Research On Pyrolysis Properties And Constitutive Model Of EPDM Insulation

As the insulation material of solid rocket motors, ethylene propylene diene monomer (EPDM) is located between the motor case and propellant grain. During the rocket motor working process, change regularity of the burning area of grain could be effectively controlled by insulation to get the internal ballistic performance in demand,which ensures the normal work of the rocket motor. In addition, EPDM insulation also play an important role in buffer action of the stress and strain transfer between the motor case and propellant grain,which reduce the influence on propellant combustion from shock and vibration of the moter case.Which requires EPDM insulation should have good thermal stability and mechanical properties. In this research, thermal protection mechanism and constitutive model of EPDM insulation are inspected in the study. The study contents are as follows:(1) Research on thermal stability experiments of EPDM insulationThermal degradation temperature of EPDM is determined by thermogravimetric analysis. Microstructure, phase change, element composition and content of pyrolytic material are researched by energy dispersive spectrometer (EDS) and scanning electron microscope(SEM). Thermal protection mechanism of EPDM insulation is analyzed based on experiment results and theoretical foundation of the change of mechanical behavior of EPDM insulation at different pyrolytic state is provided.(2) Research on mechanics experiments of EPDM insulationEquilibrium response curves and tensile curves at different pyrolytic states are obtained by multi-step relaxation and uniaxial tensile tests. Viscous flow feature, strain rate sensitivity and super-elastic propertie are analyzed based on experiment results to determine the material mechanics category. Which provides the experimental basis on establishing constitutive model of EPDM insulation at different pyrolytic states.(3) Research on constitutive model of EPDM insulationBased on quasi-static mechanics experimental results of EPDM insulation, hyperelastic model was adopted to fitting the equilibrium response curve, based on which, the parameter function of strain and strain rate are introduced to describe the rate effects of EPDM insulation under the uniaxial tension tests. And in this way, visco-hyperelastic constitutive model is established to describe and predict the mechanical response characteristics of EPDM insulation as long as the strain rates are in a limited range.(4) Research on constitutive model of EPDM insulation pyrolytic layerBased on quasi-static mechanics experimental results that EPDM at the state of initial pyrolysis has obvious visco-hyperelastic feature, a visco-hyperelastic constitutive model consists of parallel composition with a hyperelasticity unit containing pyrolytic temperature and a viscoelastic unit containing damage; Based on mechanics experimental results of EPDM at the state of late pyrolysis, Ogden(n=2) model was tried to establish to describe the superelastic-brittle features. The fitting results show that mechanical behavior of EPDM at different pyrolysis state could be described by visco-hyperelastic constitutive model and superelastic-brittle constitutive model respectively, which provides a basis for the transmission of stress and strain of EPDM insulation during solid rocket motor working process.