Abstract
The shock response of two-dimensional model high explosive crystals with various arrangements of circular voids is explored. We simulate a piston impact using molecular dynamics simulations with a Reactive Empirical Bond Order (REBO) model potential for a sub-micron, sub-ns exothermic reaction in a diatomic molecular solid. In square lattices of voids all of one size, reducing that size or increasing the porosity while holding the other parameter fixed causes the hotspots to consume the material more quickly and detonation to occur sooner and at lower piston velocities. The early time behavior is seen to follow a very simple ignition and growth model. The hotspots are seen to collectively develop a broad pressure wave (a sonic, diffuse deflagration front) that, upon merging with the lead shock, transforms it into a detonation. The reaction yields produced by triangular lattices are not significantly different. With random void arrangements, the mean time to detonation is 15.5% larger than with the square lattice; the standard deviation of detonation delays is just 5.1%.
Highlights
Heterogeneities such as inclusions, voids, cracks, and other defects enhance the shock sensitivity of high explosives by causing additional shock dissipation that creates small regions of high temperature called hotspots [1]
Chemical reactions initiated in the hotspots emit pressure waves that merge with the lead shock and strengthen it, so that further hotspots are created with more vigor
We observe that the extent of reaction closely follows a very simple ignition and growth model
Summary
Heterogeneities such as inclusions, voids, cracks, and other defects enhance the shock sensitivity of high explosives by causing additional shock dissipation that creates small regions of high temperature called hotspots [1]. Chemical reactions initiated in the hotspots emit pressure waves that merge with the lead shock and strengthen it, so that further hotspots are created with more vigor. This positive feedback is the principal mechanism of the shock-to-detonation transition in inhomogeneous explosives [2]. Spherical voids are an often-studied, common defect in explosives [2,3,4,5] They can become hotspots upon collapsing under shock loading, and may cause hotspots elsewhere by their partial reflection of the lead shock and by emitting further shocks upon their collapse and explosion. Inert beads produce similar effects; while they do not collapse violently enough to become hotspots, their reflected shocks are not weakened by immediately following rarefactions and so may more create hotspots where they collide
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