Protecting Planes with Fabric
by David Pescovitz
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Tarek Zohdi (Left) and George Johnson hold a piece of Zylon ballistic fabric. Its super-strength and light weight make it an ideal shield. (David Pescovitz photo)
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In 1989, a United Airlines DC-10 made a crash landing in Sioux City, Iowa, killing 112 people. One of the engine rotors tore apart its metal housing, releasing a spray of shrapnel that pierced the plane's hydraulic steering lines. Thanks to a well-prepared crew in the air and on the ground, more than half of the passengers survived the catastrophe. But that was not the first or the last time this kind of mechanical failure would bring down a plane. UC Berkeley professors Tarek Zohdi and George Johnson are working on a fabric-based system that may prevent it from ever happening again.
The researchers are continuing a long collaboration with Boeing and the Federal Aviation Administration to study a ballistic fabric manufactured by Toyoba Corporation. The lightweight materials, called Zylon, could potentially be used as a super strong lining to protect key components such as fuel lines and hydraulic lines. Several times tougher than the Kevlar used in most bulletproof vests, Zylon is woven from yarn consisting of 350 individual polymer microfibers. One square inch of the fabric contains more than 400,000 of those fibers.
"We're helping determine the best design of the fabric, from how many threads make up each piece of yarn to the tightness of the weave to how layers can be combined into a shield," Zohdi says.
The idea is that a fabric shield built inside the rotor casing, for example, would contain the shrapnel if the blade itself were to fail as it did in the Sioux City crash. That particular catastrophe resulted from a phenomenon within the engine called "creep." Radial forces can pull the titanium rotor blades outward over time, Zohdi explains. Eventually, the blade may collide with the casing, shattering it as it did over Iowa.
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In this video (.avi), a projectile just barely breaks through the ballistic fabric, gets pulled back in as the fabric retracts to its original shape, and then is finally propelled forward on the second "bounce" of the fabric. (courtesy the researchers)
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"The rotor breaks into tiny pieces that travel at extremely fast speeds, upwards of 1,500 feet per second," Zohdi says. "The effect is like spraying the hull with tiny knife fragments or firing a shotgun repeatedly at the plane."
Indeed, that's not so different from the way Zohdi and Johnson ultimately test their theories. First though, Zohdi runs computer simulations to predict how various configurations of the fabric and mounting systems might stop a projectile. His simulations are based on a numerical method tailored specifically for the Zylon. While the simulations are highly complex, representing the dynamics millions of fibers, they take just a few minutes to compute.
"This makes it easy to test different fabric designs and configurations," Zohdi explains.
The Ballistics Laboratory's custom-built powder gun accommodates a 50 caliber shell into which the projectile is inserted. (David Pescovitz photo)
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Once a concept proves out on the screen, the researchers head into the College of Engineering 's basement ballistics laboratory. Built by the late professor Werner Goldsmith, a world-renowned ballistics expert who spearheaded the Zylon research, the laboratory is a shooting gallery for the study of materials. Johnson and his students load up massive stationary guns with 1.5 inch long steel cylinders. After they pull the trigger, a high-speed videocamera captures the projectile hitting the fabric swatches. Based on the response of the Zylon and the dynamics of the projectile, Zohdi and Johnson then tweak the configuration of the material.
There are myriad ways in which the material can be arranged, and each variation affects its ability to absorb energy. For example, folding a sheet is not as effective as mounting several layers a slight distance apart. The researchers are also exploring how different mounting mechanisms affect the material's ability to stop the projectile.
"If the projectile is penetrating the material, you might think that it makes sense to grab the edges more tightly," Johnson says. "That's actually the worst thing you can do though. You need to loosen it just enough that it slows down the projectile."
The aim is to spread the load across the fabric, he says. With enough layers, the projectile should be stripped of all its kinetic energy. The problem, though, is that most of the energy is highly-localized at the spot where the projectile actually hits.
One possible way to slow down the projectile is to coat each layer in the shield with an adhesive. That way, the projectile drags the first sheet with it as it attempts to pierce a hole in the next layer. This enormous amount of friction should stop the projectile cold, Zohdi says.
Once the simulations and experimental observations are complete, Zohdi and Johnson expect that the final product will be an amalgam of the high-tech fabric and old-fashioned steel that blunts the edges of the projectiles before they hit the cloth. Getting the mix right though will require many more trips between the computer and the shooting gallery.
"There's metal analysis involved, fabric analysis, simulation, and experimentation," Zohdi says. "Combining the materials in the right way."
Tarek Zohdi's home page
George Johnson's home page
Werner Goldsmith, 1924-2003 (Lab Notes, November 2003)
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