and plant fibers. The interlaminar toughening and damping enhancement mechanisms of CNC, PSF, nano-fibrous membranes and flax fibers on the carbon fiber reinforced composites are also revealed. On the basis of aforementioned experimental results, the finite element simulation and calculation methods provided to lay the foundation of interlaminar toughening and damping optimization of the carbon fiber reinforced composites.
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