Poisson’s ratio, put very simply, is the measure of how much the width or diameter of a material will change whenever it is pulled lengthwise. Or, in more technical terms, it is the measure of the change in lateral (transverse) strain over the change in linear (axial) strain. The simplest example of this is seen when pulling a rubber band. As the rubber band is stretched in one direction, its width gets noticeably thinner. That is evidence of a large Poisson’s ratio.
Since Poisson’s ratio is a measure of one strain divided by another strain, it is a unitless number. Very flexible, low-modulus materials, like rubber, typically have Poisson’s ratios as high as 0.5, whereas very stiff, high-modulus materials, like concrete, typically have Poisson’s ratios very close to 0. Some materials, however, even have Poisson’s ratios as low as -1, meaning that as they are pulled lengthwise, the width or diameter of the material will increase. The most relatable example of this is a Hoberman sphere, which can be found in any toy store.
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