Rigid Material

Add the node and select one or more geometrical objects to make them a rigid body. is a material model, with only one material property: the mass density. It can be used for

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Domains in the Solid Mechanics interface (3D, 2D, and 2D axisymmetry).

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Boundaries in the Shell interface.

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Boundaries (2D) and edges (3D) in the Beam interface.

By default, an node is added (see Initial Values (Rigid Material)).

You can add functionality to a rigid domain through the following subnodes:

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Fixed Constraint (Rigid Material) to fully constrain the rigid domain.

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Prescribed Displacement/Rotation to prescribe the displacement of individual degrees of freedom.

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

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Applied Moment (Rigid Material) to apply a moment.

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

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

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


	In 2D axisymmetric interfaces, the rigid domain has only a single degree of freedom: translation along the Z-axis. In this case it is not possible to give moment loads, mass moment of inertia, or rotational springs.

Shell Properties


	This section is only present when Rigid Material is used in the Layered Shell interface. See the documentation for the Rigid Material node in the Layered Shell chapter.

Density

The default ρ is taken . In this case the material assignment for the domain supplies the mass density. For enter another value or expression.

If any material in the model has a temperature dependent mass density, and is selected, the list will appear in the section. As a default, the value of Tref is obtained from a . You can also select to enter a value or expression for the reference temperature locally.


	The density is needed for dynamic analysis or when the elastic data is given in terms of wave speed. It is also used when computing mass forces for gravitational or rotating frame loads, and when computing mass properties (Computing Mass Properties).

See also

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Mass Density and Volume Reference Temperature.

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Using Common Model Input

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Default Model Inputs and Model Input in the COMSOL Multiphysics Reference Manual.

Center of Rotation

Select a — , , or . The center of rotation affects how displacements are interpreted, and is also used as the default in various subnodes.

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For , the center of rotation is taken as the center of mass of the rigid domain.

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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 rigid domain 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 , enter the , Xc, in the table.

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.


	This section is not available in 2D axisymmetric components.

Formulation

Some contributions from a rigid domain 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 to force all matrix contributions from the rigid domain and its subnodes to be symmetric.


	Using an approximative stiffness matrix will in general require more iterations. However, since the computational cost per iteration will be cut at least by a factor of two if a symmetric matrix can be used, it is usually more efficient to ignore a weak nonsymmetry. In particular, if the rotation of the rigid domain per time step or parameter increment is small, there will in general be no increase in the number of iterations at all if this option is used. When the global stiffness matrix is nonsymmetric for other reasons, then there is nothing to be gained from symmetrizing the contribution from the rigid domain.

Constraint Settings

When a rigid domain shares a boundary with 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 .

Advanced

Select — (default), , or , to choose how to group the dependent variables added by the feature in the solver sequence.


	Rigid Material


	Modeling Rigid Bodies: Application Library path Structural_Mechanics_Module/Connectors_and_Mechanisms/rigid_domain

Location in User Interface

Context Menus

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Physics tab with selected:

Physics tab with or selected:

Physics tab with selected: