Stainless steel composite metal foam offers more protection than other forms of armour

Armour made from stainless steel composite metal foam can block blast pressure and fragmentation at 5,000ft/s from high explosive incendiary rounds that detonate 457mm away.

CMF panel after test. Black marks are fragments trapped inside panel

These are conclusions from research conducted by North Carolina State University and the US Army's Aviation Applied Technology Directorate, which are detailed in the Journal of Composite Structures.

"In short, we found that composite metal foam (CMF) offers much more protection than all other existing armour materials while lowering the weight remarkably," said Afsaneh Rabiei, senior author of a paper on the work and a professor of mechanical and aerospace engineering at NC State. "We can provide as much protection as existing steel armour at a fraction of the weight - or provide much more protection at the same weight.

"Many military vehicles use armour made of rolled homogeneous steel, which weighs three times as much as our steel-CMF," Rabiei said. "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 would reduce vehicle weight significantly, improving fuel mileage and vehicle performance."

For this study, researchers fired a 23×152mm HEI round - often used in anti-aircraft weapons - into an aluminium strikeplate that was 2.3mm thick. Steel-CMF plates measuring 254mm by 254mm - and either 9.5mm or 16.75mm thick - were placed 457mm from the aluminium strikeplate. According to NC State, the researchers assessed that the steel-CMF held up against the wave of blast pressure and against the copper and steel fragments created by the exploding round, as well as aluminium from the strikeplate.

"Both thicknesses of steel-CMF stopped the blastwave, and the 16.75mm steel-CMF stopped all of the fragments from 15mm2 to over 150mm2 sizes," Rabiei said. "The 9.5mm steel-CMF stopped most, but not all, of the fragments. Based on the results, a 10mm steel-CMF plate would have stopped all of the frag sizes."

The researchers also developed computer models of how the steel-CMF plate would perform. When compared to the experimental results, the model is said to have matched very closely. The researchers then used the model to predict how aluminium 5083 armour - a type of armour that has a similar weight and thickness to the 16.75mm steel-CMF - would perform against HEI rounds.

The model showed that, while aluminium armour of similar weight to the steel-CMF panels would stop all of the frags, the aluminium armour would buckle and allow fragments to penetrate much deeper.

This would result in more damage to the panel, transferring large amounts of stress to the soldiers or equipment behind the armour. The steel-CMF absorbs the energy of the blast wave and flying fragments through local deformation of hollow spheres, leaving the steel-CMF armour under considerably less stress.

Next steps include testing the steel-CMF against improvised explosive devices and high-calibre, mounted ballistics. The researchers have already tested the CMF's performance against hand-held assault weapons, radiation and extreme heat.