Scientists have demonstrated a lightweight type of foam made of hollow metallic spheres that can withstand the incredible impact of .50 calibre rounds: the kinds of bullets fired by heavy machine guns and sniper rifles.

While weighing only about half as much as conventional steel armour, this engineered material – called composite metal foam (CMF) – protects just as effectively against these deadly projectiles, even when tested against armour-piercing ballistics.

"The CMF armour was less than half the weight of the rolled homogeneous steel armour needed to achieve the same level of protection," says engineer and materials scientist Afsaneh Rabiei from North Carolina State University (NCSU).

"In other words, we were able to achieve significant weight savings – which benefits vehicle performance and fuel efficiency – without sacrificing protection."

Rabiei helped lead the development of composite metal foam at NCSU about 15 years ago, and these days heads up the university's Advanced Materials Research Lab (AMRL).

In the lab, her team spends a lot of time experimenting, trying to figure out just what this metallic foam is capable of.

CMF, which is manufactured using patented processes, is fabricated from metals including aluminium and steel, which are riddled with hollow air pockets throughout, like other kinds of foam.

In previous research, Rabiei's team has shown CMF can do things like obliterate medium-size bullets, protected against the blast of high-explosive rounds, in addition to shielding against fire, heat, and a number of kinds of rays and radiation.

In the new work, the researchers wanted to see how CMF would cope against the deadly force of .50 calibre rounds measuring 12.7 x 99 mm – among the largest kinds of bullets commonly used in conventional machine guns and long-range rifles.

Their experiments consisted of firing ball and armour-piercing .50 calibre rounds at the CMF, at speeds from 500 metres per second up to 885 metres per second.

In the tests, the CMF acted as an active core in the armour, covered by a ceramic faceplate at the front, with a thin back plate of aluminium.

(North Carolina State University)

The results showed that the CMF layer can absorb 72–75 percent of the kinetic energy of the ball rounds, and absorbed 68–78 percent of the kinetic energy of the armour-piercing rounds – and prevented projectile penetration at speeds up to 819 metres per second.

While the researchers say there's still room to optimise the material, it's amazing to think that this engineered foam filled with pockets of air is able to stop projectiles travelling at over 800 metres per second.

"There is additional work we could do to make it even better," says Rabiei.

"For example, we would like to optimise the adhesion and thickness of the ceramic, CMF and aluminium layers, which may lead to even lower total weight and improved efficiency of the final armour."

Even in its current form, we're looking at a kind of engineered material that can stop some of the deadliest bullets used in war – and in a material that weights only half as much as standard protection, meaning things like military vehicles could be lighter, and more manoeuvrable.

It's pretty clear there's a desperate need for these kinds of material advantages – as long as arms manufacturers keep making bullets, that is.

The findings are reported in Composite Structures.