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

Oasys LS-DYNA Environment: User Guide (Version 8.1)
10.0 RESTRAINTS & CONSTRAINTS
This section describes the options for restraining nodes (clamping) or constraining them (tying
them to other nodes). Similar options are available for rigid bodies, but these are entered in a
different way and are described in Section 8.
RESTRAINTS
Restraints work by setting the accelerations of nodes to zero in the restrained directions.
The usual method is to use SPC Restraints (*BOUNDARY_SPC) with defined coordinate
systems (*DEFINE_COORDINATE_SYSTEM). The coordinate system can be aligned with the
global x,y and z axes, or not, as required.
Restraints in global x, y and z directions can also be entered on the node cards.
CONSTRAINTS
Any number of nodes can be tied together in any of the global x, y and z directions - see Section
3.31 of this manual. Constraints work by summing the forces on the constrained nodes, summing
their masses, and hence calculating the motion of the constrained set. This motion is then
imposed on all of the constrained nodes. There is no "master" node which the other nodes have
to follow - all the nodes in the set are treated equally and overall equilibrium of the constraint set
is preserved.
Another type of constraint is *CONSTRAINED_SPOTWELD. Two nodes (which must not be
coincident) are linked by a massless rigid bar. Failure forces can then be defined at which the
spotweld breaks and the nodes become free. If more than two sheets are connected, use
*CONTRAINED_GENERALISED_WELD_SPOT.
Other features which work by imposing motion on the nodes (such as rigid bodies, restraints, etc)
should not be applied to constrained nodes. In particular, it is wrong to define the same nodal
degree of freedom in more than one constraint set; both sets attempt to impose a different motion
on the same node. This does not cause the program to crash, but the supposedly constrained
nodes drift apart. Instead, all the nodes which are required to move together should be defined
in a single constraint set. The global x, y and z degrees of freedom are treated independently, so
it is possible to restrain a node in x and constrain the same node in y, etc.
It is not possible to constrain nodes in directions other than global x, y or z, nor to constrain
rotational degrees of freedom.
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