Foam. Light, soft and fluffy, right? Wrong. Watch what this wall of foam does to an armor-piercing bullet.

Dr. Afsaneh Rabiei, professor of mechanical and aerospace engineering at North Carolina State University, has invented a new type of metal foam that can stop bullets. And that’s only the beginning of its long list of possible uses.

Metal foams are a relatively new class of materials that are lighter and can absorb more energy than solid metal. Metal foams are composed of metal and air, with the metal forming a matrix of empty cells.

Dr. Rabiei has spent her career studying metal foams and their unusual properties. They are incredibly useful in that they are light but still strong and flexible and can therefore absorb more energy than solid metal.

High Performance Composite Metal Foam from MMC Productions Inc on Vimeo.

However, metal foams do have drawbacks. The cells in the matrix are all different sizes, with cell walls of varying thicknesses that change shape differently from one another under pressure.

Dr. Rabiei invented a composite metal foam (CMF) using hollow metal spheres to generate cells of an even size, instead of allowing the metal matrix to develop randomly. The spheres are surrounded by metal of the same or a different type, creating the strongest type of metal foam yet. This new composition means the CMF can absorb much more energy before breaking or changing shape.

This is how CMFs can stop a bullet that can easily punch through Kevlar. The bullet used in the test was a 7.62 x 63 millimeter M2 armor-piercing projectile, but it didn’t even make the CMF break a metaphorical sweat.

The foam “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 says. “To put that in context, the [National Institute of Justice] standard allows up to 44 a millimeter indentation in the back of an armor.”

CMFs are more than just strong. Last year, Dr. Rabiei showed that CMFs are effective at shielding against X-rays, gamma rays and neutron radiation. Earlier this year, she also demonstrated that they can handle fire and heat twice as well as the solid metals from which they are made.

This means that there are numerous applications for CMF beyond armor, from architecture to space exploration to nuclear waste transport to biomedical engineering [“biomedical engineering”, article: http://phys.org/news/2010-02-material-mimics-bone-biomedical-implants.html#nRlv].

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