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Detection of Rigid Body Modes

The unconstrained bodies utility detects any rigid (or free) body modes of bodies that are not adequately supported by fixtures, connectors, or contact conditions.

In a static study, open the Contact Visualization Plot PropertyManager, and click the Unconstrained bodies tab.

It is recommended to define realistic materials, loads, and boundary conditions on your model, before you run the unconstrained bodies utility. The study properties should reflect, as accurate as possible, the operating loads and boundary conditions of the model you are trying to analyze.

The unconstrained bodies utility can detect local singularities (elements with zero or almost zero stiffness) in the decomposed global stiffness matrix that could be indicative of a rigid body motion. The algorithm is able to detect situations where the contrast between neighboring stiffness terms in the global stiffness matrix is legitimately very large and does not necessarily lead to singularities. Such models can include:
  • Parts with tapered geometry where the stiffness changes significantly across the part’s profile.
  • Assemblies with parts that have large discrepancies in their material stiffness properties.
  • Parts with localized extreme rigidities at locations where connectors or remote loads are applied.
  • Thin parts modeled as shells that have inherently very small rotational stiffness.
The above cases are not treated as singularity sources for the global stiffness matrix by the unconstrained bodies utility, if the parts are sufficiently constrained.

For each part of an assembly model, the algorithm checks the existence of free translations and rotations in the global X, Y, and Z direction. It is also able to detect instability issues in assemblies with chain (or hinge) mechanisms between parts. In cases where it detects free body modes, the program animates them accordingly by adjusting the corresponding stiffness and force in that direction to produce a bounded response that reflects a rigid (or free body) motion.

It is recommended to stabilize the detected free body modes of the parts with the appropriate translational or rotational restraints, before you proceed with the analysis.

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