| Carbon fiber reinforced epoxy composites(CF/EP)are widely used in the field of aerospace due to the light weight and excellent mechanical properties.The epoxy matrix in the interlaminar region plays an important role in bonding and transferring load.However,the epoxy resin is inherently brittle due to the highly crosslinked network structure,which results in the bad fracture toughness and easy cracking and delamination.The interleaved carbon fiber reinforced epoxy composites by thermoplastic/carbon nanotubes hybrid interleaves combine the advantages of outstanding mechanical properties of carbon nanotubes,high ductile of thermoplastic resin and the toughening effect of thermosetting/thermoplastic/carbon nanotubes triphase structure,which shows the potential to comprehensively improve the mechanical properties.However,up to now,the excellent properties of carbon nanotubes have not been revealed.What’s more,the complex synergistic toughening and strengthening mechanism of thermoplastic resin and carbon nanotube is not clear.Therefore,the thermoplastic resin Polyaryletherketone with Cardo(PEK-C)and carbon nanotubes(CNTs)were used to interleave the carbon fiber reinforced epoxy composites,and their toughening effect and mechanism were studied in this subject.What’s more,the interlaminar microstructure of the interleaved composites was regulated based on the study of the correlation between fracture toughness and microstructure of EP/PEK-C/CNTs triphase system to further improve the fracture toughness.The main research contents and results are as follows:Firstly,the toughening effect and mechanism of the PEK-C modified CF/EP composites were studied.The PEK-C film interlayer was proved to be more efficient and stable in the fracture toughness than the matrix toughening and PEKC particles interleaving.Secondly,the correlation between fracture toughness and structure of the film interleaved composites was analyzed.It was found that the layered dualphase structure composed of wrinkled PEK-C homogeneous phase,nodular inversion phase and sea-island structure was formed.The three phase structures were all helpful to improve the fracture toughness.The interlaminar cracks propagated in the layered structure back and forth,which further consumed the fracture energy and increased the GIC by 90.6%.Secondly,the CNTs were introduced into PEK-C film to study the microstructure and properties of the interleaved composites by PEK-C/CNTs hybrid interlayers.First of all,the PEK-C/CNTs hybrid film was used to interleave the composite.It was found that the PEK-C/CNTs hybrid film further increased the interlaminar fracture toughness(GIC)by 16.9%and compensated the flexural properties.The CNTs were scattered in the central of wrinkled PEK-C homogeneous phase,enhancing the the interaction between PEK-C and epoxy matrix.And then the influence of PEK-C/CNTs hybrid structure and sandwich structure on the toughening effect and interlaminar structure was studied.It was found that the PEK-C/CNTs sandwich film produced a multi-layer structure in the interlaminar region and prevented the concentrated distribution of CNTs in the PEK-C homogeneous phase.The cracks propagated back and forth under mode I tensile load,dissipating the fracture energy and resulting in the best toughening effect.The GIC was increased by 138.11%compared with the blank composites.Thirdly,in order to explore the influence of CNTs on the EP/PEK-C dualphase structure and clarify the synergistic mechanism of PEK-C and CNTs,the fracture toughness and microstructure of CNTs/PEK-C/EP triphase system were studied.It was found that the particle size in dualphse tended to be smaller by the presence of CNTs and so the toughening effect of dualphase was weakened.The improvement of fracture toughness of triphase system mainly came from the bulk effect of CNTs.In order to further simulate the interlaminar region of the interleaved composites,PEK-C film-EP layered structure and PEK-C/CNTs film-EP layered structure were prepared and the diffusion and phase separation were studied by mircrostructure characterization.It was proved that the resin diffusion stopped at the beginning of EP curing and the introduction of CNTs affected the mutual diffusion of resin.Finally,based on the above research conclusions related with the microstructure of the layered structure,the interleaved composites by PEK-C/CNTs hybrid film were prepared by reducing the curing temperature and increasing the curing time,and the interlaminar fracture toughness was tested.It was proved that reducing the curing temperature could further improve the mode I interlaminar fracture toughness of the interleaved composites by 14.7%because the curing reaction of epoxy resin was delayed by reducing the curing temperature and resin diffusion and phase separation were more sufficient,which was benefited to form the efficient interlaminar structure,further improving the fracture toughness. |
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