Geometric Nonlinearity for the Solid Mechanics Interface

In the settings for each material model in the Solid Mechanics interface, you can 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. In this situation, you can force a geometrically nonlinear analysis for a certain study step by selecting the check box in the section of the study step.

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

If any active feature in the model requires the analysis to be geometrically nonlinear, the check box in the study step is selected automatically, and it cannot be cleared. The following physics features force this behavior:

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Moving mesh (when at least one deforming domain is active).

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There is an important difference between using uppercase (X, Y, Z, R) and lowercase (x, y, z, r) coordinates in expressions. The lowercase coordinates represent the deformed position, and this introduces a dependency on the solution.

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Many features, such as coupling operators, can be specified as operating in either the material (X, Y, Z) or the spatial (x, y, z) frame. This setting does not make a difference unless a geometrically nonlinear analysis is performed. In most cases you would want to do the operation in the material frame.

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The strain representation changes from using engineering strains to Green–Lagrange strains, unless is selected in the list of a certain material.

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Where Green–Lagrange strains are used, Second Piola–Kirchhoff stresses are also used. This means that material data must be given in terms of these quantities. This distinction is important only when the strains are actually large.

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For each material model, the list determines how inelastic deformations are treated. It is possible to choose an additive decomposition of strains or a multiplicative decomposition of deformation gradients. This option is not available when the formulation is set to , for which an additive strain decomposition is used.