Composite Metal Foams

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The CMF works similar to a prince Ruperts drop in how it stops a bullet. Here's a video about prince Ruperts Drop, which is a piece of glass with a very very high internal compression force ballanced precisely against a very very high materials strength limit. see youtube .
A CMF presumably works similar in that is would have to contain areas of extremely high compressive forces internally balanced by contractile forces from cooling the outside of the material faster than the inside. One the outer shell forms it stabilizes in size, but inside the solid shell of recently cooled material is hot liquid that eventually decreases in volume causing the shell surrounding the center to experience extreme compressive force near the limits of the material strength. Imagine having a CMF with lots of highly compressed bubbles in the empty space that simply explode when disturbed. The explosion from the release of stored potential energy caused by rupturing the foam is directed out towards the point of disruption. In effect the "armor" is made up of millions of compressed energy bombs that explode when hit by a bullet and direct the energy back at the bullet.

This would probably make the momentum transfer slightly higher since much of the explosive material ejected from the ruptured armor would be directed opposite the direction of the bullet. But the momentum would be well distributed by the armor so it wouldn't feel any stronger than a gun recoil. Mostly bullets kill with the shock wave impacting the body organs from the initial bullet hole. That's why they make hollow tip bullets, so that the kinectic energy of the bullet gets dispersed into the body organs doing serious rupturing harm. With this armor, the kinetic energy is largely canceled out by the armor exploding in the direction of the bullet.
 

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