Bulletproof vest technology-Carbon fiber?


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Colt46
August 23, 2004, 06:50 PM
Sorry, I couldn't seem to decide on where this topic belonged.

I got into a discussion with someone on the makeup of body armor and he said that newer vests had a Kevlar/Carbon Fiber/Ceramic plate composite. I knew that Ceramics were commonly used, but the first I've heard of carbon fiber. Anybody here know how credible this is?
Thanks

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Muzzleflash
August 23, 2004, 06:58 PM
Paging Skunk, Paging Skunk..

rayra
August 23, 2004, 06:59 PM
to my understanding, carbon fiber has great tensile strength but NOT shearing strength. Woven, in a hard resin matrix, bonded with the other ingredients into some kind of hard plate - sure. Loose weave? I don't think so.
Haven't done any reading on the specific question, though, so I can't say with any certitude.

sendec
August 23, 2004, 08:31 PM
after seeing what happens to things like carbon fiber car parts that it would be used in the ballistic portion of body armor. Ceramic would also indicate that this is supposed to be upper level stuff, so I would be dubious.

>SHOCK<^>WAVE<
August 23, 2004, 09:46 PM
Carbon fibers very brittle it's used in the form of composites to lesson weight and not lesson strength in jet airplanes and race cars. It's used in rocket thrusters for it's ability to withstand heat of rocket fuel.

Cf is very strong when molded in a carbon to carbon matrix bonded with a resin like stealth jet wings.

It seems like it would crack into a zillion pieces in a fabric like Kevlar unless it wasn't in Kevlar but bonded in the separate ceramic trauma plates.

A while back I had to procure carbon fiber woven in to a fabric used for an electric car battery project, it was used for electrodes in an experimental battery.
http://www.evworld.com/view.cfm?section=communique&newsid=213

GunnySkox
August 23, 2004, 10:03 PM
Paging Dr. Skunky. Dr. Skunky to the CF Thread, stat.

Bah, somebody already made this joke, but I don't know squat about carbon fiber.

~Slam_Fire

mcneill
August 23, 2004, 10:08 PM
I believe that Shock Wave is right on. CF is very brittle and probably would not work unless bonded to/imbedded in the ceramic plate.

Jim

>SHOCK<^>WAVE<
August 23, 2004, 10:27 PM
here's something



Composite Nanotube Fibres Spun with Record Strength and Toughness
Researchers at the University of Texas at Dallas and Ireland’s Trinity College have reported that they have spun carbon nanotube composite fibres that are tougher then any other man-made or natural polymer fibre.

The tough composite fibres were produced using a new continuous spinning process.

The authors of the paper that describes the material and process claim that their fibres have a toughness 17 times that of Kevlar and 4 times that for spiders silk. Compared to steel fibres, they are twice as stiff and strong, while being 20 times tougher.

Potential applications of the composite fibres include:

· Energy storing electronic textiles, which can be used to power electronic devices

· Bullet-proof vests

· Synthetic muscles

· Distributed fibre sensors



http://www.azom.com/details.asp?ArticleID=2188



Sandia Researchers Develop Kevlar Reinforced Gauntlets to Protect Soldiers Arms


Researchers at the National Nuclear Security Administration’s Sandia National Laboratories have created gauntlets that will aid in saving arms of military personnel riding atop Humvees and other military vehicles during combat.

The shoulder-length Sandia Gauntlets are made of layers of heavy Kevlar - reinforced material used in bulletproof vests and tyres - with carbon-composite forearm and upper arm protective inserts.

http://www.azom.com/details.asp?ArticleID=2564

GigaBuist
August 23, 2004, 11:04 PM
Paging Skunk, Paging Skunk..

My foughts edzachary. :D

Deavis
August 23, 2004, 11:51 PM
Ok, let’s clear up some things very quickly before people get the wrong idea about carbon fiber.

Carbon fibers very brittle

First off, carbon fiber is indeed brittle at the atomic level. It has an amorphous crystalline structure that fails spectacularly when sheared. You can think of carbon fiber as wanna-be graphite where its planes are unable to slip past each other (the reason graphite is a great lubricant!). If you line up those planes properly you end up with a really strong fiber in the axis in which the majority of the planes line up. (fibers can have an elastic modulus of hundreds of GPA along the fiber axis with tens of GPa perpendicular to the axis)

However, that does not mean that carbon fiber is not flexible at long length scales and small diameters. You can take a mat of woven carbon fiber and crumple it up just like a sheet of paper without the mat breaking into a million pieces. This is a consequence of making the fibers small in diameter. As the diameter of the fiber decreases its elastic modulus, strength, and flexibility all increase. This increase in strength is due to the decrease in cross-sectional area limiting the number of places for a large flaw to propagate through the fiber. Another important thing is that the fibers be long (called critical length)

Now another cool property of carbon fiber is that each individual fiber is independent of its neighbors. This is very different, from say steel, where the crack is propagating through a single crystal lattice and after the initial crack has formed it is easily propagated through the rest of the crystal. You must break every single carbon fiber individually. Breaking one in no way affects the strength of the next fiber in your weave. It turns out that the matrix, see below, also helps stop crack propogation.

