Response of thermites to dynamic high pressure and shear

Abstract
The paper begins with a review of the literature on high pressure and shear properties of materials. This is an area in which further interest was recently stimulated by published Russian work on structural bond energy release. The research reported in this paper was performed on five different thermite compositions. Thermites are a subgroup of the class of pyrotechnics and consist of a metal and a metal oxide that are formulated to release a large quantity of heat (but little gas) when they react. There has been relatively little research on their impact and shock properties. Initial drop–weight impact experiments on thermite powders showed that they are relatively insensitive to impact (compared with other energetic materials), and ignition events could be obtained only when ‘grits’ were added. The most suitable ‘grits’ for hot–spot ignition were found to be small disks of polycarbonate, which fail by fracture and shear. Ground–glass ‘grits’ had no discernible effect. However, ignition events were relatively easily obtained when the thermites were pressed into disk form. Three different geometries were studied: (i) normal impact on right–circular disks; (ii) inclined impact on right–circular disks; and (iii) normal impact on ‘pre–sheared’ disks (with 30 inclined sides). We found that ‘pre–sheared’ disks were the most sensitive and inclined impacts on right–sided disks the least. In general, the samples were only partly consumed for the range of drop–weight impact conditions studied. Having established which of the compositions was the most reactive, we performed some shock studies using a plate–impact facility. The Hugoniot of this thermite and the temperature reached during shock were both measured. Several high–speed photographic sequences (microsecond time resolution) are included showing the impact deformation of the thermites, the points of ignition, and extent of reaction. The data provided should be useful for those interested in modelling the deformation and reactive properties of thermites.

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