Soldier armor, such as bulletproof vests and helmets, depends upon high-performance ballistic fibers for its strength and effectiveness. Army researchers are experimenting with how they can improve armor by making it lighter and stronger. At the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, researchers examine high-performance fibers in an innovative soft armor research program. By understanding how fibers behave under stress, researchers can predict their performance during a ballistic event. With this information, researchers can leverage a materials-by-design approach to create stronger, more effective body armor.
“The fibers we’re talking about are about 15-30 micrometers in diameter, or half the size of a human hair,” said Dr. Kenneth Strawhecker, a materials engineer at the laboratory. “The fibers can be layers of woven fabric or cross-plies of unidirectional sheets.”
The Army researchers partner with U.S. industrial fiber manufacturers to study the structure inside single fibers to see how it influences the strength and stiffness of the fibers. “High-performance fibers have a stringy structure like string cheese, where the strings are called fibrils,” Strawhecker said. “We have learned how to use a gallium-ion beam [a device that resembles a scanning electron microscope] to mill notches in the fiber so that we can carefully peel it open and then use atomic force microscopy to observe the fibrils inside.”
Atomic force microscopy examines material surfaces at the extremely small scale - on the order of fractions of a nanometer. Strawhecker uses the analogy of a tiny version of a blind man tapping his cane along the inside of the fiber. This is how atomic force microscopy maps the fibrils for length and width, he said.
“If we tap the cane -- the AFM probe -- harder, we can determine how stiff the fibrils are,” he said. “Or, if we push a probe in between the fibrils we determine the strength between the fibrils.” The researchers discovered that these fibrils tend to form bundles of fibrils.
“We measured the energy to separate the bundles for different fibers,” Strawhecker said. “The result we found is that some bundles offered more resistance to separation than others, depending upon the chemical makeup or the processing of the material.” These findings leave the researchers optimistic that as they look at smaller and smaller structures within a fiber, they will understand what holds them together.
“Long polymer molecules form crystals which group into nano-fibrils, which group into fibril bundles, which group into single fibers,” Strawhecker said. “At each size we have found that there is a similar resistance to separation between these parts for a fiber with given chemical makeup and processing history.”
Understanding this structure means knowing how molecules are arranged together to form structures that ultimately will stop a bullet for the soldier. “Determining the best way to arrange them can be based upon these models and it depends on finding the strongest, stiffest overall fiber structure due to the arrangements within,” he said.