Research On Thermal Expansion Properties Of EP And CFRP At Ultra-low Temperature

As the main container for liquid fuel,cryogenic storage tanks account for approximately 70%of the dry weight of launch vehicle.the use of ultra-low temperature rocket fuel storage tanks made of carbon fiber reinforced resin matrix composites(CFRP)to replace traditional metal storage tanks is expected to reduce the weight of rocket fuel tank by 25-30%and the cost by 20-25%.the mismatch of low temperature linear expansion between carbon fiber(CF)and resin(EP)leads to microcrack which is the main reason for leakage,and leakage is the main failure mode of CFRP tank.In this regard,this paper analyzes the evolution mechanism of the low-temperature thermal expansion behavior of EP and CFRP and its influencing factors,and gives the best design window for preparing low-linear expansion coefficient EP and good low-temperature thermal expansion CFRP.In order to study the variation of the coefficient of thermal expansion(CTE)of EP at low temperature and the effect of molecular structure on CTE,the CTE of EP at room temperature to-196?was measured.Based on the data,a model was established according to the relevant literature,and an effective analytical expression was given by regression analysis.Combined with the test results of DMA and DSC,it is verified that the temperature dependence of EP is interval.CTE decreases with the descending of temperature,and multiple secondary transformations occur at low temperature.Integrated with DMA and infrared tests,the influence of molecular structure such as the degree of cross-linking,flexible and rigid groups on CTE is analyzed.Nano-silica particles(n-SiO₂)were used to modify EP to prepare low linear expansion coefficient epoxy resin,and to explore the influence of n-SiO₂ on the low-temperature linear expansion behavior of EP.the temperature dependence of n-SiO₂/EP is similar to that of EP.the addition of nanoparticles reduces the CTE of EP and at the same time causes a positive shift in the transition temperature.Combined with XRD,DSC,DMA,SEM and other test methods,the mechanism of the effect of nanoparticles on the linear expansion coefficient of EP was analyzed.For the purpose of exploring the low temperature thermal expansion behavior of CFRP and the influence of material design and structure design on CTE,the reinforcement effect of CF system on CFRP is analyzed.Based on the research of unidirectional fiber-reinforced composite materials,so as to reduce the anisotropy of the material's CTE and improve the leakage resistance,the CFRP layer is designed to find that the leakage resistance has been improved but when the angle between fibers is 90°the anisotropy still exists.In order to solve the limitations of the ply design,after introduction of n-SiO₂ to prepare hybrid composite materials,it is found that the anisotropy characteristics were significantly reduced,and the helium leakage rate was reduced by 54.7%.Combining the droplet debonding experiment and low-temperature fracture SEM to characterize the CFRP interface is for analyzing the mechanism of the impact of nanoparticles on the CTE of the hybrid composite material through the low-temperature interface.Combining the low-temperature linear expansion and leakage resistance of CFRP,the process window for preparing micro-expanded CFRP with better near-isotropic anti-leakage performance is given.