Researchers Confirm Method for Assessing Body Armor Failure

NIST researchers used positron annihilation lifetime spectroscopy (PALS) to view fibers on a molecular level.

Scientists at the National Institute of Standards and Technology (NIST) explain in a published paper a new way to assess damage to the high-performance fibers used in body armor. They used positron annihilation lifetime spectroscopy (PALS), described as an intense positron beam facility at North Carolina State University's PULSTAR Nuclear Reactor, to view the fibers on a molecular level and have discussed their results in the Journal of Polymer Science, with NIST reporting their research may help increase confidence in the apparel that protects military personnel, police, and public figures from gunfire and also may lead to the development of lighter materials for body armor in the future.

"High-performance polymer fibers have been used in ballistics applications for more than 40 years. Traditionally, these fibers are woven together into a fabric and then layered 15-20 times over to make a vest with a thickness of anywhere from about 6 to 13 millimeters (a quarter to half an inch). Although effective at stopping or slowing down bullets, users have sometimes found these vests, which are worn either under or over clothing, to be heavy and bulky—akin to wearing 15 to 20 shirts at once on a hot summer day. Many would like a more comfortable alternative," NIST reported this week. "The testing of soft body armor has been a big concern because the deployment of a new kind of fiber—believed to be superior to the previous material—unexpectedly failed in 2003, resulting in the death of a police officer. That and other incidents prompted a 2005 recall of some of the vests made with the new material. Although the performance of these vests was superior when they were fresh out of the box and in pristine condition, tests later showed that the mechanical properties of the fibers inside the vests began to deteriorate after a few months of normal wear."

The new vests were eventually removed from market. The U.S. Department of Justice asked NIST to help evaluate the problem and determine why the vests were failing.

"The fibers in these ballistic applications cannot fail [in the field], period,” said Gale Holmes, a materials research engineer at NIST. "But previously, we had no way to know if they were changing over time as people were wearing and using them." To do this, the researchers needed an analytical method to characterize structural or chemical differences in the fibers that would account for their loss in performance and to characterize materials for their withstand a bullet's impact, especially after field use.

NIST's news release said PALS has been used to test materials in other sectors. For this research, they injected positrons into ballistic fibers in order to determine whether voids were created during folding on a scale of less than 5 nanometers; more voids would mean a higher chance of fiber failure.

"It allowed us to characterize changes in the fibers that you cannot see with other techniques. We were surprised during our research at how sensitive the technique was," Holmes said. "Before, we didn't have a really good way to discriminate why some materials broke during folding tests and some didn't,” said researcher Christopher Soles. "This is the first materials characterization tool that gives insights into why some materials can be folded and still maintain their strength."

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