By Katherine Norton-Hewins, PhD, Body Armor Consultant and Researcher, Shrivenham,Oxfordshire, United Kingdom
n recent months, U.S. President Barack Obama has been put under increasing pressure to tighten gun laws after such devastating incidents as the one on December 14, 2012, at Sandy Hook Elementary School in Newton, Connecticut.
The United States has the highest gun ownership rate in the developed world with approximately 90 guns for every 100 citizens and 8,583 firearms-related fatalities in 2011 (63 of which were law enforcement officers). In 2012, 49 U.S. law enforcement officers were reported feloniously killed in the line of duty as the result of a firearm.1
The type and level of threat against law enforcement officers in other parts of the world is clearly different. In the United Kingdom, only six law enforcement officers were killed by a firearm since the year 2000—three of which were feloniously killed in 2012. Knife crime is a leading cause of concern in the United Kingdom, and the body armor worn by U.K. law enforcement officers reflects this reality.
A Vital Tool
Personal body armor is becoming an increasingly vital tool for both law enforcement and military personnel throughout the world. Initially manufactured to prevent a projectile penetration, ballistic personal body armor is also now designed to protect against Behind Armor Blunt Trauma (BABT), including back-face signature injuries (penciling). BABT describes the spectrum of injuries that are received by an individual wearing armor after a nonpenetrating ballistic impact. The extent of these injuries can range between mild grazing to organ damage and, in extreme cases, even death. A back-face signature injury occurs when the body armor retains the round, but does not effectively dissipate the energy efficiently, causing a narrow section of the armor to deform backward into the body, penetrating the skin and causing a “penetrative like” injury, see figures 1 and 2.
|Figure 1: Left to Right. Before impact; typical blunt trauma injury; and penciling injury.|
|Figure 2: Photograph showing penciling on the back-face of armor after testing.|
Currently, the U.K. Home Office Center for Applied Science (CAST) and the U.S. Department of Justice, National Institute of Justice (NIJ) both have ballistic resistant standards in place to assess the protective levels of armor for penetrative and non-penetrative trauma. Both of these standards have been developed through close ongoing collaboration between the two countries. Additionally similar standards exist for knife and spike resistant armors, again developed through U.S. and U.K. collaborated efforts.2
The standards that are currently in place in the United States and the United Kingdom are unique, as they are developed in conjunction with serving officers, to reflect the specific needs of the end users. While there are similarities between the requirements of a U.S. law enforcement officer and a U.K. law enforcement officer, there are also crucial differences.
There are many factors influencing the similarities and differences between the two standards. The majority of U.K. officers do not carry conventional firearms on a daily basis, and access to firearms by criminals in the United Kingdom is far less prevalent than in the United States. Although ballistic threats do exist in the United Kingdom, a knife attack is the greater risk to a U.K. law enforcement officer. Because the primary threat in the United Kingdom to law enforcement officers is from a knife attack, the majority of U.K. armor for routine patrol is both ballistic and knife resistant. Whereas, in the United States, the greater threat is from firearms. As such, the majority of the armor for U.S. law enforcement is for ballistic protection only.
The CAST and NIJ have developed standards to test the performance of personal body armor.3 The ballistic standards are designed to test whether armor is capable of stopping specific threat rounds while being able to limit the amount of back face deformation exhibited in the clay backing material. The body armor manufacturers must demonstrate compliance of their products with these standards for them to be certified.
|Figure 3: Plastilina block testing in accordance with HOSDB standards. The specified box size is slightly different in the NIJ standard. However, the difference is unlikely to affect the results obtained during testing.|
Both the standards for the NIJ and the Home Office Scientific Development Branch (HOSDB) use Roma Plastilina No. 1 (a type of oil-based clay) as a backing material when testing soft body armor, as seen in figure 3. The clay is conditioned to a specific temperature and is calibrated using a dropped weighted mass to check the depth of the deformation caused.
