Research On Dewetting Mechanical Behaviors Of HTPB Propellant At Low Temperatures Under Uniaxial Tension Loading

With the diversification of the requirements in deep space exploration and normalization of fighter flight carrying bomb,the low temperature mechanical properties and safety of solid rocket motor(SRM)becomes an important technical issue during development.The solid propellant is more susceptible to dewetting damage at low temperature,thus the ignition process of SRM at low temperature is more complex and dangerous than that at normal temperature.Therefore the structural integrity of propellant grain at low temperature is one of the key problems to be solved urgently during the research and production of SRM.Domestic and foreign researches have stated that the quantitative characterization of dewetting damage,failure mechanism and mecromechanical behavior of the composite solid propellant are of great significance to effectively analyze the structure integrity of propellant grain during ignition at low temperatures.However,the research es on the dewetting damage of solid propellant at low temperature are still insufficient,especially on the quantitative characterization of the dewetting evolution process.Therefore,it is urgent to carry out corresponding research through the innovation of experimental test method and model theory.Based on the above mentioned reasons,the tri-component Hydroxyl-terminated polybutadiene(HTPB)propellant used in the typical SRM is chosen as the study object in this thesis.Then the researches on dewetting mechanical behavior of the propellant at low temperatures have been systematically studied by means of experiment,theory and numerical simulation,which contain the low temperature mechanical property under uniaxial tensile condition,dewetting damage characterization,dewetting evolution law and microscopic damage nonlinear constitutive models.The details are as follows:(1)According to the damage theory,the concept of dewetting had been redefined.The dewetting is the phenomenon that leads to the failure of volume elements when the mechanical properties of materials were deteriorated caused by the action of external factors.It expands the scope of its definition and lays a foundation for the subsequent quantitative characterization and evolution analysis of microscopic damages.(2)Experimental studies on mechanical properties of HTPB propellant at low temperatures.A new type of small dumbbell-shaped specimen is designed by combining simulation with test,which takes into account both macroscopic mechanical properties test and microscopic damage characterization.The low temperature mechanical properties of HTPB propellant under quasi-static uniaxial tension are obtained by analyzing the stress-strain curves of new dumbbell-shaped specimens under different thermal aging time.The difference of mechanical properties between low temperature and normal temperature,and the change rule of mechanical properties at low temperatures before and after thermal aging are discussed respectively.(3)Research on the characterization method of dewetting damage of HTPB propellant based on micro CT.Through the testing principle and performance parameter analysis of micro-CT equipment,and considering the microstructure characteristics of HTPB propellant,three damage quantitative characterization methods,such as gray value,porosity and fractal dimension,are expound profoundly,and their applicable ranges are analyzed respectively.(4)Research on in-situ scanning experiment of HTPB propellant by using micro CT.The in-situ scanning of HTPB propellant under uniaxial tension is realized by using self-designed small dumbbell specimens and the improved loading device of micro-CT equipment.The CT images of the specimens under different strain loads are obtained.The distribution of composition and defect of HTPB propellant before and after tensile test are discussed by statistical analysis of CT gray image.(5)Analysis of dewetting damage evolution of HTPB propellant at low temperatures.Based on three characteristic parameters(gray value,porosity and fractal dimension),the micro-CT in-situ scanning images of HTPB propellant specimens under different conditions are analyzed,and the relationships between dewetting damage and strain,aging time and temperature are discussed.The functional relationships between the dewetting damage characteristic parameters and tensile strain are established.The dewetting evolution law and mechanical behavior mechanism with aging time and temperatures of HTPB propellant are obtained.(6)Research on the constitutive model of HTPB propellant at low temperatures.According to deformation characteristics and dewetting damage of HTPB propellant under quasi-static uniaxial tension at low temperatures,the constitutive models including single dewetting damage and compound dewetting damage are constructed.Then the validity and applicability of the constitutive model are verified through comparing the results predicted by the constitutive model with experimental results.(7)Study on numerical simulation of dewetting evolution of HTPB propellant at low temperatures.Based on the formulation of HTPB propellant,the filling model of propellant micro-particles is established by using molecular dynamics algorithm.The dewetting micro-damage evolution processes of HTPB propellant under different aging time and low temperatures are simulated by the extended finite element method.Then the evolution mechanism of dewetting damage of HTPB propellant under loading conditions is revealed,and the influencing mechanism of meso-damage on macro-mechanical properties of HTPB propellant is well explained.