Computational analysis of hot-spot formation by quasi-steady deformation waves in porous explosive

Abstract

The impact and shock sensitivity of porous (granular) high-explosives is related to the formation of small mass regions of elevated temperature within the material called hot-spots by dissipative mechanisms such as plastic and friction work. Because of their small size, hot-spots are difficult to experimentally interrogate, particularly for high volumetric strain rates (ϵ̇V>10,000 s−1). In this study, simulations are performed for large ensembles of deformable particles (≈4000 particles) using a combined finite and discrete element technique to characterize statistical distributions of hot-spot intensity, geometry, and spatial proximity within and behind quasi-steady, piston supported uniaxial waves in granular HMX (C4H8N8O8). Emphasis is placed on examining how the material's initial particle packing density, characterized by its effective solid volume fraction ϕ¯s,0, affects hot-spot statistics for pressure dominated waves corresponding to piston speeds within the range 300≤Up≤500 m/s. Predictions indicate that hot-spot intensity is only marginally affected by initial porosity (1−ϕ¯s,0) for all piston speeds, whereas hot-spot size, number density, volume fraction, and volume specific surface area appreciably increase with porosity and exponentially increase with piston speed. Minor variations in particle shape are predicted to be largely inconsequential. Joint distributions of hot-spot intensity and size are combined with thermal explosion data to identify and examine critical hot-spots that quickly react behind waves. These results indicate that the observed increase in sensitivity with initial porosity for sustained loading is likely due to an increase in hot-spot size and number rather than intensity.