The External Stress-Strain Relation is a special type of material model where the computation of second Piola-Kirchoff stress is delegated to external code which has been compiled into a shared library. External libraries must first be imported into an
External Material feature under
Global Definitions>Materials.
The External Stress-Strain Relation node is only available with some COMSOL products (see
http://www.comsol.com/products/specifications/).
Select an External material from the list of compatible External Materials added under
Global Definitions>Materials. For a material to be compatible with this External Material material model node, its
Interface type must be set to a type whose required input quantities are all defined by this External Material. Allowed required inputs include Green-Lagrange strains, the deformation gradient, as well as all standard model inputs.
The use of an External Stress-Strain Relation will always force the study to be geometrically nonlinear. The default behavior is then to use a large strain formulation in all domains. There are, however, some cases when the use of a small strain formulation for a certain domain is needed. This will for example be the case if the material model you have implemented is formulated using engineering strains.
In such cases, select the Force linear strains check box. When selected, a small strain formulation is always used, independently of the setting in the study step.
