O-168 Projection Criteria for Insensitive Munitions and Hazard Classification

A study has been conducted on the origin of projection criteria for Insensitive Munitions and Hazard Classification. The distance-mass relations from the UN Orange book could be well reproduced with the software TRAJCAN, assuming the maximum impact distance reached by a natural steel fragment launched from 1 m height. The analysis also showed that the fragment shape and material have a significant influence on the throw distances. For fuzes comparable impact distances have been predicted as for natural steel fragments.

Further analysis showed that at the maximum distance, the impact energy is generally much smaller than the launch energy. This is an issue, as the latter energy is often used as a criterion. For the launch energies of interest (8, 20 and 79J) the height reached by the projectiles is not enough to reach the terminal velocity and energy before impacting the ground. The most relevant energy to know with respect to hazard is that of impact, possibly with an exposed fire fighter at 15m distance. For this reason, new distance-mass relations were developed that match the criteria based on impact energy, instead of launch energy.

For near vertical projections the impact distance does not provide any information about the launch energy or impact energy, although the latter will never be larger than the terminal energy. It is important, during IM and HC tests, to verify the presence of such projections with high speed video. An impact distance may also be reached by high velocity shallow trajectories. This will often appear to be unrealistic due to ricochet effects.

To assess the hazard of projections to persons, both blunt injury and penetration injury have to be taken into account. State of the art blunt injury models predict only minor injury for a 20J impact. For a 79J blunt impact there is a small probability of lethality and major injury is likely to occur. These conclusions can be drawn for a standing person with a frontal exposure to the event, and given that the body is hit once. Lethal penetration injury by smaller projections may take place at lower energy levels. The smallest projectile masses in the distance-mass relations (25g to 100g) are in the transition region from penetration injury to blunt injury. An assessment based on blunt injury alone is not conservative in this region. The 20J criterion is however sufficient to avoid lethal penetration injury for steel fragments larger than 25g. 

It has been suggested to harmonize the projection criteria with the definition of the Inhabited Building Distance (IBD) in [AASTP-1, 2015]. This means that the 20J criterion would be changed into a 79J criterion and that the hit probability would be limited to 1%. A rule of thumb was developed to ensure a hit probability below 1% at 15m; the total number of projectiles counted between the 20J and 79J distance-mass relation should stay below 25. This rule of thumb assumes that each launch angle is equally likely and that trajectories are well approximated by straight lines up until 15m. It is however not straight forward how to scale a test result to realistic, full scale, accident scenarios.

We recommend changing the distance-mass relation that distinguishes a munitions burning response to a 20 J impact energy criterion at 15 m. Maintaining 20J as an energy value is necessary to guarantee projectile related non-lethal effects and minimal probability of major injuries, which one would expect from a burning reaction. Furthermore, we recommend distinguishing between projectile shapes and materials; at least between steel and aluminium projections. Including a hit probability to the projection criteria is not recommended because of the aforementioned scaling issues.