Nearly a century after its conception, Afsaneh Rabiei, Professor of Mechanical and Aerospace Engineering out of North Carolina State University, has developed a composite metal foam capable of turning armor-piercing bullets into dust upon impact.
Essentially a metal sponge consisting of hollow metal beads within solid metal, composite metal foam (CMF) generally retains some physical properties of its base materials. While its defining characteristic is ultra-high porosity, CMF boasts 5 to 6 times greater strength as well as over 7 times higher energy absorption than previously developed metal foams. Typically created by melting aluminum around hollow metal spheres, it is impressively 70% lighter than sheet metal and 80 times more energy absorbent than steel.
Rabiei and her team tested the newly created CMF by firing 7.62 x 63 millimeter M2 armor piercing projectiles, in accordance with National Institute of Justice standards, and the results were staggering.
“We could stop the bullet at a total thickness of less than an inch, while the indentation on the back was less than 8 millimeters,” says Rabiei. “To put that in context, the NIJ standard allows up to 44 millimeters indentation in the back of an armor.”
In layman’s terms, as a high-speed object impacts the CMF, the air pockets inside of the hollow metal spheres collapse, like bubble wrap, absorbing the energy instead of transferring it through the metal.
“When a bullet hits something, normally it penetrates into it. But since the bullet cannot penetrate our material, it hits it and bounces off. The force that our material applies to the bullet breaks it,” said Professor Rabiei. “In our experiment, we found that the bullet totally shattered.”
Unlike other metal foams, the increased strength of Rabiei’s composite metal foam can be attributed to a uniform size of the hollow spheres as well as a metal “matrix” between them. “If the bubbles are not the same size, the larger ones will buckle first and the material will fail early,” explains the professor. And while other companies are developing metal foams for low energy use, Rabiei understands the boundless potential of her newly created material.
“If tomorrow I can reach out to an investor in car companies and they want to put it behind the bumpers of cars, I’d be more than happy with that,” she says. “Or, if someone wanted to put it in body armor or vehicle armor, I would sure be delighted to help that happen. Any of these applications can save lives.”
Professor Afsaneh Rabiei first developed metal foam for transportation and military use. Curious about the potential for nuclear or space application, Rabiei and her colleagues began researching the material further. In 2015, the team discovered CMF to be highly radiation-resistant. Rabiei saw promise in the early research results.
“Our foams have the advantage of being non-toxic, which means that they are easier to manufacture and recycle,” she explains. “In addition, the extraordinary mechanical and thermal properties of composite metal foams, and their energy absorption capabilities, make the material a good candidate for various nuclear structural applications.”
In addition to being extrememly radiation-resistant, researchers have found CMF capable of handling fire and heat twice as efficiently as the base metals it is comprised of. These characteristics alone mean that composite metal foam has the potential to be used for applications that require strong, lightweight materials capable of blocking radiation and withstanding extremely high temperatures, such as space exploration or nuclear waste transportation.
Currently used in mechanical, aerospace, and manufacturing industries, researchers are finding the potential of metal foams to be boundless. Another metal foam developed at Cornell University is able to be transformed from one shape to another, and even has the potential to be used within biological systems — such as artificial knees, for example.
Although a fairly new substance — the first metal foam prototype wasn’t developed until the 1950s — with such bizarre characteristics, one can only imagine what discoveries will come from further research on the material.