Glass differs from crystalline materials in internal atomic arrangement. It is irregular in glass as against crystals where the atoms are arranged with perfect regularity. This regularity leads to the definite possibility of spatial repetition of atoms over very long distances inside the material. In glass, this long-range order does not exist, but there is a kind of order known as the short-range order. Often this short-range order is the cause of locked-in stresses inside the material in random locations and in random directions. Therefore when an external stress is applied, the net effective stress is the total of the internal and the external stresses. The net effective stress becomes less than the applied stress, rendering the glass tougher than most of the crystalline materials. And glass can be made extra tough by specialized heat treatments. Such treated glasses are termed as toughened glass.
Bullet-proof glass is made by binding alternate layers of such toughened glass and polycarbonate laminates. If the force exerted on the front layer of the glass by an object like a fast moving bullet, carrying a large amount of momentum and energy, exceeds the breaking strength, a crack develops in the glass layer. But immediately, the energy deposited by the impact gets distributed in many directions because the initial crack multiplies into many cracks in different directions. Importantly, the polycarbonate layer below further absorbs the impact force, distributes it laterally and renders the crack impervious to the next layer of glass. Thus, the bullet loses its energy and momentum, it stops. The force of the bullet from outside is spread out by the tough plastic layer and the bullet is stopped even if the front glass layer shatters. A bullet fired from in side, however, can puncture the polymer layer easily before breaking the glass locally, only slowing down the bullet slightly.
Source: thehindu.com
