Auxetics defy common sense. Unlike conventional materials, which narrow when pulled lengthwise, they widen when pulled lengthwise.
Credit: N. Hanacek/NIST
Imagine pulling on the long ends of a rectangular piece of rubber.
It should become narrower and thinner.
But what if, instead, it got wider and fatter?
Now, push in on those same ends. What if the rubber became narrower and thinner?
Such common-sense-defying materials do exist. They’re called auxetics, and they have a raft of unique properties that make them well-suited for sneaker insoles, bomb-resilient buildings, car bumpers and clothing.
Despite this great potential, auxetic products have been slow to market. Researchers at the National Institute of Standards and Technology (NIST) and the University of Chicago hope to change this.
In a new study published in NPJ Computational Materials, they announced they’ve developed a new tool that makes designing materials with auxetic properties easier and faster. An algorithm, the tool enables precise three-dimensional design of auxetics.
“It’s a huge advance for auxetics,” said NIST materials research engineer Edwin Chan, a study co-author. “We can actually optimize the material to have whatever particular mechanical properties and behavior that you want.”
The behavior of elastic materials is partially described by Poisson’s ratio, which explains how the material changes shape when you stretch or squeeze it in one direction.
Most materials have a positive Poisson’s ratio, which means squeezing them in one direction will make them wider and/or thicker in other directions. Stretching them makes them narrower and/or thinner.
Auxetics have a negative value of Poisson’s ratio and do exactly the reverse. ..
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