High strain-rate constitutive models for solid rocket propellants

A constitutive model that incorporates mechanical damage and nonlinear viscoelastic material response for solid composite rocket propellants has been developed for high strain-rate impact loading conditions. The numericalconstants in this model could be related to the physics of the propellant and were obtained by nonlinear least-squares fitting of the data obtained from Hopkinson Bar experiments in the strain-rate range from 10 3 to 10 4 s - 1 . Damage was taken into account using an approach based on correcting the viscoelastic function with a stress softening function. A simple method for calibrating the stress softening function using the recoverable strain energy density has been developed. This energy-based approach has the advantage of being able to capture damage more adequately and allows for bulk inelastic behavior. Predictions of the stress response of two different composite propellants for different loading conditions (temperature and strain rate) and histories (and, therefore, different damage levels) were made using the constitutive model. Good agreement between the predicted and experimentally measured stresses for strain levels up to 30-40% was obtained. The model was able to predict the stress response quite reasonably outside the temperature range used to develop the constitutive equation, but inside the range of the reduced strain-rate, indicating the validity of the time-temperature superposition principle for high strain-rate conditions. The model was also able to reproduce quite well the mechanical deformation (both in magnitude and shape of the stress-strain curve) of predamaged propellants.