Research On The Mesoscopic Damage Mechanism And Nonlinear Viscoelastic Constitutive Model Of Composite Propellant

As demands for firing range and motility of rockets weapons improve continuously,the high-energy propellant has been increasingly applied in engineering field widely.NEPE is a new type composite propellant which has high specific impulse,high density and excellent mechanical properties at low temperatures.These advantages make it become a viable option to meet the requirements of extending range for rocket weapons.As a consequence,NEPE propellant has become one of the most potential heterogeneous propellant in aerospace field.However,the rising of energy content means a more complex structure in meso-scale,which leads to a more inconclusive mechanical behavior when compared with the traditional composite propellant.The problem limits the application of NEPE propellant in greater scope.Therefore,for popularizing NEPE propellant in real products and intensive studying the inherent reason of macroscopic mechanical behavior of NEPE propellant,the computational micro-mechanics method was used in this paper to reveal the mesoscopic damage mechanism and evolution of NEPE propellant.The mesoscopic research findings were used for following constitutive modelling purpose.Concretely speaking,the main research contents are listed as follows.(1)The mesoscopic morphology of NEPE was observed by scanning electron microscope.Then the constant rate uniaxial tension and stress relaxation tests of NEPE propellant with customized formula were conducted at room temperature.The tests show that the mechanical property of NEPE propellant is rate dependent,and the main mechanical parameters are affected by strain rate noteworthy.The results of comparison between experimental curves of propellant with different formula indicate that the change of filling rate have no effect on stress relaxation behavior of propellant,but only make difference to quantitative value of the relaxation modulus.What is more,relative to change of particle size gradation,the relaxation modulus is more sensitive to variation of volume fraction of solid.Finally,the constant strain rate tests of binder films were conducted at normal temperature,and a viscoelastic-superelastic constitutive relation was established to describe its complicated mechanical behavior.The model can provide a good prediction of uniaxial mechanical response for binder materials.(2)The packing model of NEPE propellant was generated by event-driven molecular dynamics method and the mechanical behavior of interface between particles and binder was modeled by exponential type cohesive zone method.The meso model was generated in strict accordance with the real composition of NEPE propellant.The effect of mesostructure and component property on macro mechanical performance of NEPE propellant was studied by FEM calculation.The computed results indicate that the macro mechanical behavior of NEPE rely heavily on its mesostructured.The macro initial modulus of NEPE depends on the binder property and particle volume fraction,while the dewetting process is controlled by particle size gradation and interface property.Then,based on the established simulation model,the simulative load was applied and the damage evolution process of NEPE propellant was obtained by numerical calculation.The computed results basically show the micro-morphology and component states of NEPE propellant under uniaxial load in four stage,which can partly reveal the root cause of the macro-mechanical characters of NEPE propellant.(3)To overcome the absence of the interface parameters obtaining tool in meso scale,a parameter inversing identification method based on testing measures and Hook-Jeeves optimization algorithm was developed.On account of the diversification of particle type in NEPE,the uniaxial tension tests of customized propellant specimens with different packing particles were used to obtain the interface parameters of AP and RDX by inverse analysis step by step.Whereafter,to consider the viscoelastic characteristic of interface between particles and binder,a rate-dependent cohesive zone method was developed and the model parameters were acquired by inverse calculation.Next,the interface parameters of AP and RDX under different temperatures were calculated and analyzed,the results show that the mechanical property of interface was largely influenced by temperature.(4)Based on the free-volume theory and refer to the time-temperature superposition principle,the time-temperature-damage superposition principle was proposed.The principle was described by the time-temperature-damage equation,which was expanded from WLF model.The relaxation tests under different strain levels at different temperatures were conducted,and the relationship between temperature,damage and viscoelastic performance of NEPE was studied.Then,combining time-temperature and time-damage superposition principle,two master curves without damage were obtained respectively by horizon shifting along the logarithmic time scale at reference temperature.The damage of propellant was quantified by accumulative damage model.The two master curves were verified by real long-time relaxation test of NEPE propellant.The two curves coincide well with the experimental one,which shows that the developed equivalent model can give a good description of the relationship between temperature,damage and viscoelastic performance of NEPE propellant.(5)The microscopic study shows that particle dewetting is a key factor in determining the macro mechanical behavior of NEPE.However,the existing phenomenological model unable to take dewetting into consideration.Therefore,in order to solve this problem,the damage due to dewetting was introduced into viscoelastic constitutive model by the reduced time based on viscoelastic dewetting criteria and proposed time-damage superposition principle,to establish the proposed nonlinear constitutive model which can reflect the effect of meso-structure change on macro mechanical behavior.In order to verify the accuracy of the proposed model,the constant strain rate tests at different temperatures and combined load tests were conducted,while the proposed nonlinear constitutive model was programmed through Matlab software and evaluated with corresponding tests.The predicted results well matched the measured ones.The difference between predicted values and experimental data is within the limits of 20%,which shows that the proposed nonlinear constitutive model is capable of describing the viscoelastic mechanical behavior of NEPE propellant within a certain range of strain rate and provides a simple means to predict the macroscopic mechanical behavior of solid propellant with complicated mesostructure.In the end,the research objectives of this thesis are the analysis of damage mechanism in meso scale and the modeling of macro mechanical behavior of NEPE propellant with consideration of dewetting process.The research target was achieved by means of the experiment,simulation and inversion method.The mesoscopic mechanical behavior of NEPE propellant was studied,which has revealed the internal damage mechanism and evolution process of NEPE propellant.Through the research,the changing law of macroscopic mechanical performance along with the meso-structure of NEPE propellant was obtained.Results indicate that the mechanical properties of NEPE propellant is highly depend on its meso-structure,and the macroscopic mechanical response of NEPE propellant can be predicted by mesoscopic simulation model.Then,based on the mesoscopic research achievement,a constitutive equation of NEPE propellant involved dewetting factor was developed and the established model is capable of describing the mechanical response of NEPE propellant in macroscale.The outcome of this thesis can provide theoretical guidance for formulation design of NEPE propellant and for structure integrity analysis of grains.