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Tied, Tied Offset, and Tiebreak Contacts

Constraint-based / Penalty-based Contacts

Tied contacts fall into 2 major categories: constraint-based and penalty-based. Those with TIEBREAK, OFFSET, or BEAM_OFFSETin the name are penalty-based.  All others are constraint-based. A node, whether it be on the slave or master side, cannot be involved in more than one constraint-based contact.  Also, rigid bodies cannot be included in a constraint-based contact.

For tied contacts without the offset option, any slave nodes that meet the criteria to be tied will be moved to the master surface during initialization thus eliminating gaps between slave nodes and master segments.  If you see such a gap when viewing the model, the node is not tied. A warning message is written to the d3hsp/messag files whenever a slave node is not tied.

The criteria that must be met for a successful tie are:

  • Slave node must be within the normal projection of a master segment
  • The offset of the slave node from the master segment in the normal direction cannot exceed a certain tolerance.You can change this tolerance by using negative values for the contact thickness. 

(See Remark 2 on p. 7.42 of the 971 User's Manual.)

There are various tied contacts in LS-DYNA. Which you choose will depend on the application. If the surfaces to be tied are not rigid or otherwise constrained, then a constraint-based tied contact can be used, e.g.,

*CONTACT_TIED_NODES_TO_SURFACE

(if nodes do NOT have rotational DOF, e.g., as in solids or thick shells)

*CONTACT_TIED_SHELL_EDGE_TO_SURFACE  (if any nodes being tied do have rotational DOF, e.g., as in thin shells)
*CONTACT_TIED_SHELL_EDGE_TO_SURFACE_CONSTRAINED_OFFSET  (if any nodes being tied do have rotational DOF, e.g., as in thin shells AND a physical gap between the tied surfaces is to be retained)

 

Rigid Parts

If a rigid part is being tied, then a penalty-based tied contact could be used but not a constraint-based tied contqact. This can be accomplished by adding the word "OFFSET" to the contact name, e.g., *CONTACT_TIED_NODES_TO_SURFACE_OFFSET.  (The exception is when "CONSTRAINED_OFFSET" is used.)
See the notes on p. 7.4 of the 971 User's Manual regarding OFFSET, BEAM_OFFSET, and CONSTRAINED_OFFSET tied contacts.

As an alternative to using tied contact for permanently bonding a deformable body to a rigid body, one could use *CONSTRAINED_EXTRA_NODES.

 

Tiebreak Contact

"TIEBREAK" contacts have the added feature of including a failure criterion.  These contact types are usually penalty-based.  A constraint-based tied contact with a failure criterion is *CONTACT_TIED_SURFACE_TO_SURFACE_FAILURE.

Automatic tiebreak contacts have some special features (see OPTION parameter) and behave as automatic contacts after failure. 
       tiebreak option for automatic contact
            eq.-3:see 3, moments are transferred
            eq.-2:see 2, moments are transferred
            eq.-1:see 1, moments are transferred
            eq. 1:stick
            eq. 2:tiebreak for nodes initially in contact
            eq. 3:tiebreak after sticking
            eq. 4:sliding only with friction and failure
            eq. 5:elastoplastic interface with damage
            eq. 6:rigid-plastic interface with damage
            eq. 7:DYCOS Discrete Crack Model
            eq. 8:see 6, with thickness offset (Delam model)  (v. 971 only)

 

Offset Tied Contacts

 
Regular tied offset contacts use a penalty method and do not transmit forces/moments in a beam-like manner. This can lead to nonphysical rotational constraints.  Preferred to these regular offset contacts are BEAM_OFFSET and CONSTRAINED_OFFSET tied contacts which DO transmit forces/moments between the slave node and master segment in a beam-like manner.  The BEAM_OFFSET option is penalty-based and applies only to TIED_SHELL_EDGE_TO_SUFACE contact whereas the CONSTRAINED_OFFSET option is constraint-based and is available for TIED_NODES_TO_SURFACE,  TIED_SURFACE_TO_SURFACE,  and TIED_SHELL_EDGE_TO_SURFACE contacts. 
Be aware that *CONTACT...CONSTRAINED_OFFSET redistributes the mass of the slave nodes to the master surface but (at least) in doing so the rotary inertia of the slave nodes is transformed. In *CONTACT_TIED_SHELL_EDGE_TO_SURFACE_BEAM_OFFSET, mass of the slave nodes stays put.  

 
 

Trouble-shooting: Tied contact doesn't appear to work


Look for the word "Warning" in your message file to see if there are any messages about slave nodes not being tied. If slave nodes are too far away from the master surface, the slaves will not be tied.    This threshold distance is defined in Remark 2 on p. 7.42 of the 971 User's Manual.  Note that by setting the slave and/or master contact thickness to a negative value, the threshold distance can be manually controlled.
 
Unfortunately, there's no convenient way to display unconstrained nodes.  A suggestion -- use the unix command grep to get a listing of unconstrained nodes, e.g.,
grep "slave node ID" d3hsp >list

Then edit this listing to make a node set via *SET_NODE_COLUMN. Insert this node set into your input deck and read the deck into LS-PrePost in order to view the node set.