A three-dimensional nonlinear viscoelastic constitutive model of solid propellant

A nonlinear viscoelastic constitutive model for the solid propellant is proposed. In their earlier work, the authors have developed an isotropic constitutive model and veri®ed it for one-dimensional case. In the present work, the validity of the model is extended to three-dimensional cases. Also, implementation of the model into a commercial code for the use of the constitutive model in the ®nite element analysis of solid propellant is discussed. Large deformation, dewetting and cyclic loading e€ects are treated as main sources of nonlinear behavior of the solid propellant. The softening of the solid propellant due to dewetting is treated by the modulus decrease. The nonlinearities during cyclic loading are accounted for by the functions of the octahedral shear strain measure. The constitutive equation is implemented into a ®nite element code for the analysis of propellant grains. A commercial ®nite element package `ABAQUSʹ is used for the analysis and the model is introduced into the code through a user subroutine. The model is evaluated with di€erent loading conditions and the predicted values are in good agreement with the biaxial test results