Shock-Absorbing, Reusable Body Armor

Mechanical engineers from Johns Hopkins University in Baltimore have found a new way to build body armor with a lightweight elastomer material that relies on a complex liquid crystal structure. The resulting armor is “lighter, stronger, and reusable,” according to the university’s press release. That could be a game-changer in the highly deformable world of body armor.

Sung Hoon Kang—senior author of the new paper, published in February in the journal Advanced Materials—is part of the tantalizingly named Hopkins Extreme Materials Institute (HEMI), established in 2012 to study “science associated with materials and structures under extreme conditions and demonstrating extreme performance.” Its projects are funded by organizations like the U.S. Department of Energy and the National Science Foundation, with areas of study including things like how materials behave in Earth’s mantle.

It’s easy to see how Earth’s mantle is extreme, with some of the highest temperatures and pressures on the planet. Body armor is a different application, but something that is still very extreme: absorbing gunshots, for example, and spreading that energy out in a way that does not harm the wearer is no small feat.

We are excited about our findings on the extreme energy absorption capability of the new material,” Kang says in the statement.

The idea of a material that can outperform today’s helmets and car bumpers piqued Kang’s curiosity. One of the major areas for improvement is deformation, which is the way the force of an impact presses material way out of shape. Think of a car’scrumple zone,” which is literally designed to collapse to absorb impact; you’re not exactly “reusing” that portion of the car afterward, especially in higher-speed crashes.

Many helmet and impact-absorbing materials dissipate energy through inelastic mechanisms, such as plastic deformation and fracture and fragmentation. However, these materials can become permanently damaged after one-time usage and are not suitable for repeated use,” the researchers write.

So if the non-reusable mechanisms are inelastic—which makes sense, the opposite of elastic and therefore unable to “bounce back”—how do we do things differently? This is where the idea of metamaterials comes into play. A metamaterial is something carefully engineered on the micro-scale to have properties that a simple layer of plywood or metal would not have. The goal is to build better functionality starting at the atomic level.


Super Body Armor

According to ancient lore, Genghis Khan instructed his horsemen to wear silk vests underneath their armor to better protect themselves against an onslaught of arrows during battle. Since the time of Khan, body armor has significantly evolved — silk has given way to ultra-hard materials that act like impenetrable walls against most ammunition. However, even this armor can fail, particularly if it is hit by high-speed ammunition or other fast-moving objects.

Researchers at Texas A&M University have formulated a new recipe that can prevent weaknesses in modern-day armor. By adding a tiny amount of the element silicon to boron carbide, a material commonly used for making body armor, they discovered that bullet-resistant gear could be made substantially more resilient to high-speed impacts.

For the past 12 years, researchers have been looking for ways to reduce the damage caused by the impact of high-speed bullets on armor made with boron carbide,” said Kelvin Xie, assistant professor in the Department of Materials Science and Engineering. “Our work finally addresses this unmet need and is a step forward in designing superior body armor that will safeguard against even more powerful firearms during combat.”

Boron carbide, dubbed “black diamond,” is a man-made material, which ranks second below another synthetic material called cubic boron nitride for hardness. Unlike cubic boron nitride, however, boron carbide is easier to produce on a large scale. Also, boron carbide is harder and lighter than other armor materials like silicon carbide, making it an ideal choice for protective gear, particularly ballistic vests.

Despite boron carbide’s many desirable qualities, its main shortfall is that it can damage very quickly upon high-velocity impact.

Boron carbide is really good at stopping bullets traveling below 900 meters per second, and so it can block bullets from most handguns quite effectively,” Xie said. “But above this critical speed, boron carbide suddenly loses its ballistic performance and is not as effective.”

Scientists know high-speed jolts cause boron carbide to have phase transformations — a phenomenon where a material changes its internal structure such that it is in two or more physical states, like liquid and solid, at the same time. The bullet’s impact thus converts boron carbide from a crystalline state where atoms are systematically ordered to a glass-like state where atoms are haphazardly arranged. This glass-like state weakens the material’s integrity at the site of contact between the bullet and boron carbide.

When boron carbide undergoes phase transformation, the glassy phase creates a highway for cracks to propagate,” Xie said. “So, any local damage caused by the impact of a bullet easily travels throughout the material and causes progressively more damage.”

Previous work using computer simulations predicted that adding a small quantity of another element, such as silicon, had the potential to make boron carbide less brittle. Xie and his group investigated if adding a tiny quantity of silicon also reduced phase transformation.

Xie and his collaborators found that even with tiny quantities of silicon, the extent of phase transformation went down by 30%, noticeably reducing the damage from the indentation.