Poisson's Ratio

Poisson's ratio σ_p (often ν) measures lateral contraction during axial stretching: σ_p = −(lateral strain)/(longitudinal strain).

When you stretch a rod axially, it gets THINNER laterally — both effects related by σ_p.

Range: 0 ≤ σ_p ≤ 0.5 for isotropic materials. Most metals: 0.25-0.35. Cork: ~0. Rubber: ~0.5 (essentially incompressible).

σ_p = 0.5: material is incompressible (volume unchanged when stretched). Rubber, soft tissues approach this.

σ_p = 0: no lateral contraction at all. Cork has ~0.04 — useful for bottle stoppers.

Some special materials (auxetics) have NEGATIVE Poisson's ratio — they expand laterally when stretched.

Links Young's modulus, bulk modulus, and shear modulus: B = Y/[3(1−2σ_p)], G = Y/[2(1+σ_p)].

Dimensionless quantity — pure number.