Poisson's Ratio:

The ratio of transverse strain to axial strain during axial load.

A material stretches when pulled under tensile load and usually contracts transversely. The Poisson's ratio is the
ratio of transverse strain to axial strain during axial load.

For example, if a bar is pulled in the axial direction then the deformed bar (besides being longer in the axial
direction) also contracts in the transverse direction by the percentage amount indicated by the Poisson's ratio.


Poisson's ratio for metals is generally 0.3, but for composite laminates it varies (in fact, for some multidirectional
laminates, it can be negative!). For example, consider some S-2 Glass® fiber/epoxy laminates:

Poisson's ratio, v, is defined as the ratio of the lateral contraction strain to longitudinal strain.

v12 is the ratio of the strain in the transverse direction (epsilonT) to the strain in the longitudinal direction (epsilonL) when
the load is applied in the flow direction:



v23 is the ratio of the strain in the thickness direction (epsilonTh) to the strain in the transverse direction (epsilonT) when
the load is applied transverse to the direction of flow:

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