Figure of Insensitivity

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Figure of Insensitiveness (or F of I) is an inverse scale of measure of the impact sensitiveness of an explosive substance. In this context, sensitiveness refers to the likelihood of initiation by accidental means, for example by impact, friction, electrostatic discharge, application of flame, etc. A mnemonic sometimes used is: "the 'ess' in sensitiveness refers to 'Safety'". The term 'Sensitivity', when applied to explosives, is a quantitative measure of the level of stimulus required to produce initiation (usually by shock) and is used where deliberate initiation of explosives is being discussed.

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 prepared anvil and this is loaded into the drop tower. A weight is then dropped on the test specimen from a measured height and the specimen is observed to determine whether initiation or non-initiation occurs. This test is repeated many times, varying the drop height, to determine the drop height at which a 50% likelihood of initiation will occur.

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 FofI of 80.

The scale was originally defined using TNT as the reference standard, with TNT having, by definition, an FofI of exactly 100. On this original scale, RDX yielded an FofI of around 80. Following Word War II, when more sophisticated explosive compositions replaced pure TNT as the most common energetic component of weapon systems, RDX was adopted as the reference standard.