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Poisson's ratio is a measure of the Poisson effect, that describes the expansion of a material in directions perpendicular to the direction of compression. The value of Poisson's ratio is the negative of the ratio of transverse strain to axial strain. In mathematical formula, the ratio is usually denoted by the Greek letter ν (nu).
For small values of these changes, ν is the amount of transversal expansion divided by the amount of axial compression. Most materials have Poisson's ratio values ranging between 0.0 and 0.5. Incompressible materials such as rubber, have a ratio near 0.5.