Study On Hybrid Veil Synergistic Interlaminar Toughened Carbon Fiber/epoxy Composite Based On Polyethersulfone Porous Fibers

Carbon fiber/epoxy（CF/EP）composites have the advantages of light weight and high strength,corrosion resistance,and designability,which are beginning to be used on a large scale in the fields of aerospace,rail transportation,and sports.However,CF/EP composites are mainly prepared by prepreg/hot press tank process,which has high material cost,long process flow and large equipment investment,resulting in high cost of use.The liquid molding process represented by vacuum-assisted resin infusion molding（VARI）,on the other hand,does not require low-temperature storage of raw materials,has a small mold investment,and can realize the overall molding of large and complex structures,which gradually becomes one of the main development directions for low-cost manufacturing.In practical engineering applications,CF/EP composites are usually in the form of laminates,and the carbon fiber layers only rely on epoxy resin for bonding and load transfer,while the epoxy resin is bad brittle and toughness after curing,resulting in poor interlaminar toughness and easy delamination damage of the composites.Therefore,how to improve the interlaminar toughness of composite materials without affecting the liquid molding process has become a key problem that needs to be solved urgently.In this thesis,polyethersulfone（PES）was selected to prepare PES porous fibers efficiently and stably by wet spinning.on this basis,different hybrid fiber veils were designed to interlayer toughen the CF/EP composites prepared by VARI process,and the effects of different hybrid fiber veil on the interlayer structure and properties of CF/EP composites were systematically investigated,and the interlayer toughening mechanism of hybrid fiber veil was investigated and elucidated.Firstly,in this paper,PES porous fibers were prepared by wet spinning,and PES short-cut fiber veil（PESV）with different areal densities were prepared by wet-laid method,and the structure and properties of PESV interlayer toughened CF/EP composites and the toughening mechanism were systematically analyzed.The results showed that the finger-like microporous structure existed inside the PES porous fibers prepared by wet spinning from inside to outside,and the fibers were compatible with epoxy resin.The interlaminar fracture toughness of CF/EP composites showed a trend of increasing and then decreasing with the increase of PESV areal density,and the highest interlaminar toughness was achieved at the PESV areal density of 31.6 gsm,which increased by54.4%and 62.2%compared with the reference G_IC and G_IIC,respectively.SEM analysis revealed that the toughening mechanism was mainly due to the dissolution and diffusion of PES porous fibers in the epoxy resin and the formation of uniformly distributed PES microspheres in the epoxy resin matrix through phase separation,which triggered plastic deformation of the matrix,PES microsphere pullout and crack deflection and termination to improve the interlaminar toughness of the composite.The in-plane mechanical properties tests showed that the low areal density of PESV did not negatively ARfect the tensile,flexural and interlaminar shear properties of the CF/EP composites.The tensile strength,flexural strength,flexural modulus and interlaminar shear strength of the composites were increased by 10.8%,4.0%,7.7%and 2.9%,respectively,compared to the reference samples at a PESV areal density of 21.6 gsm.Subsequently,PES/CNT composite fibers were prepared by introducing carbon nanotubes（CNT）into PES dissolution and by wet spinning,and PES/CNT fiber veil with different CNT contents（referred to as PES/CNT-V）with a areal density of 21.9 gsm were prepared by wet-laid method,and the effects of PES/CNT-V on the structure and properties of CF/EP composites were investigated,and the interlayer synergistic toughening mechanism of CNT and PES was explored.The results showed that the G_IC and G_ⅡC of the CF/EP composites toughened by PES/CNT-V with different CNT contents showed a trend of increasing and then decreasing with the increase of CNT contents,and the interlaminar fracture toughness of PES/CNT-V with 100-1.8（mass ratio of PES to CNT）reached the best value,and the G_IC and G_ⅡC increased by 36.0 and 35.5%,respectively,compared with the reference samples.SEM studies revealed that PES microspheres and CNT were uniformly distributed in the interlaminar epoxy resin region,in which,in addition to the above-mentioned interlaminar toughening effect of PES microspheres,the bridging,pull-out and fracture of CNT at the nanoscale as well as to crack deflection and termination could further consume energy,and the superposition and synergistic effect of the two toughening mechanisms significantly improved the interlaminar toughness of the composites.In-plane mechanical property tests showed that the tensile properties,flexural properties and interlaminar shear properties of CF/EP composites toughened with different CNT contents of PES/CNT-V interlaminar showed different degrees of improvement;among them,the tensile strength of the composites was improved by 31.1%after100-1.8 PES/CNT-V toughening;100-2.4 PES/CNT-V after toughening,the flexural strength and flexural modulus of the composites increased by 2.4%and 4.4%,respectively;after toughening with100-1.8 PES/CNT-V,the interlaminar shear strength of the composites reached a maximum value of 67.2 MPa,which increased by 5.5%compared with the reference sample.Finally,in this paper,PES porous fibers and high fracture toughness aramid（AR）fibers were selected to prepare hybrid fiber veil with different mass ratios of 21.9 gsm areal density（referred to as PES/AR-V）by wet-laid method,and the effects of PES/AR-V on the interlaminar fracture toughness and other mechanical properties of the composites were systematically investigated,and the interlaminar synergistic toughening mechanism of PES/AR-V was analyzed.The results showed that the G_IC and G_ⅡC of CF/EP composites toughened with different mass ratios of PES/AR-V plies were improved to different degrees.1-1（mass ratio of PES to AR）PES/AR-V toughened composites reached the maximum G_IC,which was 60%higher than that of the reference sample;2-1 PES/AR-V toughened composites reached the maximum G_ⅡC,which was 60%higher than that of the reference sample;2-1 PES/AR-V toughened composites reached the maximum G_ⅡC,which was60%higher than that of the reference sample.SEM characterization revealed that PES microspheres dissolved in epoxy resin and formed by reaction-induced phase separation were uniformly distributed in the interlaminar resin region,which improved the interlaminar toughness by consuming energy for crack deflection or termination and plastic deformation;high toughness AR were distributed in the interlaminar resin region,which improved the interlaminar toughness by bridging action The high toughness AR are distributed in the interlaminar resin region,and the energy is consumed through bridging and tearing of AR,so PES/AR-V can play a good synergistic toughening effect on CF/EP.The in-plane mechanical property tests showed that 2-1 PES/AR-V toughening had the least effect on the tensile properties,flexural properties and interlaminar shear strength of the composites.Among them,the tensile strength of the composites increased by 5.8%compared with the reference sample,and the flexural modulus was basically the same,while the tensile modulus,flexural strength and interlaminar shear strength of the composites decreased by11.6%,12.0%and 15.9%,respectively,compared with the reference sample.