Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of poisson's ratio. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
About 0.3 for most metals, 0.35 to 0.4 for plastics, and approaching 0.5 for rubber. It describes why a bar gets thinner when pulled and why materials bulge when compressed.
It appears wherever a stress state has more than one axis — it is what converts a uniaxial tensile test into the shear modulus through G = E ÷ 2(1 + ν).
Common misuse
Ignoring it in a biaxial stress analysis. Assuming zero lateral contraction overestimates the stiffness of a constrained part.