
Metal foam obliterates bullets - WellDressed
http://phys.org/news/2016-04-metal-foam-obliterates-bullets.html
======
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.

~~~
rmelly
Dragon Skin
([https://en.wikipedia.org/wiki/Dragon_Skin](https://en.wikipedia.org/wiki/Dragon_Skin))
claimed to withstand multiple hits without loss of performance, although there
was some controversy around it.

[https://www.youtube.com/watch?v=aYaSRIbPWkM&nohtml5=False](https://www.youtube.com/watch?v=aYaSRIbPWkM&nohtml5=False)

~~~
mlhaufe
Given that it weighed ~50 pounds and failed in a variety of environmental
conditions, it's no surprise it wasn't adopted.

[1] <[https://www.quora.com/Why-was-Dragon-Skin-armor-rejected-
by-...](https://www.quora.com/Why-was-Dragon-Skin-armor-rejected-by-the-U-S-
Army?share=1>)

~~~
hga
Problem is, US Army procurement is sufficiently corrupt you can't trust
testing of any physical item you hand to them, this goes back at least to the
'50s when they caused the AR-10 to fail by replacing screws with wire
springs....

That said, the design is obviously way too heavy without anyone even needing
to weigh it.

------
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!

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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)

~~~
matheweis
That would be outstanding if it could ... talk about the perfect material for
shielding supercritical spacecraft components. Radiation resistance,
projectile resistance, lighter than normal metal, temperature resistant...
What's not to like?

~~~
AaronFriel
You still need to make it fit, and this shielding may require more volume and
more mass than a Whipple shield.

~~~
matheweis
I was thinking of a small amount to protect supercritical components;
navigation computers, comms, life support (if applicable); not necessarily the
entire spacecraft.

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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.

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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...](http://www.sciencedirect.com/science/article/pii/S0969806X15300104)

~~~
Wingman4l7
The subreddit /r/scholar or the Twitter hashtag #icanhazPDF can help out.

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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.

~~~
SEJeff
This is exactly how a Soviet style RPG works. Ironically, it is also why the
"bird cage armor" works so well against the RPG. Here is a picture of a
Stryker combat vehicle from when I was in Mosul Iraq circa 2004ish:

[http://www.digitalprognosis.com/album/images/iraqpics/the-
st...](http://www.digitalprognosis.com/album/images/iraqpics/the-stryker.jpg)

The slats are smaller than the height of the outer explosive and prematurely
detonate it. As a result, the penetrator simply bounces off the armor
harmlessly.

~~~
refurb
I thought the slats were to provide stand off distance for explosively formed
projectiles.

If the explosively formed jet forms more than a few inches from the armor,
it's way less effective.

~~~
SEJeff
Nope, we had them in Iraq before EFPs were really a thing (Thanks Iranain Quds
Force for that). They were specifically for RPGs, which was one of the few
things that could take out a Stryker without it. They even tried those Soviet
parachute anti-tank grenades, but the Stryker is much faster than a tank and
just moved away in time. That being said, the I'm sure this slat (birdcage as
we called it) armor would help with that as well.

~~~
grkvlt
I thought an RPG _was_ an EFP? The round is just a shaped charge with a copper
liner that penetrates the armour. So the slatted armour stops both RPGs and
EFP-based IEDs.

~~~
hga
The usual RPG with a HEAT shaped charge doesn't destroy by forging a
penetrator that then potentially travels some distance to the target, it
produces an intense, narrow jet that'll cut through a lot of material.

------
perlpimp
[http://www.youtube.com/watch?v=wfFcs25KmMc](http://www.youtube.com/watch?v=wfFcs25KmMc)
more about the inventor.

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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.

~~~
jdfellow
The effect is called "spalling" and current steel and ceramic armor plates are
coated in a layer of rubber material to catch the fragments.

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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."

~~~
PhantomGremlin
What about vehicle armor?

Something like this would be a great improvement over the very heavy homebrew
plating that our guys in Iraq and Afghanistan were forced to improvise on
their Humvees? I.e. before MRAP type vehicles made it into the field.

Lighter and better armor isn't just for people, it's also for vehicles.

~~~
InclinedPlane
It's actually very similar in a lot of ways to chobham armor, which is layers
of ceramic and high strength metal in a composite structure. This seems to
work similarly but with a more consistent mixture of both materials throughout
a material, which might make it easier to manufacture and to work with, and
possibly lighter as well.

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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.

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

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swehner
What if you shoot twice at the 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.

~~~
dredmorbius
No. You're confusing _consistency_ with _accuracy_.

To hit someone in the eye, you've got to aim at their eye and land a round
there. That is, _accuracy_ at hitting a _pre-specified_ target.

To hit armour in the same location twice, you need either two (or more rounds)
delivered _consistently_ on _whatever happens to be where the first round
lands_. It's a case where being a Texas Sharpshooter actually works (painting
bullseyes around the bullet holes you just shot into a barn).

A compound impactor with a leading and secondary round might work, for
example. Difficult to package into a small-arms round, but possible. Keeping
everything on the same impact point would be the crucial element.

~~~
PhasmaFelis
At that point, wouldn't it be easier and just as effective to use a single,
larger round? .50-cal rifles already exist; no need to invent a big bullet
that turns into two smaller bullets.

(Unless you mean something that starts out the same size as the one in the
video, in which case I can't imagine that two halves of a .30-06 would
penetrate any better than one whole .30-06.)

~~~
dredmorbius
There's often an advantage do a dual-mode strike. That's how many existing
armour-piercing weapons operate. E.g., tungsten-alloy armor piercing rounds:

[http://www.tungsten-alloy.com/armor-piercing.htm](http://www.tungsten-
alloy.com/armor-piercing.htm)

Another alternative, depleted uranium, relies on the exceptionally dense, and
self-sharpening DU penetrator to defeat armour. Rather effectively.

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jp555
"showed that CMFs are very effective at shielding... gamma rays..."

Really? That would be quite an achievement.

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

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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 :)

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visarga
And a big ass gun /

~~~
dang
We detached this comment from
[https://news.ycombinator.com/item?id=11464777](https://news.ycombinator.com/item?id=11464777)
and marked it off-topic.

