When a sample of material is stretched in one direction, it tends to get thinner in the other two directions. Poisson's ratio (ν, $\backslash mu$), named after Simeon Poisson, is a measure of this tendency. It is defined as the ratio of the contraction strain normal to the applied load divided by the extension strain in the direction of the applied 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 materials, mostly polymer foams, have a negative Poisson's ratio; if these auxetic materials are stretched in one direction, they become thicker in perpendicular directions.

A Poisson's ratio greater than 0.5 does not make sense because at a certain strain the material would reach zero volume, and any further strain would give the material "negative volume".

See also

• Young's modulus
• Coefficient of thermal expansion

External links

• Meaning of Poisson's ratio
• Negative Poisson's ratio materials
• More on negative Poisson's ratio materials (auxetic)
• Poisson's ratio

Continuum mechanics | Structural engineering | Physical quantity | Dimensionless numbers

Poissonzahl | Coeficiente de Poisson | Coefficient de Poisson | Coeficiente de Poisson | 푸아송 비 | Poisson-factor | ポアソン比 | Współczynnik Poissona | Poissonovo število | 蒲松比