Metal foam obliterates bullets

[dkbrk]

This doesn't have much in the way of interesting details, you need to go to the paper for that.

The armour is a composite sandwich, with boron carbide on the front face (a hard ceramic), followed by the metal foam, and either aluminium or kevlar on the rear face.

The boron carbide layer blunts the bullet and distributes the compressive stress over a large area of the metal foam. The metal foam is made of 2mm-diameter hollow steel spheres in a stainless steel matrix (created using powder metallurgy). The metal foam deforms plastically under the compressive stress, absorbing the kinetic energy of the projectile (i.e. the spheres get crushed). The backplate provides tensile strength and stops the foam from tearing apart due to residual tensile stresses.

[chillydawg]

Presumably very expensive and one-time usable. I guess that's true of most types of armour anyway.

[the8472]

There's a 4th layer: The whole thing is vacuum-wrapped in epoxy-fiberglass composite to catch spall.

[kragen]

It's unfortunate that the researchers couldn't find someone who speaks English fluently to write the paper with. Here's the abstract: "The application of advance [sic] materials to manufacture hard armor systems has led to high performance [sic] ballistic protection. Due to its [sic] light-weight [sic] and high impact energy absorption capabilities, composite metal foams have shown good potential for applications as ballistic armor. A high-performance light-weight composite armor system has been manufactured using boron carbide ceramics as the strike face, composite metal foam processed by powder metallurgy technique [sic] as a bullet kinetic energy absorber interlayer, and aluminum 7075 or Kevlar™ panels as backplates[,] with a total armor thickness less than 25 mm. The ballistic tolerance of this novel composite armor system has been evaluated against the 7.62 × 51 mm M80 and 7.62 × 63 mm M2 armor piercing projectiles according to U.S. National Institute of Justice (NIJ) standard 0101.06. The results showed that composite metal foams absorbed approximately 60–70% of the total kinetic energy of the projectile effectively and stopped both types of projectiles with less depth of penetration and backplate deformation than that specified in the NIJ 0101.06 standard guidelines. Finite element analysis was performed using Abaqus/Explicit to study the failure mechanisms and energy absorption of the armor system. The results showed close agreement between experimental and analytical [sic] results."

I'm hoping they carried out their experiments (physical and simulation; in my book, FEA simulation results aren't "analytical") more carefully than they wrote the paper.

The foam does sound like a pretty interesting material! I've read about metal foams since my childhood, but the hollow-sphere-based foam seems significantly stronger than the more irregular foams.

[redacted]

I did modelling work for a group researching metal foams in my past academic life. Very interesting stuff with a lot of applications if the manufacturing processes can be nailed down.

Our funding was a mixture of European Space Agency and a car conglomerate: light armour for space vehicles and crumple zones / reinforcement respectively (less acceptable to research military applications in EU in my experience).

[fudged71]

I'd love to chat more about these materials, could you message me? tom[at]printtopeer[dot]com cheers!

[colordrops]

Would this be effective against micrometeoroids and orbital debris?

[paulsutter]

I asked Dr Rabiei that very question by email yesterday, and she is working on it:

"Let me start by saying that it is not a crazy idea and is actually what I have been working on recently. I even have a proposal in DARPA on testing the performance of the material at supersonic speeds. At this time, our data covers up to a speed close to 1Km/s as you mentioned. We have not tested the performance at higher speed and that is what I was hoping to conduct soon."

(I asked if it could be used for space debris at 8-15km/s, and apologized for asking a crazy question)

[patio11]

This presumably depends mostly on what your delta-V is. You can be wearing Epic Foam Platemail of the Whale +5 but if your orbital velocity is 7 km/s and you hit a 100g rock at the same velocity but going the other direction physics says you're about to have a very bad day.

[ChuckMcM]

That would be a winning application. Something that I expect you could get NASA/SpaceX on board with, send some sheets up as ride along payloads for a resupply mission, put them in the bay behind the Dragon module on the second stage with some cameras, then use dragon as an uplink to look for space debris impact on the second stage as it re-enters and monitor the results. I would have to know more about what is available in the remains of the second stage to know how much mass you would end up adding for the experiment but I expect it would be less than a couple of kilograms.

[venomsnake]

The current designs already use dual hull - the first makes the projectile explode to dust, the second stops the debris. The critical problem is weight. That construction that is shown looks to be quite heavy relatively.

[lolc]

The part about shielding radiation sounds dubious to me. I thought shielding from radiation was mostly about mass. How is that foam supposed to shield better than cast metal of the same weight? Maybe the "foam" deteriorates less?

[rdtsc]

> I thought shielding from radiation was mostly about mass.

Not for all types of radiation. Neutron radiation for example is absorbed better by a hydrogen rich material like say polyethylene. But then during absorbtion it would emit gamma rays sometimes, so metal shielding is needed as well.

