Testing the Validity of the NIJ Clay Standard for Approving Body Armor With a Preliminary Insight Into Injury Correlation

The non-penetrating energy dispersion of a projectile into body armor, or behind armor blunt trauma (BABT), creates injuries from the back-face deformation (BFD) of the body armor into the wearer resulting in blunt-type injuries to include death.  The pass/fail of body armor is almost solely dependent on its BFD, coming from the National Institute of Justice (NIJ), and using an oil-based clay (Roma Plastilina No.1) as a backing material and measuring standard. The passing limit of the armor is set at 44mm:  the BFD cannot exceed 44mm in more than 20% of the hits, and the armor fails if BFD reaches 50mm.  However, this BFD limit has never been correlated with injury risk or severity.  Furthermore, this clay standard is extremely temperature dependent, and any variation in the preparation of the clay can result in drastically different results.

A rig was constructed to simulate the effect of BABT while reducing the risks associated with traditional ballistics testing; an impactor was created to mimic the exact shape and dimensions of body armor’s BFD resulting from a high-caliber round.  The need for firepower and body armors has been negated in narrowing the experimentation to the single injury-causing interaction.  The cylindrical carbon fiber impactor is hollow to accommodate a wireless accelerometer sensor/data acquisition system. The test rig is comprised of a 6m steel launch tube and a pressurized locking mechanism to load the impactor.  Two different velocities were utilized:  a low armor-deformation speed of ≈28m/s and a high armor-deformation speed of ≈64m/s.  The clay was prepared and preconditioned according to the NIJ standard (2008).  Eight tests were conducted: five at the low velocity and three at the high velocity.  The temperature of the clay was determined to be ≈27.22°C achieved from the preconditioning methods.  One test at the low and high velocity was done at ≈25.56°C to investigate the effect of prolonged exposure to a room temperature setting.  For a preliminary look into boney injuries resulting from these body armor BFD velocities, a whole-body post-mortem human specimen (PMHS) was likewise tested.  The PMHS received two hits at the lower velocity and one at the higher. 

The low-velocity clay tests at 27°C had average velocity 27.72m/s and clay depth 16.63mm, well within the NIJ standard.  The two PMHS low-velocity tests had velocities and peak accelerations 28m/s 1338g and 29m/s 1118g with impacts in the upper right and lower left thoracic quadrants respectively.  As expected, these hits did not result in any injury.  The 25.5°C, low-velocity hit onto the clay resulted in a significantly different BFD measure of only 9mm.  While still being within the passing region of the NIJ standard, it is important to note for cases where accuracy is a necessity.  For the high-velocity tests on the clay at 27°C, the average depth of penetration was 47.75mm – outside of the 44mm standard, but within the 50mm hard limit.  The PMHS high-velocity test in the upper left thoracic quadrant had velocity 59m/s and peak acceleration 6843g which resulted in a catastrophic failure of the ribcage.  The autopsy results for the whole body PMHS revealed 16 different rib fractures including a fracture pattern indicative of flail.  Being post-mortem, areas that may have contained contusion cannot be determined, but visual damage to the pleura was observed at the impact site.  The number of fractures is concerning since the upper limit of the NIJ standard was not reached, and that alone demands further investigation into the validity of the clay surrogate.  The high-velocity clay test done at 25.5°C had a BFD of 37mm, indicating a passing value.  Knowing that in the testing of body armor the clay may be sitting outside the conditioning chamber and open to room temperature for an extended length of time, this extreme difference in response needs to be addressed.  The measure of BFD through depth of penetration into clay may very well be critically under-predicting injuries received from BABT.