Once you have these nice skinny fibers you can place them in a very brittle matrix, say an epoxy, and increase the strength of the material. The epoxy is only there to transfer the load to the fibers and to keep them from bending out of axis. If you took a small piece of solid epoxy, you could break it very easily. Imagine a 1”x12”x1/4” piece of epoxy. Grab the ends, bend it (tensile and compression being applies) and it will break. However, if you place fibers in it that are longer than the critical fiber length, they will carry the load being applied and keep the epoxy matrix from breaking. To give you an idea, you could not break a piece of carbon fiber of those dimensions, if properly engineered, easily. Stand on it, bend it like a coat hanger, it won't break.

It seems like it would crack into a zillion pieces in a fabric like Kevlar unless it wasn't in Kevlar but bonded in the separate ceramic trauma plates.

As I pointed out, a mat of carbon fiber will not break in that fashion. Due to the strength of the carbon fiber matrix, it takes quite a bit of energy to destroy it. When you see an F1 car hit the wall and it breaks into a zillion pieces, you are seeing an engineer smile. Those cars are meant to explode like that because it dissipates huge amounts of energy. If only Nascar would pay attention!

This is also one of the reasons that you place a ceramic plate in a bulletproof vest. When it breaks it dissipates a large amount of energy by propagating cracks through the material and not into your body. You absolutely want the material to crack because it is getting rid of energy that would otherwise be used to penetrate your body. Now there is a fine line of how much cracking that you want. Too brittle and the plate crumbles and subsequent shots go right through. There is a tradeoff there that is very well thought out and it is all about energy transfer.

One of the big things Kevlar has going for it is cut resistance. I won't go into depth because I've already made this too long. Bascially, it is really hard to cut Kevlar and that makes it very attractive for vests. It is worth noting that Kevlar is DuPonts brandname for an aramid fiber, specifically a polyamide. There are other aramids that would be better suited than Kevlar but the industry doesn't seem to want to use them. Kevlar can also be spun into an incredibly tight weave of fibers without breaking the chains (i.e. reducing critical length in the case of carbon fiber).

However, as Shock pointed out nanotubes could change that. Nanotubes are pretty cool, except they are very difficult to make in long lengths. It was just recently that we have been able to make a nanotubes of considerable length (microns of length was considered incredible a few years ago). They have this horrible propensity for sticking together due to surface energy and they are usually put into a solution of soaps to keep them from sticking. The solutions have a very low weight of nanotubes for a given volume and that makes them very unattractive for commercial production methods that are employed for fibers like Kevlar. All of that is changing though!

Anyhow, just thought I’d throw out a little info there… If I missed something, please feel free to correct me.

>SHOCK<^>WAVE<
August 24, 2004, 12:18 AM
Here's some stuff if anybodies interested, I worked with the pitch based precursor brittle fiber's but there are other types of precursors.

CARBON FIBERS
http://www.engr.utk.edu/mse/pages/Textiles/CARBON%20FIBERS.htm

Matt G
August 24, 2004, 02:22 AM
Good stuff.

I've been wondering when nanotube technology would/could be made to make things like vests. What substance are we looking at using for the nanotubules? Carbon?

schizrade
August 24, 2004, 05:09 AM
Carbon Nanotubes are the strongest material we can find/make. Going to be an awesome revolution when we can mass produce them affordably. Space Elevators, truley bullet proof vests. Crap, you would die from the blunt force trauma from a .50 BMG with a carbon fibre nanotube vest. Look up Space Elevators and research Carbon Fibre Nanotubes to learn more. Support Science!

hso
August 24, 2004, 10:31 AM
BTW - Thanks Deavis and Shock.

Carlos Cabeza
August 24, 2004, 11:01 AM
Science is fun ! Thanks for the information guys.:cool:

Skunkabilly
August 24, 2004, 12:18 PM
I'm all ears but heck if I'm doin' a Rich Davis on this one :eek:

benEzra
August 24, 2004, 09:42 PM
Theoretically (based on tensile strength calculations), I think a thin monofilament of carbon nanotubes could lift a pickup truck, and IIRC a 2-inch cable could lift a good-sized merchant ship. Not sure how well the tensile strengths will scale up to the macroscopic scale, but it's promising.

As far as brittleness lessing with smaller-diameter fibers, look at fiberglass. Tiny threads of glass (yes, glass, like window glass) embedded in a resin matrix, and you can make ballistic panels out of it.

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