Nylon and Kevlar vests have been protecting police officers and military personnel for decades, but new weapons technology means that manufacturers must continually improve such vests to keep up. With the help of software created by Associate Professor David Roylance of the Department of Materials Science and Engineering, the best designs can now be identified before expensive field tests are performed.
The goal in designing protective vests, Professor Roylance explained, is finding the right balance between the effectiveness of the material and its bulk or weight, depending on the vest's intended use. The heavier the material, the better it may be at stopping projectiles such as bullets and shrapnel, but the more ineffective the person wearing it becomes. Heavy material cuts down on mobility and also traps heat, which quickly leads to exhaustion during physical activity. Because World War II airplane crewmen didn't have to move around much, he said, they could wear nylon vests weighing nearly 30 pounds to protect them from fuselage-penetrating anti-aircraft flak.
When a person is hit with a projectile such as a bullet, stress waves propagate outward from the site of impact, transferring energy from the bullet to the medium it hits, Professor Roylance explained. The wavespeed increases as the material is made stiffer, so stiff materials can distribute the projectile's energy over a wide area more quickly. However, increasing a material's stiffness often reduces its toughness-the amount of energy the material can absorb without fracturing. Ceramic materials, for example, are significantly stiffer than even steel, but they have little toughness-they are brittle and therefore subject to fracturing. "It's not always clear which is the more important parameter to optimize," he said. "When you improve one property, you pay for it with another."
Since the 1970s, Professor Roylance has been developing and refining a software program called FABRIC that helps designers at research sponsor Dupont (which makes nylon and Kevlar, the protective material most widely used today) select the best vest design for maximum effectiveness. The program does not suggest new materials; instead, it predicts the effects of an impact on various actual or proposed armor designs. It also provides accurate models of the blunt deformation caused by a projectile such as a bullet, taking into account the bullet's initial and residual velocity, how far it will penetrate into the material, the shape of the indentation and the amount of force it will impart.
With this knowledge, designers can test various weaves, thicknesses and layer arrangements of Kevlar or other material on a computer before doing so experimentally. "It costs several hundred thousand dollars to characterize the ballistic response for a single material," Professor Roylance said.
Kevlar, an aromatic polyamid or aramid, is two-thirds the density of steel but only one-fifth as heavy. However, it costs $10-$20 per pound compared to about $1 per pound for nylon, Professor Roylance noted. It is also used in radio tower guy wires and in fiberglass and other composites. It replaced nylon as the protective material of choice in the late 1960s, around the time Professor Roylance received his PhD and began his military service at the Army Materials and Research Center in Watertown, MA. He was later assigned to duty in Vietnam, and protective materials "went from being academically interesting to being a very personal interest," he said, adding that he and other soldiers often sat on several vests while riding in Jeeps in case the vehicle ran over a land mine.
An early type of protective garment, chain mail, was an effective barrier against swords and arrows, but became obsolete with the invention of longbows and gunpowder. Today's Kevlar vests can save the life of a policeman shot with some kinds of weapons, but they are not completely bullet-proof against higher-caliber guns and rifles. Even if bullet penetration can be prevented, enough momentum can cause fatal injuries from blunt trauma. On the battlefield, more injuries are caused by shrapnel from shells, land mines and other explosive devices than by rifle bullets, Professor Roylance said.
As vests are upgraded to protect against new types of weapons, Professor Roylance has changed the code in FABRIC to allow testing of new characteristics. For example, the program may be altered to evaluate slippage, whereby more slippery projectiles can slide between Kevlar fibers and thus achieve greater penetration effects. Teflon-coated "cop-killer" bullets have been designed to do just this, making vests less effective against them, he said. The program can also model the effects of needle-shaped projectiles, another potential "improvement" on larger, blunter bullets.
A version of this article appeared in MIT Tech Talk on January 31, 1996.