Researchers at NC State have designed a super-tough material that's being tested by the US military
January 3, 2020
Time to say “Wow” once again for a product designed by engineers at NC State. UNC-TV Science first told you about composite metal foam in the fall of 2018. It now appears a discovery that is being tested by the US military as possible battle armor to keep passengers and cargo safe on airplanes flying across the country.
How it works
First a little background. Composite metal foam is made up of hollow metallic spheres- made of materials such as stainless steel or titanium- embedded in a metallic matrix of steel, aluminum, or some type of metal alloy. It looks something like tapioca pudding. Multiple tests showed metal foam is amazingly tough: it’s able to withstand .50 caliber rounds, high temperatures and even explosions. Part of the reason the material is so strong is because it is based on the same concept at bubble wrap.
“That cushionability comes from the bubbles, “ said Afsaneh Rabiei , Ph.D, a professor of mechanical and aerospace engineering at North Carolina State University when we talked with her last year. “It’s what protects your vase when you wrap it in bubble wrap. The cushionability comes from the hollow spears, but now at a higher concentration and at a higher load level.”
Think again about bubble wrap. The packing material works because the air pockets in the wrap absorb the energy of an impact by changing the shape of the bubble. It’s the same thing with Styrofoam. The packing material may be crushed, but the cushions of air (the space between the material) absorbs the energy of an impact.
Metal foam can protect aircraft wings
New research shows composite metal foam (CMF) protects the leading edges of aircraft wings. But instead of using steel-steel CMF, in which both the spheres and matrix were made of steel, researchers immersed the steel-steel CMF into an epoxy resin, making sure the epoxy filled the hollow sphere and any open spaces in the matrix.
Tests then compared how well the metal foam compared with aerospace grade aluminum in three areas:
1. Contact angle-how well water streams off the material.
2. Insect adhesion-how well bug parts stuck to the material.
3. Particle wear-how well the material stands up to erosion.
Remember we’re talking about the wings of a plane. If water or bug parts builds up, or if erosion is eating away at the edges of a wing, the plane won’t fly as well. That’s not good! It turns out CMF held up remarkably well in all of the tests, which means the metal foam could provide the plane wings a longer lifetime at a lower cost- because of less maintenance and replacement. Both materials weigh about the same.
“While aluminum is the metal of choice for the leading edge of aircraft wings, we can show CMF might provide a valuable replacement,” said Rabiei. “But the tests also suggest we could use different materials for the matrix or spheres that may also provide the same strength but at a fraction of the weight. Either way you are improving performance and efficiency.”
Frank Graff is a producer/reporter with UNC-TV, focusing on Sci NC, a broadcast and online science series.