Stress Induced Anisotropy in Pressurized Thick Walled Cylinders

Abstract

The most important mechanical features of propellants arise from the presence of a highly packed array of granular particles (filler), and a distribution of adhesive strengths between the rubbery binder and these particles. The first factor leads to dilatation and the formation of voids in any stress field other than pure hydrostatic compression. The second factor virtually guarantees that the pullaway of the binder from the filler is nonuniform, leading in extreme cases to the so-called "zebra-stripe" effect, or localized dewetting. This factor also is associated with stress relaxation due to the slow flow of the binder from regions of high strain concentration into regions of low concentration or into voids. Finally, because the binder is incompressible, and the filler is for all practical purposes infinitely rigid, most of the macroscopically applied load is concentrated as large strains near the binder-filler interfaces leading to non-linear behavior. At ambient temperature or thereabouts, viscoelasticity as associated with polymer chain uncoiling plays no role in the mechanical behavior of the propellant. Summarizing, the important mechanical features to be expected are 1. Dilatation with void formation when the stress is tensile. 2. Localized dilatation because of nonuniformity of adhesion strengths. 3. Stress relaxation due to binder flow and perhaps due to particle movement at a very slow rate determined by frictional and adhesive effects . 4. Nonlinear stress-strain relations due to high local strains at binder-filler interfaces.