Poisson's ratio
/ PWA-sonn /
Stretch a rubber band and watch it get not only longer but also thinner. Squeeze a block and it bulges out at the sides. Poisson's ratio measures this sideways reaction: how much a material contracts across its width when you stretch it along its length. Almost everything does it, because materials tend to keep their volume roughly constant while changing shape.
It is defined as nu = minus (lateral strain) / (axial strain): the width strain divided by the length strain, with a minus sign so the ratio comes out positive for normal materials. If a bar stretched 0.1 percent longways narrows by 0.03 percent sideways, then nu = 0.03 / 0.1 = 0.3. Most metals sit near 0.3, rubber approaches 0.5 (it barely changes volume at all), and cork is near 0 (it hardly bulges, which is why a cork pushes into a bottle neck without jamming). A value of exactly 0.5 means perfectly volume-conserving.
Poisson's ratio ties the elastic constants together: for an isotropic material E, the shear modulus G, and nu are linked by G = E / (2 x (1 + nu)), so knowing two gives the third. It matters wherever sideways effects count — a squeezed rubber seal pushing outward against a groove, or the bulging of a pressurised pipe. A curious footnote: a few engineered auxetic materials have a NEGATIVE Poisson's ratio and get FATTER when stretched, which is unusual but real.
Pull a wide rubber eraser lengthwise and its middle visibly narrows; a metal bar does the same but so slightly (nu about 0.3) you would need instruments to see the width shrink.
Poisson's ratio links stretching one way to thinning the other — near 0.3 for metals, near 0.5 for rubber.
A value of 0.5 means no volume change (rubber); it cannot exceed 0.5 for ordinary materials. Rare auxetic structures have a negative ratio and thicken when pulled.