Figure of Insensitivity (or 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 by accidental means e.g. impact, friction, electrostatic discharge, application of flame, etc. It is a quantitative measure of the level of stimulus required to cause initiation, typically by shock/impact.
The Figure of Insensitiveness 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 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 sensitiveness 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 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, when more complex explosive compositions replaced pure TNT as the most common energetic component of weapon systems, RDX was adopted as the reference standard.