# Stiffness¶

Type: 4-Tensor, or section [v_1, …, v_81] -/- optional

Defines stiffness tensor also called elasticity tensor. Units are pascals, , which corresponds to force per unit area.

This stiffness tensor relates the strain of a body to the stresses within the body. In the used linear form this relation is given by:

where is the initial strain of the body, i.e., the strain for which the body is free of stresses.

When the Poisson ratio (see PoissonRatio) and the Young modulus (see YoungModulus) are set, a stiffness tensor is implicitly defined by the following expression when not otherwise specified: