I mean, it’s designed to stop a bullet which delivers much more force than a human being can ever produce during stabbing
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The short answer is because knives are sharp and bullets are blunt.
Kevlar has exception tensile strength for a relatively lightweight fabric. Which means if you were to take a strand of it and pull it would be very hard to break compared to common fabrics like cotton or nylon.
When a pistol bullet hits (Kevlar is generally only rated to stop pistol rounds) a vest it tries to just tear through with its momentum. That woud be like trying to just tear the individual strands apart. That’s very hard to do. Some of them will break, but you layer it a half inch or inch thick or whatever (not sure what the specs on real vests are) and it’s just too much to get through for a blunt Kevlar. Usually.
Stab it with a knife though, and the blade simply cuts the fibers, it doesn’t have to tear them. It’s still going to be harder than stabbing through like an equivalent amount of cotton or whatever, but it’s very possible to simply stab through kevlar.
I don’t see this being mentioned but I’m not reading every branching thread here, but in case it got missed.
The bullet gets all of its energy in an instant and cannot get anymore.
If you’re stabbing with a knife, you can… like… keep pushing.
In terms of actual energy you’re imparting? I can put a LOT more force into a knife than one powder-charge puts into a bullet, I just can’t do it as quickly.
It’s mostly because of how bullet resistant vests (BRVs) work. As weird as it sounds, they’re actually designed to break.
The idea is that when you shoot a bullet, it smacks into the BRV, which spreads the energy of that bullet across a larger area.
How they do that varies on design, but in some cases, it’s just a ceramic plate that shatters on impact, and spreads the energy across the cracks
In essence, they turn a 9mm bullet wound into a bicycle-crash wound. The energy is being spread across all of your torso instead of a single small hole.
Now, what’s the difference between a bullet and a knife? Once a bullet leaves the gun, there’s nothing pushing it forward.
A knife is constantly being pushed forward by the hand that holds it. So it smacks into the vest, and the stabber can just keep pushing his hand forward.
Do you understand sharpness? Also all my vests would protect from both, however my head and neck (great targets) are available to both.
The constant force applied to a much lower surface area means more parting of the individual fibers. The energy stays applied for a longer time and increases instead of immediately disappearing. Whereas a bullet is blunt and the energy quickly dispersing.
If you step on a brick with all your weight, it won’t penetrate your foot. There’s a lot of force there, but it’s spread over a wide area.
If you have a nail sticking up through your floor and you step on the point with all your weight, it’ll easily go in the bottom of your foot and out the top. Same amount of force, but much more concentrated on a teeny little area.
The point of a knife is, ordinarily, very, very small. Because of this, the force is all concentrated on a tiny little spot, one or two fibers. A bullet is comparatively much larger, covering hundreds, maybe thousands of fibers. To get in, the bullet has to break ***through*** all those super strong fibers.
The bullet gets stopped while the knife point can wiggle and thread its way ***between*** the fibers; it doesn’t have to brute force through them, it can go between and push them apart to make a path.
It comes down to PSI. The tip of a knife can deliver a much higher PSI then a bullet can. You are talking about the needle like tip of a knife versus 1/8 square inch of a bullet head.
Kevlar is really good at resisting stretching. Like 5-10 times stronger than steel kind of good. But it’s made of tiny fibers and those fibers can be cut (although they’re tough, they’re not so tough that a steel edge can’t cut them).
Compare it to a fishing net. It’s a really strong fishing net, strong enough to catch bullets. But a knife is a knife, so it cuts the fibers and goes through.
Let’s start with the simplest demonstration of the problem. A bed of nails.
Any one nail would go through a standard vest, if you put the weight of a person on it. However, if you collect a large number of nails together, a person can lay on them without any damage, even without wearing a vest.
Because the force is distributed across all the nails.
Now back to our example:
There are many kinds of bullets, many kinds of vests. Let’s simplify first:
Assume this is not an armor piercing bullet and the vest is Kevlar with no plating.
The bullet strikes the vest and flattens out a little. It looks a bit like a mushroom. The vest spreads this impact out over a much broader area. The force transfers from the bullet to the vest. It’s the same amount of force, but spread out over a large space. The force from this larger impact still transfers to the body, but it does not penetrate.
A knife does not spread the force out. The pressure is applied at the sharp point of the blade, which cuts through the fibers of the vest. The vest cannot distribute the force over a larger area since it’s physically being cut apart.
Take your palm and press it against your cheek as hard as you can. Now take your index finger and do the same.
Which was more unpleasant? That’s because you’re using a similar amount of force on a smaller area.
Bulletproof vests can withstand a bullet because the force is spread out amongst a larger area. A knife will exert force on a teeny tiny area and go right through.
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