Solid Rocket Motor Containing The Defective Propellant Viscoelastic Mechanical Behavior Analysis

Propellants are typical viscoelastic materials. Their failure and fragmentation could result in the suddenly increase of the conflagrant area of propellants. It can cause the pressure increase of the chamber and affect the propellent force of rocket engines. Thus, the explosion of rockets or other catastrophic incidents might take place. the analysis of the viscoelastic mechanics behavior of the propellant is very important, especially when some deffects exist in propellants under axial over laoding and complicated temperature load.In this paper, according to the normal conditions of the axial overload and the complex temperature variations in solid rocket propellants during the process of the launch and flight of rockets, a hexahedral propellant specimen with defect and a test model of a certain solid rocket engine propellant with defects are uinvestiagted. By the reviews of the viscoelastic theory of propellants, the fracture theory of propellants with defects, the factors affecting the performance of propellants and the finite element method for the thermal viscoelastic structures, some main numerical simulation results for the two visoelastic propellant models are obtained as shown in the foliowings:1. The viscoelastic material model of the propellants is established reasonably. An generalized Maxwell linear viscoelastic model is built from the propeties of the propellant materials based on its studies by the other reseach works. The viscoelastic model consists of 20 parallel Maxwell components paralled with a spring component. The model can express more complicated viscoelastic behavior of the propellants under complex force loads and thermal environment.2. Two 3D finite models of the hexahedral propellant specimen with defect and the test model of the solid rocket engine propellant with defects are established. Five load conditions including uniaxial tension, axial overload, cooling process after curing, cryogenic tensile and ignition emission process, are simulated numrically.3. For the hexahedral propellant specimen with defect, the viscoelastic mechanics behavior is studied for three load conditions including the uniaxial tension, the cooling process after curing and the cryogenic tensile, respectively. The stress field and strain field variations of the long term time under the uniaxial tension are calculated. The positions of the maximum stress and strain and the high strain gradients region in the crack front are found. The crack propagation direction can be evaluated according the numerial results. The stress field and strain field variations of the long term time under the cooling process after curing are calculated. The thermal stress field, thermal strain fields are obtained. The cooling shrinkage might increase the stress magnitudes in crack front zone. The thermal and mechanical coupling stress field and strain field are obtained as well for the load condition of the cryogenic tensile. It can be found that the thermal stress is a major stress and the mechanical stress is a secondary stress. When the temperature change is same, the stress and the strain diturbutions under the cooling process after curing are similar to thoses of the cryogenic tensile process.4. For the solid rocket engine propellant model with defects, a viscoelastic mechanical model of a half of the structure is established according to the structural symmetry property. Three load conditions including the axial overload, the cooling process after curing and the ignition emission process are investigated. The stress field and strain field varing with the time are calculated for the uniaxial tension,. The positions of the maximum stress and strain and the high strain gradients region oin the front of the crack are found. The direction of the crack propagation can be predicted. The temperature field, the thermal stress field and the thermal strain field varing with the time is getten for the cooling process. The cooling shrinkage can increase the stress manigutude near the crack. The transient thermal and mechanical coupling stress field and strain field were obtained for the ignition emission process. It can be found that the thermal stress is the major stress and the mechanical stress is the secondary stress.