External Stress-Strain Relation

The 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 feature under .


	See also External Material and Working with External Materials in the COMSOL Multiphysics Reference Manual.

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

Material

Select an from the list of compatible external materials added under . For a material to be compatible with this model node, its 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.


	The stress and strain measures that are used will depend on the kinematics. For example, in a total Lagrangian formulation, strains will be interpreted as Green–Lagrange strains, and output stresses are expected to be second Piola–Kirchhoff stresses.

Geometric Nonlinearity

The settings in this section control the overall kinematics, the definition of the strain decomposition, and the behavior of inelastic contributions, for the material.

Select a — (default), , or to set the kinematics of the deformation and the definition of strain. When is selected, the study step controls the kinematics and the strain definition.

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With the default , a total Lagrangian formulation for large strains is used when the check box is selected in the study step. If the check box is not selected, the formulation is geometrically linear, with a small strain formulation.

To have full control of the formulation, select either , or . When is selected, the physics will force the check box in all study steps.

When inelastic deformations are present, such as for plasticity, the elastic deformation can be obtained in two different ways: using additive decomposition of strains or using multiplicative decomposition of deformation gradients.

Select a — (default), , or to decide how the inelastic deformations are treated. This option is not available when the formulation is set to .

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When is selected, a multiplicative or additive decomposition is used with a total Lagrangian formulation, depending on the check box status in the study step.

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Select to force an additive decomposition of strains.

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Select to force a multiplicative decomposition of deformation gradients. This option is only visible if is set to .

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

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There are some cases when a small strain formulation could be useful for a certain domain, even though the study step is geometrically nonlinear. One such case is contact analysis, where the study step is always geometrically nonlinear, but where a geometrically linear formulation is sufficient for the material.

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When a multiplicative decomposition is used, the order of the subnodes matters. The inelastic deformations are assumed to occur in the same order as the subnodes appear in the model tree.


	See Lagrangian Formulation, Deformation Measures, and Inelastic Strain Contributions in the Structural Mechanics Theory chapter. See Modeling Geometric Nonlinearity in the Structural Mechanics Modeling chapter. See Study Settings in the COMSOL Multiphysics Reference Manual.

Transverse Shear Strains


	This section is only present in the Membrane interface and in 2D Plane Stress in the Solid Mechanics interface.

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 check box to store state variables also for the two transverse shear strains.

Location in User Interface

Context Menus

Ribbon

Physics tab with selected: