When a material is compressed in one direction, it tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect. Poisson's ratio is a measure of the Poisson effect.

Poisson’s ratio bounded by two theoretical limits: -1 < ≤ 0.5. In 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 this range: 0 < ≤ 0.5. For perfect incompressible material we have theoretically . However, we suggest =0.495 in order to avoid too small time steps.