Poisson’s ratio mechanics Written and fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Nov. 20, 2025 •History Poisson’s ratio, in mechanics, a quantity describing how a material behaves when it is stretched or compressed. It is expressed as the negative of the ratio (ν) of the transverse strain (perpendicular to an applied stress, εtrans) on a solid to the axial strain (parallel to an applied stress, εaxial) on it: ν = - ε t r a n s ε a x i a l .Strain is the change in length of a...

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