Geometric Nonlinearity for the Solid Mechanics Interface
In the settings for each material model in the Solid Mechanics interface, you can select a Formulation — From study step (default), 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. In this situation, you can force a geometrically nonlinear analysis for a certain study step by selecting the Include geometric nonlinearity check box in the Study Settings section of the study step.
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 check box in all study steps.
If any active feature in the model requires the analysis to be geometrically nonlinear, the Include geometric nonlinearity 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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Contact, because the deformation between the contacting boundaries is part of the solution.
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Moving mesh (when at least one deforming domain is active).
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Hyperelastic materials, which are always formulated for large strains.
For geometrically nonlinear analysis, the behavior of several features differs from the geometrically linear case:
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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 Geometrically linear is selected in the Formulation 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 Strain decomposition 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 Geometrically linear, for which an additive strain decomposition is used.
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Pressure loads are interpreted as follower loads, so that the direction of the load as well as the loaded area depend on deformation.
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Rigid connectors take finite rotations into account.
Studies and Solvers in the COMSOL Multiphysics Reference Manual