Taking wing. Stiffer materials could be used in future spacecraft.

Breaking the Stiffness Barrier

Engineers thought they knew how to make materials as stiff as possible. But now a trick has shattered the old bounds. The finding may lead to new materials that could be used in cars, planes, and spacecraft.

Stiffness is a material's capacity to push back when pushed, like a spring digging into your hand when you squash it. This behavior determines the material's strength and ability to dampen vibrations. Engineers can calculate the maximum stiffness of a given material in a given shape; for instance, they know how to give airplane wings their maximum strength using a given alloy. But these upper limits are calculated using equations that only take into account springlike, or positive, stiffness. Some materials, however, have "negative stiffness": Their structure has been buckled or contorted in such a way that if pressure is applied, their stored energy only causes more compression in the same direction. Imagine a spring that, as you begin to press it, collapses all the way down on its own.

Mixing tiny amounts of these bizarre materials with compounds that have positive stiffness can result in materials that are stiffer than anybody thought possible, says physicist Roderic Lakes and his team at the University of Wisconsin, Madison. In a series of papers last week in Nature, Physical Review Letters, and Philosophical Magazine Letters, the team outlined that theory--and proved that it could work.

For instance, they made rods of tin to which negatively stiff bits of a ceramic called vanadium dioxide had been added. At certain temperatures, they found that the combined material's stiffness was greater than the conventionally calculated maximum--greater even than if they had embedded the tin with diamond. Lakes says that the negative bits essentially kick back against the surrounding material to neutralize force applied to the rods.

The work is still in its early stages, but new superstiff, superdamping materials might be used to make the quietest cars and airplanes ever, experts say. The materials could improve spacecraft, says structural dynamicist Keats Wilkie of the NASA Langley Research Center in Hampton, Virginia, if they're light enough to lower launch costs, yet stiff enough to protect sensitive, pricey equipment. Says Wilkie: "It's very interesting stuff."

Related sites

Abstract of the paper in Physical Review Letters
Roderic Lakes's home page