Relationship Between Moduli
For all isotropic polymeric materials where Hook's law is valid, simple relationships exist between the elastic constants such as Young's modulus E, shear modulus G, bulk modulus B, and Poisson's ratio ν) As long as two of them are known, all others can be predicted. There are many different elastic constants from which we can choose. Often the relation between two moduli is expressed as a function of the Poisson ratio:
and for the Poisson's ratio
From these relations it follows that −1 < ν < 1/2 are the classical bounds to the Poisson’s ratio. However, almost all classical materials lie within 1/5 < ν < 1/2.1 For isotropic weakly compressible materials such as liquids and rubbers, the Poisson's ratio approaches the upper bound ν = 1/2. In that case the elastic tensile modulus is three time the shear modulus and the bulk modulus is much larger than the Young's modulus,
E ≈ 3 G and B »E
The Poisson's ratio of rubber is never exact 0.5 because this means the elastic bulk modulus would go to infinity which is not possible. However, values very close to 1/2 have been reported (0.4999)1. In the case of quasi-isotropic polycrystalline polymers and other rather stiff isotropic polymers like polystyrene (polymers with bulky side groups), the Poisson's ratio reaches a value of about ν ≈ 1/3.
E ≈ 8/3 G and E ≈ B
For most engineering plastics, ν is in the range 0.35 < ν < 0.45 (see table of Poisson's ratio's).
For a (hypothetical) material with no lateral contraction, ν = 0, the elastic tensile modulus is about two times the shear modulus and three times the bulk modulus:
E ≈ 2 G = 3 B
However, most common plastics have much larger Poisson's ratios, ν > 0.3.
P. H. Mott and C. M. Roland, Physical Review B 80, 132104 (2009).