Predictions for weak mechanical ignition of strain hardened granular explosive

Abstract

Predictions are given for the coupled bulk and grain scale response of initially unstressed, strain hardened granular HMX (C4H8N8O8) due to mild piston impact (impact speeds <100 m/s). Importantly, this response depends on the material’s strain history as the stress necessary for bulk inelastic compaction (crush up) increases with the solid volume fraction. Although the quasistatic compaction behavior of HMX is well characterized, the influence of strain history on the bulk and grain scale dynamic loading response has largely been unexplored. In this study, the initial solid volume fraction of the unstressed material is varied over the range of φf⩽φ0⩽1, where φf=0.655 is its free pour value. A Hugoniot analysis for the bulk material identifies three dispersed compaction wave structures that depend on the impact speed and initial solid volume fraction, and are analogous to elastic-plastic waves in dynamically loaded solids. For increasing impact speed, these structures consist of (1) a single viscoelastic wave; (2) a leading viscoelastic wave and a trailing viscoplastic wave; and (3) a single viscoplastic wave. It is shown that the resulting localized heating near intergranular contact surfaces can trigger sustained combustion of the material. Predictions for the grain scale thermochemical response indicate that significant bulk viscoplastic heating is required for ignition of materials with φf⩽φ0⩽0.88, whereas bulk viscoelastic heating leads to the ignition of denser materials (φ0>0.91). Both viscoelastic and viscoplastic heating are predicted to be important for ignition of materials having 0.88⩽φ0⩽0.91. Within this transition range there is predicted a sharp increase in impact sensitivity as the power input needed for ignition rapidly decreases to a value close to that for the free pour density (0.40 MW/cm2) before increasing again. This result is important for assessing the impact sensitivity and deflagration-to-detonation transition of damaged explosives.