General Mechanical

General Mechanical

pressure distribution in the contact zone of a spherical joint/ ball joint of a hip implant

    • ArthurKraus
      Subscriber

      Hello,


      I am currently struggeling with a static mechanical analysis with ANSYS WORKBENCH 17.2. I am using a ball joint as a part of a hip implant. In the picture you can see the socket of the joint with an area that is marked red.


      contact zone at the inner edge of the socket


      This area has the highest local pressure. I assume that the socket transfers its force to the ball through the edge instead of the transfering it through the areal contact. How do I avoid the local pressure maximum being in the edge of the socket?


      Is use four parts: a femoral bone, the shaft of a hip implant and a ball + socket for the ball joint.


      The bone is fiexed at specific points. The shaft of the implant is connected to the bone and the ball. The contact between the ball and the socket has a friction coefficient of 0.11. The outer plane of the socket has a force and an external displacement with degrees of freedom set to "0" for rotation and "free" for translation, sothat the socket won't drift of the ball. the ball and the inner area of the socket have the same radius.


       


      boundary conditions


      I appreciate every help I can get here.

    • Sandeep Medikonda
      Ansys Employee

      This looks like a good candidate for spherical joints, why not use that instead of contact?  Maybe you are, I can't quite tell because of the language.


      I would recommend you right-click on the Solution, insert a Contact Tool and look at the contact pressure that is being applied. This will also give you an idea of why the stress is high where it is. If you are using anything other than Program Controlled in the Formulation for your contact, maybe changing that back and using an Augmented-Lagrange with a small Normal Stiffness factor (0.1, for example, should help). This will allow for some penetration but might not apply a higher force on the contact body.


      Regards,
      Sandeep
      Guidelines on the Student Community

    • ArthurKraus
      Subscriber

      Hello and thank you for you fast reply. Unfortunately I still have that problem. I will try to give more input on my setup, I hope there won't be too many translation issues.


      I've been trying a lot of different setups. For example:


      - contact with friction


      - spherical joint (body on body)


      - contact with friction + spherical joint at the same time


      I also used all the setups with and without the external displacement.


       


      Here is what I have been using as a contact setup for the simulation:


      Contact : socket


      Target: Ball


      Contact with friction


      coefficient of friction 0,11


      ...


      Algorythm: Augmented-Lagrange


      Contact stiffness: 0,01 (I was experimenting with that value - it worked out a little better until the mesh got finer)


      refresh contact stiffness: every iteration


      ...


      Contakt treatment: adapt to touch/contact


      ...


      All the "..." options are options I didn't touch, so they are program controlled in most cases.


       


      The contact tool in the solution gave me quite a similiar pressure distribution - highest pressure is near the inner edge of the socket.


      contact tool


      Also, if I cut the two bodys, you can see, that the equivalent von mises stresses seem to go through the edge and not through the whole surface.


      equivalent stress (von mises)


      Also the bodies penetrate. not sure if that is because of the contact stiffness i used. (quite hard to see, but the picture quality reduces a lot when I upload a picture)


      contact penetration


       


      I have no ideas anymore, it seems to be a very basic problem. Maybe it's the mesh. And I have about a week from now on to solve that problem.


      I also tried to use two different diameters for the two bodies (difference of 2mm) and then I made them touch in one point. Right now it does not seem too bad, but I remember trying that out before and at some point I got a bunch of errors when I tried to use a finer mesh. That's how it looks like (again high stresses in near the edge although there is a gap)


      mises stress


      But at least my contact pressure seems right:


      contact pressure


       


      I will try to make the mesh finer once more until I have a convergence for my maximum contact pressure (now that I know how to use the contact tool). I will let you know if that works out for me.


       


      Regards


      Arthur


       


       

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