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 by impact, friction, electrostatic discharge, application of flame, etc.
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 intention of these tests is to develop safety policies/rules that will govern the design, manufacturing, handling and storage of the explosive and any munitions containing it.
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.