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The Science Behind Bulletproof Materials

The science behind bullet-resistant materials: how fibers, ceramics and layered glass take a round's energy, what NIJ, NIST, the Air Force and UL say about them, and why ratings belong to tested products.

ArmorList Editorial·October 9, 2024·6 min read

Bullet-resistant materials sit where materials science meets testing. This article sets out what the sources say about how they work, from metal armor to modern fibers, ceramics and layered glass, and how they are rated.

The word bulletproof is common, but NIJ's Body Armor User Guide says no body armor is bulletproof, and NIJ titles its standard Ballistic Resistance of Body Armor. A rating means a product resisted the named threats in a defined test, not every threat.

History of bulletproof materials

Protection against weapons is ancient: for most of history, armor meant metal or hardened material, such as mail and plate. Firearms changed that. The Metropolitan Museum of Art describes European armor declining through the seventeenth century: field armor was made heavier to be bulletproof against ever more accurate firearms, and with that weight and changes in tactics, most fighting men gave up all but the metal protecting vital parts such as the head, torso and hands.

In the 20th century came synthetic fibers. The Science History Institute records that DuPont chemist Stephanie Kwolek created the first of the aramid family of fibers in 1965, and DuPont commercialized it as Kevlar, a brand that now belongs to Arclin. NIJ says that in the early 1970s, with the U.S. Army, it began developing lightweight body armor woven from Kevlar, and it records the first save in a 1975 field test. For more, our article How Effective Is Kevlar in Body Armor? covers what the sources say about the fiber.

Polymers and ceramics followed. In its 2011 report for the Army, the National Research Council counts ceramics and polymers and polymer fibers among the materials that have reduced the weight of armor for personnel.

How bulletproof glass works

Bullet-resistant glass, also called transparent armor, works by layering. The Air Force Research Laboratory describes the transparencies used in armored vehicles as traditionally multi-layered glass, and describes a newer design with a hard transparent ceramic strike plate, a middle section of glass and a polymer backing.

UL Solutions describes UL 752 as covering materials, devices, assemblies and fixtures used to create bullet-resistant barriers, and describes "bullet-resisting" as no complete penetration, and no fragments or spalling that would injure a person directly behind. Whether a given glazing resists repeated hits is a matter for its test: a rating covers the shots in the test.

To explore vehicles, see our post on Are Bulletproof Car Doors Effective?.

Materials used in bulletproof glass

The components named by the sources above are glass, polymers and, in newer designs, transparent ceramics. Glass alone is brittle. In layered glazing, polymer layers sit between and behind the glass, as the Air Force Research Laboratory describes for its design.

Transparent ceramics are one development. The Air Force Research Laboratory has tested aluminum oxynitride (ALON), a ceramic, as a strike plate to replace traditional multi-layered glass transparencies in armored vehicles; in its 2005 announcement it described ALON as having high compressive strength and durability. For more on the materials used in armor, see Materials Used in Armor Plates Explained.

A rating belongs to the tested product, not to the materials in it.

The science of force distribution

When a round strikes, its force is concentrated in a small area. The materials the sources describe deal with it in different ways.

In soft body armor, NIJ's Selection and Application Guide to Personal Body Armor describes a handgun bullet being caught in a "web" of very strong fibers that absorb and disperse its energy, with each successive layer absorbing more until the bullet is stopped. In a ceramic plate, the ceramic breaks up and blunts the round as it shatters, and a fiber backing catches the fragments.

Glass itself can be made far more resistant to breaking by stress in its surface. Prince Rupert's drops, made by dripping molten glass into water, are an old example. Purdue University describes a 2016 study in Applied Physics Letters (Aben et al.) which found that the drop's surface cools faster than its interior, leaving the surface in compression and the interior in tension; compressive stress makes it hard for cracks to grow, which is why the head of a drop resists breaking.

Levels of bulletproof protection

Not all bullet-resistant products are rated the same way, and the rating system depends on the product.

For barriers and glazing, the standard is UL 752. The current (12th) edition replaced the earlier terminology with designations beginning UL-HG (handgun), UL-RF (rifle) and UL-SG (shotgun), with the threats it names for each. UL states that UL 752 does not address personal protective equipment, such as body armor, helmets and shields.

Body armor is rated under other standards. NIJ certifies body armor for law enforcement model by model, under NIJ 0101.06 and 0101.07, with the levels defined in NIJ 0123.00; other standards, such as VPAM and the UK Home Office standard, set their own. Levels do not convert one to one between systems. For body armor levels, check out Breaking Down Hard Body Armor Levels.

Our advice: start from the threats you need to resist, then check the exact product's rating, under which standard, and the test report behind it.

Applications of bulletproof glass

The Air Force Research Laboratory's work is for transparencies in ground and air armored vehicles. UL Solutions lists places that use UL 752 barriers, from banks and courthouses to schools and infrastructure facilities. In each case, the rating that matters is the one on the installed product.

Innovations and future directions

Lighter armor at the same protection is a research goal the National Research Council discusses in its 2011 report for the Army. Transparent ceramics such as ALON, tested by the Air Force Research Laboratory, are one direction. NIST's Security Technologies Group studies how the high-strength fibers used in body armor age over time. Claims about new products are the maker's to support with test data.

Conclusion

Bullet-resistant materials work in different ways: fibers catch and spread a round's energy, ceramics break it up, and layered glass combines hard and flexible layers. The sources describe the mechanisms; the ratings belong to tested products, under named standards.

On ArmorList, a body armor product's record shows its NIJ CPL status with the date the list was read, and the standards it was tested to.

Sources

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