Figure of Insensitivity
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Figure of Insensitivity (or FofI) is an inverse scale of measure of the sensitivity of an explosive substance. In this context, sensitivity refers to how sensitive an explosive is to factors such as rough handling, elevated storage temperature etc.
Thus an explosive substance with a smaller FofI will be less able to withstand rough handling or poor storage conditions.
The scale was once defined such that TNT had an FofI of exactly 100. However in the latter part of the 20th century, the scale was redefined to reflect the fact that RDX had overtaken TNT as the shell filling of choice. As RDX was about 20% more sensitive, the scale was altered such that RDX had an FofI of exactly 80.
Note: 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 to detonate it.
See also: Safety Testing of Explosives