Ar500 steel plate can stop rifle bullets. In my brain I’m thinking like “well if kevlar is stronger than steel, then slab of kevlar of the same size of an ar500 plate should stop rifle bullets too”. Clearly it doesn’t and I don’t understand why
In: Engineering
Kevlar is a fabric consisting of very strong fibers. Ever poked through your shirt with a pencil? Ever tried it with the eraser? Rifle bullet is pencil tip, pistol bullet is eraser, shirt is kevlar. Rifle bullet has to break less fibers before it can start pushing them aside instead, which is easier. They also tend to be travelling a lot faster.
Metals are mostly homogenous on the scale of a bullet. Rifle bullets still do a better job of penetrating them due to speed/shape difference above, but they resist deformation in every direction whereas fabrics resist deformation only in line with their fibers. This is an example of isotropy vs anisotropy.
I used to make bulletproof windows, similar situation. You need to absorb a lot of energy in a very small area. Hard surfaces aren’t great at absorbing energy but are great at spreading it. Soft materials like vinyl (used in windows) deform and absorb energy, but don’t spread it well (and they puncture). Ideally you use both, and in a window you layer the hard and soft spreading and absorbing energy layer by layer.
I suspect kevlar would be better able to absorb energy, but steel better to spread it. Perhaps a combination of both would be better than either alone.
Each fiber may be stronger than steel, but the fabric they make up is not as puncture resistant. This is also why some kevlar struggles with stopping knives.
Rifle rated vests use ceramic plates (or steel, but ceramic is better) to deal with this problem.
Steel is cheaper than ceramic, which is why you see AR500 used for civilian market rifle armor.
A rifle induces more spin to the projectile than a handgun. Kevlar is a weave of materials and that construction is meant to diffuse the velocity of the projectile and spread the impact laterally. If the projectile is larger and spinning more that cannot be accomplished. Steel is condensed and tempered material that has tightly packed atoms. Steel relies upon density to deflect projectiles.
Big difference? Speed. A typical rifle round leaves the firearm at 1600-2000MPH. A typical handgun round is doing about 800MPH.
If I accidentally toss a baseball at a window, there’s a good chance it won’t even break. A MLB pitcher takes the same baseball and flings it? it’s going through that window and maybe the sheetrock across the room!
The top answer is pretty sot on. The kevlar would need to be at least 6″ thick to stop rifle rounds, and that depends on what rifle round. AR500 abrasion steel is strong and breaks up the bullet on impact sending the bullet flying wherever it can. That is why you see some sort of polyurea spray liner (truck bed liner) on steel plates.
Ceramic actually has a higher hardness that steel, which is why all level IV plates (top level under NIJ 0101.06 standard) are all ceramic/composite. Of course this ceramic isn’t like your toilet or flower vase. It’s designed for armor. You’ll have a ceramic strike face with some sort of composite backer, typically a type of polyethylene. The ceramic breaks the bullet up and the polyethylene catches the broken pieces. There’s also different types of ceramic and many different types of polyethylene.
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