What does Poisson’s Ratio mean in material science ?
Poisson’s ratio is defined as the negative sign of the ratio of lateral stress to axial stress for a uniaxial strain state. If a tensile load is applied to a material, the material will elongate in the direction in which the load is applied (perpendicular to the stress plane). Under compression load the axial dimension will decrease. If the volume is constant, there must be a lateral contraction or expansion corresponding to the strain. This lateral change should show a constant relationship with the axial change. The described relation of axial-lateral shape change is called the Poisson ratio, as the name of its discoverer.
The Poisson’s ratio is sometimes expressed as the ratio of the absolute values of axial and lateral strains. Since both strain values are unitless, the Poisson ratio is also unitless. For stresses in the elastic range, this ratio is approximately constant. Poisson’s ratio is 0.25 for a perfectly isotropic elastic material. However, for most materials, this value is in the range of 0.28-0.33. The Poisson’s ratio for steels is about 0.3. This expression means that if there is a 1 mm deformation in the direction where the force is applied, 0.3 mm deformation will occur on the side perpendicular to the direction of force application.
Rubber has a Poisson’s ratio close to 0.5 and is therefore almost incompressible. Theoretical materials with a Poisson’s ratio of exactly 0.5 are truly incompressible since the sum of all strains causes zero volume change. Cork, on the other hand, has a Poisson’s ratio close to zero. This makes the cork useful as a bottle stopper. A cork under axial load does not swell laterally to resist its placement in the bottle cap.