IBBM 2017

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Thread: reaction force and rigid joint reaction force

  1. #1
    Join Date
    Sep 2015
    Posts
    2

    Default reaction force and rigid joint reaction force

    Hello,

    I have a problem when using the rigid joint function.

    I created two beams connected by a rigid hinge joint. The rigid joint is driven by rotation angle. One beam is fixed in all DOFs. The other beam is applied by a load to its distal surface to create a torque. I want to see the reaction force and moment of the rigid joint. But the output reaction force and moment of the rigid joint is too high. Please see the file here trr3.feb.


    I also tried to rotate the beam through the rigid material instead of through the rigid joint. But the output results are all zero (see the input file here trr2.feb).

    Please could someone help give me some suggestions. Many thanks.

    Regards,
    Jerry

  2. #2
    Join Date
    Dec 2007
    Posts
    639

    Default

    Hi Jerry,

    Here are the changes I made to get your model (trr3.feb) to run:

    1) Since the two bodies are connected by a revolute joint, I deleted the rigid body constraints you placed on body 2. (I don't think this was critical.)
    2) I changed your force_penalty and moment_penalty values from 1e+099 to 1e+04. This is critical.
    3) I set the analysis to use a non-symmetric stiffness matrix, by adding <symmetric_stiffness>0</symmetric_stiffness> in the <Control> section.
    4) Since the BFGS method does not work well with non-symmetric matrices, I changed your <max_refs> to 25 and <Max_ups> to 0. (Alternatively, you can add <qnmethod>2</qnmethod> in the <Control> section to use Broyden's method instead of BFGS, which works with non-symmetric matrices.)

    I believe that the reactions should look okay now, but please check and let me know.

    Best,

    Gerard

  3. #3
    Join Date
    Sep 2015
    Posts
    2

    Default

    Dear Gerard,

    Many thanks for the message. I changed the input file according to your suggestions. Now the model is much improved. The reaction forces and moments are consistent with theoretical calculations.

    I further checked each of the parameters you suggested and realized that the too high force penalty and moment penalty mainly accounts for the zero reaction forces and moments.

    Regards,
    Jerry

  4. #4
    Join Date
    Dec 2007
    Posts
    639

    Default

    Hi Jerry,

    I am glad it worked out. I have updated the User's Manual to revise the large penalty parameters suggested in the examples and to provide more guidelines on how to select those parameters. These changes will appear in the next release.

    Best,

    Gerard

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