Cryo-thermal Cycling Behavior Of Carbon Fiber/Epoxy Composites For Cryogenic Fuel Tanks

The development of heavy-lift launch vehicles and reusable launch vehicles is critical for human kind in implementing space activity and exploiting space resource.At the same time,design fuel tanks based on composite materials is an effective way to reduce the structural weight of launch vehicles and improve its payload capacity.However,the working conditions of fuel tanks are extremely harsh,and the composites for fuel tank should be capable to maintain their structural integrity after cryo-thermal cycling,which is particularly important for resuable launch vehicles.In addition,large difference in coefficient of thermal expansion between carbon fiber and polymer matrix is tending to generate cracks in composites after cryo-thermal cycling,which would result in structural failure.According to the above problems,five types of cryo-thermal cycling,including the combination of long cycling and short cycling and accelerated aging with water treatment,were designed in this thesis,which were employed to treat the unidirectional and orthogonal carbon fiber/epoxy composites.Subsequently,the effect of cryo-thermal cycling treatment on the performance and microstructure of composites were systematically studied.The main results are as follows:Based on the differential scanning calorimetry and viscosity analysis of epoxy resin,the parameters of autoclave process of composites were determined.Then,carbon fiber/epoxy prepregs and unidirectional and orthogonal carbon fiber/epoxy composite laminates were prepared by hot-melt method and autoclave process,respectively.The specimens of unidirectional carbon fiber/epoxy composites for 90° tensile test were treated with 4 types of cryo-thermal cycling.After being treated by cycling type I and III,the damage types of composites were mainly involved with interface cracks between carbon fiber and epoxy resin,and matrix cracks caused by interface cracks.After treated with cycling type ? and ?,the damage types of composites were dominated by voids and matrix cracks.In addition,the 90° tensile strength of unidirectional carbon fiber/epoxy composites decreased with the increase in cycle number,for the 4 types of cryo-thermal cycling.In comparison with original samples,the 90° tensile strength of unidirectional carbon fiber/epoxy composites treated with cycling type I to IV was descread by 20.3%,22.8%,14.5%,and 26.1%,respectively.The interlaminar shear specimen of carbon fiber/epoxy composites were treated with5 types of cryo-thermal cycling.The results showed that the unidirectional composites exhibited various damages including interface cracks,interface debonding,gaps,matrix cracks and voids.Besides these damages mentioned,de-laminations also were observed in orthogonal composite samples.Moreover,after treated by cycling type I to V,the ILSS of composites monotonically decreased with the increase of cycle number,due to thermal stress generated during cycling treatment or the shear stress generated by moisture absorption and swelling of epoxy resin.In comparison with original samples,the ILSS of unidirectional carbon fiber/epoxy composites treated with cycling type I to ? was descread by 4.9%,13.2%,5.1%,12.8% and 5.6%,respectively,while the ILSS of orthogonal carbon fiber/epoxy composites treated with cycling type I to ? was decreased by 17.1%,18.1%,17.3%,17.9% and 17.6%,respectively.