Stuff gets stiffed by unstiff inserts

Stiff materials resist deforming when you poke them. In contrast, some materials give way so dramatically to the slightest touch that they are said to have an exotic property known as negative stiffness. When pressed, such materials break apart or suddenly rearrange themselves internally, ending up as materials with ordinary, so-called positive stiffness.

Now, scientists have found that morsels of negative-stiffness materials can remain intact within larger chunks of positive-stiffness material. Moreover, the inserts give the host material a positively awesome boost in stiffness.

“If you combine negative stiffness with positive stiffness, you get very unusual behavior,” says Roderic S. Lakes of the University of Wisconsin-Madison.

In one experiment, Lakes’ team used rolling and casting to blend vanadium dioxide particles into pure tin. The crystal structures of those particles undergo changes associated with negative stiffness. Within a limited temperature range, the tin’s stiffness jumped by nearly 4 percent, or roughly twice the boost in stiffness that could be achieved with an equivalent addition of the stiffest material known–diamond. The scientists report their findings in the March 29 Nature.

As well as becoming exceptionally stiff, such composites perform remarkably well at quelling vibrations. Both effects could prove useful, Lakes says. The extra stiffness might make structures such as aircraft wings more rigid. Moreover, the materials’ ability to dampen vibrations might lead to new auto-parts coatings that would make car interiors quieter. Positive-negative stiffness combinations may already occur in natural substances such as rocks and bones, Lakes speculates.

In the February Philosophical Magazine Letters, Lakes reported a remarkably simple way of using negative stiffness to boost a structure’s positive stiffness. He glued a straight, thumb-length tube of silicone rubber to an identical tube with a kink, which endowed it with negative stiffness. The result was an overall structure stiffer than the straight tube alone.

According to accepted theory about composite solids, materials in these experiments shouldn’t become as stiff as they actually do. However, in the March 26 Physical Review Letters, Lakes reports that by simply removing the theory’s assumption that stiffness is always positive, the theory predicts that materials as stiff as these new composites ought to exist.

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