Some Observations About the Drop-weight Explosive Sensitivity Test
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RESEARCH PAPER
Some Observations About the Drop‑weight Explosive Sensitivity Test P. J. Rae1 · P. M. Dickson1 Received: 22 May 2020 / Accepted: 25 September 2020 © The Author(s) 2020
Abstract The apparent simplicity of the drop-weight apparatus for explosive sensitivity testing hides the reality that it is actually a complex integrated test of both ignition and growth of deflagration. Although the drop-weight test is undeniably a useful screening test for explosive properties, a misunderstanding of the technique’s limitations has blinded many researchers to its limited wider applicability. This monograph discusses how the test actually works, the significant engineering difficulties with standardization between machines, which types of explosives are suited to the test and which are not, and finally offers a few suggestions for alternatives when a more quantified understanding of a material’s response is required for other applications. Keywords Explosive · Sensitivity · Impact The drop-weight test (sometimes called the drop-hammer test) is superficially one of the simplest explosive sensitivity tests that can be conceived [1,2]. At its most basic, it involves trapping a small sample of explosive between two parallel, hard, high melting-point and rigid surfaces, one of which is dropped from varying heights to compress the sample. The drop-height is varied until a defined level of explosive violence is obtained from the sample and this height is recorded as a figure of sensitivity (or insensitivity). Usually this threshold is for a 50% probability of a violent ignition (known as a “go”). This seeming simplicity obscures the fact that the process of producing chemical reactions in the explosive sample in this way involves a very complex series of events that are still not fully understood, but requires both high pressure and high shear rates. Additionally, it is extremely uncommon that in day-to-day usage, transport or storage a thin layer of energetic material is ever trapped between two parallel, hard, high meltingpoint and rigid surfaces that then compress the material quickly enough for reaction to start. It is therefore both surprising and serendipitous that the drop-weight test has any real utility as a safety screening test. Indeed, the very applicability of the test is somewhat suspect when it is realized that the ranking of individual * P. J. Rae [email protected] 1
M-6, Los Alamos National Laboratory, PO Box 1663, MS‑P917, Los Alamos, NM 87545, USA
explosives is often different depending on the machine design. Table 8.3 in Reference [2] succinctly summarizes the problem. In that table the absolute ranking of several common explosives are compared against four types of machine. The general ranking is consistent (e.g., TNT is always less sensitive than PETN), but in two cases RDX is less sensitive than HMX, while in one example that order is reversed and in another they are equal. Additionally, in four cases, lead azide is found to be more sensitive than PETN, but in one machine that order is also rever
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