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Eye Master
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Interesting - it likely uses the bullet's energy against itself.

Rifle bullets are typically rotating so fast, like 250,000RPM, that centrifugal force is on the edge of tearing them apart. Indeed, when they hit an unarmored person, the impact with your skin is enough disturbance that a fracture starts and the bullets 'explode'. This is what makes the 223 so lethal at close range.

By getting metal, albeit with a lot of voids, you get the fracture to start at the outside of the plate, so the bullet expands/explodes, and has a lot more surface area. Well, larger surface area is easier to stop, so a thicker/lightweight plate will take the energy. Softer armor won't initiate the fracture. Thinner armor won't stop it in time.

As they say, 'anything is bulletproof, as long as it is thick enough'. So there is some curve you can plot of hardness versus thickness that defines bulletproof.

I had an idea along these lines 35 years ago, where my prof in materials was on the NASA steering committee, and told us as freshmen that if we had a cool enough experiment, he could get it done on the space shuttle. Problem with making metal foam is gravity, as the metal wants to sink down and the bubbles go up. My idea was to make metal foam in space, so there is no gravity, and the bubbles will distribute. Alas, I never saw the benefit, and was too busy drinking beer to want to be the nerdy kid working on the space shuttle, otherwise I could have been famous and maybe made hundreds of dollars from it.
 

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Interesting - it likely uses the bullet's energy against itself.

Rifle bullets are typically rotating so fast, like 250,000RPM, that centrifugal force is on the edge of tearing them apart. Indeed, when they hit an unarmored person, the impact with your skin is enough disturbance that a fracture starts and the bullets 'explode'. This is what makes the 223 so lethal at close range.

By getting metal, albeit with a lot of voids, you get the fracture to start at the outside of the plate, so the bullet expands/explodes, and has a lot more surface area. Well, larger surface area is easier to stop, so a thicker/lightweight plate will take the energy. Softer armor won't initiate the fracture. Thinner armor won't stop it in time.

As they say, 'anything is bulletproof, as long as it is thick enough'. So there is some curve you can plot of hardness versus thickness that defines bulletproof.

I had an idea along these lines 35 years ago, where my prof in materials was on the NASA steering committee, and told us as freshmen that if we had a cool enough experiment, he could get it done on the space shuttle. Problem with making metal foam is gravity, as the metal wants to sink down and the bubbles go up. My idea was to make metal foam in space, so there is no gravity, and the bubbles will distribute. Alas, I never saw the benefit, and was too busy drinking beer to want to be the nerdy kid working on the space shuttle, otherwise I could have been famous and maybe made hundreds of dollars from it.
That's not really a problem... in fact the literature on how to make metal foam commercially relies on gravity.

What I was curious about was on the size of the bubbles. How would a foam with much smaller bubbles work when compared to one with larger bubbles?
 
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