Poisson's Ratio of Steel

Stretch a steel bar by 0.001 along its axis and it narrows by 0.0003 in both transverse directions. That ratio, 0.30, holds across essentially all carbon and alloy steels in the elastic range, and it is the third leg of the isotropic elastic triangle: any two of E, G and ν fix the third through G = E / [2(1 + ν)]. It also fixes the bulk modulus, K = E / [3(1 − 2ν)], which for steel comes to about 167 GPa.

The value is only elastic. Once steel yields, plastic flow conserves volume, and the effective ratio climbs toward 0.5 — a necking tensile specimen is behaving with ν ≈ 0.5, not 0.3. Poisson's ratio is also what makes thick-walled pressure vessels, shrink fits and biaxially loaded plates behave differently from their one-dimensional intuition: a plate stretched in one direction while restrained in the other develops transverse stress of νσ that a uniaxial calculation never sees, and strain-gauge rosette reduction is wrong without it.