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
Steel is much more dense then kevlar. The steel plate used in armour is much heavier then the kevlar plate carrier it is mounted in. You can get thin ar500 steel armour but they are not going to be able to protect against the same rounds.
In addition to this kevlar and hardened steel are very different materials with different properties. Kevlar is very strong, but it will easily bend and you can even cut it with some effort. A bullet hitting kevlar might not punch through the kevlar at once but push the kevlar into the body behind it creating a huge wound. The forces concentrated in that spot may eventually cut the kevlar as well. Ar500 is very hard, a slightly different property. Unlike kevlar it will not deform and therefore are able to distribute the force of incoming projectiles over a larger area. Basically it is impossible to push the entire plate into the body behind it.
However Ar500 is brittle. When a projectile hits it then it might cause steel spalling on the back to shoot out at lethal speeds. This is why an armour plate need the kevlar plate carrier, to contain the spalling. Either one is lethal to get shot with.
Kevlar is made of fibers, rifle rounds are able to lodge themselves between the fibers stretching them until they pass through
This is why hard armor has plates such as steel or cemeric while soft armor, made for handguns) doesn’t
This is also why it can be bulletproof but not stab proof
Kevlar is considered soft armor. It’s light, thin, and easy to wear. We wear plate carriers for larger calibers. These just hold steel or ceramic plates. 10 pounds of Kevlar is better at stopping something than than 10 pounds of steel but steels a lot heavier and we can wear more steel plates than we can wear kevlar.
KE (Kinetic Energy) = 1/2 * m * v^(2)
Rifle rounds travel much much faster than handgun rounds. This velocity is squared in the KE equation. So, a bullet from a rifle of the same weight might have 4-10 times the energy of the handgun bullet.
Kevlar can stop a rifle bullet, the issue comes from the soft squishy thing behind the vest.
Handgun rounds are slow and don’t have a ton of energy. It’s like getting hit with a baseball comparatively. If someone throws a baseball at you and it hits you on the shirt in the stomach, the baseball won’t go through the shirt. When you pull up your shirt you’re going to have a baseball sized welt on your body right where it hit. This is ok for something like a baseball, it might hurt but it’s not a huge deal.
Rifle rounds have a lot of energy. They’re like a sledgehammer. Now replace the baseball with a 10lb sledgehammer. When someone hits you in the shirt, the shirt will stop the sledgehammer but all the sledgehammer force is going directly into the space it hit on your stomach and you’re going to have a very very bad day. Broken bones, internal bleeding, maybe some organ ruptures. You have a pretty good chance of being severely injured/dying from blunt force trauma.
How do we fix this? We put a big slab of very hard material under your shirt on the stomach. When you get hit with the sledgehammer, now all the sledgehammer force is distributed evenly along your entire stomach instead of just the sledgehammer face. Getting hit with the sledgehammer is still going to hurt, but it’s more like getting hit with a bunch of baseballs all over your stomach.
It’s because all of the soundbites exclude key information that only engineers would understand.
Kevlar is stronger than steel… but in tension? compression? by weight? by volume? And what does “stronger” mean? Elastic modulus? Toughness/deformation?
Kevlar is not stronger than steel vs a very sharp knife, for example.
Its not only about stopping the bullet, but also about dispercing the force to a greater area. Kevlar might hold the rifle bullet, but its to flexy to disperce it around and damage will be significant. Steel plate is rigid and impact will be spreaded through all its surface. Even then its still a huge impact and very painfull.
an ant (pound for pound) is stronger than a human. I can crush an ant with my pinkie, but not a human. so being stronger in that sense doesn’t really mean anything.
a bullet from a rifle is travelling way faster than one fired from a handgun. You wouldn’t wear just a kevlar vest if you are expecting machine gun fire. In the military we wear a ceramic armor plate inside of a kevlar vest. I wouldn’t even want to wear a steel plate because that’s transferring the kinetic energy of a bullet strike straight into your chest whereas a ceramic plate is meant to crack and absorb energy.
Kevlar is great for things like law enforcement or personal protection details where mobility and/or concealment is critical. It does a good job at stopping rounds from a handgun, which you are most likely to encounter in those duties.
When materials fail, there are two different types: brittle failures and ductile failure. A ductile failure is where a material is stretched to the point where it can no longer support itself, whereas a brittle failure is where a load is rapidly applied and the material snaps under the stress. You can see this if you’ve ever played with silly-puddy as a kid, if you stretch it slowly it will get thinner and thinner until it eventually comes apart, whereas if you yank it apart instead of forming a long, wispy tail it snaps and forms a nice flat edge.
The way kevlar works is essentially like a catcher’s mit, when the bullet hits the surface it causes the fibers to stretch, and since kevlar has really high tensile strength it’s very resistant to ductile failures. However, there’s a limit to this depending on impact velocity, and if a bullet is traveling fast enough instead of causing the fibers to stretch, they brittle fracture. This is why, despite carrying roughly the same amount of energy in the form of momentum, 44 Magnum will usually be stopped by a kevlar vest, but a 223 Remington with a much lighter bullet and traveling more than twice as fast, will pass right through.
Steel works in a different way than kevlar. When a bullet hits steel, it not only compresses the material in front of it, it also has to move material out of the way perpendicular to its direction of travel. This is why craters have a ridge around their perimeter, this is the material that got pushed out of the way. Because you’re not stretching the material, and instead compressing it in multiple directions, this makes solid steel much more resistant to penetration than kevlar.
3/8 steel weights about 15lbs per sqft.
Typical person has about 10 sqft of coverage on their torso so we’re talking about a 150lbs of plate to cover the chest/back.
A similar kevlar vest weights 5lbs, plus about 10lbs of sacrificial ceramic plates
The plates are providing most of the protection by dispersing the impact on your body from the size of a bullet up to the size of a baseball. Still hurts, but it’s the difference between getting hit with a club and a club with a nail in it.
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