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    • Through the years aside from chemical weapons warheads have

    2018-11-12

    Through the years, aside from chemical weapons, warheads have also been designed to generate either fragments or blast shock waves as their primary damage mechanism. Thermobaric explosives (TBXs) have been predominantly used in a blast role (rather than for their fragmentation characteristics) due to their enhanced blast effect, which is a direct result of the secondary combustion of additives [74]. The shock wave generated by the detonation of TBXs is of a lower amplitude but has a longer period than that of conventional secondary explosives. The work by Jaansalu and coworkers dwells on an investigation in which using a flash X-ray imaging technique, the ability of TBXs to shatter metal casings and to propel the resulting fragments have been reported [74]. During the investigation, three casing materials were used. Those were AISI 1026 steel, gray NS-398 iron and ductile iron while two different TBX compositions were employed with C4 serving as a benchmark. The fracture behavior of the casings, as a function of explosive fill and material characteristics, was mostly as expected. They used C4 (RDX/plasticizer (91/9)), TBX-1 (monopropellant/magnesium) and TBX-3 (monopropellant/aluminum/RDX). One TBX formulation exhibited a run distance to detonation. The well known Gurney equation was employed to get a correlation and compare the final fragment velocities. It was found that in the case of two of these TBX compositions, as compared to similar amount of C4, a larger fraction of the available energy of explosive was converted to mechanical energy to propel the fragments. This fraction of energy was influenced by the confinement of the detonation products as well as the ignition delay of the metal powders. These two factors had a greater influence on the fragment velocities than did material characteristics. Jaansalu et al. also investigated and discussed the fragmentation characteristics, influence of explosive material, fragmentation velocity, influence of casing thickness, etc. [74]. Within the testing experiments, the X-ray images captured the fracture behavior of the casings as a function of fill and material characteristics. The casings fragmented as expected. The X-ray images also provided information on the run-up distance of the explosive fills used. The run distance for the TBX-3 formulation, containing liquid monopropellant, aluminum powder, and RDX, is about 20 mm. The run distance for the TBX-1 formulation (monopropellant/magnesium) is less than 20 mm, such that no indications of asymmetric expansion are observed. Note that the Gurney equation assumes that the fraction of energy propelling the fragments of any charge is roughly the same. The results obtained in this work have been found to be consistent with the conclusion that a larger fraction of energy is available in TBX (liquid propellant/metal) formulations. Furthermore, this fraction of energy is dependent upon the confinement of the detonation products as well as the ignition delay of the metal powders used. It has been firmly concluded that those two factors have a substantial influence on the fragment velocities of the casing than do its material characteristics [74]. In the investigation by Fair, a technique called “Twin Screw Extruder” (TSE) was used [75]. The failures in manufacturing of advanced explosives containing large amounts of metal powders to improve performance, such as PAX-3, have proven how difficult the production stage is. According to the article, the old manufacturing processes had low yield which resulted in a high cost per unit and questionable product uniformity. A group of researchers (TSE team) who were investigating the use of a TSE machine to mix and extrude an aluminum base explosive (PAX-3) was mentioned. The TSE team had successfully demonstrated this concept on a new formulation (coded 02-02-06). This material had been processed using a smaller concentration of green solvents in comparison to the conventional batch processing and additionally, the product was more uniform. The TSE method mentioned above uses a base material consisting of coated HMX (PAX-2 or PAX-2A), made by conventional means, and reprocessing it into its aluminized corollary. The article claims that this manufacturing process is extremely flexible, allowing for the reformulation of a base material into a number of different explosives with designed and tailored characteristics. It was also claimed that this new technology cut the cost of manufacturing. The loss of organic solvents to the environment and waste treatment requirements would also be greatly reduced. It is anticipated that the concentration of the organic solvents to be employed will be reduced by as much as 50% as compared to traditional batch processes [75].