A temperature/strain-rate-dependent finite deformation constitutive and failure model for solid propellants

The stress-strain response under progressive damage and the ultimate failure of solid propellants are two key issues affecting the integrity of solid rocket motors. Previous research primarily focused on the progressive damage in solid propellants during production and storage. However, they failed to take the temperature/strain-rate-dependent ultimate failures into consideration. The failure strains of solid propellants are experimentally observed to show strong temperature/strain-rate dependence and exhibit an abnormal evolution at low and high temperatures, respectively. With increasing loading strain rate, the failure strains decrease at low temperatures near the glass transition temperature (T _g) but increase at high temperatures far above T _g. In this study, we introduce the glassy and rubbery failure criteria based on strain energy densities at ultralow and ultrahigh temperatures, respectively, into a viscoelastic constitutive model and build a unified model for the progressive damage and the ultimate failure of solid propellants. With the introduction of these two additional criterion parameters, the developed model can effectively predict the yield-type stress-strain responses, microscopic damage-induced volume dilatations, and temperature/strain-rate-dependent ultimate failures of the solid propellants by comparing the model predictions with the experimental results. The competition between the glassy failure and the rubbery failure results in the propellants exhibiting a maximum break strain near the glass transition temperature. Consequently, when the strain rate is increased, the propellants exhibit a predominantly glassy response, which shifts the failure envelope toward a higher temperature. This induces an abnormal evolution of failure strains by making the propellants stretchable at high temperatures and brittle at low temperatures.