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As part of the Body Armor Safety Initiative, NIJ examined both new and used Zylon®-based bullet-resistant armor. The results thus far are reported in the Third Status Report to the Attorney General on Body Armor Safety Initiative Testing and Activities. Research findings in the report include:
- Ballistic-resistant material, including Zylon®, can degrade due to environmental factors, thus reducing the ballistic resistance safety margin that manufacturers build into their armor designs.
- The ultimate tensile strength  of single yarns removed from the rear panel of the Forest Hills armor was up to 30 percent lower than that of yarns from "new" armor supplied by the manufacturer. Ultimate tensile strength is the maximum stress (force per unit area) that a material, in this case a Zylon® yarn, can withstand prior to failure. All Zylon® yarns were nominally 500 denier, i.e., the yarns did not vary in linear density or effective cross-sectional area.
- Artificially aged armor of the same type that failed in the Forest Hills incident was ballistically tested but no bullet penetrations occurred. NIJ continues to study the Forest Hills body armor penetration to resolve the cause of that failure.
Ballistic and mechanical properties were tested on 103 used Zylon®-containing body armors provided by law enforcement agencies across the United States. Results of the tests include:
- Sixty of the 103 used armors (58 percent) were penetrated by at least one round during a six-shot test series. Of the armors that were not penetrated, 91 percent had backface deformations in excess of that allowed by the NIJ Standard for new armor. This means that while the bullets did not penetrate the vest, the impact of one or more bullets created a dent of more than 44 mm (almost 2 inches) into the clay in back of the vests during testing, a depth that may cause serious injury.
- Only four of the used Zylon®-containing armors met all performance criteria expected under the NIJ Standard for new body armor compliance.
These results do not conclusively prove that all Zylon®-containing body armor models have performance problems, but the results clearly show that used body armor that contains Zylon® may not provide the intended level of ballistic resistance. In addition, the results imply that a visual inspection of body armor and its ballistic panels does not reveal whether a particular piece of Zylon®-containing body armor has maintained its ballistic performance.
Properties of Zylon®
Part of the Body Armor Safety Initiative is an applied research component that examined material properties of Zylon® to understand the causes of the ballistic failures. Preliminary findings from the applied research effort indicate that:
- Zylon® fibers show a systematic loss in tensile strength, tensile strain , and ballistic performance correlated with the breakage of specific bonds in the chemical structure of the material.
- It is likely that the ballistic performance degradation in Zylon®-containing armors is closely related to the chemical changes in poly-p-phenylene benzobisoxazole, the chemical basis of Zylon® fiber. The breakage of one particular part of the PBO molecule, known as the oxazole ring, correlates with degradation of the mechanical properties of Zylon® fibers. The breakage in the oxazole ring can be monitored using an analysis technique known as Fourier transform infrared spectroscopy.
- Preliminary investigations into Zylon® degradation mechanisms have suggested that oxazole-ring breakage occurs as a result of exposure to both moisture and light.
- When there was no potential for external moisture to contact Zylon® yarns, there was no significant change in the tensile strength of these yarns. External moisture may be necessary to facilitate the degradation of Zylon® fibers.
See Third Status Report to the Attorney General on Body Armor Safety Initiative Testing and Activities (pdf, 47 pages) for greater detail on these findings.
 Tensile strength is the maximum stress (force exerted over a given cross-sectional area) that a material can withstand prior to failure (as defined in Supplement I: Status Report to the Attorney General on Body Armor Safety Initiative Testing and Activities (pdf, 15 pages).
 Tensile strain is a measure of the amount of stretch in fiber before it breaks (as defined in the Third Status Report to the Attorney General on Body Armor Safety Initiative Testing and Activities (pdf, 47 pages).