Rigid Connector

The is a boundary condition for modeling rigid regions and kinematic constraints such as prescribed rigid rotations. A rigid connector can connect an arbitrary combination of boundaries, edges, and points which all will move together as being attached to a virtual rigid object.

You can add the node at the boundary, edge, and point levels.

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When added at the boundary level, you can connect boundaries, edges (3D), and points as long as at least one boundary is selected. Selecting edges and points is optional.

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When added at the edge level (3D only), you can connect edges and points as long as at least one edge is selected. Selecting points is optional.

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When added at the point level, you can connect a set of points to each other.

When the selection consists of boundaries only, you can also choose to remove the assumption of rigidity, while still respecting force and moment equilibrium. With this formulation, it is possible to avoid artificial constraint effects at the connected boundaries.

If the study step is geometrically nonlinear, the rigid connector takes finite rotations into account.

You can add functionality to the rigid connector through the following subnodes:

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Applied Force (Rigid Connector) to apply a force in given point.

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Applied Moment (Rigid Connector) to apply a moment.

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Mass and Moment of Inertia (Rigid Connector) to add extra mass and moment of inertia in a given point.

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Spring Foundation (Rigid Connector) to add a translational or rotational spring or damper in a given point.

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

Shell Properties


	This section is only present in the in the Layered Shell interface, where it is described in the documentation for the Rigid Connector node.

Interface Selection


	This section is only present in the in the Layered Shell interface, where it is described in the documentation for the Rigid Connector, Interface node.

Boundary Selection

This section is present when the node has been added at the boundary level. Select one or more boundaries to be part of the rigid region.

Edge Selection

This section is present in 3D when the node has been added at the boundary or edge level, and is .

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When the is added at the edge level, select one or more edges that form part of the rigid region.

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When the is added at the boundary level, this section is initially collapsed. Here, you can add optional edges to the rigid region. The edges cannot be adjacent to the selected boundaries.

Point Selection

This section is always present when is .

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When the is added at the point level, select a number of points that form the rigid region.

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When the is added at the boundary or edge levels, this section is initially collapsed. Here, you can add optional points to the rigid region. The points cannot be adjacent to the selected boundaries or edges.

If your selection is too small, you may introduce rigid body motions in the model. No exact rules can be given because there are many possible configurations where, for example, domains are connected to each other or affected by other constraints. Also, you can, by providing proper constraints for the rigid connector, suppress rigid body motions.

Some suggestion for ensuring stability are

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At the point level in 3D, at least three points, not located on a straight line, are needed.

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At the point level in 2D, at least two points are needed.

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At the edge level in 3D, a single straight edge is not sufficient since it can act as a hinge.

Pair Selection

If this node is selected from the menu, choose the pair to use. An identity pair has to be created first. The rigid connector applies to the common part of the boundaries, and makes the parts behave as if there were an infinitely stiff layer between them.

Coordinate System Selection

The is selected by default. The list contains any applicable coordinate systems that the model includes. Prescribed displacements and rotations are specified along the axes of this coordinate system. It is also used for defining the axis directions of the moment of inertia tensor of the subnode.

Connection Type

Select or . When the connection type is rigid, the whole rigid connector acts as a virtual rigid object. In the flexible formulation, the selected boundaries are allowed to have internal deformations, and the kinematic constraints are fulfilled only in an average sense. The flexible formulation is useful for example when applying loads, since it will reduce local constraint effects.

If a selected boundary is located on a rigid domain, the connection type setting does not matter. The rigid formulation is always used,

The flexible formulation can be used when the selection only consists of boundaries.

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The section is shown only when the has been added at the boundary level.

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When the connection type has been set to , the and sections are hidden.

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Any edges or points that have been selected prior to changing from to will be ignored.


	With the flexible formulation, some extra degrees of freedom are added for each rigid connector. In order to get good convergence properties for a nonlinear solution, these variables must have a good scaling. When a new solver sequence is generated, then an appropriate manual scaling is automatically set for these variables. If, however, an existing solver sequence was generated while Connection Type was set to Rigid, and you then change to Flexible, no such scaling will be present. In this case, you either have to regenerate the solver sequence, or set the scaling manually under the Dependent Variables node in each study step.

Center of Rotation

The center of rotation serves two purposes.

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If you prescribe the displacement of the rigid connector, this is the location in space where it is fixed.

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Results are interpreted with respect to the center of rotation.

Select a — , , or .

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For the center of rotation is at the geometrical center of the selected geometrical objects of the highest geometrical dimension.

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For select an — , , or . The available choices depend on physics interface and geometrical dimension. The center of rotation is located at the centroid of the selected entities, which do not need to be related to the rigid connector itself. As a special case, you can select a single point, and thus use that point as center of rotation.


	Once chosen, a default Center of Rotation: Boundary, Center of Rotation: Edge, or Center of Rotation: Point subnode is automatically added.

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For , in the XC table enter coordinates based on space dimension.

Select the check box to add an optional offset vector to the definition of the center of rotation. Enter values for the offset vector Xoffset.

The center of rotation used is the sum of the vector obtained from any of the input methods and the offset vector.

Prescribed Displacement at Center of Rotation

To define a prescribed displacement at the center of rotation for each space direction, select one or several of the available check boxes then enter values or expressions for the prescribed displacements. The direction coordinate names can vary depending on the selected coordinate system.

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u0x

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u0y

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For 3D components: u0z

Prescribed Rotation at Center of Rotation

Specify the rotation at the center of rotation. Select from the list: , , or .


	For 2D components, the Constrained rotation and Prescribed rotation at center of rotation is always about the z-axis, so no component selection is necessary.

Constrained Rotation (3D Components)

For select one or more of the available check boxes to enforce zero rotation about the corresponding axis in the selected coordinate system:

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Prescribed Rotation at Center of Rotation

For enter an

. For 3D components also enter an Ω for the , , and coordinates.

Reaction Force Settings

Select to compute the reaction force caused by a prescribed motion. The default is to not compute the reaction force. When selected, the prescribed motion is implemented as a weak constraint.

Select to use a unidirectional constraint for enforcing a prescribed motion. The default is that bidirectional constraints are used. This setting is useful in a situation where a bidirectional constraint would give an unwanted coupling in the equations. This would happen if the prescribed value of the motion is a variable solved for in other equations.

Constraint Settings

On the boundaries where the rigid connector are coupled to a flexible material, all nodes on that boundary are constrained to move as a rigid body. As a default these constraints are implemented as pointwise constraints. If you want to use a weak constraint formulation, select .

This formulation cannot be combined with the formulation of the rigid connector, which in itself is a special form of weak constraint.

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You can add a Harmonic Perturbation subnode for specifying a harmonic variation of the values of the prescribed displacements and rotations in a frequency domain analysis of perturbation type.

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You can activate and deactivate the rigid connector by assigning it to a constraint group. See Load Cases in the Structural Mechanics Modeling chapter.

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You can assign the value of the prescribed displacement and rotation to a load group. See Load Cases in the Structural Mechanics Modeling chapter.

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Rigid Connector Theory

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Harmonic Perturbation

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Load Cases


	Assembly with a Hinge: Application Library path Structural_Mechanics_Module/Connectors_and_Mechanisms/hinge_assembly

Location in User Interface

Context Menus

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Physics tab with selected: