The Study Of Cryogenic Mechanical Properties Of T700 Graphite/Epoxy Composite Materials

Currently, reusable launch vehicle (RLV) has been an urgent need in the field of aerospace applications. Cryogenic propellant tank is the main structure of RLV, whose quality and volume account for great proportion of RLV. Therefore, the successful manufacture of high strength and light weight cryogenic tanks is the critical technology for RLV. With high mechanical performance as well as good thermal properties, carbon fiber reinforced plastics (CFRP) may be an alternative material for the cryogenic tank structure. However, the working condition of reusable liquid oxygen propellant tanks is very complex. The tanks are usually working with conversion between liquid oxygen temperature and room temperature, which significantly changes the mechanical properties of composites. So it is necessary to study the mechanical properties and damage progress of CFRP at cryogenic temperatures.To simulate the actual condition of cryogenic propellant tank, the ultra-low temperature mechanical properties of unidirectional and cross-ply T700/epoxy composite laminates were investigated using an environmental test chamber. Besides, the specimens were submerged in liquid oxygen (LO2) for different time and immersed in LO2 and returned to room temperature for different thermal cycles. After a certain immersing time or thermal cycle, the micro-morphologies of specimens were observed by environment scanning electron microscope (SEM). The specimens after cryogenic treatment were also tested bending properties to study the relationship between damage of micro-morphologies and mechanical properties.The result shows that the tensile modulus increases at ultra-low temperature, while the tensile strength decreases at ultra-low temperature. And bending strength and bending modulus both increase significantly. Fiber debonding and micro-cracks appear in the inner of the unidirectional composite laminates after immersing cycling. With the increasing of immersing time and cycle index, the number of fiber debonding and micro-crack increases. Correspondingly, the tensile strength of specimens after immersing and thermal cycles first rise and then fall. However, most of the micro-cracks are through individual layer for the cross-ply composite laminates. With the increasing of immersing time and cycle index, the amount of micro-crack increases and the density of micro-crack in outer layer increases greater than that in the inner layer. However, the tensile strength of treated specimens first fall and then rise with the increasing of micro-cracks'amount. The blending modulus of specimens has the same variation trend with blending strength.