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Poisson's ratio
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Lateral to longitudinal strain.
ν = −ε_lat/ε_longWhen stretched, a material thins sideways by ν times the axial strain. ν ≈ 0.3 for most metals, 0.5 for rubber (incompressible).
A rod 1% longer becomes 0.3% thinner.
Cork has ν ≈ 0, so wine corks fit bottles.
In more detail
Poisson's ratio is the ratio of transverse strain to axial strain when a material is stretched. Most materials have values between 0 and 0.5, and rubber is close to 0.5.
Real-world uses
- Engineers use Poisson's ratio to predict how structures bulge or thin under load.
- Rubber seals rely on its near-incompressible behaviour.
- Stretching a rubber sheet makes it thinner across its width, and Poisson's ratio tells how much it narrows.
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Frequently asked questions
Why is it negative for some materials?
Auxetic materials expand sideways when stretched, giving a negative Poisson's ratio.
What is the value for cork?
Close to zero, which is why cork can be pushed into bottles without bulging.
What is a typical Poisson's ratio for metals?
Often around 0.3, meaning a 10 percent stretch narrows the metal by about 3 percent across its width.
Related topics in Fluids & Elasticity
Bulk modulus · Shear modulus · Stress–strain curve · Elastic potential energy density · Bending of beams · Torsion of a cylinder