Figure of Insensitivity (''F of I'') is an inverse scale of measure of the impact sensitivity of an
explosive substance. In this particular context the term 'Insensitivity' refers to the likelihood of initiation/
detonation
Detonation () is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations propagate supersonically through shock waves with s ...
by impact,
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
,
electrostatic discharge, application of
flame
A flame (from Latin '' flamma'') is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density they ...
, etc. It is a quantitative measure of the level of stimulus required to cause explosive decomposition.
The figure of insensitivity is determined from impact testing, typically using a drop-weight tower. In this test, a small sample of the explosive is placed on a small steel anvil which is slotted into a recess in the base of the drop tower. A cylindrical, 1 kilogram steel weight (mounted inside a tube to accurately guide its descent to the impact point in the centre of the anvil) is then dropped onto the test specimen from a measured height. The specimen is monitored both during and after this process to determine whether initiation occurs. This test is repeated many times, varying the drop height according to a prescribed method. Various heights are used, starting with a small distance (e.g. 10 cm) and then progressively increasing it to as high as 3 metres. The series of drop heights and whether initiation occurred are analysed statistically to determine the drop height which has a 50% likelihood of initiating the explosives. The intention of these tests is to develop safety policies/rules which will govern the design, manufacturing, handling and storage of the explosive and any munitions containing it.
A reference standard sample of
RDX
RDX (abbreviation of "Research Department eXplosive") or hexogen, among other names, is an organic compound with the formula (O2N2CH2)3. It is a white solid without smell or taste, widely used as an explosive. Chemically, it is classified as a ...
is currently used to calibrate the drop tower, so that the drop height to produce 50% likelihood of initiation in this material is measured and recorded. The drop height required to initiate other explosives can then be related to the RDX standard, so that a ready comparison of impact sensitivity between different explosives can be made. By convention, explosives having a 50% initiation drop height equal to that of RDX are given a F of I of 80.
The scale was originally defined using
TNT
Trinitrotoluene (), more commonly known as TNT, more specifically 2,4,6-trinitrotoluene, and by its preferred IUPAC name 2-methyl-1,3,5-trinitrobenzene, is a chemical compound with the formula C6H2(NO2)3CH3. TNT is occasionally used as a reagen ...
as the reference standard, with TNT having, by definition, an F of I of exactly 100. On this original scale, RDX yielded an F of I of around 80. Following
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
, when more complex explosive compositions replaced pure TNT as the most common energetic component of weapon systems, RDX was adopted as the reference standard.
Sensitivity should not be confused with ''Sensitiveness'' which is a measure of how easy it is to detonate an explosive. In this context an explosive with more sensitiveness requires a smaller
detonator or
gayne
{{unreferenced, date=August 2011
An explosive booster is a sensitive explosive charge that acts as a bridge between a (relatively weak) conventional detonator and a low-sensitivity (but typically high-energy) explosive such as TNT. By itself, the ...
to detonate it.
See also
*
Safety testing of explosives
The safety testing of explosives involves the determination of various properties of the different energetic materials that are used in commercial, mining, and military applications. It is highly desirable to measure the conditions under which exp ...
External links
"Safety Cases for Explosive Sites" by Robert D. Wilcox, 2003 (This paper states that TNT has a defined F of I equal to 100.)
{{DEFAULTSORT:Figure Of Insensitivity
Explosives engineering