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
Bullet’s tend to be wider than a sharp knife blade. That increase in surface area makes it easier for the Kevlar to prevent it from penetrating. Most vests however have inserts for plates which can protect against knife attacks/larger caliber rounds.
The answer is that a bullet-proof vest can protect you from knife wounds, depending on the vest. A lot of them have metal or ceramic plates inside the vest, and those plates would protect against a knife’s cutting edges. But some vests are also made of just kevlar fibers, and a knife would be able to “push apart” and/or cut these fibers; these vests would NOT protect against a knife attack.
Materials behave differently at different speeds, and a good example of this is water. You can very easily slide into water from standing at the edge of the pool, but jumping from a bridge or from a helicopter so you impact with a lot of speed, that will be lethal to you; the water will feel like a cement surface at the moment of impact.
And a lot of vests are designed like this, they will break apart the bullet and stop its huge speed, but something as slow and sharp as the tip of a knife will just easily push apart the fabric fibers and penetrate into your chest. The difference is that the kevlar material has inertia and does not “move out of the way” at bullet speeds (it feels like a solid surface), but easily moves out of the way when trying to work the point of a knife into it.
Knife stabs tend to keep pushing after facing resistance, since they’re still in the attacker’s hand, while if a bullet faces resistance, it’ll lose energy. That helps a stab get through material that would stop a bullet. Worth noting that bullet-proof vests WILL give way if you get shot in the same spot a few times.
Bullets are tiny and spins, most bullet vest are kevlar, which is a fabric that will entangle the bullet and force the energy to bleed out thus trapping the bullet. A knife is designed to cut, so when they stab the kevlar, because it doesn’t spin, it just cuts the fabric (provided there’s enough thrust) and enters the person.
In layman’s term, a bullet does piercing damage, a kevlar vest provides piercing protection, a knife does slashing damage, which a kevlar vest does nothing for. you require slash protection (plate or armor).
Two things
>delivers much more force than a human being can ever produce during stabbing
Nope if this were true newton’s 3rd law says I could not hold on to the gun when it fires.
A 9mm generates 500 ish newton of force. while a average adult man can punch with about twice that. pro boxers can hit with 5000 newtons of force.
Second getting a knife through a IIIA vest, Assuming it’s not cut resistant, is NOT trivial. You need a sharp strong knife and a lot of upper body strength.
If it is cut resistant you don’t stand a chance in hell.
In this video the guy is standing over the vest and stabling down into it. So every advantage possible given to the knife. It barely pokes through.
Ok.
Take a piece or cardboard and place it on a piece of meat.
Then hit it with a hammer and stab it with a knife.
Which do you recon will break through the cardboard and damage the piece of meat?
—
Note: I know fuck all about guns, so if I get a term wrong keep in mind: I’m not an American, I speak English as 2nd language and I’m an mechanical engineer.
Now this doesn’t mean that cardboard will protect you from getting hit by a hammer. It just means that those two have different kind of impact profile. And therefor the deliver force differently.
A bullet meant for stopping soft targets – as in humans – shouldn’t be sharp. This seems counter-intuitive at first but it isn’t if you follow the logic I presented earlier.
Lets imagine you have a bullet that is more sharp; like a medical blade or an injection needle but bigger. And lets pretend this can somehow survive the forces that would result in it being shot. What would happen when it hits a person? It would basically just go through. It is sharp enough not to pull tissues but cut through them. It would actually do very minimal harm all things considered. It doesn’t deliver force but rather pushes through with it
Now if you consider some small calibre bullet, like under 9mm. You look at the tip, it is rather round or at time it has 0 sharpness,as it is flat (used for target shooting if I’m not mistaken); or “negative sharpness” as in it is concave “hollow point”.
So why is that? Why would “negative sharpness” be desirable quality? Because you want the bullet to deliver as much kinetic force to the target as you possibly can. It is this force that stops people. It is this force that causes massive destruction inside the human body as it causes a shockwave that destroys our more fragile organs. But in reality even this is actually lot of the time undesirable – you don’t always want to kill but you want to stop or disable otherwise. Imagine getting kicked or hit to the stomach so that it knocks the wind out of you. You are helpless and unable to act for tens of seconds, even minutes.
—
So what is the difference between these two types of protective gear?
One is made of fibre that resists laceration, as in the individual strains are tough and the weave is made tight so that it is hard to get even a sharp object in to it, to even cause a cut to start. It doesn’t have to stop the blade, just ensure that it doesn’t go through to your skin.
The other is made of fibres the stretch and deform without breaking, and from a flexible weave that once they meet a fast flying object, they slow it down draining energy. In only has to slow it down enough to ensure that the bullet doesn’t penetrate your skin. The bullet can and usually does go through the fabric, but it has lost enough energy to not go through your skin or damage your organs. It’ll still hurt like hell and bring you down. In reality bullet proof vests are made by stacking these fabrics or top of each other and/or impact plates. Depending on what level of resistance is desired. But whatever method(s) are used, the point is still the same: Slow down the kinetic object so it doesn’t penetrate.
Now we use the methods used for stab resistance and bullet resistance for other things also. When making explosion resistant structure for industrial use, blasting, or even high pressure handheld water jet operation (Hydrodemolition – aka really powerful power washers). Or when protecting wires and cables in machinery; or people who work with sharp objects or machinery in industry and construction who need laceration resistant PPE. The design principles remain the same.
Kevlar bulletproof vests are made of many layers of fabric (Kevlar) that are very strong against forces which pull them. The layers are sewn tight together, so that when something impacts them, the Kevlar is stretched by the impact. When a bullet impacts the vest, each layer has to sequentially stretch until it goes past it’s breaking point before the bullet can go through. Once a layer is broken, it loses all of its strength. That’s why bullet proof vests are a one time defense.
Knives are sharp enough that the blade can fairly easily cut the fabric. They still put up a lot of resistance, but Kevlar is not great against cutting.
A knife attack also tends to be repeated stabs at very close range in the same area. Most guns are either very accurate but take a while to aim, or they fire rapidly and are much less accurate. As I mentioned earlier, once the vest is damaged, it is completely ineffective. Getting shot from distance once gives you time to take cover. Getting stabbed usually means multiple stabs before you have a chance to get away.
Bullets and knives work on very different principals. Bullets use velocity to impart so much energy into an object that they overcome the tensile and compressive strength of and rip right through. Knives, on the other hand, use a sharp blade to minimize the area that the force is being exerted upon to overcome the shearing strength of whatever you’re cutting.
Basic bulletproof vests are made of Kevlar, which has a very high tensile and compressive strength but very low shear strength, making it difficult for a bullet to tear through but easy for a knife to pierce. More advanced armor uses metal or ceramic plates, which can also resist knives.
“I mean, it’s designed to stop a bullet which delivers much more force than a human being can ever produce during stabbing.”
Any action causes an equal and opposite reaction. If a person can hold the gun, a person can generate the same force.
Most non military bullets are made to fragment or be soft pointed if you’re cheap. As in rip up your insides. Penetration is kinda secondary when a guy is wearing a t shirt. Bullet smashes into body armor it spreads it’s energy everywhere around it because it’s thinking it’s about to hit as many organs in your soft shitty body.
Penetrating rounds and knives are not supposed to break up and go through whatever.
Latest Answers