The External Stress–Strain Relation is a special type of material model where the computation of 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/).

The Global coordinate system is selected by default. The Coordinate system list contains all applicable coordinate systems in the component. The coordinate system is used for interpreting directions of orthotropic and anisotropic material data and when stresses or strains are presented in a local system. The coordinate system must have orthonormal coordinate axes, and be defined in the material frame. Many of the possible subnodes inherit the coordinate system settings.

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 strain, the deformation gradient, as well as all standard model inputs.

Select a Formulation — From study step, Total Lagrangian, or Geometrically linear to set the kinematics of the deformation and the definition of strain. When From study step is selected, the study step controls the kinematics and the strain definition.

When From study step is selected, a total Lagrangian formulation for large strains is used when the Include geometric nonlinearity checkbox is selected in the study step. If the checkbox is not selected, the formulation is geometrically linear, with a small strain formulation.

To have full control of the formulation, select either Total Lagrangian, or Geometrically linear. When Total Lagrangian is selected, the physics will force the Include geometric nonlinearity checkbox in all study steps.

Select a Strain decomposition — Automatic, Additive, or Multiplicative to decide how the inelastic deformations are treated. This option is not available when the formulation is set to Geometrically linear.

The Strain decomposition input is only visible for material models that support both additive and multiplicative decomposition of the deformation gradient.

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 nonisotropic materials where the normal to the surface is a principal direction. If this is not the case, select the Transverse shear strains checkbox to store state variables also for the two transverse shear strains.

Select the Reduced integration checkbox to reduce the integration points for the weak contribution of the feature. Select a method for Hourglass stabilization — Automatic, Manual, or None to use in combination with the reduced integration scheme. The default Automatic stabilization technique is based on the shape function and shape order of the displacement field.

Control the hourglass stabilization scheme by using the Manual option. Select Shear stabilization (default) or Volumetric stabilization.

When Shear stabilization is selected, enter a stabilization shear modulus, Gstb. The value should be in the order of magnitude of the equivalent shear modulus.

When Volumetric stabilization is selected, enter a stabilization bulk modulus, Kstb. The value should be in the order of magnitude of the equivalent bulk modulus.

Select how to define Equivalent elastic constants — Automatic, or User defined. When Automatic is selected, the equivalent elastic constants are computed from the output of the external material. In some cases this may lead to unexpected results or expensive evaluations when used in for example contact conditions. It is possible to instead manually define the equivalent elastic constants by selecting the User defined option. When selected enter values for the equivalent bulk modulus Keq and the equivalent shear modulus Geq.

Physics tab with Solid Mechanics selected:

Physics tab with Membrane selected: