When a sample of material is stretched in one direction, it tends to get thinner in the other two directions. Poisson's ratio (ν) is a measure of this tendency. It is defined as the ratio of the strain in the direction of the applied load to the strain normal to the load. For a perfectly incompressible material, the Poisson's ratio would be exactly 0.5. Most practical engineering materials have ν between 0.0 and 0.5. Cork is close to 0.0, most steels are around 0.3, and rubber is almost 0.5. Some polymer foams have a negative Poisson's ratio; if they are stretched in one direction, they become thicker in perpendicular directions.

Poisson's ratio is named for Simeon Poisson.

External links

• Meaning of Poisson's ratio (http://silver.neep.wisc.edu/~lakes/PoissonIntro.html)
• Negative Poisson's ratio materials (http://silver.neep.wisc.edu/~lakes/Poisson.html)
• Poisson's ratio (http://www.webelements.com/webelements/properties/text/definitions/poissons-ratio.html)

de:Poissonzahl ja:ポアソン比 sl:Poissonovo število pl:Wsp�łczynnik Poissona