BANG! The shot rings out. And the bullet bounces harmlessly away. How? Glass that's purpose-built to stop penetrating projectiles.

But this brilliant blocker didn't happen overnight. It took years of research, risk and repeated efforts to create a substance capable of stopping bullets. To understand the strength, let's break down the science of building bulletproof barriers.

How Does Bulletproof Glass Work?

Also known as ballistic glass, safety glass or security glass, this bulletproof barrier got its name from — you guessed it — the ability to stop bullets.

But how does bulletproof glass work? The principle is fairly straightforward: Where typical glass will shatter into shards on impact from a bullet or other hard object, its bulletproof counterpart layers on materials such as polyvinyl butyral (PVB), polyurethane (PU) or acrylic that make it possible to absorb significant amounts of bullet momentum and stop shots from getting through. Thicker glass also helps to reduce the risk of objects breaking through. While standard car windows are around 3mm thick, bulletproof glass may be 10mm thick (or more) per sheet.

Worth noting: The more layers added, the greater the chance of optical distortions in the glass, which in turn reduce the amount of light let in and the clarity of images. It's also important to note that the layering process is critical for glass to deliver on its bullet-stopping potential. Air bubbles between glass and acrylic, PVB or PU layers can weaken the bond and reduce the efficacy of the finished product.

The History of Building a Better Barrier

The notion of a nigh-unbreakable barrier got its start with tadpole-shaped pieces of glass known as Prince Rupert's Drops. These gained popularity in the 17th century when they were given to King Charles of England by Prince Rupert of Germany. Made by tossing red-hot pieces of molten glass into water, these drops have two unusual properties. If you smash the head of the drop with a hammer or shoot it with a bullet, it won't break. Alternately, bend the thin tail with your fingers, and the entire drop disintegrates.

Although the drops have been around for centuries, it took until 2017 to fully understand the forces at work. It started with research from 1994, which found that the drops are in a state of "unstable equilibrium" which sees the head's surface under highly compressive stress, while the interior is under massive tension. Then, in 2017, scientists found that this surface stress was much higher than originally believed. Experiments reported the compressive stress at 700 megapascals — or almost 7,000 times standard atmospheric pressure. As a result of this stress, it's almost impossible to crack a drop by striking the head, since any cracks created will simply grow parallel to the surface instead of breaking through into the tension zone. Striking the tail, meanwhile, provides a path to the interior tension zone which in turn disrupts the unstable equilibrium and destroys the droplet.

In 1903, another approach to creating bullet blockers was discovered by Edouard Benedictus, a French chemist. The (possibly apocryphal) story says that Benedictus accidentally dropped a glass flask on the ground while working, but instead of shattering, the glass simply cracked. Why the save? Because the flask held a liquid nitrate solution, which coated the interior wall with a thin plastic layer, enabling the force of the drop to be distributed rather than concentrated at a single point. The combination of these two techniques — glass hardening and chemical layering — set the stage for bulletproof materials development through the 19th and 20th centuries.

Today, the bulletproof glass market is worth more than $6 billion worldwide.

The Science of Stopping Shots

Force equals mass times acceleration. While bullets don't have much in the "mass" category, they've got plenty of acceleration. As a result, they have a substantial amount of force concentrated into a very small area. And when this force meets a rigid object — such as glass — it tends to smash right through with little resistance. In practice, this means that a bullet fired at a standard car or house window has no problem passing through. Although processes such as tempering and laminating glass can help to reduce the risk of random jagged shards, the glass itself remains no barrier to bullets.

Meanwhile, bulletproof glass looks to more effectively distribute imparted force.

Consider catching a ball coming toward you at a high speed. If you hold your hand motionless and grab the ball when it arrives, chances are it's going to hurt. Why? Because all the force carried by the ball is directly transferred into your hand. Put on a few pairs of fuzzy gloves and it won't hurt as much because the material in each pair absorbs and distributes some of the force before it reaches your hand. Allow your hand to move with the ball when it arrives, and the force is further reduced since you're slowing the ball down over a larger distance.

Bulletproof barriers work on the same principle. By layering thicker sheets of glass with PVB, PU or acrylic layers, the force carried by the bullet is distributed across a larger area rather than a single point. And by increasing the thickness of the glass, the bullet naturally slows down over greater distance.

This isn't to say that the glass doesn't take damage. Watch a video of bulletproof testing on cars and you'll see a host of cracks and splinters in the glass, but bullets won't get through. The type and amount of bullets blocked depends on how the glass is made and how many layers are used. To help standardize glass protection ratings, the Underwriter's Laboratory (UL) created a 10-level scale:

Level 1 — Can stop three bullets from small-caliber (9mm) handguns.
Level 2 — Can stop three bullets from large-caliber (.357) handguns.
Level 3 — Can stop three bullets from larger guns such as .44 magnums.
Level 4 — Can stop one bullet from a .30 caliber rifle.
Level 5 — Can stop one bullet from a rifle firing a 7.62mm full metal jacket round.
Level 6 — Can stop five bullets from small-caliber handguns.
Level 7 — Can stop five bullets from a 5.56mm rifle.
Level 8 — Can stop five full metal jacket rounds from a 7.62mm rifle.
Level 9 — Can stop one armor-piercing bullet from a .30-06 rifle.
Level 10 — Can stop one bullet from a .50 caliber rifle.

New Approaches to Stopping Bullets Short

In addition to bullets, ballistic glass will also stop blunt-force impacts. As a result, protective glass is now used in a wide variety of protective applications. For example, bulletproof windows are common on military or law enforcement vehicles and are often used in government buildings, banks and even schools. Meanwhile, improved production processes and lowered costs have made these barriers viable for venues such as arenas or stadiums to help protect staff from unruly crowds.

New approaches are also on the horizon for this technology. Synthetic ceramics with reduced weight but the same stopping power as glass are now being tested — for example, the U.S. Air Force is currently testing a ceramic composed of aluminum oxynitride (ALONtm) for use in vehicles against armor-piercing weapons. Work with air gaps is also underway. Instead of eliminating these gaps, this approach looks to create a consistent air gap layer between a sheet of laminated glass and one of polycarbonate. The hard glass layer deforms the bullet, the air gap provides some resistance and the polycarbonate layer then stops the bullet. This approach provides a 35% reduction in weight compared with traditional bulletproof options.

Ballistic barriers? Scientists have cracked the code with bulletproof glass — but there's always room for a better blocker build.

Check out Northrop Grumman career opportunities to see how you can participate in this fascinating time of discovery in science, technology, and engineering.

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Breaking Barriers: The Science Behind Bulletproof Glass

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