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Thread: Modeling mechanical tensile testing of a nerve using ANSYS workbench.

  1. #1

    Modeling mechanical tensile testing of a nerve using ANSYS workbench.

    Hello everyone.

    I am trying to model mechanical tensile testing of a nerve using ANSYS workbench.
    The setup:
    Thus far, the model is almost cylindrical (as in the cross sections are not circles, but closed loop splines). The model is quite slender as the length of the structure is about 92mm, but the total cross section is roughly about 7.14 square millimeters. You could say that I have several cylinders inside the larger cylinder (constituting the funicular areas of the nerve). The nerve's sheath volume has a different modulus of elasticity than the funicular volumes. Roughly my average mudulus of ealsticity is very soft: 46 MPa and my Poisson ratio is 0.49 (like an elastomer).

    The problem:
    When I simulate this structure using a total 108 Newton force, the software yields a total deformation of 208.5 mm. This is HUGE! This corresponds to a length increase of 126%! While I do understand that this has to do with the slenderness of the model and the elasticity moduli, I know that this result is incorrect, as real nerve tensile testing has given us a maximum deformation of 26% before failure of the specimen. At the moment when the simulation converges and the result is displayed, the software gives a warning message saying that the elongation is too large compared to the model's bounding box and that turning "large deflection on" should resolve the problem. After this, I turn "large deflection ON" and the model fails and won't converge. I have tried to resolve this problem by changing several options in the workbench software. I have tried changing the solver type and also changing the non-linear solution setting. Despite of this, the simulation still won't converge.

    The question: What can I possibly do to make this model converge having the "large deformation" ON? Either that, or how can I obtain realistic simulation results having "large deformation" OFF? Should I change line search? Or, what does it mean to implement the hyperelastic idealization? Like I know that the hyperelastic idealization is the correct thing to do for modeling tissues (as an example). But how to do this in Ansys Workbench? I know that there's ton of options of things that I can change to obtain correct results. However, the options are sooo many that this would result in a huge matrix of combinations that would never end... Can anyone offer some advice as to how to resolve this problem in a quicker fashion without having to go through the changing of sooo many options?

    Any input from anyone would be greatly appreciated.

    Good day.

  2. #2
    Join Date
    2000-02
    Location
    Boston, USA
    Posts
    3,359
    You really should use the "large deformations" options since the strains are finite. You should also use a hyperelastic material model not linear elasticity. I recommend that you start with the neo-Hookean model since it is so simple.

    -Jorgen
    Jorgen Bergstrom, Ph.D.
    PolymerFEM Administrator

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