| Carbon fiber reinforced polymer composites(CFRP)are widely used in aerospace and rail transportation industries because of their high strength and modulus,excellent corrosion resistance and low density.However,due to the poor interlaminar toughness,CFRP composites are prone to delamination damage when subjected to out-of-plane mechanical loads(such as low-velocity impact).The electrical conductivity of CFRP composites is poor,so they are vulnerable to lightning damage,In addition,the internal delamination of laminates is not easily detected,which seriously threatens the safety of aircraft.Therefore,in order to meet the requirements of structure-function integration of CFRP composites,it is necessary to realize online damage monitoring in service state while improving their mechanical properties and electrical conductivity.In summary,this paper aims to explore the preparation of functional interlayers capable of industrial mass production through low-cost and simple processes to achieve the synergistic improvement of mechanical properties,conductive properties and delamination monitoring sensitivity of carbon fiber composites.The main research contents and conclusions are as follows:Polyethersulfone(PES)films were prepared by solution molding method,and the interlaminar fracture toughness and delamination monitoring sensitivity of CFRP composites were simultaneously improved.The results showed that the initial and propagation mode I interlaminar fracture toughness(GⅠC,ini and GⅠC,prop)and the modeⅡ interlaminar fracture toughness(GⅡC)of carbon fiber composites are 207.60 J/m2,221.75 J/m2 and 1823.14 J/m2,respectively.The PES film interlayer with an areal density of 15 g/m2 increased the GⅠC,ini,GⅠC,prop and GⅡC of the laminate by 32.25%,55.00%and 9.59%,respectively.The toughening mechanism of PES film was mainly due to the formation of a phase separation structure composed of PES shell and epoxy resin inclusions,then the plastic deformation of PES increased the fracture energy dissipation.Meanwhile,by measuring the resistance change during the delamination process of CFRP composites,the in-situ monitoring of the initiation and evolution of mode I de lamination and the initiation of mode Ⅱ delamination were realized.The mode I delamination sensitivity coefficient(k)of the control sample is 0.14,and the relative resistance change(ΔR/R0)of mode Ⅱ delamination initiation is 4.30%.The PES film interlayer increased the k value of mode I delamination to 4.55.However,the electrical conductivity of the laminate decreased from 0.24 S/m to 0.07 S/m after the insertion of the insulating PES film.In order to improve the mechanical properties and the delamination monitoring sensitivity without reducing the electrical conductivity of composites,based on the above studies,multi-walled carbon nanotubes(MWCNTs)with high electrical conductivity were introduced into the PES to prepare the PES/CNT functional films.The functional films simultaneously improved the interlaminar fracture toughness,impact resistance,electrical conductivity and delamination monitoring sensitivity of CFRP composites.The results showed that the PES/CNT functional film with a CNT content of 5 wt%(P@5CNT)increased the GⅠC,ini and GⅠC,prop of the CFRP composite by 27.10%and 9.79%,respectively,the damage area under 20 J impact energy was reduced by 14.90%.The PES/CNT film with a CNT content of 10 wt%(P@10CNT)increased the GⅡC of composites by 98.00%.The toughening mechanism was mainly due to the plastic deformation of PES in the phase separation structure,the pull-out,fracture and bridging of CNTs,which improved the resistance to crack evolution.At the same time,the PES/CNT functional film with CNT content of 2.5 wt%increased the k of mode I delamination to 1.71,and increased the ΔR/R0 of mode II delamination to 20.91%.The increase of CNT content improved the electrical conductivity of functional films,and the electrical conductivity of the P@10CNT film increased to 107.96 S/m,the electrical conductivity of the prepared laminates increased by 62.50%.However,it reduced the k of the mode I delamination to 0.89,and reduced the ΔR/R0 of mode Ⅱ delamination onset to 6.20%(still higher than the Control).It is confirmed that functional films with different toughening effects,electrical conductivity,and delamination monitoring sensitivity can be tailored by adjusting the CNT content in the functional films.In order to solve the problems of easy agglomeration and relatively high cost of CNTs,low-cost and easily dispersed carbon black(CB)was introduced into PES to prepare PES/CB functional film interlayers,which achieved the simultaneous improvement of interlaminar fracture toughness,impact resistance and delamination monitoring sensitivity of CFRP composites.The results showed that the PES/CB functional film with CB content of 7.5 wt%increased the GⅠC,ini,GⅠC,prop and GⅡC of carbon fiber composites by 99.16%,82.83%and 74.04%,respectively.The damage area of the composite decreased by 27.17%at 20 J impact energy The toughening mechanism was mainly due to the plastic deformation of PES,the pull-out of CB,which consumed more fracture energy.Conductive CB improved the electrical conductivity of the functional films,and the electrical conductivity of the PES/CB functional film with a CB content of 10 wt%increased to 2.39 S/m,which also increased the electrical conductivity of the laminate by 12.50%.The PES/CB functional film significantly improved the delamination monitoring sensitivity of the laminates.The PES/CB functional film with 2.5 wt%CB content increased the k value of mode I delamination and the ΔR/R0 of mode Ⅱ delamination onset to 3.26 and 20.10%,respectively.In addition,the mathematical model of relative delamination increment(Δa/c),ΔR/R0 and electrical conductivity was further deduced,confirming that for interlayers with a specific electrical conductivity,the delamination size can be predicted based on the monitored ΔR/R0 values.Nickel-coated carbon fiber(NiCF)veil,as a metal and non-metal hybrid material,has excellent mechanical and electrical properties.NiCF veil interlayers improved the interlaminar fracture toughness,impact resistance,electrical conductivity and electrical stability during mode I delamination of CFRP composites.The results showed that the NiCF veil with low areal density(17 g/m2,N17)significantly improved the GⅠC,ini and GⅠC,prop of the laminate by 74.75%and 54.46%,respectively.The NiCF veil with high areal density(34 g/m2,N34)increased the GⅡC of the laminate by 36.22%.The toughening mechanism was mainly attributed to the pull-out,debonding and fracture of the NiCFs,and the peeling and fracture of the nickel coating.The effect of NiCF veil on the low-velocity impact resistance of the laminate was studied by combining experiment and finite element simulation.After inserting a layer of N34 NiCF veil in the mid-plane of the laminate,the impact peak impact force and the residual compressive strength increased by 4.39%and 16.15%,respectively,and the damage area decreased by 30.92%.Since the NiCF veil has higher electrical conductivity than the above functional films,the insertion of a layer of N17 and N34 increased the electrical conductivity in the thickness direction of the laminate by 125.05%and 220.49%,respectively.The ΔR/Ro of mode Ⅱ delamination onset was also improved by 14.20%and 11.56%.However,the high-conductivity NiCF veil interlayer reduced theΔR/R0 of the laminate during mode I delamination,this means that the NiCF veil improved the electrical stability of the mode I delamination.In addition,a structurefunction relationship expression was established to qualitatively verify the relationship among ΔR/R0,interlayer resistance and Δa/c.In this paper,the effects of polymer/carbon nanomaterial functional films and NiCF veil interlayers with hybrid metal and non-metal structures on the interlaminar fracture toughness,impact resistance,electrical properties and delamination monitoring sensitivity of carbon fiber composites were systematically studied.The feasibility of customizing interlayer materials with different toughening effect/delamination monitoring sensitivity or different toughening effect/electrical stability is proved,which has application potential in the fields of interlaminar toughening,structural health monitoring and lightning strike protection of carbon fiber composites. |
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