Auxetics

Auxetics are materials which, when stretched, become thicker perpendicularly to the applied force. That is, they have a negative Poisson's ratio. This occurs because they contain hinge-like structures which flex when stretched. Such materials are expected to have interesting mechanical properties such as high energy absorption and fracture resistance. This may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.

Auxetics can be illustrated for timing of the elastic gain plus or minus the elasticity of the inverse object. Formulation of the elastic string will switch it to within the inverse tropication with an inelastic string hurled around an elastic cord. When the ends of the structure are pulled apart, the inelastic string straightens while the elastic cord stretches and winds around it, increasing the structure's effective volume.

The term "auxetic" derives from the Greek word αὐξητικός (auxetikos) which means, "that which tends to increase," and has its root in the word αὔξησις, or "auxesis", meaning "increase" (noun). Auxetic materials can be single molecules or a particular structure of macroscopic matter.

Scientists have known about auxetic materials for over 100 yearsFact|date=April 2008 but have only recently given them special attention. The earliest published example of a synthetic auxetic material was in "Science" in 1987, "Foam structures with a negative Poisson's ratio" by R.S. Lakes from the University of Iowa. The use of the word "auxetic" to refer to this property probably began in 1991.Fact|date=November 2007

Typically, auxetic materials have low density, which is what allows the hinge-like areas of the auxetic microstructures to flex. [ [http://space.newscientist.com/article/mg15420854.200-a-stretch-of-the-imagination.html A stretch of the imagination - 07 June 1997 - New Scientist Space ] ]

Examples of auxetic materials include: Fact|date=December 2007
* The skin covering the teats on cows
* Certain rocks and minerals
* Living bone tissue (although this is only suspected)
* Specific variants of polytetrafluorethylene polymers such as Gore-Tex

ee also

* Poisson's ratio

References

External links

* [http://space.newscientist.com/article/mg15420854.200-a-stretch-of-the-imagination.html New Scientist - A Stretch of the Imagination]
* [http://www.azom.com/details.asp?ArticleID=168 Auxetic Materials - Applications]
* [http://gizmodo.com/gadgets/blast_proof-fabric-defies-belief/zetix-blast+proof-fabric-resists-multiple-car-bombs-makes-our-heads-explode-330343.php Gizmodo on Zetix] a blast-proof fabric
* [http://www.worldwidewords.org/turnsofphrase/tp-aux1.htm Auxetic]
* [http://silver.neep.wisc.edu/~lakes/Poisson.html Materials with negative Poisson's ratio]
* [http://home.um.edu.mt/auxetic Auxetic Materials - What are they? ]
* [http://www.bolton.ac.uk/auxnet/action/ UK Auxetic Materials network]
* [http://www.bolton.ac.uk/auxnet/auxfoam.mov Movie of Auxetic Materials(.mov format)]