Ranking the Drop-Weight Impact Sensitivity of Common Explosives Using Arrhenius Chemical Rates Computed from Quantum Molecular Dynamics Simulations

Abstract

Drop-weight impact tests are used routinely to characterize the handling safety of explosives. Numerous studies have sought to connect various physical and chemical properties of the energetic molecules and materials to their measured impact sensitivities. Wenograd in the early 1960s demonstrated that there is a strong dependency of the drop-heights on the critical temperatures required for explosives to undergo prompt reactions. Reactive quantum molecular dynamics simulations with the lanl31 density functional tight binding model have been used to compute the delay time before the thermal explosion of the secondary explosives erythritol tetranitrate (ETN), pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), trinitrotolune (TNT), and 3,3′-diamino-4,4′-azoxyfurazan (DAAF) as a function of the initial temperature and pressure. The delay time to explosion data are consistent with Arrhenius chemical kinetics, which is expected for thermally activated processes in materials and in accord with experimental measurements. The critical temperatures required for the materials to undergo prompt explosions display the same dependence on drop height as was observed by Wenograd. Hence, quantum-based reactive molecular dynamics simulations are potentially a tool for ranking the drop-weight impact sensitivity and handling safety of explosives.

Analysis of Ignition Sites for the Explosives 3,3′-Diamino-4,4′-azoxyfurazan (DAAF) and 1,3,5,7-Tetranitro-1,3,5,7-tetrazoctane (HMX) Using Crush Gun Impact Testing

“…Previous drop-weight impact test results with DAAF have indicated that the explosive is quite insensitive to a nonshock-induced impact, with drop heights at the upper limit of the test (>320 cm) . However, in recent drop-weight testing by the authors, DAAF exhibited DH 50 values (the height from which 50% of the drops exhibited an audible “go”) at significantly lower values than those previously recorded . Higher DH 50 values correspond to more energy required to generate a reaction, or “go”, using sound meters.…”

“…In contrast, we have observed that DH 50 values for PETN and other standard explosives typically increase on the bare anvil relative to the values collected using grit. 6,60 Sound level data for each impact test drop of DAAF and HMX can be seen in Table 3 and Figure 5, where the sound threshold for a "go" is represented by the horizontal lines in each plot. The plots with HMX have a clearly defined separation between a "go" (above 117 dB) and a "no go" (well below 117 dB) with better separation apparent in the gritcontaining samples, likely due to reliable sources of ignition through friction in those samples.…”

“…54 Recently, we reported drop-weight impact test data on different particle sizes of DAAF both with and without the use of grit paper. 6 Surprisingly, audible "go" responses were recorded at drop height values lower than expected for an insensitive energetic. To investigate this further, herein we report the imaging of DAAF during subshock impacts along with the more sensitive standard explosive HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazoc-tane) (Scheme 1) in order to probe the ignition and propagation behavior that occur during the drop hammer test.…”

“…To ensure the proper handling and storage of each unique energetic material, handling sensitivity tests are conducted in order to assess the material’s response to various stimuli, such as thermal, impact, friction, and spark . While computational models and simulations have been developed in order to predict energetic material sensitivity, − empirical data are still required for accurate assessments. Many methods are available for the sensitivity testing of energetics; − however, varying analysis and instrumentation can make comparison of those results difficult.…”

“…DAAF has recently been investigated for its synthesis, − physical properties, − explosive sensitivity, particle morphology, explosive properties, , and plastic-bonded formulations (PBX) . Recently, we reported drop-weight impact test data on different particle sizes of DAAF both with and without the use of grit paper . Surprisingly, audible “go” responses were recorded at drop height values lower than expected for an insensitive energetic.…”