How Reliable Are Bulletproof Vests?

In August 2017, 30-year-old NYPD officer Hart Nguyen was shot while responding to a call about a 29-year-old Andy Sookdeo, a man in emotional distress. As Nguyen approached the bedroom where Sookdeo had barricaded himself, he was struck by three shots — one in the arm and two in his bulletproof vest. He survived. "His vest undoubtedly saved his life," tweeted NYPD Commissioner James O'Neill after the incident.

In 2016, 23-year-old Joaquin Mendez from Tampa, Florida, lost his life to a bullet wound to the chest. The local CBS affiliate reported that Mendez had sat in a chair, donned a bulletproof vest, and questioned its effectiveness. His friend, 24-year-old Alexandro Garibaldi, took out a pistol and fired at the vest to test it. The bullet penetrated the vest, fatally wounding Mendez.

As illustrated by these two cases, while bulletproof vests undoubtedly save lives, they aren't always truly bulletproof or flawless. Can bulletproof vests deteriorate over time, and is failure possible?

Let's Start with a Little History

The first bulletproof vests were primarily constructed from poly-paraphenylene terephthalamide, more commonly recognized as Kevlar. It was created by Stephanie Kwolek, a chemist at DuPont during the early 1960s.

DuPont had already pioneered the creation of Nylon and Spandex, and was now seeking the next generation of advanced fibers. These synthetic fibers are made up of polymers — enormous molecules composed of repeating smaller units, essentially forming a molecular chain. For example, Nylon is a polymer derived from hexamethylenediamine, an organic compound, and adipic acid, which is commonly found in products like Jell-O. In contrast, Kevlar is formed from molecules like para-phenylenediamine, which is used in antioxidants and dyes, and terephthaloyl chloride.

What Kwolek discovered would eventually become Kevlar, a material that would later be used in various applications including tires, racing sails, and bulletproof vests.

Today, bulletproof vests are crafted using polyethylene fibers — a widely used plastic polymer found in everyday items like grocery bags, toys, and trash bins. These polyethylene fibers (often abbreviated as 'PE') are intricately woven into layers that form the core of a protective vest. This material is not only more affordable but also stronger than Kevlar. In 1989, AlliedSignal introduced a Kevlar alternative called Spectra Shield, initially intended for sailcloth. Today, polyethylene fibers are used to create a lighter yet stronger non-woven material that complements traditional Kevlar in bulletproof vests.

Vests Tailored for Different Firearms

Tom Nardone, president and founder of BulletSafe, a company specializing in bulletproof vests, explains that the layers inside a vest are engineered to slow down a bullet's speed to subsonic levels — below the speed of sound. This gives the fibers enough time to react to the impact and provide their protective strength. The more layers in a vest or the tighter the weave of those layers, the greater the fibers' toughness — meaning their ability to bend without breaking — and their effectiveness in stopping a bullet.

Most handguns fire bullets at speeds just over the speed of sound. A 9mm bullet, for example, travels at approximately 984 feet (380 meters) per second, which is about 164 feet per second (50 meters per second) faster than the speed of sound. Slugs from shotguns travel even slower. A good bulletproof vest doesn't need to work very hard to slow down a 9mm bullet or a shotgun shell enough to allow the fibers to effectively absorb the impact.

Rifle bullets, on the other hand, pose a completely different challenge. Depending on the quality of the propellant and bullet grain, a shot from an AR-15, for example, can travel at a speed of approximately 3,300 feet per second (1,005 meters per second), nearly three times the speed of sound. Stopping such a bullet requires a much denser arrangement of ballistic fibers.

"Supersonic rounds require plates to break the bullet into fragments," explains Nardone. Most vests built to stop rifle projectiles are reinforced with high-density insert plates, typically made from ceramics and polyethylene, to fragment a bullet and capture its pieces.

"A PE vest encountering a [rifle] round would have the bullet traveling faster than the speed of sound for such an extended period that it would pierce through until the vest failed," says Nardone.

The materials used in vests are also susceptible to wear and degradation over time. For example, Kevlar weakens when exposed to salt, such as the salt found in human sweat. Over the years, as a person wears a Kevlar bulletproof vest, the likelihood of failure increases. Similarly, PE-based vests also deteriorate when exposed to UV light over time.

"Vests have a lifespan of about five years. Some people buy older, used vests, particularly from police work, which might not offer the same level of protection as newer models," Nardone adds.