Effect Of Loading Stress And Frequency On Fatigue Life Of Solid Propellant

During the service of the solid rocket motor,the propellant grains will be subjected to different forms of alternating loads,which will not cause macroscopic damage to the propellants in a short period of time.However,for a long period of cyclic loading,it has bad influences on the mechanical properties of propellants and the structural integrity of the solid rocket motor is destroyed.Based on a research project,uniaxial tension and stress relaxation tests of HTPB propellant raw materials were carried out.The mechanical properties under different strain rates of materials were obtained,and the fatigue tests of propellants under different stress levels and loading frequencies were carried out.The fatigue life equation and damage evolution law of the materials were studied.The results could provide a reference for the analysis of the integrity of the charge structure and the safety design of the engine.The main contents are as follows:(1)The uniaxial tensile and stress relaxation tests under different strain rates of HTPB propellant before fatigue were completed by using material universal testing machine,and the stress-strain curves and stress relaxation curves under different strain rates were obtained.The relaxation modulus of the material was obtained through the fractional index model fitting.The relaxation integral constitutive model was established and the damage factor related to the strain rate and strain range was introduced to describe the mechanical response of the material under different stress rates.(2)The fatigue tests of different stress levels and loading frequencies of HTPB propellants were carried out by using dynamic thermo-mechanical analyzer.The variation rules of mechanical parameters such as stress,strain and hysteresis loop in fatigue process were analyzed.Fatigue life and temperature changes of HTPB propellants during fatigue were monitored by infrared thermal imager in real time.The laws of heat production of materials during fatigue and the fatigue fracture location and fatigue limit were determined.Through the fatigue uniaxial tensile test combined with electron microscopy,the mechanism of fatigue damage of HTPB propellant was explained from the microscopic level.(3)Based on fatigue test data,the fatigue equation of HTPB propellant in double logarithmic coordinate system was obtained.The relationship between fatigue life and maximum loading stress and loading frequency was analyzed,and the fatigue life equation with frequency correction was established.From the deterioration of macroscopic mechanical properties as a starting point,the dynamic modulus attenuation of HTPB propellant is defined as the damage variable.Based on the continuum damage mechanics,a three-stage model of fatigue damage with frequency effect that can consider the development of macro-cracks is established.The damage evolut’ion of HTPB propellants at different loading stress and loading frequency at room temperature is well characterized.(4)Fatigue tests of HTPB propellant under luffing loads were carried out,the influences of loading order and loading amplitude on the dynamic modulus and fatigue temperature of the material were analyzed.The method and formula for estimating the fatigue life of Miner’s law,Miner’s law and Corten-Dolan’s theory are given,the cumulative damage theory based on fatigue damage model and fatigue temperature rise are established.The applicability and accuracy of these fatigue cumulative damage theories to predict the residual life of HTPB propellants are discussed.