General Mechanical

General Mechanical

Adding spring elements (Combin14) at midside nodes

    • Flo
      Subscriber

      Hi,

      I'm currently modelling an indenter that is glued to a substrate and loaded at its tip (ANSYS Workbench 2020R2) . To include the effect of the elastic substrate I added spring elements (combine14) at every node of the bottom side of the glue layer. Following the instructions of other discussions I managed to include three of them (for every direction) at every node of the bottom side (included in a named selection). The other "open" end of the spring elements is fixed. Unfortunately by loading the tip the midside nodes of the selection doesn't seem to have "spring support" because the deformation there is higher (see picture). I found another small discussion on reddit where someone mentioned not to add springs at midside nodes (https://www.reddit.com/r/ANSYS/comments/letp4k/is_there_a_way_to_connect_a_shell_element_with_a/). Is there a reason for this advice?

      (PS: I know having only one element layer representing the glue isn't that good.)

    • Flo
      Subscriber
      I think I found a answer to my own question (as always shortly after posting the discussion). In the ANSYS Mechanical APDL 2020 R2 documentation under "2.2. Choosing Between Linear and Higher Order Elements" it is described how linear and quadratic elements differ and when quadratic elements should be avoided. Reaction forces, masses, load values, etc. are different at midside nodes and it is also mentioned that you shouldn't use nodal-based contact elements at faces with midside nodes. If the nodal-based contact is necessary, the midside nodes should be removed, if possible.
      Maybe someone can confirm or disagree with my asumption otherwise I will mark the discussion in a few days as solved.
    • Sheldon Imaoka
      Ansys Employee

      Sorry for the late reply, but you're correct about the reason - it's due to the shape function, so that is why you don't want to put a discrete element like a spring directly on a midside node.
      Using only springs at corner nodes isn't great, either. If one end of the spring is fixed to 'ground', have you considered looking to see if an Elastic Support does what you want? It provides a stiffness only in the normal direction, but it will be uniform and work with quadratic elements.
      Regards Sheldon

    • Bill Bulat
      Ansys Employee
      Yes, that's exactly right. If there isn't a meshing feature in the Mechanical GUI that allows you to do so, you can use the EMID command in /PREP7 to remove midside nodes from the element faces on a surface before creating the LINK14s (so that the LINK14 elements are attached to the faces without midside nodes as shown in the figure on the right below):
      Best regards
      Bill
    • Flo
      Subscriber
      Hi,
      thanks a lot Sheldon and Bill. Yes I also considered using an "Elastic Support" but I also wanted to include a stiffness value in tangential direction. I compared my mechanical deformations etc. between linear and quadratic elements in general and decided to go with linear elements + spring elements because the difference wasn't that high. I also tried to only remove the midside nodes in the interaction area with the EMID command but got some other error messages due to the removal of the nodes if I remember correctly...
      But it worked also with linear elements in general + springs so I didn't follow this approach much longer.
      Best regards
      Flo

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