Bullet Proof Glass: Going Brittle vs Staying Loose

Acrylic and laminated bullet proof glass look pretty awful after being shot. That clear sheet is almost invariably rendered totally opaque by a froth of spiderweb cracks, and has often lost big hunks of glass–as well as countless tiny shards and flakes. It usually comes as a shock to realize that the glass has actually stopped the bullet, because it looks absolutely demolished.


Bullet proof glass after being shot


In most regards, acrylic and traditional glass are very different materials–glass is much harder, acrylic much more malleable, and they share no common constituent materials or manufacturing methods–but both are brittle, and this brittleness is the key to their bullet-stopping power. In the case of traditional laminated bullet proof glass, individual sheets of tempered glass (which are pretty hard in their own right; just ask any hothead who’s ever tried to dramatically punch out a window) are adhered to each other with intervening layers of thin, rubbery epoxy. When a bullet strikes this laminated bullet proof glass, the hard surface of the first sheet flattens the bullet, then shatters. The rubbery layer of epoxy then saps away some of the rushing bullet’s momentum. If the bullet still has enough energy, it might shatter the next pane of glass–flattening even more in the process–only to get further sapped by the next layer of epoxy. After a few layers of glass that bullet is out of steam–and the window looks like hell.

Monolithic acrylic bullet proof “glass” (really a thermoplastic) is very similar to traditional laminated bullet proof glass, but it does it all in a single layer (and is thus “monolithic”): A bullet strikes the very hard surfaces of the acrylic bullet proof glass, breaking off all manner of sharp, thin flecks and shards (called “spall”). In the process, the bullet mushes itself into an exceptionally non-aerodynamic shape, and wastes a good deal of its energy. A normal sheet of ballistic acrylic is over an inch and a half thick, so even after shedding these chunks there is still enough acrylic left to deflect the bullet.


Meanwhile, polycarbonate bullet proof glass is a very different beast. This is graphically demonstrated in the following video clip, extolling the virtues of Oakley “Plutonite” sunglasses lenses. “Plutonite” is Oakley’s trademarked version of high-quality optical-grade polycarbonate. You’ll see how, when a projectile strikes polycarbonate, even a thin layer of totally non-laminated material will wiggle, wobble, rebuff the projectile, rather than cracking and letting it through.



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