General Mechanical

General Mechanical

Why uniaxial, biaxial and shear curves for the Neo-Hookean material model do not appear properly?

    • fx213
      Subscriber

      Hello,


      I am trying to set up a transient structural analysis for an elastomeric cylindrical sample that subjected to compression stress.


      In Engineering data part though, we first have to define the properties.I want to use the Neo-Hookean hyperelastic material model parameter 5 for example to obtain the uniaxial, biaxial and shear curve, that generated through the curve fitting of the model.


      Following the steps from a relevant video tutorial, I insert the stress strain data and then I determine the respective uniaxial, biaxial and shear data in order to be appeared in the Chart data of the model curves` of the above deformation modes in different colours. This doesn`t happens in my case.


      Thus, I would like to make a query on why these modes from the material model do not appear in the chart? You can find attached a screenshot.


      Thank you.


       

    • Sandeep Medikonda
      Ansys Employee

      Hi you have a -ve shear modulus input and that is the reason why you don't see anything in the chart. Check your experimental data and the way you are fitting it once?


      Regards,
      Sandeep
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    • fx213
      Subscriber

      Dear Sandeep Medikonda,


      Thank you for your reply, I will try to check the data and fitting again.                                                                                                                                                  Could I ask you, from your experience, which type of data fitting you think is the best option? So far, I have used linear polynomial and investigate the exponential fitting as well.


      Kind regards,


      FXanthaki

    • Sandeep Medikonda
      Ansys Employee

      Aya, 


      That is a very broad question and my short answer would be that it depends.


      Me and peter have had some good discussions with zack on hyper-elastic materials in general, please go over his recent discussions. You might find them useful.


      Regards,
      Sandeep

    • fx213
      Subscriber

      Dear Sandeep,


      Concerning to your first reply, saying that I 'have a -ve shear modulus input and that is the reason why you don't see anything in the chart', do you mean that I should remove shear value from the properties to see all the three curves in the graph? Bulk modulus and shear modulus values appeared once I input the stress/strain data.


      So, we firstly consider the case of incompressibility that assists with the convergence of the curve for such analyses.


      Assuming that we want to account for compressibility case, 


      we use the relations in the articles you cited in ''Nonlinear Material with Reduced Integration- Tubing Convergence'' to determine and deal with incompressibility and find the incompressibility constant D or d and Jel (from volumetric data and and hydrostatic pressure (stress). Is that correct?


      Still, one more question; for the case of biaxial and shear(test data), we use the same stress/ strain test data we used for uniaxial? This is what I did and I wanted to be sure that I understood well and determined the properties correctly.


      Thank you very much for the quite useful discussion activity you forwarded to me and for the to-the-point articles you attached as well. 


      Kind regards,


      FXanthaki

    • Sandeep Medikonda
      Ansys Employee

      FXanthaki,


        Please open a new discussion for new questions? Closed and long discussions get less attention and often don't get the help you need. Please take a moment to review the Guidelines.


      Coming to your problem: Yes, you can use the PVT test to determine incompressibility parameters. This test should give off the volumetric strain vs hydrostatic stress, using the slope of which you should be able to determine the parameters as suggested in that article.


      Also, you can't use the same data for different tests. They are different:



      Regards,
      Sandeep

    • fx213
      Subscriber

      Dear Sandeep Medikonda,


      Thank you for your suggestions.


      I am sorry for the long discussions.Thank you for your time.


       


      Kind regards, 


      FXanthaki


       

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