A Visco-hyperelastic Constitutive Model For NEPE Propellant And Its Application

To increase the range and to decrease the passive mass of the rocket weapon system,high energy solid propellants are widely used.NEPE(Nitrate Ester Plasticized Polyether)propellant,with unlimited potential,represents the development direction of solid propellant,due to its excellent mechanical capabilities,high volume density and high specific impulse.To investigating on the mechanical properties,present a proper constitutive model and resolve the structural integrity is the premise of its application.However,the complicated microstructure leads to its complex mechanical performances,which multiply the difficulties on the task.To address the problem,study on the macro-mechanical performances of the NEPE propellant was carried out.The main contents of this thesis are as follows:(1)Mechanical experiments on the NEPE propellant were carried out.Uniaxial tensile experiments under serial temperature at different rates verified the rate dependence and temperature dependent of its mechanical properties.Yield stress,initial modulus,rupture stress were linear correlation with the logarithm of strain rates.Relaxation modulus in the form of Prony series was obtained by the Meng-Sorvari method.The discrepancy between the equilibrium stress in multi-step relaxation experiment and the results in the ultra-low rate tensile experiment got more significant with the increase of the strain,because the damage in multi-step relaxation experiment was much larger than that in ultra-low rate tensile experiment.The overstress and stress ratio were grabbed to characterize the relaxation behavior in multi-step relaxation experiment.The corresponding overstress versus strain behavior showed an initial slop increase and then gradually stable.The stress ratio at different strain were not statistically significant different.It was turned out that the time characteristics of the relaxation behavior were unchanged at all strain conditions.Stress softening effect and instantaneous residual strain phenomenon appeared in loading-unloading experiments.(2)Based on the fundamental theory of constitutive model,a visco-hyperelastic constitutive model coupled with volume dilatation damage was presented.Shear modulus damage function and bulk modulus damage function were grabbed to characterize the distortion part and volume part changes of mechanical properties due to damage respectively.The comparison between the experimental data and predicted results showed that the visco-hyperelastic constitutive model was able to accurately characterize the mechanical behavior of NEPE propellant.(3)Study on the application of the visco-hyperelastic constitutive model of NEPE propellant were carried out.Based on the Abaqus finite elements software,a UMAT subroutine was programmed to simulate the uniaxial constant-rate and variable-rates tensile processes.The simulation results were well fitted with the experimental data,which illustrated the validity of the constitutive model and its numerical method.(4)The numerical method was applied to different working condition.The simulation results could describe the stress concentration and strain concentration of a specific structural component,the reaction forces of which was approximately equal in both experiment and simulation.Both simulation and experiment showed that stress concentration factor had a distinct effect on the notch strength of the NEPE propellant,which demonstrated its sensitivity to notch.The UMAT subroutine was grabbed to simulate the star perforation grain of NEPE propellant with different sizes.The simulation results revealed the stress and strain response under inner pressure and acquired the variation laws of star perforation grain strength along with the structural parameter.Preliminary results revealed that the numerical method of this work could precisely characterize the mechanical properties of NEPE propellant,and could be used effectively in the finite element simulation,which would be a theoretical foundation for grain structural integrity analysis.