O-167 Analysis of the IM type V reponse descriptor_plus annex
The IM type V response descriptors given in AOP-39 Ed. 3 have raised a number of questions. MSIAC undertook a study to answer these questions.
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 TP16 legacy natural steel fragment launched from 1 m height. Besides the distance-mass relations for launch energies of 8J and 20J a new one was determined for 79J. The calculations agree well with an analytical solution in the limit of large projectile mass and small launch energy. The analysis showed that projectiles with a different shape or material may reach significantly smaller distances. For fuzes we predict comparable impact distances as for natural steel fragments.
Further analysis showed that at the maximum distance and for a vertical launch, the impact energy is generally much smaller than the launch energy. 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 is the impact energy of projectiles at 15 m distance. New distance-mass relations were therefore developed that match energy criteria at 15 m.
Modeling of injury and lethality due to projectile impact has a long history. A distinction is made between blunt injury and penetration injury. 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 relatively small steel fragments takes 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 for these projectile masses. The 20J criterion is however sufficient to avoid lethal penetration injury for fragments larger than 25g.
It has been suggested to harmonize the IM type V response descriptor with the definition of the Inhabited Building Distance (IBD) in AASTP-1. This means that the 20J criterion should be changed into a 79J criterion and that the hit probability should 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. A remaining question is whether the result from an IM test (with a small amount of ammunition) can be scaled to represent realistic accident scenarios (e.g. a fully loaded shipping container). It is not straight forward how to scale the test results and requirements to match a full scale situation.
Four possibilities for a way forward have been proposed:
- Maintain distance-mass relation based on 20 J launch energy criterion
- Change to distance-mass relation based on 20 J impact energy criterion at 15 m
- Change to distance-mass relation based on 79 J impact energy criterion at 15 m
- Change to distance-mass relation based on 79 J impact energy criterion at 15 m, combined with a 1% hit probability criterion at 15 m..
Other possibilities combine any of the options above with more detail about the projectile shape and material.
The MSIAC recommendation is to choose option 2, for which we have the following reasons:
- It seems reasonable to move from a launch energy criterion to an impact energy criterion at the location of a possibly exposed fire fighter (15m).
- Keeping 20J as an energy value is necessary to guarantee non-lethal effects and minimal probability of major injuries for a fire fighter at 15m. This seems, to the author and MSIAC staff, to be consistent with a burn type of response.
- To include the hit probability (option 4) causes scaling issues between an IM test, its configuration, and a full scale situation which are not easy to solve. Also it can be questioned whether it really makes sense to harmonize the IM type V response descriptor with the IBD.
Furthermore MSIAC recommends extending option 2 with distance-mass relations for additional projectile shapes and materials. The additional projectile types investigated in this report resulted in more restrictive distance-mass relations compared to natural fragments (steel). Applying the distance-mass relations for natural fragments (steel) also to other projectile types therefore potentially underestimates the hazard. MSIAC recommends distinguishing at least between steel and aluminium fragments. It is possible to extend to more types of projectiles, but the additional experimental effort should also be considered.