Elementary stiffness matrices
=================================

Tangent and secant stiffness matrices are available for this law. By default in code_aster, the tangent matrix is used. To use the secant matrix, or discharge matrix, you must add the keyword PAS_MINI_ELAS with a value greater than the interval between two calculation times in the keyword factor NEWTON for non-linear operators.

The secant matrix is calculated from the secant module in each main direction :math:`i`:


* :math:`{E}_{i}^{s}\mathrm{=}{\sigma }_{i}\mathrm{/}{\varepsilon }_{i}` if :math:`{\epsilon }_{i}\ne 0`


* :math:`{E}_{i}^{s}\mathrm{=}E` if :math:`{\epsilon }_{i}=0`