Experimental And Applied Research On Constitutive Model Of Composite Solid Propellant Under Superimposed Pressure And Changing Rates

In recent years,the structural integrity of SRM during ignition is one of the most critical part which draws attention of the design department and the application department for SRM.Solid propellant is not only sensitive to the change of temperature and strain rate,but also to pressure variation.It is very frequent that the mechanical performance tests of propellants under atmospheric pressure can completely meet the requirements while problems still occur during test or missile launch,which has become a major problem in the research field of mechanical properties of propellants.For the SRM with wall pouring,the propellant is under superimposed pressure during the ignition,so it is very important to evaluate the mechanical response characteristics of SRM under superimposed pressure.This study focuses on the mechanical properties of composite propellant,a test system for propellant under superimposed pressure loading conditions was developed.The mechanical response of the propellant under superimposed pressure is studied systematically through the superimposed pressure loading system,and a nonlinear constitutive model considering the influence of dewetting and superimposed pressure is proposed.It is used to analyze the structural integrity of SRM by Abaqus after numerical discretization.The main research contents are as follows:The stress-strain response law of the propellant under wide temperature and strain rates range was found,and the mechanism of rate-dependent “dewetting” damage and double peak phenomena of the propellant was revealed.Damage characteristics was found during rate-dependent tensile test under different temperature and strain rates.Specifically,double peak phenomenon was found under room and low temperature during fast stretching.The “dewetting” damage mechanism was studied through numerical analysis based on combining molecular dynamics method,finite element model with the definition of cohesion model on the interface of particles and binder.It is found that both the quantity of damaged interfaces and the extent of damage evolution of the interfaces have an impact on the mechanical response characteristics of the propellant.Based on the experimental characteristics and damage mechanism,the law of ratedependent stress-strain response of the propellants was predicted.The design scheme of the superimposed pressure loading system for propellant was proposed,and the "pneumatic" superimposed pressure loading system for propellant with wide temperature and strain rate loading range was invented.Based on the estimation of the technical index of the superimposed pressure loading test system,the preliminary design scheme of the superimposed pressure testing system was proposed.The design,assembly and commissioning process of the superimposed pressure testing system was explained in details.The built superimposed pressure test system has fully considered the extreme service environment of SRM and takes the target ambient temperature,pressure and strain rate as the main parameter index.Repeated tests show the system has good consistency and high stability,which can be used for the test on the mechanical properties of viscoelastic materials under superimposed pressure.The mechanical response characteristics and laws of composite propellant under superimposed pressure were obtained,and the damage and failure mechanism of propellant under superimposed pressure were revealed.By performing uniaxial test for propellant under different superimposed pressure,temperature and strain rates,it was found that the “dewetting” damage can be restrained under superimposed pressure by noticing there are no obvious “dewetting” point on stress-strain curve.The damage mechanism analysis based on damage interface extraction was used to compare the damage interface of propellant under atmospheric pressure and superimposed pressure,it was found that the damage interface of propellant under superimposed pressure was significantly reduced when comparing to that of under atmospheric pressure.With the help of mesoscopic mechanics simulation and electron microscope scanning test,it was found that with the increase of superimposed pressure,the microscopic damage form of propellant has gradually changed from particle “dewetting” dominated to particle breakage dominated.The extrusion between particles under superimposed pressure leads to the successively crushing of particles with bigger size,and the matrix and the remaining particles are too weak to resist the continuous force of the applied load.The sudden drop in stiffness eventually results in the sudden fracture of the propellant.The mechanical properties of propellant under superimposed pressure are sensitive to the size and overall proportion of AP particles of propellant.According to the mechanical response characteristics of propellant under superimposed pressure,a nonlinear rate-dependent constitutive model of propellant with the consideration of superimposed pressure and “dewetting” effect was established.Based on developed Zhu-Wang-Tang nonlinear viscoelastic constitutive model,the nonlinear viscoelastic constitutive model by considering the effect of superimposed pressure and "dewetting",as well as wide range of strain rates,was proposed.The parameters for the constitutive model was obtained by piecewise fitting through the test data under different superimposed pressures and room pressures.The constitutive model was validated by the experimental data of different superimposed pressures and strain rates.The obtained constitutive model shows very good consistency to the test data and can simulate mechanical response characteristics of propellant at different loading conditions.Based on three-dimensional finite element numerical discretization and the secondary development in Abaqus,the structural integrity of propellant during ignition was analyzed.Through the three-dimensional numerical discretization of the established nonlinear viscoelastic constitutive model of propellant,the structural integrity of the propellant during ignition was analyzed by the secondary development through Abaqus UMAT interface to correlate it with the Abaqus solver.The results show that the constitutive model of propellant after numerical discretization well reflects the mechanical response characteristics of rate correlation and superimposed pressure correlation of propellant during ignition.The tensile hoop strain of motor grain is likely to be the main reason for the structural integrity destruction.From the view of the structural integrity of motor grain,it is suggested to develop a propellant formula with a higher elongation at “dewetting” point to achieve a higher elongation at break under superimposed pressure.The result of this paper can provide theoretical basis and data support for structural integrity analysis of SRM,it has important scientific significance and engineering application value.