Rigid element translation difficulty

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  • Matt Szelistowski
    Junior Member
    • Mar 2008
    • 8

    Rigid element translation difficulty

    Steve,

    When I analyze the attached file of a rigid element with prescribed displacement, I keep getting the following negative jacobian error until the file quit iterating:

    "Negative jacobian was detected at element 1 at gauss point 1
    jacobian = -1.#IND"

    When I change the model's material to linear elastic, the file analyzes completely. I do get a strange unassigned rotation toward the end of the analysis though.

    What am I not doing right?

    Thanks,
    Matt
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    Hi Matt,

    When using the linear elastic model your model may run, but it is underconstraint. You apply a displacement in the z-direction but there are no constraints in the x and y direction, which is probably the cause for your unexplained rotation. Keep in mind that FEBio does not have any dynamics so no inertia term to prevent things some flying off in space. If your system is underconstraint you may see (and usually will) strange (incorrect) behavior. So, on the other hand, if you see this kind of behavior you can usually assume that your system is underconstraint. Try fixing some nodes (one or two may do the trick) in the x and y direction and I bet things run much better.

    As far as the rigid material is concerned, you need to prescribe the displacements directly on the rigid body and not on the nodes since prescribed displacements are ignored for rigid nodes. When I try this the file runs fine, so I was wondering if you could send me the file that gives you that negative jacobian.

    Thanks,

    Steve.
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

    Comment

    • Matt Szelistowski
      Junior Member
      • Mar 2008
      • 8

      #3
      Thanks; I've gotten a full simulation to run with the rigid element model after I had constrained all degrees of freedom, and then applied a displacement curve to one of the degrees of freedom under the curve editor.

      Thanks,
      Matt

      Comment

      • Matt Szelistowski
        Junior Member
        • Mar 2008
        • 8

        #4
        Steve,

        Here is the file I recieved the error message from.

        Thanks,
        Matt

        Comment

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