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 stronger than steel in tension relative to its weight. That means a certain mass of Kevlar can catch a faster/heavier bullet than the same mass of steel^(1).
However, Kevlar is a fabric, so has basically no compressive strength. This means that while the bullet doesn’t go through it, the force of the bullet does in just one place. For even small handgun rounds this can mean broken ribs.
You could theoretically make a Kevlar vest that’s able to stop high-powered rifle rounds, however, it would be useless as you’d die shortly afterwards from internal bleeding.
That’s why they usually only make soft (Kevlar) body armour rated up to around IIIA, once you get to rounds that would penetrate that, they’re almost just as lethal whether they actually penetrate you or not. Instead an AR500 steel plate not only stops the bullet from penetrating, it also distributes the force across a very wide area, meaning minimal damage. However, it does this at the cost of weight, it’s not as strong as kevlar so you need more of it (by weight).
In addition, steel plates suffer from spalling which is where fragments of the bullet fly out along the plane of the plate which can be very dangerous in its own right and often requires high-density polymer bonded to the plate to catch any fragments.
It’s the trade-off that needs to be made if you want to protect against high-powered rifle rounds.
^(1) *This is only a general rule, Kevlar body armour design is incredibly complex and its ability to stop a bullet is significantly affected by things like weave density, weave pattern, layering, and many other factors.*
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