Once calibrated, the clay is flattened, smoothed, and placed a specific distance from the muzzle of the test barrel depending on the level of armor being tested. The armor is then strapped to the front of the clay box, the test barrel is loaded, and the armor is shot.
The NIJ standard stipulates a maximum post-impact static indentation of 44mm for all levels of armor. The CAST standards stipulate a maximum level of post-impact static indentation in Plastilina of 44mm at the HG1A level of protection, and 25mm for the remaining protection levels. The HG1A level is designed to offer protection against both 9mm and 0.357 ammunition fired from handguns.
See figure 4 for a simplification of the HOSDB standard measurement method for back face deformation and figure 5 to see the typical indentation observed in clay after testing.
|Figure 4. The HOSDB method of measuring the back-face signature in Plastilina after a ballistic impact.|
The 44mm criterion was originally established after a BABT study using goats. The animals were shot at with handgun ammunition (.22- and .38-caliber) while wearing aramid jackets and physiological data was obtained before and after firing. The distension of these jackets was then compared with the distension of the jackets when tested with Plastilina, and it was concluded that 44mm was a sustainable amount of deformation before severe injury occurred.4 The 44mm criterion was originally tested against goats, using low caliber ammunition, with limited bullet energy levels. However, the criterion has been extended to cover armors that are designed to protect against higher velocity, larger caliber rounds without examining how much more energy would be transferred to the underlying tissues. The allowable back-face signature for body armor has been controversial from its introduction in the first NIJ test standard, and the debate on the relative importance of penetration-resistance versus back-face signature continues in the medical and testing communities.
|Figure 5: An example of the back deformations in the clay shown here with plaster of paris molds.|
The use of Plastilina as a testing medium in ballistic testing has many advantages: it is easy to use due to its malleability, it is reusable, it is relatively in-expensive, and it has a long shelf life. Plastilina can also provide repeatable data on how the armor deforms, allowing for comparisons of armor to be made. Clay is not a suitable medium to attempt to represent the human body’s response and structure; it is a simple comparator that can be improved with a better understanding of the biomechanics it should simulate.
There has been much controversy with the use of clay and there are acknowledged limitations relating to the use of Plastilina: The clay needs to be heated before each test and calibrated before and after each batch of armor testing; the clay does not have any simulant-like properties and as such cannot be related to human tissue directly, the current testing method does not involve the measurement of the rate of deformation, which may be of great importance.
Some believe that the use of clay as a testing material for ballistic testing and the use of the 44mm pass/fail criteria lack relevance as the measurements are too different from those occurring in the human body.6 Knudsen (who was a member of the NATO specialist team on body armor) further stated that he “found it strange that the simple depression in a backing material like clay can be expected to explain comprehensively what goes on at the moment of impact by a certain number of centimetre.” He went on to state that “we remain wholly unconvinced that this attitude could be maintained with bullets of the same weight and double the velocity.”7
More recently studies conducted by multiple research groups suggest that clay shows only the flexibility of the armor and that the future of body armor testing is with a simulator rig that can better show the physiological effect of a blow to the body after a non-penetrating impact.8
No other material has been acknowledged as a suitable replacement for Plastilina. However, Plastilina limitations, including the lack of biofidelity and the inability to provide detailed measurements—excluding maximum deformation—have led to challenges of the use of clay as a suitable test medium.
While acknowledging the limitations of Plastilina, it is also crucial to note that Roma Plastilina has stood the test of time, thearmors that have been tested against this medium have saved thousands of lives.
Research in to alternative backing materials and measurement techniques is ongoing, with many potential innovative and novel solutions and methods being developed.
