Poisson's Ratio
The ratio of transverse lateral strain to axial longitudinal strain in a material subjected to uniaxial stress.
Governing formula
ν = -ε_transverse / ε_axial = -(Δd / d₀) / (ΔL / L₀)
SI unit
Dimensionless Ratio (-1 to 0.5)
In depth
When stretched longitudinally, most materials expand or contract laterally. Incompressible materials have ν = 0.5; typical metals have ν ~ 0.3. Auxetic metamaterials possess negative Poisson's ratio (ν < 0), expanding laterally when stretched.
Examples in the real world
Rubber exhibiting a Poisson's ratio close to 0.5 (nearly incompressible volume), while cork has ν ≈ 0.