Matt Shipman, NCSU wrote:
Metal Foam Obliterates Bullets – and That’s Just the Beginning
April 5, 2016 | Matt Shipman
Composite metal foams (CMFs) are tough enough to turn an armor-piercing bullet into dust on impact. Given that these foams are also lighter than metal plating, the material has obvious implications for creating new types of body and vehicle armor – and that’s just the beginning of its potential uses.
Afsaneh Rabiei, a professor of mechanical and aerospace engineering at NC State, has spent years developing CMFs and investigating their unusual properties. The video seen here shows a composite armor made out of her composite metal foams. The bullet in the video is a 7.62 x 63 millimeter M2 armor piercing projectile, which was fired according to the standard testing procedures established by the National Institute of Justice (NIJ). And the results were dramatic.
“We could stop the bullet at a total thickness of less than an inch, while the indentation on the back was less than 8 millimeters,” Rabiei says. “To put that in context, the NIJ standard allows up to 44 millimeters indentation in the back of an armor.” The results of that study were published in 2015.
But there are many applications that require a material to be more than just incredibly light and strong. For example, applications from space exploration to shipping nuclear waste require a material to be not only light and strong, but also capable of withstanding extremely high temperatures and blocking radiation.
Last year, with support from the Department of Energy’s Office of Nuclear Energy, Rabiei showed that CMFs are very effective at shielding X-rays, gamma rays and neutron radiation. And earlier this year, Rabiei published work demonstrating that these metal foams handle fire and heat twice as well as the plain metals they are made of.
Now that these CMFs are becoming well understood, there could be a wide array of technologies that make use of this light, tough material. Armor, if you’ll forgive the pun, barely scratches the surface.
No mention of how much it weighs; which seems kind of important. But since they're even considering body armor I'm going to assume it's fairly light.
Now give me my power armor, dammit.
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That . . . is astounding. Newton's impact approximation, as I understand it, would suggest the opposite result. I guess the foamed metal DOESN'T act like a continuous low-density material?
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Zeropoint: The impact approximation doesn't really apply to armor concerns, which are more concerned with energy and force exchanges in a solid regime than transfer of momentum in a liquid regime. Exceptions for shaped charges, I suspect.
As far as the material's utility as body armor... after a brief guestimate comparison, I'm skeptical about any groundbreaking qualities, although it's definitely a technology with some interesting applications to armor.
A foam engineered for strength should have the maximum density achievable while maintaining the foam structure, even if lightness is your goal - strength of the material should go roughly as the square of density, so denser but thinner gets you the best strength-to-mass ratio. The densest a metal foam tends to get is about 20% of the parent metal, which is presumably mostly steel. This stopping plate is 1 inch thick and seems to stop bullets about as well as 0.3 inches of normal steel body armor. So you get a 30% weight reduction, and you pay for it by making the armor three times more bulky, considerably more expensive, and probably much less resilient against multiple impacts (I assume the foam structure breaks down when you shoot it).
Useful? Definitely, under certain circumstances. Likely to replace steel as the stopping plate of choice? I sincerely doubt it.
Last edited by Feil on 2016-04-09 06:05am, edited 1 time in total.
Newton's impact depth approximation holds when impact velocity significantly exceeds the speed of sound in the material being impacted. When I was doing kinematic simulation, we used "double or more" as our benchmark for "significantly exceeds", because at that sort of impact velocity, the impacted material effectively doesn't have any cohesion - starting to act like a fluid, regardless of phase - and the impactor is essentially a blunt object, for purposes of gross calculation. Bullet speed is nowhere close to the speed of sound in metal, even foam metal. In the regime at which bullets impact armor, cohesion is a key component of bullet resistance. Kevlar, for example, makes good armor because of high cohesion in addition to high tensile strength.
