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THE ORIGIN OF BULLET RESISTANT GLASS

The earliest bullet resistant glass was deployed on the battlefields of World War II.  By stacking sheets of tempered glass and laminating them to each other with layers of epoxy, Army engineers could craft bullet resistant glass to use in bunker and battle vehicle windows.  But these early bullet resistant glass windows were very unwieldy: Usually about four inches thick, weighing more than 50 pounds per square foot, and limited to being cut into small rectangular windows.

BULLET RESISTANT GLASS COMES TO BANKS

After the war, structural engineers began to consider other applications for bullet resistant glass, such as bank security.  These pioneers were greatly aided by advancements in another war-time material technology: transparent acrylic.  Acrylic was first discovered in the late 1800s.  Useful transparent acrylic was patented in 1933, and soon after marketed for use in early non-shattering “safety glass” windows.  During the war clear acrylic was used for submarine and airplane windows and canopies. These were not bullet resistant by any means, because early acrylic was quite brittle.  But the post-war discovery of advanced plastic additives and fillers resulted in acrylic that is suitable for fabricating bullet resistant glass.  This acrylic bullet resistant glass is thinner than the WWII-era laminated bullet resistant glass–about an inch for comparable stopping power–and considerably lighter, just eight pounds per square foot.  As an added bonus, since this transparent acrylic bullet resistant glass is a single sheet of thermoplastic, rather than many layers of tempered glass, it can be milled, routed, drilled, and flame polished, making it a remarkably flexible and attractive building material.

MANY OPTIONS IN BULLET RESISTANT GLASS

While acrylic, with its relatively low cost and flexible material qualities, is the preferred bullet resistant glass in many applications, it’s not the only option.  Monolithic acrylic is great for stopping shots from a 9mm, .357 Magnum, .44, or shotgun, but those who need security from something heavier–an AK-47, M16, or hunting rifle–will want layered polycarbonate (another softer thermoplastic, capable of catching and holding a bullet), or even glass-clad polycarbonate, which is composed of laminated layers of tempered glass and polycarbonate, and can stop many shots from an assault rifle, and even withstand hurricane-force winds.

 

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Test standards

Bullet-resistant materials are usually tested by using a gun to fire a projectile from a set distance into the material in a set pattern. Levels of protection are based on the ability of the target to stop a specific type of projectile traveling at a specific speed. Experiments suggest that polycarbonate fails at lower velocities with regular shaped projectiles compared to irregular ones (like fragments), so that testing with regular shaped projectiles probably gives a conservative estimate of its resistance.[7] When projectiles do not penetrate, the depth of the dent left by the impact can be measured and related to the projectile’s velocity and thickness of the material.[8] Some researchers have developed mathematical models based on results of this kind of testing to help them design bulletproof glass to resist specific anticipated threats.

Well known standards for categorizing ballistic resistance include the following:

• U.S. Department of Defense specifications for purchase of transparent armor – includes standards for bullet resistance (ATPD 2352P).

• U.S. National Insititute of Justice (NIJ) standard for ballistic resistant protective materials (NIJ Standard 0108.01).

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