What's Metal Foam?

A stainless still composite foam developed at NCSU offers new protection to military members, by blocking blast pressure and fragmentation at close range.

Think for just a few seconds about bubble wrap. Yes, bubble wrap. That magical packaging material that not only protects valuable items during shipping, but also makes that oh-so-satisfying popping noise when you squeeze and squish the bubbles. And it’s those bubbles that make bubble wrap work. The air pockets in the wrap absorb the energy of an impact by changing the shape of the bubble. The same is true for cardboard and Styrofoam and packing foam.

The packing material may be crushed but the cushions of air, those spaces between the material, absorb the energy of an impact. And it’s that concept inspired the creation of composite metal foam. “That cushionability comes from the bubbles,” said Dr. Afsaneh Rabiei, professor of Mechanical and Aerospace Engineering at North Carolina State University. “That same concept that protects your materials is used here, but at a higher concentration and at a higher load level. The biggest different is that we introduced the concept to metals, so it could be used at higher loads and heat.”

A video from the US Army testing center shows the higher heat and blast loads. The video shows an explosion from a high explosive incendiary round. It hits the metal foam at 5000 feet per second. The shells were shattered but the foam was hardly damaged.

Rabiei admits she was nervous watching the test. “I was eager to see how it worked and when I heard the foam wasn’t hurt, I kind of relaxed,” she said . “But then on my last test, they fired these huge shells and when I saw a thin layer of the material stopped everything, I ran to the testing officer and hugged him!”

And it’s all because of the bubbles. “All of the spheres are packed down and compressed and absorb the energy,” said Rabiei, as she showed me a roughly six inch block of metal foam followed by a roughly one inch block. “This larger block was compressed down to the smaller block because it absorbed all of the energy from a test,” added Rabiei . And it turns out those spaces not only give metal foam its strength. Those spaces also reduce the weight.

“This is stainless steel,” said Rabiei holding up a shiny rectangular block of the metal. It was about six inches long, four inches wide, and about ¾ of inch thick. She placed it on a scale. “You can see the block weighs almost 1200 grams, but when you weight the composite metal foam, it only weighs 400 grams, which is about one third of the weight and the composite is about 400 gram, so about 1/3 the weight. So one third the weight, but stronger. Yes much more energy absorption almost two order of magnitude, more in absorption and compression but mainly compression because those spheres are giving the cushion-ability.  

 —Frank Graff 


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