Rubber properties : Compressibility

Rubberlike material have very little compressibility compared to their shear flexibility. The numerical solution can be quite sensitive to the degree of compressibility for three-dimensional solid, plane strain, and axisymmetric analysis elements. In cases where the rubber material is highly confined (such as an rubber seals and rubber gaskets), the compressibility must be modeled correctly to obtain accurate results. In applications where the material is not highly confined, the degree of compressibility is typically not crucial; for example, it would be quite to assume that the rubber materials is fully incompressible : the volume of the material cannot change except for thermal expansion. Another class of rubberlike materials is elastomeric foams, which is elastic but very compressible.

We can assess the relative compressibility of a rubber material by the ratio of its initial bulk modulus, K, to its initial shear modulus, G. This ratio can also be expressed in terms of Poisson's ratio, , since

Table 1 represent some Poisson ratio

K/G	Poisson’s ratio
10	0.452
20	0.475
50	0.490
100	0.495
1,000	0.4995
10,000	0.49995

Since typical unfilled rubber material have K/G ratios in the range of 1,000 to 10,000 (Poisson’s ratio = 0.4995 to 0.49995) and filled rubber material have K/G ratios in the range of 50 to 200 (Poisson’s ratio = 0.490 to 0.497). If rubber material is modeled to imcompressible material it have K/G ratios in nearly infinity and Poisson’s ration = 0.5