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TIED_NODES_TO_SURFACE vs _OFFSET vs _CONSTRAINED_OFFET

    • Jacob Ellis
      Subscriber

      My understanding is that TIED_NODES_TO_SURFACE is a constraint based formulation. Adding _OFFSET to they keyword makes it a penalty formulation. And that adding CONSTRAINED_OFFSET makes it once again a constraint based formulation? This is quite confusing...  Is this true? If so, what's different between TIED_NODES_TO_SURFACE and TIED_NODES_TO_SURFACE_CONSTRAINED_OFFSET?

      And for a non-technical question, if anyone knows, what is the historical reason for the word "OFFSET" implying penalty based.

    • Ram Gopisetti
      Ansys Employee

      Hi Jacob, 

      Have an read on the artiel posted in the follwing link 

      https://www.dynasupport.com/howtos/contact/tied-tied-offset-and-tiebreak-contacts

      cheers, ram 

    • Andreas Koutras
      Ansys Employee

      Hi Jacob,

      To add to the previous response,  a non-OFFSET tied contact will project the slave nodes on the master surface during initialization and thereby change the initial geometry (no element stresses are introduced by this procedure). Instead, any tied contact with the OFFSET designation appearing in its keyword name will maintain any initial offsets (i.e. gaps) existing in the mesh between the tied nodes and the master surface without perturbing the initial geometry.

    • Jacob Ellis
      Subscriber

      Thanks, Ram.

      Andreas, by changing the geometry do you mean the element volume (and therefore mass, moment of inertia) etc are changed?

    • Andreas Koutras
      Ansys Employee

      Hi Jacob, in non-OFFSET tied contacts, if there is any initial penetration or initial gap, the tied nodes will be move onto to the master surface during initialization. Indeed, this will change the volume, and thereby mass, of the elements attached to those slave nodes. However, any unintentional initial penetrations or gaps existing in the mesh should be small from the first place, thereby the change in the initial coordinates of the slave nodes will be minimal. To avoid this, you can either use an OFFSET tied contact (penatly-based) or a CONSTRAINED_OFFSET tied contact (constraint-based). 

       

    • Jacob Ellis
      Subscriber

      Ah! That clears things up perfectly. _CONSTRAINED_OFFSET is used when there is a gap, you wish to use a constraint based formulation, and you wish to not change the element volume artificially. Perfect!

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