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 you say „Kevlar is stronger than steel“, you refer to pulling a string (I think). But a bullet approaching a plate (or a vest) is not the same thing. This is more of a push/pressure situation which may depend a lot on the shape of the bullet.
Rifle rounds are more powerful, and some have armor piercing jackets, meant to penetrate light armor.
Well, “stronger by weight” is not “stronger by volume”.
Google says AR500 plate is 3/8 in thick. Steel is 7.8 g/cubic cm. Kevlar is 1.4 g/cubic cm. So an equivalent weight of kevlar would need to be 5-6in thick. I bet 5-6in of kevlar would stop a bullet better than a 3/8 in steel plate, but it sure wouldn’t be wearable as a bullet-proof vest.
“Per pound” a 30# Kevlar vest might would but it’d be YUGE!
The idea is making structural pieces the weight the kevlar beam can hold up when it weighs just a couple pounds is equivalent to what would require many, many more pounds of structure if made of steel.
It’s stronger than steel in tension loads, as in you can hang more weight from it like a rope. It is MUCH weaker than steel in compression and torsion loads.
Simple case of people cherry picking some words out of context to be sensationalist when they say StRoNgEr ThAn StEeL because that is technically true in one specific situation.
Can you point to a video showing this?
Kevlar (aramid) fiber has a very high tensile strength, meaning it is extremely strong when you pull it along the strong axis like a rope.
It has no flex resistance, so when you stab it the fibers just move aside and let the blade/pointy bullet through.
To combat this, ballistic vests will stack layers of aramid fiber and metal/ceramic plates that will blunt the tip of an incoming bullet.
The issue is that you dont understand strength; strength is not a simple thing; kevlar has a higher tensile strength than steel but is weaker in other meaurements, ie its less hard.
Rope has excellent tensile strength (pull on it and it wont tear) but terrible resistance to bending (you can bend a piece of rope easily)
“On equal weight basis” is the easiest answer to this. The Ar500 steel plate is ~8 pounds for less than a square foot. That’s heavier than most kevlar vests, which themselves cover more than a square foot.
The other part of this answer is that “five times stronger than steel” isn’t actually a meaningful statement. Kevlar has a very high *tensile* strength. That doesn’t mean it’s more resilient than steel for all applications.
Kevlar is woven, and ultimately there are much larger gaps in the material than the gaps between molecules in a steel plate. That’s why we use it, in fact. When Kevlar stops a bullet, it deforms and uses tensile strength to resist the bullet.
Steel doesn’t use tensile strength to stop bullets. It uses *Impact* strength.
TLDR The mechanics of how these materials stop bullets are very different and you’re not comparing apples to apples.
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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