Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
15.0 IMPOSED MOTION
Certain features in LS-DYNA work by imposing motion on nodes. If two different features
attempt to impose motions on the same node, problems will arise. Not all such clashes give rise
to error messages. The problem rarely causes the analysis to crash but almost always gives
wrong results. Examples of "imposed motion" features are:
*BOUNDARY all types, including all restraints and displacement, velocity and
acceleration boundary conditions
*CONSTRAINED all types including SPOTWELD but not JOINT
*CONTACT types 1, 2, 6, 7, 17 and 18 but not other types
Rigid bodies (i.e. elements made of parts which reference a *MAT_RIGID)
*INTERFACE
*RIGIDWALL
No node should have more than one of the above attached to it. For example, it is an error to
include the same node on two rigid bodies, or to attempt to impact a rigid body against a
stonewall (but note possibility of using RWPNAL on *CONTROL_CONTACT), or to include
a restrained node in a set of constrained nodes, or to spotweld onto a rigid body. However, the
x, y and z degrees of freedom are treated independently, and it is possible, for example, to
restrain a node in x and constrain the same node in y.
Some error checking is done for clashes between "imposed motion" features, but the checking
is not comprehensive. A typical symptom of the problem is that supposedly constrained nodes
drift apart.
Options are available for rigid bodies which overcome certain of these problems - see Section
7.
The most commonly used contact surface types (3, 13 and 5) do not impose motion and hence
do not suffer from the problems described above. They can be used with rigid bodies and can
be overlapped such that nodes are on more than one contact surface.
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