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[FAQ]

Tied contacts in LS-DYNA can have either a constraint-based or a penalty-based formulation. The penalty-based formulation is invoked by including the OFFSET option in the contact keyword name. Exception is the CONSTRAINED_OFFSET type, which is still constraint-based. While a node (of a deformable or of a rigid body) can be constrained by multiple penalty-based tied contacts at the same time, a node of a deformable body can be a node of just a single constraint-based tied contact definition. If the same node is found in more than one constraint-based tied contacts, LS-DYNA will constrain that node only with the constrained-based tied contact that appears first in the input file. The remaining tied contacts will not be considered for that node and appropriate warning messages will be written in the message and d3hsp files (namely, Warning 40552 in SMP and Warning 30455 in MPP). The IPBACK flag (3rd field, Optional Card E of *CONTACT) is only applicable to constraint-based tied contacts. By activating IPBACK, an identical penalty-based contact is generated with the OFFSET type (or in the case of the SHELL_EDGE constrained-base contact, a BEAM_OFFSET type is generated). The ID of the generated interface will be set to the ID of the original interface plus 1 if that ID is available, or it will be otherwise set the maximum used contact ID plus 1. For nodes successfully tied by the constraint-based contact there will an extra redundant penalty tying, but for nodes dropped from the constraint contact due to other conflicting constraints (including those of rigid bodies) the tying will be handled solely by the automatically generated penalty-based tied contact. In MPP, nodes successfully tied with the constraint-based contact will be skipped by the penalty-based contact.