| Carbon fiber reinforced polymer composite(CFRP)is widely used in modern industry for its advantages of lightweight,high strength,anti-fatigue,anti-corrosion and high designability.Due to the special needs of weight reduction,integrated design and multi-function of aerospace vehicles,the application range and usage of CFRP composite materials in aircraft have been greatly increased in recent years.However,compared with the metal material such as aluminum,titanium alloy,the electrical conductivities of CFRP composites are relatively weak and exhibit obvious anisotropy,which makes large amount of Joule heat generate locally and instantaneously when lightning current flows through the composite material.For this reason.CFRP composites become more sensitive to the lighning environment.On the other hand,as with metal or other non-metallic composites,the supersonic and ultra-high temperature plasma discharge channel during lightning has significant ablation and impact effects on CFRP composites.In general,CFRP composites will be subjected to the coupled electrical-thermal-mechanical direct effect during the lightning process,and the CFRP composite structure will exhibit the electrical-thermal-chemical-mechanical coupling response,which puts forward high requirements for theoretical research and numerical simulation.At present,there have been numerous researches about lightning damage of CFRP composites,however,most of the work is based on the electrical-thermal coupling analysis on the Joule heating effect of lightning,and there are relatively few researches on the complex action mechanism of lightning on CFRP composites,the damage mode of composites and the evolution laws of material properties containing damage.In addition,the simulation of lightning damage of CFRP composites is a multifields coupling problem,and many factors need to be considered.It is difficult to get accurate results simply through the electrical-thermal coupling analysis.In this paper,the lightning damage mechanism of CFRP composites was studied by combining experimental and numerical methods.In terms of experimental research:(1)The key properties of TR50S-15L/YPH308 composite material directly related to lightning damage analysis including kinetic parameters of pyrolysis reaction,anisotropic electrical conductivities,modulus,strength and fracture toughness were tested and studied.This provided input parameters for the electrical-thermal-chemical-mechanical coupling analysis of lightning damage,and helped understand the electrical,thermal and mechanical properties of CFRP composites comprehensively.(2)For TR50S-15L/YPH308 composite laminates,the artificial simulated lightning tests including the nonstandard waveform(35/100 ?s)with low current peak(10?50 kA),standard waveform(lightning component D)with high current peak(80?100 kA)and combined current waveform of standard lightning components(D and C)were carried out.The post-lightning specimens were characterized by a variety of methods(apparent,SEM,ultrasonic C/B scan,etc.)to reveal the damage characteristic of CFRP composites and its cause of formation under the action of lightning strike.It was found that CFRP composites had unique damage characteristics when they were subjected to transient electrical-thermal-mechanical coupling effects of lightning,and it was impossible for CFRP composites under the action of conventional single factor to form the gas escape channel(bare carbon fibers warped up)and thickened fiber fracture.(3)The damage characteristics of CFRP composite laminates under the action of combined current waveforms of standard lightning components D+C and single current waveform of standard lightning component D were studied comparatively,and the damage analysis was conducted based on the interaction mechanism of lightning discharge channel and composite structure.The results showed that the in-plane damage morphology of CFRPs under impulsed current waveform was diamond,which was determined by anistropic electrical conductivities of CFRPs;after that,the continuous current waveform induced a conical ablation pit,which was determined by the lightning arc heat and had nothing to do with the anisotropy character of composite material.In addition,the comparative study also found that the application of continuous current waveform of component C after component D did not increase the in-plane damage area formed during the action stage of component D,but aggravated the damage degree of the in-plane damage region,and significantly increased the damage depth at the same time.In the aspect of numerical simulation,based on the artificial lightning tests,the coupled electrical-thermal-chemical-mechanical lightning damage analysis model of CFRPs was established by the finite element commercial software ABAQUS and its user defined subroutines step by step.The detailed establishment process and research conclusions are as follows:(1)Based on the basic theory of electrical-theraml coupling analysis and pyrolytic reaction kinetics theory,the pyrolysis reaction kinetics model of the composite material was introduced by user-defined subroutines USDFLD and HETVAL in electrical-thermal coupling analysis module of ABAQUS in order to realize the process that the physical properties degraded with pyrolysis degree and then affected the electric field and temperature field.The electrical-thermal-chemical coupling lightning damage analysis model of CFRP composites was established,and then the electrical,temperature and pyrolysis degree fields and the evolution laws in time and space were obtained.The application of electrical-thermal-chemical coupling analysis model avoided the problem of premature material degradation in the simulation caused by applying temperature as the degradation criterion of composite material.The equations of current and heat flux varying with the lightning current waveform in time and space were defined according to the radius expansion law of lightning arc root.In addition,the relevant assumptions and equivalent treatment methods were put forward to solve the problems of thermal load lowering,high temperature gas dissipation and cooling.