Editor's note: This column has been updated to provide attribution to original sources.
Dr. Afsaneh Rabiei is a professor of mechanical and aerospace engineering at North Carolina State University in Raleigh, and the sponge-like metal foam for which she holds multiple patents is as lightweight as aluminum but as strong as steel. After receiving a bachelor's degree in Iran and a Ph.D. in Japan, Rabiei worked as a post doctoral researcher at Harvard University. (She's fluent in English, Japanese, and Persian.)
Metallic foams are man-made, porous metal materials that contain bubbles, which can be created by injecting air or gas into a molten metal, or by adding foaming agents into a metal powder or molten metal. The process is similar to adding baking soda to a cake to make it "fluffy."
Think of a steel bar, but with thousands of little air pockets, just like a sponge. Rabiei's isn't the first metal foam, but it's the strongest because her technology uses uniform spheres surrounded by a matrix. The spheres offer "cushionability" and are "light weight," Rabiei said to the AFCEA, whereas the matrix provides the load-bearing strength and bonds the spheres together.
"The problem with other foams is that the structure is very uncontrolled, and as a result, they can deform randomly, and the large porosity can buckle under loading. That's where my kind of metal foam comes into the picture, with controlled spherical structure and added matrix between porosities to increase strength," Rabiei told the AFCEA.
Rabiei's research first gained widespread attention in 2008, when the National Science Foundation announced the creation of the world's strongest metal foam. Rabiei envisioned the material being used primarily for biomedical replacement parts (like knees) and high-impact collision protection (like car bumpers). A crash at 28 mph feels like an accident at only 5 mph with her metal foam.
Rabiei and her team added tungsten, steel, and vanadium, so-called "high-Z materials" that effectively block radiation. The metal foam can block X-rays, low-energy gamma rays, and neutron radiation much more effectively than traditional solutions and can help develop safer systems for transporting nuclear waste.
"We are working to modify the composition of the metal foam to be even more effective than lead at blocking X-rays," Rabiei said in a press release in 2015. "And our foams have the advantage of being non-toxic," she added, " which means that they are easier to manufacture and recycle.
Now, Rabiei is working with the military on the possibilities for a bullet-proof shield that is much lighter than current body armor, which can weigh up to 30 pounds and severely hamper the movement and speed of the wearer.
A video shows a bullet being obliterated by armor made out of Rabiei's composite metal foams. The bullet in the video is a 7.62 x 63 millimeter M2 armor-piercing projectile, which was fired according to the standard testing procedures established by the National Institute of Justice (NIJ). "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," Rabiei told NC State. "To put that in context, the NIJ standard allows up to 44 millimeters indentation in the back of an armor."