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.
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