(2)The pyrolytic damage of CFRP laminate under combined lightning current waveforms D+C was calculated by the established electrical-thermal-chemical coupling analysis model,and then was compared with the experiment results.It was found that the electrical-thermal-chemical coupling analysis model could predict the in-depth damage well,but for in-plane damage,the predicted damage area by the simulation was smaller than the experiment.This indicated that pyrolytic damage was only one of the damage forms for in-plane damage but contributed to most of the in-depth damage.Furthermore,by comparing the electric potentials and pyrolysis degrees of specific nodes in the model at specific moments,the electrical conduction mechanism of CFRPs related to resin pyrolysis was revealed.Moreover,by comparing to the results calculated by the Joule heat model,the different effects between Joule and arc heating during lightning components D and C were discriminated.The results showed that the Joule heating with transient characteristic dominated the temperature rise of CFRPs during component D,whereas the arc heating effect played a leading role during component C as there was enough time for heat conduction.(3)Based on the electrical-thermal-chemical coupling analysis model,on the one hand,the effects of characteristic parameters of lightning current waveforms on pyrolytic damage were studied.The results showed that the varying curves of pyrolysis degree with action integral under different waveform parameters were coincidence,and the influence of waveform parameters on the pyrolytic damage was mainly reflected by action integral;there was a strong exponential relationship between the pyrolysis degree and the action integral.On the other hand,the effects of electrical conductivities on the pyrolytic damage were studied.The results showed that the pyrolytic damage decreased significantly with the increase of the electrical conductivities in the fiber direction,and the electrical conductivity along the vertical fiber direction in the plane had no effect on the pyrolytic damage;especially,there was a threshold value for the influence of electrical conductivity along thickness direction on the damage depth of pyrolysis.Only when the electrical conductivity along thickness direction was greater than the threshold value,increasing the electrical conductivity along thickness direction had the positive effect of reducing the damage depth.(4)The lightning damage constitutive models of CFRPs were constructed,which were especially suitable for the electrical-thermal-chemical-mechanical coupling analysis.For intralaminar damage,the effect of pyrolytic damage on mechanical properties was regarded as the reduction of effective bearing area of the material unit according to continuum damage mechanics,and the lightning damage variable considering both pyrolytic and mechanical damages was introduced by user-defined subroutine VUMAT,based on Hashin failure criterion,the intralaminar constitutive model of progressive lightning damage considering pyrolytic damage was constructed;For interlaminar damage,in oder to avoiding the unit-type cohesive model affecting the calculation of electrical-thermal-chemical coupling analysis,the contact-type cohesive model was adopted to establish the interlaminar mechanical damage constitutive model.The temperature and pyrolysis degree fields changing with time and space calculated by the electrical-thermal-chemical coupling analysis model were imported as the predefined fields into the explicite analysis module of ABAQUS in order to realize the sequential coupling and data transfer between electrical-thermal-chemical analysis and thermal-chemical-mechanical analysis.At last the electrical-thermal-chemical-mechanical coupling lightning damage analysis model of CFRPs was established,and the cooperative computing of pyrolytic and mechanical damages was realized.(5)Based on the electrical-thermal-chemical-mechanical coupling analysis model,the mechanical response and damage of CFRPs under lightning current waveform D when considering thermal stress and lightning impact was calculated and analyzed.The results showed that the deformation of the body of CFRPs presented saddle-shape,and kepted damped vibration after lightning strike;there were several bulged layers at the central region on the side of facing lightning,and obvious delamination damage was observed.Moreover,the delamination damage between adjacent layers could not be completely covered by the pyrolytic damage at the same time.Furthermore,by comparing the simulation results between only considering the thermal stress and only considering the lightning impact,it was found that the thermal stress was the main factor leading to the complex stress state as well as the uplift and local bulge at the central region on the side of facing lightning.The thermal stress led to the delamination damage,increased the friction energy,and thus accelerated the consumption of mechanical energy of CFRPs in the lightning process;the lightning impact pressure was the main factor leading to the stress state and downward bending deformation at the edges of CFRPs,and it was the main source of mechanical energy in the lightning process and determined the amplitude and mode of damping vibration In addition,the effects of pyrolytic coefficient on the mechanical response and damage of CFRPs were analyzed by using the finite element method.The results showed that the pyrolytic coefficient had a great influence on the mechanical response and mechanical damage of CFRPs;with the increase of pyrolytic coefficient,mechanical damages would decrease in different rates.It was very important to determine the influencing weight of the pyrolytic damage and mechanical damage on the degradation of material properties in the mechanical analysis of CFRPs subjected to lightning strike.In this paper,with comprehensively considering of lightning current,heat flow and impact,the electrical-thermal-chemical-mechanical coupling analysis of CFPRs subjected to lightning was established,and the involved electrical,thermal,chemical and mechanical processes were analyzed systematically at the same time.The work greatly deepens the research on the lightning damage mechanism of CFRPs,and it has great significance for more accurately assessing the lightning damage,reducing the cost of lightning tests and optimizing the design of lightning protection technology. |
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