Prescribed Displacement (Thin Layer)

The node adds a condition where the displacements are prescribed in one or more directions to the boundary.

If a displacement is prescribed in only one direction, this leaves the thin layer free to deform in the other directions.

You can also define more general displacements as a linear combination of the displacements in each direction, see Prescribed Displacement.


	If a zero displacement is applied in all directions, this is the same as a Fixed Constraint (Thin Layer).


	For details, see Prescribed Displacements, Velocities, and Accelerations.

Coordinate System Selection

The is selected by default. The list contains all applicable coordinate systems in the model. Prescribed displacements and rotations are specified along the axes of this coordinate system.

Prescribed Displacement

For the displacement in each direction, select a setting from the list — , , or . Select:

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(the default) to leave the displacement component unconstrained.

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to constrain the displacement component to a given value. Enter a scalar value for the component of the prescribed displacement u0.

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to set a maximum and a minimum limit for the displacement component. Enter values for the maximum displacement u0,max and the minimum displacement u0,min. By default, they are set to Inf and -Inf, which corresponds to no active constraint.

If any displacement component is set to , an additional section is visible. Select the used to implement the weak inequality constraint — or . For both methods, enter a kp.

By default, the method is suggested, which in principle enforces the maximum and minimum limits for the displacement by adding nonlinear springs with a stiffness equal to kp when the limits are exceeded. This method is usually robust, but the accuracy is directly dependent on the chosen penalty factor.

The method adds extra degrees of freedom to improve the accuracy of the constraint. Here, the penalty factor is a numerical parameter, and has less impact on the accuracy of the constraint compared to when using the penalty method. The implementation of the augmented Lagrangian method puts no restrictions on the solver sequence, but for good convergence, proper scaling of the extra degrees of freedom can be important.

The default value for the kp on what type of entity the node is added. In the expressions below, <phys> is the tag of the physics interface, and <pd> is the tag of the node.

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For edges, the default expression is 100*<phys>.Eequ

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For boundaries, the default expression is 100*<phys>.Eequ/<phys>.<pd>.charLen

The variable <phys>.<pd>.charLen is by default equal to the length of the mesh element.


	For details about setting maximum and minimum limits for the displacements, see Limited Displacement


	In versions prior to 6.2, you could chose to formulate the constraint in two ways: standard notation or general notation. Since it was rarely used, the general notation option is no longer supported. There are more natural way of specifying identical constraints. If you have a model created in version 6.1 or earlier, where general notation was used, the settings for this option are visible, and are still functional. You should, however, consider to reformulate the constraints, since the general notation may no longer be supported in future versions. For reference, the documentation for the general notation is provided below: You can specify the displacements using a general notation that includes any linear combination of displacement components. For example, for 2D components, use the relationship For H matrix H select Isotropic, Diagonal, Symmetric, or Full and then enter values as needed in the field or matrix. Enter values or expressions for the R vector R. For example, to achieve the condition u = v, use the settings , which force the domain to move only diagonally in the xy-plane.

Constraint Settings

To display this section, click the button (

) and select in the dialog box. For more information about this section, see Constraint Settings in the COMSOL Multiphysics Reference Manual.

Excluded Points

To display these sections, click the button (

) and select in the dialog box. For more information about these sections, see Excluded Surfaces, Excluded Edges, and Excluded Points in the COMSOL Multiphysics Reference Manual.

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Modeling Thin Layers in the Structural Mechanics Modeling chapter.

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Thin Layers in the Structural Mechanics Theory chapter.

Location in User Interface

Context Menus

Ribbon

Physics tab with node selected in the model tree: