Metallic Foams Annihilate Bullets, Block Radiation

By Nathaniel Scharping | April 6, 2016 4:23 pm
Afsaneh Rabiei checks on metallic foam created in her lab.

Afsaneh Rabiei checks on metallic foam created in her lab. (Credit: Rabiei/North Carolina State University)

The word foam doesn’t necessarily convey strength, but seeing a bullet explode upon impact with composite metal foam certainly does.

Researchers at North Carolina State University created a bubble-filled metal composite that combines the strength of steel with the airiness and heat-resistant properties of foam. In tests with armor-piercing bullets, a one-inch layer of the material stopped the projectile in its tracks while allowing only an 8-millimeter indentation on the other side. Afsaneh Rabiei, a professor of mechanical and aerospace engineering at North Carolina State University, claims her version of metallic foam is the strongest yet.

The Secret is the Bubbles

Foams are made mostly of air pockets — that’s what makes them so light. They derive their strength from the composition and structure of the material that surrounds the air pockets, a steel alloy in this case. Rabiei has worked with composite metal foams (CMFs) for years to understand their unique properties and find the optimal configuration of materials and bubbles to maximize results.  

“If you look around, you will see a ton of porous materials in nature: wood, bone, leaves, a bird’s wings, anything that you want to be lightweight or have cushionability, you’ll see that it has air bubbles,” says Rabiei. “It’s a waste of material and energy to carry a solid bulk of materials with you, when you can actually use something much lighter and accomplish even more.”

In her latest study, published in the International Journal of Thermal Sciences, Rabiei describes two different methods of making her metal foam, both of which rely on hollow steel spheres to form the “bubbles.” The first method casts a metal with a low melting point, like aluminum, around the spheres, creating a matrix of hollow spaces within. For metals with higher melting points, the spheres are packed in by a metal powder that is compressed in a process called sintering to form a solid.

It Can Take the Heat

But it all comes down to the bubbles, says Rabiei. By keeping the diameter of the spheres constant, she says, any stress will impact them equally, increasing the material’s strength. The spherical shape of the bubbles confers strength as well, by distributing the force evenly around the structure. Where other metal foams buckle as their bubbles collapse, Rabiei’s maintains its structure thanks to the uniform nature of the components.

Because the spheres are filled with air, the material is also resistant to heat and heat-related expansion. In her latest experiment, she compared a block of solid stainless steel to the bubble-infused steel as both were heated by an 800-degree flame on one side. The CMF took twice as long to reach 800 degrees as the solid block, a result of the air-filled bubbles inside.

Putting CMFs to Work

The unique properties of CMFs make them ideal for use in body armor, as the bullet demonstration makes plain. Rabiei also sees uses for them in the space industry, where lightweight, durable materials are in high demand.

Additionally, by adding tungsten and vanadium to the composite, Rabiei says her CMF blocks radiation, including gamma rays, X-rays and neutron radiation. This makes the material ideal for transporting nuclear waste, she says, and also increases its utility for space travel, where harmful radiation poses a danger to human health.

In the future, Rabiei would like to test how well her metal foam stands up to vibrations, a property that could carry further benefits for vehicle construction dampen sound-proofing. Ultimately, her end game is to save a person’s life.

“If I can make one person walk out of an accident because of my materials, I will have accomplished my goal,” she says.

MORE ABOUT: materials science
  • Uncle Al

    Syntactic foams are commonplace, e.g. curing epoxy filled with about 30 vol-% of hollow glass microbubbles (e.g., 3M Scotchlite K20,~500 psi isostatic crush strength). Here, one desires a tough matrix with hard inclusions and crack propagation tip blunting. Play out momentum in time and space via strain. For an interesting projectile mucker, add 1/8″ carborundum grit to her syntactic foams – with surface wetting.

    a one-inch layer of the material stopped the projectile in its tracks” Try a lead and especially steel air gun pellet launched from a .22 rifle gun with a .22 nail gun blank (Power Level 4, yellow, in brass casing). Personally, I’d be embarrassed if my sooperdooperpooper foam were holed by 14 grain diablo air pellet ammo not even using the hot blank loads for power-actuated tools.

    If there is human meat on the far side, maybe a matrix of the alloy Al_20Li_20Mg_10Sc_20Ti_30, doi:10.1080/21663831.2014.985855

    • Uncle Al

      If the hollow steel balls are made by the coated lost foam method, one could substitute polystyrene foam with a ceramic foam (precursor, re rice hulls to silicon carbide) to create a syntactic foam filled with ceramic projectile shatterers.

      About that bullet shattering…was it a frangible round? MythBusters Episode 34: “Bulletproof Water,” 13 July 2005 All supersonic bullets (through .50-caliber) disintegrated in less than 3 feet of water. Pistol lower velocity bullets propagate up to 8 feet.

      • polzzlop

        What about “guns for destroying underwater mines”?

        • Uncle Al

          giggle Try a supercavitating DU round.

          • polzzlop

            We need something very dense and strong, with a hole in the middle to act as a ramjet.
            Propellant would need to make a lot of gas, some released at the front.

          • Uncle Al

            Shkval Russian supercavitating torpedoes. Yeah, we need nose gas ejection, so a long rod penetrator bigger than a mere bullet. Any ship can be a minesweeper – once.

          • polzzlop

            Energy/volume versus resistance to motion.
            Are there very energetic reactions with water to
            produce bubbles to propel, similar to a ramjet?

          • Uncle Al

            KISS – Keep It Simple, Stupid. Complexity invites failure. Consider solar, wind, and cow fart electricity generation.

          • polzzlop

            Future is Entropy reduction power generation.
            We need for you to come up with a 50% efficient solar cell
            to be coupled to the 230% efficient Light emitting diode.

          • Uncle Al

            Energy too cheap to meter! I favor energy too expensive and unreliable to use to Save Our Childen in 2500 AD.

          • polzzlop

            I see your point. Children need to mature and learn basic survival.
            Chop wood, get water, repair the roof, kill a chicken for supper.

  • joseph2237

    This is good science helping protect people. We need so much more of it can’t begin to say.


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