| When a sample of material is stretched in one direction, it tends to get thinner in the other two directions. Poisson's ratio,
named after Simeon Poisson, is a measure of this tendency. Poisson's ratio is the ratio of the relative contraction strain,
or transverse strain (normal to the applied load), divided by the relative 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 ? 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.
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