| Three-dimensional(3D)braided composites have excellent structural integrity,lightweight,high-strength,anddelamination resistance.During service in the atmospheric environment,the strengthdegradation of 3D braided composites caused by thermo-oxidative aging is inevitable.In addition,the braided composites after aging anddegradation may also be affected by various of impactdamage,such as bird impact and hail impact.The agingdegradation of structural parts contribute to a significant influence on thedynamic impact resistance.It is of great significance to explore and reveal thedynamic mechanical response of 3D braided composites underdifferent thermal and oxygen aging environments for thedurability,safety service performance,and life prediction of braided composite components.This project aims to investigate the impact fracture properties of three-dimensional four-direction(3D4d) and threedimensional five-direction(3D5d) braided composites before and after thermaloxidative aging.We employed experimental tests and finite element analyses to investigate the effects of aging time and temperature on the degradation of impact fracture strength of braided composites.The mechanism of impact fracture damage of braided composites after thermal oxidation aging has been revealed,which provides a theoretical and experimental reference for the safe service of braided composites in the atmospheric environment.The main contents are as follows.(1)Thermo-oxidative aging times and temperatures of epoxy resin and braided composites weredetermined by the glass transition temperature of the epoxy resin polymer,which was measured bydynamic thermal mechanical analysis(DMA).The aging temperatures of braided composites were set as 90? and 180?,and the isothermal aging times were 0,2,4,8,and 16days.(2)The surface morphologies and chemical properties of aged epoxy resin polymer were characterized by the optical camera and Microscope-infrared spectroscopy(Micro-FTIR)analysis.Electron scanning microscope(SEM)was employed to observe the interfacedamage of aged 3D braided composites.The mechanical response of epoxy resin after thermal-oxidative aging was characterized by uniaxialdynamic and static compression tests.To investigate the effect of thermo-oxidative aging on fracture behaviors,dynamic and static fracture tests were performed on 3D4d and 3D5d braided composites.(3)Multi-scale finite element models were established based on the real interweaving of braided yarns in composites.By introducing the properties of the aged resin,the material properties of the microstructure yarn model,mesoscale model(inner cell,surface cell,and corner cell),and macro-scale homogeneous model were predicted.Based on the results,a meso-macro structure cross-scale finite element model was proposed to explore the effects of thermal-oxidative aging on the impact fracture mechanical properties of 3D4d and 3D5d braided composites.Thedegradation mechanism of thermal-oxidative aging impact strength of the composites has been revealed.The main contents are as follows(1)The interaction between temperature and oxygen accelerates the aging process of epoxy resin.When aging at lower temperatures(90?)or the shorter exposure time,the post-curing leads to the enhancement of the mechanical properties of epoxy resin.On the contrary,while aging at high temperatures(180?),the thermaldecomposition of macromolecular in the epoxy resin and the oxidation of small molecules in the surface layer become the main factors affecting the mechanical properties of epoxy resin.(2)During the thermal-oxidative aging process,thedegradation of the braided composites is not only affected by the pyrolysis and oxidation of the resin matrix,but also by the weakening of the interface.Aging temperature and exposure time have significant effects on the impact fracture properties of the 3D braided composites.When ageing at lower temperatures(90?) for 16days,the aged resindominates the change of mechanical properties of composites.In this case,the impact fracture strength of braided composites is enhanced by the post-curing of the resin matrix.While ageing at high temperatures at 180? for 16days,the properties of the yarn/matrix interfacedecreased.The thermal-oxidativedegradation of the resin matrix and the weakening of interface properties lead to the rapiddeterioration of impact fracture properties of the braided composites.(3)Based on the multi-scale finite element method,we successfully predicted the macro homogeneous model material properties of braided composites which has been aged at 90? and 180? for 16days.According to the actual interweaving of yarns in braiding structure,the inner cell,surface cell,and corner cell of the braided structure were established in mesoscale,and the periodicdisplacement boundary condition application equations of surface cell and corner cell model arederived.By introducing the elastic parameters of aged resin,the properties of micro-scale yarn cell,mesoscale braided inner cell,surface cell,and corner cell were predicted.Finally,the properties of the material of mesoscale cell model were given to the homogenization model,and the macro homogeneous model material properties of 3D4d and 3D5d braided composites which has been aged at 90? and 180? for 16days were obtained.The multi-scale finite element analysis exhibits important guiding significance for thedesign of braided composite parts in aging environment.(4)Based on the proposed meso-macro structure cross-scale finite element simulation scheme,thedynamic fracture mechanical response anddamage morphologies of the aged 3D4d and 3D5d braided composites have been effectively simulated.The results show that the load-displacement curves of 3D4d and 3D5d braided composites were in good agreement with the experimental results.For the 3D5d braided composite model,the stress level of axial yarns was higher than that of the braided yarns.Due to the weakening of the interface between the yarns and resin matrix,the composites aged at 180? showed lower stress levels and the most serious fracturedamage.Serious interfacedamage of braided composites aged at 180 ? will hinder the load transmission and then lead to lower stressdistribution in the braided yarns.During thermo-oxidative aging,the impact fracture propertydegradation of 3D4d and 3D5d braided composites is attributed to both yarn-matrix interfacialdamage and resindegradation.The introduction of axial yarns improves the impact fracture resistance of3D5d braided composites.From the perspective of specific energy absorption,the axial yarns contribute to the maximum energy absorption capacity in the process of impact fracture.These results on the above revealed the effects of aging temperature and exposure time on impact fracture behaviorsdegradation of 3D4d and 3D5d braided composites.Thedegradation mechanism of impact fracture properties of 3D4d and 3D5d braided composites has been further showed by the thermal-oxidativedegradation of the resin matrix and thedamage of the yarn/matrix interfaces.We hope our investigations could provide a significant guide on structuraldesign and material selection of braided structural composites under long-term service in the thermal-oxidative aging environment. |
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