Damage in Polymer Bonded Energetic Composites: Effect of Loading Rate

Abstract

Particulate composites are widely used in the materials world. An understanding of their damage behaviour under a variety of loading conditions is necessary to inform models of their response to external stimuli. In the present experimental study, fine and coarse grained RDX-HTPB composites have been used to investigate the effect of loading rate on the degree of damage produced in polymer bonded explosives subjected to varying degrees of uniaxial compression. High strain rate loading (4 × 103 s−1) was achieved using a direct impact Hopkinson pressure bar and low strain rate loading (1 × 10−2 s−1) using an Instron mechanical testing machine. The causal metrics are the degree to which the samples were strained and the mechanical energy expended in straining them. The damage metric is the residual low rate compressive modulus of the samples. The quantitative, physically based, results discussed in terms of the Porter-Gould activated debonding damage model clearly demonstrate that for both fine and coarse fills there is a marked reduction in residual moduli as a function of imposed strain, and substantially less specific energy is required to cause the same level of damage at the lower strain-rate. In the case of the coarse grained composite there is some evidence for a change in damage mechanism at the higher strain-rate. We obtain a value for the measured work of adhesion and a measure of the effective modulus local to the damage site, as damage is actually occurring. The observed underlying behaviour should be broadly applicable to particulate composites, whenever stiff filler particles are held in a viscoelastic matrix.