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Thread: Need help in modeling indentation stress relaxation of cartilage

  1. #41
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    Dec 2007
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    Hi Lewis,

    Young's modulus controls the equilibrium response to a creep problem (such as the example you are analyzing): It determines the relation between the applied (constant) load and the equilibrium creep deformation. So, under a constant load, increasing E decreases the creep deformation.

    The permeability and ksi control the transient creep response (how long it takes to reach equilibrium, and the shape of the transient response). More specifically, the characteristic time constant for reaching equilibrium is proportional to the square of a characteristic measure of length (e.g., the indenter radius or the cartilage thickness) divided by the product of ksi and permeability.

    Best,

    Gerard

  2. #42
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    Oct 2019
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    Magic, that helps explain what I was observing in post view.

    If you have time could you please take a look at my model (iv attached) and tell me what i'm doing wrong in terms of loads. Iv made sure to negate the load curve of the indenter and also to have an initial overlap of -1e-03 in the z axis. But it still won't show any deformation when running the analysis. I also found it difficult to get the sphere to form a wedge centre but does this even matter if its rigid?

    Many thanks, Lewis
    Attached Files Attached Files

  3. #43
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    Hi Lewis,

    The load you applied does not cause too much deformation. I was able to increase it by a factor of 10 and also by a factor of 100 and in both cases the analyses converged and the solutions shows more significant deformation.

    When you choose a non-symmetric stiffness matrix, you should switch the quasi-Newton method from BFGS to Broyden. But the best results for contact analyses is achieved using full-Newton iterations (set max_ups=0).

    Best,

    Gerard

  4. #44
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    Oct 2019
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    Great thank you.

    I question about symmetry. I have made two spherical indentation models. One model is 1/4 tissue layer and 1/4 indenter. The other model is a 3 degree tissue layer and 1/4 indenter. The idea behind using the 3 degree wedge was reduced computational time. Will my output analysis results be identical in both models? Ideally I would like to make the spherical indenter also 3 degrees but I have trouble doing this with the editable mesh. Or does it not matter how much of the indenter is present as its a solid?

    Many thanks, Lewis

  5. #45
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    Hi Lewis,

    In principle (i.e., if the penalty parameter for the symmetry plane is high enough), the axisymmetric analysis results between a 1/4 (90-degree) model and a narrow wedge (3-degree) model should be the same for all the nodal and element variables (pressure, displacement, stress, etc.). The indenter load response will differ by a factor of (90 degrees / 3 degrees) = 30.

    It is okay if the mesh of the rigid spherical indenter spills over the 3-degree wedge, that should not affect the analysis results.

    Best,

    Gerard

  6. #46
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    Oct 2019
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    Magic. So I have increased my penalty parameter in the symmetry plane. And also switched from BFGS to Broyden. My two models are now both with a 45 degree indenter but one model is the 3 degree tissue wedge and the other is the 90 degree tissue wedge. However I'm still confused with what value my load curve should be for both.

    For the 3 degree wedge should it be 1/120 of the force I want to apply. And for the 90 degree wedge should it be 1/4 of the force I want to apply?

    Or for both models is the load curve 1/8 of the force I want to apply as the indenter is only 45 degrees?

    Iv attached my files if that's any help.
    Attached Files Attached Files

  7. #47
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    Dec 2007
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    Hi Lewis,

    For the 3 degree wedge should it be 1/120 of the force I want to apply.
    Yes.
    And for the 90 degree wedge should it be 1/4 of the force I want to apply?
    Yes.

    Best,

    Gerard

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