Response of thermites to dynamic high pressure and shear

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 rec ently stimulated by published Russian work on structural bond energy releas e. The research reported in this paper was performed on five different ther mite compositions. Thermites are a subgroup of the class of pyrotechnics an d consist of a metal and a metal oxide that are formulated to release a lar ge quantity of heat (but little gas) when they react. There has been relati vely little research on their impact and shock properties. Initial drop-wei ght impact experiments on thermite powders showed that they are relatively insensitive to impact (compared with other energetic materials), and igniti on events could be obtained only when 'grits' were added. The most suitable 'grits' for hot-spot ignition were found to be small disks of polycarbonat e, which fail by fracture and shear. Ground-glass 'grits' had no discernibl e effect. However, ignition events were relatively easily obtained when the thermites were pressed into disk form. Three different geometries were stu died: (i) normal impact on right-circular disks; (ii) inclined impact on ri ght-circular disks; and (iii) normal impact on 'pre-sheared' disks (with 30 degrees inclined sides). We found that 'pre-sheared' disks were the most s ensitive and inclined impacts on right-sided disks the least. In general, t he samples were only partly consumed for the range of drop-weight impact co nditions studied. Having established which of the compositions was the most reactive, we performed some shock studies using a plate-impact facility. T he Hugoniot of this thermite and the temperature reached during shock were both measured. Several high-speed photographic sequences (microsecond time resolution) are included skewing the impact deformation of the thermites, t he points of ignition, and extent of reaction. The data provided should be useful for those interested in modelling the deformation and reactive prope rties of thermites.