Rigid Connector
The Rigid Connector is a boundary condition for modeling rigid regions and kinematic constraints such as prescribed rigid rotations. The selected points will move as a single rigid object.
You can add the Rigid Connector node at the edge (2D: boundary) and point levels.
If the study step is geometrically nonlinear, the rigid connector takes finite rotations into account.
Rigid connectors are available in the Solid Mechanics, Multibody Dynamics, Shell, Beam, and Pipe Mechanics interfaces. Rigid connectors from different interfaces can be attached to each other.
You can add functionality to the rigid connector through the following subnodes:
Applied Force (Rigid Connector) to apply a force in given point.
Mass and Moment of Inertia (Rigid Connector) to add extra mass and moment of inertia in a given point.
Spring Foundation (Rigid Connector) to add a translational or rotational spring or damper in a given point.
When physics symbols are shown, a rigid connector is represented by a symbol at the center of rotation, together with a set of lines connecting the center of rotation with the centers of gravity of the selected entities.
This section is present when the Rigid Connector node has been added at the edge level. Select one or more edges to be part of the rigid region.
This section is present when the Rigid Connector node has been added at the boundary level. Select one or more edges to be part of the rigid region.
Point Selection
This section is always present.
When the Rigid Connector is added at the point level, select a number of points that form the rigid region.
When the Rigid Connector is added at the edge (2D: boundary) level, 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.
Coordinate System Selection
The Global coordinate system is selected by default. The Coordinate system list contains all applicable coordinate systems in the model. Prescribed displacements or 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 Mass and Moment of Inertia subnode.
Center of Rotation
The center of rotation serves two purposes.
Select a Center of rotationAutomatic, Centroid of selected entities, or User defined.
For Automatic the center of rotation is at the geometrical center of the selected points. The constraints are applied at the center of rotation.
For Centroid of selected entities a subnode for selection of the points is added to the Model Builder.
For User defined, in the Global coordinates of center of rotation XC table enter coordinates based on space dimension.
Once Centroid of selected entities is chosen, a default Center of Rotation: Point subnode is added.
Select the Offset 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.
For 3D components: Prescribed in z direction u0z
Prescribed Rotation
Specify the rotation at the center of rotation. Select from the By list: Free (the default), Constrained rotation, or Prescribed rotation.
For 2D components, the Constrained rotation and Prescribed rotation is always about the z-axis, so no component selection is necessary.
Constrained Rotation (3D Components)
For Constrained rotation select one or more of the available check boxes to enforce zero rotation about the corresponding axis in the selected coordinate system:
Constrain rotation about x-axis
Constrain rotation about y-axis
Constrain rotation about z-axis
Prescribed Rotation
For Prescribed rotation enter an Angle of rotation . For 3D components also enter an Axis of rotation Ω for the x, y, and z coordinates.
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.
Released Degrees of Freedom
In some cases it can be useful to not constrain the displacement in a certain direction. To do so, select a local Coordinate system for specifying the directions in which the degree of freedom will be released. The Coordinate system list contains only applicable coordinate systems in the model.
Select one or more of the available check boxes to release the displacement in the corresponding axis in the selected coordinate system:
For 3D components: Release displacement in x3 direction
For 3D components: Release rotation around x1 direction
For 3D components: Release rotation around x2 direction
Note that the Rigid Connector solves for global displacement degrees of freedom (DOFs) and global rotational DOFs – unless they are explicitly prescribed. Since releasing certain displacement field components reduces the number of equations used to solve for the global DOFs, it may become necessary to constrain some global displacement or rotation components to achieve static determinacy.
The section Released Degrees of Freedom is only shown if the check box Use weak constraints for rigid-flexible connection in the section Constrain Settings is not enabled.
Reaction Force Settings
Select Evaluate reaction forces 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 Apply reaction only on rigid body variables 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.
Formulation
Some contributions from a rigid connector will, under geometric nonlinearity, result in a nonsymmetric local stiffness matrix. If all other aspects of the model are such that the global stiffness matrix would be symmetric, then such a nonsymmetric contribution may have a heavy impact on the total solution time and memory usage. In such cases, it is often more efficient to use an approximative local stiffness matrix that is symmetric.
Select Use symmetric formulation for geometric nonlinearity to force all matrix contributions from the rigid connector and its subnodes to be symmetric.
Constraint Settings
On the boundaries where the rigid connector is coupled to a flexible material, all nodes on such a 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 Use weak constraints for rigid-flexible connection.
This formulation cannot be combined with the Flexible formulation of the rigid connector, which in itself is a special form of weak constraint.
Constraint Settings
On the points where the rigid connector is coupled to a flexible material, all nodes 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 Use weak constraints for rigid-flexible connection.
Advanced
It is possible to couple rigid connectors to each other. In the Connect to list, you can select any rigid connector defined in the Solid Mechanics, Multibody Dynamics, or Shell interfaces as being rigidly connected to the current one.
Select Group dependent variables in solverFrom physics interface (default), Yes, or No, to choose how to group in the solver sequence the dependent variables added by the Rigid Connector feature.
Location in User Interface
Context Menus
Ribbon
Physics tab with Beam or Pipe Mechanics selected: