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
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.
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