However, what is clear is that in order to move forward successfully, any new and improved backing material or measurement technique must address all, not just some, of the limitations seen with Plastilina. Moving to an alternative method that has not been fully validated and demonstrated as effective would be a step backward that could potentially endanger lives. Both NIJ and CAST embark on an ongoing review process for their standards, as well as evaluating the threats faced by officers—and the alternatives to Plastilina. ♦
1“Honoring Officers Killed in 2012: Line of Duty Deaths,” Officer Down Memorial Page, www.odmp.org/search/year?year=2012 (both accessed July 1, 2013).
2National Institute of Justice, Law Enforcement and Corrections Standards and Testing Program, Stab Resistance of Personal Body Armour: NIJ Standard-0115.00, www.ncjrs.gov/pdffiles1/nij/183652.pdf; and John Croft and Daniel Longhurst, HOSDB Body Armor Standards for UK Police: Part 3: Knife and Spike Resistance, publication no. 39/07/C (Sandridge, U.K.: Home Office Scientific Development Branch, 2007), www.bsst.de/content/PDF/39-07-C_-_HOSDB_Body_Armour1.pdf (both accessed July 1, 2013).
3John Croft and Daniel Longhurst, HOSDB Body Armour Standards for UK Police: Part 2 Ballistic Resistance, publication no. 39/07/8 (Sandridge, U.K.: Home Office Police Scientific Development Branch, 2007), www.bsst.de/content/PDF/39-07-B_-_HOSDB_Body_Armour1.pdf; National Institute of Standards and Technology, Office of Law Enforcement Standards. Ballistic resistance of body armor NIJ Standard -0101.06 (Washington, D.C.: National Institute of Justice, 2008), www.ncjrs.gov/pdffiles1/nij/223054.pdf (both accessed July 1, 2013).
4Victor R. Clare et al., Body Armor Blunt Trauma Data, data from Edgewood Arsenal: Aberdeen Proving Ground, Maryland (Washington, D.C.: National Institute of Law Enforcement and Criminal Justice, Law Enforcement Assistance Administration, 1976).
5Jack C. Roberts et al., “Assessing behind Armor Blunt Trauma in Accordance with the National Institute of Justice Standard for Personal Body Armor Protection Using Finite Element Modeling,” The Journal of Trauma 62, no. 5 (May 2007): 1127–1133; Modeling for Military Operational Medicine Scientific and Technical Objectives, by James H. Stuhmiller et al., prepared for the U.S. Army Medical Research and Materiel Command, Fort Detrick, Maryland, (San Diego, Calif.: Jaycor, 2005).
6Peter Juel Thiis Knudsen, 1996. “Conclusions of STBA,” in Proceedings of the 3rd Meeting of the Specialist Team on Body Armor (San Diego, Calif., April 23–25, 1996).
7Phil L. Gotts and Paul M. Fenne, “Use of the BABT Simulator Rig for Soft Body Armor Assessment,” in Personal Armor Systems Symposium (Quebec City, Canada, September 13–17, 2010
8Katherine Hewins et al., “The Validation of a Thoracic and Abdominal Test Rig for BABT Soft Body Armour Testing,” 24th International Symposium on Ballistics (New Orleans, September 22–27, 2008), www.dtic.mil/ndia/2008ballistics/Thursday/Hewins.pdf (accessed July 1, 2013).
|Dr. Katherine Norton-Hewins has studied injury biomechanics in the area of ballistic trauma throughout her career. She completed her PhD in the field of Behind Armour Blunt Trauma at Cranfield Defence and Security, The Defence Academy of the United Kingdom. She then moved to Detroit, Michigan, where she began her postdoctoral studies at Wayne State University in the Ballistic Impact Research laboratory led by Professor Cynthia Bir. Dr. Norton-Hewins has worked with both the National Institute of Justice and the Centre for Applied Science and Technology—formally known as the Home Office Scientific Development Branch (HOSDB). She has delivered papers at International conferences and has worked with both military and law enforcement agencies in the United States and the United Kingdom.|
Please cite as:
Katherine Norton-Hewins, "Body Armor Standards: Home and Away," The Police Chief 80 (August 2013): 26–29.