5 Replies Latest reply on Nov 23, 2016 10:24 AM by Bill McEachern

    "Free Contact" as Opposed to "Allow Penetration"?

    John Willett

      I have been making very thin extruded cuts purely to prevent automatic bonding between parts in certain areas.  Of course this results in poor mesh quality.  What is the best way to prevent parts that are coincident from being bonded in a static study, especially when I want most other coincidences (even between the same parts) to be bonded with compatible mesh?

       

      Should I be placing split lines around the coincident areas that I don't want to be bonded and then using "Allow Penetration" contact sets to enforce this?  Does this effectively prevent the program from checking for contact, thereby saving processing time?

       

      I just noticed a cryptic reference to "Free Contact" in SW Help (nothing useful in Knowledge Base), but I don't understand exactly what it does, nor how to invoke it without losing compatible mesh at other bonded contacts.  Is this equivalent to, or better in some way than, "Allow Penetration" for my application?

       

      I'm running SW Premium 2016 SP5.0 under Windows 7 SP1.  The idea here is to represent welds that seep between part, but not all, of contacting faces, without making the geometry more complex with thin cuts.  (Ideally the weld would both add a bonded "fillet" on the outer edge between the contacting parts and bond a short distance into the contacting faces between them.  I haven't found a way to add a fillet between parts at the assembly level, but it's relatively easy to add an extruded or revolved boss on the edge of one of the two parts that will be coincident with the other.  The problem is controlling the bonding of the contacting face between the parts...)  Any guidance would be most welcome! -- John Willett

       

      Message was edited by: John Willett for clarification and correction of typos

        • Re: "Free Contact" as Opposed to "No Penetration"?
          Bill McEachern

          apply an "allow penetration" contact set.

            • Re: "Free Contact" as Opposed to "Allow Penetration"?
              John Willett

              Just for completeness (in case anyone else has wondered), here are my conjectures, now confirmed by my VAR's tech support:

               

              1) "Free Contact" is not a contact condition per se, but simply a lack of contact conditions.  In other words you would have to suppress the default Global Contact (-Bonded-) Component Contacts condition.  Then any coincident faces that are not specified in Contact Sets would effectively be "Free."  The downside of this is that Bonded contacts (where specified in Contact Sets) will have incompatible mesh -- usually a bad thing.

               

              2) The only way to enforce compatible mesh for Bonded contacts is to keep the Global Contact (-Bonded-) Component Contacts condition and to check the "Compatible Mesh" radio button under Component Contact/Bonded/Options.  When that is active, all Bonded contacts, even those specified in Contact Sets (e.g., for the purpose of getting accurate Free-Body forces across those joints), are made with compatible mesh.

               

              3) "Allow Penetration" apparently accomplishes exactly the same result as "Free Contact" without the downside and with little or no additional baggage.

               

              I hope this helps somebody else, as SW doesn't seem to specify this information clearly in Help or KB. -- John Willett

                • Re: "Free Contact" as Opposed to "Allow Penetration"?
                  Bill McEachern

                  Hi John,

                  Here are some comments on your summary:

                  1) "Free Contact" is not a contact condition per se, but simply a lack of contact conditions.  In other words you would have to suppress the default Global Contact (-Bonded-) Component Contacts condition.  Then any coincident faces that are not specified in Contact Sets would effectively be "Free."  The downside of this is that Bonded contacts (where specified in Contact Sets) will have incompatible mesh -- usually a bad thing.

                   

                  Incompatible mesh is not always a bad thing nor even frequently. The advanced bonding option usually does a decent job of dealing with differences in element sizes across on bonded contact. The problem arise when the elements are vastly different in size and the connections between nodes can make the connection to stiff. It is easy to illustrate with the simple bonding option and two different size meshes. Most other codes like ANSYS and ABAQUS that is the only option you have.

                   

                   

                  3) "Allow Penetration" apparently accomplishes exactly the same result as "Free Contact" without the downside and with little or no additional baggage.

                   

                  It might be better put as "no interaction"

                    • Re: "Free Contact" as Opposed to "Allow Penetration"?
                      John Willett

                      Bill -- Thanks for your always very helpful explanations.

                       

                      >>Incompatible mesh is not always a bad thing nor even frequently. The advanced bonding option usually does a decent job of dealing with differences in element sizes across on bonded contact.<<

                       

                           I think you mean (Static) "Study/Properties/Incompatible bonding options/More accurate (slower)."  I usually leave this radio button selected.  Any harm in this with bonded compatible meshes?  But when I do bond incompatible meshes, Simulation (in SW Premium) appears to take much longer to run than with compatible meshes.  I have not experimented with the relative accuracy.

                       

                      >>["Allow Penetration"(?)] might be better put as "no interaction"<<

                       

                           Yes, that would be much clearer, but I guess it doesn't fit into their naming convention.

                       

                      Thanks again! -- John Willett

                        • Re: "Free Contact" as Opposed to "Allow Penetration"?
                          Bill McEachern

                          No harm in having it checked for compatible meshes, its a good thing full stop and yeah it takes awhile for the solver to put it all together if there a lot of them.

                          If you want to do some experiments just bond two solid bars with big difference in cross sections together and put vastly different element sizes on each and then compare compatible and incompatible mesh options with simple and advanced bonding options. It will likely illustrate what happens.

                          And "no interaction" does not fit in their adopted lexicon.