Models that are not adequately restrained can translate or rotate freely. In general, each component has 3 translational and 3 rotational rigid body modes. For an assembly, the 6 rigid body modes of every component should be prevented.
The following factors contribute to preventing rigid body modes:
- Displacement restraints
- Study properties (soft spring and inertia relief flags)
- Contact conditions
- Connectors
The soft spring option can be used as a preliminary tool to investigate stability. It is recommended NOT to use it in the final study. The inertia relief flag can be used in certain cases where the external loads are balanced.
Adequate Restraints for Solid Models
The deformation of a solid model is completely defined by 3 translations at each node. The rotations of a solid model are implicitly defined by the translations of the nodes. If you apply a fixed restraint to a solid, the 3 degrees of freedom (displacements) of each node belonging to the edge, vertex, or face are restrained.
Example
Consider a solid model of a cube:
- If you fix one vertex, the model is not stable because it can rotate about the fixed vertex.
- If you fix two vertices, the model is not stable because it can rotate about the line connecting the two vertices.
- If you fix an edge, in general a straight edge, the model is not stable because it can rotate about the fixed edge.
- If you restrain two orthogonal faces in the normal directions, the model is not stable because it can slide in the third direction.
- If you fix three vertices, the model is stable.
Fixing any number of vertices along a straight line is not adequate to stabilize a solid model.
- If you fix a face, the model is stable.
- If you fix an edge and a vertex, that is not part of the edge, the model is stable.
- If you restrain three orthogonal faces their normal directions, the model is stable.
For another example, consider a hollow cylinder:
- If you restrain a cylindrical face in the radial direction, the model is not stable because it can rotate and slide .
- If you restrain a cylindrical face in the tangential direction, the model is not stable because it can slide in the axial direction.
- If you restrain a cylindrical face in the tangential direction and a vertex in the axial direction, the model is stable.
- If you fix any face, the model is stable.
Adequate Restraints for Shells
The deformation of a shell model is completely defined by 3 translations and 3 rotations at each node. The Immovable and Fixed restraint conditions are different for shells. Immovable sets translations to zero but does not restrain rotations. Fixed sets all translations and rotations to zero.
Example
Consider a plate meshed with shell elements:
- If you make a vertex immovable, the model is not stable because it can rotate about this vertex.
- If you make an edge immovable, the model is not stable because it can rotate about this edge.
- If you fix an edge or more than one vertex, the model is stable.
It is NOT recommended to run shell models with one fixed vertex. Although fixing a vertex theoretically stabilizes a shell model, the numerical simulation may lead to incorrect results.