aramid fiber
/ AIR-uh-mid /
The golden-yellow fiber best known as Kevlar, the stuff of bulletproof vests. It is a polymer fiber whose long, stiff molecules line up along the fiber, giving it remarkable toughness and resistance to being cut or torn; it absorbs impact by stretching and pulling rather than shattering.
Aramid is an aromatic polyamide; its rigid rod-like chains are highly aligned and held sideways by hydrogen bonds, so the fiber is strong and stiff for a polymer: modulus about 70 to 130 GPa, tensile strength about 3 to 3.6 GPa, and density only about 1.44 g/cm^3 (lighter than glass or carbon). It stays tough and does not shatter, so it excels at absorbing impact and stopping penetration.
Aramid fills the toughness niche between glass and carbon: body armour, helmets, ropes, cut-resistant gloves, and impact skins on aircraft. Honest limits: it is strong in tension but weak in compression (fibers kink and buckle), it soaks up moisture, degrades in sunlight and UV, and is notoriously hard to cut and machine cleanly.
A bulletproof vest is many layers of woven aramid: the tightly bonded, aligned chains catch and spread a bullet's energy across the weave, blunting and stopping it rather than being punched through.
A tough, impact-absorbing fiber that is strong in tension but weak in compression.
Aramid is a star in tension and impact but poor in compression (it buckles), so it is often blended with carbon in hybrid composites rather than used where a part is squeezed or bent hard.