General Mechanical

General Mechanical

Radial Stent Expansion and Force Measurement

    • Jayden
      Subscriber

      Hi, 


      I'm trying to expand a stent and measure the radial force required for the expansion and then i want to apply a force in the axial direction and measure the force reaction due to that displacement. 


      However I'm having trouble trying to constrain the stent properly so i was wondering if someone could help. 


      I have cut the model in half and applied a symmetry condition on the 3 ends as shown. I have applied a -2mm displacement to the inner face using the "Normal to" command and large deflection and weak springs is turned on. However the force reaction from this displacement condition alone is too small and not realistic


      I then tried adding a second displacement condition based on a new coordinate system added to the middle of the stent. I set the x direction (axial) to zero and the y and z directions to be free. this gave a force of approx 7 N which seems more realistic.


      However, when i try and add a 3rd displacement in the axial direction, the solution fails and won't converge. 


      the material used is Cobalt Chromium MP35N with bilinear hardening and Yield stress of 450 MPa and Tangent modulus of 1705 MPa and i have it set up as a 2 step simulation and i have specified substeps for each displacement process.


      If someone could provide some assistance for this it would be much appreciated. I have attached some pictures from the model. 


      Thanks  

    • peteroznewman
      Subscriber

      Please review this post.


      Also, ANSYS staff are not permitted to open attachments, so please also use the Insert Image button to put the image into the post.


    • Jayden
      Subscriber

      thanks for the response and no problem. 


      As i am not using a cylinder to expand the stent so it is not a contact problem, i want to just expand the stent uniformly using a displacement and measure the force due to that displacement.


      However when i apply the displacement to the 3 vertices in the centre like suggested in the linked forum, I got a conflicting DOF error. 


      Is there another way you suggest to constrain it?


       

    • peteroznewman
      Subscriber

      If you got an error, you have a mistake. This is a proven constraint pattern. 

    • Jayden
      Subscriber

      The white dots resemble the 3 vertices i've selected, 


      However, i cannot use the normal to displacement on the inner face to expand the stent because of the DOF error, however im restricted to only using displacements for the stent  and if i change the radial direction to a constant value it displaces the whole stent rather than expanding. So i am not quite sure where my error is


       


       

    • peteroznewman
      Subscriber

      The Cylindrical Coordinate System does not have the Z axis along the stent axis. That is your mistake.  Rotate the Cylindrical Coordinate System using the Rotate buttons on the Coordinate System tab.

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