I mean, it’s designed to stop a bullet which delivers much more force than a human being can ever produce during stabbing
In: 784
When you calculate the force on the tip of a knife generated when someone 150-200 pounds stabs with all their force it comes out around 30tons of pressure that easily passes between the weaves of kevlar. It is also worth noting that a blade is what is used when cutting kevlar when they manufacture armor.
Most vests provide a lot of knife protection. Even an nij lvl1 trauma pad is pretty tough to stab through. But people stab faces, necks and hands… which dont have protection.
In Stargate, Goa’uld’s personal defense shields deflection capability is directly proportional to the amount of kinetic energy being directed at it. Thus, a bullet moving at high velocity will be repelled by the shield, but a much slower-moving projectile, such as a dart, will successfully penetrate it
If you’ve ever seen a bullet going into a Kevlar vest, it’s not like the movies where the bullet smushes onto the outside. As long as it’s a bullet the vest is rated to stop, on contact it will make a nest into the Kevlar weave because of the bullet’s velocity and rotation. (The larger the handgun round rating, the thicker and heavier the vest weave) Essentially the bullet goes in, gets tangled, and velocity is reduced to 0. The weave of course does not have the same effect to stop a knife blade.
Attempting an ELI5
Bullets are dull, incredibly lightweight and soft but move super fast.
Knives are sharp, much heavier than bullets, very hard and are slow.
Just because the end result is a type of hole in a type of object doesn’t mean it’s helpful to imagine them as similar things. They are very different.
Kevlar fibers are a special material that specialize in very quickly spreading out the force of a fast dull object. This is the common material in bullet proof vests that has limited use against knives. The most important interaction between a bullet and kevlar fibers is that when the bullet first impacts the fibers the both the bullet and the fibers begin to deform.
The soft lead of the bullet tip spreads out against the resisting kevlar, and the fibers compress into an even tighter clump of overlapping nets as the energy of the bullet spreads out from the initial point of impact. The important aspect of the fibers are that they are incredibly long and interconnected forcing the bullet to overcome the resistance of a much larger amount of fabric than just the small area of fibers it is in contact with. The bullets energy from its initial firing is going into three events right now. The deformation of the bullet’s metal, the very wide amount of resisting fibers and the flesh of the person in contact with those resisting fibers. That is a lot of energy absorption.
Now look at the stabbing motion of a knife. When the point of the knife contacts the fibers of the kevlar, the point of the knife, many times harder than a bullet, does not deform in the slightest. It is also many times sharper and as the kevlar collapses under the point of the knife none of the knifes energy is spreading to a wider area, it stays on the point of the knife, and the fibers are severed before they can absorb much energy behind the blade. The knifes energy is not being spread out over a wide area and is only coming in contact with materials that will separate rather than resist.
Now kevlar is not the only material used in bullet proof vests. Ceramic plates are common additions that make a protective material more useful against knife attacks as well. Part of the reason for this is that bullets capeable of overcoming kevlar start to have some of the traits that make knives useful at overcoming kevlar, Bullets that are heavier, made of more hardened materials that deform less, have sharper points and travel faster are effective at overcoming kevlar fibers without additional fortifications to supplement the job the fibers are meant to do.
I don’t know what you mean by “vest” I’m assuming you are talking about soft armor though. Reading quite a few of the answers here, many are assuming the same thing. I can assure you though, no one is stabbing a knife through lvl 4 ceramic plates.
You know oobleck right?
Kinda like that
They can, a steel or ceramic plate will protect you from both.
It’s only an issue with cevlar and other similar materials.
No vest is entirely protective of any attack, if you get shot or stabbed above or bellow the plates then you’re still toast.
The contact surface of a bullet(a circle) is much more than the contact surface of a knife(a line). Pressure is force divided by area. Less contact surface for same force makes more pressure.
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