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Auxetic Materials exhibit the unusual property of getting fatter when stretched and narrower when compressed (Fig. 1). This unusual property can be described mathematically in terms of the Poisson’s ratio which gives a measure of the changes in shape that accompany uniaxial loading of materials. 

More specifically, for loading in the y-direction, the Poisson’s ratio nyx  is given by:

This ratio is normally positive since stretching in the y-direction (+ve strain) will normally be accompanied by a shrinkage in the transverse x-direction (-ve strain). However, in auxetic materials, this ratio is negative since stretching in the y-direction (+ve strain) will normally be accompanied by an expansion the transverse x-direction (+ve strain).

Auxetic Materials Research Group
c/o Prof. Joseph N. Grima
Dept. of Chemistry, University of Malta, Msida, MSD 06, MALTA
Tel: +356 2340 2274 / 5  or +356 3240 2550 -  Fax: +356 2540 1091
e-mail: auxetic@um.edu.mt - www: http://home.um.edu.mt/auxetic