# Dictionary:Poisson’s ratio

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{{#ifeq:Dictionary |Dictionary| The ratio of the fractional transverse contraction to the fractional longitudinal extension when a long isotropic rod is stretched.

Equivalently: the negative of the ratio of axial strain to radial strain in a long cylinder (with stress-free sides) of an isotropic medium.

Named for Siméon Poisson, French physicist.
Specifying Poisson’s ratio is equivalent to specifying the ratio of *V*_{S}/*V*_{P}, where *V*_{S}, and *V*_{P} are S- and P-wave velocities. Values ordinarily range from 0.5 (no shear strength, e.g., fluid) to 0 (but theoretically from 0.5 to –1.0; see Debski and Tarantola, 1995).^{[1]}

See *elastic constants* and Figure E-6.

## References

- ↑ Debski, W. and Tarantola, A., 1995, Information of elastic parameters obtained from the amplitudes of elastic waves: Geophysics, 60, 1426–1436.