Issue with Prescribed Motion on Rigid Body Master Node in LS-DYNA
Sabin Acharya
I just wanted to make two master nodes at the end of my model, and wanted to slave all the circumferential nodes to that master node. Moreover, I wanted to apply unit displacement at the first/leftmost master node in the Y-direction and try to restrain the other DoF at that master node, and I also made all DoF constrained at the second/rightmost master node. And I need the reactions arising due to this unit displacement. For that, I have used different options, and finally, I think the relevant one would be *CONSTRAINED_Nodal_Rigid_Body_SPC* to make the master and slave node as well as to constrain the desired DoF. Within this command, my *NSID* is the node set of circumferential node, *PNODE* is the middle node, *CMO* 1.0, and I have applied *CON1* and *CON2* accordingly. And for unit displacement, I applied the *BOUNDARY_PRESCRIBED_MOTION_NODE* command and applied unit displacement at the middle/leftmost node in Y-translation (DoF=2) with VAD=2. With this, I got warnings such as: " Attempt to apply local prescribed motion to globally constrained node.
Prescribed motion on node 1 in direction 2 is ignored."
Additionally, the combination of *NODAL_RIGID_BODY* to make master and slave, and *BOUNDARY_SPC_NODE* to apply boundary restraint, and with *BOUNDARY_PRESCRIBED_MOTION_NODE* to apply unit deformation, also did not work. The unit definition does not apply to model with this.
My Intention:
1. To apply unit deformation one by one to each DoF at the two end master nodes and to see the reactions due to this in those nodes in other DoF, to basically see what the 6X6 stiffness matrix of this 3D structure would look like.
Can you share some insights on this problem and what could be better and accurate DYNA commands to do such a task?
Model snip for reference attached.
Regards,
Sabin Acharya
PhD Scholar, CEE, University of Nevada, Reno.
James Kennedy
Dear Sabin,
See if this information is perhaps of some help.
2.4 Constraints and Rigid Bodies
Avoid the use of *CONSTRAINED_NODE_SET unless nodes in the node set are coincident.
To rigidly connect two or more deformable, non-coincident nodes, use *CONSTRAINED_NODAL_
RIGID_BODY instead of *CONSTRAINED_NODE_SET so that nonphysical resistance to rotation
is not imposed. A rigid body can be defined in LS-DYNA in several ways:
• A part that references *MAT_RIGID is a rigid body.
• A set of nodes referenced by *CONSTRAINED_NODAL_RIGID_BODY forms a rigid body.
• A node or set of nodes referenced by *CONSTRAINED_EXTRA_NODES is added to or becomes
a rigid body.
Rigid bodies are subject to certain rules of modeling to which deformable bodies are not. For example:
• With few exceptions, *CONSTRAINED_OPTION cannot be applied to any element or part that is
rigid or to any node that is included in a rigid body. The exceptions are * CONSTRAINED_RIGID_
BODIES, * CONSTRAINED_JOINT, and * CONSTRAINED_LAGRANGE_IN_SOLID (if penalty-
based coupling is used).
• Constraint-based contact algorithms, predominately used in LS-DYNA in tied (not tiebreak) contact
types, cannot be used on rigid bodies.
• Prescribed motion cannot be applied to more than one node of a rigid body. The preferred method of
prescribing motion to a rigid body is via
*BOUNDARY_PRESCRIBED_MOTION_RIGID in which the motion is defined with respect to the
center-of-mass of the rigid body.
• Though nodal single-point-constraints (*BOUNDARY_SPC) on nodes of rigid bodies will internally
tbe converted by LS-DYNA into an equivalent set of constraints on the rigid body’s center-of-mass, the
preferred method of constraining rigid body motion is via Card 2 of *MAT_RIGID, or if the rigid body
is defined as a nodal rigid body, via
*CONSTRAINED_NODAL_RIGID_BODY_SPC. *BOUNDARY_PRESCRIBED_MOTION_
RIGID would be another acceptable means of constraining a rigid body (velocity or displacement could
be prescribed as zero). The mesh of rigid bodies should generally be no coarser than that of a deformable
body, unless the rigid body’s motion is fully constrained and/or prescribed, or unless the mass properties
are specified directly via *PART_INERTIA. Because mass is lumped at the nodes, a coarse mesh will
often give highly inaccurate inertia values.
Sincerely,
James M. Kennedy
KBS2 Inc.
August 2, 2025
--
You received this message because you are subscribed to the Google Groups "LS-DYNA2" group.
To unsubscribe from this group and stop receiving emails from it, send an email to
ls-dyna2+u...@googlegroups.com.
To view this discussion visit
https://groups.google.com/d/msgid/ls-dyna2/f99077b5-217b-4b99-9327-20fcab07d468n%40googlegroups.com.