A bar of an isotropic, elastic material ordinarily shrinks laterally when it is stretched longitudinally. The lateral contracting strain divided by the applied tensile strain is Poisson's ratio, which varies from material to material.

Th e ratio of the strain in the transverse direction to the strain in the longitudinal direction.

The ratio of the strain in the traverse direction to the strain in the longitudinal direction.

for elastic deformation, the negative ratio of lateral and axial strains that result from an applied axial stress.

When a material is stretched, its cross-sectional area changes as well as its length. Poisson's Ratio is the constant relating these changes in dimensions, and is defined as the ratio of the change in width per unit width to the change in length per unit length.

When a sample of material is stretched in one axis, it tends to get thinner in the other two axes. Poisson's ratio (Î½) 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.

The ratio of transverse (lateral) strain to the corresponding axial (longitudinal) strain resulting from uniformly distributed axial stress below the proportional limit of the material; the value will average about 0.2 for concrete.

If a square bar is stressed in a testing machine in the direction of its length so that the length increases, there is a contraction in each opposite direction, which produces a decrease in the thickness of the bar. The ratio between the contraction at right angles to a stress and the direct extension is called the Poisson's ratio. Its value in steel is in the order of 0.28.

')(After Simeon Poisson 1781-1840) The ratio of the change in strain perpendicular to the direction of loading to the change in strain caused in the same direction. y / d (for to loading or unloading in the x-direction). For undrained loading of saturated soil, u = 0.5 For drained loading or unsaturated soil,´ = 0.2 - 0.5

In addition to the strain in the z direction, the applied stress causes strains in the x and y directions. The ratio of the deformation in the x direction to that in the z direction is known as Poisson's Ratio. = Lx L z

The ratio of the second principal strain 2 in the transverse direction to the principal strain 1 in the axial direction when a uniaxial tension or compression is applied.

the absolute value of the ratio between linear strain changes, perpendicular to and in the direction of a given uniaxial stress change.

When a sample of material is stretched in one direction, it tends to get thinner in the other two directions. Poisson's ratio (Î½, \mu), 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, or axial strain (in the direction of the applied load).