Damage Constitutive Model Of Composite Solid Propellant Under Prestrain And Its Application

During the curing process and the cooling process of the solid rocket motor grain after curing,due to the mismatch of the thermal expansion coefficient of the propellant,insulation layer and case,the shrinkage deformation of the propellant grain is restricted,and consequently led to the residual thermal strain in the propellant grain.Generally,the residual strain formed by curing and cooling process is defined as the prestrain of the propellant grain.The propellant grain is in a prestrain state throughout the storage period,which will affect the structural integrity and reliability of the propellant grain.The key to evaluate the effect of prestrain during storage on the propellant performance and the structural integrity of the propellant grain is to obtain the mechanical properties of the solid propellant under prestrain,and to establish a constitutive model with damage that can describe the mechanical response of the propellant reasonably,and then apply it to the structural integrity analysis of solid motor grain.In this paper,the HTPB composite propellant was researched.The mechanical properties of solid propellant under prestrain were explored by experimental method,and the viscoelastic constitutive model considering the damage effect of prestrain and aging was established.Then the structural integrity analysis and safety margin assessment methods of the propellant grain under superimposed loads were discussed and verified for engineering applications purpose.The main research contents are as follows.(1)The aging tests for HTPB composite propellant under the interaction of prestrain and temperature were designed and carried out.Then the uniaxial tensile tests and the dynamic mechanical analysis(DMA)tests of HTPB propellant stored under different prestrain conditions were conducted.Combined with scanning electron microscopy(SEM)tests of HTPB propellant section and tensile fracture surface,the effects of prestrain on the macro mechanical properties and dynamic mechanical properties of HTPB propellant were obtained.(2)Based on the linear viscoelastic constitutive and the continuous damage theory,a viscoelastic constitutive model with damage effect was established.The prestrain and aging time were incorporated into the damage function.The principal curve of relaxation modulus of HTPB propellant was obtained through the stress relaxation tests of HTPB propellant.Then the Prony series expression of the relaxation modulus and the WLF equation were fitted.The constitutive model was discretized by the dimensionless processing of the relaxation modulus.Then the material subroutine of the solid propellant constitutive model was developed based on the UMAT interface of the ABAQUS platform.The uniaxial tensile tests of HTPB propellant were simulated subsequently by the constitutive model.The results show that the mechanical response of HTPB propellant aged under different prestrain conditions can be reflected accurately by the established constitutive model,which can provide support for structural integrity analysis of propellant grains.(3)The structural integrity analysis and safety margin assessment for an analog tube motor grain under typical loads was carried out,and the structural integrity verification tests were conducted to verify the structural integrity evaluation method.Then a structural integrity assessment method for the propellant grain under typical loads was established,which realized the engineering application of the constitutive model with damage.The results show that the propellant constitutive model of solid propellant with damage can be used for structural analysis of propellant grain.The safety factor and safety margin of propellant grain under the cooling load and ignition pressure load can be calculated accurately by the combined safety factor evaluation method.It can be applied to the structural integrity assessment of solid rocket motors under superimposed loads.(4)Based on the constitutive model of propellant with damage and the structural integrity assessment method,the structural integrity analysis for the finocyl grain of a solid rocket motor in service were performed,which can provide a useful reference for life cycle structural integrity assessment and storage life estimation of solid rocket motors.The research results of this paper can provide useful reference and guidance for both understanding the mechanical properties of solid propellant aged under prestrain and the structural integrity assessment and storage life management of composite propellant grains,which has important theoretical and engineering application value.