For Your Protection, a New Kind of Metal

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Up next, metal foam. That’s what I said.

Think of a porous material, like Styrofoam, but made out of steel, aluminum, or another metal. We have a great video on our website of ballistic tests showing this material shattering and armor-piercing bullets shot at it. It’s amazing, it just– boom– goes away. Go to sciencefriday.com/metalfoam to see it.

But metal foam has promises way beyond military applications. There’s evidence that this same material could be used to minimize the damage in car crashes or shield spacecraft from heat and radiation– all kinds of things I don’t think people have really thought about totally yet, because it’s so new. Here with details on this new space-age material, as we used to like to say, is Afsaneh Rabiei. And he is Professor of Mechanical and Aerospace Engineering at North Carolina State University in Raleigh. Welcome to Science Friday.

AFSANEH RABIEI: Thank you very much.

IRA FLATOW: Let’s start with the video of a bullet striking this metal foam. Describe what’s happening there.

AFSANEH RABIEI: Well, we have a composite armor made of our material, with some front page and back plate. And we do ballistic testing, all sorts of different impacts from type three, type four– which is the different sizes of bullet– and then 50 cal. And we were happily surprised to see that even the hard core of the bullet is stuck in those bubbles that we have created inside the metal.

IRA FLATOW: Why do the bubbles in the metal, in the foam, work better then just the plain old armor that we us today?

AFSANEH RABIEI: Well, if you look at the armor that people are using, normally people are looking at the back of the armor and provide something like Kevlar to take care of the tensile force in the back. And front part of armor is where the compressive forces is applied. For that, normally, people are using either heavy steel or ceramic. Steel is heavy and ceramic is also, to some extent, heavy– but more important problem with ceramics is that they shatter in the first strike. So for the second strike, you have nothing.

So we came up with the idea– I originally started making this material, not necessarily for armors, but more for cars and trains to squeeze, like a sponge, but in a heavy-duty form of a sponge. So we can put it in front of a car or high-speed train and take care of an impact. But when we saw the performance of the material, we started thinking how about ballistic and bullet? So I tested those and you saw what you saw. So that the material can perform.

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

IRA FLATOW: It’s like a balloon. You press it a little bit and it gives way until it bursts. So it absorbs the force of your finger. It’s counterintuitive to think that you’re adding a lot of bubbles to the metal, that would make it stronger and able to resist the bullet penetrating further– or the egg. But that’s how it works. Is there any secret to how you arrange the bubbles inside that make it better?

AFSANEH RABIEI: Well, actually, that is the main purpose. Because I’m not the first person who ever made metal foam. There have been other studies where people blow air into a molten metal and create a metal foam, similar to a bubble bath. But with a molten metal in the bath. Or adding some foaming agent, which is working similar to baking soda in a cake batter.

But the problem with other metal foam has been– a, it creates a bunch of bubbles– we call it porosities or air pocket– that are in different sizes. They are randomly connected and there is no structure to hold those bubbles. But our material has a very uniform bubble structure. Just no larger or smaller.

So 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 foam and it can perform well in [INAUDIBLE] or [INAUDIBLE].

IRA FLATOW: And not only– I understand that it also can work to be resistant to radiation in spacecraft. Would it block radiation coming in?

AFSANEH RABIEI: Correct. It can block the radiation. And the reason for that– and not only radiation, but also heat. And the reason for that is the air that is trapped inside those porosities that provide a scattering of the radiation, as well as providing the heat shield.

It’s very similar to the cup that you hold hot beverage inside– so you have all those porosities inside the Styrofoam. And it’s similar, but in a metal form. So more heavy-duty.

IRA FLATOW: Sort of a Styrofoam metal.

AFSANEH RABIEI: Correct.

IRA FLATOW: How soon are we going to see this? Are we going to see it showing up anywhere in actual use?

AFSANEH RABIEI: Well, I’m working on it, believe it or not. I’m working on it from both aspects of research. And I’m hoping that we can bring in investors and business people, because I am by no means– I am not a business person. I’m a scientist. But I definitely welcome–

IRA FLATOW: I could see DARPA, I could see all kinds of people becoming interested in this. I want to thank you very much for taking time to be with us, Afsaneh.

AFSANEH RABIEI: Thank you for having me.

IRA FLATOW: Afsaneh Rabiei is Professor of Mechanical and Aerospace Engineering at North Carolina State University right there in Raleigh.

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