Kevlar - how does it work?

[Legion]

Would you say it absorbs impact or distributes it? explain.

 

[AlphaWolf]

Kevlar is a material with a very high tensile strength, when woven into a fabric it merely resists puncture and distributes the impact. It can be very thin, it's not absorbing anything.

 

[Ty]

Kevlar is a material with a very high tensile strength, when woven into a fabric it merely resists puncture and distributes the impact. It can be very thin, it's not absorbing anything.

What's the difference? Surely it's conserving the inertia so isn't it technically "absorbing" the energy because it has to go somewhere. I think the trick is that it does a good job of distributing the energy it IS absorbing across a wide area.

 

[drspangle]

It just distributes the pressure throughout the whole garment. Take the force of the bullet over a square cm (getting shot without kevlar) and divide it by the total area of the vest. Or something like that

 

[AlphaWolf]

Kevlar is a material with a very high tensile strength, when woven into a fabric it merely resists puncture and distributes the impact. It can be very thin, it's not absorbing anything.

What's the difference? Surely it's conserving the inertia so isn't it technically "absorbing" the energy because it has to go somewhere. I think the trick is that it does a good job of distributing the energy it IS absorbing across a wide area.

To me absorbing would imply that most of the impact was not transferred through the garment to the wearers body.

 

[DemoCoder]

First, Kelvar protects you by not allowing the bullet to penetrate your body and rip a supersonic shockwave through your guts.

Second, since the bullet doesn't doesn't penetrate, you are going from a situation of

__ (vest, no impact)

V (vest after impact)

so that your shoulders, side, and back are providing counterforce on the edges of the vest, therefore, some of the force has been distributed.

That said, you can still take serious internal injuries.

 

[Legion]

how much inertial energy is being distributed and how much would you say is being absorded.

What exactly is inertial energy in a physical sense.

 

[Nathan]

http://www.dupont.com/kevlar/europe/lifeprotection/lps_softbody_set.html

 

[jvd]

if u get shot while wearing this u will hurt. U will hurt alot . THey say the best thing to do when hit is to fall back with the force. Its not a fun experiance and most cops pass out from the shock

 

[VtC]

And wind up with a nasty bruise and a lot of aches and pains for a few days. Still beats having a hole in you, though.

 

[digitalwanderer]

if u get shot while wearing this u will hurt. U will hurt alot . THey say the best thing to do when hit is to fall back with the force. Its not a fun experiance and most cops pass out from the shock

It's also why you see a lot of really thick sets of kelvar armor, they stick a lot of heavy padding in it to help lessen the force of impact and help distribute it more.

You could just have a thin layered shirt made of kelvar that would stop a bullet from penetrating you, but your body is still gonna have to deal with the blunt force impact of the bullet which is more than enough to break bones/rupture blood vessels.

I like the thin solid-plate filled kelvar stuff best. The kelvar stops penetration, the plate helps to spread impact.

 

[MuD]

I'm a bit rusty on the details, but if I'm not mistaken, the maximum force (penetration) allowed by kevlar vests is 4.4 cm (roughly 2 inches), without actually killing you. More details are available here: http://people.howstuffworks.com/body-armor.htm

 

[digitalwanderer]

Hey MuD, welcome to Beyond3D!

2" without padding? Hell, that would kill you if it hit the wrong spot without breaking thru the kelvar! (Could you take a 2" push straight in on your ticker or wouldn't it kind of force a bit of rib/breastbone thru your main pump? :| )

 

[MuD]

Thanks for the warm welcome Dig.

Could you take a 2" push straight in on your ticker or wouldn't it kind of force a bit of rib/breastbone thru your main pump?

Yes, you see, that's the problem. The regulations are way too relaxed on this issue. That's why sometimes, people *still* die even though they have body armor. Since making a better body armor would mean thicker, heavier materials and a higher price, they make a trade-off. Sad but true.

 

[Legion]

Here is what the dupont site states:

Soft body armour made with KEVLAR® brand fibre works by “catchingâ€￾ a bullet in a multilayer web of woven fabrics. The engaged fibres absorb and disperse the energy of the impact to other fibres in the fabric weave. This transfer occurs at “crossover points,â€￾ where the fibres are interwoven. Additional energy is absorbed by the other layers in the body armour, reducing the amount of transferred energy that causes “blunt trauma.â€￾ These woven fabrics are strong enough to perform, while offering the additional advantage of being more comfortable to wear than traditional plastic-based shield products.

I have to ask, what is the physical definition of "absorb" and how does it relate to the phrasing here?

 

[PC-Engine]

It works the exact same way a trampoline works...

 

[I.S.T.]

You jump up and down on Kevlar?

 

[Nathan]

It works the exact same way a trampoline works...

Sort of. To extend the analogy; when a very heavy person jumps on a trampoline, they elastically deform the trampoline initially. As they travel downwards, they transfer their energy into the trampoline mat/springs by stretching the atomic bonds out of their equilibrium position.

If they aren't heavy enough to break the trampoline, then the majority of this energy can be recovered, and used to hurl the jumper back into the air. But if they do manage to break the trampoline, all the energy stored in the trampoline suddenly gets released as heat, sound, motion in a different direction, plastic deformation, and new free surfaces. The jumper will then fall to the ground with a much lower velocity than if the trampoline had never been there.

Think of a kevlar vest as multiple trampolines. Each layer takes some of the energy from the bullet until it finally hits a layer that it can't break through.

 

[Legion]

well ok but what about an explanation of "absorb"

 

[PC-Engine]

well ok but what about an explanation of "absorb"

Honestly your body would do A LOT of the absorbing. If it didn't then you wouldn't feel a thing.

It's just that the vest spreads the the impact out to a wider area in the vest itself which in turn transfers that energy over to your body (torso). Absorption is proportional to mass ie the larger the mass the more it can absorb. Think of a small heatsink compared to a large heatsink...same thing. Instead of all the energy being concentrated into the tip of the bullet, it tries to spread it out to say the size of a fist. So think of it as getting punched real hard in the chest or stomach or whatever.