Testing polymer fibers for body armor
The ideal mechanical properties for these vests and other gear include a combination of high stiffness, large tensile strength, and a significant strain-to-failure in order to absorb the impact of the bullet. Initial work by Holmes revealed that the natural creasing and folding that a vest would normally encounter while in use led to a significant degradation of these critical mechanical properties, especially in humid environments.
While the degradation in the mechanical properties was self-evident, what was missing was an analytical technique to characterize the structural or chemical differences in the fibers that would account for their loss in performance. Although there is no material that could be completely “bulletproof” in every circumstance, researchers did want a way to characterize materials for their varying ability to mitigate a bullet’s impact, especially after field use.
The characterization method selected by Holmes and Christopher Soles at NIST made use of an intense positron beam facility at North Carolina State University’s PULSTAR Nuclear Reactor.
The positron annihilation lifetime spectroscopy (PALS) technique provides a molecular-level view of the structure of materials. It has been used for testing materials in other sectors, including porous membranes and semiconductor insulators. For this work, positrons were injected into ballistic fibers and enabled researchers to determine if any voids were created during folding on a scale of less than 5 nanometers.
Using PALS, Holmes and Soles discovered that void levels are very sensitive indicators of damage sustained by the fibers after folding; a larger population of voids means a better chance of fiber failure. The team previously suspected that void creation was a critical component of mechanical degradation, but the small angle X-ray scattering measurements that had been used in the past tended to be less sensitive to voids smaller than 5 nanometers and proved to be inconclusive. The critical damage was occurring on much finer length scales.
“It allowed us to characterize changes in the fibers that you cannot see with other techniques,” Holmes said. “We were surprised during our research at how sensitive the technique was.”
“Before, we didn’t have a really good way to discriminate why some materials broke during folding tests and some didn’t,” said Soles. “This is the first materials characterization tool that gives insights into why some materials can be folded and still maintain their strength.”
The results may act as a design cue for those wanting to develop new alternatives to the current body armor. It may also help fine-tune the amount of fibers currently prescribed for these products, making for more comfortable vests.
— Read more in John A. Howarter et al.,”Nanostructural evidence of mechanical aging and performance loss in ballistic fibers,” Journal of Polymer Science Part B: Polymer Physics 55, no. 23 (28 September 2017): 1711-17 (DOI: 10.1002/polb.24417)