28 DECEMBER 2006
With the weekly attacks on U.S. soldiers in Iraq surpassing 600, roadside bombs remain the weapon of choice for Iraqis intent on killing Americans. But beyond protecting soldiers on the ground, the U.S. military is also concerned about protecting against the much stronger impact of a missile striking a ship.
Most armor relies on mass to dissipate shock, but massive ships are
slow ships. In a light, speedy warship, armor is often sacrificed for
speed and maneuverability. 
Here's
a brainstorm that could break that deadly equation: a "decorated, tapered,
and highly nonlinear granular chain."
This is not a new-fangled Christmas-tree ornament, so we suppose some explanation is in order. Fasten your safety straps: elementary physics ahead!
A simple "tapered chain." The larger balls in foreground strike the smaller balls behind them, and they bounce away with greater velocity, but without absorbing all the energy, as would happen in a Newton's cradle. This structure, combined with a bunch of smaller balls, may play a role in advanced blast protection and shock absorption. Courtesy Stéphane Job, Institut Superior de Mechaniques, Paris
To protect against impact, you need to dissipate the impact's kinetic energy. Typically, that's done with strong, heavy armor, but as missiles speed up, you may need so much mass that eventually the missile will either win, or the ship will sink.
Surajit Sen, a professor of physics at the State University of New York at Buffalo, has hit upon another idea: building armor of small, elastic spheres that will bounce around in a chaotic fashion. Because the balls have different masses, they move erratically, and much of the incoming kinetic energy gets transformed into heat and noise.
To survive the physics that follows, keep one idea in mind: if you want to punch someone out, you don't wrap your fist in a beanbag.
Starting simple
As with so much in the science of mechanics, the starting point is Isaac
Newton, father of the study of motion. Remember the toy called "Newton's
cradle"? You drop the first ball, it bangs against the next, transferring
almost all its energy. 
It's
a chain reaction: The ball at the opposite end bounces out, falls back,
and starts the whole process in reverse.
"Newton's cradle" is simple. It's got almost perfect energy transfer. And it helped inspire a new form of armor. Photo: Penn State
As you know, the balls bounce for quite a while – meaning the kinetic energy is conserved. Simple as it is, Sen says the cradle was a starting point for his study of shock absorption, because conservation is the opposite of dissipation.
"Years ago, I basically got into the opposite problem, of shock transmission," Sen says. "I was studying solitary waves, also called solitons, energy lumps that move through granular material." A structure like Newton's cradle, he says, "is the perfect energy transmitter, the perfect shock transmitter, and that got me wondering, if you taper the size of the balls, could you break down the shock waves?"
It sounds simple, and it is: When a heavy particle strikes a lighter one, the lighter one moves faster than the particle that hit it, but it also carries less kinetic energy. "If you progressively shrink the particle size, you would be taking a large shock wave and reducing the kinetic energy," he says.
Sen found that varying the size and mass of the balls did indeed impede the energy transfer from one ball to the next. As the kinetic energy moves down the chain, even more energy is absorbed.
A simulation shows how shock moves through the
new material. Red and yellow show the brunt of the force. The sound wave
(green) generated by the shock strikes deeper grains before the main shock
does, showing that the shock is already being dissipated. (Image
slightly modified) Courtesy Surajit
Sen, State University of New York at Buffalo
Highly decorated?
So we have the tapered part of " decorated, tapered, and highly nonlinear granular chain" taken care of. What's with "decorated"?
Sen and his colleagues "decorated" the chain by nesting a bunch of smaller balls with the larger ones shown in the photo, and that further boosted shock absorption. "The kinetic energy has to cross these dinky bridges from one particle to the next, and there is a pileup on the bridge," Sen says. "The small particles ... have to move fast, and they rattle around so much that they dissipate a lot of energy as sound and heat."
The new study shows that "decorating can improve shock absorption by more than 50 percent," Sen says.
Finally, what about "non-linear"? That simply means the kinetic energy "just hops from one grain to the other as opposed to causing multiple grains to collectively swish around," Sen says.
The Penguin is a helicopter-launched anti-ship
missile. As the U.S. Navy develops bigger, better ship killers, it is
desperately seeking beefier protection against same. Photo:
US
Navy
Ship, protect thyself
Funding for the recent research came from the U.S. Department of Defense, which is eager to protect against ever-faster incoming missiles. "I was told by people from the DOD ... that the typical speed of an advanced missile is above several hundred meters per second," Sen says, "and they have nothing that comes close to withstanding that."
A NATO Sea Sparrow missile on aircraft carrier USS
Abraham Lincoln. The Sea Sparrow is a medium-range missile that can destroy
planes and anti-ship missiles. But if this anti-missile missile fails,
can Abe's armor protect against incoming missiles? Photo:
U.S.
Dept. of Defense
Future tests of the new shock-absorber will focus on a series of cylinders about six inches long, stacked next to each other, each holding "tapered, decorated" grains.
But the technology should work at essentially any thickness, Sen suggests, and that could have civilian uses, such as dampening shock in artificial knees. "We live in a world of granular material, like soil, and we are fairly insulated from things that are going on around us. Granular material provides natural shock absorption, and is perhaps an untapped frontier in shock absorption."
— David Tenenbaum
Bibliography
•
Decorated, Tapered, and Highly Nonlinear Granular Chain, Robert Doney
and Surajit Sen, PRL 97, 155502 (2006), PHYSICAL REVIEW LETTERS.
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