A Study on Creep Behavior of Composite Solid Propellants Using the Kelvin-Voigt Model

A Kelvin-Voigt model consisting of a spring and a dashpot in parallel was applied for the viscoelastic characterization of solid rocket propellants. Suitable values of spring constants and damping coefficients were employed by a least squares fit of the errors to generate creep curves using a Dynamic Mechanical Analyzer (DMA) for composite solid propellants. Three different composite propellant formulations based on HTPB/AP/Al having burning rates of 5 mm/s, 15 mm/s and 20 mm/s were tested under different stress levels varying from 0.1 MPa to 3 MPa and at different temperatures varying from 35 °C to 85 °C. Creep behavior with recovery was studied and analyzed to evaluate the viscoelastic properties. The change in spring constants, representing elastic deformation, was very small compared to the damping coefficients for the propellants studied. For a typical propellant formulation, when the stress level was increased, the spring and damping coefficient both increased significantly whereas for an increase in temperature, they remained nearly constant. However, the ratio E/η was observed to be constant and independent of stress level. It was also observed that the variation of E and η varied linearly with increase in stress whereas their ratio showed a logarithmic variation. A mathematical correlation was developed to evaluate the viscoelastic properties during creep of composite propellants.