| Short fiber reinforced polymer composites(SFTRPC)structure parts with complicated shape,light-weight,good corrosion resistance,high accuracy and good performance stability could be manufactured by high efficient extrusion compounding and injection molding techniques.Recently,SFTRPCs have a good potential in the application of aerospace engineering,military equipment and machinery equipment.When aerospace composites are subjected to harsh service extreme temperature and loading,the variation in interfacial stress transfer determines the macroscopic mechanical behavior of SFTRPCs.Most of previous reports about SFTRPCs focused on the mechanical properties at room temperature.However,the mechanical properties at extreme temperatures are quite scarce.The present dissertation is dedicated to proposing extreme-temperature-resistant fiber sizing agents for composites in aerospace engineering.And the influencing mechanisms by fiber modifications for the mechanical properties of SFTRPCs at various temperatures were revealed.The main research contents and outcomes are summarized as follows:(1)Graphene oxide(GO)-sized short carbon fibers(GO@SCFs)were prepared via a dip coating method.SCF/PEI composites were manufactured by extrusion compounding and injection molding.The effects of GO content on the mechanical properties of SCF/PEI composites at room temperature were studied.The results indicate that the tensile strength of GO@SCF/PEI composites increases with the increase of GO sizing content,and achieves a maximum enhancement at the GO content of 0.4 wt%.Subsequently,the temperature-dependent tensile properties and fracture modes of GO@SCF/PEI composites were systematically investigated from-70 ℃ to170 ℃.The results show that the GO treatment plays a positive role in enhancing the tensile strength of SCF/PEI composites at-70 ℃ and 30 ℃ whereas a negative role at100 ℃ and 170 ℃.Morphological observations indicate that the fiber-matrix interfacial adhesion is enhanced at-70 ℃ and 30 ℃ but deteriorated at 100 ℃ and 170 ℃ due to the thermal expansion mismatch between SCFs(or GO)and PEI.(2)In order to overcome the issue of the degradation in the mechanical properties at high temperatures of SCF/PEI composites by GO sizing agent,thermally stable thermoplastic polyimide hybridized with GO sizing agent(GO-TPI)was proposed to coat carbon fiber surfaces.The effect of sizing agent on the temperature-dependent(-60 ℃,20 ℃ and 170 ℃)tensile behaviors of SCF/PEI composites were systematically investigated.The results indicate that the hybrid GO-TPI sizing leads to the optimum reinforcement effectiveness,attributing to the blocking effect of GO and the miscibility of TPI and PEI.Single fiber push-in test was conducted and corresponding numerical simulation was made to reveal the effect of GO-TPI sizing on the interfacial properties of SCF/PEI.In addition,the finite element modeling of single carbon fiber composite fragmentation with different sizing treatments was developed to clarify the temperature-dependent interfacial failure mode of PEI composites based on three types of coated SCFs.(3)The hybrid sizing agents of carbon nanotube(CNT)and polydopamine,which possesses good adhesive properties,fabrication ease and environmental-friendship,were developed to coat carbon fiber surfaces.A single fiber fragmentation test was experimentally carried out and the results were numerically analyzed to elucidate the effects of CNT-PDA sizing layer on the interfacial failure behavior.Furthermore,the effects of hybrid sizing agent on the temperature-dependent mechanical properties of PEI composites were investigated from both experiment results and theoretical predictions.The PEI composites with CNT-PDA treated SCFs exhibit the improved tensile strength at 25 ℃ and-60 ℃ due to the enhanced interfacial properties between SCFs and PEI after CNT-PDA treatment.However,CNT-PDA hybrid sizing treatment plays a negative role in enhancing the mechanical properties of SCF/PEI composites at170 ℃.The Fu-Lauke model predictions were conducted to interpret the reinforcing mechanisms of coated SCFs by hybrid sizing agent.Finally,the short term creep behavior of PEI composites was investigated by using Burger’s model and the results indicate that creep resistance of the PEI composites was obviously enhanced by the CNT-PDA sizing treatment at 25 ℃.(4)SFTRPC structure parts may experience violent temperature variation during servicing in outer space.Therefore,the effects of cryo-thermal cycling on the SCF/PEI composites were systematically investigated.Firstly,the influences of cryo-thermal cycling(-196 ℃～170 ℃)on the mechanical properties at 25 ℃ and 170 ℃ of SCF/PEI composites were analyzed.The results show that after cryo-thermal cycling,the tensile strength at 25 ℃ of SCF/PEI composites was apparently improved and the modifying effect of sizing treatment on the composites was still maintained.After cryo-thermal cycling,the mechanical properties at 170 ℃ of SCF/PEI composites were also enhanced.More importantly,the negative effect of 0.25CNT-PDA sizing agent on the mechanical properties at 170 ℃ was improved by cryo-thermal cycling process.Subsequently,in order to reveal the enhancing mechanisms for the mechanical properties at 25 ℃ and 170 ℃ of SCF/PEI composites during cryo-thermal cycling,the effect of cryo-thermal cycling on the SCF reinforcement and strength of PEI were analyzed.Finally,the effects of cryo-thermal cycling on the creep resistance and thermal stability of SCF/PEI composites were investigated. |
