FEM MaterialMechanicalNonlinear

Description
Adds nonlinear mechanical material model. Currently only plasticity with simple hardening is available.

Usage

 * 1) To define nonlinear mechanical material model do the following:
 * 2) * Add the first and select it.
 * 3) * Click the button or choose the  option from the menu.
 * 4) To edit an existing MaterialMechanicalNonlinear object:
 * 5) * Click it in the Tree view.
 * 6) * Select the material model (currently only simple hardening is available).
 * 7) * Define yield points (stress vs plastic strain). The first point must have a zero plastic strain specified.