Thermo-mechanical Coupling Of Solid Composite Propellant Under Dynamic Loading

The solid composite propellant is increasingly used in aerospace and military fields and is subjected to dynamic loadings,including impact loading and cyclic loading.The composite propellant not only exhibits nonlinear mechanical properties under dynamic loading,but also has a self-heating effect,which increase the temperature of the material.Since mechanical behaviors of composite propellant are highly sensitive to temperature,the self-heating effect in turn affects the mechanical behavior,indicating that the composite propellant is significantly thermo-mechanically coupled under dynamic loadings.In order to fully study the thermo-mechanical coupling of composite propellant under dynamic loadings,the experimental and theoretical analysis are carried out.The main contents of this dissertation are listed as follows:(1)The mechanical and thermal responses of composite propellant under impact loading were obtained simultaneously by SHPB(Split Hopkinson Pressure Bar)and transient infrared measurement technology.The stress-strain relationships of composite propellant under impact loading are rate-dependent with large deformation and accompanied by an increase in its own temperature.When subjected to impact loading,almost all mechanical energy of composite propellant is dissipated into heat.To describe the high strain rate mechanical properties of composite propellant,the Zhu-Wang-Tang nonlinear viscoelastic constitutive model is combined with the Mooney-Rivlin hyperelastic constitutive model to build an isothermal visco-hyperelastic constitutive model.In order to take the self-heating effect into account,the proposed model is later modified to a thermo-visco-hyperelastic constitutive model.The experimental data agrees well with model predictions.(2)The fatigue tests of strain-controlled mode were carried out.Meanwhile,in order to obtain the temperature field during deformation,the surface temperature of tested propellant specimen during fatigue was detected by an infrared camera.The results show that the composite propellant sees self-heating effect under dynamic loading,manifesting as an increase in internal and superficial temperature.When subjected to cyclic loading with high frequency and high strain amplitude,the temperature rise of the composite propellant can reach up to tens of degrees.The self-heating effect under cyclic loading accelerate the accumulation of fatigue damage,weakening mechanical responses of the composite propellant,which is manifested by decrease of the average dynamic modulus with increasing loading cycles.(3)Using the concept of “intrinsic dissipation” and detected temperature evolution,the fatigue limit of composite propellant is successfully obtained.According to the basic principles of thermodynamics,the energy balance equation in the fatigue process of composite propellant is established.The energy storage rate during fatigue is obtained by measuring the dissipation rate.The energy storage rate reflects the cumulative damage in the fatigue of composite propellant and can be used as an indicator to quickly predict the fatigue life.The proposed method is more efficient than the conventional one and can offer acceptable results.(4)The linear(thermo)viscoelastic constitutive model derived from the irreversible thermodynamic framework,basic laws of continuum mechanics and time-temperature equivalent principle of the thermo-rheological simple material is introduced.Based on the Schapery model,a nonlinear viscoelastic constitutive model is proposed by introducing two nonlinear viscoelastic functions.The constitutive model considers the maximum von Mises equivalent stress during the deformation history,and requires less parameters compared with other models.The proposed nonlinear thermo-viscoelastic constitutive model has been numerically implemented in a finite element package,Abaqus,through its user defined subroutine,UMAT.Furthermore,in order to simulate the thermomechanical coupling,another subroutine UMATHT is introduced to work collectively with UMAT.The results show that the numerical simulations agree well with experimental ones,which validated the accuracy of proposed model.This dissertation deepens the understanding of the thermo-mechanical coupling of composite propellant under dynamic loading and provides an important theoretical basis for the grain design and service processing of solid rocket motors.