Hot-spot generation and growth in shocked plastic-bonded explosives studied by optical pyrometry

Abstract

The aggregate behavior of hot spots in shocked plastic-bonded explosives (PBX) was studied by nanosecond optical pyrometry. The averaged thermal emission spectra from at least 25 tiny (50 μg) explosive charges of a pentaerythritol tetranitrate PBX, at several impact velocities from 1.5 to 4.5 km/s, was used to determine average temperatures and emissivities. Individual spectra were analyzed to determine the distribution of hot spot temperatures in individual charges with unique microstructures. Understanding shocks in tiny charges with different microstructures is needed to understand shocks in large PBX charges which sample many microstructures as they propagate. The initial hot spot density was several percent, and the average initial hot spot temperature of 4000 K was, surprisingly, independent of impact velocity. With underdriven shocks, the initial hot spot temperatures clustered around 4000 K, but with overdriven shocks, there were both hotter and colder hot spots. The initial hot spot density increased quadratically with impact velocity. The generation of hot spots was described by a model with a threshold energy to trigger hot spot formation and a distribution of energetic barriers to hot spot formation.

Influence of Void Configurations on Hot Spot Temperature in PBXs under Impact Loading

“…The presence of elongated triangular voids makes PBXs more sensitive. It is worth noting that recently, Bassett et al [41] used nanosecond optical pyrometry to observe that under intense shock loads, the average hot spot temperature in a PBX ex- plosive can reach as high as 4000 K, which is in agreement with the numerical results discussed above.…”

“…However, with a further increase in shock intensity, the effects of chemical reactions should be considered, which would result in the generation of a large amount of reaction product gases and changes in the mechanism of void collapse. As observed by Bassett et al [41], the average initial hot spot temperature of 4000 K is independent of the impact velocity in the range of 1500 to 4500 m/s. The impact velocity considered in this study was below this range.…”

Hot Spot Chemistry in Several Polymer‐Bound Explosives under Nanosecond Shock Conditions

“…At the higher impact velocities used here, all formulations yielded limited radiance after the initial ∼40 ns. This fast cooling is likely due to rapid adiabatic expansion from a detonating charge, as recently seen in X‐PETN .…”

“…In the lower velocity regime, the temperature dynamics appear consistent with previously published results on PETN‐based PBXs where single shots were shown rather than the dynamics shown here which were taken from the averaged spectral radiance of many shots . The high impact velocity regime shows a greatly increased cooling rate at early times due to driving a shock near the von‐Neumann spike pressure and generating an overdriven detonation condition which has recently been discussed .…”

“…The formulations appear to follow an exponential growth (solid lines) in F and the exponential growth parameter of X-PETN, X-RDX, and X-TNT are 1.00 � 0.04, 0.96 � 0.08, and 1.42 � 0.07 s km À 1 , respectively. We recently described the emissivity dependence on impact velocity in X-PETN as quadratic, indicating a linear relationship between the energy barrier to collapse a hot spot-yielding pore and the input shock energy [20]. The alternate paradigm presented here, which is equally well-fit over the range of impact velocities for X-PETN and a better fit when including X-RDX and X-TNT, would suggest that the energy barrier is exponential in shock energy rather than linear.…”