Lightning Strike Protection Of Carbon Fiber Reinforced Polymer Composites

Carbon fiber reinforced polymer(CFRP)composites are widely used in aerospace field because of their high mechanical strength and rigidity,excellent fatigue resistance and corrosion resistance,as well as the lower density for weight loss compared with metal materials.It is an inevitable trend that the CFRPs will be widely applied in large aircraft,because their high-strength and light-weight can remarkable increase the passenger capacity and fuel efficiency of aircraft.However,unlike metallic structural parts that can quickly release lightning current,CFRPs are vulnerable to lightning strike damage due to their poor conductivity,leaving a serious threat to aircraft safety.In order to ensure the safety of CFRPs in aircraft applications,lightning strike protection(LSP)of CFRP is required.Therefore,it is of great significance to study the lightning strike damage and protection of CFRPs,and to improve their LSP capability,which can promote the application safety of CFRP in aircraft.In view of the above background,this research subj ect was proposed.In this study,several issues on LSP of CFRPs were deeply studied by the method of combining experiment and simulation,where the experiment was the main method while the theoretical simulation was supplemented.Firstly,the lightning damage behavior and mechanism of CFRP were studied by artificial lightning strike experiments and finite element method(FEM)simulation.Secondly,taking expanded metal foils(EMFs)and nickel-plated carbon fiber nonwoven veils(Ni-CFNVs)as examples,the LSP effectiveness and mechanism of different LSP layers were studied.Finally,taking fiberglass isolation as an example,the lightning protection effect and mechanism of the anticorrosion-used isolation layer was explored.The main research contents and results are as follows:The lightning tests of CFRPs showed that the lightning damage of CFRPs included three main forms:resin ablation,carbon fiber fracture and delamination,and the main damage driving force was the high temperature and mechanical force generated by lightning strike.After the CFRP specimens subjected to lightning strike,severe resin ablation and fibers breakage damage occurred on their surface,and a large amount of delamination damage occurred inside the composite.However,the surface damage area and the internal layer damage area were inconsistent.Both the surface damage and internal damage of the composite material gradually extended toward both the layering direction and the thickness direction from the lightning strike point.The CFRPs had different damage characteristics and damage mechanisms under different lightning waveforms.When the CFRP was subjected to the standard Waveform D(impulse current waveform,100 kA),the damage was concentrated on one lightning attachment point,with small area and great depth.When the CFRP was subjected to the standard Waveform C(continuing current waveform.200 Coulomb),the damage dots were randomly distributed,with large area and small depth.The standard Waveform C had a typical thermal ablation effect,while the standard Waveform D had a typical electro-thermal-mechanical impact effect.In this thesis,all the parameters of the pyrolysis dynamic equation of CFRP were obtained from analyzing the thermogravimetric curves at different heating rates by Friedman method,and the expression of the thermal pyrolysis degree a was derived by the difference method.Then,by coupling the electro-thermal field and the pyrolysis kinetic constitutive,the lightning strike damage model of CFRP was established,which can simulate thermal ablation process of CFRP during lightning strikes well.The FEM simulation results are in good agreement with the experimental results,indicating the accuracy and applicability of the FEM model.Then,by comparing the simulation results of lightning damage and the experimental results,the universal lightning damage criterion of CFRPs was established,i.e.,the in-plane damage of the lightning strike can be evaluated by the temperature field,and the in-depth damage of the lightning strike can be evaluated by the pyrolysis field.The EMFs presented high lightning protection efficiency under the artificial simulated lightning strike of standard Waveform C and Waveform D.Although the EMFs were ablated by lightning,the underlying CFRP layers remained substantially intact.Compared with the unprotected CFRP specimen,the lightning damage of the specimen with expanded foils was significantly reduced,and the residual strength and modulus after lightning strike were improved.The LSP efficiency of the expanded metal mesh increased as the areal density increased.The LSP efficiency of the expanded aluminum foil was significantly higher than that of the expanded copper foil of the same areal density,even higher than the expanded copper foil whose surface density is twice.The LSP efficiency of the lightning protection film depended mainly on the areal density of the internal EMF rather than the resin film layer.The damage morphology of the specimen after lightning strike indicates that the conductivity of the EMFs layer was anisotropic.In this thesis,FEM calculation by RVE model was used to prove the anisotropy of the electrical conductivity of the EMFs.The conductivity in the Long Width of Diamond(LWD)direction,i.e.,Long diagonal direction of the diamond grid,of the EMFs is 2-5 times than that of the Short Width of Diamond(SWD)direction,i.e.,Short diagonal direction of the diamond grid.The anisotropic electrical conductivity of the expanded foils is mainly due to the fact that its mesh structure has diamond-shaped(LWD>SWD).By adjusting LWD/SWD,the degree of conductivity anisotropy(the ratio of ?LWD/?SWD)of the EMFs can be designed.Taking the EMF as an example,this study proposes a new method to improve the LSP efficiency by using LSP layer with anisotropic conductivity.In the lightning strike Zone 1 and Zone 3 of an aircraft,the LWD direction of the EMFs needs to be designed to be aligned with the lightning current path.In the lightning strike Zone 2,the LWD direction of the EMFs should be designed to be vertical to the aircraft flying directionAs a metal and non-metal integrated(hybrid)material,Ni-CFNV has excellent 2 lightning protection performance.Lightning strike experiments showed that 70 g/m2 Ni-CFNV had higher LSP efficiency than the 73 g/m2 commercial expansion copper foil under both standard Waveform C and Waveform D.Therefore,the Ni-CFNV can replace the metal mesh as the LSP scheme for CFRP structures in lightning Zone 2 of aircraft.The Ni-CFNVs are made of nickel-plated carbon fiber with core-sheath structure,and they have two LSP networks:the nickel coating network and carbon fiber framework.The LSP mechanism can be explained by the synergy of these two networks:nickel coating network makes the Ni-CFNV more conductive,and the carbon fiber skeleton increases the ablation resistance of the Ni-CFNV.In addition to the electrical conductivity of the protective layer,the ablation resistance of the protective layer is also an important factor affecting the LSP efficiency of the LSP layer for CFRPs.An excellent LSP scheme must have both electrical conductivity and ablation resistance.The lightning strike tests proved that the fiberglass isolation layer used for electrochemical corrosion protection were able to improve the LSP effect of CFRPs.The LSP effect of the fiberglass isolation layer is based on an "isolation" mechanism,which can be explained by its electrical insulation,thermal insulation and fire resistance.As an electrical insulator,the fiberglass isolation layer prevents lightning arcs,sparks and current from penetrating down to the underlying CFRP layer,preventing the CFRP from being ablated by the internal Joule heat.As a thermal insulator,the glass fiber can prevent the lightning plasma,arc or spark and the Joule heat of the protective layer or the heat of the flame from being transferred to the underlying CFRP layers,inhibiting the ablation of the CFRP layers by external heat.The fire resistance of the fiberglass isolation layer can effectively extend the service life of the fiberglass,isolate the charge and heat of the lightning.and inhibit the combustion of the CFRP.The isolation layer is important for LSP of CFRPs and it must be electrically insulating,thermally insulating and fire resistant.Inserting an electrically insulating,thermally insulating and fire resistant isolation layer between the LSP layer and the CFRP layer not only can eliminate the electrochemical corrosion of the LSP layer,but also can improve the LSP effectiveness of CFRP.In conclusion,on the basis of the lightning damage mechanism of CFRPs,the roles of LSP layers and isolation layers in lightning protection were studied in the thesis,revealing the LSP mechanisms and laws of CFRPs,which provide important guiding significance to the LSP optimal design for the CFRP structures on aircraft.