[dubyah]

They outperformed aluminum because of the iron content in the steel of the sphere walls or the matrix itself of the steel-steel composite metal foam.

The composite metal foam that was comparable to lead utilized steel with higher concentrations of tungsten and vanadium.

[SixSigma]

"foam" is the right way of thinking, it's not some thin bubbles of mostly air, from comment above :

> The metal foam is made of 2mm-diameter hollow steel spheres in a stainless steel matrix (created using powder metallurgy).

[feelix]

if you have something that interrupts the wave it blocks the whole wave. that's why the gauss on the microwave front door works yet you can see through it, because microwaves (the actual waves) are in the order of centimetres wide.

[ceejayoz]

> 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.

Designers of spacecraft and lunar/Mars bases are likely pretty happy about this development.

[hyperion2010]

Here's the doi for that paper: doi:10.1016/j.radphyschem.2015.07.003

Unfortunately I could only find the paywalled version here: http://www.sciencedirect.com/science/article/pii/S0969806X15...

[kibwen]

Neat, though of course this raises the question: what would it take to engineer a foam-piercing bullet?

[blisterpeanuts]

Two projectiles. First one explodes on impact and compromises the target. Second one penetrates the target.

[PhasmaFelis]

I'm not an expert, but after some Googling, I think the bullet they're using there consists of a steel core inside a lead envelope inside a copper or copper-nickel jacket. So, at a guess, you'd want to replace the steel with something much harder and/or denser. Tungsten, maybe? I dunno. Depleted uranium would probably do it; I've never heard of it being used in anti-personnel rounds, but in cannon sizes it pokes holes in tanks quite nicely.

[userbinator]

A bullet made out of metal foam?

[perlpimp]

http://www.youtube.com/watch?v=wfFcs25KmMc more about the inventor.

[viraptor]

I found the post-impact part of the video interesting. The article says "turn an armor-piercing bullet into dust on impact". But in the video, it looks like it turns it into a lot of metal particles / shrapnel. As a body armor it looks like it could be increase the danger to those around a person getting hit.

[fiatmoney]

In practice, you'd probably wrap the plate in fabric that would tend to catch the fragments, if nothing else because you need a plate carrier.

[PhasmaFelis]

It's hard to say for sure since there's only five frames of video post-impact, but it looks like those fragments are slowing down really fast. In the first couple of frames they're moving faster than the bullet was, but in the last couple they barely move at all. It seems possible that they're only dangerous within a foot or two of the impact point, which seems like a very acceptable trade-off for the benefits.

Even if I'm wrong, unless it catches somebody in the eye, I don't think those little fragments could do worse than a flesh wound.

[personjerry]

Yes, I understood that as "turns bullet into shrapnel" which sounds rather problematic.

[ZanyProgrammer]

Are there any potential downsides to this stuff? It seems too good to be true.

[patio11]

It's going to be expensive, may not meaningfully improve survival in actual combat situations, and may not actually be successfully deployed.

How could this be extraordinarily effective at stopping bullets but not meaningfully improve survival? Fairly few American soldiers are killed by gunfire which directly strikes them; even fewer of these soldiers are killed by gunfire which directly strikes them on their armor. (This is partly because armor is fairly effective, partly because bullets are often non-fatal, and partly because medical care is very good.) Improved body armor may not meaningfully improve survival against other threats such as, for example, IEDs (once 60%+ of fatalities, down these days) or "the helicopter impacted terrain at a high rate of speed."

[camoby]

This reminds me of an old Mythbusters experiment where they showed that covering a wall in the same stuff people put in the bottom of their trucks can protect the wall from explosions.

[tim333]

Looking at the ammo channel for I think the same ammo the results don't seem that remarkable. The foam article quotes "less than an inch", the ammo test article has about 1 inch for mild steel and 0.319 inches for AR500 plate

http://www.ammochannel.com/30-06-30-cal-m2-ap-armor-piercing...

[swehner]

What if you shoot twice at the same spot?

[Tloewald]

Armor of all kinds doesn't behave as well when hit a second time in the exact same spot.

[PhasmaFelis]

If you can reliably hit a target that small, you could just shoot the other fellow in the eye and not worry about armor.

[jp555]

"showed that CMFs are very effective at shielding... gamma rays..."

Really? That would be quite an achievement.

[aftbit]

Sweet - when can I buy level IV plates made out of this stuff?

[guard-of-terra]

I read about it as a child. On earth, molten metals readily lose gas by the force of gravity, but in space, you can easily make metal foams that will be very strong and light. Sounds like a good material for spacecrafts.

Also, how about glass foam? I imagine you can blow large and cheap hulls from foamed molten glass.

[onetimePete]

bubbling asteroids :)

[visarga]

And a big ass gun /

[dang]

We detached this comment from https://news.ycombinator.com/item?id=11464777 and marked it off-topic.