Study On The Effect Of Confining Pressure On Mechanical Properties Of N15 Solid Propellant

In order to meet the high thrust demand of solid rocket motors used in the power propulsion devices of the current strategic and tactical weapons and commercial space launch vehicles,N15 solid propellant has been widely applied because of its advantages of high specific impulse,low characteristic signal and stable performance.At present,the engine is designed to meet the requirements of the indicators,but the failure of the launch and even the explosion caused by the destruction of the internal structure during the ignition work is a problem for the researchers.N15 propellant works in a high-temperature and high-pressure environment during ignition.Nevertheless,regarding studies on the structural integrity of grain,the impact of environmental pressure on the mechanical properties of N15 propellant is rarely considered,and the related research results cannot truly describe the mechanical characteristics of propellant under working pressure.So as to explore the mechanics behaviors of N15 propellant under confining pressure environment and improve the propellant grain structure integrity research under confining pressure environment,this dissertation carried out the effect of confining pressure on mechanical properties of N15 propellant,aiming to study the influence of the confining pressure on N15 propellant mechanical behaviors,and reveal the influential mechanism of confining pressure on the mechanical behaviors of N15 propellant,thus establish the damage-containing constitutive model of N15 propellant to describe the effects of confining pressure.The main contents of this dissertation are as follows:(1)Study on the constitutive model of viscoelastic materials with damage.Based on the theory of continuum mechanics and irreversible thermodynamics,the mechanical properties of viscoelastic materials are introduced,then the integral and differential constitutive models describing the stress-strain relationship of viscoelastic materials are briefly introduced.Internal state variables are introduced through damage mechanics to describe the damage evolution.In the process,based on the potential energy theory proposed by Schapery,a dual energy density function is established,a constitutive model of the elastic body with damage and its damage evolution law are derived,a pseudo-variable theory is introduced,and an elastic-viscoelastic correspondence principle is adopted.The equations derived from the potential theory of elastic materials are generalized to viscoelastic materials,and the stress-strain relationship of viscoelastic materials is simplified to a constitutive equation similar to that of elastic materials.The constitutive model of elastic materials with damage is derived by reference in the process,a damage-containing constitutive model suitable for viscoelastic materials under confining pressure environment is derived.(2)Design and verification of confining pressure test system for viscoelastic materials.To meet the requirements for testing the mechanical properties of N15 propellants in the confining pressure environment,the confining pressure environment test device of petroleum,clay,coal and other materials is used for reference.According to the functional requirements and design indicators of the confining pressure test system,a confining pressure test system is proposed.In the overall plan,appropriate pressure supply parts are selected to assemble the pressure supply device.The design standards of the pressure vessel are used,and the design and strength check of each component of the confining pressure device are performed.The finite element software ABAQUS is applied to find the key parts under different working conditions.According to the simulation results of the structure,the structure design is optimized accordingly.The nitrogen confining pressure test system was built with proper sealing structure and electronic universal testing machine as loading device,and passed the performance test.(3)Experimental and influential study on the mechanical properties of N15 propellant under confining pressure.The relaxation test of N15 propellant at room temperature and atmospheric pressure is carried out,and the relaxation modulus in the form of Prony series is obtained.The numerical simulation of the grain under ignition pressure is performed using ABAQUS software,and the deformation characteristics of the grain are obtained,combined with the asymmetry of tension and compression of the propellant,the more vulnerable position of the propellant under ignition pressure is determined.Uniaxial tensile test of N15 propellant under confining pressure environment is performed,and the stress-strain response under different working conditions is obtained.and the mechanical property parameters of the material are obtained.The correlation between the confining pressure,strain rate and the material parameters is analyzed by the method of no repetition two factor variance analysis.The pressure sensitive index is introduced,and the Eyring model is improved,which can describe the relationship between the mechanical property parameters of the material and the confining pressure.Using scanning electron microscope,the cross-sectional morphology of the test specimen is obtained under different working conditions,and the mechanism of strengthening effect of environmental pressure on propellant is explained from the microstructure.(4)The establishment and verification of a viscoelastic-damage constitutive model for N15 propellants.A non-linear viscoelastic constitutive model with damage considering the effect of confining pressure on N15 propellant is established.Based on results of uniaxial tensile test the propellant under confining pressure,the damage function of one-dimensional viscoelastic constitutive model and the functional relationship between each parameter which represents the damage model and confining pressure are fitted.According to the obtained model parameters,the mechanical responses under different working conditions are calculated.In comparison with the experimental results,the accuracy of the model is verified.The influential law of confining pressure on the mechanical properties of N15 propellant is obtained.A viscoelastic-damage constitutive model of N15 propellant under confining pressure is established,which laid the foundation for the study of the mechanical properties of solid propellants under confining pressure,and provided a reference for the study of the viscoelastic constitutive model considering the influence of confining pressure,and finally laid the theoretical foundation for the research on the grain structural integrity of the solid rocket motor.