01/22/2013 09:40:51

Notes from the Back Row: "Engineering with Impact"

If you hit something hard enough, it will break, and the consequences can be catastrophic. A space rock roughly the size of Pasadena killed the dinosaurs when it hit the Earth at about 45,000 miles per hour, but even something as small as a bird hitting a turbine blade can bring down an airplane. The damage occurs in the blink of an eye as unimaginable pressures are fleetingly focused on the hapless chunk of rock or metal. The key to survival is to disperse those forces. But how? Caltech professor Ravi Ravichandran is trying to find out.

Guruswami "Ravi" Ravichandran is the John E. Goode, Jr., Professor of Aerospace and professor of mechanical engineering and the director of the Graduate Aerospace Laboratories at Caltech. His PhD thesis on the fracture dynamics of metals under extreme impacts, written at Brown University in 1986, remains one of the classic papers in the field.

At Caltech, Ravichandran studies impacts that pack a wallop of up to a million times the pressure of Earth's atmosphere. Such extreme pressures are actually quite mundane: a head-on collision at 65 miles per hour exerts a force of some 7,000 atmospheres during the millisecond that the vehicles' steel frames buckle. (By contrast, the pressure at the bottom of the Mariana Trench in the western Pacific, the deepest point in the world's oceans, is a mere 1,000 atmospheres.) In a typical experiment, a reconditioned naval gun from World War II shoots an aluminum projectile at a copper plate, compressing it by as much as 30 percent for a millionth of a second. Meanwhile, a laser "camera" records the ripples created by the projectile's kinetic energy as it turns into pressure waves within the copper plate.

The best way we know to dissipate these waves is to pass them through alternating layers of very stiff and very elastic materials. This is the principle behind body armor and bulletproof glass, as Ravichandran vividly demonstrated during his talk by showing a video clip produced by an armored-car company. In the clip, the company's CEO stood behind a bulletproof windshield while his assistant peppered it with three rounds from an AK-47. Spiderwebs of cracks formed in the inner layer of glass and license-plate-sized fragments of the outer layer were blasted free, but the layer of polymer sandwiched between the glass sheets stopped the slugs. If one layer of elastic is good, more layers should be even better. The logical extreme—an infinite number of layers—presents certain manufacturing challenges, so "we're extending this idea of layered media into particulate composites in order to make realistic engineering materials for shock-protection applications," Ravichandran says. Think high-tech sandbags, in other words.  

"Engineering with Impact" is available for download in HD from Caltech on iTunesU. (Episode 13)

Written by Douglas Smith