Viscoelastic Analysis Of Solid Rocket Motor Grain Considering Time-Dependent Poisson’s Ratio, Aging And Damage Effects

During a long term service process, solid rocket motor(SRM) undergoes the test of complex environment, and it is inevitable to degrade the propellant performance of SRM. How to accurately estimate the structural integrity of SRM grain is a vital subject for both designers and users. A significant approach to solve the above-mentioned problem is to accurately characterize the mechanical behavior of propellant during the degradation process and to establish a reasonable constitutive model in structura l analysis of SRM grain. An emphasis of this dissertation is placed on establishing a viscoelastic constitutive model with aging and damage behaviors of SRM propellant, formulating a numerical analysis method for the model and illustrating relevant engineering application. Main achievements are summarized as follows:Two kinds of viscoelastic constitutive models are proposed for solid propellant either with constant Poisson’s ratio or with time-dependent Poisson’s ratio, respectively. Based on existing aging and damage variables as well as their evolution equations, the effects of aging and damage are introduced into the constitutive model under the material equivalent hypothesis. By introducing constant or time-dependent Poisson’s ratio, two kinds of nonlinear viscoelastic constitutive models with aging and damage evolution for solid propellant are formulated respectively, both of which are capable of dealing with thermoviscoelasticity, thermochemical aging and mechanical damage. These models are suited for structural analysis of grain for different environmental or loading cases during its service life.An experimental study on propellant Poisson’s ratio is made by using the Digital Image Correlation(DIC) method. According to the time-dependent properties of viscoelastic materials, the measuring principle for Poisson’s ratio of propellant is discussed. Based on the DIC method, a non-contact measuring system for Poisson’s ratio of propellant is established, and relevant experiments are conducted. Using the obtained images and DIC method, the traverse and longitudinal strains during loadin g process are obtained, and then an expression for Poisson’s ratio is fitted by the least square method, which provides an important parameter of Poisson’s ratio for further structural analysis of SRM grain. The present measure method and designed measure system of Poisson’s ratio of propellant are helpful for measuring Poisson’s ratio of other viscoelastic materials.A finite element analysis method is presented to investigate a thermoviscoelastic constitutive model with constant Poisson’s ratio, aging and damage. Based on the integration algorithms, the constitutive model is numerically discretized into its incremental form, and then its corresponding Consistent Tangential Stiffness(CTS) is obtained. According to the incremental method, the numerical analysis of reduced time and aging variable are operated. Furthermore, nonlinear constitutive equations are solved by using Newton iterative method, and the stress-updating is achieved. User material subroutine is compiled based on the secondary-development technology of Abaqus code; finally, a complex constitutive model is implemented into three-dimensional finite element analysis.A finite element analysis method is formulated for a new thermoviscoelastic constitutive model with time-dependent Poisson’s ratio, aging and damage. Based on the numerical analysis of the previous constitutive model with constant Poisson’s ratio, a time-dependent Poisson’s ratio, as well as the aging and damage effects, is introduced into a thermoviscoelastic constitutive model. It is numerically analyzed by the integration algorithms, then incremental nonlinear constitutive equations are solved by using the Newton iterative method, and the stress updating is implemented. Based on the secondary-development technology of Abaqus code, the constitutive model is implemented into three-dimensional engineering application. This work provides a key foundation of numerical model and program for further developing the structura l analysis module for SRM grain.A structural analysis module for SRM grain is developed, including the effects of time-dependent Poisson’s ratio, aging and damage, and some illustrative examples are given. Based on existing structural analysis system of SRM, a new structural analysis module is developed, in which eight different kinds of constitutive models can be provided through different combination effects of Poisson’s ratio, aging and damage. Using the structural analysis module developed, the influences of time-dependent Poisson’s ratio, aging and damage on the structural responses of SRM are analyzed under representative loads all throughout the service life.The present work provides a technical support for further structural integrity analysis and life estimate of practical SRM, and is of great value in theory and engineering application.