ballistic trauma, especially those that might be relevant to injuries associated with bullets and shrapnel striking body armor. This section presents the physics, biophysics, and methods of studying nonpenetrating blunt trauma with the goal of optimizing the design and testing of manufactured body armor.

A bullet is a localized source of energy that can cause high local compression and shear forces, penetrating protective layers. The most effective bullets deposit energy, shear, and momentum rapidly in the target. One general strategy for protection is to blunt the penetration of the incoming round, picking up as much mass as possible in the body armor while decreasing the round energy and increasing the contact area. Thus, the protective effect of any ballistic protective vest is provided by increasing the area of impact, thus transferring energy and momentum to the vest. However, effective transfer of large amounts of energy and momentum from the incoming round into the body armor generally implies some deformation of the rear, or back face, of the body armor.

The BABT deformation is generally larger under soft body armor for a given incoming round. An interesting comparison of energy and momentum scales may be seen by comparing characteristics of various rounds, as shown in Table 8-1. Energy varies by a factor of over 30 between the relatively slow 9 mm handgun round and the .50 caliber (12.7 mm) rifle round due to the differences in mass and velocity.

TABLE 8-1 Muzzle Parameters for Various Types of Rounds

Device Muzzle Velocity (m/sec) Round Mass (g) Energy (kJ) Momentum (kg m/sec)
9 mm 358 8 0.5 2.86
5.56 x 45 M193 ball 991 3.6 1.7 3.57
7.62 x 51 NATO ball 838 9.6 3.4 8.13
12.7 mm 50 M2 890 42 16.6 37.4

SOURCE: Ness, 2011.

A further elucidative comparison may be made between the impact energy and momentum scales of low-rate blunt trauma events such as automobile impacts and high- rate impact events such as BABT. The energy and momentum for various potential blunt trauma situations are shown in Table 8-2 and are plotted in Figure 8-1. It is apparent from Figure 8-1 that a nonpenetrating ballistic impact involves much lower total momentum transfer than typical low-rate blunt impacts. However, the energy transfer is comparable, depending on the round and impact velocity. This implies increased localization of energy transfer and shorter interaction time and likely increased localization of injury.

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