| Carbon fiber reinforced plastic?CFRP?laminated composites have been widely used in aviation-space structures because of its excellent mechanical properties.However,the interlaminar relative damage/failure behavior of laminated composites can seriously affect the loading capacity and service life of structures,herein,how to improve its toughness and anti-delamination capability has been a research focus among scholars all over the world.So the research of toughening method,toughening mechanism and relevant theory will be very significant.Recently,most investigations have proved that only a few amount of micro/nano particles can significantly improve interlaminar fracture toughness of laminated composites due to their outstanding multi-characters such as small size,large specific area and high area energy,which make the micro/nano particles toughened composites become the hot research point in composites field.The toughening characteristic and mechanisms of nano-rubber particles,carbon nanotubes?CNTs?and short carbon fibers?SCFs?on laminated composites were investigated by experimental characterization,fracture morphology observation and theoretical analysis.The theoretical models for predicting interlaminar fracture toughness of laminated composites incorporated by nano-fillers were also established.The detailed work is shown as follows:?1?To obtain the toughening effects of nano-rubber on epoxy resin under different strain rate,the fracture toughness of nano-rubber modified and unmodified epoxy resin under different strain rate(5×10-4s-1,1×10-1s-1,2.5×10-1s-1,90 s-1)were obtained though quasi-static test on Instron apparatus and dynamic test on Charpy apparatus.The experimental results showed that the 6wt%nano-rubber particles can give the highest toughening ability under static loading rate compared to dynamic loading rate,and furthermore,the nano-rubber toughening effect on epoxy resin was reduced with strain rate increase.The microscope observations indicated that the void plastic growth after rubber cavitaiton and matrix shear band is the main toughening mechanism on nano-rubber toughened epoxies resin,the extent of plastic void growth and rubber cavitations is limited by the high strain rate,which finally decrease the oughening effect.?2?To investigate nano-rubber toughening effects on epoxies and epoxy/carbon fiber composites under different temperatures,the fracture toughness of nano-rubber toughed epoxy and nano-rubber toughed epoxy/carbon fiber composites were obtained respectively under low temperatures?-80oC,-50oC,0oC?,room temperature?20oC?and high temperatures?50 oC,80 oC?.It is demonstrated that the addition of nano-rubber particles can significantly toughen the epoxies and epoxy/carbon fiber composites at room temperature.However,it was found that sub-zero severe temperatures?-50oC and-80oC?lead to remarkably reduced effect of nano-rubber toughening both on epoxies and epoxy/carbon fiber composites,whereas elevated temperature?50oC?also causes diminishing nano-rubber toughening effect on epoxy/carbon fiber composites.Scanning electron microscopy?SEM?examinations also revealed that both low temperature and high temperature can limit the capacity of void plastic growth after rubber cavitations and finally lead to decreasing on the toughening effect.?3?The CFs@CNTs hybrid structure was constructed through CNTs grown on CF using flame method,and then the CFs@CNTs hybrid structure toughening effect on epoxy/carbon fiber composites was investigated under delamination test.The microscope analysis revealed that the CNTs grown on CF improved interface between CF bundles and matrix finally leading to the cohesive failure mode on the fracture surface after delamination.More importantly,CNTs detached from CF,CNTs pullout from matrix,CNTs breakage and matrix deformation was found on the fracture surface of laminate as the crtical toughening mechanisms.?4?To further investigate the influence of CNT@Cf hybrid structure on delamination properties of epoxy/carbon fiber composites,combined with generalized beam theory and energy based approach,an analytical model was developed based on CNT pullout law,CNT detachment law and CNT sword-in-sheath law.The numerical results show CNTs with long length,large diameter,low modulus and high strength grown on the CF can significantly increase interlaminar fracture toughness of epoxy/carbon fiber composites,increasing attachment strength between CNTs and the fibres led to further fracture toughness improvement.?5?To obtain simple and low-cost method to toughen laminated composites,the non-woven carbon tissues?NWCT?composed of short carbon fibers was made using chopped and filtration procedure.The delamination,flexural and electricity conductivity experimental results show the non-woven carbon tissues can remarkable increase the interlaminar fracture toughness and electrical conductivity without compromising of flexural modulus and flexural strength.Microscopy analysis indicated that the short chopped carbon fibers can easily form into a 3D network witth more fibers aligned along through-the-thickness of laminate which benefit the higher fracture toughness of the laminated composites.SEM observation and theoretical analysis indicated that the pullout of the short carbon fibers from matrix and matrix plastic deformation are the main toughening mechanisms for laminated composites reinforced by NWCT interleave. |
PDF Full Text Request