Constitutive equations for solid propellants

Mechanical behavior of the redesigned Space Shuttle solid rocket motor propellant is studied from a phenomenological point of view. Motivated by the study of the experimental data three initially isotropic constitutive models have been developed.; All models represent the effect of strain rate, superimposed hydrostatic pressure, and cyclic loading on the stress and dilatation response of the material. A particular emphasis is given to the prediction of volume dilatation. A free energy function which leads to significantly coupled volumetric and deviatoric responses is employed for this purpose. Viscoelasticity effects are represented through a single, strain independent relaxation function. Both deviatoric and volumetric responses obey the superposition implied by this. The use of different measures of constitutive variables in the convolution integral results in three different models.; The formation and growth of voids, a dominant form of damage in propellants, are incorporated through a dilatation model which is based on the analysis of void-containing elastic materials. Softening of the material under deformation is modeled by a damage function which depends on the maximum volume fraction of voids up to the present time. Cyclic effects are incorporated by modifying this function for unloading and reloading. A criterion to distinguish different states of loading is incorporated.; The model resulting in the best representation of the available data is calibrated using only a few tests. The predictions of the model are compared with experiments for several loading conditions not used in the calibration. The comparison of three models regarding their distinguishing features and calibration are discussed.