| As the key component of solid rocket motor(SRM),the structural integrity of composite solid propellant(CSP)grain seriously affects the reliability of SRM,and becomes one of the bottlenecks restricting the further development of SRM technology.During the ignition pressurization process of SRM,the internal gas pressure would increase instantaneously.Under the dynamic impact of ignition pressure,the propellant grain surface is prone to generate microcracks and other defects.Moreover,with the increase of the grain’s modulus(fraction),the stress concentration phenomenon of CSP grain is more likely to appear,and the problem of structural integrity of charge becomes more serious.Becasue the traditional analysis technology of structural integrity of charge adopts the mechanical parameters of CSP under standard atmospheric pressure condition and ignores the influence of confining pressure caused by the hot gas on the mechanical properties of CSP,it is unable to accurately evaluate the structural integrity of SRM charge under ignition pressurization load.Therefore,to provide a guidance for the design of high grain’s modulus(fraction)of SRM,there is an urgent to carry out experimental study of CSP under confining pressure condition,establish a constitutive model and develop a refined analysis and integrity evaluation method of charge structure considering confining pressure effect.In this dissertation,a combined study of experimental study,theoretical modeling and numerical development have been carried out for mechanical properties of CSP under confining pressure condition.The main work of this dissertation includes the following:(1)A self-made active confining pressure experimental platform was established to realize the coupled loads of congining pressure and strain rate for solid propellant.The constant speed tensile tests of CSP considering the coupled effects of strain rate and confining pressure were carried out,and the stress-strain responses of CSP were obtained.The results show that confining pressure and strain rate have significant effects on mechanical properties of CSP,and the variation law of various mechanical parameters are complex,as well as the value of saturation confining pressure depends on strain rate.Based on time-pressure superposition principle(TPSP),the two master curves of maximum tensile strength and fracture energy density were established,which can predict the maximum tensile strength and fracture energy density under wide ranges of confining pressures and strain rates,respectively.It is believed that the two master curves can lay a foundation for the structural integrity evaluation of CSP grain in numerical analysis.(2)The scanning electron microscope(SEM)tests of the fracture cross-section of tested CSP under different confining pressures were carried out to explore the influence of confining pressure on the failure mode of CSP.The results show that with the increase of strain rate,the failure mode of CSP under various confining pressures changed from only particles dewetting to a combined failure mode dominated by particles dewetting accompanied by particles transcrystalline fracture.Moreover,the constant speed tensile meso-damage of CSP under various confining pressures were simulated based on meso-damage numerical simulation method.The meso-damage process of CSP under different confining pressures were obtained.The results show that confining pressure has a certain suppression effect on the initiation and evolution of dewetted damage of CSP.(3)Based on the irreversible thermodynamic framework,assuming that the nonlinear response of solid propellant was induced by damage,a damage dissipation potential function considering confining pressure effect was constructed,a damage evolution model based on strain energy density was derived,and a viscoelastic-viscodamage constitutive model of CSP was established.The model parameters identification methods of the one-dimensional constitutive model were introduced in detail.According to TPSP,the main curve of damage initiation parameter was established,which can predict the critical transition point of CSP from linear response to nonlinear response under wide ranges of confining pressures and strain rates.Based on the experimental data obtained in this work and the classical experimental results in the literature,the constitutive model was validated,and the error analysis and discussion were carried out.(4)Based on the finite element method,the viscoelastic-viscodamage constitutive model was numerically discretized,and its stress update formulation and Jacobian matrix were obtained.Based on the secondary development interface of Abaqus software,the viscoelasticviscodamage constitutive model was compiled into UMAT subroutine.Furthermore,the constitutive model and UMAT subroutine were validated through a series of constant speed tensile tests,multi-step variable speed tensile tests and complex stress load tensile tests.The results show that the constitutve model and UMAT subroutine can well predict the complex mechanical response of CSP under various experimental conditions.(5)Based on the UMAT subroutine considering confining pressure effect developed in this work,using the fracture energy density considering confining pressure effect as the failure criterion of CSP grain,the structural integrity of SRM charge during the ignition pressurization process was researched.The influences of confining pressure effect,ignition pressurization time,pressurization peak value and grain’s modulus on the mechanical response and the integrity evaluation result of charge structure were analyzed.The results show that due to confining pressure has a suppression effect on damage initiation and evolution of CSP grain,the Von Mises stress of grain calculated with confining pressure effect is greater than that without considering confining pressure effect,while the Von Mises strain is on the contrary.The safety factor of grain structure with confining pressure effect is much higher than that without confining pressure effect.With the decrease of ignition pressurization time,the Von Mises stress and safety factor of grain increase,while the Von Mises strain decreases.With the increase of ignition pressurization peak value,the Von Mises stress and strain of grain increase,while the safety factor decreases.As the charge grain’s modulus increases,the Von Mises stress and strain of grain increase,while the safety factor decreases.In conclusion,through the research this dissertation,a refined analysis and integrity evaluation method of charge structure considering confining pressure effect has been established,and the mechanical response and structural integrity of SRM grain during ignition pressurization process have been discussed,which can provide guidance for the design of high grain’s modulus(fraction)of SRM,and has important theoretical significance and practical application value. |
