poisson’s ratio
TRANSCRIPT
It’s obvious that when an element is stretched linearly, it has a tendency to get thinner in the other two directions. In simple words, the change in longitudinal and lateral strains are opposite generally opposite to each other.
Well, what is Poisson’s ratio? Denoted as ν, Poisson’s ratio is named after Simeon Poisson, which is a measure of the tendency of both the strains acting opposite to each other. It is defined as the ratio of the contraction strain normal to the applied load divided by the extension strain in the direction of the applied load. As most common materials become thinner in cross section on being stretched, Poisson’s ratio has a positive value for them.
For a perfectly incompressible material, the Poisson’s ratio is exactly 0.5. Most basic engineering materials have ν in the range between 0.0 and 0.5. Cork is around 0.0, most steels are about 0.3, and rubber is approximately 0.5. A Poisson’s ratio more than 0.5 can’t be managed for large amounts of strain because at a certain strain the material would reach zero volume, and any further strain would give the material a negative volume.
Some materials, say the polymer foams, have a negative Poisson’s ratio; if these materials are extended in one direction, they become thicker in perpendicular directions. Foams with negative Poisson’s ratios were made from conventional low-density open-cell polymer foams by causing the ribs of each cell to permanently extend inward, ensuing in a re-entrant structure.
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An example of the practical application of a particular value of Poisson’s ratio is the cork of a beer bottle. The cork should be easily inserted and removed, moreover, it also must withstand the pressure from within the bottle. Rubber, with a Poisson’s ratio of 0.5, can’t be used for the same purpose because it would expand when compressed into the neck of the bottle and would jam the mouth. Cork, which has a Poisson’s ratio of nearly zero, is ideal for this application.
It is supposed that re-entrant foams can be used in such utilities as sponges,robust shock absorbing material, air filters, and fasteners. Negative Poisson’s ratio materials can undergo slow decay of stress according to Saint-Venant’s principle. These materials are called anti-rubber, auxetics or dilatational and are an example of extreme materials.
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