Structural Integrity And Reliability Evaluation Of Motor Grain Based On Stochastic Constitutive Model

Structural integrity is one of the main factors affecting the life of solid rocket motor.At present,the deterministic constitutive model is often used to analyze the structural integrity of propellant grain.Due to the existence of propellant composition ratio and manufacturing process error,the mechanical properties and constitutive curves of propellant show obvious dispersion,which makes the evaluation conclusion of grain structural integrity have certain errors.In this paper,aiming at the randomness and dispersion of the mechanical properties of propellant materials,a random constitutive model was constructed,and the structural integrity assessment of the grain based on the random constitutive model was carried out.Aiming at the reliability evaluation problem of random variables and interval variables in the grain structure,the probability-non-probabilistic hybrid reliability model was used to calculate the reliability of the grain under ignition pressurization conditions.The main research contents of this paper are:1.Based on the ZWT constitutive model,the random parameters were introduced to establish the random constitutive model of HTPB propellant.The non-random parameters and random parameters were calibrated by five sets of uniaxial tensile stress-strain curves under the same test conditions.According to the distribution law of random parameters fitted by the test curve,the confidence upper and lower limits of random parameters were obtained by given probability,which constitutes the stress interval predicted by the random constitutive model.According to the interval,five random parameters are randomly predicted to form a random curve.Comparing the random curve with the test curve,it is found that the five sets of curves predicted by the random constitutive model basically coincide with the test curve,and the test curve mostly falls within the 95 % probability interval,indicating that the random constitutive model can effectively reflect the dispersion of propellant materials.2.The incremental form and Jacobian matrix of the random constitutive model were derived by the three-dimensional discrete method.The uniaxial tensile test process of the propellant billet sample was numerically simulated.The correctness of the model derivation process and subroutine writing was verified by comparing with the experimental constitutive curve.The results show that the random constitutive model can effectively simulate the stressstrain response of the propellant.The random constitutive model based on the test constitutive curve fitting and the random constitutive model based on the probability interval prediction were studied.The results show that the random constitutive model based on the probability interval prediction can accurately predict the mechanical response of the grain under the ignition pressurization load,and the error does not exceed 1 %.3.The sensitivity of material parameters and load to the mechanical response of grain after ignition pressurization was analyzed.The results show that the three parameters with the most significant influence on the maximum equivalent stress of the grain are Poisson’s ratio,ignition pressure and random parameters,and the three parameters with the most significant influence on the maximum equivalent strain of the grain are Poisson’s ratio,random parameters and ignition pressure.4.The response surface model of the grain after ignition pressurization was constructed,and the reliability of the grain was solved by different reliability models.The results show that the calculation results of conventional probability model and non-probabilistic model are single reliability,and the calculation results of hybrid model are a reliability interval.The results calculated by different reliability models are quite different,which indicates that the dispersion of uncertain variables has a great influence on the results.Therefore,there is a certain risk in the method of simplifying the uncertain parameters lacking experimental statistical data into random variables according to empirical assumptions.It is more reliable to deal with the reliability problem of grain structure with both random variables and interval variables by hybrid reliability model.The structural integrity analysis method based on random constitutive model and the reliability analysis method based on probability-nonprobability mixed model proposed in this paper have certain reference significance for the structural integrity and reliability evaluation of solid rocket motor charge.