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Thread: Parameter Optimization for Biphasic Unconfined Compression Stress-Relaxation

  1. #11

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    Hi Prof. Ateshian,

    why we use only one element in axial direction and many elements in radial direction? Because of the p=0 in radial direction?

    Best regards,

    Thomas

  2. #12
    Join Date
    Nov 2015
    Posts
    2

    Default

    Hi Prof. Ateshian,

    I hope this threat is not dead by now.

    I tried to run the example files in FeBio2; but with the message:
    FATAL ERROR: the variable Cartilage.solid.ground.E is not recognized

    Failed initializing the task: optimize

    I am trying to find out what could be the reason; but first searches gave me material from 2 years ago. Back these days it was often referred to not actual versions of FeBio- i cannot imagine that I (after two years) would have an outdated version though.
    I thought about a problem caused by changed syntax.

    Can you give me a hint for my search?


    Thanks in advance and best regards,

    Alex

  3. #13
    Join Date
    Dec 2007
    Posts
    1,020

    Default

    Hi Alex,

    I checked the FEBio source code and it appears that changes were made to the method by which material parameters are read. In other words a bug crept into FEBio 2.4.0 and we will need to fix that. (The bug is fixed in the current development version, but we need to transfer that fix to a new release version 2.4.1.) In the meantime, you should be able to run this analysis with an earlier version.

    Best,

    Gerard

  4. #14
    Join Date
    Dec 2007
    Posts
    1,020

    Default

    Hi Thomas,

    The reason for using only one element along the axial direction is that the solid strain and fluid pressure are homogeneous in that direction. One element is enough to capture this response. (Strain is not homogeneous along the radial direction because p=0 as you surmised, leading to an inhomogeneous radial fluid flux, fluid pressure and strain.) This type of simplification only occurs for specialized geometries and boundary conditions, as in this example of unconfined compression with frictionless platens.

    Best,

    Gerard

  5. #15
    Join Date
    Dec 2008
    Location
    Salt Lake City, UT
    Posts
    376

    Default

    Hi Thomas,

    The bug has been fixed and I've uploaded version 2.4.1 to the web site.

    Cheers,

    Dave
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

  6. #16
    Join Date
    Jun 2017
    Posts
    2

    Default

    Hi,

    I have been trying to optimise a ramp (0.5mm in 5s) and hold (100s) in FEBio2. It will run one iteration and have a fatal error in iteration 2. I have attached my Optimisation file (Optim.txt) and the Model file (A_B_Matrix.feb). I greatly appreciate any help.

    Thank you,
    Jasia
    Attached Files Attached Files

  7. #17
    Join Date
    Apr 2019
    Posts
    3

    Default

    Just a heads up for anyone using FEBio2.8.6 to get this working I had to change the optimization file for the new parameter naming structure.

    These were the changes I made which seemed to work:
    Attached Files Attached Files

  8. #18
    Join Date
    May 2019
    Posts
    3

    Default

    Hi,

    I have been trying to optimize Ogden parameters for a compression test but the optimization always ends with an error. The model is a cylindrical object compressed by a plate (Rigid Body) by prescribed displacement. I notice that the objective value is too high(10e7). Also, I tried to use the log_level but it does not make difference in the output. Lastly, I am not sure if I input negative or positive values for Force in the optimization file.

    I would be glad if somebody analyze the files attached and help me with this problem.

    Thanks,
    Fernando

    S1_RigidBodyDispl_Curve.febOptimization_Ogden_Force.feb

  9. #19
    Join Date
    Nov 2007
    Location
    Salt Lake City, UTAH
    Posts
    2,269

    Default

    Hi Fernando,

    If you set the print_level parameter to 1 (not the log_level):
    Code:
    <print_level>1</print_level>
    FEBio will print the values it is finding for the objective function. You will see that the calculated values are much bigger than the ones you specified, and that they have a different sign. So, I suggest you modify the initial values for the parameters so that you get forces that are of the same order of magnitude, and flip the signs on the target values.

    Best,

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

  10. #20
    Join Date
    May 2019
    Posts
    3

    Default

    Thanks very much Steve!

    Your instructions help me to improve the parameterization. I could minimize the obj. value until 408, then there is a fatal error. In case you are interested, please see the graph below with the comparison. The parameters were input as below:
    <param name="fem.material('Material2').c1">0.0739, 0.0069, 0.79,</param>
    <param name="fem.material('Material2').c2">-5.01e-7, -10, 10,</param>
    <param name="fem.material('Material2').m1">10.3721, 0, 100,</param>
    <param name="fem.material('Material2').m2">30.4608, -10, 1000,</param>
    optimization.jpg

    best,
    Fernando

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