LS Dyna

LS Dyna

My Mooney Rivlin material keeps expanding rapidly on impact! Any solutions?

    • MadzRedpath
      Subscriber
      I am trying to model a hip with soft tissue impacting the ground using ANSYS 18 Explicit. From a paper, the soft tissue should be modelled as hyperelastic and it used the Mooney-Rivlin model. The values it states are:



       
      C10 = 85.5kPa
      C01 = 21.38kPa
      I then calcuated d1 to be 9.39e-8.
       
      But when the soft tissue impacts the floor it expands rapidly and the femur just continues to travel through the tissue!
       
      Any ideas why this is happening or how I can fix it?
       
      Thanks Maddy
    • peteroznewman
      Subscriber

      Hello Maddy,


      Do you mean the soft tissue is the body mass around the hip, from the skin inward?


      What is the impact velocity? What other BCs have been applied?


      The Body Interactions item should be sufficient to keep elements from penetrating, but you can also add contact to help the interface from penetrating.


      If you attach a Workbench Project Archive file (.wbpz) to your reply, I will take a look at your model. The archive file has to be < 120 MB to attach. If it is larger, you can Clear Generated Data on the mesh, save and the archive will be smaller.

    • MadzRedpath
      Subscriber

      Hello Peter,


      Yes, by soft tissue I mean all soft tissue surrounding the hip. The initial velocity is 3.8m/s and the BCs are the fixed support at the floor. Either side of the femur I have tried to represent the body as a mass-spring damper system using data from a paper. 


      The file I have attached is a perfect example of what I mean! Unfortunately I had to delete the results as the file was too big. Hopefully you can help. Thank you!

    • peteroznewman
      Subscriber

      Hello Maddy,


      I am solving your model on 18.2.  What version are you using?


      I didn't understand why different bodies had different initial velocities, so I set them all to 3.8 m/s.


      I also turned off element erosion and turned up the output to 2000 frames. I will check back when it has finished.


      Regards,
      Peter

    • MadzRedpath
      Subscriber

       Hello Peter,


      I am using version 18.1. 


      Thank you so much for looking at this for me, any advice would be greatly appreciated. 


      Kind regards,


      Maddy 

    • peteroznewman
      Subscriber

      Hello Maddy,


      I stopped the above simulation after the first 10 milliseconds to review the results so far.


      A few other observations.


      1) You have a gap between the floor and the soft tissue. That is just wasting solve time moving them into contact. Make the floor tangent with the soft tissue. I can't change that because I don't have the geometry.


      2) I expect the soft tissue to be bonded to the thigh bone and pelvic bone, not left with frictionless contact. I think the soft tissue is just sliding down the bone. I have added bonded contact.


      3) I would change the Body Interaction to frictional, since the soft tissue sees friction with the floor.


      4) I changed the Mesh size from Default to 10 mm and it made a much nicer mesh and didn't change the solution time much.


      5) When you cut the thigh soft tissue, you are removing the support the missing tissue provided. Maybe better to keep the whole thigh. I can change that because I don't have the geometry.


      I will rerun these combined with the common velocity changes mentioned above and see what I get.


      Regards.


       

    • peteroznewman
      Subscriber

      Hello Maddy,



       


      After you look at that video, you can see that the point mass on the left side of the screen is having an excessive influence on the model. It needs its own spring to impact the floor since it "keeps going" past where it should have been supported by a floor that isn't there.


      Regards,
      Peter


       

    • MadzRedpath
      Subscriber

      Hello Peter,


      I want to thank you so much! You save my final year project and I would love to put you in my acknowledgments section of my report. Are you associated to a university or company? 


      Thank you so much again!


       


      Maddy 

    • peteroznewman
      Subscriber

      Hello Maddy,


      I'm glad to hear your project was a success! Thanks for thinking of putting me in the acknowledgements.  You can list me as:


               Peter Newman, ANSYS Student Community Moderator


      I am not associated with a university. I design products for a medical device manufacturer of digital imaging equipment. I love to learn how to simulate different types of problems, which is why I enjoy helping out here, but I do that in my own time.


      Warm regards,


      Peter

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