How does bulletproof glass work?
There is no simple answer to this question. How bullet proof glass works depends very much on the type of bullet and the construction principle of the bulletproof glazing. The impact of a bullet on bulletproof glass is a complex physical process that takes place in a very short time involving large forces.
In comparison to metals, wood or sand, glass has a completely different penetration behavior when struck by bullets. The problem lies in the breaking behavior of glass.
Glass breaks faster than a bullet flies
Glass breaks at a speed of several thousand meters per second. A bullet fired from a rifle such as the AK47 or the NATO rifle G3 has a lower speed of up to 1000 meters per second. This means that glass breaks faster than the bullet itself is traveling.
The fractures in the glass out-speed the bullet. During penetration, the bullet hits already destroyed glass only. Of course, this considerably reduces the resistance of glass to future projectiles. To counteract this behavior, SILATEC uses not only glass, but also highly transparent plastics such as polycarbonate.
In principle, bulletproof glass converts the point load of the bullet into a surface load in order to stop it. The more the projectile deforms after impact, the larger its cross-sectional area becomes.
This increases the energy output of the projectile and slows it down accordingly. Whether the projectile deforms, disintegrates or breaks up depends on its construction and velocity. Hard-core bullets – armor piercing (AP) bullets – generally do not deform when impacting bullet proof glass.
Soft-core bullets do, however. For this reason, hardened AP bullets are more difficult to stop than softer projectiles. It also bears mentioning that different projectiles can be fired from one and the same weapon. With very different effects on bulletproof glass. This is why the following frequently asked question can only be answered conditionally:
glass protects against the Kalashnikov
It depends strongly on which ammunition is used. Armor piercing (AP) projectiles or soft-core ammunition? Bulletproof glass that protects against hard-core AP ammunition is usually thicker than variants that stop “only” soft-core bullets. SILATEC offers both types. SILATEC AK47 NS 36/76 protects against “standard” ammunition. And the glass type SILATEC AK47 NS 62/141 offers protection against hard-core AP ammunition.
By the way, we also test such panes in the home country of the Kalashnikov – in Russia.
AK47 vs. Bulletproof glass
The angle of impact
The angle of impact of the bullet on bulletproof glass also influences the protective effect of the glazing. Put simply, the smaller the angle of impact, the less critical it is for bulletproof glass, as more material is opposed to the projectile. Bulletproof glass receives the highest load at a 90° angle, which is why bulletproof glass is usually tested at a 90° angle of fire.
Another decisive factor is how and whether the bulletproof glazing splinters on the defense side – the side facing the protected space This is why a distinction is made between splinter-free glazing (NS = no spall) and glazing with splinter exit (S = spall).
Freedom from splintering (NS) can be achieved through a final layer of plastic – ideally polycarbonate – instead of glass on the defense side. Another possibility is to make the bulletproof glazing thicker so that the bullet does not break the last glass plate. But thicker means more weight. Bulletproof glass from SILATEC is particularly thin and weighs 50% less than conventional armor glass.