goats, or 44 mm of deformation in clay, is approximately 4 percent.) It was reasonable to assume that use of the blunt impactor tests to model the injury behavior from behind-armor body trauma was “likely” conservative for this impact velocity range. However, outside the velocity range typical of handgun rounds (i.e., 240 m/sec) into soft body armor, the relationship between injury and deformation response in the clay is much less certain. More recent measurements in sheep using velocities of about 800 m/sec showed significant lethality even with 34-mm deep indents (cf. Chapter 8 and Gryth et al., 2007).
Clay BFDs were tailored to mimic such an indent. From Chapter 4 in this report, and as introduced in Chapters 2 and 3, the Roma Plastilina #1 (RP #1) modeling clay backing material used in armor testing has two important purposes. The first is to simulate the tissue response beneath the point of impact so that ballistic data generated in laboratory tests can be correlated to effects seen on the human body. The second purpose is to denote the extent of BFD during ballistic testing (Prather et al., 1977).
Multiple materials are available to simulate a body; in fact, at the time it was introduced, modeling clay was recognized to only approximate tissue response, and empirical correlations were needed to develop a probability for lethality or injury. The chief advantage of modeling clay over other materials available at the time was that it better served the function of recording BFDs; that is, when impacted, modeling clay deforms plastically, and a permanent cavity (also termed “indent,” “impression,” or “crater”) is developed under the point of impact. Correlations were developed between the geometry of the cavity and the probability of lethal injury.
The U.S. Army Aberdeen Test Center (ATC) has set the maximum acceptable BFD value at 44 mm for body armor plates tested using clay. This value appeared reasonable based upon the past measurements. As noted in Chapter 3, the Army does not have the medical outcomes to know whether 44 mm is a conservative value.
Measurement instruments were used to verify the test results, as directed by the procurement specifications. Digital calipers and then laser-based instruments were used to better measure the BFD under nonideal conditions (i.e., offset and side/edge indents) (Walton et al., 2008). However, different instruments may give different BFD readings due to each instrument having