The unconstrained bodies utility detects any rigid (or free) body
modes of bodies that are not adequately supported by fixtures, connectors, or interaction
conditions.
In a static study, open the Interaction Viewer
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 verifies 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.