Why are bulletproof vests not stabproof?
In: Engineering
A bullet designed for soft targets like humans is relatively soft often made of lead. they are also relative blunt compared to a knife-edge.
A knife is very hard and has a narrow edge.
Bulletproof west is often made of a material like kevlar that is a strong fiber. The material in may was a fabric of very strong fibers.
So a bullet will deform and press relative evenly on a quite large area. A common 9mm handgun bullet will press on an area that is larger than a circle with 9mm diameter. So the fiber is stretched.
A kife has a shape point and then a sharp thin edge so that it can cut the fiber. So you apply the force on a very small area. So perpendicular fibers to the knife can be cut and the parallel fiver is just pushed to the side.
You can compare it to regular fabric and how they react. Stretch out the fabric over something relatively soft to simulate a humanlike a thick pillow. Hit it with a hammer and stab it with a knife. It is a lot harder to get through the material with a hammer because you just get the material behind to deform and absorb energy when the fabric move. A knife can just slice through the fibers.
It is not a preference comparison but give you an idea of the difference.
There are bullets designed to penetrate bullet-proof west, the common way is to include a hard steel penetrator that do not get deformed.
They are worse then a soft bullet if you hit someone without a west because the can wit eas pass trough the individual and transfer a lot less energy. So armor penetration results in less damage on a target with no west.
When you start to look are bulletproof west that can hand rifled ammunition and is made of a layer of solid metal or stiff ceramics plates the will be knife proof. The problem is in a thinner flexible vest of kevlar and similar fibers.
Regular bullets susch as those fired from a pistol have a way bigger contact surface than the tip of a knife so the pressure from a kife is higher.
For a start, vests are not really bulletproof. They are bullet resistant. They work one of two ways. The lightest and most flexible ones use multiple layers of material, usually kevlar, that are strong enough to resist the bullet traveling through them. With each layer, the force gets spread over a wider and wider area, until it reaches the user and is (hopefully) spread out enough to not do lethal damage. A knife, though, just cuts the fibers; that being said, the vest will slow a blade down.
The other method uses high-strength armor plates, usually ceramic that absorb the impact but crack (and must be replaced); these plates are quite stab-resistant.
A lot of vests are combination stab and ballistic vests, but it really has to do with the forces involved, bullets are relatively soft and deform when the hit the vest which dissipates the energy. A knife is (generally) made from hardened steel, instead of deforming it cuts through the kevlar. To combat this most combination, ot stab vests use a more tightly woven kevlar to prevent it being cut.
A bullet proof vest is like a fishing net. Its designed to catch a bullet with a rounded top – like a fish. But if we are talking about a sword fish it can use its sword to cut the net and escape.
Basically a bullet proof vest has woven fibers – like a net – that are very strong when it comes to the tension they can withstand. But if you have something that is trying to cut them as opposed to ripping them apart, they won’t work.
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