The Study On Cryogenic Mechanical Properties Of Epoxy Resin And Its Composites

With the development of aerospace industry,the requirements of light weight for aircraft are getting higher and higher,as the main structure of the launch vehicle,the propellant tank has a large volume and a high mass proportion.In order to meet the needs of weight loss,the development of high-strength,low-density tanks has become the primary goal of the power countries in spaceflight.High performance carbon fiber reinforced resin matrix(CFRP)composites have been widely used in aerospace industry due to their high specific strength,high specific modulus,low density,low thermal conductivity,corrosion resistance and fatigue resistance,and it is gradually replacing the key parts of metal materials used in the manufacture of aerospace vehicles.However,the development of composite cryogenic tank needs to investigate cryogenic temperature mechanical properties of the materials in depth.In this paper,the mechanical properties of bisphenol F epoxy resin(NPEF-170)and T700 carbon fiber reinforced epoxy resin were tested under different cryogenic temperature environment(from-180°C to 20°C),to study how cryogenic temperature influences the stiffness,strength and elongation of epoxy resin and CFRP.Meanwhile,SEM was used to observe the fracture surface of epoxy resin and CFRP,analyzing the damage mechanism under low temperature.A lot of finite element model of CFRP with random distribution of fiber were established.The properties of epoxy resin at low temperature are used as input parameters to predict the elasticity and stress-strain curves of CFRP at cryogenic temperature.Then the stress-strain curves were compared with the previous experimental results.At the same time,the residual thermal stress of carbon fiber reinforced epoxy composite plate at cryogenic temperature and the damage mechanism under low temperature were analyzed and studied.The experimental results show that the modulus,tensile and compressive strength of the epoxy resin increase with the decrease of temperature,but the fracture strain decreases with the decrease of temperature.The transverse tensile modulus and strength of CFRP are higher at cryogenic temperature.The longitudinal tensile strength is still higher at cryogenic temperature,but the modulus show no change with temperature.The macroscopic and microscopic fracture morphology of the samples shows that the epoxy resin is plastic at room temperature while brittle at cryogenic temperature.The simulation results show that with temperature decreases from 20°C to-180°C,the thermal residual stress of fiber mainly shows compressive stress,and the matrix mainly shows tensile stress.At the same time,the areas with high stress are basically concentrated in the areas where fibers and fibers are nearer.The predicted elastic modulus and strength of composites agree well with the experimental results.By simulating the tensile damage of CFRP and combing the macroscopic microscopic morphology of the experimental damage,it is shown that the interfaces of CFRP are strengthened at low temperature,and destruction extend in the resin along the boundary of the interfaces.