The External Stress-Strain Relation is a special type of material model where the computation of second Piola–Kirchhoff 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 https://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 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.

As a default, it is assumed that the transverse shear strains are zero. For a state of plane stress, this is true for an isotropic material, and for non-isotropic materials where the normal to the surface is a principal direction. If this is not the case, select the Transverse shear strains check box to store state variables also for the two transverse shear strains.

Physics tab with Solid Mechanics selected:

Physics tab with Membrane selected: