Stopping a Speeding Bullet…With Foam

This article is based on a Science Friday interview and was originally published on PRI.org.

What would Styrofoam be like if it were made out of steel? Afsaneh Rabiei at North Carolina State University has been working for years to develop and perfect metal foams, the product of a manufacturing process that embeds hollow metal spheres in solid metal.

The resulting material is light, strong, heat, and radiation-resistant, and, when incorporated in a bulletproof vest, for example, capable of shattering bullets on impact without injuring the person wearing it.

When Rabiei started experimenting with the material, however, she wasn’t looking to make a new kind of armor.

“I originally started making this material not necessarily for armors but more for cars and trains,” Rabies says. “[I wanted it] to squeeze like a sponge, but in a heavy duty kind of form of a sponge so we could put it in front of the car or a high speed train and take care of the impact. But when we saw the performance of the material, we started thinking about ballistics and bullets. And so I tested those and we saw that…the material can perform.”

Watching a slow-motion video of the material being hit with a bullet is incredible: Bullets don’t bounce off the metal foam; they disintegrate on impact.

“Basically, what we have here is a bunch of air bubbles that are already embedded in the material, and when you hit that material, it will squeeze the bubbles, and by squeezing the bubbles, it will absorb the energy,” Rabiei says.

Rabiei and her colleagues are not the first ones to attempt to create a durable metal foam. Other innovators have tried blowing air into molten metal to create a foam similar to a metal bubble bath. Some have also tried adding foamy agents to metal to create porosity. The problem with these experiments, Rabiei says, is that the random assortment of bubbles lacks structure.

“Our material has a very uniform bubble structure,” Rabiei says. “So the load or the force is uniformly distributed between all of those porosities. At the same time, we added some matrix to hold those bubbles together to work in force-bearing or load-bearing. So as a result, the material has become the strongest metal and it can perform well in load-bearing.”

But the metal foam can do more than protect from impact; it can also filter out radiation.

“It’s very similar to the cup that you hold a hot beverage inside,” Rabiei says. “It can block the radiation…and not only radiation, but also heat, and the reason for that is the air that is trapped inside those porosities that provides a scattering of the radiation as well as providing…a kind of heat shield.”