| The balance of compression and tension performance was the important development direction of third-generation composites,and relatively poor compression performance of carbon fiber composites have limited the implementation as primary and secondary structural components in aerospace filed.Therefore,improvement of longitudinal compressive strength of carbon fiber reinforced resin composites（CFRCs）has become a focus in academia and industry.During the compression failure process of composites,lateral supporting of resin to carbon fiber was lost and elastic buckling initially occurred as the increase of external compressive load,and then plastic kinking was gradually evolved till the fiber broke,further the delamination occurred at poor interfacial bonding,resulting in premature compressive failure.Thus,the compression failure of composites was correlated to the three elements:fiber,resin and interface.The fiber buckling resistance can be enhanced through increasing elastic modulus and diameter of the fiber,and increasement of elastic and shear modulus of resin were helpful to improve the lateral support of the fiber,while increasing the interfacial bonding strength of composites can avoid premature compression failure caused by delamination.In this paper,the inorganic nano-stiffening resin system were designed,and the correlation between rigid/flexible nano interphase and resin matrix modulus was investigated.The“organic-inorganic synergistic”stiffening resin matrix were prepared,and the correlative mechanism of resin modulus on interfacial properties and compressive strength of composites was established.The interlayer reinforced carbon fiber composites with carbon nanotube film were prepared,and the mechanism of interlayer interface and stiffness on compressive strength of composites was established.The multilevel enhancement of longitudinal compressive strength of CFRCs were realized by resin and interlaminar stiffening.1.Flexible polyethylene glycol diglycidyl ether（PE）,rigid alicyclic epoxy（CE）and semi-rigid di-glycidyl ether of bisphenol A（DE）grafted nano-silica（SiO2）were designed and prepared,and the effects of different grafting chain structures on surface conformation and characteristics of epoxide grafted silica were studied,and the wettability between epoxide grafted silica and epoxy resin was evaluated,as well as the dispersion in ethanol and epoxy resin.According to molecular simulation and measurement results,PE grafting chains curled on SiO2surface,and the thickness of grafting chain layer was thin and dense,while CE grafting chains exhibited a stretched conformation with thicker and looser grafting chain layer.DE grafting chains showed half-stretched and half-curled conformation,and thickness and density of grafting chain layer were between the PE and CE grafting chains;Compared with un-grafted SiO2,the surface energy of SiO2-PE,SiO2-CE,SiO2-DE were improved,and the dispersibility and stability in ethanol and epoxy resin were optimized.Inorganic stiffened epoxy with above epoxide grafted silica were prepared（TDS,TDSP,TDSC,TDSD）,and relations between conformation of grafting chains and structure and properties of interphase were studied,and influence of elastic modulus of nano-stiffening resin on longitudinal compressive strength of carbon fiber composites was also investigated.Compared with TDS resin system,the interpenetration and interfacial chemical reaction between the resin molecules and epoxide grafting chains on silica surface of TDSP,TDSC,and TDSD resin system occurred,which resulted in the thickness of interphase enlarging to 44±5,70±12 and 53±4nm,respectively,with increasing elastic modulus of interphase of 25.55,37.65and 33.59 GPa.Due to the increased thickness and modulus of interphase,the elastic modulus of TDSP,TDSC and TDSD resin were increased to 4126±51,4277±51 and 4220±37 MPa.Compared with CF/TDS composites,the longitudinal compressive strength of CF/TDSP,CF/TDSC and CF/TDSD composites were improved by 14.7%,19.3%and 19.1%,ascribing to increased lateral support of fiber from increasement of resin modulus,which reduced the fiber buckling during compression failure of composites.2.The un-stiffened epoxy resin（TD）,organic amide acid（AA）stiffened resin（TDA）and synergistically stiffened with organic amide acid and epoxide grafted silica（SiO2-DE）（TDAS）were prepared,and the“??organic-inorganic synergistic”stiffening mechanism of epoxy resin was analyzed,as well as the influence of resin modulus on interfacial properties of carbon fiber composite.Based on model calculation and experimental verification,the influence of resin modulus and interfacial property on compressive strength of composites was investigated,and compressive elastic-plastic failure model of carbon fiber composites was established.In comparison with un-modified TD resin（3529±125 MPa）,the modulus of organic stiffened TDA resin was increased by 18.9%,while the modulus of TDAS resin was increased by34.4%due to the synergistic effect of AA and SiO2-DE.The interfacial bonding and failure modes were improved in CF/TDA and CF/TDAS composites as the resin modulus increased,and transverse fiber bundles tension（TFBT）strength were increased by 27.1%and 63.5%,respectively,while the interlaminar shear strength（ILSS）of composite were separately increased by 36.4%and 61.8%.According to Jumahat models and experimental verification,the theoretical compressive strength of CF/TDA and CF/TDAS composites were separately increased by 10.9%and 21.0%in comparison with CF/TD composites,which was consisted with the measurement results.The fiber micro-buckling in the elastic stage was delayed with the increasement of resin modulus,and the debonding between fibers and resins were also declined as the improvement of interfacial properties,resulting in the simultaneous increasement of longitudinal compressive elastic and plastic stress of carbon fiber composites.3.Based on CNT film（p-CNT-f）from chemical vapor deposition（CVD）,CNT/epoxy composite film（c-CNT-f）was prepared by hot-melting pre-impregnation method,and the interlayer reinforced composites with composite film were prepared.The effect of CNT and composite film interlayer on the interlaminar interface and rigidity of composites were investigated,and the correlation mechanism between interlaminar reinforcing and stiffening and compressive failure of composites was established.Compared with p-CNT-f,the surface chemical characteristics were improved in c-CNT-f,and the infiltration and wettability between CNT film and resin matrix were optimized,as well as the tensile properties of CNT film.Compared with CF/TDA composites,ILSS of CF/TDA/p-CNT-f composites was almost unchanged,while that of CF/TDA/c-CNT-f composites was increased by 18.5%,and in-plane shear modulus of CF/TDA/p-CNT-f and CF/TDA/c-CNT-f composites were increased by 12.4%and 16.0%,respectively,and longitudinal compressive strength were separately increased by 16.3%and 22.3%.The bridging effects of c-CNT-f in the interlaminar area improved the interlaminar interface bonding,thereby the delamination during the compression process was reduced;Meanwhile,the introduction of the CNT film improved the rigidity of interlaminar resin-rich area,which increased the transverse supporting of carbon fiber,leading to enhanced longitudinal compressive strength of composite. |
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