Mechanical Response and Shear Initiation of Double-Base Propellants

The intense shearing that occurs in propellants during impulsive loading can lead to initiation. In an effort to determine useful shear initiation criteria, the U.S. Army Research Laboratory has developed a dynamic shear punch test using a modified split- Hopkinson bar. Varying the striker bars velocity and length controls the shear rate and duration. Shear velocities approaching 100 ms and durations as long as 0.2 ms are possible. Experimental results have been obtained for several energetic materials and a nonreacting polymer, polycarbonate PC. This report presents a detailed analysis used to obtain constitutive behavior and shear initiation for double-base propellants and computational results of the shear punch test. For the simulations, the viscoSCRAM constitutive model was used to describe viscoelasticity, cracking and ignition in the propellant when subjected to dynamic shear loading conditions. First, we will present the analysis used to obtain viscoelastic material parameters. The stress relaxation function for the linear viscoelastic response was obtained by using time-temperature superposition to generate a master curve from Dynamic Mechanical Analysis DMA data. Next, the effect of initial crack size and critical hot spot duration on the ignition threshold temperature was examined. The validity of the constitutive relation, failure criterion, and shear initiation is determined based on their ability to predict the observed response from the dynamic shear punch test.