| A rod-like specimen subjected to uniaxial
tension will exhibit some shrinkage in the lateral direction for
most materials. The ratio of lateral strain and axial strain is
defined as Poisson's ratio n,
The Poisson ratio for most metals falls between 0.25 to 0.35.
Rubber has a Poisson ratio close to 0.5 and is therefore almost
incompressible. Theoretical materials with a Poisson ratio of
exactly 0.5 are truly incompressible, since the sum of all their
strains leads to a zero volume change. Cork, on the other hand, has
a Poisson ratio close to zero. This makes cork function well as a
bottle stopper, since an axially-loaded cork will not swell
laterally to resist bottle insertion.
The Poisson's ratio is bounded by two theoretical limits: it must
be greater
than -1, and less
than or equal to 0.5,
The proof
for this stems from the fact that E, G, and K
are all positive and mutually dependent. However, it is rare to
encounter engineering materials with negative Poisson ratios. Most
materials will fall in the range,
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