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.
Highlights
The drop-weight test is superficially one of the simplest explosive sensitivity tests that can be conceived [1,2]. 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
This threshold is for a 50% probability of a violent ignition. 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
Summary
Things are slightly different in the model with strength, but where melting is suppressed (C). As the explosive is compacted into the thin disc, many materials with low to moderate melting points as a result of viscoplastic heating are observed to melt and transition from opaque to clear (e.g., PETN) [5,6] Around the time this occurs the rate of radial expansion accelerates rapidly from meters per second to hundreds of meters per second. The abrupt unloading (load-drop) has been attributed to either sample melting or sintering and rapid plastic or viscous flow that occurs relatively late in the drop-weight energy transfer process [5,6] Ignition, if it occurs at all, is observed only after this load-drop and the delay is both explosive compositionand machine design-dependent. Because transparent anvil drop-weight designs can only support the anvils at the edges and the elastic moduli of glass is approximately three times lower than steel, the effect is usually more severe in these designs
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