Abstract: In order to characterized mechanical properties of modified double base propellant, a viscoelastic-viscoplastic constitutive model for modified double base propellant is derived. A serious of creep-recovery tests are done to obtain model parameters, the total strain of material is separated into viscoelastic strain and viscoplastic strain, the least square method is used to get viscoelastic parameters, while Nelder-Mead simplex optimization algorithm combining with backword Euler method are used to get viscoplastic parameters. A serious of creep-recovery tests under different stress levels and different loading times are done to verify the model, the results show that the change trend is consistent between model prediction and test value under lower stress levels or shorter loading time，the percentage of viscoelastic strain and viscoplastic strain in total strain obtained from model agree with test. Viscoelastic-viscoplastic constitutive model for modified double base propellant can describe the mechanical properties of the material within limits.

Key words: propulsion system of aviation & aerospace, solid mechanics, modified double base propellant, viscoelatic-viscoplastic model, creep-recovery