# Stiffness¶

Type: | 4-Tensor, or section |
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Range: | [v_1, …, v_81] |

Default: | -/- |

Appearance: | 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: