General Mechanical

General Mechanical

Linear elastic knee joint simulation

    • OmidNazeri1997


      I'm working on the knee joint simulation. I need to apply different force magnitudes to the model, ranging from 2900 N to 7800 N and to get the stress values as the results.

      After I complete the solution, I get 3 warning messages.

      1) Contact status has experienced an abrupt change ... .

      2) One or more contact pairs exceeds contact small sliding assumption ... .

      3) The deformation is large compared to the model bounding box.

      I wanted to know that do these things affect the results accuracy?

    • peteroznewman

      Those warning messages indicate that the solution accuracy is poor. If you look at the Total Deformation result using a display scale factor of 1.0 True Scale, you might even find the solution is complete garbage.

      Item #3 is the most telling warning, it is saying something deformed further than the entire dimension of your model! Frictional contact can be tricky to get started. The warnings above imply that the contact was not established and the surfaces may have passed through each other as if no contact was defined. You should be able to see that if you look for it.

      The corrective action is to have the following Analysis Settings:

      1. Large Deflection ON
      2. Auto Time Stepping ON
      3. Inital Substeps = 100
      4. Maximum Substep = 1000
      5. Minimum Substeps = 1 unless you are using a nonlinear material model.

      Before you solve, insert a Contact Tool under the Connections folder and Generate Initial Contact Status.  Check that the Frictional Contact is Closed.If it is Near Open with a tiny gap, such as 1e-3 or smaller, then you can edit that contact and set the Interface Treatment to Adjust to Touch.

      With the above changes, the solution may solve without those warnings and the result should have good accuracy.

    • OmidNazeri1997

      Hi peter. Thanks a lot. I'm using linear-elastic materials.
      I defined 12 contacts between the bones and the ligaments. All of the contacts were bonded except two of them, frictionless. Actually, I had used the contact tool for the model and all of them are orange and grey every other one.
      I will try these and let you know if the problem is resolved.

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