The External Strain subnode allows you to provide inelastic strain contributions to the material models Linear Elastic Material, Nonlinear Elastic Material, and Hyperelastic Material on a variety of formats, including using external coded functions.

Select a Coordinate system. All inputs that you give below are interpreted in the given coordinate system. This also implies that external code should assume that strains are passed with its local orientations. Deformation gradients are rotated by the local system in both indices.

Select the type of Strain input — External material, Strain tensor, Deformation gradient, Deformation gradient, inverse, or Stretches. The option Strain tensor is not available for the Hyperelastic Material.

For External material, the computation of an additional inelastic strain contribution is delegated to external code which has been compiled into a shared library. External libraries must first be imported into an External Material node under Global Definitions>Materials.

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 Strain subnode, its Interface type must be set to a type whose required input quantities are all defined in this node. Allowed required inputs include Green-Lagrange strains, the deformation gradient, second Piola-Kirchoff stress as well as all standard model inputs. Select Inelastic residual quantity — Strain or Deformation gradient to define the type of quantity by which you want communicate with the external code.

For Strain tensor, enter an inelastic strain contribution εext. From the Strain tensor list, you can choose User defined, or any strain tensor which is announced by another physics interface. If you select User defined, enter values or expressions for the upper diagonal part of the symmetric strain tensor.

For Deformation gradient, enter an inelastic deformation gradient contribution Fext. From the External deformation gradient list, you can choose User defined, or any deformation gradient tensor which is announced by another physics interface. If you select User defined, enter values or expressions for the components of the deformation gradient tensor.

For Deformation gradient, inverse, enter an inelastic inverse deformation gradient contribution . From the External deformation gradient inverse list, you can choose User defined, or any inverse deformation gradient tensor which is announced by another physics interface. If you select User defined, enter values or expressions for the components of the inverse deformation gradient tensor.

For Stretches, enter values or expressions for the three principal stretches. Entering data on this form is convenient for some simple geometries and strain states, but in general it is difficult to provide suitable a coordinate system for the principal orientations.

The External Strain node is only available with some COMSOL products (see http://www.comsol.com/products/specifications/).

 Solid Mechanics>Linear Elastic Material>External Strain
 Solid Mechanics>Nonlinear Elastic Material>External Strain
 Solid Mechanics>Hyperelastic Material>External Strain

Physics tab with Linear Elastic Material, Nonlinear Elastic Material, or Hyperelastic Material node selected in the model builder tree:
Attributes>External Strain