Composite metal foam stops .50-caliber rounds as well as steel

A hard armor system crafted from composite metal foam takes hits as well as conventional steel armor, but at half the weight, a development that could potentially help revolutionize the design of military vehicles by improving their armor protection, without making them heavier.

North Carolina State University (NCSU) researchers have shown that vehicle armor made out of composite metal foam (CMF) can stop ball and armor-piercing .50-caliber rounds as well as conventional steel armor, even though the CMF weighs less than half as much. CMF is a foam that consists of hollow metallic spheres, which can be crafted from stainless steel or titanium, embedded within a matrix made of steel, titanium, aluminum, or other metallic alloys.

For this study, the researchers used steel-steel CMF, which means that both the spheres and the matrix are made of steel. They created a hard armor system that consists of a ceramic face plate, a CMF core, and a thin backplate made of aluminum. They tested it using a .50-caliber ball and armor-piercing rounds fired at impact velocities from 500 meters/second up to 885 meters/second. The armor system held up impressively, as the CMF layer of the armor was able to absorb 72% to 75% of the kinetic energy of the ball rounds and 68% to 78% of the kinetic energy of the armor-piercing rounds.

“The CMF armor is less than half the weight of the rolled homogenous steel armor needed to achieve the same level of protection,” explains Afsaneh Rabiei, a professor of mechanical and aerospace engineering at NCSU.

Rabiei – who first developed the strong metal foam for use in transportation and military applications – has spent years developing and testing CMF materials. She and her collaborators were able to achieve significant weight savings without sacrificing protection.

The group’s work shows that CMF “offers a significant advantage for vehicle armor, but there is still room for improvement,” Rabiei points out. “These findings stem from testing armors we made by simply combining steel-steel CMF with off-the-shelf ceramic face plates, an aluminum backplate, and adhesive material. We simply optimized our CMF material and replaced the steel plate in standard vehicle armor with steel-steel CMF armor.”

Additional work could be done to make the CMF even better: “For example, we would like to optimize the adhesion and thickness of the ceramic, CMF, and aluminum layers, which may lead to even lower total weight and improved efficiency of the final armor.”

In related work announced earlier this year, Rabiei and her collaborators demonstrated that CMFs can block blast pressure and fragmentation at 5,000 feet/second from high-explosive incendiary rounds detonating only 18 inches away. Her team also showed that CMFs are able to stop a 7.62 by 63 mm M2 armor-piercing projectile at a total thickness of less than an inch, while the indentation on the back was less than 8 mm. For context, the National Institute of Justice standard allows for up to 44 mm of indentation in the back of armor.

As part of this earlier work, the team discovered that steel-CMF “offers much more protection than all other existing armor materials while lowering the weight remarkably,” Rabiei says. “We can provide as much protection as existing steel armor at a fraction of the weight, or provide much more protection at the same weight.”

This finding is notable because “many military vehicles use armor made of rolled homogenous steel, which weighs three times as much as our steel-CMF,” she notes. “Based on tests like these, we believe we can replace that rolled steel with steel-CMF without sacrificing safety; better blocking not only the fragments but also the blast waves that are responsible for trauma such as major brain injuries. That could reduce vehicle weight significantly, and improve fuel mileage and vehicle performance.”

Beyond the strength findings, Rabiei’s group has also shown that CMFs are extremely effective at shielding X-rays, gamma rays, and neutron radiation and are able to handle fire and heat twice as well as the plain metals they’re made of.

“In short, CMFs hold promise for a variety of applications – space exploration to shipping nuclear waste, explosives, hazardous materials, military and security applications, and even cars, buses, and trains,” Rabiei says.

This most recent work was done with support from the Department of Defense’s Joint Aviation Survivability Program, under award number W911W6-15-D-0001-0001.

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Figure 1 | A sample of an early composite metal foam developed in Rabiei’s research group. Credit: Afsaneh Rabiei.