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Poisson's ratio

When a bar is stretched along its length, it contracts slightly across it. The ratio between the lateral contraction and the axial strain is Poisson's ratio ν\nu, a material constant. For most metals ν\nu is around 0.3.

ν=−εlatεaxial\nu = -\dfrac{\varepsilon_{lat}}{\varepsilon_{axial}}Poisson's ratio

Symbols

ν\nuPoisson's ratio
εlat\varepsilon_{lat}lateral strain
εaxial\varepsilon_{axial}axial strain

Example

Steel with εaxial=1\varepsilon_{axial} = 1 ‰ and ν=0.3\nu = 0.3:

εlat=−0.3⋅1=−0.3\varepsilon_{lat} = -0.3\cdot 1 = -0.3 ‰.

The minus sign in the definition makes ν\nu positive for ordinary materials that get thinner as they stretch.
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Part of Strength of Materials: Stress and strain.